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Electrochemical reduction of 1,10-dihalodecanes at mercury cathodes in dimethylformamide
129
J. Electroanal. Chem., 280 (1990) 129-144 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
Electrochemical reduction of l,lO-dihalodecanes cathodes in dimethylformamide John C. Bart and Dennis G. Peters Department (Received
of Chemistry,
at mercury
l
Indiana University, Bloomington,
27 June 1989; in revised form 21 September
IN 47405 (U.S.A.)
1989)
ABSTRACT
In dimethylformamide containing tetramethylammonium perchlorate, reduction of l,lO-diiododecane yields two irreversible voltammetric waves at mercury, whereas reduction of l.lO-dibromodecane gives only one irreversible wave. Quantitative comparison of this behavior with that known for I-iodo- and 1-bromodecane suggests that l,lO-diiododecane is reduced in a pair of two-electron steps and that l,lO-dibromodecane undergoes a single four-electron reduction. Preparative-scale electrolyses of l,lO-diiododecane at a potential on its first reduction wave reveal that an organomercury polymer is the only product. At potentials corresponding to the second wave for l,lO-diiododecane and to the first wave for l,lO-dibromodecane, the electrolysis products include decane, 1-decene, 1,9-decadiene, I-decanol, and 9-decen-1-01 along with telomers formed by attack of solvent-derived radicals on either l-decene or 1,9-decadiene. When electrolyses of l,lO-diiodoand l,lO-dibromodecane are performed in the presence of a proton donor (diethyl malonate), the yield of decane increases, but no alcohols or unsaturated compounds are formed; instead, products such as diethyl n-decylmalonate and diethyl n-eicosylmalonate are found which arise from attack of the diethyl malonate anion on unreduced starting material.
INTRODUCTION
There have been numerous publications [l-9] dealing with the electrochemical reduction of cY,w-dihaloalkanes at mercury cathodes. Zavada et al. [l] investigated the polarographic behavior of 1,3-dibromopropane, 1A-dibromobutane. 1,5-dibromopentane, and 1,6-dibromohexane in dimethylformamide containing tetramethylammonium bromide and 5% water; on the basis of logarithmic analysis of the polarographic waves, these workers concluded that 1,3-dibromopropane undergoes a two-electron reduction and that the other three compounds are reduced in a pair of two-electron steps which cannot be resolved into discrete waves. In a subsequent
l
To whom correspondence
0022-0728/90/$03.50
should
be addressed.
0 1990 Elsevier Sequoia
S.A.
130
paper, Dougherty and Diefenderfer [2] reported that 1,4-dibromobutane exhibits two polarographic waves in nominally dry dimethylformamide containing tetramethylammonium perchlorate; electrolyses done at potentials corresponding to each of the two waves revealed that the reduction products are I-bromobutane and butane, respectively. In some of the earliest work concerning the electrochemistry of a,o-dihaloalkanes, Rifi [3-51 synthesized cyclopropane by electrolyzing 1,3-dibromopropane at mercury in dimethylformamide containing tetra-n-butylammonium bromide; in similar fashion, he obtained cyclobutane (along with butane) by reducing 1,4-dibromobutane, but only straight-chain alkanes were produced from either 1,5-dibromopentane or 1,6-dibromohexane. Rifi suggested that electroreductive cyclization of 1,3-dibromopropane and 1,Cdibromobutane proceeds via (a) a transition state, formed by uptake of two electrons, which resembles the final carbocyclic product or (b) a bromoalkyl carbanion which cyclizes faster than it is protonated. Brown and Gonzalez [6] performed controlled-potential electrolyses of 1,3-dibromopropane at mercury in acetonitrile containing tetraethylammonium bromide. They observed a potential-independent product distribution - cyclopropane (30%) and propane (61%) - which differed from that seen by Rifi [3]. In addition, when Brown and Gonzalez reduced 1,Cdibromobutane in dimethylformamide containing either tetraethylammonium bromide or tetra-n-butylammonium perchlorate, the products (which did not include cyclobutane) were butane, I-bromobutane, and dibutylmercury along with butadiene and a solid characterized as a butylmercury oligomer - again, results differing dramatically from those obtained by Rifi. To account for the formation of the various products, Brown and Gonzalez proposed that 1,4-dibromobutane is reduced to bromobutyl radicals and butyl diradicals, both in an adsorbed state, which undergo subsequent polymerization, reduction, and protonation; similar processes were invoked to describe the electrochemistry of 1,3-dibromopropane. Although inconsistencies found in these two reports may be largely due to the different solvents and electrolytes employed and, more importantly, to the use or lack of potential control, a clear picture of the behavior of 1,3-dibromopropane and 1,4-dibromobutane is not available. Uncertainties in the electrochemistry of 1,3-dibromopropane and 1,4-dibromobutane are only amplified by the results of a study by Wiberg and Epling [7], who investigated the reduction of these two dibromoalkanes at mercury and platinum in dimethylformamide containing tetraethylammonium bromide. According to these workers, a cyclic voltammogram for 1,3-dibromopropane exhibits two waves that correspond to stepwise formation of the 3-bromoprop-l-y1 radical and carbanion, respectively. Preparative-scale electrolyses afford mainly propane when performed at a potential on the first wave, whereas cyclopropane is the major product at potentials on the second wave. Electrolysis of 1,4-dibromobutane shows a similar pattern; butane is more important at less negative potentials, whereas cyclobutane is the dominant product at more negative potentials. Ruling out the intermediacy of organomercury species, Wiberg and Epling suggested that the carbocycles arise either (a) via one-electron reduction of the initially generated
131
monohaloalkyl radical followed by ring closure or (b) via direct two-electron reduction of the starting material to yield the carbocycle in a single step. Two relatively recent papers have been concerned with the electrolysis of cw,w-dibromoalkanes to produce organomercury species. Casanova and Rogers [8] demonstrated that controlled-potential reduction of cw,w-dibromoalkanes at mercury in dimethylformamide containing tetraalkylammonium salts leads to the formation of dialkylmercury compounds in yields ranging from 49% to 96% when the carbonchain length is at least four. On the other hand, Avaca, Gonziilez, and Stradiotto [9] concluded that linear polymers of the type RHg(RHg),R are obtained from electrolyses of 1,4-dibromobutane or l,%dibromopentane at mercury in dimethylformamide or acetonitrile containing a variety of supporting electrolytes. In the work described herein, we have investigated the electrochemical behavior of l,lO-diiodoand l,lO-dibromodecane at mercury cathodes in dimethylformamide for several reasons. First, no study of the reduction of an ~,~-diiodoalkane has been reported previously. Second, because carbon-iodine bonds are generally easier to reduce than carbon-bro~ne bonds, electrolyses of an ff,~-diiodoalkane can be done at less negative potentials; under such conditions, radical intermediates are more likely to be generated so that their chemistry and electrochemistry can be elucidated. Third, this research is an extension of earlier publications [lo-121 from our laboratory pertaining to the reduction of l-iodo- and l-bromodecane. EXPERIMENTAL
Reagents Tetramethylammonium perchlorate, purchased from the G. Frederick Smith Chemical Co. and employed as supporting electrolyte, was recrystallized from distilled water before use and was stored in a desiccator. Burdick & Jackson “distilled-in-glass” dimethylformamide used as solvent was subjected to further purification; it was vacuum distilled as described in earlier work [13] to remove impurities such as formic acid and N-methylformamide, it was dried over Davison 4-A molecular sieves to minimize trace amounts of water, and it was stored in the dark under an atmosphere of argon. Commercially available l,lO-diiododecane and l,lO-dibromod~ane (Aldrich Chemical Co.) were used without further purification. Triply distilled mercury (Troy Chemical Corp., Wood Ridge Chemical Division) was used for the preparation of working electrodes. Deaeration of solutions for all electrochemical experiments was done with argon (Air Products UHP).
Instrumentation and procedures Described in prior work [ 131 is a cell designed for polarography and cyclic voltammetry. Equipment employed for normal pulse polarographic studies included a Princeton Applied Research Corp. (PARC) Model 174 polarographic analyzer and a PARC Model 174A/70 drop timer. For cyclic voltammetric work we used a PARC Model 173 potentiostat-galvanostat with a PARC Model 176 current-tovoltage converter and a PARC Model 175 universal programmer to produce the
132
desired waveforms. To record polarograms and cyclic volta~ograms, we utilized a Houston Instruments Model 2000-5-5 X-Y plotter. A cell for controlled-potential electrolyses with a mercury pool cathode has been described in a previous paper [14]. Electrolyses were accomplished with the aid of the aforementioned potentiostat-galvanostat and current-to-voltage converter that provided iR compensation. We controlled the course of each electrolysis with an IBM personal computer interfaced to the potentiostat via a home-built analog-todigital converter; the current-time curve was monitored, stored, and integrated to provide the coulometric n value for the electrolysis. Other details concerning the procedure for controlled-potential electrolyses are presented in an earlier publication [14]. All potentials are quoted with respect to a reference electrode consisting of a saturated cadmium amalgam in contact with dimethylformamide saturated with both cadmium chloride and sodium chloride [15,16]; this reference electrode has a potential of - 0.75 V versus the aqueous saturated calomel electrode at 25 o C. To prepare a solution which has been electrolyzed for subsequent gas chromatographic analysis, the solution was partitioned between water and diethyl ether; the ether phase was washed five times with a saturated solution of sodium chloride to ensure the complete removal of dimethylformamide and supporting electrolyte, dried over anhydrous magnesium sulfate, and concentrated by rotatory evaporation to a volume of l-2 ml. Gas chromatographic analyses were performed with a Varian Model 3700 dual-column instrument equipped with flame ionization detectors. For the separation, identification, and quantitation of simple electrolysis products (decane, 1-decene, l,Pdecadiene, I-decanol, and 9-decen-l-ol), we employed a medium-polarity, wide-bore capillary column (10 m X 0.53 mm, RSL-300, Alltech Associates); a 50-cm length of the same capillary column was utilized to obtain chromatograms for high-molar-mass species (either alkylated diethyl malonate or telomers formed by attack of solvent-derived radicals on the simple olefinic products). To quantitate various products, known amounts of non-electroactive internal standards (dodecane and triacontane) were added to each solution prior to the start of an electrolysis. Gas chromatograp~c response factors were determined experimentally with authentic samples of the various electrolysis products, and product yields are reported in terms of the absolute percentage of starting material incorporated into a particular compound. Product id~nti~jc~tj~~
Decane, 1-decene, 1,9-decadiene, l-decanol, and 9-decen-l-01 were all identified as products by comparison of their gas chromatographic retention times with those of commercially available authentic samples. To characterize and identify the solid organomercury polymer, both the organic and mercury content of the material were established. For the analysis of the organic portion of the polymer, we used a previously described procedure [17]. A portion of the solid was treated for 10 min with 1 ml of a saturated solution of bromine in glacial acetic acid; bromine cleaves each carbon-mercury bond and
133
converts the hydrocarbon part of the polymer into a mixture of l-bromodecane and l,lO-dibromodecane. Then the mixture was extracted with diethyl ether. Next, the ether extract was washed with a solution of sodium thiosulfate to reduce any excess bromine, washed three times with water, dried over anhydrous magnesium sulfate, and concentrated by evaporation to a volume of a few ml. Finally, the quantities of 1-bromodecane and l,lO-dibromodecane in the ether extract were determined by means of gas chromatography. A modified version of the Volhard procedure [18] was employed for measurement of the mercury content of the organomercury polymer. After some of the material was recovered from an electrolyzed solution, the solid was dried by being washed with diethyl ether; gold foil was used to collect and remove spherules of elemental mercury clinging to the organomercury polymer. A 99.2-mg sample of the dried material was treated with 1 ml of concentrated nitric acid; the mixture was heated gently to decompose and dissolve the polymer, 9 ml of distilled water was added, and the solution was boiled to expel oxides of nitrogen. Next, 10 ml of distilled water was added, the solution was cooled below room temperature, 1 ml of a 0.3 M solution of iron(II1) nitrate was introduced as an end-point indicator, and the solution was titrated with 4.794 ml of 0.1117 M potassium thiocyanate. These data show that the organomercury polymer contains 54.14% mercury. If the polymer is taken to have the formula RHg(RHg),R, where R is a decyl (or decamethylene) group, and if we assume that the polymer consists essentially of decyl groups and mercury in a 1 : 1 molar ratio, we calculate that the mercury content of the polymer should be approximately 58.9%. However, if the polymer has the formula RHg,(RHg,),R, a substance with close to a 1 : 2 decyl group-to-mercury ratio, the mercury content should be about 74.1%. Thus, we conclude that the former one-to-one nature of the organomercury polymer is correct. Gas chromatograms for concentrated ether extracts of electrolyzed solutions of the dihalodecanes showed peaks attributable to a family of at least nine telomers formed by reaction of 1-decene or 1,Pdecadiene with solvent-derived radicals. A few of these species were identified with reasonable certainty by means of gas chromatography-mass spectrometry. A 2 : 1 telomer composed of two decyl moieties and a dimethylamino group, C,,H,,N(CH,),, exhibited a parent peak (m/e = 325) as well as mass spectral peaks for alkyl fragments ranging from five to nine carbons in length; because there were no fragments with 10 or more carbons, we believe that the two decyl chains are not connected end-to-end, but perhaps via a bond between interior carbons. Another substance appeared to be a 3 : 1 telomer composed of three decyl moieties and a dimethylamino group; although a parent peak (m/e = 465) was not detected, peaks were observed for alkyl fragments containing from five to twenty-one carbons. Finally, we saw a third prominent gas chromatographic peak having a retention time close to that of the aforementioned 2 : 1 telomer; although this species probably contains two decyl groups, its mass spectrum did not show a parent peak and other portions of the mass spectrum were ambiguous. Alkylated diethyl malonate species were identified with the aid of gas chromatog-
134
raphy-mass spectrometry and, in the case of diethyl n-decylmalonate, by direct comparison with the mass spectrum of authentic, synthesized material. Diethyl n-decylmalonate was prepared by dissolution of sodium metal (2.3 g, 0.10 mol) in 50 ml of absolute ethanol; next, diethyl malonate (18.1 ml, 0.12 mol) was s/uwly dripped into the solution and, finally, 1-bromodecane (20.7 ml, 0.10 mol) was added. After the solution was stirred for 30 min, it was treated with diethyl ether; the ether solution was washed twice with water, dried over anhydrous magnesium sulfate, and concentrated by evaporation. Pure (> 97%) diethyl n-decylmalonate was recovered by means of vacuum distillation: b.p. 121-126O C (0.01 Torr); mass spectrum, m/e 300, M+ (1.2%); 255, Mf - OC,H, (12%); 173, M” - C,H,, (57%); (19%); 132, M+ - C,,H, (11%); 159, M+ - C,,H,, (100%); 133, M+ -&f-I,, 127, C,H& (14%). These mass spectral data are virtually identical to those for one of the compounds produced el~trolytically. Diethyl n-eicosylmalonate, another significant product, has the following mass spectrum: m/e 313, M+ - C,H,, (4%); 312, M+ - C,H, (6%); 285, M+ - C,,H,, (9%); 267, M+ -(C,H500QzCHCHz (5%); 239, M+ -(C,H,OOC),CH(CH,), (13%); 173, M+ - C,9H39 (84%); 159, M+- C&H,, (100%); 133, M+ - C,,H,, (41%); 132, M+ - C,,H, (10%); 127, C,H&, (17%). For the latter compound, the intensities of signals corresponding to M+ and M+ + 1 species were so small compared to the major peaks listed that the assignments of masses and relative abundances were not possible; on the other hand, the existence of the malonate fragment and a twenty-carbon chain is strong circumstantial evidence that this substance is diethyl n-eicosylmalonate. Small amounts of another alkylated diethyl malonate were detected; unfortunately, mass spectral data provided no conclusive information about its identity. No mass spectrometric evidence was found that any of the alkylated diethyl malonate species is halogenated. RESULTS
AND DISCUSSION
Figure 1 shows normal pulse polarograms for 1.00 mM solutions of l,lO-diiodoand l,lO-dibromod~e in dimethylfo~~de cont~ning 0.1 M tetramethyl~monium perchlorate. For the reduction of l,lO-diiododecane, the polarogram depicted in Fig. 1A exhibits two well defined waves with half-wave potentials of - 0.93 and - 1.36 V, and the nearly identical diffusion currents indicate that an equal number of electrons is transferred in the reaction for each wave. For otherwise identical conditions, comparison of Fig. 1A to a pulse polarogram for 1-iododecane - known to undergo two sequential one-electron reductions 1121- reveals that the current for each wave for l,lO-diiododecane has twice the magnitude of that for the waves seen for the monoiodo species. Therefore, each wave for reduction of l,lO-diiododecane corresponds to a two-electron process; moreover, as revealed later in this paper, the first wave is due to reductive cleavage of each carbon-iodine bond to produce a decyl diradical, whereas the second wave is attributable to reduction of the decyl diradical to a dianionic species.
135
4
-0.2
-0.6
-1.0
-1.4
-1.8
!.2
POTENTIAL/V
Fig. 1. Normal pulse polarograms for reduction of 1 mM solutions of l,lO-dihalodecanes in dimethylformamide containing 0.1 M tetramethylammonium perchlorate recorded at a scan rate of 5 mV s-‘. A controlled drop time of 0.5 s was used; the pulse width and sampling period were 56.7 and 16.7 ms, respectively. (A) l,lO-diiododecane; (B) l,lO-dibromodecane.
In contrast to the behavior observed for l,lO-diiododecane, only one wave with a half-wave potential of - 1.42 V is seen in the pulse polarogram for l,lO-dibromodecane (Fig. 1B). If the latter polarogram is compared with a pulse polarogram for a 1.00 mM solution of I-bromodecane, which undergoes one-step, twoelectron reduction, we find that, other conditions being the same, the height of the wave for the dibromo species is twice the height of the wave for the monobromodecane. Apparently, l,lO-dibromodecane is reduced to the decyl dianion in a single four-electron process. Whenever the potential is negative enough to permit reduction of a dihalodecane to the decyl dicarbanion, it should be noted that the process must occur via an intermediate radical which may have only transient existence. It is noteworthy that the half-wave potentials mentioned above for stepwise reduction of l,lO-diiododecane at mercury in dimethylformamide containing 0.1 M tetramethylammonium perchlorate are precisely the same as those for 1-iododecane measured under comparable conditions [12], except for the already indicated difference in the diffusion currents. Similarly, the half-wave potential for reduction of l,lO-dibromodecane ( - 1.42 V) is similar to those observed in earlier studies for I-bromodecane (-1.40 V) [lo] and 1-bromo-Sdecyne (-1.41 V) [19] in the same supporting electrolyte-solvent, although the diffusion current for the dibromo compound is twice that for either of the monobromo species when the concentrations of all substances are identical.
136
POTENTIAL/V
Fig. 2. Cyclic voltammograms for reduction of 1 mM solutions of IJO-dihafodecanes in dimethyiformamide containing 0.1 M tetramethylammonium perchlorate recorded at a scan rate of 200 mV s-’ with a hanging mercury drop electrode. (A) l,l~D~~~~ne; (B) l,l~dibrom~~ne.
Cyclic voltammetric
behavior of I, 1O-dihalodecanes
Figure 2 presents cyclic voltammograms recorded at 200 mV s -’ for 1.00 mM solutions of l,lO-diiodo- and l,lO-dibromodecane in dimethylformamide containing 0.1 M tetramethylammonium perchlorate. In Fig. 2A, we attribute the abnormal (almost gaussian) shape of the first voltammetric wave for l,lO-diiododecane to the fact that decyl diradicals generated during the first stage of reduction are adsorbed onto the surface of the hanging mercury drop electrode. These adsorbed radicals effectively prevent diffusing starting material from being reduced at the electrode, and the current falls nearly to the baseline level at - 1.2 V; however, at potentials more negative than - 1.3 V, the current rises, peaks, and decays normally because adsorbed decyl diradicals undergo further reduction to nonadsorbed dianions. On the other hand, a cyclic voltammo~am (Fig. 2B) for reduction of l,lO-dibrom~ecane is without any unusual feature; there is only a single irreversible wave with a peak potential of - 1.46 V. Controlled-potential
electrolysis of I, I O-diiododecane at - I.10 V
We performed electrolyses of l,lO-diiododecane at a mercury pool cathode held at - 1.10 V in dimethylformarnide containing 0.1 M tetramethylammonium perchlorate. For concentrations of starting material below 3 mM, the coulometric n value is very close to two and the only product is a gray solid; gas chromatographic examination of a concentrated diethyl ether extract of the electrolyzed solution showed that no soluble product containing the decyl moiety is formed. It should be noted that electrolyses done with concentrations of starting material higher than 3
137
mM were always incomplete due to fouling of the mercury electrode by the gray solid. When treated with a saturated solution of bromine in glacial acetic acid, the gray solid was converted into l,lO-dibromodecane (98%) and l-bromodecane (2%); l,lO-dibromodecane must be derived from decamethylene groups in the body of the polymer, whereas l-bromodecane can only arise from polymer-ending decyl moieties. Separate analysis of the polymer for mercury revealed the presence of essentially one mercury atom for every decyl or decamethylene group. Thus, the organomercury polymer appears to be a material having the general formula
CH,(CH,),Hg[(CH,),,Hgl,(CH,),CH,. We propose that the first step in the mechanism for production of this polymer is one-electron reduction of each carbon-iodine bond of l,lO-diiododecane to form a diradical, which is adsorbed onto the cathode I(CH,),,I
+ 2 e- -+‘CH,(CH,),CH;(ads)
and which very quickly diradical: ‘CH,(CH,),CH;(ads)
incorporates
+ 2 Imercury
to give an adsorbed
(1) organomercury
+ Hg+‘CH,(CH,),Hg’(ads)
(2)
Besides the cyclic voltammetric evidence provided earlier in this paper (Fig. 2A), considerable precedence is found in earlier work [6,8-10,12,17,20-221 that alkyl radicals can be adsorbed onto mercury and that mercury can be incorporated into the absorbed species. Although the lifetime of an adsorbed organomercury diradical is not known, there is no reason to suspect that this species survives longer (10 ms) than more well investigated [30] adsorbed radicals such as ethylmercury and n-pentylmercury before being transformed into a stable and isolable material: (n + 2) ‘CH,(CH,),Hg’( +Hg
a d s) 3
CH,(CH,),Hg[(CH,),,Hg]
&H&H, (3)
Presumably, the solvent (dimethylformamide) provides hydrogen atoms to form the terminal methyl groups which limit the extent of polymer growth. On the basis of the relative yields of l,lO-dibromodecane (98%) and l-bromodecane (2%) obtained by degradation of the gray solid in the presence of bromine, conceivably as many as 100 decylmercury diradical units are linked together to form a single polymer chain. On the other hand, if the mercury content (54.14%) of the polymer is used to determine its length, as few as five decylmercury diradical units may constitute a single organomercury polymer chain. Although the analytical data cannot establish a precise chemical formula for the polymer unambiguously, it is nevertheless clear that the material is properly represented as shown in reaction (3). It is interesting to compare the results we have obtained for the reduction of l,lO-diiododecane at - 1.10 V with previous findings of several other groups of workers. Gowenlock and Trotman [23] reported that electrolysis of several organomercury halides at a platinum cathode in liquid ammonia results in the production of isolable solids characterized as being organic metals comprised of a three-dimensional network in which each mercury atom is bound to six organic
138
moieties; these authors rejected alternative ways to describe the solids, including the possibilities that the materials consist of organomercury radicals, of amalgams of mercury with organic free radicals, or of organic calomels (RHgHgR). Using a galvanostatic method, Ershler et al. 1241 examined the behavior of a series of alkylmercury acetates in water and aqueous methanol containing potassium acetate; they concluded that each alkylmercury acetate is reduced to adsorbed organomercury radicals which, after reacting with more starting material to yield a metastable organic calomel, are converted into dialkylmercury species. Among the products obtained by Brown and Gonzalez [6] in their study of the reduction of 1,4-d& bromobutane at mercury was dibutylmercury along with a white solid determined with the aid of elemental analysis and mass spectrometry to be a butylmercu~ compound having the formula C,H,HgC,HsHgC,+H,. In a similar study, Avaca et al. [9] performed controlled-potential electrolyses of 1,4-dibromobutane and Wdibromopentane at mercury in either dimethylformamide or acetonitrile which contained tetraalkylammonium perchlorates. From both of the dibromoalkanes, these investigators obtained high yields of solids that, upon mass spectrometric examination, were discovered to be most likely alkylmercury polymers of the general composition RHg(RHg),R. Cyclic alkylmercury polymers, (RHg),, as well as organic calomels, RHg,(RHg,),R, were ruled out as probable identities for the solids. Finally, in a paper by Casanova and Rogers [8], controlled-potential electrolyses of a series of dibromoalkanes ranging from 1,4-dibromobutane to l,lZdibromododecane at mercury cathodes in dimethylformamide containing tetraalkylammonium salts demonstrated that dialkylmercury compounds can be obtained in substantial yield; moreover, evidence was offered - including the isolation of di-n-hexyldimercury after reduction of l,&dibromohexane - that organic calomel species are involved at least as intermediates in the formation of diorganomercury species. Controlled-potential
electrofyses of I, IO-dihaiodecanes
at - I. 70 V
To examine the behavior of l,lO-dihalodecanes under conditions where the carbon-halogen bond at each end of the molecule can undergo reductive cleavage to form a carbanion - a process involving transfer of four electrons to each molecule - we performed controlled-potential electrolyses at - 1.70 V. Table 1 lists coulometric n values and product dist~b~tions at this potential for electrolyses of l,lO-diiodo- and l,lO-dibromodecane at mercury pool cathodes in dimethylformamide containing 0.1 M tetramethylammonium perchlorate. In contrast to the single product, an organomercury polymer, obtained from l,lO-diiododecane at - 1.10 V, the electrolysis of both of the l,lO-dihal~~anes at - 1.70 V affords a number of species that result from (a) direct two-electron reduction of the carbon-halogen bonds, (b) non-electron-consuming substitution and elimination reactions, and (c) attack of solvent-derived dimethylamino radicals on olefinic substances formed from starting material. Note that consumption of the dihalodecanes by non-electrochemical processes is responsible for the fact that the coulometric n values for both compounds are significantly less than four (Table 1).
139 TABLE
1
Coulometric data and product distributions l,lO-dibromodecane in dimethylformamide -1.70 v n value
Compound
2.08 b 1.83
for electrolytic reduction of 2.5 mM l,lO-diiododecane containing 0.1 M tetramethylammonium perchlorate
Product
distribution
Decane
I-Decene
1,9-Decadiene
I-Decanol
32 20
26 17
10 10
<5
l,lO-Diiododecane l,lO-Dibromodecane
a
a Results b Results
from five separate electrolyses. from three separate electrolyses.
obtained obtained
To account reactions: X(CH,),,X X(CH,
for the formation
+ 2 e- + X(CH,),i
),& + Hz0 -+ X(CH,),CH,
X(CH,),CH3
%
of decane,
9-Decen-l-01 4
5
we postulate
c5
the following
+ X-
Telomers S-5 > 10
sequence
of
(4)
+ OH-
+ 2 e- + - CH,(CH,),CH,
-CH,(CH,),CH,
and at
(5) + X-
+ H,O + CH,(CH,),CH,
(6)
+ OH-
(7)
Despite efforts to eliminate it, water present as a contaminant in dimethylformamide serves as the principal donor of protons for carbanions electrogenerated in steps (4) and (6). To produce 1-decene, starting material is converted to l-halodecane according to steps (4) and (5) above; once formed, the 1-halodecane undergoes an E2 elimination reaction with the hydroxide ion arising from deprotonation of water in reactions (5) and (7): X(CH,),CH,
+ OH-
+ CH,=CH(CH,),CH,
Alternatively, hydroxide can displace substitution to yield l-decanol: X(CH,),CH,
+ OH-
+ HO(CH,),CH,
+ X- + H,O
halide
ion from a 1-halodecane
+ X-
(8) via an S,2
(9)
Of course, any of the 1-halodecane that escapes these chemical reactions should undergo reduction to form decane [lo-121. Among products obtained from electrolyses of both l,lO-diiodoand l,lO-dibromodecane at -1.70 V, 1-decene is approximately five times more abundant than 1-decanol (Table 1). Moreover, 1,9-decadiene (the product of a double elimination reaction) is formed in significant quantities, whereas l,lO-decanediol (the product of a double substitution reaction) is not observed at all. Thus, elimination reactions are much more favorable than substitution reactions under the conditions of our experiments. Formation of 1,Pdecadiene is caused by a pair of E2 elimination reactions involving attack of hydroxide ions on starting material: X(CH,),,X
+ 2 OH-
+ CH,=CH(CH,),CH=CH,
+ 2 H,O + 2 X-
(10)
140
In addition, one can postulate the formation of 9-decen-l-01 by attack of two hydroxide ions, one via an elimination reaction and one via a substitution reaction, on a molecule of unreduced l,lO-dihalodecane: X(CH,),,X
+ 2 OH- --, CH,=CH(CH,),CH,OH
+ H,O + 2 X-
(11)
Again, due to the relative rarity with which substitution reactions occur under the conditions of our electrolyses, 9-decen-l-01 is formed only in small quantities (Table I). Two other points should be mentioned. First, eicosane was not detected as an electrolysis product in any of our experiments. Second, although water is implicated as the major source of protons for carbanions, the tetramethylammonium cation has been shown previously [25] to be a viable proton donor under conditions which prevail during electrolyses of l,lO-dihalod~anes. An experiment was performed to establish the plausibility of the postulated chemical attack of hydroxide ion on carbon-halogen bonds of l,lO-dihalodecanes or 1-halodecanes. Tetraethylammo~um hydroxide (2.70 mM) was added to a 2.50 mM solution of l,lO-diiodod~ane in dimethylforma~de containing 0.1 M tetramethylammonium perchlorate; the tetraethylammonium hydroxide was used instead of tetramethylammonium hydroxide because of the comparatively low solubility of the latter salt in dimethylform~de. This solution was treated exactly as if it were to be electrolyzed at a mercury pool, except that no potential was applied to the electrode. After several hours, the solution was subjected to the usual procedure for separation and determination of products by means of gas chromatography. We found three products - 1-iodo-Pdecene (35%), 9-decen-l-01 (16%), and 1,9-decadiene (14%) - along with unreacted l,lO-diiododecane (24%). Such a result proves that hydroxide ion is capable of attacking carbon-halogen bonds via both E2 and S,2 reactions under conditions prevailing during an electrolysis and, moreover, that the E2 process is preferred over the S,2 displacement. One can explain the absence of 1-decene and 1-decanol by the fact that no electrolysis was actually done; reactions involving the carbon-iodine bonds were chemical, not electrochemical in nature. On the basis of product distributions listed in Table 1, it is evident that decane, 1-decene, 1,9-decadiene, 1-decanol, and 9-decen-l-01 account for not more than 77% of the electrolyzed l,lO-diiododecane and for less than 57% of the electrolyzed l,lO-dibromodecane. In an earlier investigation [lo] of the reduction of l-iodo- and I-bromodecane at mercury in dimethylfo~a~de containing tetra-n-butylammonium perchlorate, significant amounts of the starting materials were found to be incorporated into a family of species (including C,,H,,N, CJ2Hb7N, and C,,H,,NO) apparently formed by telomerization of I-decene that was initiated by solvent-derived radicals. Thus, it was appropriate to search for analogous materials that might be produced during electrolyses of l,lO-dihalodecanes. Concentrated ether extracts of solutions that had been electrolyzed gave gas chromatograms showing the presence of no fewer than nine telomers, three of which were especially prominent. With the aid of mass spectrometry, two of the latter
141
species were found to consist of two decyl groups and one dimethylamino group (2 : 1 telomer) and of three decyl groups and one dimethylamino group (3 : 1 telomer), respectively. It appears that most, if not all, of the telomers contain at least one dimethylamino moiety derived from dimethylformamide; procedures for the synthesis of some of the most likely teiomers are outlined elsewhere [lo]. We propose that the initiating reaction involves electrolytic generation of an alkyl radical intermediate by one-electron reduction of a carbon-halogen bond of the starting material. Such a radical can abstract a hydrogen atom from a solvent molecule to yield a dimethylcarbamoyl radical which, in turn, can decarbonylate to give a dimethylamino radical. Once formed, the dimethylamino radical could attack any unsaturated substance (1-decene, 1,9-decadiene, and perhaps even 9-decen-l-01 or 1-halo-9-decene) in solution to produce a variety of telomers. A possible mechanism for the formation of a 2 : 1 telomer is as follows: ‘N(CH,),
+ C,H,,CH=CH,
C,H,,CHCH,N(CH,),
+ C,H,,CHCH,N(CH,),
(12)
+ C,H,,CH=CH2
(13) However, a 2 : 1 telomer with a different carbon-chain branching could arise via attack of a dimethylamino radical on 1,Pdecadiene. Similarly, several different types of 3 : 1 telomers can be postulated. Moreover, there can be telomers with carbon-carbon double bonds, with more than one dimethylamino group, with multiple chain-branching, and with combinations of these functionalities. So, even if the most prevalent telomers incorporate only a few percent of the starting material, it is definitely possible that all of the telomers taken together could account for the consumption of 20-30% or more of the original quantities of l,lO-dihalodecanes. We believe that the reactions leading to formation of the telomeric species must begin with attack of an alkyl radical on the solvent to remove a hydrogen atom rather than with attack of an anion on the solvent to abstract a proton. Previous research [25] has shown that, in competition with water and the tetramethylammonium cation, dimethylformamide is a poor proton donor. Furthermore, even though the potential ( - 1.70 V) employed for electrolyses of the l,lO-dihalodecanes is in the region where anions are the expected species, alkyl radicals must nevertheless be formed at least transiently, so that hydrogen atom abstraction from the solvent becomes a conceivable event. One of the reviewers raised an interesting question. If alkyl radicals are key precursors in the production of telomers, why (as revealed in Table 1) is the yield of telomers obtained from l,lO-dibromodecane apparently higher than that derived from l,lO-diiododecane? It is possible to offer a somewhat speculative answer to this query. In previous studies of the electrochemical behavior of alkyl monohalides at mercury cathodes [17,19], we found that alkyl bromides tend to exhibit more carbanion character in their reductions than do alkyl iodides. If this trend is
142
applicable to the dihalodecanes, we would expect carbanions to arise more readily from IJO-dibromodecane than from l,lO-diiododecane. Consequently, for the dibromo compound, there might be more deprotonation of adventitious water by carbanions to form hydroxide ions, more hydroxide-promoted dehydrohalogenation of unreduced starting material to give 1,Pdecadiene (or 1-decene from partially reduced starting material), and more olefins to undergo radical-initiated telomerization. Moreover, in Table 1 the n value for electrolyses of l,lO-dibromodecane (1.83) is significantly lower than that for the reduction of IJO-diiododecane (2.08) - a result linked to more extensive chemical consumption of the dibromo species to afford telomers. Controlled-potential electroiyses of I,lO-dihalodecanes at - 1.70 V in the presence of diethyl malonate
To block the elimination and substitution reactions by which hydroxide ion attacks unreduced starting material, we added diethyl malonate to the system; previous work [12,14,17,19,26] has shown that diethyl malonate effectively protonates hydroxide ion before the latter can engage in the E2 and S,2 processes which consume starting material non-electrolytically. Parenthetically, it should be mentioned that cyclic volt~mograms for the reduction of l,lO-diiodo- and l,lO-dibromodecane in dimethylformamide containing 0.1 M tetramethylammonium perchlorate and 25 mM diethyl malonate are indistinguishable from those seen in Fig. 2. Table 2 is a collection of data obtained from controll~-potential electrolyses of 215 mM solutions of the l,lO-dihalodecanes in dimethylformamide which contained 0.1 M tetramethylammonium perchlorate and 25 mM diethyl malonate. Current-time curves for these electrolyses exhibited an essentially perfect exponential decay, indicating that diffusional mass transport is current-limiting and that all of the starting material is consumed. As expected, coulometric n values for both starting materials are noticeably higher in the presence of the proton donor than when no diethyl malonate is added (Table 1); presumably, this trend is a direct consequence of the fact that diethyl malonate does block chemical attack of
TABLE 2 Couiometric data and product distributions for electrolytic reduction of 2.5 mM IJO-diiododecane and l,l~dibrom~~ane in dimethylfo~amide containing 0.1 M tetramethylammonium perchlorate and 25 mM diethyl malonate at - 1.70 V Compound
l,lO-Diiododecane a I,lO-Dibromodecane b
n value
2.80 3.10
Product dist~bution % Decane
Diethyl n-decylmalonate ’
Diethvl n-eicosvlmalonate ’
40-45 45-50
20-25 20
10-15 10-15
’ Results obtained from four separate electroiyses. b Results obtained from five separate electrolyses. ’ See text for discussion concerning other possible alkylated diethyi malonate species.
143
hydroxide ion on unreduced starting material. However, because the n values are still well below the theoretical four electrons, other reactions must be occurring which consume starting material via non-electrochemical processes. For electrolyses done in the presence of diethyl malonate, decane - which is the only simple product observed - accounts for up to 50% of the starting material and is most logically formed according to reactions (4) through (7). Hydroxide ion, generated by deprotonation of water when decane is produced, is quickly protonated by the excess diethyl malonate, so that I-decene, 1.9-decadiene, 9-decen-l-of, and l-decanol cannot form via the aforementioned elimination and substitution reactions; the absence of unsaturated products precludes the formation of telomers. However, the diethyl malonate anion is free to attack nucleophilically a molecule of unreduced dihalod~~e; this process starts a sequence of reactions that lead to several products which we believe are alkylated diethyl malonate species: X(CH2),,X
+ - CH(COOC,H&
-+ X(CH,),,CH(COOC,H5),
X(C~*),*CH(COOC*H~)~
+ 2 e- -+ - (CH~),~CH(COOC~H~)*
-(CH,),,CH(COOC,H,)2
+ H+ -+ CH,(CH&H(COOC,H,),
x(cH,),,x
+ - (CH,),,CH(COOC,H,),
+ X-
(14)
+ X-
(15) (16)
+ x(cH,),,CH(COOC,H,)~
+
x(17)
x(~H,),,CH(COOC,H~)~
+ 2e-+
- (cH~)~,CH(COOC~H,),
-(CH,),,CH(COOC,H,),
+ H+ -+ CH,(CH,),,CH(COOC,H,)2
+ x-
(18) (19)
Diethyl n-decylmalonate and diethyl n-eicosylmalonate, two compounds definitely observed as products, account for from 30% to 40% of the starting material. Previous investigations [12,26] have revealed that the electrolysis of 1-iododecane at mercury in dimethylforma~de containing an excess of diethyl malonate does lead to diethyl n-decylmalonate. Although diethyl n-eicosylmalonate is the most highly alkylated by-product found in the present work, linkage of a 30-carbon alkyl group to the diethyl malonate moiety is conceivable. Moreover, it is possible to postulate species which have a diethyl malonate group at each end of an alkyl chain. Indeed a gas chromatogram for a concentrated ether extract of an electrolyzed solution shows six or seven tiny peaks along with those for the two products named above. Some alkylated diethyl malonate species could be so strongly retained on and so difficult to elute from the capillary column employed in this research that we have been unable to detect them. Nevertheless, we are convinced that the key features of the electrochemical reduction of l,lO-dihalodecanes at mercury in dimethylformamide containing an excess of diethyl malonate have been satisfactorily elucidated. REFERENCES 1 J. Zfivada, J. KrupZka and J. &her, Collect. Czech. Chem. Commun.. 28 (1963) 1664. 2 J.A. Dougherty and A.J. Diefenderfer, J. Electroanal. Chem.. 21 (1969) 531. 3 M.R. Rifi, J. Am. Chem. Sot., 89 (1967) 4442.
144 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
M.R. Rifi, Tetrahedron Lett., (1969) 1043. M.R. Rifi, Collect. Czech. Cbem. Commun., 36 (1971) 932. O.R. Brown and E.R. Gonz&lez, J. Electroanal. Chem., 43 (1973) 215. K.B. Wiberg and G.A. Epling, Tetrahedron Lett., (1974) 1119. J. Casanova and H.R. Rogers, J. Am. Chem. Sot., 96 (1974) 1942. L.A. Avaca, E.R. Gonzalez and N.R. Stradiotto, Eiectrochim. Acta, 22 (1977) 225. G.M. McNamee, B.C. Wiliett, D.M. La Perriere and D.G. Peters, J. Am. Cbem. Sot., 99 (1977) 1831. D.M. La Perriere, B.C. Willett, W.F. Carroll, Jr., E.C. Torp and D.G. Peters, J. Am. Chem. Sot., It30 (1978) 6293, D.M. La Perriere, W.F. Carroll, Jr., B.C. Willett, E.C. Torp and D.G. Peters, J. Am. Chem. Sot., 101 (1979) 7561. K.L. Vieira and D.G. Peters, J. Electroanal. Chem., 1% (1985) 93. K.L. Vieira and D.G. Peters, J. Org. Chem., 51 (1986) 1231. L.W. Marpie, Anal. Chem., 39 (1967) 844. C.W. Manning and W.C. Purdy, Anal. Cbim. Acta, 51 (1970) 124. MS. Mbarak and D.G. Peters, J. Org. Chem., 47 (1982) 3397. I.M. Kolthoff and V.A. Stenger, Volumetric Analysis, Vol. 2, 2nd ed., ~nte~cience, New York, 194’7, pp. 336-338. R. Shao, J.A. Cfeary, D.M. La Perriere and D.G. Peters, J. Org. Chem., 48 (1983) 3289. A.N. Kashin, A.B. Ershler, V.V. Strelets, K.P. Butin, I.P. Beletskaya and O.A. Reutov, J. Organomet. Chem., 39 (1972) 237. B.C. Willett, W.M. Moore, A. Salajegheh and D.G. Peters, J. Am. Chem. Sot., 101 (1979) 1162. B.C. Wiflett and D.G. Peters, J. Electroanal. Chem., 123 (1981) 291. B.G. Gowenlock and J. Trotman, J. Chem. Sot., (1957) 2114. A.B. Ershler, V.V. Strelets, K.P. Butin and A.N. Kashin, J. Electroanal. Chem., 54 (1974) 75. K.L. Vieira, M.S. Mubarak and D.G. Peters, J. Am. Chem. Sot., 106 (1984) 5372. J.A. Cleary, MS. Mubarak, K.L. Vieira, M.R. Anderson and D.G. Peters, J. Electroanat. Chem., 198 (1986) 107.
J. Electroanal. Chem., 280 (1990) 129-144 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
Electrochemical reduction of l,lO-dihalodecanes cathodes in dimethylformamide John C. Bart and Dennis G. Peters Department (Received
of Chemistry,
at mercury
l
Indiana University, Bloomington,
27 June 1989; in revised form 21 September
IN 47405 (U.S.A.)
1989)
ABSTRACT
In dimethylformamide containing tetramethylammonium perchlorate, reduction of l,lO-diiododecane yields two irreversible voltammetric waves at mercury, whereas reduction of l.lO-dibromodecane gives only one irreversible wave. Quantitative comparison of this behavior with that known for I-iodo- and 1-bromodecane suggests that l,lO-diiododecane is reduced in a pair of two-electron steps and that l,lO-dibromodecane undergoes a single four-electron reduction. Preparative-scale electrolyses of l,lO-diiododecane at a potential on its first reduction wave reveal that an organomercury polymer is the only product. At potentials corresponding to the second wave for l,lO-diiododecane and to the first wave for l,lO-dibromodecane, the electrolysis products include decane, 1-decene, 1,9-decadiene, I-decanol, and 9-decen-1-01 along with telomers formed by attack of solvent-derived radicals on either l-decene or 1,9-decadiene. When electrolyses of l,lO-diiodoand l,lO-dibromodecane are performed in the presence of a proton donor (diethyl malonate), the yield of decane increases, but no alcohols or unsaturated compounds are formed; instead, products such as diethyl n-decylmalonate and diethyl n-eicosylmalonate are found which arise from attack of the diethyl malonate anion on unreduced starting material.
INTRODUCTION
There have been numerous publications [l-9] dealing with the electrochemical reduction of cY,w-dihaloalkanes at mercury cathodes. Zavada et al. [l] investigated the polarographic behavior of 1,3-dibromopropane, 1A-dibromobutane. 1,5-dibromopentane, and 1,6-dibromohexane in dimethylformamide containing tetramethylammonium bromide and 5% water; on the basis of logarithmic analysis of the polarographic waves, these workers concluded that 1,3-dibromopropane undergoes a two-electron reduction and that the other three compounds are reduced in a pair of two-electron steps which cannot be resolved into discrete waves. In a subsequent
l
To whom correspondence
0022-0728/90/$03.50
should
be addressed.
0 1990 Elsevier Sequoia
S.A.
130
paper, Dougherty and Diefenderfer [2] reported that 1,4-dibromobutane exhibits two polarographic waves in nominally dry dimethylformamide containing tetramethylammonium perchlorate; electrolyses done at potentials corresponding to each of the two waves revealed that the reduction products are I-bromobutane and butane, respectively. In some of the earliest work concerning the electrochemistry of a,o-dihaloalkanes, Rifi [3-51 synthesized cyclopropane by electrolyzing 1,3-dibromopropane at mercury in dimethylformamide containing tetra-n-butylammonium bromide; in similar fashion, he obtained cyclobutane (along with butane) by reducing 1,4-dibromobutane, but only straight-chain alkanes were produced from either 1,5-dibromopentane or 1,6-dibromohexane. Rifi suggested that electroreductive cyclization of 1,3-dibromopropane and 1,Cdibromobutane proceeds via (a) a transition state, formed by uptake of two electrons, which resembles the final carbocyclic product or (b) a bromoalkyl carbanion which cyclizes faster than it is protonated. Brown and Gonzalez [6] performed controlled-potential electrolyses of 1,3-dibromopropane at mercury in acetonitrile containing tetraethylammonium bromide. They observed a potential-independent product distribution - cyclopropane (30%) and propane (61%) - which differed from that seen by Rifi [3]. In addition, when Brown and Gonzalez reduced 1,Cdibromobutane in dimethylformamide containing either tetraethylammonium bromide or tetra-n-butylammonium perchlorate, the products (which did not include cyclobutane) were butane, I-bromobutane, and dibutylmercury along with butadiene and a solid characterized as a butylmercury oligomer - again, results differing dramatically from those obtained by Rifi. To account for the formation of the various products, Brown and Gonzalez proposed that 1,4-dibromobutane is reduced to bromobutyl radicals and butyl diradicals, both in an adsorbed state, which undergo subsequent polymerization, reduction, and protonation; similar processes were invoked to describe the electrochemistry of 1,3-dibromopropane. Although inconsistencies found in these two reports may be largely due to the different solvents and electrolytes employed and, more importantly, to the use or lack of potential control, a clear picture of the behavior of 1,3-dibromopropane and 1,4-dibromobutane is not available. Uncertainties in the electrochemistry of 1,3-dibromopropane and 1,4-dibromobutane are only amplified by the results of a study by Wiberg and Epling [7], who investigated the reduction of these two dibromoalkanes at mercury and platinum in dimethylformamide containing tetraethylammonium bromide. According to these workers, a cyclic voltammogram for 1,3-dibromopropane exhibits two waves that correspond to stepwise formation of the 3-bromoprop-l-y1 radical and carbanion, respectively. Preparative-scale electrolyses afford mainly propane when performed at a potential on the first wave, whereas cyclopropane is the major product at potentials on the second wave. Electrolysis of 1,4-dibromobutane shows a similar pattern; butane is more important at less negative potentials, whereas cyclobutane is the dominant product at more negative potentials. Ruling out the intermediacy of organomercury species, Wiberg and Epling suggested that the carbocycles arise either (a) via one-electron reduction of the initially generated
131
monohaloalkyl radical followed by ring closure or (b) via direct two-electron reduction of the starting material to yield the carbocycle in a single step. Two relatively recent papers have been concerned with the electrolysis of cw,w-dibromoalkanes to produce organomercury species. Casanova and Rogers [8] demonstrated that controlled-potential reduction of cw,w-dibromoalkanes at mercury in dimethylformamide containing tetraalkylammonium salts leads to the formation of dialkylmercury compounds in yields ranging from 49% to 96% when the carbonchain length is at least four. On the other hand, Avaca, Gonziilez, and Stradiotto [9] concluded that linear polymers of the type RHg(RHg),R are obtained from electrolyses of 1,4-dibromobutane or l,%dibromopentane at mercury in dimethylformamide or acetonitrile containing a variety of supporting electrolytes. In the work described herein, we have investigated the electrochemical behavior of l,lO-diiodoand l,lO-dibromodecane at mercury cathodes in dimethylformamide for several reasons. First, no study of the reduction of an ~,~-diiodoalkane has been reported previously. Second, because carbon-iodine bonds are generally easier to reduce than carbon-bro~ne bonds, electrolyses of an ff,~-diiodoalkane can be done at less negative potentials; under such conditions, radical intermediates are more likely to be generated so that their chemistry and electrochemistry can be elucidated. Third, this research is an extension of earlier publications [lo-121 from our laboratory pertaining to the reduction of l-iodo- and l-bromodecane. EXPERIMENTAL
Reagents Tetramethylammonium perchlorate, purchased from the G. Frederick Smith Chemical Co. and employed as supporting electrolyte, was recrystallized from distilled water before use and was stored in a desiccator. Burdick & Jackson “distilled-in-glass” dimethylformamide used as solvent was subjected to further purification; it was vacuum distilled as described in earlier work [13] to remove impurities such as formic acid and N-methylformamide, it was dried over Davison 4-A molecular sieves to minimize trace amounts of water, and it was stored in the dark under an atmosphere of argon. Commercially available l,lO-diiododecane and l,lO-dibromod~ane (Aldrich Chemical Co.) were used without further purification. Triply distilled mercury (Troy Chemical Corp., Wood Ridge Chemical Division) was used for the preparation of working electrodes. Deaeration of solutions for all electrochemical experiments was done with argon (Air Products UHP).
Instrumentation and procedures Described in prior work [ 131 is a cell designed for polarography and cyclic voltammetry. Equipment employed for normal pulse polarographic studies included a Princeton Applied Research Corp. (PARC) Model 174 polarographic analyzer and a PARC Model 174A/70 drop timer. For cyclic voltammetric work we used a PARC Model 173 potentiostat-galvanostat with a PARC Model 176 current-tovoltage converter and a PARC Model 175 universal programmer to produce the
132
desired waveforms. To record polarograms and cyclic volta~ograms, we utilized a Houston Instruments Model 2000-5-5 X-Y plotter. A cell for controlled-potential electrolyses with a mercury pool cathode has been described in a previous paper [14]. Electrolyses were accomplished with the aid of the aforementioned potentiostat-galvanostat and current-to-voltage converter that provided iR compensation. We controlled the course of each electrolysis with an IBM personal computer interfaced to the potentiostat via a home-built analog-todigital converter; the current-time curve was monitored, stored, and integrated to provide the coulometric n value for the electrolysis. Other details concerning the procedure for controlled-potential electrolyses are presented in an earlier publication [14]. All potentials are quoted with respect to a reference electrode consisting of a saturated cadmium amalgam in contact with dimethylformamide saturated with both cadmium chloride and sodium chloride [15,16]; this reference electrode has a potential of - 0.75 V versus the aqueous saturated calomel electrode at 25 o C. To prepare a solution which has been electrolyzed for subsequent gas chromatographic analysis, the solution was partitioned between water and diethyl ether; the ether phase was washed five times with a saturated solution of sodium chloride to ensure the complete removal of dimethylformamide and supporting electrolyte, dried over anhydrous magnesium sulfate, and concentrated by rotatory evaporation to a volume of l-2 ml. Gas chromatographic analyses were performed with a Varian Model 3700 dual-column instrument equipped with flame ionization detectors. For the separation, identification, and quantitation of simple electrolysis products (decane, 1-decene, l,Pdecadiene, I-decanol, and 9-decen-l-ol), we employed a medium-polarity, wide-bore capillary column (10 m X 0.53 mm, RSL-300, Alltech Associates); a 50-cm length of the same capillary column was utilized to obtain chromatograms for high-molar-mass species (either alkylated diethyl malonate or telomers formed by attack of solvent-derived radicals on the simple olefinic products). To quantitate various products, known amounts of non-electroactive internal standards (dodecane and triacontane) were added to each solution prior to the start of an electrolysis. Gas chromatograp~c response factors were determined experimentally with authentic samples of the various electrolysis products, and product yields are reported in terms of the absolute percentage of starting material incorporated into a particular compound. Product id~nti~jc~tj~~
Decane, 1-decene, 1,9-decadiene, l-decanol, and 9-decen-l-01 were all identified as products by comparison of their gas chromatographic retention times with those of commercially available authentic samples. To characterize and identify the solid organomercury polymer, both the organic and mercury content of the material were established. For the analysis of the organic portion of the polymer, we used a previously described procedure [17]. A portion of the solid was treated for 10 min with 1 ml of a saturated solution of bromine in glacial acetic acid; bromine cleaves each carbon-mercury bond and
133
converts the hydrocarbon part of the polymer into a mixture of l-bromodecane and l,lO-dibromodecane. Then the mixture was extracted with diethyl ether. Next, the ether extract was washed with a solution of sodium thiosulfate to reduce any excess bromine, washed three times with water, dried over anhydrous magnesium sulfate, and concentrated by evaporation to a volume of a few ml. Finally, the quantities of 1-bromodecane and l,lO-dibromodecane in the ether extract were determined by means of gas chromatography. A modified version of the Volhard procedure [18] was employed for measurement of the mercury content of the organomercury polymer. After some of the material was recovered from an electrolyzed solution, the solid was dried by being washed with diethyl ether; gold foil was used to collect and remove spherules of elemental mercury clinging to the organomercury polymer. A 99.2-mg sample of the dried material was treated with 1 ml of concentrated nitric acid; the mixture was heated gently to decompose and dissolve the polymer, 9 ml of distilled water was added, and the solution was boiled to expel oxides of nitrogen. Next, 10 ml of distilled water was added, the solution was cooled below room temperature, 1 ml of a 0.3 M solution of iron(II1) nitrate was introduced as an end-point indicator, and the solution was titrated with 4.794 ml of 0.1117 M potassium thiocyanate. These data show that the organomercury polymer contains 54.14% mercury. If the polymer is taken to have the formula RHg(RHg),R, where R is a decyl (or decamethylene) group, and if we assume that the polymer consists essentially of decyl groups and mercury in a 1 : 1 molar ratio, we calculate that the mercury content of the polymer should be approximately 58.9%. However, if the polymer has the formula RHg,(RHg,),R, a substance with close to a 1 : 2 decyl group-to-mercury ratio, the mercury content should be about 74.1%. Thus, we conclude that the former one-to-one nature of the organomercury polymer is correct. Gas chromatograms for concentrated ether extracts of electrolyzed solutions of the dihalodecanes showed peaks attributable to a family of at least nine telomers formed by reaction of 1-decene or 1,Pdecadiene with solvent-derived radicals. A few of these species were identified with reasonable certainty by means of gas chromatography-mass spectrometry. A 2 : 1 telomer composed of two decyl moieties and a dimethylamino group, C,,H,,N(CH,),, exhibited a parent peak (m/e = 325) as well as mass spectral peaks for alkyl fragments ranging from five to nine carbons in length; because there were no fragments with 10 or more carbons, we believe that the two decyl chains are not connected end-to-end, but perhaps via a bond between interior carbons. Another substance appeared to be a 3 : 1 telomer composed of three decyl moieties and a dimethylamino group; although a parent peak (m/e = 465) was not detected, peaks were observed for alkyl fragments containing from five to twenty-one carbons. Finally, we saw a third prominent gas chromatographic peak having a retention time close to that of the aforementioned 2 : 1 telomer; although this species probably contains two decyl groups, its mass spectrum did not show a parent peak and other portions of the mass spectrum were ambiguous. Alkylated diethyl malonate species were identified with the aid of gas chromatog-
134
raphy-mass spectrometry and, in the case of diethyl n-decylmalonate, by direct comparison with the mass spectrum of authentic, synthesized material. Diethyl n-decylmalonate was prepared by dissolution of sodium metal (2.3 g, 0.10 mol) in 50 ml of absolute ethanol; next, diethyl malonate (18.1 ml, 0.12 mol) was s/uwly dripped into the solution and, finally, 1-bromodecane (20.7 ml, 0.10 mol) was added. After the solution was stirred for 30 min, it was treated with diethyl ether; the ether solution was washed twice with water, dried over anhydrous magnesium sulfate, and concentrated by evaporation. Pure (> 97%) diethyl n-decylmalonate was recovered by means of vacuum distillation: b.p. 121-126O C (0.01 Torr); mass spectrum, m/e 300, M+ (1.2%); 255, Mf - OC,H, (12%); 173, M” - C,H,, (57%); (19%); 132, M+ - C,,H, (11%); 159, M+ - C,,H,, (100%); 133, M+ -&f-I,, 127, C,H& (14%). These mass spectral data are virtually identical to those for one of the compounds produced el~trolytically. Diethyl n-eicosylmalonate, another significant product, has the following mass spectrum: m/e 313, M+ - C,H,, (4%); 312, M+ - C,H, (6%); 285, M+ - C,,H,, (9%); 267, M+ -(C,H500QzCHCHz (5%); 239, M+ -(C,H,OOC),CH(CH,), (13%); 173, M+ - C,9H39 (84%); 159, M+- C&H,, (100%); 133, M+ - C,,H,, (41%); 132, M+ - C,,H, (10%); 127, C,H&, (17%). For the latter compound, the intensities of signals corresponding to M+ and M+ + 1 species were so small compared to the major peaks listed that the assignments of masses and relative abundances were not possible; on the other hand, the existence of the malonate fragment and a twenty-carbon chain is strong circumstantial evidence that this substance is diethyl n-eicosylmalonate. Small amounts of another alkylated diethyl malonate were detected; unfortunately, mass spectral data provided no conclusive information about its identity. No mass spectrometric evidence was found that any of the alkylated diethyl malonate species is halogenated. RESULTS
AND DISCUSSION
Figure 1 shows normal pulse polarograms for 1.00 mM solutions of l,lO-diiodoand l,lO-dibromod~e in dimethylfo~~de cont~ning 0.1 M tetramethyl~monium perchlorate. For the reduction of l,lO-diiododecane, the polarogram depicted in Fig. 1A exhibits two well defined waves with half-wave potentials of - 0.93 and - 1.36 V, and the nearly identical diffusion currents indicate that an equal number of electrons is transferred in the reaction for each wave. For otherwise identical conditions, comparison of Fig. 1A to a pulse polarogram for 1-iododecane - known to undergo two sequential one-electron reductions 1121- reveals that the current for each wave for l,lO-diiododecane has twice the magnitude of that for the waves seen for the monoiodo species. Therefore, each wave for reduction of l,lO-diiododecane corresponds to a two-electron process; moreover, as revealed later in this paper, the first wave is due to reductive cleavage of each carbon-iodine bond to produce a decyl diradical, whereas the second wave is attributable to reduction of the decyl diradical to a dianionic species.
135
4
-0.2
-0.6
-1.0
-1.4
-1.8
!.2
POTENTIAL/V
Fig. 1. Normal pulse polarograms for reduction of 1 mM solutions of l,lO-dihalodecanes in dimethylformamide containing 0.1 M tetramethylammonium perchlorate recorded at a scan rate of 5 mV s-‘. A controlled drop time of 0.5 s was used; the pulse width and sampling period were 56.7 and 16.7 ms, respectively. (A) l,lO-diiododecane; (B) l,lO-dibromodecane.
In contrast to the behavior observed for l,lO-diiododecane, only one wave with a half-wave potential of - 1.42 V is seen in the pulse polarogram for l,lO-dibromodecane (Fig. 1B). If the latter polarogram is compared with a pulse polarogram for a 1.00 mM solution of I-bromodecane, which undergoes one-step, twoelectron reduction, we find that, other conditions being the same, the height of the wave for the dibromo species is twice the height of the wave for the monobromodecane. Apparently, l,lO-dibromodecane is reduced to the decyl dianion in a single four-electron process. Whenever the potential is negative enough to permit reduction of a dihalodecane to the decyl dicarbanion, it should be noted that the process must occur via an intermediate radical which may have only transient existence. It is noteworthy that the half-wave potentials mentioned above for stepwise reduction of l,lO-diiododecane at mercury in dimethylformamide containing 0.1 M tetramethylammonium perchlorate are precisely the same as those for 1-iododecane measured under comparable conditions [12], except for the already indicated difference in the diffusion currents. Similarly, the half-wave potential for reduction of l,lO-dibromodecane ( - 1.42 V) is similar to those observed in earlier studies for I-bromodecane (-1.40 V) [lo] and 1-bromo-Sdecyne (-1.41 V) [19] in the same supporting electrolyte-solvent, although the diffusion current for the dibromo compound is twice that for either of the monobromo species when the concentrations of all substances are identical.
136
POTENTIAL/V
Fig. 2. Cyclic voltammograms for reduction of 1 mM solutions of IJO-dihafodecanes in dimethyiformamide containing 0.1 M tetramethylammonium perchlorate recorded at a scan rate of 200 mV s-’ with a hanging mercury drop electrode. (A) l,l~D~~~~ne; (B) l,l~dibrom~~ne.
Cyclic voltammetric
behavior of I, 1O-dihalodecanes
Figure 2 presents cyclic voltammograms recorded at 200 mV s -’ for 1.00 mM solutions of l,lO-diiodo- and l,lO-dibromodecane in dimethylformamide containing 0.1 M tetramethylammonium perchlorate. In Fig. 2A, we attribute the abnormal (almost gaussian) shape of the first voltammetric wave for l,lO-diiododecane to the fact that decyl diradicals generated during the first stage of reduction are adsorbed onto the surface of the hanging mercury drop electrode. These adsorbed radicals effectively prevent diffusing starting material from being reduced at the electrode, and the current falls nearly to the baseline level at - 1.2 V; however, at potentials more negative than - 1.3 V, the current rises, peaks, and decays normally because adsorbed decyl diradicals undergo further reduction to nonadsorbed dianions. On the other hand, a cyclic voltammo~am (Fig. 2B) for reduction of l,lO-dibrom~ecane is without any unusual feature; there is only a single irreversible wave with a peak potential of - 1.46 V. Controlled-potential
electrolysis of I, I O-diiododecane at - I.10 V
We performed electrolyses of l,lO-diiododecane at a mercury pool cathode held at - 1.10 V in dimethylformarnide containing 0.1 M tetramethylammonium perchlorate. For concentrations of starting material below 3 mM, the coulometric n value is very close to two and the only product is a gray solid; gas chromatographic examination of a concentrated diethyl ether extract of the electrolyzed solution showed that no soluble product containing the decyl moiety is formed. It should be noted that electrolyses done with concentrations of starting material higher than 3
137
mM were always incomplete due to fouling of the mercury electrode by the gray solid. When treated with a saturated solution of bromine in glacial acetic acid, the gray solid was converted into l,lO-dibromodecane (98%) and l-bromodecane (2%); l,lO-dibromodecane must be derived from decamethylene groups in the body of the polymer, whereas l-bromodecane can only arise from polymer-ending decyl moieties. Separate analysis of the polymer for mercury revealed the presence of essentially one mercury atom for every decyl or decamethylene group. Thus, the organomercury polymer appears to be a material having the general formula
CH,(CH,),Hg[(CH,),,Hgl,(CH,),CH,. We propose that the first step in the mechanism for production of this polymer is one-electron reduction of each carbon-iodine bond of l,lO-diiododecane to form a diradical, which is adsorbed onto the cathode I(CH,),,I
+ 2 e- -+‘CH,(CH,),CH;(ads)
and which very quickly diradical: ‘CH,(CH,),CH;(ads)
incorporates
+ 2 Imercury
to give an adsorbed
(1) organomercury
+ Hg+‘CH,(CH,),Hg’(ads)
(2)
Besides the cyclic voltammetric evidence provided earlier in this paper (Fig. 2A), considerable precedence is found in earlier work [6,8-10,12,17,20-221 that alkyl radicals can be adsorbed onto mercury and that mercury can be incorporated into the absorbed species. Although the lifetime of an adsorbed organomercury diradical is not known, there is no reason to suspect that this species survives longer (10 ms) than more well investigated [30] adsorbed radicals such as ethylmercury and n-pentylmercury before being transformed into a stable and isolable material: (n + 2) ‘CH,(CH,),Hg’( +Hg
a d s) 3
CH,(CH,),Hg[(CH,),,Hg]
&H&H, (3)
Presumably, the solvent (dimethylformamide) provides hydrogen atoms to form the terminal methyl groups which limit the extent of polymer growth. On the basis of the relative yields of l,lO-dibromodecane (98%) and l-bromodecane (2%) obtained by degradation of the gray solid in the presence of bromine, conceivably as many as 100 decylmercury diradical units are linked together to form a single polymer chain. On the other hand, if the mercury content (54.14%) of the polymer is used to determine its length, as few as five decylmercury diradical units may constitute a single organomercury polymer chain. Although the analytical data cannot establish a precise chemical formula for the polymer unambiguously, it is nevertheless clear that the material is properly represented as shown in reaction (3). It is interesting to compare the results we have obtained for the reduction of l,lO-diiododecane at - 1.10 V with previous findings of several other groups of workers. Gowenlock and Trotman [23] reported that electrolysis of several organomercury halides at a platinum cathode in liquid ammonia results in the production of isolable solids characterized as being organic metals comprised of a three-dimensional network in which each mercury atom is bound to six organic
138
moieties; these authors rejected alternative ways to describe the solids, including the possibilities that the materials consist of organomercury radicals, of amalgams of mercury with organic free radicals, or of organic calomels (RHgHgR). Using a galvanostatic method, Ershler et al. 1241 examined the behavior of a series of alkylmercury acetates in water and aqueous methanol containing potassium acetate; they concluded that each alkylmercury acetate is reduced to adsorbed organomercury radicals which, after reacting with more starting material to yield a metastable organic calomel, are converted into dialkylmercury species. Among the products obtained by Brown and Gonzalez [6] in their study of the reduction of 1,4-d& bromobutane at mercury was dibutylmercury along with a white solid determined with the aid of elemental analysis and mass spectrometry to be a butylmercu~ compound having the formula C,H,HgC,HsHgC,+H,. In a similar study, Avaca et al. [9] performed controlled-potential electrolyses of 1,4-dibromobutane and Wdibromopentane at mercury in either dimethylformamide or acetonitrile which contained tetraalkylammonium perchlorates. From both of the dibromoalkanes, these investigators obtained high yields of solids that, upon mass spectrometric examination, were discovered to be most likely alkylmercury polymers of the general composition RHg(RHg),R. Cyclic alkylmercury polymers, (RHg),, as well as organic calomels, RHg,(RHg,),R, were ruled out as probable identities for the solids. Finally, in a paper by Casanova and Rogers [8], controlled-potential electrolyses of a series of dibromoalkanes ranging from 1,4-dibromobutane to l,lZdibromododecane at mercury cathodes in dimethylformamide containing tetraalkylammonium salts demonstrated that dialkylmercury compounds can be obtained in substantial yield; moreover, evidence was offered - including the isolation of di-n-hexyldimercury after reduction of l,&dibromohexane - that organic calomel species are involved at least as intermediates in the formation of diorganomercury species. Controlled-potential
electrofyses of I, IO-dihaiodecanes
at - I. 70 V
To examine the behavior of l,lO-dihalodecanes under conditions where the carbon-halogen bond at each end of the molecule can undergo reductive cleavage to form a carbanion - a process involving transfer of four electrons to each molecule - we performed controlled-potential electrolyses at - 1.70 V. Table 1 lists coulometric n values and product dist~b~tions at this potential for electrolyses of l,lO-diiodo- and l,lO-dibromodecane at mercury pool cathodes in dimethylformamide containing 0.1 M tetramethylammonium perchlorate. In contrast to the single product, an organomercury polymer, obtained from l,lO-diiododecane at - 1.10 V, the electrolysis of both of the l,lO-dihal~~anes at - 1.70 V affords a number of species that result from (a) direct two-electron reduction of the carbon-halogen bonds, (b) non-electron-consuming substitution and elimination reactions, and (c) attack of solvent-derived dimethylamino radicals on olefinic substances formed from starting material. Note that consumption of the dihalodecanes by non-electrochemical processes is responsible for the fact that the coulometric n values for both compounds are significantly less than four (Table 1).
139 TABLE
1
Coulometric data and product distributions l,lO-dibromodecane in dimethylformamide -1.70 v n value
Compound
2.08 b 1.83
for electrolytic reduction of 2.5 mM l,lO-diiododecane containing 0.1 M tetramethylammonium perchlorate
Product
distribution
Decane
I-Decene
1,9-Decadiene
I-Decanol
32 20
26 17
10 10
<5
l,lO-Diiododecane l,lO-Dibromodecane
a
a Results b Results
from five separate electrolyses. from three separate electrolyses.
obtained obtained
To account reactions: X(CH,),,X X(CH,
for the formation
+ 2 e- + X(CH,),i
),& + Hz0 -+ X(CH,),CH,
X(CH,),CH3
%
of decane,
9-Decen-l-01 4
5
we postulate
c5
the following
+ X-
Telomers S-5 > 10
sequence
of
(4)
+ OH-
+ 2 e- + - CH,(CH,),CH,
-CH,(CH,),CH,
and at
(5) + X-
+ H,O + CH,(CH,),CH,
(6)
+ OH-
(7)
Despite efforts to eliminate it, water present as a contaminant in dimethylformamide serves as the principal donor of protons for carbanions electrogenerated in steps (4) and (6). To produce 1-decene, starting material is converted to l-halodecane according to steps (4) and (5) above; once formed, the 1-halodecane undergoes an E2 elimination reaction with the hydroxide ion arising from deprotonation of water in reactions (5) and (7): X(CH,),CH,
+ OH-
+ CH,=CH(CH,),CH,
Alternatively, hydroxide can displace substitution to yield l-decanol: X(CH,),CH,
+ OH-
+ HO(CH,),CH,
+ X- + H,O
halide
ion from a 1-halodecane
+ X-
(8) via an S,2
(9)
Of course, any of the 1-halodecane that escapes these chemical reactions should undergo reduction to form decane [lo-121. Among products obtained from electrolyses of both l,lO-diiodoand l,lO-dibromodecane at -1.70 V, 1-decene is approximately five times more abundant than 1-decanol (Table 1). Moreover, 1,9-decadiene (the product of a double elimination reaction) is formed in significant quantities, whereas l,lO-decanediol (the product of a double substitution reaction) is not observed at all. Thus, elimination reactions are much more favorable than substitution reactions under the conditions of our experiments. Formation of 1,Pdecadiene is caused by a pair of E2 elimination reactions involving attack of hydroxide ions on starting material: X(CH,),,X
+ 2 OH-
+ CH,=CH(CH,),CH=CH,
+ 2 H,O + 2 X-
(10)
140
In addition, one can postulate the formation of 9-decen-l-01 by attack of two hydroxide ions, one via an elimination reaction and one via a substitution reaction, on a molecule of unreduced l,lO-dihalodecane: X(CH,),,X
+ 2 OH- --, CH,=CH(CH,),CH,OH
+ H,O + 2 X-
(11)
Again, due to the relative rarity with which substitution reactions occur under the conditions of our electrolyses, 9-decen-l-01 is formed only in small quantities (Table I). Two other points should be mentioned. First, eicosane was not detected as an electrolysis product in any of our experiments. Second, although water is implicated as the major source of protons for carbanions, the tetramethylammonium cation has been shown previously [25] to be a viable proton donor under conditions which prevail during electrolyses of l,lO-dihalod~anes. An experiment was performed to establish the plausibility of the postulated chemical attack of hydroxide ion on carbon-halogen bonds of l,lO-dihalodecanes or 1-halodecanes. Tetraethylammo~um hydroxide (2.70 mM) was added to a 2.50 mM solution of l,lO-diiodod~ane in dimethylforma~de containing 0.1 M tetramethylammonium perchlorate; the tetraethylammonium hydroxide was used instead of tetramethylammonium hydroxide because of the comparatively low solubility of the latter salt in dimethylform~de. This solution was treated exactly as if it were to be electrolyzed at a mercury pool, except that no potential was applied to the electrode. After several hours, the solution was subjected to the usual procedure for separation and determination of products by means of gas chromatography. We found three products - 1-iodo-Pdecene (35%), 9-decen-l-01 (16%), and 1,9-decadiene (14%) - along with unreacted l,lO-diiododecane (24%). Such a result proves that hydroxide ion is capable of attacking carbon-halogen bonds via both E2 and S,2 reactions under conditions prevailing during an electrolysis and, moreover, that the E2 process is preferred over the S,2 displacement. One can explain the absence of 1-decene and 1-decanol by the fact that no electrolysis was actually done; reactions involving the carbon-iodine bonds were chemical, not electrochemical in nature. On the basis of product distributions listed in Table 1, it is evident that decane, 1-decene, 1,9-decadiene, 1-decanol, and 9-decen-l-01 account for not more than 77% of the electrolyzed l,lO-diiododecane and for less than 57% of the electrolyzed l,lO-dibromodecane. In an earlier investigation [lo] of the reduction of l-iodo- and I-bromodecane at mercury in dimethylfo~a~de containing tetra-n-butylammonium perchlorate, significant amounts of the starting materials were found to be incorporated into a family of species (including C,,H,,N, CJ2Hb7N, and C,,H,,NO) apparently formed by telomerization of I-decene that was initiated by solvent-derived radicals. Thus, it was appropriate to search for analogous materials that might be produced during electrolyses of l,lO-dihalodecanes. Concentrated ether extracts of solutions that had been electrolyzed gave gas chromatograms showing the presence of no fewer than nine telomers, three of which were especially prominent. With the aid of mass spectrometry, two of the latter
141
species were found to consist of two decyl groups and one dimethylamino group (2 : 1 telomer) and of three decyl groups and one dimethylamino group (3 : 1 telomer), respectively. It appears that most, if not all, of the telomers contain at least one dimethylamino moiety derived from dimethylformamide; procedures for the synthesis of some of the most likely teiomers are outlined elsewhere [lo]. We propose that the initiating reaction involves electrolytic generation of an alkyl radical intermediate by one-electron reduction of a carbon-halogen bond of the starting material. Such a radical can abstract a hydrogen atom from a solvent molecule to yield a dimethylcarbamoyl radical which, in turn, can decarbonylate to give a dimethylamino radical. Once formed, the dimethylamino radical could attack any unsaturated substance (1-decene, 1,9-decadiene, and perhaps even 9-decen-l-01 or 1-halo-9-decene) in solution to produce a variety of telomers. A possible mechanism for the formation of a 2 : 1 telomer is as follows: ‘N(CH,),
+ C,H,,CH=CH,
C,H,,CHCH,N(CH,),
+ C,H,,CHCH,N(CH,),
(12)
+ C,H,,CH=CH2
(13) However, a 2 : 1 telomer with a different carbon-chain branching could arise via attack of a dimethylamino radical on 1,Pdecadiene. Similarly, several different types of 3 : 1 telomers can be postulated. Moreover, there can be telomers with carbon-carbon double bonds, with more than one dimethylamino group, with multiple chain-branching, and with combinations of these functionalities. So, even if the most prevalent telomers incorporate only a few percent of the starting material, it is definitely possible that all of the telomers taken together could account for the consumption of 20-30% or more of the original quantities of l,lO-dihalodecanes. We believe that the reactions leading to formation of the telomeric species must begin with attack of an alkyl radical on the solvent to remove a hydrogen atom rather than with attack of an anion on the solvent to abstract a proton. Previous research [25] has shown that, in competition with water and the tetramethylammonium cation, dimethylformamide is a poor proton donor. Furthermore, even though the potential ( - 1.70 V) employed for electrolyses of the l,lO-dihalodecanes is in the region where anions are the expected species, alkyl radicals must nevertheless be formed at least transiently, so that hydrogen atom abstraction from the solvent becomes a conceivable event. One of the reviewers raised an interesting question. If alkyl radicals are key precursors in the production of telomers, why (as revealed in Table 1) is the yield of telomers obtained from l,lO-dibromodecane apparently higher than that derived from l,lO-diiododecane? It is possible to offer a somewhat speculative answer to this query. In previous studies of the electrochemical behavior of alkyl monohalides at mercury cathodes [17,19], we found that alkyl bromides tend to exhibit more carbanion character in their reductions than do alkyl iodides. If this trend is
142
applicable to the dihalodecanes, we would expect carbanions to arise more readily from IJO-dibromodecane than from l,lO-diiododecane. Consequently, for the dibromo compound, there might be more deprotonation of adventitious water by carbanions to form hydroxide ions, more hydroxide-promoted dehydrohalogenation of unreduced starting material to give 1,Pdecadiene (or 1-decene from partially reduced starting material), and more olefins to undergo radical-initiated telomerization. Moreover, in Table 1 the n value for electrolyses of l,lO-dibromodecane (1.83) is significantly lower than that for the reduction of IJO-diiododecane (2.08) - a result linked to more extensive chemical consumption of the dibromo species to afford telomers. Controlled-potential electroiyses of I,lO-dihalodecanes at - 1.70 V in the presence of diethyl malonate
To block the elimination and substitution reactions by which hydroxide ion attacks unreduced starting material, we added diethyl malonate to the system; previous work [12,14,17,19,26] has shown that diethyl malonate effectively protonates hydroxide ion before the latter can engage in the E2 and S,2 processes which consume starting material non-electrolytically. Parenthetically, it should be mentioned that cyclic volt~mograms for the reduction of l,lO-diiodo- and l,lO-dibromodecane in dimethylformamide containing 0.1 M tetramethylammonium perchlorate and 25 mM diethyl malonate are indistinguishable from those seen in Fig. 2. Table 2 is a collection of data obtained from controll~-potential electrolyses of 215 mM solutions of the l,lO-dihalodecanes in dimethylformamide which contained 0.1 M tetramethylammonium perchlorate and 25 mM diethyl malonate. Current-time curves for these electrolyses exhibited an essentially perfect exponential decay, indicating that diffusional mass transport is current-limiting and that all of the starting material is consumed. As expected, coulometric n values for both starting materials are noticeably higher in the presence of the proton donor than when no diethyl malonate is added (Table 1); presumably, this trend is a direct consequence of the fact that diethyl malonate does block chemical attack of
TABLE 2 Couiometric data and product distributions for electrolytic reduction of 2.5 mM IJO-diiododecane and l,l~dibrom~~ane in dimethylfo~amide containing 0.1 M tetramethylammonium perchlorate and 25 mM diethyl malonate at - 1.70 V Compound
l,lO-Diiododecane a I,lO-Dibromodecane b
n value
2.80 3.10
Product dist~bution % Decane
Diethyl n-decylmalonate ’
Diethvl n-eicosvlmalonate ’
40-45 45-50
20-25 20
10-15 10-15
’ Results obtained from four separate electroiyses. b Results obtained from five separate electrolyses. ’ See text for discussion concerning other possible alkylated diethyi malonate species.
143
hydroxide ion on unreduced starting material. However, because the n values are still well below the theoretical four electrons, other reactions must be occurring which consume starting material via non-electrochemical processes. For electrolyses done in the presence of diethyl malonate, decane - which is the only simple product observed - accounts for up to 50% of the starting material and is most logically formed according to reactions (4) through (7). Hydroxide ion, generated by deprotonation of water when decane is produced, is quickly protonated by the excess diethyl malonate, so that I-decene, 1.9-decadiene, 9-decen-l-of, and l-decanol cannot form via the aforementioned elimination and substitution reactions; the absence of unsaturated products precludes the formation of telomers. However, the diethyl malonate anion is free to attack nucleophilically a molecule of unreduced dihalod~~e; this process starts a sequence of reactions that lead to several products which we believe are alkylated diethyl malonate species: X(CH2),,X
+ - CH(COOC,H&
-+ X(CH,),,CH(COOC,H5),
X(C~*),*CH(COOC*H~)~
+ 2 e- -+ - (CH~),~CH(COOC~H~)*
-(CH,),,CH(COOC,H,)2
+ H+ -+ CH,(CH&H(COOC,H,),
x(cH,),,x
+ - (CH,),,CH(COOC,H,),
+ X-
(14)
+ X-
(15) (16)
+ x(cH,),,CH(COOC,H,)~
+
x(17)
x(~H,),,CH(COOC,H~)~
+ 2e-+
- (cH~)~,CH(COOC~H,),
-(CH,),,CH(COOC,H,),
+ H+ -+ CH,(CH,),,CH(COOC,H,)2
+ x-
(18) (19)
Diethyl n-decylmalonate and diethyl n-eicosylmalonate, two compounds definitely observed as products, account for from 30% to 40% of the starting material. Previous investigations [12,26] have revealed that the electrolysis of 1-iododecane at mercury in dimethylforma~de containing an excess of diethyl malonate does lead to diethyl n-decylmalonate. Although diethyl n-eicosylmalonate is the most highly alkylated by-product found in the present work, linkage of a 30-carbon alkyl group to the diethyl malonate moiety is conceivable. Moreover, it is possible to postulate species which have a diethyl malonate group at each end of an alkyl chain. Indeed a gas chromatogram for a concentrated ether extract of an electrolyzed solution shows six or seven tiny peaks along with those for the two products named above. Some alkylated diethyl malonate species could be so strongly retained on and so difficult to elute from the capillary column employed in this research that we have been unable to detect them. Nevertheless, we are convinced that the key features of the electrochemical reduction of l,lO-dihalodecanes at mercury in dimethylformamide containing an excess of diethyl malonate have been satisfactorily elucidated. REFERENCES 1 J. Zfivada, J. KrupZka and J. &her, Collect. Czech. Chem. Commun.. 28 (1963) 1664. 2 J.A. Dougherty and A.J. Diefenderfer, J. Electroanal. Chem.. 21 (1969) 531. 3 M.R. Rifi, J. Am. Chem. Sot., 89 (1967) 4442.
144 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
M.R. Rifi, Tetrahedron Lett., (1969) 1043. M.R. Rifi, Collect. Czech. Cbem. Commun., 36 (1971) 932. O.R. Brown and E.R. Gonz&lez, J. Electroanal. Chem., 43 (1973) 215. K.B. Wiberg and G.A. Epling, Tetrahedron Lett., (1974) 1119. J. Casanova and H.R. Rogers, J. Am. Chem. Sot., 96 (1974) 1942. L.A. Avaca, E.R. Gonzalez and N.R. Stradiotto, Eiectrochim. Acta, 22 (1977) 225. G.M. McNamee, B.C. Wiliett, D.M. La Perriere and D.G. Peters, J. Am. Cbem. Sot., 99 (1977) 1831. D.M. La Perriere, B.C. Willett, W.F. Carroll, Jr., E.C. Torp and D.G. Peters, J. Am. Chem. Sot., It30 (1978) 6293, D.M. La Perriere, W.F. Carroll, Jr., B.C. Willett, E.C. Torp and D.G. Peters, J. Am. Chem. Sot., 101 (1979) 7561. K.L. Vieira and D.G. Peters, J. Electroanal. Chem., 1% (1985) 93. K.L. Vieira and D.G. Peters, J. Org. Chem., 51 (1986) 1231. L.W. Marpie, Anal. Chem., 39 (1967) 844. C.W. Manning and W.C. Purdy, Anal. Cbim. Acta, 51 (1970) 124. MS. Mbarak and D.G. Peters, J. Org. Chem., 47 (1982) 3397. I.M. Kolthoff and V.A. Stenger, Volumetric Analysis, Vol. 2, 2nd ed., ~nte~cience, New York, 194’7, pp. 336-338. R. Shao, J.A. Cfeary, D.M. La Perriere and D.G. Peters, J. Org. Chem., 48 (1983) 3289. A.N. Kashin, A.B. Ershler, V.V. Strelets, K.P. Butin, I.P. Beletskaya and O.A. Reutov, J. Organomet. Chem., 39 (1972) 237. B.C. Willett, W.M. Moore, A. Salajegheh and D.G. Peters, J. Am. Chem. Sot., 101 (1979) 1162. B.C. Wiflett and D.G. Peters, J. Electroanal. Chem., 123 (1981) 291. B.G. Gowenlock and J. Trotman, J. Chem. Sot., (1957) 2114. A.B. Ershler, V.V. Strelets, K.P. Butin and A.N. Kashin, J. Electroanal. Chem., 54 (1974) 75. K.L. Vieira, M.S. Mubarak and D.G. Peters, J. Am. Chem. Sot., 106 (1984) 5372. J.A. Cleary, MS. Mubarak, K.L. Vieira, M.R. Anderson and D.G. Peters, J. Electroanat. Chem., 198 (1986) 107.