- Email: [email protected]
Catalysis by electron transfer reagents in organic electrochemistry
Journal of Molecular Catalysis, 38 (1986)
CATALYSIS BY ELECTRON ELECTROCHEMISTRY HENNING Department
203 - 226
TRANSFER
203
REAGENTS
IN ORGANIC
LUND of Organic Chemistry, Aarhus University DK-8000 Arhus C (Denmark)
Summary Reactions catalyzed by electron transfer from electrochemically generated anion radicals, dianions, anions and photochemically excited anion radicals are discussed. The reactions include reductions, cleavage reactions, substitutions and coupling reactions. Electroanalytical methods are used to establish the optimum conditions for syntheses and to determine relevant rate constants.
Introduction The shift of an electron plays a fundamental role in organic chemistry, and electrochemical methods are well suited to investigate such problems. Several entities may function as electron transfer agents; here we will be concerned with cathodically generated electron donors such as anion radicals [ 11, anions [ 21, dianions [ 31 and photochemically excited, electrochemically generated anion radicals [ 41. Electron acceptors may be generated at the anode.
Anion radicals as electron transfer reagents Addition of an electron to an organic molecule creates an anion radical; such species can react as bases, nucleophiles and electron transfer reagents. Their formation, reoxidation or further reaction may be shown by cyclic voltammetry (CV). In this technique [ 51, the current at a microelectrode is measured as a function of the potential which first is changed linearly with time to a certain value and then linearly changed back to the original value. This is illustrated in Fig. 1, which shows a CV of anthracene alone and with increasing concentrations of benzyl chloride [ 31. The compounds and conditions are chosen so that the concentration of protons is low (dry DMF) and a nucleophilic substitution is not occurring. In Fig. 1, trace a, the cyclic voltammogram shows the reversible reduction of anthracene (A) to its anion radical and further reduction to the @ Elsevier Sequoia/Printed
in The Netherlands
204
-1.0
4.5
E/V
vs Ag/AglOlml-
Fig. 1. Cyclic voltammograms of anthracene (3.4 x 10m4) in DMF/O.l M TBAI in the presence of different concentrations of benzyl chloride (a) 0 M; (b) 7.3 x 10d4 M; (c) 3.7 x 10m3 M; (d) 1.85 X 10m2 M benzyl chloride. Sweep rate 10 mV s-l [3].
dianion; as the solution is not super-dry, the very strong base, the dianion, is protonated; at the reverse scan the anion radical is reoxidized. In traces b, c and d, increasing concentrations of benzyl chloride (BX) induce a higher current for the reduction of anthracene; for some reason the electrode acts as if the concentration of anthracene were increased. The following reactions occur: A+e-SAA’
AT +B’--+A+B----t
(1) H+
A+HB
Scheme 1.
In eqns. (2) and (3), anthracene is regenerated and reduced again (l), which gives rise to the observed increased current; the benzyl chloride is reduced to toluene and some bibenzyl. The result is a catalytic reduction of benzyl chloride with antbracene as electron transferring agent (mediator). The observation of a catalytic effect is due to two factors: the transfer of an electron from the cathode to benzyl chloride (BX) requires a certain overvoltage, whereas the activation energy for the homogeneous electron transfer from anthracene anion radical (AT) to benzyl chloride is much less, and the activation energy for the heterogeneous electron transfer from the cathode to anthracene is only small; second, although AG for the homogeneous electron transfer from A’ to BX may be positive, AG for the cleavage (and solvation) of BX7 is sufficiently negative to pull eqn. (2) to the right. The mediator may be chemically bonded to the electrode, in that case a ‘chemically modified electrode’ (CME) is obtained; the system described
205
above may be regarded as a three-dimensional modified electrode. One advantage of the use of a dissolved mediator is that slower reactions between mediator and substrate may be tolerated compared to CME; another advantage is that the cell design is less critical; a disadvantage is that there might be a separation problem during work-up. By suitable choice of mediator, selective reductions may be obtained. 2Methyl-1,5-bis(benzenesulfonyl)pent-2ene (I) has two reducible groups (electrophores), which at the electrode are reduced at potentials differing by only 100 - 150 mV; this small difference makes it difficult to reduce one of the groups without reducing some of the other group. Indirect electrolysis of I using anthracene as mediator makes a selective reduction possible; the anion radical reduces the allylic benzenesulfonyl group rapidly, but the other benzenesulfonyl group considerably more slowly. During the reduction, the solution is colourless until 2 F mol-’ has passed; the solution turns blue due to the formation of stable anthracene anion radicals, the electrolysis is then stopped, and the product, 2-methyl-5-benzenesulfonyl-2-pentene (II), isolated in good yield. A+e-_AA’ C6H&S02CH2C(CHs)=CHCH2CH2S0&HS + 2 AT I
2 A + C6HSSOZ- + CH3-C(CH3)=CHCH,CH,S0,C6H, II
I
When BX is an aliphatic halide rather than benzylic or arylic, the reaction is not so simple. In Fig. 2, the CV of 3-methylisoquinoline V alone and
I -15
E/V
vs Ag/AgI
Qlml‘
.Fig. 2. Cyclic voltammograms of 3-methylisoquinoline (5.8 x 10e3 M) in DMF/O.l M TBAI in the presence of different concentrations of 1-bromobutane. (a) 0 M; (b) 8 X 10m3 M; (c) 3.2 X low2 M; (d) 6.4 X 10e2 M 1-bromobutane. Sweep rate 10 mV s-l 131.
206
in the presence of n-butyl bromide is shown becomes irreversible, and grows on addition addition does not increase the peak height. In tor) has been removed from the system. The eqns. (1) to (5). A+e-
[3]. The reversible peak of V of some n-BuBr, but further some way the catalyst (mediareactions can be described by
-AT
(1)
A’+BX8A+(BX)‘-+A+B’+X-
(2)
H+
AT +B’ -A+B-
-A+BH
(3)
AT +B’ -AB--
H+ ABH
(4)
B’+HS-BH+S’
(5)
Scheme 2.
Equations (1) to (3) have been discussed above; the coupling reaction (4) is of synthetic value and examples will be given below; reaction (5) is a parasitic reaction, which is more serious for primary halides than for secondary halides and is rather slow for tertiary halides. The catalytic reduction of BX is more pronounced for primary halides and nearly absent for tertiary halides; however, eyen for primary halides good yields of coupling products may be obtained if BX is present in excess. Cathodic reduction of pyrene in DMF in the presence of t-butyl chloride yields [6] (after reoxidation of the primarily formed dihydro derivative) 1-t-butylpyrene (III) in good yield, together with a minor amount (14%) of 4-tbutyL4,5dihydropyrene (IV) and traces of 1,6d%butylpyrene (2.5%) and l,&di-t-butylpyrene (0.8%). The EPR spectrum of the pyrene anion radical shows highest electron density at C-l, somewhat lower at C-4, and lowest at C-2, which could suggest that the t-butyl radical attacked preferentially at the position with the highest electron density. l-t-Butylpyrene is difficult to make by means other than indirect electrochemical reduction.
III
IV
VI
VII
The t-butylation of 3-methylisoquinoline (V) shows [ 71, however, that the EPR spectra are not reliable guides for prediction of the products; the highest electron density of V’ is at C-l, but the main product is 6-t-butyl-
207
5,6-dihydro-3-methylisoquinoline (VI), whereas a product, l-t-butyl-4hydroxy-3-methylisoquinoline (VII) derived from t-butylation at C-l, is isolated in 17% yield. Reductive adamantylation gives similar results. Reduction of antbracene in the presence of 1,2dichloroethane gives 9,10-dihydro-9,lOethanoanthracene (VIII). This is by far the most convenient way to prepare VIII. The reaction is probably a radical coupling (eqn. 4) followed by a nucleophilic attack by AB- on the 2chloroethyl group.
Activated olefins may also be reductively alkylated [9] ; the t-butylation of ethyl cinnamate gives t-butylation at both C-2 (IX) and C-3 (X) (and minor amounts in the phenyl group); the reaction of ethyl cinnamate with tbutylmagnesium chloride [lo] gives nearly the same yields of IX and X, which seems to indicate that the addition of this Grignard reagent goes through a single-electron transfer from the Grignard reagent to ethyl cinnamate. Other types of electrophiles may react in a similar way. Thus, reduction in DMF of anthracene in the presence of acetic anhydride [ll] produces the enol acetate of 9acetyL9,lOdihydroanthracene (XI), and activated olefins [ 12,131 may be C-acetylated; ethyl cinnamate thus produces ethyl 3-phenyl-4oxopentanoate (XII). A similar product would be formed if the acetylium anion, CH,CO-, would add to ethyl cinnamate in a Michael addition. CH 3,
,OCOCH3 C
=I NI a,) YOCH,
@-& 0
I
CHCH$OOC,H, LOCH,
N
COCH, XI
XII
XIII
Reductive acetylation of quinoxaline gives 1,4diacetyl-1,4dihydroquinoxaline (XIII), a stable derivative of a very elusive compound 1,4dihydroquinoxaline [ 141. This method provides a general way to isolate unstable reduction products. By reductive acetylation [12, 131 and carboxylation [15, 161, 1,4carbonyl compounds may be prepared which are useful for the preparation of a number of heterocyclic compounds. Scheme 3 shows how two isomers may be prepared using reductive acetylation and carboxylation. It is of interest to be able to predict from electroanalytical measurements whether a given substituent may be allowed during a carboxylation.
208
RCH =CHCOOR’
&w
?
I
RCHCH,COOR’ I COCH,
CH,COOR’ CH, RCHO
CHJOCH, 1 RCH =CHCOCH,
-
e-.CO, MeCl
RCHCHJOCH, LOOC”,
Scheme 3.
Chlorine is lost rather rapidly from an aromatic anion radical, and it might thus not be possible to carboxylate an aryl chloride. This may be investigated by cyclic voltammetry. In a reversible cyclic voltammogram the reoxidation of the anion radical AT, formed during the forward sweep, is seen as an anodic peak; if, however, A’ reacts chemically in some way, it is not available for reoxidation on the reverse scan, so only a small or no anodic peak is seen. In the usual electrochemical nomenclature, an electron transfer reaction is called E and a chemical follow-up reaction C. The process in question would thus be an EC reaction; the chemical step after a reduction in most cases would be a reaction with an electrophile, including protons, a cleavage reaction where a nucleophile is expelled, or a dimerization; for oxidation reactions with a nucleophile, loss of a proton or dimerization would be the most common follow-up reaction. The chemical follow-up reaction causes a displacement of the peak potential for reductions to less negative potentials. At the potential where the current begins to flow, the equilibrium required by the Nemst equation is disturbed by the chemical reaction. The Nernst equation requires a certain proportion between the oxidized and reduced forms at a given potential, but if the reduced form A’ is removed from the equilibrium, then the electrode tries to reestablish the required proportion A/A‘ by reducing more A, which means that the current at a given potential becomes higher than in the simple, reversible case. In a reductive EC reaction, the peak potential is thus shifted toward positive values (and toward negative values for oxidations); for a reaction with first-order kinetics, the E, (red) shifts 30 mV in the positive direction when k, is increased ten-fold. The influence of the chemical follow-up reaction depends on the ratio of the rate constant k, of the C step and the sweep rate u. The higher the u, the less influence the follow-up reaction has; for chemical reactions with
209
1.90
1.90
1.70
Fig. 3. Cyclic voltammetric peak potential of 5-chloro-8-methoxyquinoline (QCl) as a function of the sweep rate u; medium DMF/O.l M TBAI. Trace c is the cathodic peak potential of &Cl; a the anodic peak potential from the reoxidation of QC17; trace b is the cathodic peak potential of QCl in presence of carbon dioxide.
first-order rate constants k, 5 104, it is possible to ‘outrun’ the reaction and obtain a reversible cyclic voltammogram at high u. The E, (red) for a given system with first-order (or pseudo first-order) kinetics is then shifted 30 mV in the negative direction when u is increased ten-fold [17, 181. By plotting E, uersus log u, one can get curves from which the value of 12,can be obtained. This is illustrated in Fig. 3 for a reaction where the chemical step is a cleavage. Figure 3 shows a plot of log u uersus E for 5-chloro-Smethoxyquinoline (QCl), alone and in the presence of carbon dioxide [ 191. At slow sweep rates only the cathodic peak is seen; the lifetime of the anion radical is too short to influence the reverse scan. The follow-up reaction is a cleavage of the primarily-formed anion radical according to eqns. (6) and (7): QCl + e- s
&Cl:
k, QCl~-+Q’
+ Cl-
The purpose of the experiments, whose results are shown in Fig. 3, was to investigate whether the cleavage of the anion radical was faster than the reaction of QCl’ with carbon dioxide, or vice versa. If the cleavage turned out to be slower than the carboxylation, it should be possible to carboxylate QCl without loss of the Cl substituent. Curve c is the E, (red) of QCl uersus u; at u < - lo2 V s-l, dE,/d log u is - -30 mV; at u - lo2 V s-‘, the chemical reaction is being outrun, the anodic peak appears (curve a) and the voltammogram assumes the shape of a single, reversible reaction (6) without a complicating follow-up reaction (7). From the sweep rate, where the line dE,/d log u = -30 mV intersects the horizontal line k,, the first-order rate constant can be obtained [ 181. At still higher u, the peak separation increases, which means that the heterogeneous rate constant Izh is becoming small compared to u and the voltammogram assumes the shape of a quasi-reversible system.
210
Curve b shows E, (red) of QCl versus u in the presence of carbon dioxide. It is seen that E, is shifted toward positive values compared to curve c and that it is not possible to outrun the chemical follow-up reaction (eqn. (8)) within the available range of sweep rates (u 5 lo3 V s-l). QCIT + CO* -
ClQ’ - COz-
(8)
The CV investigation thus indicates that it should be possible to carboxylate 5&loro&methoxyquinoline without excessive loss of chlorine. A preparative reduction at low temperature showed that it indeed was possible [ 161. Scheme 4 shows the carboxylation of 6-chloroquinoline.
\ “ql /
CL
e-
-[ml
T
:I’
N'
b-l H*
I
or H’ (HSI
Scheme 4.
Nucleophilic aromatic substitution may proceed through different mechanisms; in the SRNl reaction [ 201, anion radicals play an important role. The scheme for SRNl is shown below (Scheme 5): ArX + e- I_ ArX7ZAr’
ArX’
(2)
+x-
(10)
Ar’ + Nu- I
ArNus
(11)
ArNu’ - e- 1-
ArNu
(12)
(and/or ArNu’ + ArX I_
ArNu + ArX’ )
(13)
Scheme 5.
The electrochemically induced SRNl reaction has been investigated in detail by Saveant [21], and Figs. 4 and 5 are taken from [20]. The dotted
211
Fig. 4. Cyclic voltammograms of 2chloroquinoline in liquid ammonia at -40 “C!; sweep rate 0.2 V s-r. (---) without added acetone enolate; () in the presence of acetone enolate, (CH$OCH$ [ 211. Fig. 5. Cyclic voltammograms of 2chloroquinoline in liquid ammonia at -40 “C; sweep rate 0.2 V s-l. (- - -) without added (CzHs0)2PO-; () in the presence of diethyl phosphite anion [ 211.
curve in Fig. 4 shows the irreversible reduction (R) of 2-chloroquinoline in liquid ammonia to quinoline and the reversible reduction of quinoline (D) to its anion radical; in the presence of acetone enolate (full line), the two peaks of 2chloroquinoline are diminished and a new, reversible system of the substitution product is visible. In this case the anion radical of the product (formed according to eqn. (11)) reduces the incoming substrate (eqn. (13)) and the peaks of 2chloroquinoline are thus diminished. The product may also be less easily reduced than the substrate, and such a case is shown in Pig. 5 [21], where CV of 2-chloroquinoline alone and in the presence of diethyl phosphite anion, (C2H,0)ZPO-, is shown. The dotted curve is again 2+hloroquinoline, and the product anion radical is oxidized at the electrode; on the next forward scan, the reduction of the product is seen. The reaction may, however, run as a catalytic reaction even when the electron transfer from the product anion radical to the substrate is an uphill process, if the cleavage of ArXr is sufficiently fast [21]. The SRNl reaction may also proceed intramolecularly; thus injection of a small charge (
212
0
I -
1
-1.0
0.5
VvsAglAgl.O.lM1°
Fig. 6. Cyclic voltammograms of S,S-diphenylbenzene-1,2dicarbothiolate (XIV)(10-2 M in DMF/O.l M TBABF4) at platinum electrode before and after a short electrolysis (20 mA for 3 min, n = 0.05 F mol-‘), sweep rate 0.4 V 6-l; (a) before electrolysis; (b) immediately after; (c) after 30 min; (d) after 1 h; (e) after 2 h; (f) solution (e) to which XV is added.
a0
COSAr
COSAr
e-_ -0a t/0COSAr
A:
A
XIV
‘SAr
-‘SAr -ArS-
xv Scheme 6.
chemically reduced to Co(I) complex in two steps coupled to extrusion of the axial ligands, the Co(I) complex behaves as a supernucleophile and reacts
213
with an alkylating agent to give an octahedral alkyl-Co(II1) complex. Further reduction, at a more negative potential, of the alkyl-Co(II1) complex in the presence of an activated olefin and a proton donor results in an addition of the alkyl group to the activated olefin and regeneration of the Co(I) complex [ 231, Br
Br
2 e-
-2 Br-
2 RY
R
#
+Y
Scheme 7.
Indirect electrolysis may take many forms [24]. An example is the reduction of activated alcohols at a mercury cathode in strong acid containing some iodide (Scheme 8). The alcohol is transformed to the alkyl iodide (eqn. (14)) which reacts with mercury (eqn. (15)); the alkyl-mercury compound is decomposed by the acid to the alkene (eqn. (16)), and the mercury iodide reduced to mercury plus iodide (eqn. (17)). The reactions are fast, and quite acceptable current densities (10 - 20 A dmm2) can be obtained 1251. RCH=CHCH20H + HI RCH=CHCH21 + Hg RCH=CHCH2HgI + HI HgI, + 2e- -+
RCH=CHCH21
(14)
RCH=CHCH2HgI
(15)
RCH2CH=CH2 + Hg12
Hg + 21-
RCH2=CHCH20H + 2e- + 2H+ Scheme 8.
(16) (17)
RCH2CH=CH2 + H20
(W
214
Anions as electron transferreagents Anions may be formed electrochemically either by reduction of a positively charged species or by a suitable cleavage reaction; reduction of a stableradical is a third possibility, but the scope is ratherlimited. l-~kyl-4me~oxyc~~nylp~id~~ iodide (XVI) may be reduced reversibly in two steps to the enolate anion of l-alkyl-1,4-dihydro-4-methoxycarbonylpyridine (Scheme 91, as shown in trace b in Fig. 7 [ 26 1. Trace a is the CV curve of l,Z~c~oro-1,2~phenylet~e (XVII); the first peak is an irreversibletwo-electron reduction to stilbene and the second peak the COOCH,
COOCH,
rl XVI Scheme 9.
0
-0.5
-1.0
-1.5Vvs.
AP,/.&~
Fig. 7. Cyclic volta~o~a~ at a platinum electrode in D~F/O,l M TBAI, sweep rate 20 mV s-l. Trace (a) 1,2dichloro-1,2-diphenylethane (1.9 x 10d2) XVII; (b) l-ethyl-4methoxycarbonylpyridinium iodide (3.0 X 10v2 M) XVI; (c) 3.0 X 10m2 M XVI + 5.6 x 1O-2 M XVII [ 261.
215
reversible reduction of stilbene to its anion radical. Trace c shows the CV curve of a solution containing both XVI and XVII. The first peak of XVI is unaffected, whereas the second peak has increased in height; this indicates that the anion XVI- has transferred electrons to XVII. Notice also that the first peak of XVII has disappeared and only the reversible peak from the reduction of stilbene is seen. The anion is thus functioning as a mediator ‘261. If XVI is reduced in presence of an alkyl halide, e.g. t-butyl bromide, an alkylation takes place at C-4 in the pyridine ring. This is an aliphatic nucleophilic substitution; a classical SN2 reaction of the large anion on t-BuBr is very unlikely to take place for steric reasons; also the fact that the anion XVI- reacts with t-butyldimethylsulfonium iodide with t-butylation rather than methylation excludes an S,2 reaction [26] ; furthermore, iodide does not react in DMF at an appreciable rate with t-BuBr, which also excludes an SN1 reaction. The nucleophilic substitution then most likely takes place through the transfer of a single electron, cleavage of t-BuBr7 and combination of the two radicals as shown in Scheme 10.
COOR
: Scheme 10.
In some cases, certain reactions have been suggested to proceed through a single-electron transfer (SET), but the SET involvement in several of these reactions has been questioned or rejected on the grounds that the rate constant for SET, calculated according to Marcus’ theory for outersphere electron transfer [2’7] from thermodynamic data and an estimate of the solvent and internal reorganization energy, was too low compared to the observed reaction rate [ 28 1. In order to postulate a SET mechanism for an aliphatic nucleophilic substitution, it is thus necessary to show that the expected rate of SET is compatible with the observed reaction rate. The idea behind the investigation is that the rate of SET to a given alkyl halide is measured as a function of the reversible redox potential of the electron donor, and from this curve the expected rate for SET can be found for any electron donor for which the reversible oxidation potential is known.
2 - Bromobutane
1.20
1.10
1.60
1.80
-E
*Iv
Fig. 8. Rate (k) of electron transfer from anion radicals to L-bromobutane in DMF/O.l M TBABFd; reference electrode AgI/O.l M I-; 2’ = 25 “C.
As discussed above, anion radicals of aromatic and heteroaromatic compounds react with aryl and alkyl halides with the transfer of an electron as the rate-determining step. In Fig. 8 the rate of the electron transfer from different aromatic anion radicals (log iz) is plotted against the reversible redox potentials of these compounds [29]. It is seen that the curve is part of a branch of a parabola, as required by the Marcus theory [ 271. For each of the alkyl halides investigated such curves are determined, and when the reversible oxidation potential of a donor is known, one can from the curve read what the expected rate for the transfer of an electron to the alkyl halide in question should be. In Table 1 [29] are given the observed rate constants of the reaction between XVT and some alkyl halides, together with the rate constant expected from the curves; in the last column, the ratio of these rate constants is shown. From Table 1 it is seen that the sterically hindered alkyl halides (tBuBr, 1-bromoadamantane, neopentyl bromide) have ratios ksua/ksxr close to 1; the small deviation from 1 is to be expected from the assumptions made and uncertainties in the determination of the rate constants. For secondary and especially primary alkyl halides, the kSUB/kSET differs significantly from 1. The results in Table 1 may be explained if it is assumed that the substitution reaction of the sterically hindered alkyl halides is a SET reaction with negligible bond formation between the nucleophile XVT and the alkyl halide in the transition state, whereas the less sterically hindered alkyl halides and the nucleophile XVI- may approach each other sufficiently so that some bond formation may take place in the transition state. According to this model, there should be a continuous spectrum of transition states
217
218
possible from the pure SET reaction with no bond formation in the transition state (pure outer sphere electron transfer) to the classical SN2 transition state (inner sphere or group transfer transition state). Parameters such as steric hindrance, oxidation potential of the nucleophile, and reduction potential of the electrophile are of importance for determining the position of the transition state on the reaction coordinate. The steric results of anion radicals with chiral reagents [ 301 may be accommodated within this model. Similar ideas have previously been expressed [26,31], and they are in accordance with the idea of regarding the classical S,2 reaction as a shift of a single electron along with group transfer rather than the two-electron shift usually depicted in the organic literature and textbooks [ 321. An example of an anion formed by electrochemical cleavage is the reduction of aliphatic disulfides to thiolates in dry DMF. In the presence of oxygen, sulfinic acids are formed according to Scheme 11. RS-SR
+ 2e- __f
RS- + O2 ) RS- + 0;
2RS
RS’ + Ot __,
RS’ + RS’ *
(19) (20)
RSC;
(21)
RSSR
(22)
Scheme 11.
The disulfide formed in eqn. (22) is again reduced and ends up as alkylsulfinic acid [ 331.
Dianions as electron transfer agents Reduction of an aromatic or heteroaromatic compound with two electrons in a truly aprotic medium produces the dianion of the dihydro derivative. Figure 9 [ 31 shows the CV of perylene (XVIII) in DMF at two sweep rates, alone and in the presence of 1,4dichlorobenzene. In trace a (sweep rate u = 400 mV s-l) and trace d (u = 10 mV s-l) the two reversible reductions of perylene are shown. In b, e, c and f increasing concentrations of 1,4dichlorobenxene are added. It is seen that the height of the first peak of XVIII is not affected, which means that the rate of the electron transfer from the perylene anion radical to the aryl halide is too slow to affect the curves. The height of the second peak is increased, which shows that the dianion is an effective electron transfer agent; notice also that the effect is more pronounced at slow sweep rate (10 mV s-i), where the dianion has more time for the electron transfer reaction before XVII12- is reoxidized at the reverse sweep, than at high sweep rate. The reactions are shown in Scheme 12. A+e-9-A’ A: + e- )
(1) AZ-
(23)
219
L
I
-10
I
-1 5
1
L/v n Ag/*p
I
- I.0
1
E/V n
-15
I
*pmg I
Fig. 9. Cyclic voltammograms of perylene (3.3 x 10v3) in DMF/O.l M TBAI at different sweep rates in the presence of different concentrations of 1,4-dichlorobenzene. (a), (b), (c) u = 400 mV s-l, (d), (e), (f) u = 10 mV s-l; (a), (d) perylene alone; (b), (e) 4.5 X 10m3 M 1,4-dichlorobenzene added; (c), (f) 3.5 x 10e2 M 1,4-dichlorobenzene added [3]. A2-+BX-_4’;+BX7-+A7+B’
+X-
(24)
Scheme 12.
When BX is an alkyl halide rather than an aryl halide as in Fig. 9, a coupling rather than a catalytic reduction takes place. This coupling could either be an ‘SN2-like’ reaction or a coupling between the anion radical and the alkyl radical according to eqn. (25) AZ-+BXG[A’+B’
+X-]-+AB-+X-
(25)
In Table 2 [28], the rate constants are given for the reactions of perylene dianion and anthraquinone dianion with t-butyl chloride and s-butyl bromide, and also the rate constants obtained from Fig. 8 and the reversible potential of A7/A2-. Table 2 shows that perylene dianion reacts with t-BuC1 and s-BuBr with the rates expected for a SET mechanism, whereas the anthraquinone dianion, where the charge is more localized at oxygen, reacts about 20 times faster than expected for a SET reaction; this might indicate a certain bond formation in the transition state in this case, whereas the perylene dianion with the more delocalized charge reacts at the same rate as an anion radical with the same redox potential.
220 TABLE 2 Bate of the substitution halides A
reaction
BX
perylene perylene anthraquinone
t-BuCl s-BuBr s-BuBr
between the dianion of perylene (Pe2-)
E2
ksuB
wa
W
-1.80 -1.80 -1.123
1 s-l
km
1
171 b 400 x 103b 48
(M-’
and alkyl
kwdkswr
s-l)
141 282 x lo3 2.2
1.21 1.42 22
aMeasured against Ag/AgI, I- = 0.10 M. bObtained by cyclic voltammetry.
Photochemically excited anion radicals as electron transfer agents Scheme 13 shows a hypothetical orbital energy diagram for the lowest singlet transition in (a) a radical cation and (b) a radical anion. The Scheme
-
t
4-k
-#-I-
+
4-k
4-k (a)
St St St
4-k St +I(b)
Scheme 13. Hypothetical orbital energy diagram for the lowest singlet transition in (a) a radical cation; (b) a radical anion.
shows that an excited state of a cation radical is a more powerful oxidation agent than the cation radical, and that the excited state of an anion radical is a better reductant (electron donor) than the anion radical. The combination of electrochemistry and photochemistry has certain complications; the excited state is generally very short-lived, and there is the possibility of back-donation of an electron to the electrode, as the electrode (for anion radicals) is less reducing than the excited state. If a compound should be able to accept an electron from a photochemically excited anion radical generated at an electrode, the electron transfer reaction must be very fast.
221
Fig. 10. Cyclic voltamrno~~~ in DMFjO.1 M TBAf of pyrene (2.3 X 10W3M) alone (b), and (a) in the presence of ~~hloro~luene (8 X 10m2 I%); (b), (d) without light; (a), (e) during i~iumination with light at 488 nm f 4 1.
In Fig. 10 such a case [4] is shown; the lower left curve (b) is a CV of pyrene in DMF, aud the lower right curve (d) pyrene in the presence of mc~orotol~ene; it is seen that the peak height of pyrene is not affected by edition of ~~~orotolueue, as the rate of electron transfer is too slow. In the upper trace to the left (a), monoc~omatic ligbt is shone on the system; the wavele~h (X = 488 nm) is chosen to correspond to the a~o~tiou maximum of the pyrene anion radical. The peak height of the pyrene is not ~fected by the light. On the other hand, if light is shone on the system containiug pyrene and mchlorotoluene, the peak height increasesvery significantly (c), The experiment thus shows that the excited state of the pyrene anion radical is able to transferan electron to na-cblorotoluenein a very fast reaction and regenerate pyrene, which then accepts an electron from the electrode to the anion radical; this is then excited etc. It has also been shown that a photochemically excited, el~~ogenera~d dianion is a better electron donor than the dianion [ 341. The rate of electron transfer from pho~hemic~y excited, electrogenerated anion radicals to a number of acceptors and to photochemic~y excited, el~t~~~erat~ cations from a number of donors has been investigated 2351 by the fluorescence quen~h~g technique [36]. It was found that the data fitted better to the values prying by the Marcustheory than to those obtained by the empiricalequation given by Weller [36], provide that a rather large value was used for the reo~~zation energy for the reaction. If the value usuahy employed in such inves~~tions 136J was used, the resuhs diverged greatly from any of the equations. C~c~a~o~s of the &rueture of the excited state of the anion radical, based on the ene~we~ht~ m~mum overlap (~~0) model, suggested reasons for the rather large reo~~zation energy for these excited statesof ion radicals [ 351. ’
222 The use of such photochemically excited anion radicals as reducing agents could have certain advantages; a strong reductant would be formed in a medium which was not very reducing, so that the electrons may be transferred ‘one by one’. If a reaction followed a sequence as in Scheme 14 [37] ((A;-)* is the excited state of a radical anion) where somewhere along the
(AT)* +B---,A+BB---+---_,
(28)
c Product I
(27)
Product II
(28)
-c e-
Scheme 14.
reaction path there is a branching, with competition between a chemical (c) and an electrochemical (e-) step, the absence of a good electron donor in the photo-electrochemical reduction might favour the chemical step (Product I, eqn. (27)) whereas the electrochemical step would be favoured at an electrode or in the presence of a conventional reducing agent (Product II, eqn. (28)). The idea was later realized [38] in a system in which anthraquinone anion radical was photochemically excited in the presence of a substituted aryl halide and anthracene. The single electron transferred from the excited anthraquinone anion radical to the aryl halide resulted in the formation of an aryl halide anion radical, which rapidly cleaved to halide ion and an aromatic radical. As no good electron donors were in the neighbourhood, the
0
o+
0
0
Scheme 15.
&I 0
0
000
Br
223
aryl radical could either abstract a hydrogen atom from the aprotic solvent or attack antbracene. The isolation of a fair yield of l-substituted arylanthracene indicated that the reaction in fact followed Schemes 14 and 15.
Electrogenerated bases as catalysts Addition of an electron to a neutral compound creates a base; in this way rather strong bases may be formed [39, 401. An advantage of electrogenerated bases (EGB) is that handling the usually employed strong bases such as butyllithium may be avoided; the use of such compounds in large quantities is not without problems. Another advantage is that the desired dose of base may easily be injected by letting a certain current run a specified period. An example of a base-catalyzed reaction is illustrated in Fig. Il. The two lower traces (a and b) are CV of azobenzene (a: u = 400 mV F’, b: 40 mV s-l); to the middle (c and d) and upper curves (e, f) benzophenone pinacol (1,1,2,2-tetraphenylethane-1,2diol) has been added in increasing amounts. Benzophenone pinacol is reduced at a potential more negative than the second peak of azobenzene. On addition of the pinacol, the peak of azobenzene diminishes and virtually disappears at the e and f curves, and a new reversible system is observed; this is the reversible reduction of benzo-
Fig. 11. Cyclic voltammograms of azobenzene at different sweep rates in the presence of different concentrations of benzophenone pinacol. (a), (c), (e) u = 0.4 V s-l; (b), (d), (f) u = 0.04 V s-l; (a), (b) 10m3 M azobenzene; (c), (d) plus benzophenone pinacol; (e), (f) plus more pinacol.
224
reversible system is observed; this is the reversible reduction of benzophenone. A simplified reaction path is given in Scheme 16. Azobenzene is reduced to the anion radical (eqn. (29)), which acts as a base by abstracting a proton from the hydroxyl group in the pinacol (eqn. (30)); in the Scheme another anion radical abstracts a proton from the other hydroxyl group, but whether the cleavage of the carbon-carbon bond occurs in the mono- or dianion is not clear yet. The cleavage of the dianion results in the formation of two anion radicals of benzophenone (eqn. (31)); these anion radicals can be oxidized to benzophenone at a potential more negative than the reduction potential of azobenzene. This means that the benzophenone anion radical is able to reduce azobenzene, which creates an electrogenerated base; at the same time benzophenone is formed. The azobenzene diffusing to the electrode is thus intercepted by the anion radical and reduced, and most of it thus never reaches the electrode surface; instead benzophenone is formed (eqn. (32)), which is seen on the CV curve. 2 C6H5N=NC&
+ 2e-9
2 (C&,N=NCBH,)’
(29)
+ (C,H,),C(OH)-C(OH)(C&I&
2 C6H,NHN’--C&l, (~6H,)2~(~-)-~(~-)(~6-I,),
(C,H,),C-O-
2 (C&I,N=NC,H,)’
+ (C&l,),C(O-)-C(O-)(C,H,), -
+ Ca,N=NC
-
(31)
2 (C,H,),~--O-
6H 5 -
(C6H,),C=0
(39)
+ (C6HSN=NC6HS)’
(32)
Scheme 16.
The Scheme is further complicated by the ability of the benzophenone anion radical to act as a base on the pinacol, so the kinetics becomes rather complicated. A different kind of catalysis is shown in Scheme 17, where an electroinactive compound (benzhydrazide) reacts with a catalyst (an aldehyde) (eqn. 34); the intermediate is reduced (eqn. 35) and the catalyst regenerated (eqn. 36) [41]. Equation (37) is the sum of eqns. (34) - (36). C,H,CONHNH, + 2e- + 2H+ ft, C6HSCONHNH2+ RR’CO -
C6H&ONH, + NH3 C6H5CONHN=CRR’ + Hz0
C6H,CONHN=CRR’ + 2e- + 2H+ RR’C=NH + Hz0 -
C6H,CONHZ + HN=CRR’
RR’CO + NH,
C,H,CONHNH, + 2e- + 2H+ -
C,H,CONH, + NH,
(33) (34) (35) (36) (37)
Scheme 17.
The reaction may be of interest in some cases where it is possible to make a substitution with the better nucleophile hydrazine, but not ammonia, and a cleavage of the N-N bond afterwards is desired.
225
In the examples presented in the preceding sections, only catalysis at the cathode has been discussed. Catalysis through the formation of cation radicals, the use of mediators in oxidation [ 241, and the formation of electrogenerated acids at the anode is also possible. Thus many types of reactions may be catalyzed by transfer of electrons, perhaps not surprising in view of the fact that all chemical reactions consist of reshuffling of electrons.
References 1 H. Lund, M. A. Michel and J. Simonet, Acta Chem. Scond., B28 (1974) 901; idem, B29 (1975) 231; J. Simonet, M. A. Michel and H. Lund, Acta Chem. Stand., B29 (1975) 489. 2 H. Lund and L. H. Kristensen, Acta Chem. Scond., B33 (1979) 495. 3 H. Lund and J. Simonet, J. Electroanal. Chem., 65 (1975) 205. 4 H. Lund and H. S. Carlsson, Acta Chem. Stand., B32 (1978) 505. 5 0. Hammerich, B. Svensmark and V. D. Parker, in M. M. Baizer and H. Lund (eds.), Organic Electrochemistry, 2nd edn., Marcel Dekker, New York; 1983, p. 78. 6 P. E. Hansen, A. Berg and H. Lund, Acta Chem. Stand., B30 (1976) 267. 7 C. Degrand and H. Lund, Acta Chem. Stand., B31 (1977) 593; U. Hess, D. Huhn and H. Lund, Acta Chem. Stand., B34 (1980) 403. 8 E. Hobolth and H. Lund,Acta Chim., 1 (1977) 35. 9 C. Degrand and H. Lund, Nouu. J. Chim., 1 (1977) 35. 10 I. Crossland, Acta Chem. Stand., B29 (1975) 468. 11 H. Lund, Acta Chem. Scam-l., B37 (1977) 424. 12 H. Lund and C. Degrand, Tetrahedron Lett., (1977) 3593; idem, Acta Chem. Stand., B33 (1979) 57. 13 T. Shono, I. Nishiguchi and H. Ohmizu, Chem. Lett., (1977) 1021; idem, J. Am. Chem. Sot., 99 (1977) 7396. 14 H. Lund and J. Simonet, C. R. Acad. Sci., Ser. C., 277 (1973) 1387. 15 S. Wawzonek and D. Wearring, J. Am. Chem. Sot., 81 (1959) 2067; S. Wawzonek and S. Gunderson, J. Electrochem. Sot., 111 (1964) 324; D. A. Tyssee and M. M. Baizer, J. Org. Chem., 39 (1974) 2819, 2823; E. Lamy, L. Nadjo and J.-M. SavBant, Nouv. J. Chim., 3 (1979) 21. 16 P. Fuchs, U. Hess, H. H. Holst and H. Lund,Acta Chem. Stand., B35 (1981) 185. 17 R. S. Nicholson and I. Shain, Anal. Chem.,. 36 (1964) 706; R. S. Nicholson, Anal. Chem., 37 (1965) 1351; idem, 38 (1966) 1406. 18 L. Nadjo and J.-M. SavBant, J. Electroanal. Chem., 48 (1973) 113;E. Lamy, L. Nadjo and J.-M. Sav&ant, J. ElectroanaLChem., 42 (1973) 189. 19 H. H. Holst, Dissertation, Aarhus, 1980. 20 J. F. Bunnett, Act. Chem. Res., 11 (1978) 413. 21 J.-M. SavBant, Act. Chem. Res., 13 (1980) 323; C. Amatore, J.-M. SavBant, C. Combellas, S. Robveille and A. Thiebault, J. Electroanal. Chem., 184 (1985) 25. 22 K. Praefcke, C. Weichsel, M. FaIsig and H. Lund,Acta Chem. Stand., B34 (1980) 403. 23 R. Scheffold, M. Dike, S. Dike, T. Herold and L. Walder, J. Am. Chem. Sot., 102 (1980) 3642. 24 J. Simonet, in M. M. Baizer and H. Lund (eds.), Organic Electrochemistry, 2nd edn., Marcel Dekker, New York, 1983, p. 843. 25 T. Lund and H. Lund, Acta Chem. Stand., B38 (1984) 387. 26 H. Lund and L. H. Kristensen, Acta Chem. Stand., B33 (1979) 495. 27 R. A. Marcus, Ann. Rev. Phys. Chem., 15 (1964) 155; L. Eberson, Adv. Phys. Org. Chem., 18 (1982) 79. 28 L. Eberson, Acta Chem. Stand., B38 (1984) 439.
226 Sand29 T. Lund and H. Lund, 12th Sandbjerg Meeting on Organic Electrochemistry, bjerg, Denmark, June 1985, Abstracts of Papers, p. 13; idem, Tetrahedron Lett., 27 (1986) 95; idem, Acta Chem. Stand., B40 (1986) in press. 30 E. Hebert, J.-P. Mazaieyrat, 2. Welvart, L. Nadjo and J.-M. Saveant, NOUV. J. Chim., 9 (1985) 75. 31 S. Bank and D. A. Noyd, J. Am. Chem. Sot., 95 (1973) 8203. Chem. Res., 16 (1983) 363; A. Pross, Act. Chem. Res., 32 A. Pross and S. S. Shaik,Acc. 18 (1985) 212. 33 C. Degrand and H. Lund, Acta Chem. Stand., B33 (1979) 512. 34 H. Lund and H. S. Carlsson, Acta Chem. Stand., 834 (1980) 409. 35 J. Eriksen, H. Lund and A. I. Nyvad, Acta Chem. Stand., B37 (1983) 359; J. Eriksen, K. A. Jdrgensen, J. Linderberg and H. Lund, J. Am. Chem. SOC., 106 (1984) 5083. 36 D. Rehm and A. Weller, Isr. J. Chem., 8 (1970) 269. Society Meeting, San Francisco, May 1983, 37 H. Lund and J. Eriksen, Electrochemical Extended Abstract p. 966. 38 P. Neileborg, H. Lund and J. Eriksen, Tetrahedron Lett., 26 (1986) 1773. and M. M. Baizer, J. Org. Chem., 31 (1966) 3885; idem, J. Electro39 J. H. Wagenknecht them. Sot., 114 (1967) 1096; R. C. Halleher and M. M. Baizer, Justus Liebigs Ann. Chem. (1977) 737. 40 P. E. Iversen and H. Lund, Tetrahedron Lett., (1969) 3523. 41 H. Lund, Electrochim. Acta, 28 (1983) 395.
CATALYSIS BY ELECTRON ELECTROCHEMISTRY HENNING Department
203 - 226
TRANSFER
203
REAGENTS
IN ORGANIC
LUND of Organic Chemistry, Aarhus University DK-8000 Arhus C (Denmark)
Summary Reactions catalyzed by electron transfer from electrochemically generated anion radicals, dianions, anions and photochemically excited anion radicals are discussed. The reactions include reductions, cleavage reactions, substitutions and coupling reactions. Electroanalytical methods are used to establish the optimum conditions for syntheses and to determine relevant rate constants.
Introduction The shift of an electron plays a fundamental role in organic chemistry, and electrochemical methods are well suited to investigate such problems. Several entities may function as electron transfer agents; here we will be concerned with cathodically generated electron donors such as anion radicals [ 11, anions [ 21, dianions [ 31 and photochemically excited, electrochemically generated anion radicals [ 41. Electron acceptors may be generated at the anode.
Anion radicals as electron transfer reagents Addition of an electron to an organic molecule creates an anion radical; such species can react as bases, nucleophiles and electron transfer reagents. Their formation, reoxidation or further reaction may be shown by cyclic voltammetry (CV). In this technique [ 51, the current at a microelectrode is measured as a function of the potential which first is changed linearly with time to a certain value and then linearly changed back to the original value. This is illustrated in Fig. 1, which shows a CV of anthracene alone and with increasing concentrations of benzyl chloride [ 31. The compounds and conditions are chosen so that the concentration of protons is low (dry DMF) and a nucleophilic substitution is not occurring. In Fig. 1, trace a, the cyclic voltammogram shows the reversible reduction of anthracene (A) to its anion radical and further reduction to the @ Elsevier Sequoia/Printed
in The Netherlands
204
-1.0
4.5
E/V
vs Ag/AglOlml-
Fig. 1. Cyclic voltammograms of anthracene (3.4 x 10m4) in DMF/O.l M TBAI in the presence of different concentrations of benzyl chloride (a) 0 M; (b) 7.3 x 10d4 M; (c) 3.7 x 10m3 M; (d) 1.85 X 10m2 M benzyl chloride. Sweep rate 10 mV s-l [3].
dianion; as the solution is not super-dry, the very strong base, the dianion, is protonated; at the reverse scan the anion radical is reoxidized. In traces b, c and d, increasing concentrations of benzyl chloride (BX) induce a higher current for the reduction of anthracene; for some reason the electrode acts as if the concentration of anthracene were increased. The following reactions occur: A+e-SAA’
AT +B’--+A+B----t
(1) H+
A+HB
Scheme 1.
In eqns. (2) and (3), anthracene is regenerated and reduced again (l), which gives rise to the observed increased current; the benzyl chloride is reduced to toluene and some bibenzyl. The result is a catalytic reduction of benzyl chloride with antbracene as electron transferring agent (mediator). The observation of a catalytic effect is due to two factors: the transfer of an electron from the cathode to benzyl chloride (BX) requires a certain overvoltage, whereas the activation energy for the homogeneous electron transfer from anthracene anion radical (AT) to benzyl chloride is much less, and the activation energy for the heterogeneous electron transfer from the cathode to anthracene is only small; second, although AG for the homogeneous electron transfer from A’ to BX may be positive, AG for the cleavage (and solvation) of BX7 is sufficiently negative to pull eqn. (2) to the right. The mediator may be chemically bonded to the electrode, in that case a ‘chemically modified electrode’ (CME) is obtained; the system described
205
above may be regarded as a three-dimensional modified electrode. One advantage of the use of a dissolved mediator is that slower reactions between mediator and substrate may be tolerated compared to CME; another advantage is that the cell design is less critical; a disadvantage is that there might be a separation problem during work-up. By suitable choice of mediator, selective reductions may be obtained. 2Methyl-1,5-bis(benzenesulfonyl)pent-2ene (I) has two reducible groups (electrophores), which at the electrode are reduced at potentials differing by only 100 - 150 mV; this small difference makes it difficult to reduce one of the groups without reducing some of the other group. Indirect electrolysis of I using anthracene as mediator makes a selective reduction possible; the anion radical reduces the allylic benzenesulfonyl group rapidly, but the other benzenesulfonyl group considerably more slowly. During the reduction, the solution is colourless until 2 F mol-’ has passed; the solution turns blue due to the formation of stable anthracene anion radicals, the electrolysis is then stopped, and the product, 2-methyl-5-benzenesulfonyl-2-pentene (II), isolated in good yield. A+e-_AA’ C6H&S02CH2C(CHs)=CHCH2CH2S0&HS + 2 AT I
2 A + C6HSSOZ- + CH3-C(CH3)=CHCH,CH,S0,C6H, II
I
When BX is an aliphatic halide rather than benzylic or arylic, the reaction is not so simple. In Fig. 2, the CV of 3-methylisoquinoline V alone and
I -15
E/V
vs Ag/AgI
Qlml‘
.Fig. 2. Cyclic voltammograms of 3-methylisoquinoline (5.8 x 10e3 M) in DMF/O.l M TBAI in the presence of different concentrations of 1-bromobutane. (a) 0 M; (b) 8 X 10m3 M; (c) 3.2 X low2 M; (d) 6.4 X 10e2 M 1-bromobutane. Sweep rate 10 mV s-l 131.
206
in the presence of n-butyl bromide is shown becomes irreversible, and grows on addition addition does not increase the peak height. In tor) has been removed from the system. The eqns. (1) to (5). A+e-
[3]. The reversible peak of V of some n-BuBr, but further some way the catalyst (mediareactions can be described by
-AT
(1)
A’+BX8A+(BX)‘-+A+B’+X-
(2)
H+
AT +B’ -A+B-
-A+BH
(3)
AT +B’ -AB--
H+ ABH
(4)
B’+HS-BH+S’
(5)
Scheme 2.
Equations (1) to (3) have been discussed above; the coupling reaction (4) is of synthetic value and examples will be given below; reaction (5) is a parasitic reaction, which is more serious for primary halides than for secondary halides and is rather slow for tertiary halides. The catalytic reduction of BX is more pronounced for primary halides and nearly absent for tertiary halides; however, eyen for primary halides good yields of coupling products may be obtained if BX is present in excess. Cathodic reduction of pyrene in DMF in the presence of t-butyl chloride yields [6] (after reoxidation of the primarily formed dihydro derivative) 1-t-butylpyrene (III) in good yield, together with a minor amount (14%) of 4-tbutyL4,5dihydropyrene (IV) and traces of 1,6d%butylpyrene (2.5%) and l,&di-t-butylpyrene (0.8%). The EPR spectrum of the pyrene anion radical shows highest electron density at C-l, somewhat lower at C-4, and lowest at C-2, which could suggest that the t-butyl radical attacked preferentially at the position with the highest electron density. l-t-Butylpyrene is difficult to make by means other than indirect electrochemical reduction.
III
IV
VI
VII
The t-butylation of 3-methylisoquinoline (V) shows [ 71, however, that the EPR spectra are not reliable guides for prediction of the products; the highest electron density of V’ is at C-l, but the main product is 6-t-butyl-
207
5,6-dihydro-3-methylisoquinoline (VI), whereas a product, l-t-butyl-4hydroxy-3-methylisoquinoline (VII) derived from t-butylation at C-l, is isolated in 17% yield. Reductive adamantylation gives similar results. Reduction of antbracene in the presence of 1,2dichloroethane gives 9,10-dihydro-9,lOethanoanthracene (VIII). This is by far the most convenient way to prepare VIII. The reaction is probably a radical coupling (eqn. 4) followed by a nucleophilic attack by AB- on the 2chloroethyl group.
Activated olefins may also be reductively alkylated [9] ; the t-butylation of ethyl cinnamate gives t-butylation at both C-2 (IX) and C-3 (X) (and minor amounts in the phenyl group); the reaction of ethyl cinnamate with tbutylmagnesium chloride [lo] gives nearly the same yields of IX and X, which seems to indicate that the addition of this Grignard reagent goes through a single-electron transfer from the Grignard reagent to ethyl cinnamate. Other types of electrophiles may react in a similar way. Thus, reduction in DMF of anthracene in the presence of acetic anhydride [ll] produces the enol acetate of 9acetyL9,lOdihydroanthracene (XI), and activated olefins [ 12,131 may be C-acetylated; ethyl cinnamate thus produces ethyl 3-phenyl-4oxopentanoate (XII). A similar product would be formed if the acetylium anion, CH,CO-, would add to ethyl cinnamate in a Michael addition. CH 3,
,OCOCH3 C
=I NI a,) YOCH,
@-& 0
I
CHCH$OOC,H, LOCH,
N
COCH, XI
XII
XIII
Reductive acetylation of quinoxaline gives 1,4diacetyl-1,4dihydroquinoxaline (XIII), a stable derivative of a very elusive compound 1,4dihydroquinoxaline [ 141. This method provides a general way to isolate unstable reduction products. By reductive acetylation [12, 131 and carboxylation [15, 161, 1,4carbonyl compounds may be prepared which are useful for the preparation of a number of heterocyclic compounds. Scheme 3 shows how two isomers may be prepared using reductive acetylation and carboxylation. It is of interest to be able to predict from electroanalytical measurements whether a given substituent may be allowed during a carboxylation.
208
RCH =CHCOOR’
&w
?
I
RCHCH,COOR’ I COCH,
CH,COOR’ CH, RCHO
CHJOCH, 1 RCH =CHCOCH,
-
e-.CO, MeCl
RCHCHJOCH, LOOC”,
Scheme 3.
Chlorine is lost rather rapidly from an aromatic anion radical, and it might thus not be possible to carboxylate an aryl chloride. This may be investigated by cyclic voltammetry. In a reversible cyclic voltammogram the reoxidation of the anion radical AT, formed during the forward sweep, is seen as an anodic peak; if, however, A’ reacts chemically in some way, it is not available for reoxidation on the reverse scan, so only a small or no anodic peak is seen. In the usual electrochemical nomenclature, an electron transfer reaction is called E and a chemical follow-up reaction C. The process in question would thus be an EC reaction; the chemical step after a reduction in most cases would be a reaction with an electrophile, including protons, a cleavage reaction where a nucleophile is expelled, or a dimerization; for oxidation reactions with a nucleophile, loss of a proton or dimerization would be the most common follow-up reaction. The chemical follow-up reaction causes a displacement of the peak potential for reductions to less negative potentials. At the potential where the current begins to flow, the equilibrium required by the Nemst equation is disturbed by the chemical reaction. The Nernst equation requires a certain proportion between the oxidized and reduced forms at a given potential, but if the reduced form A’ is removed from the equilibrium, then the electrode tries to reestablish the required proportion A/A‘ by reducing more A, which means that the current at a given potential becomes higher than in the simple, reversible case. In a reductive EC reaction, the peak potential is thus shifted toward positive values (and toward negative values for oxidations); for a reaction with first-order kinetics, the E, (red) shifts 30 mV in the positive direction when k, is increased ten-fold. The influence of the chemical follow-up reaction depends on the ratio of the rate constant k, of the C step and the sweep rate u. The higher the u, the less influence the follow-up reaction has; for chemical reactions with
209
1.90
1.90
1.70
Fig. 3. Cyclic voltammetric peak potential of 5-chloro-8-methoxyquinoline (QCl) as a function of the sweep rate u; medium DMF/O.l M TBAI. Trace c is the cathodic peak potential of &Cl; a the anodic peak potential from the reoxidation of QC17; trace b is the cathodic peak potential of QCl in presence of carbon dioxide.
first-order rate constants k, 5 104, it is possible to ‘outrun’ the reaction and obtain a reversible cyclic voltammogram at high u. The E, (red) for a given system with first-order (or pseudo first-order) kinetics is then shifted 30 mV in the negative direction when u is increased ten-fold [17, 181. By plotting E, uersus log u, one can get curves from which the value of 12,can be obtained. This is illustrated in Fig. 3 for a reaction where the chemical step is a cleavage. Figure 3 shows a plot of log u uersus E for 5-chloro-Smethoxyquinoline (QCl), alone and in the presence of carbon dioxide [ 191. At slow sweep rates only the cathodic peak is seen; the lifetime of the anion radical is too short to influence the reverse scan. The follow-up reaction is a cleavage of the primarily-formed anion radical according to eqns. (6) and (7): QCl + e- s
&Cl:
k, QCl~-+Q’
+ Cl-
The purpose of the experiments, whose results are shown in Fig. 3, was to investigate whether the cleavage of the anion radical was faster than the reaction of QCl’ with carbon dioxide, or vice versa. If the cleavage turned out to be slower than the carboxylation, it should be possible to carboxylate QCl without loss of the Cl substituent. Curve c is the E, (red) of QCl uersus u; at u < - lo2 V s-l, dE,/d log u is - -30 mV; at u - lo2 V s-‘, the chemical reaction is being outrun, the anodic peak appears (curve a) and the voltammogram assumes the shape of a single, reversible reaction (6) without a complicating follow-up reaction (7). From the sweep rate, where the line dE,/d log u = -30 mV intersects the horizontal line k,, the first-order rate constant can be obtained [ 181. At still higher u, the peak separation increases, which means that the heterogeneous rate constant Izh is becoming small compared to u and the voltammogram assumes the shape of a quasi-reversible system.
210
Curve b shows E, (red) of QCl versus u in the presence of carbon dioxide. It is seen that E, is shifted toward positive values compared to curve c and that it is not possible to outrun the chemical follow-up reaction (eqn. (8)) within the available range of sweep rates (u 5 lo3 V s-l). QCIT + CO* -
ClQ’ - COz-
(8)
The CV investigation thus indicates that it should be possible to carboxylate 5&loro&methoxyquinoline without excessive loss of chlorine. A preparative reduction at low temperature showed that it indeed was possible [ 161. Scheme 4 shows the carboxylation of 6-chloroquinoline.
\ “ql /
CL
e-
-[ml
T
:I’
N'
b-l H*
I
or H’ (HSI
Scheme 4.
Nucleophilic aromatic substitution may proceed through different mechanisms; in the SRNl reaction [ 201, anion radicals play an important role. The scheme for SRNl is shown below (Scheme 5): ArX + e- I_ ArX7ZAr’
ArX’
(2)
+x-
(10)
Ar’ + Nu- I
ArNus
(11)
ArNu’ - e- 1-
ArNu
(12)
(and/or ArNu’ + ArX I_
ArNu + ArX’ )
(13)
Scheme 5.
The electrochemically induced SRNl reaction has been investigated in detail by Saveant [21], and Figs. 4 and 5 are taken from [20]. The dotted
211
Fig. 4. Cyclic voltammograms of 2chloroquinoline in liquid ammonia at -40 “C!; sweep rate 0.2 V s-r. (---) without added acetone enolate; () in the presence of acetone enolate, (CH$OCH$ [ 211. Fig. 5. Cyclic voltammograms of 2chloroquinoline in liquid ammonia at -40 “C; sweep rate 0.2 V s-l. (- - -) without added (CzHs0)2PO-; () in the presence of diethyl phosphite anion [ 211.
curve in Fig. 4 shows the irreversible reduction (R) of 2-chloroquinoline in liquid ammonia to quinoline and the reversible reduction of quinoline (D) to its anion radical; in the presence of acetone enolate (full line), the two peaks of 2chloroquinoline are diminished and a new, reversible system of the substitution product is visible. In this case the anion radical of the product (formed according to eqn. (11)) reduces the incoming substrate (eqn. (13)) and the peaks of 2chloroquinoline are thus diminished. The product may also be less easily reduced than the substrate, and such a case is shown in Pig. 5 [21], where CV of 2-chloroquinoline alone and in the presence of diethyl phosphite anion, (C2H,0)ZPO-, is shown. The dotted curve is again 2+hloroquinoline, and the product anion radical is oxidized at the electrode; on the next forward scan, the reduction of the product is seen. The reaction may, however, run as a catalytic reaction even when the electron transfer from the product anion radical to the substrate is an uphill process, if the cleavage of ArXr is sufficiently fast [21]. The SRNl reaction may also proceed intramolecularly; thus injection of a small charge (
212
0
I -
1
-1.0
0.5
VvsAglAgl.O.lM1°
Fig. 6. Cyclic voltammograms of S,S-diphenylbenzene-1,2dicarbothiolate (XIV)(10-2 M in DMF/O.l M TBABF4) at platinum electrode before and after a short electrolysis (20 mA for 3 min, n = 0.05 F mol-‘), sweep rate 0.4 V 6-l; (a) before electrolysis; (b) immediately after; (c) after 30 min; (d) after 1 h; (e) after 2 h; (f) solution (e) to which XV is added.
a0
COSAr
COSAr
e-_ -0a t/0COSAr
A:
A
XIV
‘SAr
-‘SAr -ArS-
xv Scheme 6.
chemically reduced to Co(I) complex in two steps coupled to extrusion of the axial ligands, the Co(I) complex behaves as a supernucleophile and reacts
213
with an alkylating agent to give an octahedral alkyl-Co(II1) complex. Further reduction, at a more negative potential, of the alkyl-Co(II1) complex in the presence of an activated olefin and a proton donor results in an addition of the alkyl group to the activated olefin and regeneration of the Co(I) complex [ 231, Br
Br
2 e-
-2 Br-
2 RY
R
#
+Y
Scheme 7.
Indirect electrolysis may take many forms [24]. An example is the reduction of activated alcohols at a mercury cathode in strong acid containing some iodide (Scheme 8). The alcohol is transformed to the alkyl iodide (eqn. (14)) which reacts with mercury (eqn. (15)); the alkyl-mercury compound is decomposed by the acid to the alkene (eqn. (16)), and the mercury iodide reduced to mercury plus iodide (eqn. (17)). The reactions are fast, and quite acceptable current densities (10 - 20 A dmm2) can be obtained 1251. RCH=CHCH20H + HI RCH=CHCH21 + Hg RCH=CHCH2HgI + HI HgI, + 2e- -+
RCH=CHCH21
(14)
RCH=CHCH2HgI
(15)
RCH2CH=CH2 + Hg12
Hg + 21-
RCH2=CHCH20H + 2e- + 2H+ Scheme 8.
(16) (17)
RCH2CH=CH2 + H20
(W
214
Anions as electron transferreagents Anions may be formed electrochemically either by reduction of a positively charged species or by a suitable cleavage reaction; reduction of a stableradical is a third possibility, but the scope is ratherlimited. l-~kyl-4me~oxyc~~nylp~id~~ iodide (XVI) may be reduced reversibly in two steps to the enolate anion of l-alkyl-1,4-dihydro-4-methoxycarbonylpyridine (Scheme 91, as shown in trace b in Fig. 7 [ 26 1. Trace a is the CV curve of l,Z~c~oro-1,2~phenylet~e (XVII); the first peak is an irreversibletwo-electron reduction to stilbene and the second peak the COOCH,
COOCH,
rl XVI Scheme 9.
0
-0.5
-1.0
-1.5Vvs.
AP,/.&~
Fig. 7. Cyclic volta~o~a~ at a platinum electrode in D~F/O,l M TBAI, sweep rate 20 mV s-l. Trace (a) 1,2dichloro-1,2-diphenylethane (1.9 x 10d2) XVII; (b) l-ethyl-4methoxycarbonylpyridinium iodide (3.0 X 10v2 M) XVI; (c) 3.0 X 10m2 M XVI + 5.6 x 1O-2 M XVII [ 261.
215
reversible reduction of stilbene to its anion radical. Trace c shows the CV curve of a solution containing both XVI and XVII. The first peak of XVI is unaffected, whereas the second peak has increased in height; this indicates that the anion XVI- has transferred electrons to XVII. Notice also that the first peak of XVII has disappeared and only the reversible peak from the reduction of stilbene is seen. The anion is thus functioning as a mediator ‘261. If XVI is reduced in presence of an alkyl halide, e.g. t-butyl bromide, an alkylation takes place at C-4 in the pyridine ring. This is an aliphatic nucleophilic substitution; a classical SN2 reaction of the large anion on t-BuBr is very unlikely to take place for steric reasons; also the fact that the anion XVI- reacts with t-butyldimethylsulfonium iodide with t-butylation rather than methylation excludes an S,2 reaction [26] ; furthermore, iodide does not react in DMF at an appreciable rate with t-BuBr, which also excludes an SN1 reaction. The nucleophilic substitution then most likely takes place through the transfer of a single electron, cleavage of t-BuBr7 and combination of the two radicals as shown in Scheme 10.
COOR
: Scheme 10.
In some cases, certain reactions have been suggested to proceed through a single-electron transfer (SET), but the SET involvement in several of these reactions has been questioned or rejected on the grounds that the rate constant for SET, calculated according to Marcus’ theory for outersphere electron transfer [2’7] from thermodynamic data and an estimate of the solvent and internal reorganization energy, was too low compared to the observed reaction rate [ 28 1. In order to postulate a SET mechanism for an aliphatic nucleophilic substitution, it is thus necessary to show that the expected rate of SET is compatible with the observed reaction rate. The idea behind the investigation is that the rate of SET to a given alkyl halide is measured as a function of the reversible redox potential of the electron donor, and from this curve the expected rate for SET can be found for any electron donor for which the reversible oxidation potential is known.
2 - Bromobutane
1.20
1.10
1.60
1.80
-E
*Iv
Fig. 8. Rate (k) of electron transfer from anion radicals to L-bromobutane in DMF/O.l M TBABFd; reference electrode AgI/O.l M I-; 2’ = 25 “C.
As discussed above, anion radicals of aromatic and heteroaromatic compounds react with aryl and alkyl halides with the transfer of an electron as the rate-determining step. In Fig. 8 the rate of the electron transfer from different aromatic anion radicals (log iz) is plotted against the reversible redox potentials of these compounds [29]. It is seen that the curve is part of a branch of a parabola, as required by the Marcus theory [ 271. For each of the alkyl halides investigated such curves are determined, and when the reversible oxidation potential of a donor is known, one can from the curve read what the expected rate for the transfer of an electron to the alkyl halide in question should be. In Table 1 [29] are given the observed rate constants of the reaction between XVT and some alkyl halides, together with the rate constant expected from the curves; in the last column, the ratio of these rate constants is shown. From Table 1 it is seen that the sterically hindered alkyl halides (tBuBr, 1-bromoadamantane, neopentyl bromide) have ratios ksua/ksxr close to 1; the small deviation from 1 is to be expected from the assumptions made and uncertainties in the determination of the rate constants. For secondary and especially primary alkyl halides, the kSUB/kSET differs significantly from 1. The results in Table 1 may be explained if it is assumed that the substitution reaction of the sterically hindered alkyl halides is a SET reaction with negligible bond formation between the nucleophile XVT and the alkyl halide in the transition state, whereas the less sterically hindered alkyl halides and the nucleophile XVI- may approach each other sufficiently so that some bond formation may take place in the transition state. According to this model, there should be a continuous spectrum of transition states
217
218
possible from the pure SET reaction with no bond formation in the transition state (pure outer sphere electron transfer) to the classical SN2 transition state (inner sphere or group transfer transition state). Parameters such as steric hindrance, oxidation potential of the nucleophile, and reduction potential of the electrophile are of importance for determining the position of the transition state on the reaction coordinate. The steric results of anion radicals with chiral reagents [ 301 may be accommodated within this model. Similar ideas have previously been expressed [26,31], and they are in accordance with the idea of regarding the classical S,2 reaction as a shift of a single electron along with group transfer rather than the two-electron shift usually depicted in the organic literature and textbooks [ 321. An example of an anion formed by electrochemical cleavage is the reduction of aliphatic disulfides to thiolates in dry DMF. In the presence of oxygen, sulfinic acids are formed according to Scheme 11. RS-SR
+ 2e- __f
RS- + O2 ) RS- + 0;
2RS
RS’ + Ot __,
RS’ + RS’ *
(19) (20)
RSC;
(21)
RSSR
(22)
Scheme 11.
The disulfide formed in eqn. (22) is again reduced and ends up as alkylsulfinic acid [ 331.
Dianions as electron transfer agents Reduction of an aromatic or heteroaromatic compound with two electrons in a truly aprotic medium produces the dianion of the dihydro derivative. Figure 9 [ 31 shows the CV of perylene (XVIII) in DMF at two sweep rates, alone and in the presence of 1,4dichlorobenzene. In trace a (sweep rate u = 400 mV s-l) and trace d (u = 10 mV s-l) the two reversible reductions of perylene are shown. In b, e, c and f increasing concentrations of 1,4dichlorobenxene are added. It is seen that the height of the first peak of XVIII is not affected, which means that the rate of the electron transfer from the perylene anion radical to the aryl halide is too slow to affect the curves. The height of the second peak is increased, which shows that the dianion is an effective electron transfer agent; notice also that the effect is more pronounced at slow sweep rate (10 mV s-i), where the dianion has more time for the electron transfer reaction before XVII12- is reoxidized at the reverse sweep, than at high sweep rate. The reactions are shown in Scheme 12. A+e-9-A’ A: + e- )
(1) AZ-
(23)
219
L
I
-10
I
-1 5
1
L/v n Ag/*p
I
- I.0
1
E/V n
-15
I
*pmg I
Fig. 9. Cyclic voltammograms of perylene (3.3 x 10v3) in DMF/O.l M TBAI at different sweep rates in the presence of different concentrations of 1,4-dichlorobenzene. (a), (b), (c) u = 400 mV s-l, (d), (e), (f) u = 10 mV s-l; (a), (d) perylene alone; (b), (e) 4.5 X 10m3 M 1,4-dichlorobenzene added; (c), (f) 3.5 x 10e2 M 1,4-dichlorobenzene added [3]. A2-+BX-_4’;+BX7-+A7+B’
+X-
(24)
Scheme 12.
When BX is an alkyl halide rather than an aryl halide as in Fig. 9, a coupling rather than a catalytic reduction takes place. This coupling could either be an ‘SN2-like’ reaction or a coupling between the anion radical and the alkyl radical according to eqn. (25) AZ-+BXG[A’+B’
+X-]-+AB-+X-
(25)
In Table 2 [28], the rate constants are given for the reactions of perylene dianion and anthraquinone dianion with t-butyl chloride and s-butyl bromide, and also the rate constants obtained from Fig. 8 and the reversible potential of A7/A2-. Table 2 shows that perylene dianion reacts with t-BuC1 and s-BuBr with the rates expected for a SET mechanism, whereas the anthraquinone dianion, where the charge is more localized at oxygen, reacts about 20 times faster than expected for a SET reaction; this might indicate a certain bond formation in the transition state in this case, whereas the perylene dianion with the more delocalized charge reacts at the same rate as an anion radical with the same redox potential.
220 TABLE 2 Bate of the substitution halides A
reaction
BX
perylene perylene anthraquinone
t-BuCl s-BuBr s-BuBr
between the dianion of perylene (Pe2-)
E2
ksuB
wa
W
-1.80 -1.80 -1.123
1 s-l
km
1
171 b 400 x 103b 48
(M-’
and alkyl
kwdkswr
s-l)
141 282 x lo3 2.2
1.21 1.42 22
aMeasured against Ag/AgI, I- = 0.10 M. bObtained by cyclic voltammetry.
Photochemically excited anion radicals as electron transfer agents Scheme 13 shows a hypothetical orbital energy diagram for the lowest singlet transition in (a) a radical cation and (b) a radical anion. The Scheme
-
t
4-k
-#-I-
+
4-k
4-k (a)
St St St
4-k St +I(b)
Scheme 13. Hypothetical orbital energy diagram for the lowest singlet transition in (a) a radical cation; (b) a radical anion.
shows that an excited state of a cation radical is a more powerful oxidation agent than the cation radical, and that the excited state of an anion radical is a better reductant (electron donor) than the anion radical. The combination of electrochemistry and photochemistry has certain complications; the excited state is generally very short-lived, and there is the possibility of back-donation of an electron to the electrode, as the electrode (for anion radicals) is less reducing than the excited state. If a compound should be able to accept an electron from a photochemically excited anion radical generated at an electrode, the electron transfer reaction must be very fast.
221
Fig. 10. Cyclic voltamrno~~~ in DMFjO.1 M TBAf of pyrene (2.3 X 10W3M) alone (b), and (a) in the presence of ~~hloro~luene (8 X 10m2 I%); (b), (d) without light; (a), (e) during i~iumination with light at 488 nm f 4 1.
In Fig. 10 such a case [4] is shown; the lower left curve (b) is a CV of pyrene in DMF, aud the lower right curve (d) pyrene in the presence of mc~orotol~ene; it is seen that the peak height of pyrene is not affected by edition of ~~~orotolueue, as the rate of electron transfer is too slow. In the upper trace to the left (a), monoc~omatic ligbt is shone on the system; the wavele~h (X = 488 nm) is chosen to correspond to the a~o~tiou maximum of the pyrene anion radical. The peak height of the pyrene is not ~fected by the light. On the other hand, if light is shone on the system containiug pyrene and mchlorotoluene, the peak height increasesvery significantly (c), The experiment thus shows that the excited state of the pyrene anion radical is able to transferan electron to na-cblorotoluenein a very fast reaction and regenerate pyrene, which then accepts an electron from the electrode to the anion radical; this is then excited etc. It has also been shown that a photochemically excited, el~~ogenera~d dianion is a better electron donor than the dianion [ 341. The rate of electron transfer from pho~hemic~y excited, electrogenerated anion radicals to a number of acceptors and to photochemic~y excited, el~t~~~erat~ cations from a number of donors has been investigated 2351 by the fluorescence quen~h~g technique [36]. It was found that the data fitted better to the values prying by the Marcustheory than to those obtained by the empiricalequation given by Weller [36], provide that a rather large value was used for the reo~~zation energy for the reaction. If the value usuahy employed in such inves~~tions 136J was used, the resuhs diverged greatly from any of the equations. C~c~a~o~s of the &rueture of the excited state of the anion radical, based on the ene~we~ht~ m~mum overlap (~~0) model, suggested reasons for the rather large reo~~zation energy for these excited statesof ion radicals [ 351. ’
222 The use of such photochemically excited anion radicals as reducing agents could have certain advantages; a strong reductant would be formed in a medium which was not very reducing, so that the electrons may be transferred ‘one by one’. If a reaction followed a sequence as in Scheme 14 [37] ((A;-)* is the excited state of a radical anion) where somewhere along the
(AT)* +B---,A+BB---+---_,
(28)
c Product I
(27)
Product II
(28)
-c e-
Scheme 14.
reaction path there is a branching, with competition between a chemical (c) and an electrochemical (e-) step, the absence of a good electron donor in the photo-electrochemical reduction might favour the chemical step (Product I, eqn. (27)) whereas the electrochemical step would be favoured at an electrode or in the presence of a conventional reducing agent (Product II, eqn. (28)). The idea was later realized [38] in a system in which anthraquinone anion radical was photochemically excited in the presence of a substituted aryl halide and anthracene. The single electron transferred from the excited anthraquinone anion radical to the aryl halide resulted in the formation of an aryl halide anion radical, which rapidly cleaved to halide ion and an aromatic radical. As no good electron donors were in the neighbourhood, the
0
o+
0
0
Scheme 15.
&I 0
0
000
Br
223
aryl radical could either abstract a hydrogen atom from the aprotic solvent or attack antbracene. The isolation of a fair yield of l-substituted arylanthracene indicated that the reaction in fact followed Schemes 14 and 15.
Electrogenerated bases as catalysts Addition of an electron to a neutral compound creates a base; in this way rather strong bases may be formed [39, 401. An advantage of electrogenerated bases (EGB) is that handling the usually employed strong bases such as butyllithium may be avoided; the use of such compounds in large quantities is not without problems. Another advantage is that the desired dose of base may easily be injected by letting a certain current run a specified period. An example of a base-catalyzed reaction is illustrated in Fig. Il. The two lower traces (a and b) are CV of azobenzene (a: u = 400 mV F’, b: 40 mV s-l); to the middle (c and d) and upper curves (e, f) benzophenone pinacol (1,1,2,2-tetraphenylethane-1,2diol) has been added in increasing amounts. Benzophenone pinacol is reduced at a potential more negative than the second peak of azobenzene. On addition of the pinacol, the peak of azobenzene diminishes and virtually disappears at the e and f curves, and a new reversible system is observed; this is the reversible reduction of benzo-
Fig. 11. Cyclic voltammograms of azobenzene at different sweep rates in the presence of different concentrations of benzophenone pinacol. (a), (c), (e) u = 0.4 V s-l; (b), (d), (f) u = 0.04 V s-l; (a), (b) 10m3 M azobenzene; (c), (d) plus benzophenone pinacol; (e), (f) plus more pinacol.
224
reversible system is observed; this is the reversible reduction of benzophenone. A simplified reaction path is given in Scheme 16. Azobenzene is reduced to the anion radical (eqn. (29)), which acts as a base by abstracting a proton from the hydroxyl group in the pinacol (eqn. (30)); in the Scheme another anion radical abstracts a proton from the other hydroxyl group, but whether the cleavage of the carbon-carbon bond occurs in the mono- or dianion is not clear yet. The cleavage of the dianion results in the formation of two anion radicals of benzophenone (eqn. (31)); these anion radicals can be oxidized to benzophenone at a potential more negative than the reduction potential of azobenzene. This means that the benzophenone anion radical is able to reduce azobenzene, which creates an electrogenerated base; at the same time benzophenone is formed. The azobenzene diffusing to the electrode is thus intercepted by the anion radical and reduced, and most of it thus never reaches the electrode surface; instead benzophenone is formed (eqn. (32)), which is seen on the CV curve. 2 C6H5N=NC&
+ 2e-9
2 (C&,N=NCBH,)’
(29)
+ (C,H,),C(OH)-C(OH)(C&I&
2 C6H,NHN’--C&l, (~6H,)2~(~-)-~(~-)(~6-I,),
(C,H,),C-O-
2 (C&I,N=NC,H,)’
+ (C&l,),C(O-)-C(O-)(C,H,), -
+ Ca,N=NC
-
(31)
2 (C,H,),~--O-
6H 5 -
(C6H,),C=0
(39)
+ (C6HSN=NC6HS)’
(32)
Scheme 16.
The Scheme is further complicated by the ability of the benzophenone anion radical to act as a base on the pinacol, so the kinetics becomes rather complicated. A different kind of catalysis is shown in Scheme 17, where an electroinactive compound (benzhydrazide) reacts with a catalyst (an aldehyde) (eqn. 34); the intermediate is reduced (eqn. 35) and the catalyst regenerated (eqn. 36) [41]. Equation (37) is the sum of eqns. (34) - (36). C,H,CONHNH, + 2e- + 2H+ ft, C6HSCONHNH2+ RR’CO -
C6H&ONH, + NH3 C6H5CONHN=CRR’ + Hz0
C6H,CONHN=CRR’ + 2e- + 2H+ RR’C=NH + Hz0 -
C6H,CONHZ + HN=CRR’
RR’CO + NH,
C,H,CONHNH, + 2e- + 2H+ -
C,H,CONH, + NH,
(33) (34) (35) (36) (37)
Scheme 17.
The reaction may be of interest in some cases where it is possible to make a substitution with the better nucleophile hydrazine, but not ammonia, and a cleavage of the N-N bond afterwards is desired.
225
In the examples presented in the preceding sections, only catalysis at the cathode has been discussed. Catalysis through the formation of cation radicals, the use of mediators in oxidation [ 241, and the formation of electrogenerated acids at the anode is also possible. Thus many types of reactions may be catalyzed by transfer of electrons, perhaps not surprising in view of the fact that all chemical reactions consist of reshuffling of electrons.
References 1 H. Lund, M. A. Michel and J. Simonet, Acta Chem. Scond., B28 (1974) 901; idem, B29 (1975) 231; J. Simonet, M. A. Michel and H. Lund, Acta Chem. Stand., B29 (1975) 489. 2 H. Lund and L. H. Kristensen, Acta Chem. Scond., B33 (1979) 495. 3 H. Lund and J. Simonet, J. Electroanal. Chem., 65 (1975) 205. 4 H. Lund and H. S. Carlsson, Acta Chem. Stand., B32 (1978) 505. 5 0. Hammerich, B. Svensmark and V. D. Parker, in M. M. Baizer and H. Lund (eds.), Organic Electrochemistry, 2nd edn., Marcel Dekker, New York; 1983, p. 78. 6 P. E. Hansen, A. Berg and H. Lund, Acta Chem. Stand., B30 (1976) 267. 7 C. Degrand and H. Lund, Acta Chem. Stand., B31 (1977) 593; U. Hess, D. Huhn and H. Lund, Acta Chem. Stand., B34 (1980) 403. 8 E. Hobolth and H. Lund,Acta Chim., 1 (1977) 35. 9 C. Degrand and H. Lund, Nouu. J. Chim., 1 (1977) 35. 10 I. Crossland, Acta Chem. Stand., B29 (1975) 468. 11 H. Lund, Acta Chem. Scam-l., B37 (1977) 424. 12 H. Lund and C. Degrand, Tetrahedron Lett., (1977) 3593; idem, Acta Chem. Stand., B33 (1979) 57. 13 T. Shono, I. Nishiguchi and H. Ohmizu, Chem. Lett., (1977) 1021; idem, J. Am. Chem. Sot., 99 (1977) 7396. 14 H. Lund and J. Simonet, C. R. Acad. Sci., Ser. C., 277 (1973) 1387. 15 S. Wawzonek and D. Wearring, J. Am. Chem. Sot., 81 (1959) 2067; S. Wawzonek and S. Gunderson, J. Electrochem. Sot., 111 (1964) 324; D. A. Tyssee and M. M. Baizer, J. Org. Chem., 39 (1974) 2819, 2823; E. Lamy, L. Nadjo and J.-M. SavBant, Nouv. J. Chim., 3 (1979) 21. 16 P. Fuchs, U. Hess, H. H. Holst and H. Lund,Acta Chem. Stand., B35 (1981) 185. 17 R. S. Nicholson and I. Shain, Anal. Chem.,. 36 (1964) 706; R. S. Nicholson, Anal. Chem., 37 (1965) 1351; idem, 38 (1966) 1406. 18 L. Nadjo and J.-M. SavBant, J. Electroanal. Chem., 48 (1973) 113;E. Lamy, L. Nadjo and J.-M. Sav&ant, J. ElectroanaLChem., 42 (1973) 189. 19 H. H. Holst, Dissertation, Aarhus, 1980. 20 J. F. Bunnett, Act. Chem. Res., 11 (1978) 413. 21 J.-M. SavBant, Act. Chem. Res., 13 (1980) 323; C. Amatore, J.-M. SavBant, C. Combellas, S. Robveille and A. Thiebault, J. Electroanal. Chem., 184 (1985) 25. 22 K. Praefcke, C. Weichsel, M. FaIsig and H. Lund,Acta Chem. Stand., B34 (1980) 403. 23 R. Scheffold, M. Dike, S. Dike, T. Herold and L. Walder, J. Am. Chem. Sot., 102 (1980) 3642. 24 J. Simonet, in M. M. Baizer and H. Lund (eds.), Organic Electrochemistry, 2nd edn., Marcel Dekker, New York, 1983, p. 843. 25 T. Lund and H. Lund, Acta Chem. Stand., B38 (1984) 387. 26 H. Lund and L. H. Kristensen, Acta Chem. Stand., B33 (1979) 495. 27 R. A. Marcus, Ann. Rev. Phys. Chem., 15 (1964) 155; L. Eberson, Adv. Phys. Org. Chem., 18 (1982) 79. 28 L. Eberson, Acta Chem. Stand., B38 (1984) 439.
226 Sand29 T. Lund and H. Lund, 12th Sandbjerg Meeting on Organic Electrochemistry, bjerg, Denmark, June 1985, Abstracts of Papers, p. 13; idem, Tetrahedron Lett., 27 (1986) 95; idem, Acta Chem. Stand., B40 (1986) in press. 30 E. Hebert, J.-P. Mazaieyrat, 2. Welvart, L. Nadjo and J.-M. Saveant, NOUV. J. Chim., 9 (1985) 75. 31 S. Bank and D. A. Noyd, J. Am. Chem. Sot., 95 (1973) 8203. Chem. Res., 16 (1983) 363; A. Pross, Act. Chem. Res., 32 A. Pross and S. S. Shaik,Acc. 18 (1985) 212. 33 C. Degrand and H. Lund, Acta Chem. Stand., B33 (1979) 512. 34 H. Lund and H. S. Carlsson, Acta Chem. Stand., 834 (1980) 409. 35 J. Eriksen, H. Lund and A. I. Nyvad, Acta Chem. Stand., B37 (1983) 359; J. Eriksen, K. A. Jdrgensen, J. Linderberg and H. Lund, J. Am. Chem. SOC., 106 (1984) 5083. 36 D. Rehm and A. Weller, Isr. J. Chem., 8 (1970) 269. Society Meeting, San Francisco, May 1983, 37 H. Lund and J. Eriksen, Electrochemical Extended Abstract p. 966. 38 P. Neileborg, H. Lund and J. Eriksen, Tetrahedron Lett., 26 (1986) 1773. and M. M. Baizer, J. Org. Chem., 31 (1966) 3885; idem, J. Electro39 J. H. Wagenknecht them. Sot., 114 (1967) 1096; R. C. Halleher and M. M. Baizer, Justus Liebigs Ann. Chem. (1977) 737. 40 P. E. Iversen and H. Lund, Tetrahedron Lett., (1969) 3523. 41 H. Lund, Electrochim. Acta, 28 (1983) 395.