Electrochemical reduction of phenyl-substituted cyclopentadienes: a case of an ‘indirect father–son’ self-protonation process

Journal of Electroanalytical Chemistry 460 (1999) 160 – 175

Electrochemical reduction of phenyl-substituted cyclopentadienes: a case of an ‘indirect father–son’ self-protonation process Giuseppe Farnia a,*, Franco Marcuzzi b, Giancarlo Sandona` a a

Dipartimento di Chimica Fisica ‘A. Miolati’, Uni6ersita` degli Studi di Pado6a, 6ia Loredan 2, 35131 Padua, Italy b Dipartimento di Chimica Organica, Uni6ersita` degli Studi di Pado6a, 6ia Marzolo 1, 35131 Padua, Italy Received 12 June 1998; received in revised form 10 September 1998

Abstract The electrochemical reduction of 1,2,3,4-tetraphenyl-1,3-cyclopentadiene and 1,2,3,4,5-pentaphenyl-1,3-cyclopentadiene has been investigated in DMF. At low temperatures ( 5 − 30°C) and under anhydrous conditions cyclic voltammetry experiments indicate that both substrates are reducible in two successive, chemically reversible, one-electron steps, affording the corresponding radical anions and dianions. More complex voltammetric behaviour is detected at higher temperatures, when the radical anions are protonated by the substrate itself, giving rise, via the so-called ‘father – son’ self-protonation process, to further reducible compounds generating additional basic intermediates. In exhaustive electrolyses, 1/3 of the starting substrate is converted into dihydroreduction products and 2/3 into its conjugate bases. On the other hand, the quantitative formation of the dihydroreduction products is observed when exhaustive electrolyses are carried out in the presence of phenol, which acts as the proton donor instead of the substrate. Under voltammetric conditions, however, evidence of deprotonation of the substrate by the phenoxide anions thus generated is obtained, indicating the occurrence of an ‘indirect father – son’ self-protonation process, which takes place since phenol is kinetically more acidic, but thermodynamically less acidic, than the substrate itself. The mechanisms of decay of the radical anions are proposed on the basis of the comparison of experimental and simulated voltammetric data; this approach allowed also the determination of the characteristic kinetic rate constants, some of which are strongly dependent on the structure of the substrate, and of the importance of the homoconjugation reaction involving phenol and phenoxide anion. The stereochemistry of the dihydroreduction process and the voltammetric behaviour of the corresponding products are considered also, even in relation to the voltammetric behaviour of the starting substrates. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Cyclopentadienes; Electrochemical reduction; Dihydroreduction; Radical anions; Protonation; Self-protonation

1. Introduction The cathodic reduction of the carbon – carbon double bond is a process of valuable theoretical and technological interest, which has been investigated extensively, with several electrochemical techniques, by many researchers, since the pioneering polarographic studies by Wawzonek and Hoijtink in the early 1940s and 1950s [1,2]. Most of these studies, however, were mainly concerned with practical aspects of the hydrodimerisa* Corresponding author. Tel.: + 39-49-8275138; fax: 39-498275135; e-mail: [email protected].

tion reactions of olefins, hydrogenation of aromatics and reactions of the anionic intermediates with electrophiles [3], whereas relatively less attention was paid by the authors to other relevant mechanistic aspects, such as the systematic evaluation of the stereochemical course of the reaction, including the rearrangements of the structure, and of the role played by the electron and proton transfer processes, in spite of the fact that the knowledge of these details is essential for a correct interpretation of the whole reaction mechanism. In a series of papers on the electrochemical reduction of substituted indenes, chosen as model compounds, we have investigated both these aspects [4–12] and found

0022-0728/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 0 2 2 - 0 7 2 8 ( 9 8 ) 0 0 3 7 0 - 2

G. Farnia et al. / Journal of Electroanalytical Chemistry 460 (1999) 160–175

evidence that, although this reaction appears formally as a simple dihydroreduction of the carbon – carbon double bond of the pentatomic ring of the indenyl structure, nevertheless, several different mechanisms can operate and yield products with different stereochemistry, depending on the structure of the substrate and on the acidic character of the species present in the medium. It is well known, in fact, that the mechanism of the electrochemical reduction of the unsaturated hydrocarbons is, in general, affected by proton donors, which can be either present occasionally, even in traces, in the reaction medium or can be added on purpose to it, since such acidic species are able to protonate the basic intermediates formed during the reduction process, i.e. radical anions, monoanions and dianions. Moreover, when the substrate itself is acidic enough to protonate these intermediates, also self-protonation processes may occur, which are called, conventionally, ‘father – son’ or ‘grandfather–grandson’, depending on the kind of basic intermediate which is involved in the first protonation step, i.e. a radical anion or a dianion, respectively [13]. However, in practice the situation is not always well defined and easy to understand. For example, in one case of the indene derivatives studied by us [6,7], on the basis of kinetic and thermodynamic considerations we came to the conclusion that, contrary to expectations, the ‘grandfather–grandson’ mechanism could not operate, since the electron transfer process between the substrate and the dianion (comproportionation) was definitely faster than the proton transfer between the same two species. More generally, the possibility that this mechanism could not take place should be taken into account with all the acidic substrates which are reducible in two successive and fast one-electron transfer steps. Moreover, in work mentioned above [7] we found also that, under proper experimental conditions, an ‘indirect grandfather – grandson’ mechanism was operative, owing to the presence, in the reaction medium, of an exogenous proton donor (water) able to protonate the intermediate dianions because it is kinetically more acidic (and present also at higher concentration) than the substrate; under such conditions, the conjugate base formed was able to deprotonate the substrate, since the latter was thermodynamically more acidic than the exogenous proton donor. On this basis, one would expect that, in principle, an ‘indirect father–son’ self-protonation process could also take place, if an unsaturated substrate of proper acidity is reduced in the presence of an exogenous proton donor thermodynamically less acidic, but kinetically more acidic than the substrate itself, and able to protonate the radical anion. To our knowledge, there is only one example of such a mechanism reported in the

161

literature [14], and we thought that it could be of some interest to study it in detail, with an appropriate choice of the substrates and of the experimental conditions. For this purpose, we have studied, by cyclic voltammetry and macroscale electrolyses, the electrochemical reduction of 1,2,3,4-tetraphenyl-1,3-cyclopentadiene, 1, and 1,2,3,4,5-pentaphenyl-1,3-cyclopentadiene, 2, in DMF, both under self-protonation conditions and in the presence of phenol as an exogenous proton donor. These two substrates were chosen as model compounds because they present structural features similar to those of the phenyl-substituted indenes studied previously but have protons with a higher acidic character (for example, in DMSO pKas of 1, 2, 2,3-diphenylindene, and 1,2,3-triphenylindene are 13.7 [15], 12.5 [16], 17.7 [17], and 15.2 [17], respectively). Therefore, substrates 1 and 2 are suitable for studying the self-protonation processes, even though the system of two conjugated carbon–carbon double bonds, yielding products of the first addition of hydrogen that are further reducible, could make the study of the mechanism and of the stereochemical course of the reaction more complicated. We report in the following the result of this study.

2. Experimental

2.1. Chemicals The purification of dimethylformamide (DMF, R.P. Carlo Erba) and tetrabutylammonium perchlorate (TBAP, R.P. Carlo Erba) was described previously, as well as the activation of alumina (Merk, activity grade I) [4]. Phenol and acetic acid (R.P. Carlo Erba) were used as proton donors, in some experiments, without further purification. 1,2,3,4-Tetraphenyl-1,3-cyclopentadiene and 1,2,3,4,5-pentaphenyl-1,3-cyclopentadiene (Ega) were used after recrystallisation from acetonitrile.

2.2. Apparatus and procedures Instruments, cells, and electrodes for the electrochemical experiments are as described previously [7]. Numerical simulations of the voltammetric experiments were performed on a personal computer by a properly made program.

162 1

G. Farnia et al. / Journal of Electroanalytical Chemistry 460 (1999) 160–175

H-NMR spectra were recorded from solutions in CDCl3 on a Bruker ac 250 spectrometer operating at 250 MHz. Chemical shifts are given in d relative to Me4Si as the internal standard. Coupling constants are in Hz and refer to peak separations measured directly from the instruments for spectra not of first-order. GC-MS analyses were performed with a HewlettPackard mod. 5890 A gas-chromatograph coupled to a Hewlett-Packard mod. 5970 mass spectrometer, using an Alltech SE-30 capillary column (15 m × 0.25 mm) and the following temperature program: 100°C× 1 min, then 15°C min − 1 up to 270°C. Cyclic voltammetry (CV) experiments were carried out on a Hg electrode and macroscale reductions performed on a mercury pool. The oxidation of the anions of both substrates was detected, by CV, on a glassy carbon microelectrode. Potentials were measured with respect to an Ag AgCl reference electrode, but are referred to a saturated calomel electrode (SCE). All the experiments were carried out in DMF + 0.1 M TBAP, at various temperatures, in some cases in the presence of the appropriate concentration of proton donor. Anhydrous conditions were achieved by cycling the solvent-supporting electrolyte system through a column of activated alumina; in some cases a small amount of alumina was also added to the solution. Voltammetric kinetics under self-protonation conditions and in the presence of phenol were performed measuring the peak current of the first reduction step (corrected for the capacitive and background current) at different scan rates and substrate or phenol concentrations. The ratio C/Cref 1was obtained as the ratio between the values of I cp/62 detected, with the same electrode, in the presence of the proton donor and in the absence of any decay of the radical anions, respectively. The latter condition was achieved, in the absence of added proton donors, with scan rates high enough to obtain I ap/I cp = 1, to avoid the occurrence of the selfprotonation process. To accomplish these conditions, experiments were carried out at 0°C, a temperature which allows also the detection of the self-protonation process in voltammetric experiments carried out at the slowest sweep rates. Furthermore, with the aim of assuming a linear diffusion in the simulation, two spherical electrodes with different dimensions were used, both utilisable in the intermediate range of the sweep rates in order to compare the two different sets of data. Exhaustive controlled-potential electrolyses on a Hg pool and work up of electrolysed solutions were carried out as described previously [5]. Macroscale oxidation of hydrogen to protonate the anions R − was performed on an activated Pt gauze [18,19] and carried out with solutions obtained after exhaustive reduction of RH under self-protonation conditions, and subsequently saturated with H2 at atmospheric pressure.

2.3. Catalytic hydrogenation of 1 A solution of 1 (0.7 mmol) in acetic acid (80 ml) was hydrogenated at 1 bar and 60°C, in the presence of 10% Pd on charcoal (0.063 g) for 67 h. After separation of the catalyst, the reaction mixture was poured into an aqueous solution of NaCl (400 ml) and extracted several times with diethyl ether. The organic layer was washed with an aqueous solution of NaHCO3 and then with water to neutrality and dried over Na2SO4. Evaporation of the solvent under reduced pressure left a crude solid residue (0.250 g) formed by a complex mixture of products of partial and total hydrogenation of the pentatomic ring. Separation on a silica gel column, using a mixture of petroleum ether–dichloromethane 9:1 as the eluant, afforded, as a pure compound, the cis-1,2,3,5-tetraphenyl-1-cyclopentene 3a% (0.050 g, 20%), identified by single crystal X-ray analysis; m.p. 126–127°C (from acetone); m/z 372 (M, 90%), 281 (100), 203 (48), 115 (32), 91 (50); dH (AMX2 system) 2.09–2.16 (1 H, dt, JAM 13.4, JAX 8.2, HA), 3.17 (1 H, dt, JAM 13.4, JMX 8.9, HM), 4.42 (2H, app. t, J 8.9 and 8.2, HX), 6.9–7.3 (20 H, m, arom.).

2.4. Catalytic hydrogenation of 2 A solution of 2 (0.22 mmol) in acetic acid (100 ml) was hydrogenated at 1 bar and 65°C in the presence of 10% Pd on charcoal (0.030 g) for 6 h. After separation of the catalyst, the reaction mixture was poured into an aqueous solution of NaCl (500 ml) and extracted several times with diethyl ether. The organic layer was washed with aqueous solution of NaHCO3 and then with water to neutrality and dried over Na2SO4. After evaporation of the solvent under reduced pressure, the crude solid residue (about 80% yield) was shown to be formed by a mixture of three isomers, one of which was present in a larger amount (ca. 76%, estimated on the basis of the integration of the signals of the 1H-NMR spectrum of the crude reaction mixture), and was isolated in a pure form by column chromatography on silica gel (eluant petroleum ether–dichloromethane 9:1). This isomer was identified as the cis, cis-1,2,3,4,5pentaphenyl-1-cyclopentene 4% by single crystal X-ray analysis; m.p. 217°C; m/z 448 (M, 100%), 370 (90), 357 (67), 279 (68), 191 (60), 91 (50); dH (AB2 system) 4.59 (1 H, app. t, JAB 10.0, HA), 4.73 (2 H, app. d, JAB 10.0, HB), 6.0 and 6.9–7.3 (25 H, m, aromatics). The other two isomers were not isolated in pure form, but were obtained only in mixtures whose 1HNMR spectrum showed signals at d 4.05 (1 H, app. t, J 8.5 and 9.7), 4.73 (1 H, app. d, J 8.5), 4.88 (1 H, app. d, J 9.7), 6.8-7.3 (25 H, m, aromatics) attributable to one isomer (4¦, ca. 19% in the reaction mixture), and at d 4.37 (2 H, m), 4.74 (1 H, m), 6.0 and 6.7–7.5 (25 H, m, aromatics) attributable to the other (4§, ca. 5%).

G. Farnia et al. / Journal of Electroanalytical Chemistry 460 (1999) 160–175

The configuration to these two isomers was not assigned.

2.5. Reduction products of macroscale electrolyses As far as the reduction products, in the case of 1 the separation of the four isomers formed was not achieved and only the more abundant of them could be identified in the reaction mixture through its 1H-NMR signals by comparison with those of an authentic sample prepared by catalytic hydrogenation of 1 (see Section 3). In the case of substrate 2, the reaction mixture was formed by two isomers in a ratio approximately of 5.7:1 (estimated by integration of the 1H-NMR signals). The more abundant isomer was isolated in a pure form by column chromatography on silica gel, using a mixture of petroleum ether and dichloromethane 3:1 v/v, and was shown to be identical to the cis,cis-1,2,3,4,5-pentaphenyl-1-cyclopentene 4% obtained by catalytic hydrogenation. The second isomer was not isolated, but it showed a 1H-NMR spectrum identical to that of the isomer 4¦ obtained by catalytic hydrogenation.

2.6. Isomerisation of 1 A solution of phenol in DMF (5 × 10 − 2 M) was electrolysed exhaustively on Pt gauze at the reduction potentials of the acidic proton (−2.4 V [18,19]), generating the phenoxide ion quantitatively. Substrate 1 (1.7 × 10 − 2 M) was added under nitrogen and allowed to react for a few minutes. The reaction was quenched with a solution of phenol in excess (100:1). After the workup, the crude reaction mixture was analysed by TLC, which showed the presence of two spots; on the basis of the 1H-NMR spectrum of the mixture and, in particular, of the presence of a singlet at d 5.09, we attribute to the new product the structure of 1,3,4,5-tetraphenyl-1,3-cyclopentadiene 5 (amount present ca. 28%) and not that of 2,3,4,5-tetraphenyl-1,3-cyclopentadiene, which should display a quite different 1HNMR spectrum.

163

that expected for a reversible one-electron transfer [20]. On the contrary, the peak potential separation E ap −E cp of the second step increases with increasing sweep rate, and corresponds to the value expected for a reversible one-electron transfer only at low sweep rates (e.g. 100 mV s − 1), indicating the occurrence of a quasi-reversible electron transfer at higher sweep rates [21]. For both substrates (RH), the electron transfers hence imply the consecutive formation of the corresponding radical anions, RH’ − , and dianions, RH2 − , according to Eqs. (1) and (2), with E°1  E°2 (Table 1). E°1 (1) RH+ e − ? RH’ − E°

2 RH’ − + e − ? RH2 −

(2)

The presence of traces of water affects the voltammetric picture of both substrates, decreasing the chemical reversibility of the second reduction peak, R2. When R2 is involved in the sweep it is possible to detect, in particular under steady-state conditions (Fig. 1), a reversible one-electron peak, O3, due to the oxidation of the monoanions RH2− to the corresponding neutral radical RH2’ (Eq. (3)), being the monoanion formed by protonation of the dianion by water according to Eq. (4). E°3 (3) RH2− − e − ? RH2’ RH2 − + H2O“ RH2− + OH −

(4)

In agreement with theoretical predictions [3] and previous observations on other unsaturated hydrocarbons [4,5], the peak couple O3/R3 is located at potentials much more positive than R1 (Table 1). The presence of an excess of water implies the disappearance of peak O2, due to the quantitative decay of the dianions formed at R2, and also of peak O3 owing to the fast protonation of the monoanions RH2− to RH3, according to Eq. (5).

3. Results and discussion

3.1. Electrochemical beha6iour at low temperatures In carefully dried solutions and at low temperatures (T 5 − 30°C), substrates 1 and 2 show, in cyclic voltammetry, two successive reduction peaks, R1 and R2, (Fig. 1), which appear chemically reversible even at low sweep rates (e.g., 100 mV s − 1). For both substrates, up to 100 V s − 1 the peak potential separation between the cathodic and the anodic counterpart, E ap − E cp, of the first step does not depend on the sweep rate and its value (e.g. about 45 mV at −50°C) is close to

Fig. 1. Cyclic voltammograms of 2 (2.0 mM) at − 50°C, under anhydrous conditions: ( — ) and (---) first sweep involving different inversion potentials; ( – – – ) stationary state conditions involving both reduction steps. Voltammetric sweep rate 1 V s − 1.

G. Farnia et al. / Journal of Electroanalytical Chemistry 460 (1999) 160–175

164

Table 1 Redox potentialsa (vs. SCE) detected by cyclic voltammetry experiments, in DMF+TBAP 0.1 M, at −50°C Substrate RH 1 5 2

E°1/V (RH/RH’−) −2.07 −2.07 −2.07

E°2/V (RH’−/RH2−) −2.40 −2.32 −2.32

E°3/V (RH2’/RH− 2 ) −1.22 ca. −1.22c −1.13

Substrate RH3 3a% 6 4%

E°4/V (RH3/RH’− 3 ) −2.30 −2.60 −2.26

2− E°5/V (RH’− 3 /RH3 ) −2.63

E°7/V (RH4’/RH− 4 ) −1.27

e

e

−2.64

−1.11

E°6/V (R’/R−) \−0.2b \−0.2d 0.00

a

Obtained as (E ap+E cp)/2. The oxidation peak of R− is chemically irreversible. c The peak is covered by the corresponding peak of substrate 1. d The anion R− of 1 and 5 is the same. e Unrecognisable data since the second reduction peak of 6 is not detectable within the explorable potential range. b

RH2− +H2O “ RH3 +OH −

(5)

The presence of an excess of water causes also, in particular at low sweep rates, a lowering of the height of peak R2 and the formation of other reduction peaks at more negative potentials. The decrease of R2 is in agreement with the behaviour observed in the reduction of phenyl-substituted indenes bearing at least one acidic hydrogen: the base OH − , generated by protonation of RH2 − and RH2− by water (Eqs. (4) and (5)), deprotonates the substrate (Eq. (6)), causing a lowering of the reduction current at R2, so that an ‘indirect grandfather –grandson’ self-protonation [6,7] takes place, according to the sequence of Eqs. (1), (2) and (4)–(6). Hence, the stoichiometry of the overall process requires the consumption of 2/3 electron per molecule of RH. RH +OH − “R − + H2O

(6)

In the present case, however, the dihydroreduction products RH3 of the type 3a, 3b and 4, are, in turn, further reducible in two successive one-electron steps (Eqs. (7) and (8)), generating the corresponding radical anions and dianions, with the first peak located at potentials close to that of R2, and the second one at more negative values (Table 1).

E°4

RH3 + e − ? RH’3− E°

5 RH’3− + e − ? RH23 −

(7) (8)

Furthermore, protonation by water of the dianions RH23 − (Eq. (9)) and of the monoanions RH4− thus formed (Eq. (10)), generates additional OH − ions and this drives the indirect self-protonation process to occur to an even greater extent, with involvement of the reactions in Eqs. (1), (2) and (4)–(10) and the overall consumption of 4/5 electron per molecule of RH. RH23 − + H2O“ RH4− + OH − RH4− + H2O“ RH5 + OH −

(9) (10)

The occurrence, during the voltammetric sweep, of all these processes, as well as of homogeneous electron transfers and direct proton transfers involving the substrates and the intermediates, makes these voltammetric pictures much more complex than those investigated previously [7], so that, in this case, we have not made a quantitative study. It should also be mentioned that in the case of substrate 1 the voltammetric behaviour is further complicated by the occurrence of an isomerisation reaction (vide infra). Successive sweeps carried out without renewal of the diffusion layer show, in fact, the formation of a small reduction peak, R%2, chemically reversible under anhydrous conditions, which is attributable to the reduction of the radical anions, generated at R1, of the isomer 1,3,4,5-tetraphenyl-1,3-cyclopentadiene, 5. This attribution is in agreement with the observation of the peak couple R%2/O%2 in the first voltammetric sweep (Fig. 2) carried out on a solution containing a mixture of isomers 1 (72%) and 5 (28%) generated by chemical isomerisation of 1 (see below). The formation of the isomer 5 under voltammetric conditions can be due to deprotonation of 1 by basic species B − generated during the reduction (such as

G. Farnia et al. / Journal of Electroanalytical Chemistry 460 (1999) 160–175

165

radical anions, dianions,1 monoanions and also OH − ions formed by deprotonation of traces of water by these species), followed by reprotonation of the conjugated base R − , thus generated, by HB (Eq. (11)) and/or by the substrate itself (Eq. (12)).

(11)

(12)

The voltammetric behaviour of the isomers 1 and 5 shows that they have similar values of the redox potential E°1 , whereas the redox potential E°2 of 5 is less negative than that of 1. This could be due to a better stabilisation of the negative charge in the dianion of 5 with respect to 1, probably because of the lower steric hindrance of the former molecule, which could reach a more favourable conformation. For example, the higher thermodynamic acidity of 1,3-diphenylindene with respect to 1,2,3-triphenylindene [16] was attributed to an analogous effect.

Fig. 3. Cyclic voltammograms of 1 (20 mM) at 20°C: ( — ) and (---) before macroscale electrolysis; ( – – – ) after exhaustive electrolysis on a Hg pool at potentials of R1 ( −2.15 V); ( – · – · – ) after further exhaustive electrolysis at potentials of R5 +R%4 ( −2.70 V). Voltammetric sweep rate 10 V s − 1.

3.2. Electrochemical beha6iour at room temperature The simple voltammetric behaviour of substrates 1 and 2 observed, under anhydrous conditions, at low temperatures, becomes more complex when the temperature is raised. At 20°C, for example, a significant decrease of the chemical reversibility of R1 is observed even in the absence of traces of water. This is attributable, according to the behaviour of acidic indenes investigated previously [8] and to the results of the macroscale electrolyses reported below, to the occurrence of a ‘father–son’ self-protonation process [13] which involves the radical anions and the substrates and generates the reduction products RH3 and the

Fig. 2. Cyclic voltammograms of a mixture (2.0 mM) of isomers 1 (72%) and 5 (28%) at − 50°C, under anhydrous conditions, for different inversion potentials. Voltammetric sweep rate 1 V s − 1.

1 It should be mentioned, however, that the deprotonation reaction 2− of RH by the dianions RH , which could be responsible for the isomerisation process, practically should not take place due the presence of the competitive and very fast electron transfer involving the same species (comproportionation) [7].

Fig. 4. Cyclic voltammograms of 2 (20 mM) at 20°C: ( —) before macroscale electrolysis; (---) after exhaustive electrolysis on Hg pool at potentials of R1 ( −2.15 V); ( – – – ) after regeneration of 2 by macroscale oxidation of hydrogen on Pt (see text). Voltammetric sweep rate 10 V s − 1.

166

G. Farnia et al. / Journal of Electroanalytical Chemistry 460 (1999) 160–175

chemically irreversible reduction peak, R4, having about the same height of R1 (Fig. 5), and in the case of substrate 1 by two chemically irreversible peaks, R4 and R¦4 , with lower height (Fig. 6).

3.4. Macroscale electrolyses under self-protonation conditions

Fig. 5. Cyclic voltammograms of 2 (20 mM) at 20°C: (– – –) before, (—) and (---) after addition of phenol (0.2 M); ( – · – · –) after exhaustive electrolysis on a Hg pool at potentials of R1, in the presence of phenol ( − 2.10 V). Voltammetric sweep rate 10 V s − 1.

conjugated base R − . This aspect has been investigated quantitatively by voltammetric experiments, which are reported in a following section. The complex voltammetric behaviour detected at potentials more negative than R1 (Figs. 3 and 4), due to the occurrence of various electron and proton transfers involving the starting substrates and the species generated by reduction, was not investigated quantitatively.

3.3. Electrochemical beha6iour in the presence of exogenous proton donors Addition of proton donors such as phenol or acetic acid (in excess with respect to the substrate) makes the first voltammetric reduction peak R1 chemically irreversible at least up to the highest sweep rates investigated (1000 V s − 1). The height of this peak is higher than that observed in the absence of added acids, at the same sweep rates; in particular, the height of peak R1 in the presence of acetic acid is about 2.2 times that observed in the absence of the acid, and, in general, the peaks observed in the presence of phenol are lower than those with acetic acid. In any case, a small shift of the peak potentials towards less negative values is observed (Fig. 5). In the presence of added acidic species, at potentials more negative than R1 reduction peaks higher than those observed in their absence are detected (compare, for example, Figs. 3– 6), according to the fact that such proton donors decrease the consumption of the substrate RH via deprotonation and, therefore, increase the amount of reduction products generated. In particular, in the presence of phenol (which makes accessible a larger potential range than that allowed by the more acidic acetic acid), the chemically irreversible reduction peak R1 is followed, in the case of substrate 2 by a

For both substrates, exhaustive controlled-potential electrolyses carried out, under self-protonation conditions, at the potentials of R1, require the consumption of 2/3 electron per molecule, independently of the temperature and the degree of anhydricity of the medium. At the end of the experiments some voltammetric reduction peaks are detected at potentials more negative than those of R1 (Figs. 3 and 4). In particular, the first voltammetric peak observed in the case of compound 2, R4, located at potentials close to those of the peak R2 of the substrate, is chemically reversible even at the lowest sweep rates investigated, and its height is about 1/3 of that of starting R1 (Fig. 4). Other peaks, chemically irreversible, are detected at more negative values. In the case of substrate 1 a more complex picture is observed, with the overlapping of chemically reversible and chemically irreversible peaks. About 2/3 of unreacted substrate RH and 1/3 of reduction products RH3 (as mixture of isomers) were recovered, at the end of the electrolyses, after quenching with acetic acid (Table 2). According to all these data, the stoichiometry of the overall process occurring at potentials of R1 is that reported in Eq. (13), with the peaks observed at the end of the electrolyses attributable to the reduction of the products RH3 (see below). 3RH+ 2e − “ RH3 + 2R −

(13)

Fig. 6. Cyclic voltammograms of 1 (20 mM) at 20°C, in the presence of phenol (0.2 M): ( — ) and (---) before macroscale electrolysis; (– · – · – ) after exhaustive electrolysis on a Hg pool at potentials of R1 ( −2.10 V); ( – – – ) after further exhaustive electrolysis at potentials of R%4 ( −2.60 V). Voltammetric sweep rate 10 V s − 1.

G. Farnia et al. / Journal of Electroanalytical Chemistry 460 (1999) 160–175

167

Table 2 Results of macroscale potentiostatic electrolyses carried out at 20°C Substratea

[PhOH]/M

– 0.2 – 0.2

1 1 2 2

Further electrolysesb

Exhaustive electrolyses at R1 Electrolysis potential /V (vs. SCE)

Coulometric coefficientc

RH3 products yield %

Electrolysis potential /V (vs. SCE)

Coulometric coefficientc

RH5 products yield %

−2.15 −2.10 −2.15 −2.10

0.67 2.0 0.67 2.0

33d 100 33d 100

– −2.60e – −2.40

– 2.0e – 2.0

– 100e – 100

a

[RH]=20 mM. Electrolyses carried out at the end of those performed at R1 (see text). c Moles of electrons per mole of starting RH. d A corresponding amount of cyclopentadiene was recovered unchanged. e When the further electrolysis was performed at −2.35 instead of −2.60 V, a coulometric coefficient of 1.6 was detected, and RH3 products were still observed in the reaction mixture. b

The formation of the conjugated base R − is confirmed also, for both substrates, by the observation, on a GC electrode, of oxidation peaks O6 (for potential values see Table 1), which can be generated also by addition of TBAOH to the solutions of both substrates investigated. In the case of substrate 2, peak O6 is completely reversible even at low sweep rates (50 mV s − 1) and room temperature, whereas in the case of 1 this peak is chemically irreversible even at the higher sweep rates (1000 V s − 1) and at the lowest temperature (− 50°C) investigated. In the case of 2, peak O6 can be attributed to the one-electron oxidation of R − with formation of the corresponding neutral radical R’ (see Eq. (14)), which is stable in the time scale of the voltammetric experiments. According to the stoichiometry of reaction in Eq. (13), the height of peak O6 is 2/3 of that of peak R1 detected, before the electrolyses, on the same electrode and at the same sweep rate (high enough to render R1 chemically reversible). E°6 R − −e − ? R’ (14) In the case of 1, the chemically irreversible peak O6, whose height is, however, close to 2/3 of that of the corresponding peak R1, can be attributed also to the one-electron oxidation of the anion R − , followed, in this case, by a very fast decay of the neutral radical thus formed via dimerisation reactions2.

3.5. Macroscale electrolyses in the presence of phenol

ses carried out, in the presence of an excess of phenol, at the potentials of the first, chemically irreversible, reduction peak R1, requires the consumption of two electrons per molecule of substrate RH, and yields quantitatively the cyclopentene products RH3 as a mixture of isomers (see below and Table 2), according to Eq. (15). RH+ 2e − + 2PhOH“RH3 + 2PhO −

(15)

At the end of the electrolysis, in the case of 2, a single, chemically irreversible, reduction peak R4 (Fig. 5) is observed, whose height is similar to that of the peak R1 initially detected. Peak R4, which is the same observed as that before the electrolysis, at potentials more negative than R1, can be likely attributed to the two-electron, two-proton reduction of the products RH3, according to Eq. (16). RH3 + 2e − + 2PhOH“RH5 + 2PhO −

(16)

The more complex picture detected for 1 with respect to 2 (Figs. 5 and 6) can be due either to the occurrence of isomerisation of 1 into 5, which are both reducible in the same range of potentials3, and/or to the formation of isomeric species RH3 (e.g. 3a, 3b and 6), having very different reduction potentials (see also below). It should be mentioned, however, that the further macroscale reduction requires two electrons per molecule of starting RH to eliminate peak R4 in the case of 2 and peaks R4 + R%4 in the case of 1.

For both substrates, exhaustive macroscale electroly2

Preliminary results indicate, in fact, that in the case of substrate 2 the neutral radical R’ can be accumulated by macroscale electrolyses on a Pt gauze and gives rise to a slow decay with formation prevalently of dimeric and, to a lesser extent, of oligomeric products. In the case of 1, the neutral radicals are not accumulated by macroscale oxidation, according to the chemical irreversibility of the voltammetric peak, and dimeric species are exclusively obtained. The faster decay of the neutral radical of substrate 1 with respect to that of 2 can be attributed, in our opinion, to the lack of a phenyl group in the C5 position of the pentatomic ring of the former, which makes the access to this carbon atom easier from a steric point of view.

3

Of course, this could happen, even to a major extent, also under self-protonation conditions, where the conjugated base of 1 is formed in consistent amount. However, this aspect was not considered since the products obtained by macroscale electrolysis, under these conditions, were not further investigated, as reported above.

168

G. Farnia et al. / Journal of Electroanalytical Chemistry 460 (1999) 160–175

Isomerisation of substrate 1 in the electrolyses carried out in the presence of phenol, which is in agreement with the lower thermodynamic acidity of the latter with respect to the former (see below), was confirmed by an isomerisation experiment (see Section 2), and can be pictured by Eqs. (11) and (12) where HB in this case means PhOH.

3.6. Voltammetric beha6iour of the reduction products RH3 The voltammetric behaviour of the mixtures of isomeric products RH3 obtained by macroscale reduction and/or catalytic hydrogenation of RH (see Section 2) as well as that of pure isomers, were investigated under anhydrous conditions and low temperatures, with the aim of comparing the corresponding reduction potentials to those of the starting substrates 1 and 2. The isomer 4% shows, by CV, two chemically reversible one-electron reduction peaks attributable to the electron transfers reported in Eqs. (7) and (8), with the first step reversible at least up to 100 V s − 1 and the second one quasi-reversible, in agreement with the behaviour observed for substrate 2. The mixture of isomers 4% and 4¦, obtained by reduction of 2 in the presence of phenol, shows an identical voltammetric behaviour (Fig. 10), indicating that the reduction potentials of the two species are practically the same. This is in agreement with the fact that both isomers have a 1,2-diphenylsubstituted carbon – carbon double bond in the pentatomic ring.

On the other hand, the voltammetric behaviour of the isomeric mixture obtained by reduction of 1 in the presence of phenol is substantially different from that detected for the isomer 3a% obtained by catalytic hydrogenation (Fig. 11). In particular, whereas the latter shows two chemically-reversible one-electron reduction peaks (R4/O4 and R5/O5 couples) located at potentials similar to those detected for the isomers 4% and 4¦ (Table 1), in agreement with the presence, in its structure, of a 1,2-diphenylsubstituted carbon – carbon double bond, the mixture of isomers shows the overlapping of the peak couples R4/O4 +R5/O5 with a couple R%4/O%4 located at potentials close to those of R5/O5. We attribute peak R%4 to the first reduction step of the isomeric products 6 having only one phenyl group bonded to the carbon–carbon double bond of the pentatomic ring, in analogy with the behaviour of similar compounds; 1-phenyl-1-cyclohexene, for instance, shows,

under the same experimental conditions, only one chemically reversible peak located at potentials even more negative than R%4 (− 2.78 V vs. SCE). With regard to the voltammetric behaviour of the reduction products RH3, it should be mentioned also that chemically reversible oxidation peaks O7 are detected, at potentials about 1 V more positive than E°4 (Table 1), under the same experimental conditions which make detectable peaks O3 for substrates 1 and 2. These peaks are hence attributable to the oxidation (Eq. (17)) of the monoanions RH4− generated by protonation of the dianions RH23 − formed at R5 (Eq. (9)) E°7

RH4− − e − ? RH4’

(17)

3.7. Voltammetric kinetics under self-protonation conditions In agreement with the behaviour of indenyl compounds investigated previously [5,6,8,9], for both substrates 1 and 2 the cyclic voltammetry experiments, the coulometric results of macroscale electrolyses and the analyses of the reduction products indicate that the decay of the radical anions RH’ − occurs via protonation by the substrate RH itself (Eq. (18)), followed by reduction of the neutral radical RH2’, at the electrode (Eq. (3%)) and/or in solution by a radical anion RH’ − (Eq. (19)), and by protonation of the monoanion RH2− , thus formed, by the substrate (Eq. (20)). The possible decay via disproportionation of two radical anions with protonation of the dianion RH2 − , and of the monoanion RH2− thus generated, by two molecules of RH can be ruled out on the basis of the considerations made for indenyl substrates in our previous studies [7]. The kinetic analysis was carried out in terms of variation of the ratio between the cathodic and the anodic peak currents, I ap/I cp, of the first step with the sweep rate, 6, and the substrate concentration, cS. Owing to the relatively slow decay of the radical anions under voltammetric conditions, the variation of peak potential and current function of this peak are not significant for the investigation of the present case [13]. According to the behaviour of those indenyl substrates, which showed a relatively slow decay of the radical anions [8,13], the present experimental data fits well (Fig. 7) the theoretical working curve calculated for the so-called DISP1 pathway (Eqs. (1) and (18)– (20) with k − 18/k19 5 10 − 2 [13]), with k18 = 6×103 and 1.2× 103 M − 1 s − 1 for 1 and 2, respectively. k18

RH’ − + RHk? RH2’ + R −

(18)

− 18

E°3

RH2’ + e − ? RH2− k19

RH2’ + RH’ − k? RH2− + RH

(3%) (19)

− 19

k20

RH2− + RH“ RH3 + R −

(20)

G. Farnia et al. / Journal of Electroanalytical Chemistry 460 (1999) 160–175

It should be mentioned that the reaction in Eq. (19) is practically irreversible. In fact, from the difference E°3 −E°1 it can be obtained that K19 ( =k19/k − 19) \ 1016 and, the maximum value of k19 being the diffusion limit 5×109 M − 1 s − 1 [22], that k − 19 B10 − 5 M − 1 s − 1, that is much lower than k20, since protonation of RH2− by H2O (see above), and hence by RH, is a fast process. Furthermore, assuming the forward electron transfer in Eq. (19) is close to the diffusion limit [22], values of k − 18 5 5×107, and hence K18 ( = k18/k − 18) ]12× 10 − 5 and 2.4 ×10 − 5 for 1 and 2, respectively, can be estimated. On the basis of these values and of the zone diagram relative to the possible father-son self-protonation mechanisms [13], alternative DISP and ECE (the latter implying the electron transfer in Eq. (3%) instead of that in Eq. (19) pathways can be ruled out, in agreement with the behaviour of other unsaturated hydrocarbons investigated previously [8,13]. The occurrence of the DISP1 pathway is essentially due to the relatively slow protonation reaction of the radical anions by the substrate (forward of Eq. (18)), and to the fast, irreversible, electron transfer between the neutral radical and the radical anion (forward of Eq. (19)). Irreversibility of the reaction in Eq. (20) is likely assumed on the basis of the higher acidity of the substituted cyclopentadienes with respect to the corresponding cyclopentenes.

3.8. Voltammetric kinetics in the presence of phenol The chemical irreversibility of the first reduction peak R1 and its shift towards less negative values detected in

Fig. 8. Reduction of substrate 1 (1.0 mM) at 0°C, in the presence of phenol: variation of the ratio C/Cref (see text) with the experimental parameter reported in abscissa. Phenol concentrations: cP/mM 10 (), 25 ( ), 50 (2), 100 (). The solid lines are the working curves obtained by digital simulation for k19 =5 × 109 M − 1 s − 1, k21 =2.5 × 105 M − 1 s − 1, k22 =2.5 × 107 M − 1 s − 1, k23 =3.5× 104 M − 1 s − 1, k − 23 =1.5 × 102 M − 1 s − 1, and K23 =65 M − 1. The dashed line represents the working curve obtained for k23 =k − 23 = 0.

the presence of added proton donors such as acetic acid or phenol (in excess over the substrates), indicates the occurrence of a decay of the radical anions much faster than that observed under self-protonation conditions, hence implying a proton transfer between the proton donor and the radical anion (see, e.g. Eq. (21)), as observed previously in the case of substituted indenes [5,13]. k

21 RH’ − + PhOH“ RH2’ + PhO −

Fig. 7. Reduction of substrates 1 and 2 at 0°C under anhydrous conditions: variation of the anodic to cathodic peak current ratio, I ap/I cp, with the experimental parameter reported in abscissa. Substrate 1: cS/mM 0.45 (), 0.60 ( ), 1.5 ("), 3.2 (“). Substrate 2: cS/mM 1.0 (), 1.8 ( ), 2.4 (2), 5.2 (). The solid lines are the working curves for k − 18/k19 =10 − 2 [13] and for the values of k18 reported in Table 3.

169

(21)

Progressive additions of acetic acid causes a consistent increase of the current function of peak R1, in particular at the lowest sweep rates, up to a nearly constant value in the presence of a concentration of this proton donor at least twice that of the substrate. Under the latter conditions, the ratio between the current function detected in the presence of acetic acid and in the absence of any decay of the radical anions, C/Cref (see Section 2 for details), is close to 2.2, which is the value expected for the ratio between the current function characteristic of a two-electron, two-proton reduction process, and the current function relative to a chemically reversible one-electron transfer [23]. On the contrary, addition of phenol, even in large excess, implies not only the detection of lower peak current values with respect to those observed in the presence of acetic acid, under the same experimental conditions, but also a particular variation of the cathodic peak current function with the sweep rate. In Figs. 8 and 9, at different phenol concentrations, the

170

G. Farnia et al. / Journal of Electroanalytical Chemistry 460 (1999) 160–175

Fig. 11. Cyclic voltammograms at − 50°C, under anhydrous conditions: ( — ) and (---) isomer 3a% (2.0 mM) at different inversion potentials; ( – – – ) mixture of isomers 3a, 3b and 6 (see text). Voltammetric sweep rate 1 V s − 1. Fig. 9. Reduction of substrate 2 (1.0 mM) at 0°C, in the presence of phenol: variation of the ratio C/Cref (see text) with the experimental parameter reported in abscissa. Phenol concentrations: cP/mM 10 (), 25 ( ), 50 (2), 100 (). The solid lines are the working curves obtained by digital simulation for k19 = 5×109 M − 1 s − 1, k21 = 6.5× 104 M − 1 s − 1, k22 = 6.5× 106 M − 1 s − 1, k23 = 3.4× 104 M − 1 s − 1, k − 23 =1.0 ×102 M − 1 s − 1, and K23 = 65 M − 1. The dashed line represents the working curve obtained for k23 = k − 23 = 0.

variation of the ratio C/Cref versus the kinetic parameter log(cPRT/F6) is reported, where cP represents the analytical concentration of phenol. In particular, the part of the curves localised at the highest values of the abscissa scale (and corresponding to an increase of the ratio C/Cref with decreasing 6) resembles, especially for the highest concentrations of phenol (Figs. 8 and 9), that expected for the sequence of reactions in Eqs. (1), (21), (19) and (22) (with the

reactions in Eqs. (19), (21) and (22) chemically irreversible and Eq. (19) as the rate determining step), corresponding to k21 = 2.5× 105 and 6.5× 104 M − 1 s − 1 for substrates 1 and 2, respectively. k22

RH2− + PhOH“ RH3 + PhO −

(22)

The decrease of the current function with decreasing 6, observed in the opposite zone of the graphics (in particular, for the data detected with the lowest concentrations of phenol), is attributable to the consumption of the substrate (Eq. (23)) via deprotonation by phenoxide ions PhO − generated in the reactions in Eqs. (21) and (22). The reaction in Eq. (23) implies, in practice, the occurrence of an ‘indirect father–son’ self-protonation process, since the substrate RH is the species which eventually acts as a proton donor towards the radical anion and the monoanion. k23

RH+PhO − k? R − + PhOH

(23)

− 23

Fig. 10. Cyclic voltammograms of a mixture of isomers 4% and 4¦ (2.0 mM) at − 50°C, under anhydrous conditions: (—) and (---) first sweep involving different inversion potentials; (– – –) stationary state conditions involving both reduction steps. Voltammetric sweep rate 1 V s − 1.

According to the ‘indirect grandfather–grandson’ self-protonation process investigated previously [7], the effect exerted by the occurrence of the reaction in Eq. (23) is evident at the lowest sweep rates and, owing to the reversibility of this proton transfer, at the lowest phenol concentrations. In this regard we can observe, in fact, that the occurrence of this process is in agreement both with the significant amount of anions R − observed during the macroscale electrolysis of RH in the presence of PhOH (but lower than that observed under self-protonation conditions), and with an overall twoelectron, two-proton reduction, since the progressive consumption of RH shifts the reaction in Eq. (23) to the left, causing the complete disappearance of the (unreducible) R − species. To confirm the proposed mechanism, working curves calculated for the group of reactions in Eqs. (1), (21),

G. Farnia et al. / Journal of Electroanalytical Chemistry 460 (1999) 160–175

(19), (3%) and (23) were obtained by digital simulation (see Appendix A), assuming the following simplifications. (i) According to what is reported above, the decay of the radical anions by disproportionation or by protonation by the substrate were both neglected on the basis of the low values of the corresponding rates with respect to that of the reaction in Eq. (21). Accordingly, the reaction in Eq. (20) was neglected with respect to the reaction in Eq. (22). (ii) On the basis of the observation made above for the self-protonation mechanism, the reactions in Eqs. (21) and (22) were considered irreversible, taking into account also the presence of an excess of phenol, and the rate constant of the irreversible electron transfer in Eq. (19) assumed to be close to the diffusion limit. (iii) According to the higher basicity of the monoanions RH2− with respect to the corresponding radical anions RH’ − [9], the rate constant k22 was assumed to be at least 100 times higher than k21. With this assumption, the simulated voltammograms practically do not depend on the particular value of k21 utilised in the computation. (iv) On the basis of the higher thermodynamic acidity of RH with respect to phenol (as indicated by the excess of PhOH necessary to regenerate RH by protonation of R − ), K23 ( = k23/k − 23) is higher than 1, so that k23 \ k − 23. With these assumptions, linear sweep voltammograms were simulated using the values of k21 reported previously, and various values of k23 and k − 23. These simulations indicated that: (i) according to the experimental observations, the zone corresponding to the higher sweep rates essentially depends on the value of k21, in particular for high phenol concentrations; (ii) for a given value of k21, the remaining zone of the curves, and, in particular, the position of the maximum, depends strongly on the value of k23; (iii) the value of k − 23 affects only the data obtained with the lowest values of the sweep rate. On this basis, however, using the same set of kinetic constants it is impossible to fit satisfactorily the experimental data with the simulated ones. Different values of k23 are, in fact, necessary to fit the data detected for different phenol concentrations. In particular, lower values of the kinetic constant k23 have to be utilised in correspondence with higher concentrations of phenol. This observation should indicate that the deprotonation reaction of RH by PhO − becomes gradually less important when the amount of proton donor is increased. This fact can be attributed to the presence, in the dipolar aprotic solvent utilised, of the homoconjugation reaction in Eq. (24), according to which higher concentrations of PhOH imply the presence of lower concentrations of free phenoxide anions, and hence a deprotonation of RH to a lower extent considering that

171

the complexed species PhOHOPh − must be much less basic than the uncomplexed anions PhO − . K24

PhOH+ PhO − ? PhOHOPh −

(24)

Assuming that the equilibrium in Eq. (24) is easily achieved in the reaction layer, and taking in account also that the concentration of PhOH, this species being present in excess with respect to PhO − , is close to the analytical value cP, the different values of k23 used in the simulation without considering the reaction in Eq. (24) (and indicated as k%23) are roughly related to the real kinetic constant k23 by Eq. (25), where c% represents the concentration of phenoxide anions in the absence of homoconjugation (as assumed in the simulation) and c the real concentration of PhO − which, in turn, depends on the value of K24. k%23c%= k23c

(25)

From Eq. (25), taking into account the stoichiometry of the reaction in Eq. (24), it can be obtained easily that: 1/k%23 = 1/k23 + K24cP/k23

(26)

According to Eq. (26), a (rough) linear fit is obtained, for both substrates, reporting the experimental values of 1/k%23 against cP, with the intercept and the slope which assign to k23 a value in the range 2–4× 104 M − 1 s − 1, and to the ratio K24/k23 a value in the order of 2–3 ×10 − 3 s, respectively. To confirm the hypothesis of the occurrence of the homoconjugation reaction, simulations were performed considering the reaction in Eq. (24), and hence without restrictions on the values of the concentration of phenol and related species. However, in order to avoid the introduction of two more kinetic constants, the achievement of the equilibrium condition in the reaction layer was assumed for homoconjugation. Under these hypotheses, a good agreement between experimental and simulated data was obtained (Figs. 8 and 9) using a single value of the kinetic constant k23 for different phenol concentrations and, of course, the same value of the equilibrium constant K24 for both substrates (Table 3).

3.9. Comparison of kinetic constants First of all, it should be observed that taking into account the uncertainty of the experimental data, and the assumption that the complex PhOHOPh − has not an appreciable acidic or basic character with respect to PhOH and PhO − , respectively, the values reported in Table 3 represent a rough estimation of the constants k23, k − 23, and K24. As regards k23 and K24, in particular, it was observed that, within certain limits, the simulated data depend essentially on the value of the ratio k23/K24 rather than on the individual values of the

G. Farnia et al. / Journal of Electroanalytical Chemistry 460 (1999) 160–175

172

two constants. Furthermore, some uncertainty also accompanies the values of k − 23, since the corresponding reaction affects, to a lower extent, only the data corresponding to very low sweep rates. On the contrary, the values of k21 can be considered quite reliable, being essentially detected on the basis of experimental data (those obtained for the highest sweep rates) practically unaffected by the proton transfers reported in Eq. (23). On this basis, we can assert that the comparison of the experimental and simulated data confirm the importance of the reversible reaction in Eq. (23) and of the homoconjugation reaction in Eq. (24), yields a significant value for the kinetic constant k21 and allows an estimation of the kinetic constant k23 and of the homoconjugation equilibrium constant K24 or, better, of their ratio k23/K24. Furthermore, we can observe that the higher value of k21 for substrate 1 (ratio ca. 4:1) indicates a higher kinetic basicity of the corresponding radical anion with respect to that of substrate 2. Since the potentials of the first reduction step of both substrates is the same, this fact cannot be attributed to a lower resonance stabilisation of the radical anion 1’ − with respect to 2’ − , but rather to steric effects exerted by the phenyl group bonded at C5, taking also into account that protonation of both radical anions should occur preferentially at C1 or C4 since, in this way, the more stable allylic neutral radicals are formed. A similar situation was observed for the protonation of the radical anions of 2,3diphenylindene and 1,2,3-triphenylindene, investigated previously [8]. On the other hand, the value of self-protonation rate constant k18 depends both on the basic character of the radical anions and on the acidic character of the substrates. However, k18 is also higher for substrate 1, with a ratio between the values for the two substrates (5:1) similar to that observed for k21. This effect can still be attributed to the different basicity of the two radical anions. This hypothesis can be considered correct if the resonance stabilisation effect on the conjugated base Table 3 Values of kinetic and thermodynamic constants relative to self-protonation mechanisms of substrates 1 and 2 in DMF+0.1 M TBAP, at 0°C Constant

Substrate 1

Substrate 2

k18/M−1 s−1 k−18/M−1 s−1 K18 = k18/k−18 k19/M−1 s−1 k21/M−1 s−1 k23/M−1 s−1 k−23/M−1 s−1 K24/M−1 (k23/K24)/s−1

6×103 55×107 -5 ]12×10 9 5×10 2.5×105 3.5×104 ca. 1.5×102 65 5×102

1.2×103 55×107 -5 ]2.4×10 9 5×10 6.5×104 3.4×104 ca.1×102 65 5×102

R − , due to the phenyl group bonded at C5 in substrate 2, which increases the acidic character of 2 with respect to 1, is compensated by the steric hindrance exerted by the same substituent on C5, as well as by the presence of only one acidic proton in 2 with respect to 1. This assumption is in agreement with the similar values of k23 (or of the ratios k23/K24) for the two substrates, indicating that they have similar kinetic acidity with respect to phenoxide anions. As far as the estimated value of K24 is concerned, it can be observed that very different values are reported in the literature [24], sometimes higher than ours, depending on the method used for the determination of the constant and on the grade (in particular anhydricity) of the solvent used.

3.10. Identification of the reduction products Coulometric evidence and GC-MS and 1H-NMR analyses of the reduction products obtained by macroscale electrolyses show that the electrochemical reduction of 1 and 2 at potentials corresponding to the first step formally corresponds to the addition of two hydrogen atoms to the conjugated system of carbon– carbon double bonds of the cyclopentadienic ring. However, the isolation and identification of the reduction products was complicated by the formation of several isomers, as a consequence of the fact that the addition of hydrogen can occur either with 1,2- or with 1,4-regioselectivity and with syn- or anti-stereoselectivity; furthermore, isomerisation reactions can take place also under certain experimental conditions (see below). For both substrates 1 and 2, in the electrolyses carried out, under self-protonation conditions, at the potentials of the first reduction step, only 1/3 of the starting compounds were converted into reduction products RH3, as a mixture of isomers; these could not be separated from the large excess of starting material still present in the final reaction mixture and were not identified. In the presence of phenol, on the other hand, the exhaustive reduction of 1 and 2 at the potentials of the first reduction step consumes all the starting material. In the case of substrate 1, the GC-MS analysis of the final reaction mixture showed the presence of four products, all with Mw 372 and almost identical fragmentation patterns of 4, 22, 52 and 22%, respectively, following the order of elution. Unfortunately, all the attempts made to separate these isomers failed. Tentatively, we assigned to the more abundant isomer of the mixture the structure of a 1,2,3,5-tetraphenyl-1-cyclopentene with cis-configuration at the carbons 3 and 5, by comparing its 1H-NMR signals, well distinguishable as the most prominent signals of the spectrum of the reaction mixture, with those of the authentic sample 3a%, prepared by catalytic hydrogenation of 1 (see Sec-

G. Farnia et al. / Journal of Electroanalytical Chemistry 460 (1999) 160–175

tion 2). Isomer 3a%, in fact, displays a typical pattern of 1 H-NMR signals (AMX2 system), which can be used, as a sort of fingerprint, for its identification. Also, the macroscale electrolyses of 2 carried out in the presence of phenol afforded a mixture of products, which was shown by GC-MS analysis to be formed by two isomers with Mw 448 of 15 and 85%, respectively, following the order of elution. The more abundant isomer was isolated in pure form and identified by single crystal X-ray analysis as the cis, cis-1,2,3,4,5-pentaphenyl-1-cyclopentene 4%. The minor isomer, 4¦, could not be isolated in a pure form and was not identified. It should be mentioned also that when the exhaustive macroscale electrolyses of 1 and 2 were carried out until complete disappearance of all the reduction peaks (see above), mixtures of products formally corresponding to addition of four hydrogen atoms to the cyclopentadienic system were obtained in both cases. In fact, GCMS analyses of the mixtures generated by reduction of substrates 1 and 2 show isomers (four, in both cases), with Mw 374 and 450, respectively; in the 1H-NMR spectra of these mixtures only signals in the aliphatic and aromatic regions are detected. These products were not studied further.

3.11. Stereochemical aspects of the reduction About the stereochemical course of the electrochemical reduction, only a few comments can be made here. In fact, the complexity of the system studied and the problems encountered in the isolation and identification of the reaction products (especially those obtained under self-protonation conditions) do not allow any definitive conclusion, for the moment. As far as substrate 1, the electrochemical reduction in the presence of phenol, at potentials corresponding to the first step, seems to follow prevalently a pathway of a formal 1,4-syn-addition of hydrogen to the cyclopentadienic system, analogously to the catalytic hydrogenation (see Section 2); in fact, 3a% is the major product ( \ 50%) of the reaction and certainly it cannot derive from an isomer of 1. In the case of substrate 2, the stereochemistry of the cathodic reduction is even more selective, since only two products are formed in a ratio approximately 5.7:1; also in this case the formal syn-addition of hydrogen to the cyclopentadienic ring is favoured. This stereochemistry appears to be consistent with that found, under the same conditions, in the electrochemical reduction of the phenyl-substituted indenes studied previously [5,8,9,11,12], for which a kinetically controlled process was operating, with the insertion of the two protons on the same side of the plane of the pentatomic ring.

173

4. Conclusions The electrochemical reduction of substrates 1 and 2 leads to the formation of the corresponding radical anions and dianions; the latter are not further reducible within the accessible potential range, but are protonated easily even by very weak acidic species (e.g. water traces), affording the monoanion RH2− and eventually the dihydroreduction products RH3. The radical anions are much weaker bases than the dianions and can be protonated, under voltammetric conditions, only by proton donors more acidic than water, such as the substrate itself (giving rise, in this case, to a ‘father–son’ self-protonation process), or by an exogenous acidic species such as phenol. In both cases the neutral radicals thus formed are reduced to the monoanions, which are protonated to the products RH3. Under self-protonation conditions only 1/3 of the starting substrates afford these products, 2/3 being transformed into the conjugated base R − (which is not reducible within the accessible range of potential), so that 2/3 electron per molecule are required in exhaustive electrolyses carried out at the potentials of the first reduction step. On the contrary, two electrons per molecule are required in exhaustive electrolyses carried out in the presence of phenol, leading to quantitative formation of RH3 species. However, under appropriate voltammetric conditions the reduction of both substrates implies the overall involvement of less than two electrons per molecule, due to the occurrence of a deprotonation reaction of RH by the phenoxide anions generated from phenol in the protonation reactions of the basic species formed in the reduction. The occurrence of this reaction is due to the higher thermodynamic acidity of the species RH with respect to PhOH which, on the other hand, is more acidic than both substrates from a kinetic point of view. This means, in practice, that an ‘indirect father–son’ self-protonation process takes place, at least in part, in which phenol catalyses the protonation reactions by RH. Under exhaustive electrolysis conditions, however, a two-electron process is observed since the total consumption of the substrate shifts the proton transfer reaction between RH and PhO − towards these species. The formation, as final compounds, of RH3 species, which are further reducible at potentials more negative with respect to those of the starting substrates, makes the voltammetric behaviour of the substrates RH quite complicated, since several electron and proton transfers can occur, both in the absence and in the presence of phenol, and also because different isomeric species are formed. Furthermore, isomerisation can involve also the starting substrate in the case of 1. The comparison of experimental and simulated data confirms the proposed mechanisms and indicates also the necessity of considering the homoconjugation reac-

G. Farnia et al. / Journal of Electroanalytical Chemistry 460 (1999) 160–175

174

tion involving PhOH and PhO − ; it allows an estimation of the more significant kinetic constants, which show that the kinetic acidities of 1 and 2 with respect to the phenoxide anions are similar (nevertheless substrate 2 is thermodynamically more acidic than 1), whereas the kinetic basicity of the radical anion of 1 is higher than that of the radical anion of 2. These facts can be attributed to electronic and steric effects exerted by the phenyl group bonded at C5 of 2, taking into account also that the protonation reactions of the basic intermediates preferentially involves C1 and C4 atoms of both substrates, as indicated by the stereochemical course of the reduction process.

Acknowledgements The Ministero dell’Universita` e della Ricerca Scientifica e Tecnologica (MURST) and the Consiglio Nazionale delle Ricerche (CNR), Rome, are acknowledged for financial support.

Appendix A In the absence of homoconjugation, the process involving the semi-infinite linear diffusion of the species RH (A), RH’ − (B), RH2’ (C), RH2− (M), R − (N), and PhOH (P) toward a planar electrode and the sequence of reactions in Eqs. (1), (21), (3%), (19), (22) and (23), is presented, assuming the same diffusion coefficient (D) for all the species and the conditions of mass conservation for PhOH +PhO − ([PhOH] + [PhO − ]= cP, the analytical concentration of phenol), by the six partial differential Eqs. (A1), (A2), (A3), (A4), (A5) and (A6), with the corresponding initial (Eq. (A7)) and boundary (Eqs. (A8), (A9), (A10), (A11), (A12) and (A13)) conditions. (A/(t = D(( A/(x ) +k19BC − k − 23A(cP −P) 2

2

(A1)

+k − 23NP (B/(t =D(( B/(x ) − k21BP −k19BC

(A2)

(C/(t =D(( 2C/(x 2) +k21BP −k19BC

(A3)

(M/(t =D(( M/(x ) + k19BC − k22MP

(A4)

(N/(t =D(( N/(x ) + k23A(cP −P) −k − 23NP

(A5)

2

2

2

2

2

2

(P/(t = D(( 2P/(x 2) − k21BP −k22MP + k23A(cP − P) − k − 23NP t= 0, x ]0:

(A6)

A= cS, B = C =M =N = 0, P = cP (A7)

t \0, x “ : t \0, x =0:

A= cS, B = C =M =N =0, P=cP (A8)

A= B exp[F(E− E°1 )/RT]

(A9)

((A/(x)x = 0 = − ((B/(x)x = 0

(A10)

C= M exp[F(E−E°3 )/RT]

(A11)

((C/(x)x = 0 = − ((M/(x)x = 0

(A12)

((N/(x)x = 0 = ((P/(x)x = 0 = 0

(A13)

In this sequence of equations the capital letters A–C, M, N and P represent the concentrations of the corresponding species, which depend on the time, t, and on the distance from the electrode, x. The initial and boundary conditions express that: (i) only RH (with cS its analytical concentration) and PhOH are initially present in solution; (ii) both electrode charge transfers are Nernstian; (iii) the fluxes of the couples of species A/B and C/M at the electrode surface are bound by the corresponding electron transfer reactions; (iv) the species N and P are electroinactive. In the simulation of a linear sweep voltammetric experiment the applied potential E is related to the initial potential, Ei, and to the scan rate, 6, by Eq. (A14), and the response to be sought is the total current, expressed by Eq. (A15), as a function of the applied potential. E= Ei − 6t

(A14)

I= FSD[((A/(x)x = 0 + ((C/(x)x = 0]

(A15)

To compare experimental and simulated data (see Section 3), the values of total current I were transformed into the corresponding values of the ratio C/ Cref, according to Eq. (A16), where 0.446 represents the value of the current function expected for a completely reversible one-electron transfer, independently of the scan rate [25]. C/Cref = [I/FScS(DF6/RT)1/2]/0.446

(A16)

Computation were performed by expressing the equations in terms of dimensionless variables and parameters, and using for E°1 , E°3 , cS and cP the experimental values. Under these conditions, the solutions depend on the values of the dimensionless parameters RTcSk19/F6, RTcSk21/F6, RTcSk22/F6, RTcSk23/F6, RTcSk − 23/F6, which describe the competition between the homogeneous reactions and the diffusion. The system of differential equations was solved by means of an implicit (Crank–Nicolson) finite-difference procedure [26,27], with an exponentially expanding space grid [28]. Voltammograms were calculated for different values of the above parameters, in order to obtain the variation of the ratio C/Cref with the sweep rate. Taking into account the homoconjugation reaction in Eq. (24), assuming it is fast enough so that the equilibrium condition in Eq. (A17) is fulfilled, on the basis of the mass conservation [PhOH]+[PhO − ]+ 2[PhOHOPh − ]= cP the differential Eqs. (A1), (A5) and

G. Farnia et al. / Journal of Electroanalytical Chemistry 460 (1999) 160–175

(A6) are modified by substitution of the term (cP − P) with (cP −P)/(1 + 2K24P). [PhOHOPh − ]= K24[PhOH][PhO − ]

(A17)

Fits between experimental and calculated data are reported in Figs. 8 and 9, for the rate-constant values reported in the text. In these figures are shown also, for a comparison purposes, the theoretical working curve obtained in the absence of the reaction in Eq. (23).

References [1] S. Wawzonek, H.A. Laitinen, J. Am. Chem. Soc. 64 (1942) 1765, 2365. [2] G.J. Hoijtink, J. van Schooten, Rec. Trav. Chim. Pays Bas 71 (1952) 1089. [3] H. Kiesele, J. Heinze, in: M.M. Baizer, H. Lund (Eds.), Organic Electrochemistry, Ch. 7, 3rd edn., Marcel Dekker, New York, 1991. [4] G. Farnia, F. Maran, G. Sandona`, M.G. Severin, J. Chem. Soc. Perkin Trans. 2 (1982) 1153. [5] G. Farnia, F. Marcuzzi, G. Melloni, G. Sandona`, J. Am. Chem. Soc. 106 (1984) 6503. [6] G. Farnia, G. Sandona`, F. Marcuzzi, G. Melloni, J. Electroanal. Chem. 264 (1989) 297. [7] G. Farnia, J. Ludvı´k, G. Sandona`, M.G. Severin, J. Chem. Soc. Perkin Trans. 2 (1991) 1249. [8] G. Farnia, G. Sandona`, F. Marcuzzi, G. Melloni, J. Chem. Soc. Perkin Trans. 2 (1988) 247. [9] G. Farnia, G. Sandona`, F. Marcuzzi, in: J.C. Alexander (Ed.), Current Topics in Electrochemistry, Research Trends, vol. 3,

.

175

Trivandrum, India, 1994, pp. 441 – 457. [10] G. Capobianco, G. Farnia, G. Sandona`, F. Marcuzzi, G. Melloni, J. Electroanal. Chem. 134 (1982) 363. [11] A. Dal Moro, G. Farnia, F. Marcuzzi, G. Melloni, Nouv. J. Chim. 4 (1980) 3. [12] G. Farnia, F. Marcuzzi, G. Melloni, G. Sandona`, M.V. Zucca, J. Am. Chem. Soc. 111 (1989) 918. [13] C. Amatore, G. Capobianco, G. Farnia, G. Sandona`, J.M. Save´ant, M.G. Severin, E. Vianello, J. Am. Chem. Soc. 107 (1985) 1815. [14] M.L. Vincent, D.G. Peters, J. Electroanal. Chem. 328 (1992) 63. [15] F.G. Bordwell, personal communication, 1998. [16] F.G. Bordwell, Acc. Chem. Res. 21 (1988) 456. [17] F.G. Bordwell, G.E. Drucker, J. Org. Chem. 45 (1980) 3325. [18] G. Farnia, G. Sandona`, F. Marcuzzi, J. Electroanal. Chem. 348 (1993) 339. [19] G. Farnia, B. Lunelli, F. Marcuzzi, G. Sandona`, J. Electroanal. Chem. 404 (1996) 261. [20] A.J. Bard, L.R. Faulkner, Electrochemical Methods, Ch. 2, Wiley, New York, 1980. [21] A.J. Bard, L.R. Faulkner, Electrochemical Methods, Ch. 3, Wiley, New York, 1980. [22] H. Kojima, A.J. Bard, J. Am. Chem. Soc. 97 (1975) 6317. [23] L. Nadjo, J.M. Save´ant, J. Electroanal. Chem. 48 (1971) 113. [24] K. Izutzu, Acid – Base Dissociation Constants in Dipolar Aprotic Solvents, Chemical Data Series No. 35, IUPAC, Blackwell, Oxford, 1990. [25] A.J. Bard, L.R. Faulkner, Electrochemical Methods, Ch. 6, Wiley, New York, 1980. [26] J. Heinze, M. Sto¨rzbach, J. Mortensen, J. Electroanal. Chem. 165 (1984) 61. [27] D. Britz, Digital Simulation in Electrochemistry, Springer-Verlag, Berlin, 1988. [28] S.W. Feldberg, J. Electroanal. Chem. 127 (1981) 1.

.