Mechanism of the electrochemical reduction of benzyl chlorides catalysed by Co(salen)

JOURNAL OF

ELSEVIER

Journal of ElectroanalyticalChemistry 444 (1998) 241 245

Mechanism of the electrochemical reduction of benzyl chlorides catalysed by Co(salen) Abdirisak Ahmed Isse, Armando Gennaro, Elio Vianello

*

Dipartimento di Chimica Fisica, Universitd di Padova, via Loredan 2, 35131, Padova, Italy

Received 14 August 1997; receivedin revised form 23 October 1997

Abstract

The electrochemical reduction of benzyl and 4-(trifluoromethyl)benzyl chlorides catalysed by Co(salen) (H2salen, N,N'-bis(salicylidene)-ethane-l,2-diamine) was studied in acetonitrile. Electrogenerated (Co~(salen)) reacts with the halide to give an organocobalt(III) complex. Further one-electron reduction of the latter yields an unstable intermediate that undergoes rapid decomposition by cleavage of the Co-C bond. The mechanism of bond breaking in the one-electron-reduced organocobalt(II) complex was investigated. The results of preparative-scale electrolysis on solutions containing Co(salen) and benzyl chloride, performed under different experimental conditions, in particular in the presence of radical or carbanion scavengers, indicate homolytic cleavage of the Co-C bond. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Benzyl chlorides; Electrocatalytic reduction; Homolytic bond breaking

1. Introduction

Square-planar cobalt Schiff base complexes, CoHL, have been extensively investigated as potential catalysts for the reduction of organic halides [1-5]. Interaction between cobalt(I) and the halide yields an organocobalt(III) complex. Most frequently the latter is obtained by electrochemical (or chemical) reduction of the cobalt(II) chelate in the presence of the halide [2,3,6-8]. Further reduction of alkylcobalt(III) complexes yields the unstable (ConL(R))- species that decomposes by cleavage of the C o - C bond. Several decomposition pathways have been suggested [7,9-18]. In the case of (Con(salen)(R))- (H2salen, N,N'-bis(salicylidene)-ethane-l,2-diamine), heterolytic bond cleavage yielding R and CoU(salen) has been reported for R=C6H5 and perfluoroalkyl [12-14], whereas for * Corresponding author. Fax: + 39 49 8275135. 0022-0728/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PI1 S0022-0728(97)00572-X

R=CH3 and C2H5 decomposition yields R" and Co~(salen) - [13,14]. Some other one-electron-reduced organocobalt complexes have also been reported to cleave homolytically to R" and (CoI(L)) - [7,8,16 18]. it has been reasonably argued that the preferred bond breaking mode of (CoHL(R)) - depends mainly on the electron-donor properties of the R moiety [13]. Organocobalt complexes bearing good electron-withdrawing groups give carbanions upon decomposition whereas the radical mode is preferred with electron-donating groups. We have recently reported on the electrochemical carboxylation of arylmethyl chlorides catalysed by Co(salen) [11], showing how the mode of C o - C bond breaking in the intermediate (Co~L(R)) - may affect both the reaction mechanism and the product distribution. In this paper we report further experimental evidence as to the decomposition mechanism of benzylcobalt(lI) complexes in the catalytic reduction of benzyl chlorides.

A.A. lsse et al./Journal of Electroanalytical Chemistry 444 (1998) 241 245

242

2. Experimental 2. I. Chemicals

Acetonitrile (BDH) was distilled over CaH:~ and stored under an argon atmosphere. Tetra-n-butylammonium perchlorate (Fluka) was recrystallized twice from ethanol + water (2:1) and dried in a vacuum oven at 60°C. The complex Co(salen) was prepared as described in the literature [19]. All other products were commercially available reagents and were used as received. 2.2. Instrumentation

Electrochemical measurements were made with an E G & G Princeton Applied Research model 173 potentiostat/galvanostat equipped with a programmable function generator Amel model 568 and a 2090 digital Nicolet oscilloscope. An Amel model X - Y recorder was used for recording the cyclic voltammograms. A Hg micro-electrode made from a 2 m m diameter Pt sphere coated with Hg after electrolytic deposition of silver was used as the working electrode for cyclic voltammetric experiments. A Hg pool electrode was used for bulk electrolyses. The reference electrode was AglAgI]0.1 M n - B u 4 N I + D M F whose potential was always measured versus an aqueous calomel electrode (SCE) to which all potentials are finally referred. A platinum wire was used as a counter electrode. All experiments were performed at 25°C. The electrolysis products were analysed by using either an H P L C Perkin Elmer Series 4 liquid chromatograph equipped with a UV detector and a reversed-phase ODS2 column eluted with CH3CN + H 2 0 or a Varian 3700 gas chromatograph with a Supelco column G P 10% SP-1200 + 1% H3PO4.

where L stands for the ligand salen. Rate constants of the order of 105 M - ~ s 1 were determined for the reaction in Eq. (2) [11]. The newly formed peak is due to the one-electron reduction of the organocobalt complex (CoraL(R)) formed in the reaction in Eq. (2). (CoraL(R)) + e - ~- (ConE(R)) -

The peak potentials measured for this process at v = 0.2 V s - 1 in solutions containing CoU(salen) and RC1 in a molar ratio 1:10 are - 1.45 and - 1.38 V versus SCE for benzyl and 4-(trifluoromethyl)benzyl, respectively. The species stemming from the above electron transfer is very unstable, undergoing rapid homolytic (Eq. (4)) or heterolytic (Eq. (5)) Co C bond cleavage: (Co"L(R)) - -* R" +

(COIL) -

(4)

(Co"L(R))-- ~ R - + (Co"L)

(5)

The nature of the chemical reactions triggered by the C o - C bond dissociation is expected to be different in the two cases since they involve radical or carbanion intermediates. In the case of heterolytic bond breaking the resulting carbanion is likely to undergo proton transfer reaction with any proton-donor D H present in the reaction medium.

R

+DH~RH+D-

0

(CoUL) + e - ~---(CoiL)

(1)

(COIL)

(2)

+ RC1--* (CoraL(R)) + C1

B

a -10

m

-20

Cyclic voltammetry of Con(salen) in CH3CN + 0.1 M n-Bu4NCIO4 shows a reversible peak couple corresponding to the reduction of Co(II) to Co(I) with E ° = - 1 . 2 9 V versus SCE. Addition of arylmethyl chlorides causes an increase in the peak current of the reduction peak which becomes irreversible and is shifted to less negative potentials, while a new reduction peak appears at more negative potentials (Fig. 1). The mechanism of electrochemical reduction of the complex in the presence of RC1 has been previously described [11]. The process underlying the first reduction peak is

(6)

10

< 3. Results and discussion

(3)

b

-30

-40

-50

m

i

i

I

I

-2

-1.5

-1

-0.5

E / V vs SCE Fig. 1. Cyclic v o l t a m m o g r a m s of 1.32 m M C o " ( s a l e n ) in C H 3 C N + 0.1 M n-Bu4NCIO 4 at a m e r c u r y electrode (a) in the absence and (b) in the presence of 10.86 m M benzyl chloride, v = 0.2 V s - 1.

A.A. Isse et al./ Journal of Electroanalytical Chemistry 444 (1998) 241 245

243

Table 1 Electroreduction products of benzyl chloride catalyzed by Co(salen)

1 2 3 4 5 6 7

[RCI]/mM

[CoL]/mM

15.43 10.86 22.17 21.67 22.17 8.69 26.7

1.48 1.32 1.35 2.023 1A6

Scavengerb

[Scavenger]/mM

DPP FI

202 201

CO2

280

E~/V

RH/%

R2Hg/%

RR/%

- 1.37 - 1.40 - 1.65 - 1.40 - 1.40 -2.30 2.10

17.4 18.6 30.0 47.6 24.5 90.6 28.8 ~

22.4 17.1 8.7 n.d. a 15.3 n.d. n.d.

2.3 1.6 1.1 n.d. 0.6 n.d. n.d.

a In CH3CN+0.1 M n-Bu4NCIO4 at the Hg electrode; the yield is calculated with respect to RCI disappeared. b DPP, diphenylphosphine; FI, fluorene. c Electrolysis potential (vs. SCE). a n.d., Not detected. e 60.9% of phenylacetic acetic acid was also obtained.

Homolytic bond breaking, on the other hand, yields a radical which may undergo different reactions such as hydrogen atom abstraction from the solvent (Eq. (7)), radical-radical coupling: to give a dimer (Eq. (8)) and electron transfer from the electrode or from the reduced complexes (COIL)- and (ConL(R))- in solution to give the carbanion R-- (Eq. (9)). R" + SH ~ R H + S"

(7)

2R" --*R R

(8)

R'+e

(9)

--,R

The electrocatalytic reduction of benzyl chloride was investigated under different experimental conditions, especially in the presence of trapping agents. Preparative-scale electrolyses of CoII(salen) solutions containing excess RCI were carried out at an Hg electrode at potentials where reduction of the benzylcobalt(III) complex could take place. In order to minimize the contribution of the reaction in Eq. (9) the electrolysis potentials were usually set at values less negative than the standard reduction potential of the benzyl radical (ER./R_ = --1.43V versus SCE [20]). The results are summarised in Table 1. Most often complete deactivation of the catalyst occurred after a few catalytic cycles. The total yield of the reduction products is quite low probably because some reduction intermediate of the halide is involved in the process leading to the deactivation of the catalyst. Only the yields of the reduction products, namely toluene, bibenzyl and dibenzylmercury are reported, whereas other products such as benzyl alcohol (ROH), dibenzyl ether and hydrocinnamonitrile stemming from nucleophilic substitution of anions on RC1, although quite often observed in small yields, were omitted since they are not relevant to the discussion on the mechanism of C o - C bond dissociation. The yields reported are, however, corrected for the quantity of RC! converted to the above products when these were observed.

A survey of the data reported in Table 1 indicates intermediacy of a radical species and hence of a homolytic bond breaking of ( C o n L ( R ) ) - . The first relevant result is the formation of bibenzyl suggesting radical-radical coupling of R'. The yield of this product is always quite small but the fact that it is obtained in the catalytic process is highly significant. A second important result is the formation of dibenzyhnercury. It is well known that organic halides give organomercury products upon reduction at mercury electrodes [21-25]. The mechanism of such processes has been investigated extensively by various groups which conclude that the radical species ensuing from dissociative electron transfer to RX interacts strongly with Hg and evolves to give R2Hg as the final product [21-25]. R ' + H g - , RHg~d S

(10)

2RHg~as -* R2Hg + Hg

(11)

According to the radical mechanism, toluene would be formed by hydrogen atom abstraction of R" from the solvent, provided that the electrode potential is not negative enough to reduce the radical to the anion R . The effect of variation of the applied potential (Table 1, entries 1 3) is in agreement with radical intermediacy. As the potential becomes more and more negative the yield of toluene increases, since the radical is reduced to R - which is protonated, whereas formation of the radical-based products bibenzyl and dibenzylmercury decreases. Further support of the radical mechanism is provided by the effect of the addition of a good hydrogen atom donor such as diphenylphosphine (DPP). In this case the yield of toluene rises to ca. 50% while the formation of other radical based products such as dibenzylmercury or bibenzyl is completely suppressed. It is to be noted, however, that the interpretation of the result obtained with DPP is not unambiguous. As a matter of fact DPP, besides being a good hydrogen atom donor,

A.A. lsse et al./Journal oJ Electroanalytical Chemistry 444 (1998) 241-245

244

Table 2 Electrocatalytic reduction of 4-(trifluoromethyl)benzyl chloride ~

1 2 3

[RCII/mM

[CoL]/mM

Scavenger b

[Scavenger]/mM

E~/V

na

RH/%

10.14 10.14 10.14

1.26 1.10 1.35

DPP F1

200 200

- 1.30 - 1.30 - 1.30

t.9 1.2 1.8

45.8 9I .0 64.0

In CH~CN+0.1 M n-Bu4NClO 4 at the Hg electrode; the yield is calculated with respect to RCI disappeared. b FI, fluorene, DPP, diphenylphosphine. Electrolysis potential (vs. SCE). d Charge consumed (e /molecule of RH formed).

is also a proton donor (pK, = 23.1 in DMSO [26]), being better than H20 (pK~ = 31.2 in DMSO [271]) and CH3CN (pK~=31.3 in DMSO [27]). Therefore the result obtained with DPP is not in disagreement with a heterolytic C o - C bond breaking mode resulting in the formation of a carbanion as intermediate. In order to substantiate the above result better the trapping experiment was repeated using fluorene which, compared to DPP, is a poor hydrogen atom donor [28] but has about the same strength (pKa = 22.6 in DMSO [27]) as an acid. A comparison of the results reported in entries 2 and 5 of Table 1 shows that a slight increase in the yield of toluene is obtained when fluorene is used as trapping agent. This result suggests that DPP, which, unlike F1, causes a substantial increase of the yMd of RH, mainly acts as a hydrogen atom donor rather than as an acid. The results reported so far, i.e. formation of bibenzyl, dibenzylmercury and toluene as well as the effect of the radical scavengers, are better accounted for by a radical mechanism. In particular, the presence of bibenzyl is of paramount importance in that it is considered to be indicative of radical-radical coupling. It should be noted, however, that bibenzyl could also be formed, in principle, by nucleophilic substitution of R on RCI. R

+RCI~RR+CI

(12)

In order to check whether the reaction in E q (12) effectively contributes to the production of the dimer, a few experiments were carried out on the direct electrode reduction of benzyl chloride. The results are included in Table 1 (entries 6-7). The electrolysis potential was always set at a very negative value in order to ensure reduction of RC1 to R - . This is confirmed by the result of the experiment performed in the presence of a carbanion trap '.such as CO2 where a high yield (60.9%) of phenylacetate was obtained. It is important to note that in these experiments bibenzyl was never observed among the reduction products. We thus conclude that the nucleophilic substitution in Eq. (12) is not important in the electrocatalytic reduction of benzyl chloride being overrun by the protonation reaction in Eq. (6). Let us now examine the results of the electrocatalytic reduction of 4-(trifluoromethyl)benzyl chloride, bearing

an electron-withdrawing group on the benzylic moiety (Table 2). Also with this chloride complete deactivation of the catalyst was observed after a few catalytic cycles. Only 4-(trifluoromethyl)toluene was observed as a reduction product. The dimer 4,4'-di(trifluoromethyl)bibenzyl and the dialkylmercury compound were both absent. This result may be taken to indicate the presence of carbanions as intermediate species in the electrocatalytic process, implying heterolytic cleavage of the organocobalt(II) complex (ConL(R)) - . It should be noted, however, that, whatever the prevailing Co C bond breaking mode, the predominant presence of R as intermediate is expected with this halide because, under our experimental conditions, the 4-(trifluoromethyl)benzyl radical R" would be easily reduced to R . In fact the standard potential of the R'/Rcouple was estimated to be - 1.09 V versus SCE [11]. This value is ca. 200 mV more positive than the electrolysis potential which was set at the foot of the reduction wave of the organocobalt complex. Also the homogeneous reduction of the radical R" by the complexes (COIL) and ( C o " L ( R ) ) - is thermodynamically favoured since the standard redox potential of both complexes is considerably more negative than that of the R'/R couple. In the presence of a hydrogen atom donor such as DPP both the yield of R H and the efficiency of the electrocatalytic process are greatly improved. The electrolysis comes to an end only after total conversion of the halide takes place and the yield of 4-(trifluoromethyi)toluene increases to 91%. At the end of the electrolysis the Co(salen) complex is quantitatively recovered. Coulometric measurement of the the charge consumed in the process has shown that the electrolysis required 1.2e-/molecule of RC1 for each molecule of RH formed. For the sake of comparison, the experiment was repeated, under otherwise identical conditions, in the presence of fluorene which acts mainly as a proton donor. In this case the yield of RH increases to 64% with a charge consumption of 1.8e /molecule of RC1 for each molecule of RH formed. Also with this scavenger no deactivation of the catalyst couple was observed.

A.A. lsse et al. /Journal (~f Electroanalytical Chemistry 444 (1998) 241 245

These results can be accounted for by a mechanism involving a homolytic C o - C bond breaking (Eq. (4)). The radical R" stemming from the dissociation of (Co w L(R))- partitions into two reaction channels: reduction, either at the electrode or in solution, to R and hydrogen atom abstraction to give RH. Thus in the absence of a good hydrogen atom donor the leading path for the disappearance of the radical is its reduction to the carbanion and the electronicity of the process approaches 2e -/molecule of RC1. Instead, when DPP is added, hydrogen atom abstraction predominates with a consequent lowering of the charge consumption to values close to le-/molecule of RC1. It can be said, in conclusion, that the electrocatalytic reduction of both arylmethyl chlorides investigated involves homolytic C o - C bond cleavage of the,' organocobalt(II) complex (CoHL(R)) , in spite of the presence of the good electron-withdrawing substituenl: CF3 in one of them. This is at variance with the heterolytic bond cleavage, reported to occur when a perfluoroalkyl group is directly bound to cobalt in the organometallic complex [14]. The electron-attracting capability of the CF3 group appears to be quenched within the benzylic structure of the 4-(trifluoro-. methyl)benzyl chloride.

Acknowledgements Financial support by CNR and MURST is gratefully acknowledged.

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