Electrochemical Reduction of Benzyl Bromide in the Presence of Carbon Dioxide

Indi an Journal or Chemi stry Vol. -l2A. April 2003. pp. 75 1-7 57

Electrochemical reduction of benzyl bromide in the presence of carbon dioxide

Abdiri sak A (sse & Armando Gennaro* Department or Ph ys ical Chemi stry. Uni versity of Padova. via Loredan 2. 35 13 1 Padova. Ital y E-mail: A .Gen naro @c hfi .unipd.it Receil'ed 5 Decelll ber 2002

The ele..:trochemical reduction of benzy l bromide has bee n in ves ti gated in acetonitrile and CO2-sa turated acetonitrile by cyclic vo ltammetry and controlled-potential elec tro lysis. Electroreduction of the halide in th e abse nce or CO2 lead s to a va riety of products. th e distribution of whi ch d epe nd ~ on the elec trode material. applied potcntial and proton ava ilability in the med ium. The elec trocarboxylation proresse, ha ve bee n carried out at Hg and graphite ca th odes using bo th a two­ co mpartment ce ll and an undi vided ce ll w ith di ssolving AI anode. The best results (87 % phenylaceti c y ield) are obtained at Hg in th e undivided ce ll. Ca talysis by Co(salen ) allows th e process to be performed at potentials more positive than th ose required by direc t reducti on but gives onl y poor to moderate yields of acid. Res ults obtained rrom experiments carried out with ben zy l chloride have been compared with th ose obtained in the case of benzy l bromide.

The electrochemical fixation of CO2 into organic out at Hg and graphite electrodes using two different substrates is a convenient method of sy nthes is of cell arrangements: a two-compartment ce ll with a Pt carboxyl ic ac ids I. In particular, non-steroidal anode or an undi vided ce ll with a dissolving AI anriinflammatory agents such as 2-arylpropanoic anode. Both direct and indirect (mediated ) ac ids may be eas ily prepared by electrocarboxylation electrored uction of the halide in CO2-saturated of suitable benzy l halides. Indeed . this process has CH,CN were examined. As a catalys t a cobalt been the subject of several investigations with complex (Co(salen), sa len = lI2,2'- 11 ,2 -ethanediy lbi s­ 2 particu lar inleres t to th e optimisation of product (nitri lomethy lidyne)]bis[phenolatoIJ - ) , which has y i e ld s 2 . ~ . To this end, chlorides ha ve been largely been previously reported to have good catal ytic preferred to bromides or iod ides as starting materials effects towards the red uction of benzy l chlorides, was 6 for elec trocarboxylat ion. However. a drawback to the used . use of benzyl ch lorides is that their reduction at the most common ly used cathodes occurs at very highly Materials and Methods negative potential s, where concomitant reduction of Acetonitrile (BOH ) was distilled over Ca H2 and

CO2 may tak e place, res ulting in undesired products stored under argon atmosphere. Tetra-I/­ and a decrease of current efficiency. To circumvent butylammonium perchlorate (Fluka) was crystallized this difficulty. various cata lytic systems, mainly based twice from EtOH + H20 (2: I ) and dried in a vacuum 5 8 on transition metal complexes. have been used - . oven at 60°C. Co(salen)" and benzyl phenylacetate [t is widely known that alkyl bromides and iod ides (PhCH2C02CH2Ph )1 2 were prepared according to reduce much eas ter than th eir corresponding procedures described in th e literature. Carbon dioxide CIIl one. Ies '1·111 . For example, at a glassy carbon (99 .998%) was supplied hy Sl AO (Italy). A ll other elec trode in acetonitrile, th e reduction potential of reage nts were commercially available reagents and ben zy l bromide is 0.5 V more positive than that of we re used as received. benzy l chloride Ill. The potentials required for th e Electrochemical measurements were carri ed ou t by red uction of benzy l bromides are considerably less using an EG&G PARC (Model 173/179) negati ve than the red uction potential of CO2. Hence, potentios tatlcoulometer coupled with a uni versa l use of benzyl bromides in electrochemical programmer (M odel 175) and a LeCroy L T 322 carboxylations ;nay have th e advantage of avoiding osc illoscope. For cyclic voltaJl1metry meas ureme nts. concomitant reduction of CO2. glassy carbon or mercury was used as the working In thi s paper we desc ribe the res ults of an electrode. The counter and the reference elec trodes inves ti gation on th e electrochemical carboxylation of were a Pt wire and Ag/Agl/O. I M ( I/- C ~ H ~)" I in benzyl bromide in ace tonitrile. The study W;lS carried OMF, respec ti ve ly. A t th e end of th e ex periment. th e 752 INDIAN J Cf-IEM. SEC A, APR IL 2003

potential of th e reFerence elec trode was always o . measu red versus the saturated calomel electrode (SCE), to which all potential s are finally referred. Con trolled-potenti al electrol yses were ca rried out e either at a Hg pool Os cm ) or at a compact graphite rod (6 cm\ using two different cells: a two­ -5 0 compartment ce ll with a Pt an ode separated from th e cathodic compartment by glass Frits and Tylose-(Il­ C ~ H ( » 4 NCl0 4 -sa turat ed bridge or an undivided cell with a sac rificial AI an ode. All ex periments were - J 00 performed at 25 °C. ~--~------~------~--~ -2.0 -1.5 , 1.0 The electrolys is products were analysed by using ail HPLC Perkin-Elmer Series 4 liquid E (V vs SCE) ch romatograph. equipped with a UV detector and a reve rsed- phase LC 18- DB Supelco column. The eluent Fi g. I- Cyclic vnltam metry o j" 1.93 mM be llzy l bromide ill C H,CN + 0.1 M (II-C4 H

~ 2, 50 Results and Discussion

1.::teClmcl,elllical redllC lioll of Ph CH]Br Cyclic vo ltamlTletry ex periments were carried out in CH,CN + 0 .1 M ( Il -C 4 H () ~ NClO ~ . using mercury 100 and glassy ca rbon electrodes (GC). Figures I and 2 -2.0 -1. 5 -1. 0 -0.5 show cyclic vo ltammetric curves for the reduction of ben zyl brom ide at GC and Hg, res pectively. On both E (V vs SCE) electrodes. th e compound ex hibits a single irreversible and broad peak. The va lu es of th e peak potentials (Ep) Fig. 2- Cyclic voltam metry of 2.24 mM benzyl bromide in CH ICN + M (II -C4Hq)4NCI04 at a Hg electrode ill th e (- ) measured at a sean rate (II) of 0.2 V S- I are - 1. 82 V O.t ab ~c ll c e and (- - -) prese nce 01'0.28 M CO2. I' = 0.2 V S- I. and - 1.5 1 V \IS SCE at GC and Hg. respectively. Ep va ri es linearl y with the logarithm of II and th e slopes, The vo ltam metric beha viour of Ph CH2 Br IS aE/ alogl', obtai ned at GC and Hg are - 124 strongly affected by COc or by proton donors such as m V /decade and - 183 m V /decade, respec ti vely . The CH,C02H. For example, when CO2 is bubbled into a tran sfer coefficients (a) ca lculated from th e above solution containing Ph C H2Br. a con ~ i de r a bl e increase slopes accordi ng to the equation IJ aEI/dlog v = of th e reducti on peak of the bromide is observed - 1.1 5RT/aF are 0.24 and 0. 16 at GC and Hg, (Fig. 2). Addition of an ac id brings about a similar respectively. a was also ca lculated from th e peak effec t. It seems that, in the absence of carbanion width. th e di fference between Ep und th e potential at scavengers, reducti on of the halide is not truly a 2e­ half peak (Ep/c ), according to the equation 1.1 Ep/2 - Ep = process. To understand better th e exact stoichiometry 1.857 RT/a. The averages of th e va lues obtained at of the process, a se ri es of con trolled-potential different sca n rates in the 0.2 - 20 V S- I ran ge are elec trolys is was carried out under different 0.30 and 0.28 for GC and Hg, respec ti ve ly. These data ex perimental conditions. are typica l of a red ucti on controlled by the kineti cs of The results of th e elec trolys is are summarised in th e heterogeneous electron trans fer (ET) and are in T ab le I . A first observation on the data is that, if an ag ree ment with the mec hani sm previously reported eff icient proton donor is not present in so lution , the for th e clectrored uction of Ph CH2 Br at GC, which IS charge consumption is very close to I e-/molecule of considered to be an inert electrode material 10. Ph CH2 Br (e ntries 1-4). Addition of I fill water ISSE clal.: ELECTROCARBOXYLATIO OF BENZYL BROt\lIDE 75 :'

Table I- Elcc trochemi ca l reduction or benzy l bromide in CH,CN + 0.1 M (II -C4 H9 ).:NCI0 4 . h S. No. Elec trode IPhCH 2Bri E"rp II Product zields (%)" d mM V vs SCE RI-I I-I gR2 RCH2C ROI-I ROR d Total

Hg 11 .58 - 1. :1 1 1.0 18 78 0 2 () 98 2 Hg 8.42 - 1.40 1. 2 33 20 20 2 8 X]

3 H0o 8.42 - 1.65 1.1 49 0 15 4 15 88 4 C 10.52 - 1.85 U 57 0 10 7 16 90 5 Hg 8.42,,1 - 1.40 1.0 49 15 4 .'i 15 X8 6 Hg 8.42 ,,1 - 1.65 1.1 49 () 5 5 :1 0 Xc) 7 Hg 8.42·,2 - 1.65 1.7 82 0 0 17 () 99 8 C 8.42,,2 - 1.85 1. 7 84 0 0 11 0 96

"Added wa ter: 10. 1 M. 2 1.0 M. hCharge (F/mol) co nsumed with respect to convert ed Ph CH2Br. "Yicld is ca lculated with respect to PhC H213r disappeared. dYield represent s th e percelllage or th e ori ginal Ph CH213r incorporated into th e product significa ntly increa. es the charge consumption of the toluene does not exceed 57 % wh ile significant process, which now tend s to a 2e- red uction to toluene amounts of side products are formed (e ntries 3-4). It (e ntries 7-8). Tht' electrolys is gives, ri se to a variety is noteworthy that under such conditions appreciable of products th e distribution of which strongly depend s amounts of hydrocinnamonitrile are fo rmed. When on ex perimental conditions such as cathode material , H20 is added to the reacti on medium. the yit' ld of applied potemial and proton ava ilab ility in th e toluene increases at th e detriment of the yield of the medium. When the applied potential is negative side products, reachi ng ca 84% in the presence of I M enough to ensure reducti on of th e intermed iate benzyl H20. The yield of hydrocinnamon itrile is particularly rad ica l, toluene is formed as th e principal reducti on affected by th e presence of H20 , decreasing with product. Under such circumstance, th e maj or side increasing concentration of th e latter. In th e presence products are hydrocinnamonitrile, benzyl alcohol and of I M H20 , hydrocinnamonitrile form ati on is no dibe nzy l ether. whi ch are formed b y nucleophilic longer observed implying that reac ti on ( I ) is outpaced allack of anions stemming from the protonation of by reacti on (2). This mean s th at H20 is a mu ch more PhCH :- - at th e starting benzy l bromide. In fact, th e efficient proton donor than CH,CN. Since, however, proton donor involved in reaction could be H20 th e two proton donors have comparab le pK" va lu t's l~ . (e ither resi dual or purposely added) or CH3CN, which the difference in reactivity between th e two •n ave simi" 1a r p K" va I ues t4 . compounds should be clue to a significa nt difference between the intrinsic barriers of the proton transfer PhCH 2- + CH,C . -'• PhCH, + T H2CN ... ( I ) reactions ( I ) and (2). The effect of th e appl ied potential (Eapp) on the PhC H - + H 0 - . PhC H, + OH . .. (2) 2 2 distribution of th e products has been inves ti gated at the Hg electrode. I\s shown by the data reported in Table I ·(entries 1-3), the yield of toluene increa. e. ... (4) with decreasing Eilpp . When the electrolys is was carried out at - 1.31 V liS SCE, a potential I\lso the benzyl alcohol formed in reac tion (4) may corresponding to the foot of the reduction peak of get involved in a proton transfer react ion with PhCH ~ ­ PhCH 2 Br (see Fig. 2), dibenzyl mercury was obtained as the pri nci pal reduction produc t. Shi rt ing the to give an alkoxide Ion PhC H20 - that l11ay further electrolysis potential to nlore negative va lues results react with Ph CH2 Br. in a decrea se of the yield of (PhCH:!hHg while that of Ph C H ~ increases. At - 1. 65 V vs SCE no (PhCH :!) ~ H g Ph CH 2 - + Ph C H20H - - PilC H, + PhCH 20 - ... (5) is formed. It is worth noting thaI. although radi ca l­ rad ical coupl ing of benzy l rad ica ls has a rate con, tant of the ord er of I O ~ M- I~ I(Ref. 15), bibenzyl was The di :-. tri butiOI1 of such products is strongly never observed among th e recluction pr\)(lucts The affected by th e concen tration or 11 20 in th e reaction ben zyl radica ls arc preferen tially captured by th e Hg medium. In nominal ly dry acetonitrile th e yield of electrode'. Depending on the applied potential. th e 754 INDIAN J CHEM. SEC A. APR IL 2003

benzy lmercury radi ca l so formed may either undergo of th e acid is very low, th e maj or prod uct being th e

I e- reduction to PhCH 2 - or give dibenzy lmercury es ter, which aeco ul1lS for up to 70% of the starting through disproportionati on9 halide. When the ex periments were performed in The electrolys is at th e graphite elec trode was an undi vided cell with a sac ri fi cial A I anode ca rri ed out at a po tential corres ponding to the E" of (e ntries 4-5), th e yield of the acid increased up to 68 o/c the halide as measured at GC electrode. In thi s case while formation of th e es ter became co mpl etely J th e process in volves free benzyl radicals, which are suppressed. The A I + cati ons formed at th e anode immed iately redu ced at th e very negative potentials stabi I ise th e ca rboxy late ion. mak i ng reac tion (8) too required for th e reduction of th e starting halide. Thus, slow to occur in the time scal e of th e experiment. only trace amounts of bibenzyl were obse rved in th e The res ults of so me elec trocarhoxylation ex periment s carried out at th e graphite elec trode. It ex perimen ts on benlyl ch lori de per formecl at a Hg was also noti ced that th e nature of the ca th ode cathode in CO2-sa tu rated C H.lC are inc I uded in material ha s no significant effect on th e se lec ti vity of Table 2 (entries 3, 6). The red uction potential of th e process so long as an £ va lue negati ve enough to Ph CH2Ci at th e Hg electrode is very nega ti ve ( 'ee ensure immed iate reduction of th e intermediate Fig. 3), so th e ex periments were carri ed out at a radica ls is app li ed. potential (- 2. 16 V vs SC E) corres pond ing to th e foot of the reduction wave of th e chloride. in order to

EleCfJ'Ocarboxvlafion of PltCH ]X (X = Br. Cf) minimise th e con tribution of elec troreduction of CO2. According to the data obtai ned from the A few observa tions can be made comparing the

vo ltammetric in ves ti gati on, CO2 is a good scavenger res ults of electrocarboxylation of Ph C H2Ci with th ose of benzy l ca rbani ons. In fac t. as shown in Fi g. 2. of the same process, carried ou t under similar bubbling CO2 into a so lution of PhCH 2Br ca uses a conditions, for PhCH 2Br. Firstly, in both types of remarkable enhancement of th e peak current for th e electrochemica l ce ll, electrocarboxy lation or the red ucti on of the halide. In th e prese nce of CO2. th e chloride gi ves better chemica l yields of phenylaceti c benzy l carbani ons are rapidly trapped by CO2 (Eq. 7) acid. Secondly, onl y a sma ll quant ity of es ter is and the overall process tends to become a 2e­ formed when Ph C H2Ci is used, indica ting that the SN2 red ucti on of the halide. Under such ci rcumstances, reaction (Eq. 8) on the chl oride is quite slow. ph eny l ace tate is ex pec ted to be th e principal Red ucti on of PhCH 2Ci , however, requires very reducti on product. nega tive potential, which also in vo lves direct reduction of CO2 at th e elec trode (Fig. 3). To ... (7) minimise th e in vo lve ment of 5uch undes irable reac ti on, an applied poten tial as much posi ti ve as The res ults or preparati ve-sca le electrolysis of benzyl poss ible was se lected for th e elec trolys is. Nevertheless, th e charge consumption (see Table 2. bromide in CO2-saturated C H :1 C are reported in Table 2. The elec tro lys is were performed both in entries 3,6) is considerabl y greate r tha ll th e theoretical divid ed and undi vided ce lls using both Hg and va lue or 2e-/molec ul e of PhCH2Cl req uired for th e graphite (C) cath odes. T he main red ucti on products carboxy lati on or th e halide. we re ph enylacetic acid (RC02 H) and toluene, bibenzyl being either abse nt or detec tabl e only at Co(salen )-colOlysed eleCl rocor!Joxv!of ion trace leve ls. M os t of th e side products observed in th e The ca talytic effect of Co(sal en) 0 11 th e ex periments performed in th e absence of CO2 were elec trocarboxylati on process has also been examined. also abse nt : only small quantities of benzy l alcohol Fi gure 4 shows cyclic vo ltammogram s for th e could be obse rved. Bes ides pheny lace ti c ac id and reduction of the complex in th e prese nce of be Illy I toluene, ben zy l ph enyl acetate, whi ch is formed by bromide and COo. In th e absence of ·PhCHoBr. nu cleo philic attack of th e carboxylate ion on Co"(sa len) ex hibits a reversible pea k cOL~p l e Ph CH2Br (Eq. 8), was obtained when th e electrolys is co rres ponding to the red uction of CoO l ) to Co(l ) .\l ith was perform ed in a two-compartment cell (Table 2, C" = - 1.30 V vs SCE. Addition I' be nzy l bromide entrics 1-2). In such experiments, th e yield causes an increase in th e pea k curren l or th e reduction pea k which becomes irreversible and is shifted to Ph CH2Br + PhCH 2COc- - - more posi tive potentials, and a new redu cti on peak

PhCH2C02CH2 Ph + Br- .. . (8) appears at more negative potentials (Fig. 4b). The ISSE el al.: ELECTROCARBOX YLATION OF BE ZYL BROMIDE 755

Table 2- Elec trochelllical carboxylati on of be nzy l halides in CH,C + 0. 1 M (II -C.j H

S. o. Electrode Ce ll" RX fR X I E"pp Product yields (* )" mM V vs SCE

I Hg A PhCH , Br 10.51 - 1.65 I A 26 9 .+.+ 79 2 C A PhCH, Br 10.5 1 - 1. 85 1. 5 13 14 70 <)7 3 Hg A PhCH ,CI 10.86 - 2. 16 2.6 7 1 22 5 <)R 4 Hg i3 PhCH , Br 10.5 1 - 1.65 2A 62 22 0 R.+ 5 C B Ph CH, Br 10.5 1 - 1.85 2.2 68 16 () X'+ 6 Hg B PhCH ,CI 10.86 -2. 16 SA 87 10 () <) 7 7 C A Ph CH, i3r" 10.5 1 - l AO 1. 5 8 19 44 7 1 8 C A Ph CH,CI " 10.86 - 1.62 2. 1 36 37 I 7.+ 9 C i3 Ph CH, Br" I 0.51 - l AO 2.0 47 25 () 72 lac B Ph CHoCI " 10.86 - 1. 62 2A 49 32 0 X I "The ce ll used fo r elec tro lys is was either a t;o compartment ce ll (A) or an undi vi ded ce ll with an alullliniulll sacrificial anode ( 13 ). hl n the presence of co I mM fCo(sa len) I. "Charge (F/mol) consumed with respec t to co nverted PhCH , X. dYield is ca lcu lated with respect 10 convert ed Ph CH, X. cYield represents the percentage of th e ori ginal PhCH , X incorporated into the product.

o 20,------,

Or- -50 ~ - -20 r- / / ... -< -100 , ' 2- I I -40 r- I I -1 50 b ,/ ," -60 r- I/r- - V -2.5 -2.0 -1.5 ~ ~ Ot- ~ E (V vs SCE) ~. b " - 100 r- " Fig. 3- Cyc li c vo lt ::lInmetry of benzyl chl oride in CH,CN + 0. 1 M " (II-C.j I-l

Fig. 4- Cyclic vo itammogra ills at a glassy ca rbon electrode of rCo11(salen)] + e- [Co1(salen) r ... (9) I Co(sa len) in CH3C + 0. 1 M (II -C4 H<».j CI0 4 at v = 0.2 Vs- : (a) 0.82 mM Co(salen) (b) as (a) + 8A I mM PhCH , i3r and (c) as (b) [Co1(sa len)r + Ph CH2 Br - ~ in th e presence of 0.28 M CO, . lll I Ph CH2Co (sa len)1 + Br- ... ( IOj more negative th an that of Co(salen). The peak lll [Ph CH2Co (sa len)] + e- -~ potential meas ured for the reduction of ll lll l [PhCH2Co (salen)r ... ( I I) PhCH2Co (sa len) is -1.45 V vs SCE at v = 0.2 V s· . Reduction of th e newly formed organometallic co mplex (Eq. I I ) yields a very unstabl e cobalt( lI) spec ies, which rapidly undergoes homolytic Co-C

The elec trogenerated cobalt(l) complex reacts with bond cleavage to give Ph CH2• and [Col(salen)r Ph CH2Br according to an SN2 mechanism. Such a (Eq. 12). The chemistry and electrochemistry of th e reac ti on lead s to th e formation of an orga nocobalt benzyl rad ica l so formed are essentially th ose already compl ex (Eq. 10), which is reducible at potentials described in th e unmediated process. 756 INDIAN J CHEM. SEC A. APRIL 2003

According to th e above reaction sequence, A comparison of th e product dist ribution in th e elcc trocatal yti c reduction of Ph CH2Br ca talysed by med iated and unmediated processes shows that Co(salen) can be achieved at potentials corres ponding ca tal ys is with Co(salen) brings about a marked

to th e reduction of th e orga nocobalt complex. In fact, increase of th e RH to RC02H ratio. The add itional th e peak current observed for the latter process is amount of toluene formed in the Co(salcn)-catalysed more than twice that of th e I e- reduction of process may be attributed to th e hydro lys is of th e Co"(salen) to rCo1(sa len)r. Indeed, th e process is intermed iate organocobalt complex (Eq. 13). ca talyti c and the exchan ged number of electrons is

significa ntly greater than I . The catal yti c nature of th e [phCH2Co"(salen)r + H20 lll red uction of Ph C H2Co (s al en) becomes more PhCH, + Co"(sa len) + O H ... ( 13) apparent if th e so lution is sa turated with CO2 (see Fig. 4, curve c). Similar res ults ha ve bee n already The role played by such a reac ti on in the found for th e ca talytic reduction of benzyl chlorides elec trochemica l carboxylati on of benzy l chl orides by Co(sa len) and th e role played by CO2 in th e ca talysed by Co(salen) has bee n prev ious ly evidenced elec trocata ly ti c process has been ex plained by ex periments with D20 6. The occurrence of reaction elsew here(' . ( 13), and hence th e hi gh sensitivity of th e catal ytic T he res ults of so me preparati ve-sca le process to H20 , is ye t another point in fa vour of the electroca rboxylations of benzyl bromide ca tal ysed by unmediated process as co mpared to the Co(salen)­ Co(salen) are reported in Table 2. The experiments ca talysed one. were co nducted both in divided and undi vided cells To complete th e comparI son of th e wi th a graphite ca th ode: Hg could not be used for this elec trocarboxylati on processes of benzy l bromide purpose as direct reduction of Ph CH2 Br overlapped with th ose of the chloride. th e Co(salen)-catalysed wi th that of th e ca taly tic process. Controlled-potential electrocarboxylat ion of Ph CH2Ci was also electrol ys is, in CO2-sa turated CH,C containing co I inves ti gated. Indeed. thi s process has bee n th e subject mM Co(salen) and a 10-fold excess of PhC H2 Br, was of a prev ious study, in which modera te yiel ds of 6 carried out at - 1.4 V vs SCE. It is to be noted that this ph eny laceti c acid have bee n found . Since it has been potential is in th e ri sing portion of the reduction peak ev idenced in that study that th e yield of th e ac id of the organocobalt complex (Fi g. 4. curve b). Such a depends on th e app lied potential, the best res ults va lu e has bee n chosen for th e electrolys is in order to being ob tained at E < - 1. 60 V liS SCE. the avoid unmediated reduction of th e bromide at th e ex periments with Ph CH2Ci were perfo rm ed at - 1. 62 graphite elec trode. The applied potential, however. V I'S SC E, under oth erwise identical conditions as in ensures immediate reduction of Ph CH2• at the th e case of Ph CH:> Br. The -res ults are included in electrode since il is co mparable with th e reducti on Table 2 (e ntries 8, I 0). In good agreement with th e potential of th e radi ca l (- 1.43 VI'S SCE). The data unmediated processes, in the divided ce ll Ph CH2CI show th at bett er res ults. in terms of product yields are gives a much bett er ph enylacetic yield as co mpared to obtained by the unmediated elec trocarboxylation as th e bromide. Instead , in th e undi vided ce ll th e two compared to the Co(salen)-catalysed process. In Co(salen)-catalysed processes gi ve virtually the sa me addition, th e cobalt complex decomposes during the res ults. electrolys is; at th e end of th e elec trolys is onl y a very To sum up, ben zy l bro mide may be used as a small fraction of th e cata lyst st ill rema ined in th e starting materi al in th e electrochemica l syn th es is of act ive form . It is very likely th at th e benzyl ca rbanion ph enylaceti c acid . The process in t c undi vided ce ll is involved in th e chemical process leading to gives a sa ti sfa ctory acid yield. The chl ori de appea rs. deac tivation of the ca tal ys{" which ex plains, at least hO\A/ever, to be a better ca ndidate if th e problems in part, why th e overall yield of th e ca tal ysed related wi th th e very nega ti ve potential req uired for electrocarboxy lation is always smaller th an that of the its reduction are overcome. e.g.. by efficient corres ponding unmedi ated process . A n advantage of elec trocatal ys is. th e Co(salen)-catalysed process over th e unmediated one is that th e Fonner can be achieved at relati ve ly Acknowledgements pos itive potentials. The poss ibility of ex ploiting thi s Financial support from lhe Millislem advantage is, however, se verely limited by th e low del/'/slmzioll e, de// 'Ullilersilit c della Ricerca turnover number of th e ca tal ys t. (MIUR) is gratefully acknowledged. ISSE et al .: EL ECTROCARBOXYLATIO OF BENZYL BROMIDE 757

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