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Electrochemical Reduction of Benzyl Bromide in the Presence of Carbon Dioxide

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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 <j )4NCl04 al a glassy carbo ll elec trode at I ' was a mixture of CH,CN and H20 , ac idified with I I = 0.2 Vs- . CH, C02H (5 g L- ). At th e end of th e elec trolys is a samp le of so lution was withdrawn from the o elec troc hemi ca l ce ll and direc tl y ana lysed by HPLC. Authentic compounds were used for th e identification and quantl!'ica ti on of the products. ~ 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.
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