Electroreduction benzyl bromide

Tab. 6.16. Electroreduction of benzaldehyde, dimethyl maleate and benzyl bromide.

The formation of arylzinc reagents can also be accomplished by using electrochemical methods. With a sacrificial zinc anode and in the presence of nickel 2,2-bipyridyl, polyfunctional zinc reagents of type 36 can be prepared in excellent yields (Scheme 14) . An electrochemical conversion of aryl halides to arylzinc compounds can also be achieved by a cobalt catalysis in DMF/pyridine mixture . The mechanism of this reaction has been carefully studied . This method can also be applied to heterocyclic compounds such as 2- or 3-chloropyridine and 2- or 3-bromothiophenes . Zinc can also be elec-trochemically activated and a mixture of zinc metal and small amounts of zinc formed by electroreduction of zinc halides are very reactive toward a-bromoesters and allylic or benzylic bromides . ... 

The massive zinc (rod or plate) reacts spontaneously with activated bromides provided the preliminary electroreduction of a catalytic amount of zinc salt (ZnBr2 or ZnCl2) occurs. Reactions are carried out in nitrile solvents (CH3CN, PhCN,. ..) or their mixture with dichloromethane. An undivided cell fitted with a zinc anode and an indifferent cathode (gold, nickel, carbon, zinc,. ..) is used. As observed with benzylic bromides, the activation leads to an organozinc compound able to react with either the nitrile solvent or an electrophile reagent. The process is depicted in equation 12. 

In acetonitrile or CH2Cl2 acetonitrile mixture solid zinc becomes very reactive towards a-bromoesters, or allylic or benzylic bromides when being in the presence of catalytic amounts of zinc formed by electroreduction of ZnX2 (X = Br, Cl). Stable benzylzinc compounds are thus obtained from benzyl-bromides (Equation 8.12, Protocol 3).9... 

Scheme 8. Electroreductive pathways of (a) dimethylmalate and (b) benzyl bromide. (From ref. 196).

Using a similar type of the microflow cell, Haswell s group reported the electroreductive coupling of activated olefins and benzyl bromide derivatives (Equation 12.8) [40]. The microflow electrochemical reactor can be easily multiplexed to generate a number of parallel flow cells, which offer the performance of a single cell while increasing the volumetric throughput of the system. 

Electroreduction of benzyl bromide with Ni(I) complexes provides different products depending on the reduction potentials potential (V vs. Ag/Ag+ in MeCN) — 1.45 bibenzyl 76%, toluene 2% —1.80 bibenzyl 7%, toluene 90% [280]. 

Solid zinc activation has been used with success for the formation of stable organozinc compounds from functionalized benzylic bromides [8] and has been applied to the condensation of activated halides with nitriles [9] (Blaise reaction) or carbonyl compounds (10] (Reformatsky reaction). In this case, solid zinc is activated by catalytic generated zinc formed by electroreduction of anhydrous ZnBr2 solution in acetonitrile as solvent. Results and practical approaches have been reported [llj. 

Nickel-bpy and nickel-pyridine catalytic systems have been applied to numerous electroreductive reactions,202 such as synthesis of ketones by heterocoupling of acyl and benzyl halides,210,213 addition of aryl bromides to activated alkenes,212,214 synthesis of conjugated dienes, unsaturated esters, ketones, and nitriles by homo- and cross-coupling involving alkenyl halides,215 reductive polymerization of aromatic and heteroaromatic dibromides,216-221 or cleavage of the C-0 bond in allyl ethers.222... 

A copper(O) complex, electro-generated from Cu(acac)2, is able to undergo an oxidative addition with benzyl and allyl bromides. Further reduction leads to the coupling products bibenzyl and 1,5-hexadienes Methyl-3-hexene-l,6-dicarb-oxylate can be prepared from butadiene and CO by electroreduction if di-Fe dicyclopentadienyl tetracarbonyl is used as redox catalyst Electro-generated low-valent tungsten species are able to reductively dimerize benzaldehyde to stilbene according to Eq. 83. The reduction potential was controlled at the third wave of the WClg catalyst (V = -1900 mV/SCE)... 

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