Electroreductive acylation

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... 

The acyl radical equivalents formed by the electroreduction of acyl phosphonium salts have an interesting reactivity and the electroreduction of nonconjugated olefinic acids yields intramolecular coupling products (Scheme 28) [11, 54, 55]. 

Electroreduction of cyclohexanecarbonyl chloride (68) at a mercury cathode in acetonitrile leads to production of cyclohexanecarboxaldehyde (69) and 1,2-dicyclo-hexylethene-1,2-diol dicyclohexane carboxylate (70) as reduction products, reminiscent of phenylacetyl chloride ( equation 39). The isolation of small amounts of dicyclohexyl ketone (71) and dicyclohexylethanedione (72) strongly suggests that cyclohexanecarboxaldehyde is formed exclusively via an acyl radical intermediate65(equation 44). 

There have been a few reports describing electroreduction of acyl halides. Benzoyl chloride when reduced (Ep = —1.82 V versus Ag/O.IM AgN03) in aprotic media gives good yields (85%) of stilbenediol dibenzoate [111,112]. This seems to be a general reaction for a variety of acyl chlorides [113]. The reduction of cinnamoyl chloride leads to 2,4-dibenzyl-1,3-cyclobutadione [114] ... 

An interesting coupling of acyl halide with Michael acceptors involves an organo-metallic intermediate and photochemical step as well as an electroreduction. The electrochemical reduction, however, is used to reduce the intermediate cobalt oxidation state [115]. 

The most systematically investigated acyl anion equivalents have been the IMS ethers of aromatic and heteroaromatic aldehyde cyanohydrins, TBDMS-protected cyanohydrins, - benzoyl-protected cyanohydrins, alkoxycaibonyl-protected cyanohydrins, THP-protected cyanohydrins, ethoxyethyl-protect cyanohydrins, a-(dialkylamino)nitriles, cyanophosphates, diethyl l-(trimethylsiloxy)-phenyimethyl phosphonate and dithioacetals. Deprotonation di these masked acyl anions under the action of strong basie, usually LDA, followed by treatment with a wide varies of electrophiles is of great synthetic value. If the electrophUe is another aldehyde, a-hydroxy ketones or benzoins are formed. More recently, the acyl caibanion equivalents formed by electroreduction of oxazolium salts were found to be useful for the formation of ketones, aldehydes or a-hydroxy ketones (Scheme 4). a-Methoxyvinyl-lithium also can act as an acyl anion equivalent and can be used for the formation of a-hydroxy ketones, a-diketones, ketones, y-diketones and silyl ketones. - - ... 

Electroreduction of pyridazines in the presence of acetic anhydride gives the acylated open-chain diamines (cf. 117 -> 222). In aqueous solution a 1,2-dihydropyridazine is formed, which decomposes to nitrogen and an unsaturated hydrazino-aldehyde that polymerizes. Allylic bromi-nation of l,2-dicarbomethoxy-l,2,3,6-tetrahydropyridazine followed by de-hydrobromination gives l,2-dicarbomethoxy-l,2-dihydropyridazine. Catalytic hydrogenation gives the hexahydro compound." ... 

Electroreduction of pyridazines in the presence of acetic anhydride gives the acylated open-chain diamines.132... 

Electroreduction of styrenes or alkyl methacrylates in the presence of aliphatic acid anhydrides or N-acylimidazoles with zinc anode brought about novel one-pot vicinal double C-acylation to afford the corresponding 1,4-diketones in good to high yields. This is an example of cross-coupling reaction with no use of halogenated compounds. 

Yamamoto Y, Goda S, Maekawa H, Nishiguchi I (2003) Novel one-pot vicinal double C-acylation of styrenes and methacrylates by electroreduction. Org Lett 15 2755-2758. doi 10.1021/ol035020v... 

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