Catalytic Reduction of Phenyl-Conjugated Acetylenic Halides by Nickel(I)Salen: Cyclization versus Coupling

摘要

Cyclic voltammetry and controlled-potential electrolysis were employed to study the catalytic reduction of five phenyl-conjugated haloalkynes by nickel(I)salen electro-generated at carbon cathodes in dimethylformamide containing tetramethylamnionium tetrafluoroborate.Electrocatalytic reduction of 7-bromo-and 7-iodo-1-phenyl-1-heptyne affords the carbocyclic product,benzyhdenecyclohexane,in up to 41% yield,whereas under similar conditions reduction of 5-halo-1-phenyl-1-pentyne and 8-bromo-1-phenyl-1-octyne gives benzylidenecyclobutane and benzylidenecyclohep-tane,respectively,in very low yield(<=1 %).Dimers,alkynes,and alkenynes are other products formed from the phenyl-conjugated haloalkynes.Dimers(diphenylalkadiynes)derived from 5-halo-1-phenyl-1-pentyne and 8-bromo-1-phenyl-1-octyne are obtained in yields ranging from 85 to 93%,whereas 1,14-diphenyltetradeca-1,13-diyne(the dimer produced from 7-halo-1-phenyl-1-heptyne)is found in yields of 45-51%.To account for the formation of the various products,a mechanistic scheme that involves phenyl-conjugated alkynyl radicals arising from nickel(I)salen catalyzed cleavage of the carbon-halogen bond of each substrate was formulated.