Reaction of phenyl-substituted allyl-lithiums with secondary alkyl halides. A polar process versus single-electron transfer

Abstract

The reaction of 1-phenylallyl-lithium (1a) with optically active 2-halobutanes in ether in the presence of tetramethylethylenediamine or hexamethylphosphoramide gives exclusively 4-methyl-3-phenylhex-1-ene (5a)(coupling at the phenyl-substituted site) with essentially 100% inversion of configuration. In contrast, treatment of 1,1-diphenylallyl-lithium (1b) with (–)-2-halobutanes under the same conditions results in the formation of a mixture of 4-methyl-3,3-diphenylhex-1-ene (5b)(coupling at C-1) and 4-methyl-1,1-diphenylhex-1-ene (6b)(coupling at C-3). Moreover, C–C bond formation at the 1-position to provide (5b) is also found to proceed with complete inversion of configuration, while a small but significant loss of stereochemical integrity is observed in the case of the C-3 attack product (6b). These results suggest that a polar pathway should predominate for the formation of the C-1 attack products (5a, b), while competition between polar and single-electron-transfer processes occurs for the formation of the C-3 attack product (6b).