Free radical reactions to generate alkenes and/or ionic reactions to generate hydroximoyl chlorides when β-nitrostyrenes react with triethylaluminium or diethylaluminium chloride

Abstract

β-Nitrostyrenes 1 react with triethylaluminium or diethylaluminium chloride in diethyl ether solution and under nitrogen or argon to generate the alkenes 2 and the hydroximoyl chlorides 3 after work-up with ice-cold, conc. hydrochloric acid. The formation of the alkenes 2 is proposed to be a free-radical reaction via NO₂/alkyl substitution since the yields of the alkenes 2 are increased in the presence of benzoyl peroxide (Bz₂O₂) and decreased in the presence of galvinoxyl. Only the alkenes 2 are produced with a high stereoselectivity for the E isomers when β-nitrostyrenes react with triethylaluminium in the presence of one to two equivalents of Bz₂O₂ as free-radical initiator. The mechanism of the generation of the hydroximoyl chlorides 3 is proposed to proceed through a 1,4-addition pathway to produce nitronates A, then the protonated nitronates B or the nitroso cations C are trapped by chloride ion to form the final products. The yields of compounds 3 are also improved by the presence of Lewis acids such as MgCl₂. Medium to high yields of the hydroximoyl chlorides 3 and traces or low yields of the alkenes 2 are generated when triethylaluminium or diethylaluminium chloride react with β-nitrostyrenes in the presence of three equivalents of MgCl₂ under argon.