Radical reactions of p-nitrobenzylidene halides with aci-nitronate ions

Abstract

The radical S_RN1 and subsequent E_RC1 reactions of p-nitrobenzylidene dibromide (2b) and bromide chloride (2c) with the lithium salt (1a) and the tetrabutylammonium salt (1b) (under phase-transfer conditions) of 2-nitropropane are discussed and compared with the reactions of the dichloride (2a). The S_RN1 and E_RC1 sequence occurs in rapid succession in the reaction of (2b), in contrast to the reactions of (2a) and (2c) which give isolable amounts of the S_RN1 product, p-NO₂C₆H₄CH(Cl)-CMe₂NO₂ (3a). The monobromide p-NO₂C₆H₄CH(Br)CMe₂NO₂ (3b) cannot be detected in the S_RN1-E_RC1 sequence leading from (2b) to β,β-dimethyl-p-nitrostyrene (5). The E_RC1 reaction of the monobromide (3b) is also studied and shown to take place more slowly than the combined S_RN1-E_RC1 sequence for (2b).    

The phase-transfer technique is shown to be a convenient method for the preparation of C-alkylated derivatives like (3a), but not (3b), in both-the o- and p-nitrobenzylic systems. The dibromide p- NO₂C₆H₄CH(Br)CMe₂Br (7) was shown to undergo competitive E_RC1 and E2(H) reactions with (1a) to give the styrene (5) and p-NO₂C₆H₄C(Br)=CMe₂ (8) respectively. Iodide ion reacts with (7) to give the styrene (5) by an E2(Br) mechanism at a rate which is several orders of magnitude slower than the radical process. Alkoxide ions gave E2(H) reactions on dibromide (7) and monochloride (3a) with elimination of the elements of hydrogen bromide and nitrous acid respectively. The catalytic effect of white light, the inhibitive effect of oxygen and p-dinitrobenzene, and the effect of concentration on the radical processes are discussed.