A novel borane effect on the C/O alkylation ratio in competing S_RN1-S_N2 reactions

The presence of Et₃B in the reaction of 2-nitropropane anion (1) with p-nitrobenzyl bromide (5b) in tert-butyl alcohol results in an approximately three-fold increase in C-alkylation [yielding 1-(2-methyl-2-nitropropyl)-4-nitrobenzene (9) by an S_RN1 mechanism] versus O-alkylation (by an S_N2 mechanism, and leading thereafter to p-nitrobenzaldehyde). Dioxygen completely inhibits C-alkylation in the absence of Et₃B, but is ineffective in its presence. p-Dinitrobenzene does not significantly affect the thermal reaction (30°C) of 1 with 5b, but partially inhibits C-alkylation in reaction under photostimulation. However, p-dinitrobenzene catalyzes C-alkylation in the thermal reaction in the presence of Et₃B. Rationalizations of these novel results are presented. Qualitatively similar results were obtained for the reaction of phenylnitromethane anion (16) with 5b.