Carbanion reactivity; studies of σ ‐adduct formation from benzyltriflone anions and 4‐nitrobenzofurazan derivatives

1 H NMR studies in DMSO‐d 6 of the reaction of benzyltriflones, 2 , with 4,6‐dinitrobenzofuroxan, 4 , indicate the formation, with or without the presence of added base, of anionic σ ‐adducts. Spectra obtained from solutions of 2 and 4‐nitrobenzofurazan, 6a , in the presence of triethylamine are consistent with products formed by the elimination of trifluoromethylsulfinic acid from σ ‐adducts initially formed by carbanion attack at the 5‐position of 6a . Kinetic studies of the latter reaction in methanol allow the determination of rate constants for nucleophilic attack by the carbanions at the 5‐position. The low value of β for these reactions together with the failure to observe reaction with 1,3,5‐trinitrobenzene suggest that the benzyltriflone anions have unusually large steric requirements. Copyright © 2007 John Wiley & Sons, Ltd.