Nucleophilic Reactivities of Imide and Amide Anions
journal contributionposted on 06.08.2010, 00:00 by Martin Breugst, Takahiro Tokuyasu, Herbert Mayr
The kinetics of the reactions of amide and imide anions 2a−o with benzhydrylium ions 1a−i and structurally related quinone methides 1j−q have been studied by UV−vis spectroscopy in DMSO and acetonitrile solution. The second-order rate constants (log k2) correlated linearly with the electrophilicity parameters E of 1a−q according to the correlation log k2 = s(N + E) (Angew. Chem., Int. Ed. Engl. 1994, 33, 938−957), allowing us to determine the nucleophilicity parameters N and the nucleophile-specific parameters s for these nucleophiles. The reactivities of all sulfonamide and diacylimide anions are found in a relatively small range (15 < N < 22). Comparison with structurally related carbanions revealed that amide and imide anions are less reactive than carbanions of the same pKaH. These effects can be attributed to the absence of resonance stabilization of one of the lone pairs in the amide or imide anions. As amide and imide anions are exclusively attacked at nitrogen by benzhydrylium ions, Kornblum’s interpretation of the ambident reactivity of amide anions has to be revised.