Quantification of the Electrophilic Reactivities of Aldehydes, Imines, and Enones

The rates of the epoxidation reactions of aldehydes, of the aziridination reactions of aldimines, and of the cyclopropanation reactions of alpha,beta-unsaturated ketones with aryl-stabilized dimethylsulfonium ylides have been determined photometrically in dimethyl sulfoxide (DMSO). All of these sulfur ylide-mediated cyclization reactions as well as the addition reactions of stabilized carbanions to N-tosyl-activated aldimines have been shown to follow a second-order rate law, where the rate constants reflect the (initial) CC bond formation between nucleophile and electrophile. The derived second-order rate constants (log k(2)) have been combined with the known nucleophilicity parameters (N, s(N)) of the aryl-stabilized sulfur ylides 4a,b and of the acceptor-substituted carbanions 4c-h to calculate the electrophilicity parameters E of aromatic and aliphatic aldehydes (1a-i), N-acceptor-substituted aromatic aldimines (2a-e), and alpha,beta-unsaturated ketones (3a-f) according to the linear free-energy relationship log k(2) = s(N)(N + E) as defined in J. Am. Chem. Soc. 2001, 123, 9500-9512. The data reported in this work provide the first quantitative comparison of the electrophilic reactivities of aldehydes, imines, and simple Michael acceptors in DMSO with carbocations and cationic metal-pi complexes within our comprehensive electrophilicity scale.