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Reaction pathway and rate-determining step of the Schmidt rearrangement/fragmentation: A kinetic study
Akimoto, Ryo,Tokugawa, Takehiro,Yamamoto, Yutaro,Yamataka, Hiroshi
experimental part, p. 4073 - 4078 (2012/06/29)
The Schmidt rearrangement of substituted 3-phenyl-2-butanone with trimethylsilyl azide in 90% (v/v) aqueous TFA gave two types of product, fragmentation and rearrangement, the ratio of which depends on the substituent: more fragmentation for a more electron-donating substituent. Rate measurements by azotometry indicated the presence of an induction period, and the pseudo-first-order rate constants showed saturation kinetics with respect to the azide concentration. It was indicated that the reaction proceeds through pre-equilibrium in the formation of iminodiazonium (ID) ion and that the N 2 liberation from the ID ion is rate-determining. Under high azide concentration conditions, where the effective reactant is the ID ion, the reaction gave a linear Hammett plot with a value of -0.50. The observed substituent effects on the rate and the product selectivity imply that path bifurcation on the way from the rate-determining TS to the product states occurs, as suggested by previous molecular dynamics simulations, in a similar manner to the analogous Beckmann rearrangement/fragmentation reactions.
