Steric Effects on Silene Reactivity. The Effects ofortho-Methyl Substituents on the Kinetics and Mechanisms of the Reactions of 1,1-Diarylsilenes with Nucleophiles

Abstract

The effects of ortho-methyl substitution on the reactivity of 1,1-diarylsilenes are assessed, through the study of a series of transient 1,1-diarylsilenes bearing ortho-methyl substituents in various numbers and positions on the aryl rings. The five silenes studied in this work (ArAr‘SiCH2; 5c−g) were prepared by photolysis of the corresponding 1,1-diarylsilacyclobutanes (6c, Ar = Ar‘ = 2-MeC6H4; 6d, Ar = Ph, Ar‘ = 2,6-Me2C6H3; 6e, Ar = Ph, Ar‘ = 2,4,6-Me3C6H2; 6f, Ar = 2-MeC6H4, Ar‘ = 2,6-Me2C6H3; 6g, Ar = Ar‘ = 2,6-Me2C6H3). Isomeric 1,1-diarylsilacyclobutane(s) arising from phototransposition reaction of one of the aryl rings are also formed in significant amounts in the photolyses, but do not interfere with the analysis. UV absorption spectra and lifetimes of 5c−g were determined in acetonitrile and hexane solution at 25 °C by laser flash photolysis methods, along with absolute rate constants for their reactions with methanol, n-butylamine, and acetic acid in acetonitrile solution. Deuterium kinetic isotope effects were also determined for the reactions with methanol and acetic acid. The rate constants for decay of the five silenes and for their reaction with the three reagents vary over 3−4 orders of magnitude and in the order 5c ≫ 5d ∼ 5e > 5f ≫ 5g. The decline in reactivity throughout the series is accompanied by a change in the mechanism for methanol addition, from one that is purely first order in alcohol in the case of 5c to one that is purely second order in alcohol in the case of 5g. The results are consistent with a mechanism involving initial complexation of the nucleophile with silene, followed by proton transfer by competing unimolecular and bimolecular pathways, the latter via a protonation−deprotonation sequence. The rate constants for acetic acid addition parallel those for n-butylamine addition, verifying that both reactions proceed via a mechanism involving initial nucleophilic attack at silicon followed by proton transfer.