Primary and secondary kinetic isotope effects in the decomposition of a tertiary alkoxide

Abstract

Kinetic isotope effects have been determined for the elimination of toluene from the alkoxide of 1,2,3-triphenylpropan-2-ol (1a). In DMSO, the rate measurements on 1a and [1,1,3,3-²H₄] 1,2,3-triphenylpropanol (1b) at 30°C give k_H4/k_C4= 1.68. For the enolate products of reactions of [1,3-²H₂]- and [1,1-²H₂]-1,2,3-triphenylpropanol (1c and 1d), isolated as enol ethers, the ratio of deuteriated to undeuteriated products was 2.67 and 1.37, respectively. The ratio of undeuteriated to deuteriated toluenes from 1d was 1.46. In the reaction of [1-¹³C]-1,2,3-triphenylpropanol (1e) the ratio of ¹³C-labelled product to unlabelled was 1.064. The data are shown to be consistent with rate-limiting expulsion of a benzyl anion from the alkoxide, followed by fast proton transfers involving a solvent molecule to yield the observed products. The negative chemical ionization spectrum of 1a shows ions corresponding to deprotonated alcohol and the enolate of 1,2-diphenylethanone. The ratios of deuteriated to undeuteriated enolates in the B/E linked spectra of 1c and 1d were 2.47 and 0.72, respectively, and the ratio of ¹³C-enolate to unlabelled enolate in the fragmentation of 1e was 1.083. These data are rationalized in terms of an ionic fragmentation to an ion–dipole complex of 1,2-diphenylethanone and benzyl anion followed by a proton transfer which is at least partially rate limiting within the complex to yield the enolate.