Control of regioselectivity in reactions of dialkylstannylene acetals. Part II. NMR results and mechanistic analysis

Abstract

The 119Sn NMR spectra of dialkylstannylene acetals derived from a number of carbohydrate-based terminal 1,2-diols have been recorded. The spectra of chloroform-d solutions at room temperature of most of these dialkylstannylene acetals were consistent with the presence of single symmetric dimers only although some did contain small proportions of higher oligomers. A general mechanistic scheme is presented that explains the regioselectivity of p-toluenesulfonation reactions of dialkylstannylene acetals presented in the preceding paper. In the scheme, the reaction intermediates are the dialkylstannylene acetal dimers. For p-toluenesulfonation reactions, equilibration between the three possible dimers is faster than reaction with p-toluenesulfonyl chloride. Reaction regioselectivity depends both on the position of the dimer equilibrium and on the rates of reaction of individual dicoordinate oxygen atoms in the dimers. The general scheme was also used to explain the regioselectivity obtained for the very fast oxidation reactions of dialkylstannylene acetals with bromine or N-bromosuccinimide. In these kinetically controlled reactions, regiochemistry is determined by the initial reaction of dicoordinate oxygen atoms in the most populated dimer(s) with the electrophilic oxidizing reagent.