Intermolecular exchange and configurational stability of organotin alkoxides and phenoxides

Abstract

Chemical shift non-equivalence for the diastereotopic methyl groups of methyl-(2-methyl-2-phenylpropyl)phenyltin alkoxides and phenoxide is observed in benzene solution. Peak separation and line-width analysis show that in these compounds intermolecular exchange of OR groups (R = alkyl or phenyl) and configuration inversion at the tin atom proceed at a different rate, exchange being much faster than inversion. The change in bulk of the alkoxy-group, or its substitution with a phenoxy-group, brings about appreciable changes in the exchange rate but only marginally affects inversion. A mechanism involving intermolecular association and non-dissociative positional exchange of the groups bonded to tin is suggested. Addition of free alcohols or phenol shows that, while exchange of OR groups occurs between the tin derivative and these added compounds, the inversion rate is increased and this accounts for a different mechanism including transetherification. Experiments with alcohols or phenol containing an OR group different from that contained in the tin derivative support this point. Coalescence of diastereotopic methyls on adding dimethyl sulphoxide is observed only in the case of the organotin phenoxide. The role of steric and electronic effects of the R group is discussed.