NOVEL TEMPLATE EFFECTS OF DISTANNOXANE CATALYSTS IN HIGHLY EFFICIENT TRANSESTERIFICATION AND ESTERIFICATION

The transesterification of carboxylic esters and the esterification of carboxylic acids are effected under mild conditions under catalysis by 1,3-disubstituted tetraalkyldistannoxanes 1. Various functional groups remain unaffected and otherwise difficult to obtain esters are accessible. An ester bearing a tertiary butyl group in the carboxylic acid moiety remained unchanged in competition experiments with a less bulky ester, which undergoes transesterification quantitatively. The unique features of the reactions are attributable to the template effects of the dimeric structure of 1. The facility with which compounds 1 can be converted into alkoxydistannoxanes 2 and the synergistic effect of the proximate tin atoms of 2 play key roles in permitting smooth reactions and high selectivity. Another notable feature of compounds 1 is their unusually high solubility in organic solvents, even though the compounds have a metaloxane core as a major skeletal part. The double-layered structure of 1, in which the inorganic moiety is surrounded by eight alkyl groups, permits esterification to be driven to completion simply by heating a mixture of the carboxylic acid and the alcohol. The distannoxane-catalyzed esterification is irreversible, and thus, no hydrolysis of the product esters occurs when compounds 1 are used as catalysts.