Synthetic and mechanistic approaches to macrocyclic polystannanes

Abstract

The purpose of this study was to synthesize a macrocyclic polystannane, 8, which would potentially use the Lewis acid character of several tin atoms to create an acidic solvation ring similar to the basic ring of the crown ethers. Significant progress has been made towards the synthesis of the desired macrocycle. Employing a series of reactions we have built an acyclic polystannane which contains latent protected functional groups. Deprotection and activation of these groups will provide a species that may be used in a final ring closure step to give the desired macrocycle. The synthetic scheme we have developed may be readily modified for the synthesis of a variety of analogs. In the course of these studies a variety of new carbon-tin and tin-bromine bond formation reactions have been developed. While studies directed toward the synthesis of the tin macrocycles were underway, it became apparent that although the organotin lithium reagents are valuable reagents for the formation of carbon-tin bonds, the mechanism by which they react is still uncertain. In an effort mechanism of these reactions, we have undertaken the synthesis and evaluation of various mechanistic probes. Two probes, 11 and 12, gave no indication that free radical intermediates were involved in the reaction. We have initiated the synthesis of two sophisticated cyclopropylcarbinyl probes, 13 and 14 which have the advantage of containing stereochemical labels and which can be used to distinguish between the various mechanisms possible for nucleophilic substitution by organometal anions.