Four-membered Rings with One Silicon, Germanium, Tin, or Lead Atom

Abstract

This chapter primarily deals with the chemistry of silacyclobutanes (siletanes), which are the most stable and thoroughly studied of the cyclobutane heterocycles that contain one group 14 heteroatom. Silacyclobutanes and germacyclobutanes are generally prepared by cyclization of 3-halopropylsilyl (or -germyl) halides. Nucleophiles (Lewis bases) induce formation of siletane ‘ate’ complexes that undergo further chemistry. Notably, organolithium species or even alkali metal alkoxides react with siletanes to generate ring-opened organometallic nucleophiles. Popular uses for siletanes include ring-opening polymerization, ring-expansion and insertion reactions, Hiyama couplings and Tamao oxidations triggered by siletane ring opening, and addition reactions to aldehydes (e.g., aldol or allylations) that are designed to take advantage of the inherent ring strain of the siletane. The internal C–Si–C bond angle (ca. 80°) deviates significantly from the ideal tetrahedral bond angle of 109.5°, which imparts enhanced Lewis acidity as compared to unstrained organosilanes. Other physical parameters of siletanes, including spectroscopic and thermodynamic values, are compiled herein. This chapter focuses on the literature between 1995 and 2006, during which time the utility of silacyclobutanes in organic synthesis has increased dramatically.