CHEMISTRY OF ENOXYSILACYCLOBUTANES - HIGHLY SELECTIVE UNCATALYZED ALDOL ADDITIONS

O-(Silacyclobutyl) ketene acetals derived from esters, thiol esters, a nd amides underwent facile aldol addition with a variety of aldehydes at room temperature without the need for catalysts. The uncatalyzed al dol addition reaction of O-(silacyclobutyl) ketene acetals displayed t he following characteristics: (1) the rate of reaction was highly depe ndent on the spectator substituent on silicon and the geometry of the ketene acetal, (2) the O,O-ketene acetal of E configuration afforded t he syn aldol products with high diastereoselectivity (93/7 to 99/1), ( 3) conjugated aldehydes reacted more rapidly than aliphatic aldehydes, and (4) the reaction was mildly sensitive to solvent. In addition, th e aldol reaction was found to be efficiently catalyzed by metal alkoxi des. Labeling experiments revealed that the thermal aldol reaction pro ceeds by direct intramolecular silicon group transfer, while the alkox ide-catalyzed version probably proceeds via in situ generated metal en olates. Computational modeling of the transition states suggests that the boat transition structures are preferred, supporting the observed syn selectivity of the thermal aldol reaction. Both thermal and alkoxi de-catalyzed Michael additions were investigated, revealing a competit ion between 1,2- and 1,4-addition favoring the former.