Crystallization-Induced Asymmetric Transformation vs Quasi-Racemate Formation in Tetravalent Boron Complexes

Abstract

Crystallization-induced asymmetric transformation (AT) has been achieved with the salicaldimine complexes 8 / 9, 11/12, and 14a / 15a and with the oxazaborolidinone complexes 22b / 23b and 22c / 23c . In the case of 22a and 23a, the initially formed 3:1 mixture of diastereomers crystallizes under equilibrium conditions to afford a quasi-racemate 24, containing both diastereomers in the unit cell. Enolate formation from ent - 22b is demonstrated, and methylation occurs to give 26a . Aldol condensation of the enolate is also feasible, and hindered aldehydes afford adducts such as 27a or 27b with good diastereoselectivity. Factors that contribute to quasi-racemate formation are discussed.