Crystallization-induced asymmetric transformation vs quasi-racemate formation in tetravalent boron complexes

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.