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

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