Boron-Based Diastereomerism and Enantiomerism in Imine Complexes Determination of the Absolute Configuration at Boron by CD Spectroscopy

Boron turns out to be a stable stereogenic center in imine complexes of aryl and alkyl boronates. Diastereomerically pure complexes 7a-c are obtained from chiral imine ligands 5a,b that are derived from the amino alcohol (R)-4. The configuration at the boron atom is determined by crystal structure analyses. Racemic boronates 10a-c, available from a condensation of aryl boronic acids 6 with the achiral imine ligand 9, can be separated into stable enantiomers by HPLC on a chiral column. The racemization barrier Delta G(not equal) has been determined to amount to 105-115 kJ mol(-1). The comparison of calculated and measured CD spectra permits to assign unambiguously the absolute configuration to boron in the enantiomeric boronate-imine complex 10a. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)