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 Δ G ≠ 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 .(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)