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

✍ Scribed by Manfred Braun; Sebastian Schlecht; Marco Engelmann; Walter Frank; Stefan Grimme

• Publisher: John Wiley and Sons
• Year: 2008
• Tongue: English
• Weight: 301 KB
• Volume: 2008
• Category: Article
• ISSN: 1434-193X
• DOI: 10.1002/ejoc.200800787

✦ Synopsis

Abstract

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)