The power of electron paramagnetic resonance to study asymmetric homogeneous catalysts based on transition-metal complexes

Asymmetric synthesis involves the preparation of chiral compounds with well-defined three-dimensional molecular structure or stereochemistry. Many of these enantioselective transformations involve chiral ligands incorporating transition-metal ions. Over the years numerous synthetic, spectroscopic and theoretical approaches have been applied to understand and probe the inner workings and mode of chiral transfer in these homogeneous based catalysts. For the paramagnetic based asymmetric complexes, electron paramagnetic resonance (EPR) can provide a wealth of detail and information on the structure and mechanism of the catalysts. Despite this potential, the application and uptake of advanced EPR methodologies to this field of endeavor has been limited. The purpose of this review is to explain and illustrate, through representative examples, the enormous amount of information that can be obtained from an EPR study of the chiral metal-based complexes, ranging from electronic structure and symmetry to spatial arrangement of interacting ligands and substrates. (C) 2008 Elsevier B.V. All rights reserved.