Mechanisms of Catalytic Reactions
One of the largest commercial applications of homogeneous catalysts is the metallocene-catalyzed polymerization of simple alkenes to make polyethene, polypropene, polystyrene, etc. New homogeneous metallocenes catalysts based on Ti and Zr have revolutionized this industry, making possible new polymeric materials through exquisite control of polymer molecular weights and microstructure. Surprisingly, characterization of rate laws for the fundamental steps (initiation, propagation, and termination) of metallocene-catalyzed polymerization and a fundamental understanding of how various activators and co-catalysts affect the rates, stereospecificity, and molecular weights of catalytic polymerizations are underdeveloped. Our research encompasses new approaches to determining the number of catalyst sites that are producing polymers (i. e., active site counting), new chromatographic and mass spectrometric methods for obtaining mechanistic information, and new approaches to probing the influence of ion-pairing dynamics on the polymerization activity. An offshoot of this development work has been the development of new stopped-flow NMR methods and probes. The goal of this work is to exploit the high information content of NMR spectroscopy in the determination of fast reaction kinetics.
Chiral Ligands for Enantioselective Catalysis
The key attribute of homogeneous catalysts is selectivity. The potential of harnessing such selectivity for cost effective, sustainable manufacturing of pharmaceuticals and fine chemicals drives modern research in homogeneous catalysis. We have developed a new class of phosphine ligands, 3,4-diazaphospholanes, that are chiral, rapidly synthesized, and readily expanded into diverse collections. When bound to rhodium, bis-3,4-diazaphospholanes effect enzyme-like rates and enantioselectivities for the hydroformylation of a variety of alkenes. Catalytic hydroformylation effects the conversion of an alkene, carbon monoxide, and dihydrogen into aldehydes. Effective, practical enantioselective hydroformylation enables the production of chiral aldehydes that serve as important synthons in the production of pharmaceuticals and fine chemicals. We are actively expanding the range of chiral diazaphospholanes and their applications. More recently we have embarked on detailed studies of the hydroformylation mechanism.
Computations are a valuable complement to both catalyst design and mechanistic studies. Our goals are to develop a “Valence Bond” perspective of bonding in transition metal complexes and to apply high level, hybrid quantum mechanics/molecular mechanics methods to explore the origin of selectivity control in homogeneous catalysis. In addition to understanding fundamental issues concerning electronic structure, we recently have initiated a computational exploration of the mechanism of asymmetric hydroformylation.