Base Metal Catalysis
Homogeneous catalysis with transition metals has transformed the art and science of chemical synthesis. The industrial synthesis of most commodity and fine chemicals, including pharmaceuticals, rely on at least one, if not many, transition metal catalyzed steps. Traditionally these catalysts are based on precious metals of the second and third rows of the transition series that suffer from high cost, limited availability and pose environmental concerns. Research in our laboratory is aimed at replacing these metals in homogeneous catalysis with more abundant and inexpensive base metals such as iron, manganese and cobalt.
We have discovered a family of bis(imino)pyridine iron dinitrogen complexes that are versatile pre-catalysts for a range of C-H, C-Si and C-C bond forming reactions. The area of longest standing interest in our group has been olefin hydrogenation and hydrosilylation. Iron catalysts for alkene hydrogenation that exhibit high turnover frequencies and broad functional group tolerance have been discovered. We continue to explore new ligand architectures to improve reactivity, scope and induce enatioselectivity. In the area of olefin hydrosilylation, we are engaged in an active collaboration with Momentive Performance Materials to develop iron catalysts as replacements for existing platinum-catalyzed processes.
A second area of interest is in C-C bond forming reactions to promote novel ring constructions. A recent highlight has been the discovery of a family of iron catalysts that promote the [2 p + 2 p ] cycloaddition of ethylene and butadiene to yield vinylcyclobutane. This result stands in contrast to the expectations from orbital symmetry where [4 p + 2 p ] Diels-Alder chemistry would be expected. We are currently exploring iron-catalyzed methods to access various cyclobutanes in both inter- and intramolecular cyclizations. Kinetic and isotopic labeling studies have provided insights into the mechanisms of these unique reactions.
A final area of interest is olefin polymerization. Following Brookhart and Gibson’s seminal discovery of bis(imino)pyridine iron- and cobalt-catalyzed olefin polymerization, there has been a long-standing debate over the nature of the active species during chain propagation and the oxidation states of the metal centers following activation with aluminum alkyls. Our laboratory has prepared well-defined examples of bis(imino)pyridine iron and cobalt alkyl cations and using spectroscopic and computational studies been able to evaluate polymerization activity using well-defined single component catalysts. Current efforts are focused on understanding structure-reactivity relationships and the pathways of monomer enchainment and chain termination in these processes.
Russell, S. K.; Lobkovsky, E.; Chirik, P. J. “An iron-catalyzed intermolecular [2 p + 2 p ] cycloaddition.” J. Am. Chem. Soc. 2011, 133, 8858-8861.
Chirik, P. J.; Wieghardt, K. W. “Radical ligands confer nobility of base-metal catalysts.” Science 2010, 327, 794-795.
Tondreau, A. M.; Milsmann, C.; Patrick, A. D.; Hoyt, H. M.; Lobkovsky, E.; Wieghardt, K.; Chirik, P. J. “Synthesis and electronic structure of cationic, neutral, and anionic bis(imino)pyridine iron alkyl complexes: Evaluation of redox activity in single-component ethylene polymerization catalysts.” J. Am. Chem. Soc. 2010, 132, 15046-15059.
Sylvester, K. T.; Chirik, P. J. “Iron-catalyzed, hydrogen-mediated reductive cyclization of 1,6-enynes and diynes: Evidence for bis(imino)pyridine ligand participation.” J. Am. Chem. Soc. 2009, 131, 8772-8774.
Trovitch, R. J.; Lobkovsky, E.; Bill, E.; Chirik, P. J. “Functional group tolerance and substrate scope in bis(imino)pyridine iron catalyzed alkene hydrogenation.” Organometallics 2008, 27, 1470-1478.
Bouwkamp, M. W.; Bowman, A. C. Lokbovsky, E.; Chirik, P. J. “Iron-catalyzed [2 p + 2 p ] cycloaddition of a , w -dienes: The importance of redox-active supporting ligands.” J. Am. Chem. Soc. 2006, 128, 13340-13341.
Bart, S. C.; Lobkovsky, E.; Chirik, P. J. “Preparation and molecular and electronic structures of iron(0) dinitrogen and silane complexes and their application to catalytic hydrogenation and hydrosilation.” J. Am. Chem. Soc. 2004, 126, 13794-13807.