Iron Catalyzed C-X (X = C, Si, B, H) Bond Forming Reactions

Abstract

The synthesis of a new bis(imino)pyridine iron dinitogen compound, (iPr(tb)PDI)Fe(N2)2, was undertaken in order to explore the effect of restricting imine dissociation on catalysis. While this modification did not prevent the κ2-coordination of the ligand it did impart favorable solubility and crystallinity properties that were exploited in order to isolate and characterize catalytically relevant intermediates for intramolecular [2+2] cycloaddition and hydrogenative cyclization reactions. In the case of the [2+2] cycloaddition reactions changing the substrates allowed for the isolation and characterization of both the diene and metallacyclopentane intermediates. The diene compound was found to be best describe as high-spin iron(I) antiferromagnetically coupled to a mono reduced chelate resulting in the overall S=1 compound. The metallacycle was found to be best describe as intermediate-spin iron(III) antiferromagnetically coupled to a mono reduced chelate resulting in the overall S=1 compound. The catalytic cycle therefore occurs via an Fe(I)-Fe(III) redox event and the redox-active chelate remains mono reduced throughout this process.

The development of (tricPDI)Fe(N2)2 enabled expansion of the scope of intramolecular [2+2] cycloaddition reactions to include 1,7-octadiene and derivatives. The use of (Me(iPr)PDI)Fe(N2)]2(μ2-N2) with these substrates led to divergent “Alder-Ene” reactivity yielding 1-methyl-2-methylenecyclohexane and the respective derivatives as a mixture of the possible regioisomers. Catalytically relevant metallacycles were isolated and characterized for both of these reactions. The scope of the [2+2] reaction was further expanded to include intermolecular variants. In the case of alkene dimerization reactions, trans-1,2-substituted cyclobutanes were observed as the exclusive product. When cross-cycloadditions between dienes and alkenes were run, cis-1,3-substituted cyclobutanes were obtained resulting from an iron alkyl-allyl intermediate distinct from the iron metallacyclopentanes.

The scope of alkene hydrosilylation and hydroboration with these catalysts were further explored to include substrates of interest for flavor and fragrance applications in collaboration with Firmenich. High-throughput screening techniques were applied toward the discovery of iron catalysts for the asymmetric hydrogenation of alkenes in collaboration with Merck. Poor conversions and selectivities were observed with a variety of bidentate phosphines and iron precursors. The reactivity of related, independently prepared complexes was explored.