title: Mechanistic Investigation of Bis(imino)pyridine Manganese Catalyzed Carbonyl and Carboxylate Hydrosilylation.
abstract: We recently reported a bis(imino)pyridine (or pyridine diimine, PDI) manganese precatalyst, (Ph2PPrPDI)Mn (1), that is active for the hydrosilylation of ketones and dihydrosilylation of esters. In this contribution, we reveal an expanded scope for 1-mediated hydrosilylation and propose two different mechanisms through which catalysis is achieved. Aldehyde hydrosilylation turnover frequencies (TOFs) of up to 4900 min-1have been realized, the highest reported for first row metal-catalyzed carbonyl hydrosilylation. Additionally, 1 has been shown to mediate formate dihydrosilylation with leading TOFs of up to 330 min-1. Under stoichiometric and catalytic conditions, addition of PhSiH3to (Ph2PPrPDI)Mn was found to result in partial conversion to a new diamagnetic hydride compound. Independent preparation of (Ph2PPrPDI)MnH (2) was achieved upon adding NaEt3BH to (Ph2PPrPDI)MnCl2and single-crystal X-ray diffraction analysis revealed this complex to possess a capped trigonal bipyramidal solid-state geometry. When 2,2,2-trifluoroacetophenone was added to 1, radical transfer yielded (Ph2PPrPDI·)Mn(OC·(Ph)(CF3)) (3), which undergoes intermolecular C-C bond formation to produce the respective Mn(II) dimer, [(μ-O,Npy-4-OC(CF3)(Ph)-4-H-Ph2PPrPDI)Mn]2(4). Upon finding 3 to be inefficient and 4 to be inactive, kinetic trials were conducted to elucidate the mechanisms of 1- and 2-mediated hydrosilylation. Varying the concentration of 1, substrate, and PhSiH3revealed a first order dependence on each reagent. Furthermore, a kinetic isotope effect (KIE) of 2.2 ± 0.1 was observed for 1-catalyzed hydrosilylation of diisopropyl ketone, while a KIE of 4.2 ± 0.6 was determined using 2, suggesting 1 and 2 operate through different mechanisms. Although kinetic trials reveal 1 to be the more active precatalyst for carbonyl hydrosilylation, a concurrent 2-mediated pathway is more efficient for carboxylate hydrosilylation. Considering these observations, 1-catalyzed hydrosilylation is believed to proceed through a modified Ojima mechanism, while 2-mediated hydrosilylation occurs via insertion.
authors.affiliation: School of Molecular Sciences, Arizona State University , Tempe, Arizona 85287, United States.
authors.forename: Tufan K
authors.initials: TK
authors.lastname: Mukhopadhyay
authors.affiliation: School of Molecular Sciences, Arizona State University , Tempe, Arizona 85287, United States.
authors.forename: Christopher L
authors.initials: CL
authors.lastname: Rock
authors.affiliation: Department of Chemistry, Korea Advanced Institute of Science and Technology (KAIST) , Daejeon 34141, South Korea.|Center for Catalytic Hydrocarbon Functionalizations, Institute for Basic Science (IBS) , Daejeon 34141, South Korea.
authors.forename: Mannkyu
authors.initials: M
authors.lastname: Hong
authors.affiliation: Department of Chemistry, Indiana University , Bloomington, Indiana 47405, United States.
authors.forename: Daniel C
authors.initials: DC
authors.lastname: Ashley
authors.affiliation: School of Molecular Sciences, Arizona State University , Tempe, Arizona 85287, United States.
authors.forename: Thomas L
authors.initials: TL
authors.lastname: Groy
authors.affiliation: Department of Chemistry, Korea Advanced Institute of Science and Technology (KAIST) , Daejeon 34141, South Korea.|Center for Catalytic Hydrocarbon Functionalizations, Institute for Basic Science (IBS) , Daejeon 34141, South Korea.
authors.forename: Mu-Hyun
authors.identifier: 0000-0002-8832-8187
authors.initials: MH
authors.lastname: Baik
authors.affiliation: School of Molecular Sciences, Arizona State University , Tempe, Arizona 85287, United States.
authors.forename: Ryan J
authors.identifier: 0000-0003-4935-6780
authors.initials: RJ
authors.lastname: Trovitch