Mechanistic Investigation of Bis(imino)pyridine Manganese Catalyzed Carbonyl and Carboxylate Hydrosilylation

We recently reported a bis(imino)pyridine (or pyridine diimine, PDI) manganese precatalyst, (1412PPTDI)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(-1) have 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 PhSiH3 to ((PDI)-P-Ph2PPr)Mn was found to result in partial conversion to a new diamagnetic hydride compound. Independent preparation of ((PDI)-P-Ph2PPr)Mn](2) was achieved upon adding NaEt3BH to ((PDI)-P-Ph2PPr)MnC12 and 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 ((PDI)-P-Ph2PPr)Mn(OC.(Ph)(CF3)) (3), which undergoes intermolecular C C bond formation to produce the respective [Mn(II) dimer, mu-O,Npy-4-OC(CF3)(Ph)-4-H-(PDI)-P-Ph2PPr)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 PhSiH3 revealed 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.