Mechanistic
Investigation of Bis(imino)pyridine Manganese Catalyzed Carbonyl and
Carboxylate Hydrosilylation
Version 3 2018-03-10, 14:14
Version 2 2017-12-07, 22:45
Version 1 2017-03-27, 12:48
Posted on 2018-03-10 - 14:14
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–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 (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)MnCl2 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 (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 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.
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Mukhopadhyay, Tufan K.; Rock, Christopher L.; Hong, Mannkyu; Ashley, Daniel C.; Groy, Thomas L.; Baik, Mu-Hyun; et al. (2017). Mechanistic
Investigation of Bis(imino)pyridine Manganese Catalyzed Carbonyl and
Carboxylate Hydrosilylation. ACS Publications. Collection. https://doi.org/10.1021/jacs.7b00879
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AUTHORS (7)
TM
Tufan K. Mukhopadhyay
CR
Christopher L. Rock
MH
Mannkyu Hong
DA
Daniel C. Ashley
TG
Thomas L. Groy
MB
Mu-Hyun Baik
RT
Ryan J. Trovitch