4 years ago

The Emergence of Manganese-Based Carbonyl Hydrosilylation Catalysts

The Emergence of Manganese-Based Carbonyl Hydrosilylation Catalysts
Ryan J. Trovitch
In recent years, interest in homogeneous manganese catalyst development has intensified because of the earth-abundant and nontoxic nature of this metal. Although compounds of Mn have largely been utilized for epoxidation reactions, recent efforts have revealed that Mn catalysts can mediate a broad range of reductive transformations. Low-valent Mn compounds have proven to be particularly effective for the hydrosilylation of carbonyl- and carboxylate-containing substrates, and this Account aims to highlight my research group’s contributions to this field. In our initial 2014 communication, we reported that the bis(imino)pyridine-supported compound (Ph2PPrPDI)Mn mediates ketone hydrosilylation with exceptional activity under solvent-free conditions. Silanes including Ph2SiH2, (EtO)3SiH, (EtO)2MeSiH, and (EtO)Me2SiH were found to partially reduce cyclohexanone in the presence of (Ph2PPrPDI)Mn, while turnover frequencies of up to 1280 min–1 were observed using PhSiH3. This led us to evaluate the hydrosilylation of 11 additional ketones and allowed for the atom-efficient preparation of tertiary and quaternary silanes. At that time, it was also discovered that (Ph2PPrPDI)Mn catalyzes the dihydrosilylation of esters (by way of acyl C–O bond hydrosilylation) to yield a mixture of silyl ethers with modest activity. Earlier this year, the scope of these transformations was extended to aldehydes and formates, and the observed hydrosilylation activities are among the highest obtained for any transition-metal catalyst. The effectiveness of three related catalysts has also been evaluated: (Ph2PPrPDI)MnH, (PyEtPDEA)Mn, and [(Ph2PEtPDI)Mn]2. To our surprise, (Ph2PPrPDI)MnH was found to exhibit higher carboxylate dihydrosilylation activity than (Ph2PPrPDI)Mn, while (PyEtPDEA)Mn demonstrated remarkable carbonyl hydrosilylation activity considering that it lacks a redox-active supporting ligand. The evaluation of [(Ph2PEtPDI)Mn]2 revealed competitive aldehyde hydrosilylation and formate dihydrosilylation turnover frequencies; however, this catalyst is significantly inhibited by pyridine and alkene donor groups. In our efforts to fully understand how (Ph2PPrPDI)Mn operates, a thorough electronic structure evaluation was conducted, and the ground-state doublet calculated for this compound was found to exhibit nonclassical features consistent with a low-spin Mn(II) center supported by a singlet PDI dianion and an intermediate-spin Mn(II) configuration featuring antiferromagnetic coupling to PDI diradical dianion. A comprehensive mechanistic investigation of (Ph2PPrPDI)Mn- and (Ph2PPrPDI)MnH-mediated hydrosilylation has revealed two operable pathways, a modified Ojima pathway that is more active for carbonyl hydrosilylation and an insertion pathway that is more effective for carboxylate reduction. Although these efforts represent a small fraction of the recent advances made in Mn catalysis, this work has proven to be influential for the development of Mn-based reduction catalysts and is likely to inform future efforts to develop Mn catalysts that can be used to prepare silicones.

Publisher URL: http://dx.doi.org/10.1021/acs.accounts.7b00422

DOI: 10.1021/acs.accounts.7b00422

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