The synthesis of alkylphosphine-substituted α-diimine (DI) ligands and their subsequent addition to Ni(COD) 2 allowed for the preparation of ( iPr2PPr DI)Ni and ( tBu2PPr DI)Ni . The solid state structures of both compounds were found to feature a distorted tetrahedral geometry that is largely consistent with the reported structure of the diphenylphosphine-substituted variant, ( Ph2PPr DI)Ni . To explore and optimize the synthetic utility of this catalyst class, all three compounds were screened for benzaldehyde hydrosilylation activity at 1.0 mol% loading over 3 h at 25 °C. Notably, ( Ph2PPr DI)Ni was found to be the most efficient catalyst while phenyl silane was the most effective reductant. A broad scope of aldehydes and ketones were then hydrosilylated, and the silyl ether products were hydrolyzed to afford alcohols in good yield. When attempts were made to explore ester reduction, inefficient dihydrosilylation was noted for ethyl acetate and no reaction was observed for several additional substrates. However, when an equimolar solution of allyl acetate and phenyl silane was added to 1.0 mol% ( Ph2PPr DI)Ni , complete ester C–O bond hydrosilylation was observed within 30 min at 25 °C to generate propylene and PhSi(OAc) 3 . The scope of this reaction was expanded to include six additional allyl esters, and under neat conditions, turnover frequencies of up to 990 h −1 were achieved. This activity is believed to be the highest reported for transition metal-catalyzed ester C–O bond hydrosilylation. Proposed mechanisms for ( Ph2PPr DI)Ni -mediated carbonyl and allyl ester C–O bond hydrosilylation are also discussed.
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Anti-Markovnikov terminal and gem -olefin hydrosilylation using a κ 4 -diimine nickel catalyst: selectivity for alkene hydrosilylation over ether C–O bond cleavage
The phosphine-substituted α-diimine Ni precursor, ( Ph2PPr DI)Ni , has been found to catalyze alkene hydrosilylation in the presence of Ph 2 SiH 2 with turnover frequencies of up to 124 h −1 at 25 °C (990 h −1 at 60 °C). Moreover, the selective hydrosilylation of allylic and vinylic ethers has been demonstrated, even though ( Ph2PPr DI)Ni is known to catalyze allyl ester C–O bond hydrosilylation. At 70 °C, this catalyst has been found to mediate the hydrosilylation of ten different gem -olefins, with turnover numbers of up to 740 under neat conditions. Prior and current mechanistic observations suggest that alkene hydrosilylation takes place though a Chalk–Harrod mechanism following phosphine donor dissociation.
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- Award ID(s):
- 1651686
- NSF-PAR ID:
- 10089987
- Date Published:
- Journal Name:
- Dalton Transactions
- Volume:
- 48
- Issue:
- 2
- ISSN:
- 1477-9226
- Page Range / eLocation ID:
- 461 to 467
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/C8DT04608E
