Transition Metal-Free Direct Hydrogenation of Esters via a Frustrated Lewis Pair

Sapsford, Joshua S. and Csókás, Dániel and Turnell-Ritson, Roland C. and Parkin, Liam A. and Crawford, Andrew D. and Pápai, Imre (2021) Transition Metal-Free Direct Hydrogenation of Esters via a Frustrated Lewis Pair. ACS CATALYSIS, 11 (15). pp. 9143-9150. ISSN 2155-5435

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Abstract

"Frustrated Lewis pairs" (FLPs) continue to exhibit unique reactivity for the reduction of organic substrates, yet to date, the catalytic hydrogenation of an ester functionality has not been demonstrated. Here, we report that iPr(3)SnNTf(2) (1-NTf2; Tf = SO2CF3) is a more potent Lewis acid than the previously studied iPr(3)SnOTf; in an FLP with 2,4,6-collidine/2,6-lutidine (col/lut), this translates to faster H-2 activation and the catalytic hydrogenolysis of an ester bond by a main-group compound, furnishing alcohol and ether (minor) products. The reaction outcome is sensitive to the steric and electronic properties of the substrate; CF3CO2Et and simple formates (HCO2Me and HCO2Et) are catalytically reduced, whereas related esters CF(3)CO(2)nBu and CH3CO2Et show only stoichiometric reactivity. A computational case study on the hydrogenation of CF3CO2Et and CH3CO2Et reveals that both share a common mechanistic pathway; however, key differences in the energies of a Sn-acetal intermediate and transition states emerge, favoring CF3CO2Et reduction. The alcohol products reversibly inhibit 1-NTf2/lut via formation of resting-state species 1-OR/[1.(1-OR)]+[NTf2]-; however, the extra energy required to regenerate 1-NTf2/lut exacerbates the unfavorable reduction energy profile for CH3CO2Et, ultimately preventing turnover. These findings will assist the design of future main-group catalysts for ester hydrogenation, with improved performance.

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