Mono- and tri-ester hydrogenolysis using tandem catalysis. Scope and mechanism

The scope and mechanism of thermodynamically leveraged ester RC(O)O-R′ bond hydrogenolysis by tandem metal triflate + supported Pd catalysts are investigated both experimentally and theoretically by DFT and energy span analysis. This catalytic system has a broad scope, with relative cleavage rates scaling as, tertiary > secondary > primary ester at 1 bar H₂, yielding alkanes and carboxylic acids with high conversion and selectivity. Benzylic and allylic esters display the highest activity. The rate law is ν = k[M(OTf)_n]¹[ester]⁰[H₂]⁰ with an H/D kinetic isotope effect = 6.5 ± 0.5, implying turnover-limiting C-H scission following C-O cleavage, in agreement with theory. Intermediate alkene products are then rapidly hydrogenated. Applying this approach with the very active Hf(OTf)₄ catalyst to bio-derived triglycerides affords near-quantitative yields of C₃ hydrocarbons rather than glycerol. From model substrates, it is found that RC(O)O-R′ cleavage rates are very sensitive to steric congestion and metal triflate identity. For triglycerides, primary/external glyceryl CH₂-O cleavage predominates over secondary/internal CH-O cleavage, with the latter favored by less acidic or smaller ionic radius metal triflates, raising the diester selectivity to as high as 48% with Ce(OTf)₃.