On the Mechanism of the Ruthenium‐catalyzed β ‐methylation of Alcohols with Methanol

Selective β ‐methylation of alcohols with methanol has been recently described using a catalytic system comprising the ruthenium pincer complex [RuH(CO)(BH 4 )(HN(C 2 H 4 PPh 2 ) 2 )]‐(Ru‐MACHO‐BH) 1 and alcoholate bases as co‐catalysts. 1 Here we present a detailed mechanistic analysis for the mono‐methylation of 1‐phenyl‐propane‐1‐ol 2   a as prototypical example. Several experimentally observed intermediates were localized as stable minima on the DFT‐derived energy surface of the entire reaction network. The ruthenium complex [Ru(H) 2 (CO)(HN(C 2 H 4 PPh 2 ) 2 )] I was inferred as the active species catalyzing the de‐hydrogenation/re‐hydrogenation of substrates and intermediates (“hydrogen borrowing”). The hydrogen‐bonded alcohol adduct of this complex was identified as the lowest lying intermediate (TDI). The C−C bond formation results from a base‐catalyzed aldol reaction comprising the transition state with the highest energy (TDTS). Experimentally determined Gibbs free activation barriers of 26.1 kcal/mol and 26.0 kcal/mol in methanol and toluene as solvents, respectively, are reflected well by the computed energy span of the complex reaction network (29.2 kcal/mol).