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On the Mechanism of the Ruthenium-catalyzed beta-methylation of Alcohols with Methanol

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Leitner,  Walter
Research Department Leitner, Max Planck Institute for Chemical Energy Conversion, Max Planck Society;
Institut für Technische Chemie und Makromolekulare Chemie, Rheinisch‐Westfälische Technische Hochschule Aachen, Worringer Weg 1, 52074 Aachen, Germany;

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Citation

Kaithal, A., Schmitz, M., Hölscher, M., & Leitner, W. (2020). On the Mechanism of the Ruthenium-catalyzed beta-methylation of Alcohols with Methanol. ChemCatChem, 12(3), 781-787. doi:10.1002/cctc.201901871.


Cite as: https://hdl.handle.net/21.11116/0000-0007-B3A2-7
Abstract
Selective beta-methylation of alcohols with methanol has been recently described using a catalytic system comprising the ruthenium pincer complex [RuH(CO)(BH4)(HN(C2H4PPh2)(2))]-(Ru-MACHO-BH) 1 and alcoholate bases as co-catalysts. 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(C2H4PPh2)(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).