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On the Mechanism of the Ruthenium‐catalyzed β ‐methylation of Alcohols with Methanol
Author(s) -
Kaithal Akash,
Schmitz Marc,
Hölscher Markus,
Leitner Walter
Publication year - 2020
Publication title -
chemcatchem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.497
H-Index - 106
eISSN - 1867-3899
pISSN - 1867-3880
DOI - 10.1002/cctc.201901871
Subject(s) - chemistry , ruthenium , catalysis , pincer movement , medicinal chemistry , photochemistry , methanol , alcohol , transition state , organic chemistry
Abstract 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).