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Mechanism of the Formation of Carboxylate from Alcohols and Water Catalyzed by a Bipyridine-Based Ruthenium Complex: A Computational Study
journal contribution
posted on 2014-01-08, 00:00 authored by Haixia Li, Michael B. HallThe
catalytic mechanism for oxidizing alcohols to carboxylate in
basic aqueous solution by the bipyridine-based ruthenium complex 2 (BIPY-PNN)Ru(H)(Cl)(CO) (Nat. Chem. 2013, 5, 122) is investigated by density
functional theory (DFT) with the ωB97X-D functional. Using water
as the oxygen donor with liberation of dihydrogen represents a safe
and clean process for such oxidations. Under NaOH, the active catalyst
is 3 (BIPY-PNN)Ru(H)(CO). Four steps are involved: dehydrogenation
of alcohol to aldehyde (Step 1); coupling of aldehyde and water to
form the gem-diol (Step 2); dehydrogenation of gem-diol to carboxylic
acid (Step 3); and deprotonation of carboxylic acid to carboxylate
anion under base (Step 4). The dehydrogenations of alcohol (Step 1)
and gem-diol (Step 3) prefer the double hydrogen transfer mechanism
to the β-H elimination mechanism. The coupling of aldehyde and
water (Step 2) proceeds through cleavage of water by catalyst 3 followed by concerted hydroxyl and hydrogen transfer to
the aldehyde. The formation of the carboxylate anion occurs via direct
deprotonation of the carboxylic acid under base (Step 4), while in
the absence of base a stable carboxylic acid-addition complex 6 was formed. Added base was found to play important roles
in the generation of catalyst 3 from both the stable
carboxylic acid-addition complex 6 and its chloride precursor
complex 2. The chemoselectivity for the formation of
carboxylic acid rather than ester is ascribed to the favorable cleavage
of water and the subsequent generation of the stable carboxylate anion
that leads to carboxylic acid upon acidification.