Mechanism of Alcohol–Water Dehydrogenative Coupling into Carboxylic Acid Using Milstein’s Catalyst: A Detailed Investigation of the Outer-Sphere PES in the Reaction of Aldehydes with an Octahedral Ruthenium Hydroxide

In
aqueous basic media, the square-pyramidal complex [Ru­(PNN)­(CO)­(H)]
(1-Ru, where PNN is a dearomatized bipyridyl-CH-PtBu2 pincer ligand) catalyzes the transformation
of alcohols and water into carboxylates and H2. A previous
theoretical investigation reported the following mechanism for the
reaction: (i) metal-catalyzed dehydrogenation of the alcohol into
an aldehyde, (ii) metal–ligand cooperation (MLC) addition of
water to 1-Ru to give an octahedral ruthenium hydroxide
(2-Ru–OH), (iii) concerted MLC hydration of the
aldehyde by 2-Ru–OH to give separated 1-Ru and a gem-diol, and (iv) concerted MLC dehydrogenation of the
gem-diol by 1-Ru into an octahedral ruthenium dihydride
(2-Ru–H) and a carboxylic acid. We calculate the
outer-sphere PES in the reaction between the aldehyde and 2-Ru–OH to start with a localized coupling step yielding an
ion-pair minimum (7-ip-OH) in which the hydroxyl group
of an α-hydroxyl-alkoxide (gem-diolate) is coordinated to the
metal of a cationic square-pyramidal complex. From 7-ip-OH,
we identify a route to carboxylic acid that circumvents ligand deprotonation
involving (i) 1,1-rearrangement of the gem-diolate within the contact
ion pair through an α-OH/O– slippage TS into
the octahedral 2-Ru–OCH­(OH)­R and (ii) a second
1,1-rearrangement through an α-O–/H slippage
TS that gives a new ion-pair minimum in which the α-hydrogen
of the anion is coordinated to the metal, followed by a localized
hydride-transfer TS that gives a carboxylic acid and the octahedral
hydride complex (2-Ru–H). The net transformation
from 2-Ru–OH and the aldehyde to the carboxylic
acid and 2-Ru–H can be viewed as a H/OH metathesis
in which a hydride and a hydroxide are exchanged between the acyl
group of the aldehyde and the metal center of 2-Ru–OH.
The MLC mechanism gives the same metathesis products through the intermediacy
of a gem-diol. When the SMD solvent continuum model is applied during
geometry optimization with water as the solvent, the Gibbs free energy
profile of the slippage pathway is predicted to be much lower than
that predicted for MLC. The possibility of dissociation of the ion
pair 7-ip-OH into free ions and reassociation is also
briefly addressed. Some calculations are also performed to address
why no esters are observed in the given system.