Dehydrogenative Coupling of Aldehydes with Alcohols Catalyzed by a Nickel Hydride Complex

A nickel hydride
complex, {2,6-(iPr2PO)2C6H3}­NiH, has been shown to catalyze
the coupling of RCHO and R′OH to yield RCO2R′
and RCH2OH, where the aldehyde also acts as a hydrogen
acceptor and the alcohol also serves as the solvent. Functional groups
tolerated by this catalytic system include CF3, NO2, Cl, Br, NHCOMe, and NMe2, whereas phenol-containing
compounds are not viable substrates or solvents. The dehydrogenative
coupling reaction can alternatively be catalyzed by an air-stable
nickel chloride complex, {2,6-(iPr2PO)2C6H3}­NiCl, in conjunction
with NaOMe. Acids in unpurified aldehydes react with the hydride to
form nickel carboxylate complexes, which are catalytically inactive.
Water, if present in a significant quantity, decreases the catalytic
efficiency by forming {2,6-(iPr2PO)2C6H3}­NiOH, which causes catalyst
degradation. On the other hand, in the presence of a drying agent,
{2,6-(iPr2PO)2C6H3}­NiOH generated in situ from {2,6-(iPr2PO)2C6H3}­NiCl and NaOH can be converted to an alkoxide species, becoming
catalytically competent. The proposed catalytic mechanism features
aldehyde insertion into the nickel hydride as well as into a nickel
alkoxide intermediate, both of which have been experimentally observed.
Several mechanistically relevant nickel species including {2,6-(iPr2PO)2C6H3}­NiOC­(O)­Ph, {2,6-(iPr2PO)2C6H3}­NiOPh, and {2,6-(iPr2PO)2C6H3}­NiOPh·HOPh have been independently synthesized,
crystallographically characterized, and tested for the catalytic reaction.
While phenol-containing molecules cannot be used as substrates or
solvents, both {2,6-(iPr2PO)2C6H3}­NiOPh and {2,6-(iPr2PO)2C6H3}­NiOPh·HOPh
are efficient in catalyzing the dehydrogenative coupling of PhCHO
with EtOH.