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

A nickel hydride complex, {2,6-( i Pr 2 PO) 2 C 6 H 3 }NiH, has been shown to catalyze the coupling of RCHO and R′OH to yield RCO 2 R′ and RCH 2 OH, 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 CF 3, NO 2, Cl, Br, NHCOMe, and NMe 2, 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-( i Pr 2 PO) 2 C 6 H 3 }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-( i Pr 2 PO) 2 C 6 H 3 }NiOH, which causes catalyst degradation. On the other hand, in the presence of a drying agent, {2,6-( i Pr 2 PO) 2 C 6 H 3 }NiOH generated in situ from {2,6-( i Pr 2 PO) 2 C 6 H 3 }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-( i Pr 2 PO) 2 C 6 H 3 }NiOC(O)Ph, {2,6-( i Pr 2 PO) 2 C 6 H 3 }NiOPh, and {2,6-( i Pr 2 PO) 2 C 6 H 3 }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-( i Pr 2 PO) 2 C 6 H 3 }NiOPh and {2,6-( i Pr 2 PO) 2 C 6 H 3 }NiOPh·HOPh are efficient in catalyzing the dehydrogenative coupling of PhCHO with EtOH.