Vanadium Pyridonate Catalysts: Isolation of Intermediates in the Reductive Coupling of Alcohols

Abstract

The reductive coupling of alcohols using vanadium pyridonate catalysts is reported. This attractive approach for C(sp 3 )−C(sp 3 ) bond formation uses an oxophilic, earth-abundant metal for a catalytic deoxygenation reaction. Several pyridonate complexes of vanadium were synthesized, giving insight into the coordination chemistry of this understudied class of compounds. Isolated intermediates provide experimental mechanistic evidence that complements reported computational mechanistic proposals for the reductive coupling of alcohols. In contrast to previous mononuclear vanadium(V)/vanadium(III)/vanadium(IV) cycles, this pyridonate catalyst system is proposed to proceed by a vanadium(III)/vanadium(IV) cycle involving bimetallic intermediates.

Vanadium pyridonates: dimerization, redox behaviour, and metal–ligand cooperativity

“…However, during workup this green solution gradually changed to gold-orange with concomitant precipitation of an insoluble pink powder. To our surprise, 1 H NMR spectroscopy and X-ray crystallography verified that the identity of the pink solid was a previously reported 37 tris(pyridonate)vanadium( iii ) dimer 2a 37 rather than the expected V( iv ) complex (Scheme 2). Performing the same reaction as shown in Scheme 2 using a pyridone with steric bulk next to the C–O fragment ( 1b ) or no steric bulk ( 1c ) did not result in crystalline material, however MS analysis supported the formation of a dimeric tris(pyridonate)vanadium( iii ) complex for the reaction with 3-methyl pyridone 1b (750 m / z ).…”

“…While the reduction of vanadium compounds with simple amines is known, 42 there has been no investigation of this reactivity in the context of vanadium pyridonate complexes. In fact, beyond our recent report on vanadium pyridonates, 37 the fundamental coordination chemistry of this class of compounds is unexplored; only one pyridone or pyridonate complex of vanadium was known prior to our work. 43 As a result, the factors that influence the aggregation, redox chemistry, and metal–ligand cooperativity of vanadium pyridonate complexes specifically are not well-understood.…”

“…In one case, pyridonate ligands were used for the vanadium-catalyzed reductive coupling of alcohols, in which a C–O bond is cleaved and a new C–C bond is formed. 37 Through isolation of catalytic intermediates, it was demonstrated that the pyridonate ligand participated in both metal–ligand cooperativity and hemilability; the latter allowed for facile binding of the alcohol substrate, while the former granted formation of the alkoxide intermediate through deprotonation of the O–H bond by the pyridyl nitrogen. Importantly, reduction of a V( iv ) intermediate to V( iii ) by alcohol substrate was proposed to occur via PCET, which is supported by DFT studies on a related system.…”

Deoxygenative Transition-Metal-Promoted Reductive Coupling and Cross-Coupling of Alcohols and Epoxides

“…Recently, the catalytic oxidation/reductive coupling of alcohols by vanadium pyridonate complexes was reported (Scheme 21). 32 This work provides additional evidence for the earlier mechanistic proposals for the reductive coupling of alcohols. Benzylic alcohols gave yields of 34-99% under similar reaction conditions to those reported by our group.…”

Metal–Ligand Cooperative Proton Transfer as an Efficient Trigger for Rhodium-NHC-Pyridonato Catalyzed gem-Specific Alkyne Dimerization

“…Thus, 2-heteroatom-substituted pyridine ligands appear to be promising candidates to fulfill the requirements of a BHetA structure with tight chelate coordination . Particularly, 2-pyridonate moieties have been shown to act as versatile proton-responsive ligands which can behave as powerful internal bases as well as efficient proton shuttles . Moreover, its proven hemilability would be key for the generation of vacant sites and the proton transfer process.…”