SYNTHESIS AND STRUCTURAL CHARACTERIZATION OF A MEERWEIN PONNDORF VERLEY OPPENAUER REDOX INTERMEDIATE AT A TANTALUM(V) CENTER

The alkyne adducts (DIPP)3Ta(PhC≡CPh) (1) and (DIPP)3Ta(Me3SiC≡CMe) (2, DIPP = 2,6-diiso- propylphenoxide) are precursors to the metallacyclic products (DIPP)3Ta(CPh═CPhCPh(Me)O) (3) and (DIPP)3Ta(C(SiMe3)═CMeCPh2O) (4) from their reactions with 1 equiv of acetophenone and benzophenone, respectively. (DIPP)3Ta(PhC≡CPh) (1) also reacts with 1 equiv of benzaldehyde to form the related metallacyclic compound (DIPP)3Ta(CPh═CPhCH(Ph)O) (5) in high yield. Excess benzaldehyde reacts with 1 and 2 to form the benzyl alkoxide complexes (DIPP)3(PhCH2O)Ta(η2-CR═CRʹCPh═O) (6, R = Rʹ = Ph; 7, R = Me3Si, Rʹ = Me) with a dative bonding interaction between the ring oxygen and the tantalum(V) center. Deuterium-labeling experiments and reactivity studies of this reaction have implicated the oxophilic complex 5 as an intermediate that is capable of transferring a hydride to a weakly coordinated (probably η1, O-bound), second molecule of an aldehyde or ketone. Thus, (DIPP)3(PhCH2O)Ta(η2-CPh═CPhCPh═O) (6), (DIPP)3(CH3CH2O)Ta(η2-CPh═CPhCPh═O) (8), and (DIPP)3(CH3CH2(CH3)-CHO)Ta(η2-CPh═CPhCPh═O) (9) are formed from the reaction of 5 with benzaldehyde, acetaldehyde, and 2-butanone, respectively. The hydride-transfer step of this reaction is equivalent to the Meerwein-Ponndorf-Verley/Oppenauer reactions in which a secondary alkoxide ligand transfers a hydride to the carbonyl of a coordinated aldehyde or ketone. Support that the reaction 5 → 6 proceeds through an η1-benzaldehyde intermediate is provided in the isolation of (DIPP)3(η1-EtCN)Ta(CPh═CPhCH(Ph)O) (11) from the reaction of (DIPP)3Ta(CPh═CPhCH(Ph)O) (5) with propionitrile. An X-ray structural study of 6 reveals that it belongs to the space group P1 (No. 2) with triclinic cell parameters a = 11.552 (1) Å, b = 13.479 (1) Å, c = 20.246 (2) Å, α = 90.82 (1)°, β = 99.44 (1)°, γ = 114.82 (1)°, and V = 2810.5 Å3 for Z = 2. Final agreement factors for 8791 reflections with Fo2 > 3.0σ(Fo2) were 0.022 (unweighted) and 0.028 (weighted). The Ta—O(CH2Ph) bond length of 1.865 (2) Å, the Ta-O(metallacycle) distance of 2.182 (2) Å, the C═O bond length within the ring (1.265 (3) Å, and the O-C(H2)-Cipso angle of the alkoxide (113.2 (3)°) are all consistent with the formulation of 6 as (DIPP)3(PhCH2O)Ta(η2-CPh═CPhCPh═0) with a dative bonding interaction between the ring oxygen and the metal. © 1990, American Chemical Society. All rights reserved.