Synthesis and structural characterization of a Meerwein-Ponndorf-Verley/Oppenauer redox intermediate at a tantalum(V) center

• Published in: Organometallics 1990-01, Vol.9 (1), p.266-273
• Main Authors: Strickler, Jamie R, Bruck, Michael A, Wexler, Pamela A, Wigley, David E
• Format: Article
• Language: English
• Subjects: ALKYNES; CHEMICAL REACTIONS; Chemistry; COMPLEXES; Condensed matter: structure, mechanical and thermal properties; CRYSTAL STRUCTURE; DATA; DATA ANALYSIS; Exact sciences and technology; EXPERIMENTAL DATA; HYDROCARBONS; INFORMATION; INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; MEASURING INSTRUMENTS; MEASURING METHODS; NUMERICAL DATA; Organic chemistry; ORGANIC COMPOUNDS; ORGANOMETALLIC COMPOUNDS; Organometalloidal and organometallic compounds; Physics; Preparations and properties; REDOX REACTIONS; Structure of solids and liquids; crystallography; Structure of specific crystalline solids; SYNTHESIS; TANTALUM COMPLEXES; TRANSITION ELEMENT COMPLEXES 400200 > Inorganic, Organic, & Physical Chemistry; Transition metals derivatives
• ISSN: 0276-7333; 1520-6041
• DOI: 10.1021/om00115a039

The alkyne adducts (DIPP){sub 3}Ta(PhC{triple bond}CPh) (1) and (DIPP){sub 3}Ta(Me{sub 3}SiC{triple bond}CMe) (2, DIPP = 2,6-diisopropylphenoxide) are precursors to the metallacyclic products (DIPP){sub 3}Ta(CPh=CPhCPh(Me)O) (3) and (DIPP){sub 3}Ta(C(SiMe{sub 3})=CMeCPh{sub 2}O) (4) from their reactions with 1 equiv of acetophenone and benzophenone, respectively. (DIPP){sub 3}Ta(PhC{triple bond}CPh) (1) also reacts with 1 equiv of benzaldehyde to form the related metallacyclic compound (DIPP){sub 3}Ta(CPh=CPhCH(Ph)O) (5) in high yield. Excess benzaldehyde reacts with 1 and 2 to form the benzyl alkoxide complexes (DIPP){sub 3}(PhCH{sub 2}O)Ta({eta}{sup 2}-CR=CR{prime}CPh=O) (6, R = R{prime} = Ph; 7, R = Me{sub 3}Si, R{prime} = 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 {eta}{sup 1}, O-bound), second molecule of an aldehyde or ketone. Thus, (DIPP){sub 3}(PhCH{sub 2}O)Ta({eta}{sup 2}-CPh=CPhCPh=O) (6), (DIPP){sub 3}(CH{sub 3}CH{sub 2}O)Ta({eta}{sup 2}-CPh=CPhCPh=O) (8), and (DIPP){sub 3}(CH{sub 3}CH{sub 2}(CH{sub 3})CHO)Ta({eta}{sup 2}-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 {yields} 6 proceeds through an {eta}{sup 1}-benzaldehyde intermediate is provided in the isolation of (DIPP){sub 3}({eta}{sup 1}-EtCN)Ta(CPh=CPhCH(Ph)O) (11) from the reaction of (DIPP){sub 3}Ta(CPh=CPhCH(Ph)O) (5) with propionitrile.