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Reaction Mechanism of the C⋮N Triple Bond Cleavage of β -Ketonitriles on a Molybdenum(0) Center 1

2000; American Chemical Society; Volume: 122; Issue: 8 Linguagem: Inglês

10.1021/ja9921928

1943-2984

Yoshiaki Tanabe, Hidetake Seino, Youichi Ishii, Masanobu Hidai,

Synthetic Organic Chemistry Methods

The molybdenum dinitrogen complex trans-[Mo(N2)2(dppe)2] (2) reacted with 2−2.5 equiv of various β-ketonitriles at room temperature to afford the (nitrido)(nitrile-enolato) complexes trans-[Mo(N)(NCCR1COR2)(dppe)2] (4; R1 = H, R2 = 4-MeOCOC6H4, 4-ClC6H4, 4-Tol, 4-MeOC6H4, 2-C4H3O, 2-C4H3S, Pri; R1 = CN, R2 = Me, Ph; dppe = Ph2PCH2CH2PPh2) via the C⋮N triple bond cleavage of the nitriles on the molybdenum center. On the other hand, the reaction of complex 2 with 2 equiv of pivaloylacetonitrile at room temperature led to the isolation of the (alkylideneamido)(nitrile-enolato) complex trans-[Mo(NCHCH2COBut)(NCCHCOBut)(dppe)2] (5k), which further underwent the cleavage of the CN double bond of the alkylideneamido ligand to give the corresponding (nitrido)(nitrile-enolato) complex trans-[Mo(N)(NCCHCOBut)(dppe)2] together with 4,4-dimethyl-1-penten-3-one. Furthermore, treatment of 2 with large excess amounts of 4-chlorobenzoylacetonitrile followed by anion metathesis with [NHEt3][OTf] (OTf = OSO2CF3) resulted in the isolation of the cationic (imido)(nitrile-enolato) complex trans-[Mo{NCH2CH2CO(C6H4Cl-4)}{NCCHCO(C6H4Cl-4)}(dppe)2][OTf] (6c+[OTf]-). The solid-state structures of 4h·1.5C2H4Cl2 (R1 = CN, R2 = Me), 5k·C6H6, and 6c+[OTf]- were determined by single-crystal X-ray analyses. The detailed NMR analysis of the reaction of 2 with aroylacetonitriles revealed that the (alkylideneamido)(nitrile-enolato) complexes trans-[Mo(NCHCH2COR)(NCCHCOR)(dppe)2] (5) act as the key intermediates for the C⋮N triple bond fission, and the rate constants for the conversion of the complexes 5 into the nitrido complexes 4 showed good correlation with the Hammett σp or σa constants for the aroyl substituents, where positive ρ values were obtained (ρp, 1.42; ρa, 0.41). A reaction mechanism for the nitrido complex formation is proposed, which includes (1) the substitution of a dinitrogen ligand in 2 with a β-ketonitrile molecule, (2) the fast protonation of the nitrile ligand by a second β-ketonitrile molecule leading to the formation of complex 5, (3) the relatively slow proton shift from the α-position of the carbonyl group to the amido carbon in the alkylideneamido ligand to form an enolated imido ligand, and (4) the fast elimination of a vinyl ketone from the imido ligand giving the nitrido complex 4 as the final product.