Addition of N-aryl imines to alkyne(pentacarbonyl)chromium and tungsten - A novel route to alkenyl(amino)carbene complexes

Article abstract of DOI:10.1016/S0022-328X(00)00010-3

Photolysis of [M(CO)₆] in CH₂Cl₂ gives [(CO)₅M(CH₂Cl₂)] (M = Cr, W). Replacement of CH₂Cl₂ by arylacetylene, HC≡CC₆H ₄R-p (R = Me, H, Br), produces the thermolabile arylacetylene complexes [(CO)₅M(HC≡CC₆H₄R-p)]. Addition of N-phenyl benzylideneimines, PhN = C(C₆H₄ R′-p)H (R′ = Me, H, Cl), to solutions of these alkyne complexes affords alkenyl(amino)carbene complexes, [(CO)₅M=C(NPhH)C (C₆H₄R-p))=C(C₆H₄R′-p))H], and 2-azetidin-1-ylidene complexes, [(CO)₅M=C-NPh-C(C ₆H₄R′-p)H-C(C₆H₄R-p)H]. The formation of the alkenyl(amino)carbene complexes is favored. The ratio alkenyl(amino)carbene/2-azetidin-l-ylidene complex is 2.5-3 for M = W and 6.5-8 for M = Cr. Both types of complexes are obtained as mixtures of isomers. The 2-azetidin-1-ylidene complexes are very likely formed by cycloaddition of the imines to the C=C bond of vinylidene complexes resulting from tautomerization of the alkyne complexes. The cycloaddition is highly stereoselective. Predominantly, the syn isomer is obtained (syn/anti ≥ 9). In contrast, the alkenyl(amino)carbene complexes are presumably derived from the alkyne complexes via cycloaddition of the imines to the coordinated alkyne and subsequent 1,2-hydrogen shift and ring opening. Preferentially, the E isomers (where both aryl substituents are cis with respect to the C=C bond) are produced. The structure of the major isomer of the alkenyl(amino)carbene complex [(CO)₅W=C(NPhH)C(C₆ H₄Me-p)=C(Ph)H] has been established by X-ray structural analysis.