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

Abstract

Photolysis of [M(CO)6] in CH2Cl2 gives [(CO)5M(CH2Cl2)] (M=Cr, W). Replacement of CH2Cl2 by arylacetylene, HC=CC6H4R-p (R=Me, H, Br), produces the thermolabile arylacetylene complexes [(CO)5M(HC¡ÔCC6H4R-p)]. Addition of N-phenyl benzylideneimines, PhN=C(C6H4R'-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(C6H4R'-p)H-C(C6H4R-p)H]. The formation of the alkenyl(amino)carbene complexes is favored. The ratio alkenyl(amino)carbene:2-azetidin-1-ylidene complex is 2.5¨C3 for M=W and 6.5¨C8 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(C6H4Me-p) =C(Ph)H] has been established by X-ray structural analysis.