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

Photolysis of [M(CO)(6)] in CH2Cl2 gives [(CO)(5)M(CH2Cl2)] (M = Cr, W). Replacement of CH2Cl2 by arylacetylene, HC=CC6H4R-p (R = Me, H, Br), produces the thermolabile arylacetylene complexes [(CO)(5)M(HC=CC6H4R-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)(5)M=C(NPhH)C(C6H4R-p)=C(C6H4R'-p)H], and 2-azetidin-1-ylidene complexes, [(CO)(5)M=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 2.5-3 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 greater than or equal to 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)(5)W=C(NPhH)C(C6H4Me-p)=C(Ph)H] has been established by X-ray structural analysis. (C) 2000 Elsevier Science S.A. All rights reserved.