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)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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)5MC(NPhH)C(C6H4R-p)C(C6H4R′-p)H], and 2-azetidin-1-ylidene complexes, [(CO)5M(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≥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.