CC-Coupling Reactions of Group 6 Allenylidenes with Ynamines:  Formation of New Cyclobutenylidene and Pentatrienylidene Complexes

Abstract

The reaction of the diarylallenylidene complexes [(CO) 5 M C C C(C 6 H 4 R- p ) 2 ] (M = Cr ( 1 ), W ( 2 ); R = H ( a ), Me ( b ), OMe ( c ), NMe 2 ( d )) with the ynamines MeC⋮CNEt 2 and PhC⋮CNEt 2 affords two products: alkenylallenylidene ( 3, 5 ) and cyclobutenylidene complexes ( 4, 6 ). The alkenylallenylidene complexes [(CO) 5 M C C C(NEt 2 )C(R‘) C(C 6 H 4 R- p ) 2 ] (R‘ = Me, M = Cr ( 3 ), W ( 5 ), R = H ( a ), Me ( b ), OMe ( c ), NMe 2 ( d ); R‘ = Ph, M = Cr ( 3 ), R = OMe ( e ), NMe 2 ( f )) are formed via cycloaddition of the C⋮C bond of the ynamine to the C 2 C 3 bond of 1 and 2, respectively, and subsequent cycloreversion. The cyclobutenylidene = Me, M = Cr ( 4 ), W ( 6 ), R = H ( a ), Me ( b ), OMe ( c ), NMe 2 ( d ); R‘ = Ph, M = Cr ( 4 ), R = OMe ( e ), NMe 2 ( f )) are formed by cycloaddition of the ynamines to the C 1 C 2 bond of 1 and 2 . The compounds 3, 4a − c, 5, and 6a − c are stable at room temperature. In contrast, 4d − f and 6d decompose on contact with air, light, or silica. Complex 3d was characterized by an X-ray structural analysis. The product ratios 3 / 4 and 5 / 6 strongly depend on the solvent and the substitution pattern of both the allenylidene complexes 1 and 2 and the ynamine. In general, decreasing polarity of the solvent increasingly favors formation of cyclobutenylidene complexes. The solvent dependence indicates that the transition state for the formation of 4 and 6 is significantly less polar than that for the formation of 3 and 5 . The ratios 3 / 4 and 5 / 6 increase in the series a < b < c < d . Kinetic measurements of the reaction of 1c, d with the ynamines MeC⋮CNEt 2 and PhC⋮CNEt 2 reveal that the complex pairs 3, 4 and 5, 6 are formed in parallel pathways with an associative rate-determining step for each. The reactions follow second-order kinetics, first-order in the concentrations of the allenylidene complexes 1, 2 and of the ynamines. The activation enthalpies Δ H ⧧ are small, and the activation entropies Δ S ⧧ are strongly negative. Δ S ⧧ is more negative for the formation of the alkenylallenylidene complexes than for the formation of the cyclobutenylidene complexes.