CC-COUPLING REACTIONS OF GROUP-6 ALLENYLIDENES WITH YNAMINES - FORMATION OF NEW CYCLOBUTENYLIDENE AND PENTATRIENYLIDENE COMPLEXES

The reaction of the diarylallenylidene complexes [(CO)(5)M=C=C=C(C6H4R -p)(2)] (M = Cr (1), W (2); R = H (a), Me (b) OMe (c), NMe2 (d)) with the ynamines MeC=CNEt2 and PhC=CNEt2 affords two products: alkenylalle nylidene (3, 5) and cyclobutenylidene complexes (4, 6). The alkenylall enylidene complexes [(CO)(5)M=C=C=C(NEt2)C(R')=C(C6H4R-p)(2)] (R' = Me , M = Cr (3), W (5), R = H (a), Me (b), OMe (c), NMe2 (d); R' = Ph, M = Cr (3), R = OMe (e), NMe2 (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, a nd subsequent cycloreversion. The cyclobutenylidene complexes [(CO)(5) M=C-C(R')=C(NEt2)-C=C(C6H4R-p)(2)] (R'=Me, M = Cr (4), W (6), R = H (a ), Me (b), OMe (c), NMe2 (d); R' = Ph, M = Cr (4), R = OMe (e), NMe2 ( 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 temper ature. In contrast, 4d-f and 6d decompose on contact with air, light, or silica. Complex 3d was characterized by an X-ray structural analysi s. The product ratios 3/4 and 5/6 strongly depend on the solvent and t he substitution pattern of both the allenylidene complexes 1 and 2 and the ynamine. In general, decreasing polarity of the solvent increasin gly favors formation of cyclobutenylidene complexes. The solvent depen dence 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. T he ratios 3/4 and 5/6 increase in the series a < b < c < d. Kinetic me asurements of the reaction of 1c,d with the ynamines MeC=CNEt2 and PhC =CNEt2 reveal that the complex pairs 3,4 and 5,6 are formed in paralle l pathways with an associative rate-determining step for each. The rea ctions follow second-order kinetics, first-order in the concentrations of the allenylidene complexes 1,2 and of the ynamines. The activation enthalpies Delta H double dagger are small, and the activation entrop ies Delta S double dagger are strongly negative, Delta S double dagger is more negative for the formation of the alkenylallenylidene complex es than for the formation of the cyclobutenylidene complexes.