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

Abstract

The reaction of the diarylallenylidene complexes [(CO)5M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: alkenylallenylidene (3, 5) and cyclobutenylidene complexes (4, 6). The alkenylallenylidene complexes [(CO)5M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 C2C3 bond of 1 and 2, respectively, and subsequent cycloreversion. The cyclobutenylidene = 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 C1C2 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⋮CNEt2 and PhC⋮CNEt2 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 ΔH ⧧ are small, and the activation entropies ΔS ⧧ are strongly negative. ΔS ⧧ is more negative for the formation of the alkenylallenylidene complexes than for the formation of the cyclobutenylidene complexes.

Ferrocenyl-substituted allenylidene complexes of chromium, molybdenum and tungsten: Synthesis, structure and reactivity

“…In contrast, from the corresponding reaction of diarylallenylidene complexes with diethylaminopropyne [14] without subsequent cycloreversion) are obtained. The product ratios strongly depend on the solvent and the substitution pattern at C c .…”

Mesoionic and Related Less Heteroatom-Stabilized N-Heterocyclic Carbene Complexes: Synthesis, Catalysis, and Other Applications

“…345 pentacarbonyl complexes with ynamines. 346 In THF solution, complex 428 slowly eliminates N 2 and rearranges to 429, which limits the utility of this method to generate pyrazolylidene complexes. 4.1.2.…”

Generation of Polyunsaturated Cumulene Chains by Unprecedented Insertions of the Ynamine MeCCNEt2 in Ruthenium(II) Allenylidene Complexes

“…A subsequent cycloreversion reaction yields the alkenyl(amino)allenylidenes 3 and 4 . An analogous mechanism was also proposed in the cycloaddition of ynamines with Group 6 allenylidene complexes 2a…”

“…While simple nucleophilic and electrophilic additions are well‐studied, cycloaddition processes are scarce, in particular those involving CC dipolar substrates 2. In 1998 Fischer and co‐workers reported the synthesis of cyclobutenylidene and alkenylallenylidene Group 6 metal complexes which are formed as a mixture by the cycloaddition of ynamines to both the C α C β and C β C γ bonds of an allenylidene chain, respectively 2a. During our investigations into the reactivity of indenyl–allenylidene complexes [Ru(CCCR 1 R 2 )( η 5 ‐C 9 H 7 )(PPh 3 ) 2 ][PF 6 ] we have shown that the addition of nucleophiles to the unsaturated chain takes place regioselectively at the C γ .…”