REACTIONS OF BETA-CH AGOSTIC ALKENYLZIRCONOCENE COMPLEXES

Hydrozirconation of phenyl(trimethylsilyl)acetylene at ambient temperature yields mixtures of the β-CH agostic alkenylmetallocene complex [inline formula omitted] zirconocene dichloride (5), and the metallacyclopentadiene Cp2ZrC(SiMe3)=CPhCPh=C(SiMe3) (3a). The last product probably is formed from 2a and [Cp2Zr(H)Cl] via hydride/chloride exchange, producing the intermediate [inline formula omitted] the intermediate [inline formula omitted] (4a), which loses dihydrogen and adds 1 equiv of Me3SiC=CPh to give 3a. Photolysis of 2a also yields a mixture of 3a and 5. Complex 3a was characterized by X-ray diffraction. It crystallizes in space group C2/c with cell parameters a = 21.241 (4) Å, b = 10.881 (2) Å, c = 14.692 (3) Å, β = 116.91 (1)°, Z = 4, R = 0.035, and Rw = 0.039. The reaction of 2a with lithioacetylides LiC=CR (R = CH3, Ph) produces metallacycles as well. The complexes [inline formula omitted] (3b,c) are thought to be produced via β-CH agostic alkenylalkynylmetallocene intermediates [inline formula omitted](4b,c). Intramolecular abstraction of the acidic agostic alkenyl proton by the α-alkynyl ligand directly leads to the formation of bis(alkyne)metallocene complexes (7b,c), which undergo rapid ring closure to give 3b and 3c, respectively. Complex 3c (R = Ph) was characterized by X-ray diffraction. It crystallizes in space group P21/n with cell parameters a = 7.798 (1) Å, b = 20.695 (2) Å, c = 15.549 (2) Å, β = 100.15 (1)°, Z = 4, R = 0.035, and Rw = 0.047. From EHMO model calculations it is suggested that the “anomalous” hydrozirconation of diphenylacetylene, for example, which cleanly gives Cp2ZrCl2 and the metallacycle [inline formula omitted] also proceeds via intermediate β-CH agostic (alkenyl) Cp2Zr(X) type intermediates. © 1990, American Chemical Society. All rights reserved.