The Story of 1-Alkenyl Complexes of Early Transition Metals - A low Temperature Access to the versatile Titana-allene Building Block [Cp*₂Ti=C=CH₂]

Different rotameres of 1-Alkenyl complexes of early transition metals lead to the selective formation of diene complexes, metalla-cyclopentenes or vinylidene intermediates. In such a way the titaniumvinylidene species [Cp*₂Ti=C=CH₂] (1) can easily be generated from methylenetitanacyclobutane Cp*₂TiC(=CH₂)CH₂CH₂ (2), obtained via a Cp*₂Ti(CH=CH₂)₂ intermediate (3) in quantitative yield, or by a α-H-elimination of methane from the vinyl-methyl derivative, Cp*₂Ti(CH=CH₂)CH₃ (4). The systems 2→1 or 4→1 can be used in a great number of different organometallic reactions. Of considerable interest are syntheses of cycloaddition products, by trapping 1 with cumulenes, transition metal carbonyls, nitriles, phosphaalkynes and alkynes. The obtained products are of high thermal stability due to the stabilization effect of the α-exo-methylene group, the formation of stronger C_sp2-C_sp2-bonds in the four membered rings and the lowered electrophilicity of the titanium center caused by the strongly basic Cp*-ligands. With oxygen containing cumulenes and heterocumulenes, monomeric metallaoxetanes exhibiting a planar ring geometry are obtained for the first time. Due to its reactivity in solution the metallaoxetanes can be divided in classical (forming Ti=O) and nonclassical oxetanes (forming Ti=C). Reactions of 1 with transition metal carbonyls lead to the formation of metallaoxetanes exhibiting a nonclassical behavior. These nonclassical oxetanes rearrange into five-membered cyclic Fischer-carbene complexes upon heating. Starting from cycloaddition products of 1 which exhibit titanacyclobutane or titanacylobutene structures, ring-enlargement reactions could be achieved by using carbon monoxide, nitriles or isonitriles. The formation of trans-polyacetylene is observed in reaction of 1 with an excess of acetylene. An intermolecular carbene-carbene coupling reaction of a Fischer- and a Schrock-carbene ligand by coupling the vinylidene fragment in 1 [Cp*₂Ti=C=CH₂] with the methoxyethylidene moiety of the chromium complex (CO)₅Cr=C(OMe)Me is described. A stabilization of 1 is achieved by complexation with unsaturated copper and gold complexes, forming dinuclear σ,σ-μ-vinylidene complexes.

titanocene - vinylidene complexes - metathesis - metallacycles - binuclear complexes