Reactivity of acetylenes toward the titanocene vinylidene fragment [Cp*2Ti=C=CH2]. Formation of methylenetitanacyclobutenes and vinyltitanium acetylides. Crystal and molecular structures of Cp*2TiCR'=CR''C=CH2 (R'=R''=CH3; R'=SiMe(3), R''=C6H5) and Cp*2Ti(CH=CH2)(C CPh)

The titanocene vinylidene intermediate [Cp*Ti-2=C=CH2] (4) (Cp*, C-5(CH3)(5)), formed by ethylene or methane elimination from Cp*2TiCH2CH2C=CH2 (3) and Cp*Ti-2(CH=CH2)(CH3) (11), respectively, reacts with symmetrical alkynes such as acetylene (12a), 2-butyne (12b), 1,2-diphenylacetylene (12c), 1,2-bis(trimethylsilyl)acetylene (12d), and 1,2-bis(tri-n-butyl- stannyl)acetylene (12e) by a [2 + 2] cycloaddition to give metallacyclobutenes I Cp*(2)TiCR=CRC=CH2 (8a-e). When unsymmetrical alkynes are used, such as 2-pentyne (13g), 2,4-hexadiyne (13h), 1,4-diphenyl-1,3-butadiyne (13i), 1-phenyl-1-propyne (13j), or 1-(trimethylsilyl)-2-phenylacetylene (13k), different regioisomers can be isolated. For the reaction products of 4 with 13g and 13j, kinetic and thermodynamic products can be distinguished. In reactions of 1-pentyne (13f) and 4, a 1:1.5 mixture of the [2+2] cycloaddition product Cp*2TiCH=C(nPr)C=CH2 (17f) and the alkyne C-H bone activation product, Cp*2Ti(CH=CH2)(C=C-nPr) (19f), is formed. By using stronger acidic acetylenes, like phenylacetylene (13l), or terminal acetylenes with bulky subsituents, such as 2,2-dimethyl-3-butyne (13m) or (trimethylsilyl)acetylene (13n), the vinyl acetylides Cp*Ti-2-(CH=CH2)(C=CR)[R=Ph (19l), t-Bu (19m), SiMe(3) (19n)] are isolated in high yields. The structures of Cp*2TiC(CH3())=C(CH3)C=CH2 (8b) and Cp*2TiC(SiMe(3))=C(Ph)C=CH2 (17k) were determined. The pseudotetrahedral molecules contain planar cyclobutene rings. The X-ray structure of 191 is presented. The regioselectivity of the formation of 17 and its regioisomer 16, using unsymmetrically substituted acetylenes, is attributed to the polarity of the C=C bond, on the basis of C-13 NMR data. The reactivity of the methylenetitanacy-clobutenes depends on the substituents of the alkynes. The formation of trans-poly(acetylene) occurs via 8a with an excess of acetylene. Analogous reactions of 8b and 8c with C6H11NC are observed in the TiC(R)=C(R) sigma-bond opposite the exo-methylene group, forming five-membered rings Cp*2TiC(=NC6H11)C(R)=C(R)C=CH2[R=Me (25b), Ph (25c)].