REACTIVITY OF ACETYLENES TOWARD THE TITANOCENE VINYLIDENE FRAGMENT [CP-ASTERISK-2TI=C=CH2] - FORMATION OF METHYLENETITANACYCLOBUTENES AND VINYLTITANIUM ACETYLIDES - CRYSTAL AND MOLECULAR-STRUCTURES OF CP-ASTERISK-2TICR'=CR''C=CH2 (R'=R''=CH3 - R'=SIME(3), R''=C6H5) AND CP-ASTERISK-2TI(CH=CH2)(C-CPH)

The titanocene vinylidene intermediate [CpTi-2=C=CH2] (4) (Cp*, C-5(C H3)(5)), formed by ethylene or methane elimination from Cp2TiCH2CH2C= CH2 (3) and CpTi-2(CH=CH2)(CH3) (11), respectively, reacts with symme trical alkynes such as acetylene (12a), 2-butyne (12b), 1,2-diphenylac etylene (12c), 1,2-bis(trimethylsilyl)acetylene (12d), and 1,2-bis(tri -n-butyl- stannyl)acetylene (12e) by a [2 + 2] cycloaddition to give m etallacyclobutenes I Cp(2)TiCR=CRC=CH2 (8a-e). When unsymmetrical alk ynes are used, such as 2-pentyne (13g), 2,4-hexadiyne (13h), 1,4-diphe nyl-1,3-butadiyne (13i), 1-phenyl-1-propyne (13j), or 1-(trimethylsily l)-2-phenylacetylene (13k), different regioisomers can be isolated. Fo r the reaction products of 4 with 13g and 13j, kinetic and thermodynam ic products can be distinguished. In reactions of 1-pentyne (13f) and 4, a 1:1.5 mixture of the [2+2] cycloaddition product Cp2TiCH=C(nPr)C =CH2 (17f) and the alkyne C-H bone activation product, Cp2Ti(CH=CH2)( C=C-nPr) (19f), is formed. By using stronger acidic acetylenes, like p henylacetylene (13l), or terminal acetylenes with bulky subsituents, s uch as 2,2-dimethyl-3-butyne (13m) or (trimethylsilyl)acetylene (13n), the vinyl acetylides CpTi-2-(CH=CH2)(C=CR)[R=Ph (19l), t-Bu (19m), S iMe(3) (19n)] are isolated in high yields. The structures of Cp2TiC(C H3())=C(CH3)C=CH2 (8b) and Cp2TiC(SiMe(3))=C(Ph)C=CH2 (17k) were dete rmined. The pseudotetrahedral molecules contain planar cyclobutene rin gs. The X-ray structure of 191 is presented. The regioselectivity of t he formation of 17 and its regioisomer 16, using unsymmetrically subst ituted acetylenes, is attributed to the polarity of the C=C bond, on t he basis of C-13 NMR data. The reactivity of the methylenetitanacy-clo butenes depends on the substituents of the alkynes. The formation of t rans-poly(acetylene) occurs via 8a with an excess of acetylene. Analog ous 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 r ings Cp2TiC(=NC6H11)C(R)=C(R)C=CH2[R=Me (25b), Ph (25c)].