Reactivity of Acetylenes toward the Titanocene Vinylidene Fragment [Cp*₂TiCCH₂]. Formation of Methylenetitanacyclobutenes and Vinyltitanium Acetylides. Crystal and Molecular Structures of Cp*₂TiCR‘CR‘‘CCH₂ (R‘ = R‘‘ = CH₃; R‘ = SiMe₃, R‘‘ = C₆H₅) and Cp*₂Ti(CHCH₂)(C⋮CPh)

Abstract

The titanocene vinylidene intermediate [Cp*2TiCCH2] (4) (Cp*, C5(CH3)5), formed by ethylene or methane elimination from Cp*2TiCH2CH2CCH2 (3) and Cp*2Ti(CHCH2)(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-butylstannyl)acetylene (12e) by a [2 + 2] cycloaddition to give metallacyclobutenes Cp*2TiCRCRCCH2 (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*2TiCHC(nPr)CCH2 (17f) and the alkyne C−H bond activation product, Cp*2Ti(CHCH2)(C⋮C-nPr) (19f), is formed. By using stronger acidic acetylenes, like phenylacetylene (13l), or terminal acetylenes with bulky substituents, such as 2,2-dimethyl-3-butyne (13m) or (trimethylsilyl)acetylene (13n), the vinyl acetylides Cp*2Ti(CHCH2)(C⋮CR) [R = Ph (19l), t-Bu (19m), SiMe3 (19n)] are isolated in high yields. The structures of Cp*2TiC(CH3)C(CH3)CCH2 (8b) and Cp*2TiC(SiMe3)C(Ph)CCH2 (17k) were determined. The pseudotetrahedral molecules contain planar cyclobutene rings. The X-ray structure of 19l 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 13C NMR data. The reactivity of the methylenetitanacyclobutenes 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,c and substituted alkynes are not successful. The isomerization of the titanacyclobutenes 16 → 17 indicates a cycloreversion. Selective insertion reactions of 8b and 8c with C6H11NC are observed in the TiC(R)C(R) σ-bond opposite the exo-methylene group, forming five-membered rings Cp*2TiC(NC6H11)C(R)C(R)CCH2 [R = Me (25b), Ph (25c)].