The titanocene vinylidene intermediate [Cp*2TiCCH2] (4) (Cp*, C5(CH3)5), formed by ethylene or methane elimination from Cp*2TiCH2CH2CCH2 (3) and Cp*2Ti(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-butylstannyl)acetylene (12e) by a [2 + 2] cycloaddition to give metallacyclobutenes Cp*2TiCR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 bond 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 substituents, such as 2,2-dimethyl-3-butyne (13m) or (trimethylsilyl)acetylene (13n), the vinyl acetylides Cp*2Ti(CHCH2)(C⋮CR) [R = Ph (19l), t-Bu (19m), SiMe3 (19n)] are isolated in high yields. The structures of Cp*2TiC(CH3)C(CH3)CCH2 (8b) and Cp*2TiC(SiMe3)C(Ph)CCH2 (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)CCH2 [R = Me (25b), Ph (25c)].

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The titanocene vinylidene intermediate [Cp*2TiCCH2] (6) (Cp*:  C5(CH3)5), formed by ethene or methane elimination from Cp*2TiCH2CH2CCH2 (5) and Cp*2Ti(CHCH2)(CH3) (7), respectively, reacts with 1,3-diynes RC⋮C-C⋮CR (R = Me (13a), Ph (13b), SiMe3 (13c), CMe3 (13d)) by a [2 + 2]-cycloaddition, giving metallacyclobutenes Cp*2TiC(C⋮CR)CRCCH2 (9a−d). Using the unsymmetrical 1,3-diyne RC⋮CC⋮CR1 (R = CMe3, R1 = SiMe3 (13e)), a 9:1 mixture of the metallacyclobutenes Cp*2TiC(C⋮CR)CR1CCH2 (9e) and Cp*2TiC(C⋮CR1)CRCCH2 (9e‘) is formed. In all cases, exclusive formation of one regioisomer exhibiting the C⋮CR substituent in the α-position of the metallacycle is observed. The regioselectivity of the formation of 9 is attributed to the polarity of the C⋮C bond as well as the stereochemical conditions, proven by ab initio calculations at the Hartree−Fock (HF) level of theory with the 3-21G(*) standard basis set and molecular modeling. The X-ray structure of 9c is presented. Subsequent reactions of 9 with an excess of the titanocene intermediate 6, to form binuclear metallacycles, are not observed.

Complex a b c d e Ref.