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*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)].

Terminal Alkynes in the Coordination Sphere of Titanocene Complexes - Elementary Steps in Catalytic Dimerizations and Oligomerizations of Alkynes

“…In an attempt to achieve this, we added typical vinylidene trapping reagents, like transition metal carbonyls [9] or isocyanates [22] to the reaction mixtures. However, in the presence of transition metal carbonyls (W(CO) 6 , Mn 2 (CO) 10 [23]. The reaction of 14 with 15 b in the presence of a large excess of acetone lead to a mixture of products containing, among others, a titanacyclopentene derivative, as shown by 13 C NMR measurements.…”

“…Chemical shifts are reported in ppm and referenced to residual protons in deuterated solvents (benzene-d 6 , d = 7.15 ppm for 1 H NMR spectroscopy; benzene-d 6 , d = 127.96 ppm for 13 C NMR spectroscopy). Mass spectroscopic analyses were performed on a Finnigan MAT 95 mass spectrometer.…”

“…[1,2] Indeed, the reaction of the thermally generated titana-allene intermediate [Cp* 2 Ti=C=CH 2 ] (7) (Cp* = C 5 Me 5 ), [3±5] formed by ethene and methane elimination from 9 and 8 respectively, with alkynes yields the titanacyclobutene complexes (12) from which the formation of trans-polyacetylene is observed for R = R' = H in the presence of an excess of the unsubstituted acetylene HC≡CH [6]. Alternatively, the formation of vinyltitanium acetylides (13) is observed in reactions of 7 with acidic alkynes.…”

Regioselectivity in Iron‐Catalyzed [2+2+1] Cycloadditions: A DFT Investigation of Substituent Effects in 1,4‐Diazabutadienes

“…This reaction can also be interpreted as a [3+2] cycloaddition reaction in which a metal is involved leading to a new ferra-pyrrolidine ring and thus is closely related to the formation of titanacyclobutenes. [21] The addition of ethylene leads to the diastereomeric transition states TS 1 and TS 2 , in which a new CÀC bond is formed between ethylene and the imine functional group next to the oxazine nitrogen atom. Alternatively, the new C À C bond is established between ethylene and the imine functional group next to the oxazine oxygen atom (TS 3 , TS 4 ); these products reflect the experimentally observed regioselectivity.…”

“…[20] The most closely related reactions with respect to the subject of our work, in which a metallacycle is formed by a formal cycloaddition reaction was described by Beckhaus et al The reaction of a titanocene vinylidene complex with alkynes, diynes, or isothiocyanates results in methylene-metallacyclobutenes or methylene-metallathietane complexes, respectively. [21] The regioselectivities of the cycloaddition reactions may be attributed to the polarity of the substituted alkyne substrates or the lower polarity of the C=S bond compared to the C=N bond in the latter case.…”