Titanacycles derived from reductive coupling of nitriles, alkynes, acetaldehyde, and carbon dioxide with bis(pentamethylcyclopentadienyl)(ethylene)titanium(II)

Article abstract of DOI:10.1021/om00125a008

The reactions of bis(pentamethylcyclopentadienyl)(ethylene)titanium(II), (η-C₅Me₅)₂Ti(η-C₂H ₄) (1), with alkynes, nitriles, carbon dioxide, and acetaldehyde have been examined and the products characterized spectroscopically. The most common reaction pathway observed is the addition of 1 equiv of substrate to 1 affording the respective monomeric metallacyclopentene (η-C₅Me₅)₂TiC(R)=CMeCH₂CH ₂ (R = H and Me), metallacycloimine (η-C₅Me₅)₂TiN=C(R)CH₂CH ₂ (R = CH₃, C₂H₅, t-C₄H₉, and P-CH₃C₆H₄), metallalactone (η-C₅Me₅)₂TiOC(O)CH₂CH ₂, and oxymetallacyclopentane (η-C₅Me₅)₂TiOCH(CH₃)-CH ₂CH₂ products. In each case, the preferred (often exclusive) product from the reaction has the sterically least-hindered substituent adjacent to the titanium atom. Depending upon the reaction conditions and the particular substrate chosen, insertion into a C-H bond or substitution of the C₂H₄ ligand in 1 by the substrate may be observed, as evidenced by the formation of (η-C₅Me₅)₂Ti(C≡CR)(C ₂H₅), where R = CH₃ and t-C₄H₉, and (η-C₅Me₅)₂Ti(η-RC≡CR), where R = CH₃ and C₆H₅. Many of the Ti(IV) products are found to undergo further reactions under relatively mild conditions. For example, carbonylation of the metallacyclopentene (η-C₅Me₅)₂TiC(CH₃)=C(CH ₃)CH₂CH₂ quantitatively yields (η-C₅Me₅)₂Ti(CO)₂ and the corresponding dimethylcyclopentenone while the metallacycloimine compounds (η-C₅Me₅)₂TiN=C(R)CH₂CH ₂ undergo a 1,3 hydrogen shift to generate metallacycloenamine species (η-C₅Me₅)₂TiNHC(R)=CHCH₂, where R = CH₃ and C₂H₅. The latter rearrangement has been found to occur via an intramolecular pathway and is sensitive to the steric bulk of the R group.