Although the first attempt to prepare an organotitanium compound dates back to 1861, the first example was not reported until 1954. In that year titanocene dichloride was described by Wilkinson and Birmingham. Independently, titanium-based Ziegler–Natta catalysts were described leading to major commercial applications, for which the 1963 Nobel Prize in Chemistry was awarded. This technology underscored the technical significance of organotitanium chemistry.
Properties
The titaniumelectron configuration vaguely resembles that of carbon and like carbon, the +4 oxidation state dominates. Titanium is however a much larger element than carbon, reflected by the Ti-C bond lengths being about 30% longer, e.g. 210 pm in tetrabenzyltitanium vs a typical C-C bond of 155 pm. Simple tetraalkyltitanium compounds however are not typically isolable, owing to the large size of titanium and the electron-deficient nature of its tetrahedral complexes. More abundant and more useful than the simple tetraalkyl compounds are mixed ligand complexes with alkoxide and cyclopentadienyl coligands. Titanium is capable of forming complexes with high coordination numbers. In terms of oxidation states, most organotitanium chemistry, in solution at least, focuses on derivatives of Ti and Ti. Ti compounds are rarer, examples being titanocene dicarbonyl and Ti22. 2− is formally a complex of Ti. Although Ti is involved in Ziegler–Natta catalysis, the organic derivatives of Ti are uncommon. One example is the dimer Bistitanium chloride|2. The oxidation states −1, 0, +1 are also known in organotitanium compounds. Due to the low electronegativity of titanium, Ti-C bonds are polarized toward carbon. Consequently, alkyl ligands in many titanium compounds are nucleophilic. Titanium is characteristically oxophilic, which presents challenges to handling these compounds, which require air-free techniques. On the other hand, high oxophilicity means that titanium alkyls are effective for abstracting or exchanging organyl ligands for oxo groups, as discussed below.
Compounds
Alkyl titanium chlorides and alkoxides
Organotitanium compounds are somewhat useful reagents in organic chemistry. Many of these reagents and catalysts are incompletely defined. At least from the commercial perspective, the most useful organotitanium compounds are generated by combining titanium chloride and diethylaluminium chloride. As Ziegler–Natta catalysts, such species efficiently catalyze the polymerization of ethylene. The process is heterogeneous and no organotitanium intermediates have been well characterized for this process. Numerous organotitanium reagents are produced by combining titanium tetrachloride, titanium tetraalkoxides, or mixtures thereof with organolithium, organomagnesium, and organozinc compounds. Such compounds find occasional use as stoichiometric reagents in organic synthesis. "Methyltitanium trichloride", nominally CH3TiCl3, can be prepared by treating titanium chloride with dimethylzinc in dichloromethane at -78 °C. It delivers a methyl groups to carbonyl compounds and alkyl halides. "Methyltriisopropoxytitanium" is a related reagent. A dialkyltitanium species is implicated for Ti-promoted cyclopropanations starting from a Grignard reagent and an ester. This reaction is the basis of the Kulinkovich reaction. The "Lombardo's reagent" is a methylenation reagent. It is functionally related to the Dibromomethane-Zinc-Titanium Chloride reagent. This chemistry addresses a shortcoming of the Wittig reagent by methylenating enolisable carbonyl groups without loss of stereochemical integrity. It can for example also be applied in a conversion of a ketene into an allene:
Titanocene derivatives
A particularly rich area of organotitanium chemistry involves derivatives of titanocene dichloride. Tebbe's reagent is prepared from titanocene dichloride and trimethylaluminium. It is used as a methylenation agent for carbonyl compounds. It is an alternative for Wittig reagents when the carbonyl group is sterically challenged or when it easily forms the enol. Tebbe's reagent itself does not react with carbonyl compounds, but must first be treated with a mild Lewis base, such as pyridine, which generates the active Schrock carbene. Tebbe's reagent adds simple alkenes to give titanocyclobutanes, which can be regarded as stable olefin metathesis intermediates. These compounds are reagents in itself such as 1,1-bis-3,3-dimethyltitanocyclobutane, the adduct of Tebbe's reagent with isobutene catalysed with 4-dimethylaminopyridine. , The Petasis reagent or dimethyl titanocene is prepared from titanocene dichloride and methyllithium in diethyl ether. Compared to Tebbe's reagent it is easier to prepare and easier to handle. It is also a methylenation reagent. The Nugent-RajanBabu reagent is a one-electron reductant used in synthetic organic chemistry for the generation of alcohols via anti-Markovnikov ring-opening of epoxides, and is generated as a dimer 2 and used in situ from titanocene dichloride.
Titanocene
Early work on "titanocene" itself eventually revealed that this species was a fulvalene complex. The titanocene dimer was recognised in the 1970s but not structurally characterised until 1992, and the investigations led to many innovations on cyclopentadienyl complexes of titanium. Only in 1998 was a true titanocene derivative identified, the paramagnetic species 2Ti.
MonoCp compounds
Less useful in organic chemistry but still prominent are many derivatives of titanium trichloride, TiCl3. This piano-stool complex is obtained by the redistribution reaction of titanocene dichloride and titanium tetrachloride. With an electron count of 12, it is far more electrophilic than the 16e titanocene dichloride.
