Organotitanium compound

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Organotitanium compounds in organometallic chemistry contain carbon to titanium chemical bonds. Organotitanium chemistry is the science of organotitanium compounds describing their physical properties, synthesis and reactions. They are reagents in organic chemistry^[1][1]

Brief history

Although the first attempt at an organotitanium compound dates back to 1861, it took until 1953 for the first such compound to arrive. In that year titanium phenyl-tri(isopropoxide) was prepared by Herman and Nelson from titanium isopropoxide, phenyllithium and titanium(IV) chloride. Titanocene dichloride was invented in 1954 and the first methyltitanium compounds were introduced in 1959. Ziegler-Natta catalysts as a major commercial application soon followed for which the Nobel Prize in Chemistry of 1963 was awarded.

Properties

The titanium electron configuration ([Ar]3d²4s²) resembles that of carbon and like carbon the +4 oxidation state dominates and like carbon compounds, those of titanium have a tetrahedral molecular geometry. Their bond lengths are much longer than that of C-C bond, with a recorded 210 picometer for the Ti-C bond in tetrabenzyltitanium. Simple tetraalkyltitanium compounds are monomers in solutions and in the gas-phase while their tetra-alkoxy counterparts can be tetrameric.

Due to its low electronegativity, Ti-C bonds are very polarized towards carbon and in many titanium compounds the carbon atom behaves as a carbanion. Organotitanium compounds are lewis acids with vacant coordination sites which can be occupied by forming octahedral complexes with bidentate ligands such as bipy.

Chemistry

Organotitanium compounds are important reagents in organic chemistry.

• The Ziegler-Natta catalyst (1954) is obtained from titanium(III) chloride and diethyl aluminumchloride and important in ethylene polymerization.

• Methyl titanium trichloride CH₃TiCl₃ (1959) is a nonbasic nucleophilic reagent. It can be prepared by reacting titanium(IV) chloride with dimethylzinc in dichloromethane at -78°C. It is used in nucleophilic addition of methyl groups to carbonyl compoundsan and in SN1 methylation of alkyl halides. Methyltriisopropoxytitanium is a related reagent prepared in situ from titanium isopropoxide, titanium(IV) chloride and methyllithium^[1]

• Tebbe's reagent (1978) is related to the Ziegler-Natta catalyst and 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.

• Another methylenation reagent combination is the carbenoid Lombardo's reagent or Dibromomethane-Zinc-Titanium(IV) Chloride (1982),^[1] for example applied in a conversion of a ketene into an allene:^[1]

Image:LombardoReagent.png

• The Petasis reagent or dimethyl titanocene (1990) 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.

Image:Petasis Reagent.png

• Tebbe's reagent reacts with simple alkenes to titanocyclobutanes which can be regarded as stable olefin metathesis intermediates.^[1] These compounds are reagents in itself such as 1,1-bis(cyclopentadienyl)-3,3-dimethyltitanocyclobutane, the adduct of Tebbe's reagent with isobutene catalysed with 4-dimethylaminopyridine.

See also

• Other chemistries of carbon with other Period 4 elements: organotitanium compounds, organovanadium compounds, organochromium compounds, organomanganese compounds, organoiron compounds,organocobalt compounds, organonickel compounds, organocopper compounds, organozinc compounds, organogallium compounds, organogermanium compounds, organoarsenic compounds and organoselenium compounds.

References