- Titanocene dichloride
Titanocene dichloride Identifiers CAS number RTECS number XR2050000 Properties Molecular formula C10H10Cl2Ti Molar mass 248.96 g/mol Appearance bright red solid Density 1.60 g/cm3, solid Melting point
Solubility in water sl. sol. with hydrolysis Structure Crystal structure Triclinic Coordination
Dist. tetrahedral Hazards R-phrases , S-phrases NFPA 704 Related compounds Related compounds Ferrocene
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Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Titanocene dichloride is the organotitanium compound with the formula (η5-C5H5)2TiCl2, commonly abbreviated as Cp2TiCl2. This metallocene is a common reagent in organometallic and organic synthesis. It exists as a bright red solid that slowly hydrolyzes in air. Cp2TiCl2 does not adopt the typical "sandwich" structure like ferrocene due to the 4 ligands around the metal centre, but rather takes on a distorted tetrahedral shape.
- 2 NaC5H5 + TiCl4 → (C5H5)2TiCl2 + 2 NaCl
Cp2TiCl2 can also be prepared by using freshly distilled cyclopentadiene:
- 2 C5H6 + TiCl4 → (C5H5)2TiCl2 + 2 HCl
Applications in organic synthesis
Cp2TiCl2 is a generally useful reagent that effectively behaves as a source of Cp2Ti2+. A large range of nucleophiles will displace chloride. Examples:
- The Petasis reagent, Cp2Ti(CH3)2, is prepared from the action of CH3MgCl or MeLi on Cp2TiCl2. This reagent is useful for the conversion of esters into vinyl ethers.
- The Tebbe's reagent Cp2TiCl(CH2)Al(CH3)2, arises by the action of 2 equivalents Al(CH3)3 on Cp2TiCl2.
Cp2TiCl2 is a precursor to many Ti(II) derivatives. Titanocene, TiCp2, is itself so highly reactive that it is not known but it can be trapped by conducting the reduction in the presence of ligands. Reduction of titanocene dichloride results in the fulvalene complex shown in the figure.
Reductions have been investigated using Grignard reagent and alkyl lithium compounds. More conveniently handled reductants include Mg, Al, or Zn. The following syntheses demonstrate some of the compounds that can be generated by reduction of titanocene dichloride in the presence of π acceptor ligands.
- Cp2TiCl2 + 2 CO + Mg → Cp2Ti(CO)2
- Cp2TiCl2 + 2 PR3 + Mg → Cp2Ti(PR3)2 + MgCl2
- Cp2TiCl2 + 2 Me3SiCCSiMe3 + Mg → Cp2TiMe3SiCCSiMe3 + MgCl2
- Cp2TiCl2 + 2 CO + Mg → Cp2Ti(CO)2
With only one equivalent of reducing agent, Ti(III) species result, i.e. Cp2TiCl.
Titanocene equivalents react with alkenyl alkynes followed by carbonylation and hydrolysis to form bicyclic cyclopentadienones, related to the Pauson-Khand reaction). A similar reaction is the reductive cyclization of enones to form the corresponding alcohol in a stereoselective manner.
Reduction of titanocene dichloride in the presence of conjugated dienes such as 1,3-butadiene gives η3-allyltitanium complexes. Related reactions occur with diynes. Furthermore, titanocene can catalyze C-C bond metathesis to form asymmetric diynes.
Derivatives of (C5Me5)2TiCl2
The closest relative to titanocene-ethylene complex is that derived by Na reduction of (C5Me5)2TiCl2 in the presence of ethylene. The Cp compound cannot be made. This pentamethylcyclopentadienyl (Cp*) species undergoes many reactions such as cycloadditions of alkynes.
Titanocene dichloride was investigated as an anticancer drug. The mechanism by which it acts is not understood, but some conjecture that it might be due to its interactions with the protein transferrin.
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