Trichloro(cyclopentadienyl)titanium1

[1270-98-0]  · C5H5Cl3Ti  · Trichloro(cyclopentadienyl)titanium  · (MW 219.33)

(preparation of various cyclopentadienyltitanium(IV) complexes,1 e.g. enantioselective templates by exchanging two of the chloride ligands with chiral alkoxides;2 replacement of the third chloride by allyl groups3 or by ester enolates4 affords reagents for stereoselective allyltitanation of aldehydes2b,3 and for aldol reactions,4 respectively; combination with metal hydrides or with n-BuLi results in low-valent species, reagents for various reductions;5 catalysts for alkene polymerizations are obtained by activation with organoaluminum compounds, methylaluminoxane, or aluminosilicates;6 Ti-imido complexes derived from primary amines undergo intramolecular cyclizations with triple bonds;7 the Lewis acidity is used to cleave epoxides and acetals;8 methyl or ethyl derivatives can be used for the chemoselective alkyltitanation of aldehydes in the presence of ketones9)

Physical Data: mp 208-211 °C (benzene); crystal structure.10

Solubility: sol aromatic hydrocarbons (toluene, dichlorobenzene, xylene), CH2Cl2, CHCl3, Et2O, THF, similar aprotic solvents.

Form Supplied in: yellow-orange powder or crystals.

Analysis of Reagent Purity: 1H NMR d (CDCl3): 7.06 ppm (impurity, partial hydrolysis: 6.92 ppm); 13C NMR: 123.4 ppm (impurity: 122.2 ppm).

Preparative Methods: the title reagent CpTiCl3 (1) is most conveniently prepared by ligand redistribution between Dichlorobis(cyclopentadienyl)titanium and Titanium(IV) Chloride,1,11 or by reaction of 1-trimethylsilyl-2,4-cyclopentadiene with TiCl4.12 Another method involves chlorination of Cp2TiCl2 with Sulfuryl Chloride/Thionyl Chloride13 or titanation of cyclopentadienyl-Tl.14

Purification: by recrystallization in the presence of HCl(g)1,11 followed by sublimation in high vacuum.10

Handling, Storage, and Precautions: the crystalline solid must be protected from moisture and prolonged exposure to UV light should be prevented. Reactions should be carried out in dry equipment with absolute solvents in an inert atmosphere (Ar, N2).

Preparation of Chiral Cyclopentadienyldialkoxytitanium Chlorides.

Cyclopentadienyltitanium complexes can be prepared from CpTiCl3 (1) by exchanging one, two, or all three of the chloride ligands.1 Reaction with alcohols (ROH) results in spontaneous substitution of one chloride with evolution of HCl, affording monoalkoxides (2). For the second substitution to the dialkoxides (3) the HCl has to be withdrawn from the equilibrium (2) &ibond; (3) by neutralization with a tertiary amine, e.g. Triethylamine.3 Preferred solvents for these ligand exchanges are Et2O and hydrocarbons. In the case of diols leading to favorable cyclic Ti complexes, e.g. (R,R)-(4), the second substitution can also be effected by heating in toluene under a slow stream of Ar or N2 (eq 1).2b

The use of two identical chiral alcohols or a C2-symmetrical chiral diol leads to chiral Ti complexes without a stereogenic metal center. Chloro(cyclopentadienyl)bis[3-O-(1,2:5,6-di-O-isopropylidene-a-D-glucofuranosyl)]titanium (5)2a,3 and Chloro(h5-cyclopentadienyl)[(4R,trans)-2,2-dimethyl-a,a,a,a-tetraphenyl-1,3-dioxolane-4,5-dimethanolato(2-)-Oa,Oa]titanium ((R,R)-4)2b have been successfully used for the enantio- and diastereoselective additions of nucleophiles to achiral and chiral aldehydes.2c,d The cyclic complex (R,R)-(4) and its enantiomer are especially suited for the transfer of allyl and terminally monosubstituted allyl groups, affording homoallyl alcohols of excellent optical purity. The branched anti-isomers are obtained in the case of substituted nucleophiles.2b While the titanate (5) with ligands derived from D-glucose can be used for allyltitanations as well,3 its optimal use is for highly stereoselective aldol reactions of acetate,4a propionate,4b and glycine ester enolates.4c In this case the enantiomer is not readily available.

Reductions.

Reducing agents can be obtained by the combinations CpTiCl3/Lithium Aluminum Hydride,5a,b,e,f CpTiCl3/Sodium Borohydride,5d CpTiCl3/n-Butyllithium,5c,6a and CpTiCl3/Sodium Naphthalenide.5g The four-stage conversion of CpTiCl3 (1) and LiAlH4 has been studied by calorimetric titration; the titanium is thereby reduced to TiIII.5a CpTiCl3/LiAlH4 has been used for inter- and intramolecular pinacol couplings (eq 2),5b deoxygenations of endoxides (eq 3),5e and reduction of acetals.5f

Reaction of CpTiCl3 (1) with BuLi gives a TiII species, which catalyzes the hydrogenation of simple alkenes.5c,6a This catalyst can be immobilized on alumina.5c The combination CpTiCl3/NaBH4 allows the selective reduction of aromatic iodides in the presence of chlorides and bromides.5d The reducing power of Alumina-supported (1) and Na naphthalenide is sufficient for nitrogen fixation, i.e. the conversion of N2 to NH3.5g Similar reagents can be obtained from TiCl4, Titanium(III) Chloride, Cp2TiCl2, aluminum chlorides, or by using Potassium Hydride, Magnesium, and Magnesium Amalgam as reducing components.

Alkene Polymerization.

While CpTiCl3 (1) is inactive, efficient catalysts for alkene polymerizations are obtained in combination with methylaluminoxane (MAO).6c,d,e,g The reaction between (1) and MAO has been studied by EPR.6c Instead of MAO, Diethylaluminum Chloride6a and aluminosilicates6b,f have also been used for activation. The prominent features of these catalysts are the stereoregular syndiotactic polymerization of styrene6c,e and the cis-1,4-polymerization of certain 1,3-dienes.6e,g In contrast, a regioirregular polymer is obtained from propene.6f Stereoregular Ziegler-Natta-type polymerization catalysts, especially for polypropylene, can also be obtained from ansa-bridged titanocenes or zirconocenes with planar chirality.15

Lewis Acid Catalysis and Other Applications.

Reaction of CpTiCl3 (1) with primary amines gives highly reactive Ti-imido complexes which undergo rapid but reversible cyclodimerization. These imides undergo facile intramolecular [2 + 2] cycloadditions with triple bonds, affording highly strained titanacycles (6), which, under catalytic conditions (0.2 equiv. 1) are further transformed to 1-pyrrolines or 1-piperines,7a e.g. (7), an elaborated intermediate for the alkaloid monomorine (eq 4).7b With stoichiometric amounts of (1) the intermediate (6) can be trapped by hydrolysis, C-acylation with nitriles, or N-acylation with acid chlorides (eq 5).7a

Further applications of CpTiCl3 (1) include regioselective opening of propylene oxide8a and, in combination with Sodium Iodide, the cleavage of cyclohexanone acetals8b CpTiCl2Me can be prepared from (1) with Me2Zn, Trimethylaluminum, Methyllithium, or MeMgCl. In competition experiments it transfers its methyl group to aldehyde and ketone carbonyls (much like Methyltitanium Triisopropoxide), with high preference for aldehydes. However, an advantage over (i-PrO)3TiMe is its thermal stability, also exhibited by the ethyl analog.9


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Andreas Hafner

Ciba-Geigy, Marly, Switzerland

Rudolf O. Duthaler

Ciba-Geigy, Basel, Switzerland



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