Titanium dihalide

Titanium Diffuoride. Unlike other titanium dihalides, titanium difluoride [13814-20-5] is known only from mass spectra of gases. 

Considerable work in recent years has been devoted to the synthesis of ring-substituted derivatives of his rj -cyclopentadienyl)titanium dihalides. The ring orientation in... 

If metallic Ti is available, the compound may be easily synthesized from the elements (see the procedure for the preparation of titanium dihalides). A weighed amount (5-6 g.) of freshly distilled Br 3 is placed in a thick-wall quartz tube cooled with Dry Ice, crude Ti is added (somewhat more than the stoichiometric quantity), and the tube is sealed under high vacuum. The Brs begins to melt on removal of the coolant the reaction starts immediately and flames appear. After completion of the reaction the tube is opened and the TiBr4 is distilled off it may be purified by multiple distillation (see Til4). 

Bis(cyclopentadienyl) titanium dihalide Ethylene oxide Terephthalic 60)... 

Titanium forms dihalides TiXj, for example titanium(II) chloride, formed by heating titanium metal and the tetrachloride to about 1200 K. TiCl2 is a black solid, which disproportionates on standing to TiCl4 + Ti. Since it reduces water to hydrogen, there is no aqueous chemistry for titanium(II). A solid oxide TiO is known. 

Titanium-based reagents generated by the reduction of gem-dihalides with low-valent metal species are widely used for the alkylidenation of carbonyl compounds (Scheme 14.28). As in the case of methylidenation, the system gem-dibromide/TiCl4/Zn/TMEDA... 

Scheme 14.28. Carbonyl alkylidenation utilizing gem-dihalides and low-valent titanium reagents.

A series of reagents have been developed which are prepared in situ from a geminal dihalide or a dithioacetal [635,730] and a transition metal complex. Titanium-based reagents of this type olefinate a broad range of carbonyl compounds, including carboxylic acid derivatives (Table 3.12), and are a practical alternative to the use of isolated carbene complexes. 

The most widely studied transition metal is titanium. At various times, all oxidation states of titanium (II, III, IV) have been proposed for the active site of titanium-based initiators. Most of the evidence points to titanium (HI) as the most stereoselective oxidation state, although not necessarily the most active nor the only one [Chien et al., 1982]. (Data for vanadium systems indicate that trivalent vanadium sites are the syndioselective sites [Lehr, 1968].) Initiators based on the a-, y-, and 8-titanium trihalides are much more stereoselective (iso-selective) than those based on the tetrahalide or dihalide. By itself, TiCl2 is inactive as an initiator but is activated by ball milling due to disproportionation to TiCl3 and Ti [Werber et al., 1968]. The overall stereoselectivity is usually a-, y-, 8-TiCl , > TiCL > TiCLj P-TiCl3 [Natta et al., 1957b,c],... 

More recently, the doubly lithiated derivatives of [4.1.1]- and [3.1.1]propellasilanes underwent bridging metathesis at the bicyclobutane ends with dihalides of germanium, tin and transition metal titanium, to give the corresponding highly strained metallacyclic propellanes (equation 7)29. 

The dihalides of titanium, formed by reduction of the tetrahalides, are vigorous reducing agents and unstable T1CI2 is inflammable in air. The trihalides, though more stable than the dihalides, are effective reducing agents. Ti(TTT) occurs in aqueous solutions as Ti(1120)( 3, ... 

The trihalides of zirconium, like the dihalides of titanium, are extremely strong reducing agents, reacting even with H20. 

Mikami and Nakai et al. have developed a chiral titanium catalyst for the glyoxylate-ene reaction, which provides the corresponding a-hydroxy esters of biological and synthetic importance [7] in an enantioselective fashion (Scheme 8C.3) [8,9]. Various chiral titanium catalysts were screened [ 10]. The best result was obtained with the titanium catalyst (1) prepared in situ in the presence of MS 4A from diisopropoxytitanium dihalides (X2Ti(OPr,)2 X=Br [11] or Cl [12]) and enantiopure BINOL or 6-Br-BINOL [13], The remarkable levels of enantiose-lectivity and rate acceleration observed with these BINOL-Ti catalysts (1) [14] stem from the... 

Halides of aluminum, silicon, and phosphorus5, tin tetrachloride, titanium tetrachloride, and antimony pentachloride6 did not form complexes with diphenyl tellurium oxide, but converted it to the corresponding diphenyl tellurium dihalide. 

Titanium(III) compounds can be produced by reduction of compounds containing titanium in the +4 state. In aqueous solution Ti3+ exists as the purple Ti(H20)63+ ion, which is slowly oxidized to titanium(IV) by air. Titanium ) is not stable in aqueous solution but does exist in the solid state in compounds such as TiO and dihalides of the type TiX2. 

Titanium tetrachloride and aluminium triethyl form a hydrocarbon soluble complex at low temperatures which decomposes at —30°C to give the trichloride as a major product [32]. Complexes containing tetravalent titanium stabilized by adsorption on titanium trichloride apparently persist in catalysts prepared at Al/Ti ratios below 1.0 [33], but at higher ratios there are some Ti(II) sites present in the catalyst [34]. Analysis shows that at Al/Ti ratios above 1.0 the solid precipitate contains divalent titanium or even lower valency states of the metal [35]. Reduction of TiCl4 with AlEt2 Cl is less rapid and extensive than with AlEts and even at high Al/Ti ratios [36] reduction does not proceed much below the trivalent state. Aluminium alkyl dihalides are still less reactive and reduction to TiClj is slow and incomplete except at high Al/Ti ratios or elevated temperatures [37]. 

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