TiCl2

Tiianium ll) chloride, TiCl2- Black powder (TiCl4 plus Ti or heat on TiCl3). Strong reducing agent, immediately reduces water. Forms some complexes. 

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. 

The Stock Oxidation-Number System. Stock sought to correct many nomenclature difficulties by introducing Roman numerals in parentheses to indicate the state(s) of oxidation, eg, titanium(II) chloride for TiCl2, iron(II) oxide for FeO, titanium(III) chloride for TiCl, iron(III) oxide for Fe203, titanium(IV) chloride for TiCl, and iron(II,III) oxide for Fe O. In this system, only the termination -ate is used for anions, followed by Roman numerals in parentheses. Examples are potassium manganate(IV) for K2Mn02, potassium tetrachloroplatinate(II) for K PtCl, and sodium hexacyanoferrate(III) for Na3Fe(CN)3. Thus a set of prefixes and terminations becomes uimecessary. 

Other methods iaclude hydrogen reduction of TiCl to TiCl and TiCl2 reduction above the melting poiat of titanium metal with sodium, which presents a container problem plasma reduction, ia which titanium is collected as a powder, and ionized and vaporized titanium combine with chlorine gas to reform TiCl2 on cool-down and aluminum reduction, which reduces TiCl to lower chlorides (19,20). 

Titanium Dichloride. Titanium dichloride [10049-06-6] is a black crystalline soHd (mp > 1035 at 10°C, bp > 1500 at 40°C, density 31(40) kg/m ). Initial reports that the titanium atoms occupy alternate layers of octahedral interstices between hexagonaHy close-packed chlorines (analogous to titanium disulfide) have been disputed (120). TiCl2 reacts vigorously with water to form a solution of titanium trichloride andUberate hydrogen. The dichloride is difficult to obtain pure because it slowly disproportionates. 

TiCl2 maybe prepared either by the disproportionation of TiCl at 475°C in vacuum. 

Alternatively, the TiCl may be reduced using hydrogen, sodium, or magnesium. It follows that TiCl2 is the first stage in the KroU process for the production of titanium metal from titanium tetrachloride. A process for recovery of scrap titanium involving the reaction of scrap metal with titanium tetrachloride at >800° C to form titanium dichloride, collected in a molten salt system, and followed by reaction of the dichloride with magnesium to produce pure titanium metal, has been patented (122,123). 

Hydrate isomerism of TiCl3.6H20, yielding [TiCl2(H20)4]" CU as one of the isomers, has already been referred to (p. 965) and analogous complexes are formed by a variety of alcohols. Neutral complexes, [T1L3X3] have been characterized for a variety of ligands such... 

Apart from TiO and the lower halides already mentioned, the chemistry of these metals in oxidation states lower than 3 is not well established. Addition compounds of the type [TiCl2L2] can be formed with difficulty with ligands such as dimethylformamide and acetonitrile, but their magnetic properties suggest that they also are polymeric with appreciable metal-metal bonding. However, the electronic spectra of Ti in TiCl2/AlCl3 melts and also of Ti incorporated in NaCl crystals (prepared by... 

Mikami et al. have reported that the chiral titanium reagent 12 derived from bi-naphthol and TiCl2(0-i-Pr)2 catalyzes the Diels-Adder reaction of a-bromoacrolein or methacrolein with isoprene or 1-methoxy-l,3-butadiene to afford the cycloadducts with high enantioselectivity [18] (Scheme 1.25). 

Several titanium(IV) complexes are efficient and reliable Lewis acid catalysts and they have been applied to numerous reactions, especially in combination with the so-called TADDOL (a, a,a, a -tetraaryl-l,3-dioxolane-4,5-dimethanol) (22) ligands [53-55]. In the first study on normal electron-demand 1,3-dipolar cycloaddition reactions between nitrones and alkenes, which appeared in 1994, the catalytic reaction of a series of chiral TiCl2-TADDOLates on the reaction of nitrones 1 with al-kenoyloxazolidinones 19 was developed (Scheme 6.18) [56]. These substrates have turned out be the model system of choice for most studies on metal-catalyzed normal electron-demand 1,3-dipolar cycloaddition reactions of nitrones as it will appear from this chapter. When 10 mol% of the catalyst 23a was applied in the reaction depicted in Scheme 6.18 the reaction proceeded to give a yield of up to 94% ee after 20 h. The reaction led primarily to exo-21 and in the best case an endo/ exo ratio of 10 90 was obtained. The chiral information of the catalyst was transferred with a fair efficiency to the substrates as up to 60% ee of one of the isomers of exo3 was obtained [56]. 

In most TiCl2-TADDOLate-catalyzed Diels-Alder and 1,3-dipolar cycloaddition reactions oxazolidinone derivatives are applied as auxiliaries for the alkenoyl moiety in order to obtain the favorable bidentate coordination of the substrate to the catalyst... 

In a more recent study on 1,3-dipolar cycloaddition reactions the use of succi-nimide instead of the oxazolidinone auxiliary was introduced (Scheme 6.19) [58]. The succinimide derivatives 24a,b are more reactive towards the 1,3-dipolar cycloaddition reaction with nitrone la and the reaction proceeds in the absence of a catalyst. In the presence of TiCl2-TADDOLate catalyst 23a (5 mol%) the reaction of la with 24a proceeds at -20 to -10 °C, and after conversion of the unstable succinimide adduct into the amide derivative, the corresponding product 25 was obtained in an endojexo ratio of <5 >95. Additionally, the enantioselectivity of the reaction of 72% ee is also an improvement compared to the analogous reaction of the oxazolidinone derivative 19. Similar improvements were obtained in reactions of other related nitrones with 24a and b. 

Seebach et al., who first developed the TADDOL ligands [53, 67], have also developed a number of polymer- and dendrimer-bound TiCl2-TADDOLate catalysts derived from the monomeric TADDOLs [68]. Application of 10mol% of this type of catalysts, derived from polymers and dendrimers of 27 and 28, respectively, in the... 

The normal electron-demand principle of activation of 1,3-dipolar cycloaddition reactions of nitrones has also been tested for the 1,3-dipolar cycloaddition reaction of alkenes with diazoalkanes [71]. The reaction of ethyl diazoacetate 33 with 19b in the presence of a TiCl2-TADDOLate catalyst 23a afforded the 1,3-dipolar cycloaddition product 34 in good yield and with 30-40% ee (Scheme 6.26). 

It has been more difficult to obtain the exo isomer in the above described reaction. Application of the TiCl2-TADDOLate complex induced fair exo selectivity and up to 60% ee. This was improved by the application of succinimide as an auxiliary for the alkene. This approach has been the only entry to a highly exo selective reaction and up to 72% ee of the exo isomer was obtained. In the Pd(BF4)2-BI-NAP-catalyzed reaction which gave mixtures of the endo and exo isomers, high ee of up to 93% was in a single case obtained for the minor exo isomer. In one case it was also observed that a Zn(OTf)2-BOX complex induced some exo selectivity and up to 82% ee of the exo isomer. 

Fig, 8,9 The calculated model complexes formed between 3-acroloyl-l, 3-oxazolidin-2-one and an achiral analog of TADDOL-TiCl2,... 

Mechanism The mechanism of cracking has not been established. Even the corrosion reaction that is responsible for the initial attack has not been determined. Early work led to the suggestion that chlorine gas was generated and could cause fracture by a cyclic process requiring the formation and decomposition of TiCl2 ... 

The best procedure reported to date for the asymmetric allylation of aldehydes using tributyl(2-propenyl)stannane involves the catalyzed addition with the BINOL-TiCl2 complex as catalyst. Good yields and ee s were obtained for both aromatic and aliphatic aldehydes using 20 mol% of the catalyst127. 

Among one-component polymerization catalysts subhalides of the transition metals are most similar in composition to the traditional Ziegler-Natta catalysts. In this connection, the study of the simplest one-component catalyst of this type (especially TiCl2) is of great importance for the clarification of still disputable problems of the mechanism of polymerization by two-component catalysts. 

High molecular weight linear polyethylene was obtained from ethylene polymerization by TiCl2 (146) this polymer contained only vinyl-type double bonds (less than 0.1 per 1000 carbon atoms). 

Polypropylene formed on TiCl2 contained 25-30% of the isotactic fraction insoluble in hot heptane (146). 

Milling in a ball-mill (10, 149). In the activation of TiCl2 by ballmilling the average oxidation number of the titanium ions was changed however there was no dependence evident between the catalytic activity and the content of Ti(II) in the catalyst the proportional dependence of the activity on the specific surface was not observed either (10). 

In Benning el al. (146) some data on the kinetics of ethylene polymerization in the presence of TiCl2 activated by ball-milling are given. Polymerization was studied at 140-260°C (the solution process in cyclohexane). The first orders of the polymerization rate on the monomer and catalyst concentrations have been established. The polymerization decreased with temperature a sharp drop in rates at about 180-200°C was observed. 

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