Titanium -chloride TiCl

Titanium chloride (TiCl ) Titanium chloride (TiCl ) Zirconium chloride (ZrCl ) Zirconium chloride (ZrCl ) Cobalt chloride ((CcCl ) )... 

Schreiner et al. developed thiourea catalyst as a promising hydrogen donor, which has more benefit in solubility, synthesis and catalytic mrn over number compared with urea catalyst, in the Diels-Alder reaction of A-crotonyloxazolidinone and cyclopentadiene [22,23] (Table 9.7). A,A -Di[3,5-bis(trifluoromethyl)phenyl]thiourea accelerates the reaction and improves stereoselectivity (run 4) similar to a metal catalyst such as aluminium chloride (AICI3) (mn 2) or titanium chloride (TiCls) (run 3). 

Nilson and Pettersson prepared titanium chloride TiCl, and reduced it with sodium metal in a closed steel cylinder. They obtained titanium metal with only 5% contamination. 

Titanium forms three series of salts in which the element is respectively tetra-, tri-, and mono-valent. Thus, titanium and chlorine form titanium tetrachloride, TiCl4, titanium trichloride, TiCl3, and titanium monochloride, TiCl. The two last are unstable and readily pass into the higher chloride. Titanium tetrachloride shows a marked resemblance to tin tetrachloride it unites easily with hydrochloric acid in solution, with formation of the complex acid, ehloro-titanic acid, [TiCl6]tI2, and forms many crystalline products with other chlorides. It also unites with ammonia, forming ammines. 

The chemistry of Lewis acids is quite varied, and equilibria such as those shown in Eqs. (28) and (29) should often be supplemented with additional possibilities. Some Lewis acids form dimers that have very different reactivities than those of the monomeric acids. For example, the dimer of titanium chloride is much more reactive than monomeric TiCL (cf., Chapter 2). Alkyl aluminum halides also dimerize in solution, whereas boron and tin halides are monomeric. Tin tetrachloride can complex up to two chloride ligands to form SnCL2-. Therefore, SnCl5 can also act as a Lewis acid, although it is weaker than SnCl4 [148]. Transition metal halides based on tungsten, vanadium, iron, and titanium may coordinate alkenes, and therefore initiate polymerization by either a coordinative or cationic mechanism. Other Lewis acids add to alkenes this may be slow as in haloboration and iodine addition, or faster as with antimony penta-chloride. 

ThN404CMH42, Thorium, bis(2,4-pentane-dionato)[5,10,25,20-tetraphenylpor-phyrinato(2—)]-, 22 160 TiClC ( io,Titanium(lII)7chlorobis( q -cyclo-pentadienyl)-, 21 84 TiCl2> Titanium chloride, 24 181 TiCl]0]Ci2H24, Titanium(III), trichlorotris-(tetrahydrofiiran), 21 137 TiCl402C8H , Titanium(rV), tetrachlorobis-(tetrahydrofuran)-, 21 135 TiFeHi 4, Iron titanium hydride, 22 90 TiHNbOs, Hydrogen, pentaoxoniobatetita-nate(l-), 22 89... 

The mechanism proposed for the solid titanium chloride catalysts is essentially the same for all catalysts and it is usually referred to as the Cossee-Arlman mechanism [46]. Titanium is hexacoordinated in the TiCls or supported catalysts by four bridging chlorides and one terminal chloride that is replaced by an alkyl (P, for polymer chain) from the alkylating agent (Et2AlCl or EtsAl), and a vacancy that is available for propene coordination (see Fig. 6.12). The front and back of the complex shown are not equiveilent the "blocked" chlorine at the front causes more steric hindrance than the "exposed" one at the back [46]. 

As previously stated, the first polymerization catalysts described by Ziegler consisted of mixtures of TiCU and aluminum trialkyls, which reacted with each other continuously to give TiCls and aluminum alkyl chlorides. However, in much recent research work, and in many commercial systems where titanium chlorides are employed, a preformed suspension of TiCls crystals is prepared separately and added to the reaction vessel. There are four reasons for selecting processes of this type ... 

Titanium(IV) chloride TiCl, -54 Zinc carbonate ZnCOg -34... 

XX is generally considered as the active species formed from titanium chloride and alkyla-luminum components. The in the structure represents an unoccupied (vacant) site of the octahedral titanium complex. XX represents an active titanium site at the surface of a TiCls crystal after modification by reaction with the alkylaluminum component. The titanium atom shares four chloride ligands with its neighboring titanium atoms and has an alkyl ligand (incorporated via exchange of alkyl from the alkylaluminum for chloride) and a vacant orbital. There are molecular mechanics calculations that indicate dimeric Ti2Cl6 may be the active species instead of monomeric TiCls [Monaco et al., 2000]. Other proposals for the active species include bimetallic species that contain both titanium and aluminum [Liu et al., 2002]. To simplify matters, our discussions will center on the monomeric and monometallic titanium species, especially since the mechanistic details of stereoselectivity and activity are essentially the same for both monomeric and dimeric titanium species as well as titanium-aluminum species. 

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