Titanium tetrachloride crystal structure

The crystal structure of the adduct of titanium tetrachloride and the ester formed from ethyl 2-hydroxypropanoate (ethyl lactate) and acrylic acid has been solved. It is a chelated structure with the oxygen donor atoms being incorporated into the titanium coordination sphere along with the four chloride anions. 

Ziegler-Natta catalyst A stereospecific catalyst for polymerization reactions, consisting of titanium tetrachloride and triethylaluminum. zinc-blende structure A crystal structure in which the cations occupy half the tetrahedral holes in a nearly close packed cubic lattice of anions also known as sphalerite structure. 

The dependence of the diastereomeric ratio on the choice of Lewis acid can be understood when considering the geometry of the Lewis acid complex. In the case of the titanium tetrachloride catalysed reaction, the interaction of the ester and the catalyst is strongly supported by the first crystal structure observed of the Lewis acid with a chiral dienophile (Figure 4)118. 

FIGURE 4. The crystal structure of the Lewis add complex of a chiral dienophile with titanium tetrachloride ... 

Similar results have been obtained by polymerization of a or j3-naphthofuran (LXXIV) [200]. Like benzofuran, these monomers are not dissymmetric and it is only when the molecule has reacted that a sequence of asymmetric carbon atom is formed along the main chain they polymerize very easily in presence of aluminum chloride or titanium tetrachloride and j3-phenylalanine the use of c/-alanine or /-histidine as co-catalyst always leads to polymers with no optical activity both for benzo- and naphthofurans. Polynaphto-furan can be crystallized by thermal treatment, in contrast to polybenzofuran and its structure, at least partially, must be considered isotactic. This last work dates back to 1966. Since that time, no other results have been published in this field. 

Oppolzer s auxiliary opened, in addition, an access to a/iti-configured aldol adducts 272 (Scheme 4.62). For this purpose, silyl ketene N,0-acetal 271 was prepared from propionic sultam 92, obtained as a single diastereomer, according to the NMR spectra of the crude product, and isolated as a crystalline compound it was characterized as a cis-silicon enolate by a crystal structure analysis. For the subsequent Mukaiyama aldol addition, titanium tetrachloride was found to be the optimum Lewis acid to yield the awti-diastereomers 272 in excellent diastereoselectivity. Their formation under attack of the enolate to the Re-face of the aldehyde is consistent with an open transition state 275, wherein the Lewis acid-coordinated aldehyde is located on the face opposite to the sulfonyl group (Scheme 4.62) [136b]. An alternative approach to the a fi-aldol adducts was also elaborated, based upon cA-boron enolates 267 when they are reacted with aldehydes in the presence of titanium tetrachloride, an ti-selective aldol addition occurs leading to the products 272 rather than to sy -aldols 268 that result in the absence of the Lewis acid [136c]. 

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