Ti complexes

This conceptual link extends to surfaces that are not so obviously similar in stmcture to molecular species. For example, the early Ziegler catalysts for polymerization of propylene were a-TiCl. Today, supported Ti complexes are used instead (26,57). These catalysts are selective for stereospecific polymerization, giving high yields of isotactic polypropylene from propylene. The catalytic sites are beheved to be located at the edges of TiCl crystals. The surface stmctures have been inferred to incorporate anion vacancies that is, sites where CL ions are not present and where TL" ions are exposed (66). These cations exist in octahedral surroundings, The polymerization has been explained by a mechanism whereby the growing polymer chain and an adsorbed propylene bonded cis to it on the surface undergo an insertion reaction (67). In this respect, there is no essential difference between the explanation of the surface catalyzed polymerization and that catalyzed in solution. 

Thus, dearomatization of the furan heterocycle in the 4,5-ti complexes causes electrophilic addition mainly at the uncoordinated carbon C3. The other possibility... 

Scheme 37 shows a reaction sequence leading to a mono(amidinato) indium bia(arylimido) complex. Subsequent treatment of the latter with Ti(NMe2)4 resulted in formation of the heterobimetallic In/Ti complex [But(NPr )2]In /t-NCgH3Pri-2,6 2Ti(NMe2)2 (cf. Sections 111.B.2 and V.A.l)." ... 

Single emission at 675 (exc 455) Single emission at 575 (exc 465 nin) Figure 3.18 Au TI complexes obtained by condensation reactions and their respective luminescent behavior. 

R = t-butyl, 23 phenyl, 2 or trimethylsilyl2 ) have been characterised as Ti-complexes of various transition metal acceptors. 

In reactions similar to those observed with the Zr species 56 (Scheme 9), Waymouth used both chiral (e.g., 66) and achiral constrained geometry Ti complexes to desymmeterize the oxabicycloheptane 65 achieving good yields with modest enantioselectivity (Eq. 9) [29],... 

It should be pointed out that a structurally related bis(imine-phenoxy)Ti complex 54 (Fig. 32) having a C6F5 on the imine-N with MAO activation does not initiate living ethylene polymerization. Interestingly, DFT calculations suggested that there is virtually no interaction between the ortho-F and the 3-H (ortho-F/ 3-H distance 3.66 A) (Fig. 33) [21]. 

Recently, bis(imino)pyridyl Fe(II)-based catalysts have been reported to afford isospecific chain-end controlled propene polymerization occurring through secondary monomer insertion.138 139 Even more recently, catalytic systems based on the octahedral bis(salicylaldiminato)Ti complex have been reported to afford syndiospecific chain-end controlled propene polymerization,140 which possibly occurs through secondary monomer insertion.141... 

A possible support to the latter model comes from its geometrical similarity with the octahedral bis(salicylaldiminato)Ti complex (Figures 122c,d)... 

Cyanohydrination (addition of a cyano group to an aldehyde or ketone) is another classic reaction in organic synthesis. Enantioselective addition of TMSCN to aldehyde, catalyzed by chiral metal complexes, has also been an active area of research for more than a decade. The first successful synthesis using an (5,)-binaphthol based complex came from Reetz s group142 in 1986. Their best result, involving Ti complex, gave 82% ee. Better results were reported shortly thereafter by Narasaka and co-workers.143 They showed that by... 

The 4 A Molecular Sieves System. The initial procedure for the Sharpless reaction required a stoichiometric amount of the tartrate Ti complex promoter. In the presence of 4 A molecular sieves, the asymmetric reaction can be achieved with a catalytic amount of titanium tetraisopropoxide and DET (Table 4-2).15 This can be explained by the fact that the molecular sieves may remove the co-existing water in the reaction system and thus avoid catalyst deactivation. Similar results may be observed in kinetic resolution (Table 4-3).15... 

Use of poly(octamethylene tartrate) in place of dialkyl tartrates offers practical utility since the branched polymers yield hetereogeneous Ti complex catalysts which can be removed by filtration. Overall the work-up procedure is considerably simplified relative to the conventional Sharpless system. In addition, significant induction is shown in the epoxidation of (Z)-allylic alcohols[7] and even with homoallylic[8] species where the dialkyltartrates give very poor results Figure 5.3. Table 5.2 is illustrative of the scope using the polymer ligand. 

In the isoelectronic series (butadiene)M( j8 — CgHg) (M = Ti, Zr, Hf), the Hf complex exhibits an NMR spectrum at > 30 °C consistent with an envelope flip (AG = 17.6 kcalmol-1). The same process can be detected for the Zr complex at > 40 °C only via magnetization transfer experiments (AG > 20 kcalmol-1). The Ti complex exhibits a static structure by NMR spectroscopy16. 

---