Titanium complexes dimeric

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. 

Diethyl tartrate, the allylic alcohol, and the oxidant r-BuOOH displace the iso-propoxide groups on titanium to form the active Ti-catalyst in a complex ligand exchange pathway. From structural and kinetic studies, " Sharpless proposed that oxygen transfer occurs from a dimeric complex that has one tartaric ester moiety per titanium atom. 

The well-known Sharpless system for the enantioselective epoxidation of allyl alcohols has been investigated [23]. This system employs a tetra-alkoxy titanium precursor, a dialkyltartrate as an auxiliary, and an alkyl hydroperoxide as oxidant, to effect the enantioselective epoxidation. The key intermediate is thought to be a dimeric complex in which titanium is simultaneously coordinated to the chelating tartarate ligand, the substrate in the form of an oxygen bound / -allyl-oxide and an -tert-butylperoxide. 

The chlorotitanium calix[4]arene 83, 84 complexes were reported to be active using conventional heating and microwave (MW) irradiation for the ROP of LA under solvent-free conditions (Scheme 6.7). Nearly full conversion (95%) was observed after only 80 min under MW-heating versus 180 min for the thermal heating method. Unfortunately, a loss of control was observed with the PLAs molecular weights and PDIs. The lowest activity was observed for the dimeric complex 85, presumably due to the severe steric congestion at both titanium centres but resulting in isotactic enriched PLA up to 72% (Pm). ... 

The first hydroaminations by this mechanism were reported by Bergman with zircono-cene complexes and by Livinghouse with monocyclopentadienyl titanium and zirconium complexes. Bergman reported the intermolecular addition of a hindered aniline to an alkyne. The hindrance of the aniline was important to prevent formation of stable dimeric complexes containing bridging imido groups. Livinghouse reported intramolecular reactions that occurred at lower temperatures over shorter times. The intramolecularity of this process allows the [2+2] cycloaddition of the imido complex with the alkyne to be faster than the dimerization. 

With the tetradentate nitrilotriethoxo ligand titanium forms a dimeric complex [Ti2 /r-OC2H4)N(C2H40)2 2(OPr )2], in which one arm of the chelating Ugand forms... 

All ligands 3c through e-H2 form coordination compounds with titanium(IV) ions that possess dendrimer structures. Due to the non-polar Frechet-type dendritic substituents, the compounds are soluble only in solvents with low polarities (mixtures of methanol and chloroform). Consequently, the dimeric complexes are observed as the major species in solution. Fignre 2.6 shows the structural formula of L Lij (30)5X12 ] and the structure derived from x-ray diffraction of the anion of the GO dendrimer [Lij (30)5X12)] . Xhe central triple lithium-bridged part and the dendritic substituents decorating its periphery may be seen. 

Titanium Complexes of Unsaturated Alcohols. TetraaHyl titanate can be prepared by reaction of TYZOR TPT with aHyl alcohol, followed by removal of the by-product isopropyl alcohol. EbuUioscopic molecular weight determinations support its being the dimeric product, octaaHoxydititanium. A vinyloxy titanate derivative can be formed by reaction of TYZOR TPT with vinyl alcohol formed by enolization of acetaldehyde (11) ... 

One of the most famous chiral titanium complexes is the Sharpless catalyst (16), based on a diisopropyl tartarate complex. Nmr studies suggest that the complex is dimeric ia nature (146). An excellent summary of chiral titanium complexes is available (147). 

A new process developed by Institut Francais du Petrole produces butene-1 (1-butene) by dimerizing ethylene.A homogeneous catalyst system based on a titanium complex is used. The reaction is a concerted coupling of two molecules on a titanium atom, affording a titanium (IV) cyclic compound, which then decomposes to butene-1 by an intramolecular (3-hydrogen transfer reaction. ... 

Metal-induced reductive dimerization of carbonyl compounds is a useful synthetic method for the formation of vicinally functionalized carbon-carbon bonds. For stoichiometric reductive dimerizations, low-valent metals such as aluminum amalgam, titanium, vanadium, zinc, and samarium have been employed. Alternatively, ternary systems consisting of catalytic amounts of a metal salt or metal complex, a chlorosilane, and a stoichiometric co-reductant provide a catalytic method for the formation of pinacols based on reversible redox couples.2 The homocoupling of aldehydes is effected by vanadium or titanium catalysts in the presence of Me3SiCl and Zn or A1 to give the 1,2-diol derivatives high selectivity for the /-isomer is observed in the case of secondary aliphatic or aromatic aldehydes. 

Several mono(amidinato) complexes of titanium containing the N,N -bis(trimethylsilyl)benzamidinato ligand have been prepared either by metathe-tical routes or ligand substitution reactions as outlined in Schemes 80 and 81. Trialkoxides are accessible as well as the dimeric trichloride, which can be... 

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