Titanium complexes reductions

The cyclopentadienyl group is another interesting ligand for immobilization. Its titanium complexes can be transformed by reduction with butyl lithium into highly active alkene hydrogenation catalysts having a TOF of about 7000 h 1 at 60 °C [85]. Similar metallocene catalysts have also been extensively studied on polymer supports, as shown in the following section. 

A Et2Zn-(5, S)-linked-BINOL (21) complex has been found suitable for chemos-elective enolate formation from a hydroxy ketone in the presence of isomerizable aliphatic iV-diphenylphosphinoylimines.103 The reaction proceeded smoothly and /9- alkyl-yS-amino-a-hydroxy ketones were obtained in good yield and high enantioselectivity (up to 99% ee). A titanium complex derived from 3-(3,5-diphenylphenyl)-BINOL (22) has exhibited an enhanced catalytic activity in the asymmetric alkylation of aldehydes, allowing the reduction of the catalyst amount to less than 1 mol% without deterioration in enantioselectivity.104... 

Titanium cesium alum, 6 50 Titanium (II) chloride from disproportionation of titanium (III) chloride, 6 56, 61 Titanium(III) chloride, 6 52, 57 Titanium (IV) chloride, reduction of, with hydrogen, 6 52, 57 Titanium complex compounds, cations, with acetylacetone, [Ti-(C.H. hTiCl, and [Ti(C6H7-0,),]FeCl , 2 119, 120 Titanium(IV) oxide, extraction of, from ilmenite, 5 79, 81 to titanium powder with calcium, 6 47... 

From cyclopentadienyl titanium complexes 49 and di-p-tolylcarbodiimide, a product 50, derived from a reductive coupling reaction, is obtained. ... 

Ellis and coworkers have extensively studied the chemistry of hexacarbonyltitanate(2-), [Ti(CO)6] . Reductive car-bonylation see Reductive Carbonylation) of Ti(CO)4 (trmpe) with (cryptand 2,2,2)potassium naphthalenide at -70 °C followed by warming to room temperature gives the thermally robust [K(cryptand 2.2.2)]2[Ti(CO)6] in quantitative yield. Treatment of Ti(CO)6 with azobenzene gives [Ti(PhN=NPh)(CO)4] (equation 3) in 40-65% yield. Hydrolysis of [Ti(PhN=NPh)(CO)4] " gives 1,2-diphenyUiydrazine and the hydroxo-carbonyl titanium complex, [Ti2(/x-OH)2(CO)8] (equation 3), which was strac-turally characterized as the [K(18-crown-6)]+ salt. ... 

Mixed cyclopentadienyl-diene titanium complexes, Cp TiX(diene)(X = Cl, Br, I), have been prepared in 30-60% yield by the stoichiometric reaction of CpTiXs with (2-butene-l,4-diyl)magnesium derivatives or by the reduction of CpTiXs with RMgX (R = i-Pr, f-Bu, Et X = Cl, Br, I) in the presence of conjugated dienes, as shown in Scheme 4. The butadiene, 1,3-pentadiene, and 1,4-diphenylbutadiene complexes of Cp TiX exhibit a unique prone (endo) conformation (13), while the isoprene, 2,3-dimethylbutadiene, and 2,3-diphenylbutadiene complexes prefer the supine (exo) conformation (14). Reduction of Cp TiX(diene) with RMgX or Mg gives a low-valent species, which catalyzes a highly selective (>99%) tail-to-head linear dimerization of isoprene and 2,3-dunethylbutadiene. " ... 

The mixed cyclopentadienyl-cycloheptatrienyl ( 5-7 ) titanium complex Cp( -C7H7)Ti has been prepared in 33 -40% yield by the reduction of CpTiCb with isopropylmag-nesium bromide or Mg in the presence of cycloheptatriene. Reduction of Cp TiCb in THF with Mg in the presence of cycloheptatriene gives Cp ( -C7H7)Ti in 68% yield. Titanium 5-7 complexes exhibit sandwich structures see Sandwich Compound) with the five-membered and seven-membered rings nearly parallel to each other. 

Mixed 5-8 titanium complexes have been prepared by reduction of Cp TiCb and CpTiCb with Mg in THF in the presence of cyclooctatetraene to give Cp ( ) -C8H8)Ti... 

Enantioselective Reduction of Aromatic Ketones. Aromatic substituted ketones and a-halo ketones are reduced by (EtO>3 SiH with good enantioselectivity in the presence of bis-oxazoline titanium complex [Ti(ciY-DiPh-Box)2F2], prepared from chiral bis-oxazoline, BuLi, and Tip4 (eq 4). 

Chiral titanium complexes are also employed as effective asymmetric catalysts for other carbon-carbon bond-forming reactions, for example addition of diketene (Sch. 66) [154c,162], Friedel-Crafts reaction (Sch. 67) [163] (Sch. 68) [164], iodocar-bocyclization (Sch. 69) [165], Torgov cyclization (Sch. 70) [166], and [2 -i- 1] cycloaddition (Sch. 71) [167]. Asymmetric functional group transformations can also be catalyzed by chiral titanium complexes. These transformations, for example the Sharpless oxidation [168] or hydride reduction [169] are, however, beyond the scope of this review because of space limitations. Representative results are, therefore, covered by the reference list. 

Many soluble catalysts are known which will polymerize ethylene and butadiene. High activity soluble catalysts are employed commercially for diene polymerization but most soluble types are inefficient for olefin polymerization. A few are crystalline and of known structure such as blue (7r-C5H5)2TiCl. AlEtaCl [49] and red [(tt-CsHs )2TiAlEt2 ] 2 [50]. The complex (tt-CsHs )2TiCl2. AlEt2Cl polymerizes ethylene rapidly but decomposes quickly to the much less active blue trivalent titanium complex. Soluble catalysts are obtained from titanium alkoxides or acetyl acetonates with aluminium trialkyls and these polymerize ethylene and butadiene. Several active species have been identified, dependent on the temperature of formation and the Al/Ti ratio. Reduction to the trivalent state is slow and incomplete and maximum activity for ethylene polymerization occurs at about 25% reduction to Ti [51]. 

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