Titanium complexes reduction reactions

Among the appHcations of lower valent titanium, the McMurry reaction, which involves the reductive coupling of carbonyl compounds to produce alkenes, is the most weU known. An excellent review of lower valent titanium reactions is available (195). Titanium(II)-based technology is less well known. A titanium(II)-based complex has been used to mediate a stetio- and regio-specific reduction of isolated conjugated triple bonds to the corresponding polyenes (196). 

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... 

Most titanium(IV) alkyls tend to be reduced by aluminum alkyls in a complicated sequence of reactions accompanied by evolution of alkane and alkene. The catalytic activity of the bis(cyclopentadienyl)titanium-aluminum complexes is associated with the titanium alkyl. Hence, it is very interesting to investigate the mechanism of any reductive reaction. In order to study side reactions in the absence of polymerization, highly alkylated systems completely free of halogen are preferred. Moreover, reduction takes place much faster, the higher the alkyl-group content of the added aluminum alkyl. 

Figure 10.9. Photochemical reactions taking place at the surface of a TiO, pigment (I) photo reduction of the titanium atom resulting in the production of hydroxyl radical (II) reduced titanium atom reacting with oxygen to form an unstable complex (III) reaction of the complex with water to produce peroxyl radicals. Both the hydroxyl and peroxyl radicals can react with the polymer matrix and initiate degradation.

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

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. " ... 

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. 

Intramolecular coupling reactions of group 4 imino-acyl complexes yielding the corresponding free imines are rather rare and mainly involve titanium complexes [35 — 38]. The mechanism of these reactions still remains unclear although a concerted reductive elimination step, that initially leads to an -irnine intermediate complex of the type II, has already been postulated [35] (Fig. 4). 

The use of bis-Cp titanium complexes as catalytic systems for reductive transformations via one-electron transfer in organic reaction has been reviewed.1... 

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