Titanium complexes applications

A variety of aldehyde/alkyne reductive couplings involving the stoichiometric use of early transition metals (Ti and Zr) have been developed (Scheme 27) [68-70]. The low cost and ease of handling of titanium alkox-ides render these stoichiometric processes very practical despite the lack of catalytic turnover. Recent variants of stoichiometric processes involving titanium alkoxides have demonstrated impressive scope in relatively complex applications [71-73]. 

Besides the above electrophiles, the acetylene—titanium complexes react regioselectively with other acetylenes providing the corresponding titanacyclopentadienes. An example of a homo-coupling reaction is shown in Eq. 9.11 [30], which also displays some synthetic applications [30,31]. Especially noteworthy is the highly regioselective cross-coupling reaction of unsymmetrical internal and terminal acetylenes, which is illustrated in Eq. 9.12... 

A full account5 describes the enantioselective carbonyl-ene reaction of glyoxylate esters catalyzed by a binaphthol-derived chiral titanium complex that is potentially useful for the asymmetric synthesis of a-hydroxy esters of biological and synthetic importance.6 The present procedure is applicable to a variety of 1,1-disubstituted olefins to provide ene products in extremely high enantiomeric purity by the judicious choice of the dichloro or dibromo chiral catalyst (see Table). In certain glyoxylate-ene reactions involving removal of a methyl hydrogen, the dichloro catalyst... 

Heterolytic liquid-phase oxidation processes are more recent than homolytic ones. The two major applications are the Wacker process for oxidation of ethylene to acetaldehyde by air, catalyzed by PdCl2-CuCl2 systems,98 and the Arco oxirane" or Shell process100 for epoxidation of propylene by f-butyl or ethylbenzene hydroperoxide catalyzed by molybdenum or titanium complexes. These heterolytic reactions require less drastic conditions than the homolytic ones... 

Carreira et al. used a chiral BlNOL-derived Schiff base-titanium complex as the catalyst for the aldol reactions of acetate-derived ketene silyl acetals (Scheme 8C.29) [64a]. The catalyst was prepared in toluene in the presence of salicylic acid, which was reported to be crucial to attain a high enantioselectivity. A similar Schiff base-titanium complex is also applicable to the carbonyl-ene type reaction with 2-methoxypropene [64b], Although the reaction, when con-... 

K. Mikami, Y. Motoyama, and M. Terada, Asymmetric catalysis of Diels-Alder cycloadditions by an MS-free binaphthol-titanium complex dramatic effect of MS, linear vs positive nonlinear relationship and synthetic applications, J. Am. Chem. Soc., 116 (1994) 2812-2820. 

The Lewis acid-catalyzed conjugate addition of silyl enol ethers to a,y3-unsaturated carbonyl derivatives, the Mukaiyaraa Michael reaction, is known to be a mild, versatile method for carbon-cabon bond formation. Although the development of catalytic asymmetric variants of this process provides access to optically active 1,5-dicarbonyl synthons, few such applications have yet been reported [108], Mukiyama demonstrated asymmetric catalysis with BINOL-Ti oxide prepared from (/-Pr0)2Ti=0 and BINOL and obtained a 1,4-adduct in high % ee (Sch. 43) [109]. The enantioselectiv-ity was highly dependent on the ester substituent of the silyl enol ether employed. Thus the reaction of cyclopentenone with the sterically hindered silyl enol ether derived from 5-diphenylmethyl ethanethioate proceeds highly enantioselectively. Sco-lastico also reported that reactions promoted by TADDOL-derived titanium complexes gave the syn product exclusively, although with only moderate enantioselectiv-ity (Sch. 44) [110]. 

K. Barry Sharpless (bom 1941) received his PhD in 1968 at Stanford University. Since 1990 he is W. M. Keck Professor of Chemistry at the Scripps Research Institute in La Jolla, USA. Among several other important discoveries. Sharpless developed catalysts for asymmetric oxidations. In 1980 he achieved the catalytic asymmetirc oxidation of allylic alcohols to chiral epoxides by utilizing titanium complexes with chiral ligands (e. g. Section 3.3.2). One of the many applications of chiral epoxides is the use of the epoxide (R)-glycidol for pharmaceutical production of beta-blockers. Sharpless received the Nobel prize for chemistry in 2001 together with Knowles and Noyori. 

Matsuoka, T., Harano, K., Uemura, T., Hisano, T. Hetero Diels-Alder reaction of N-acyl imines. I. The reaction of N -thiobenzoyl-N,N-dimethylformamidine with electron-deficient dienophiles. Stereochemical and mechanistic aspects. Chem. Pharm. Bull. 1993, 41, 50-54. Mikami, K., Motoyama, Y., Terada, M. Asymmetric Catalysis of Diels-Alder Cycloadditions by an MS-Free Binaphthol-Titanium Complex Dramatic Effect of MS, Linear vs Positive Nonlinear Relationship, and Synthetic Applications. J. Am. Chem. Soc. 1994, 116, 2812-2820. 

The applications of these ligands have been limited to the work by Nakazawa et al.244 who found tris(pyrazolyl)methane titanium complexes to be high-activity catalysts for the polymerization of olefins, and the use of tris(pyrazolyl)methane zinc complexes to model zinc-containing enzymes, such as dihydrorootase and carbonic anhydrase.245 The structure of the free ligand HC(Me2pz)3 has also been reported.246... 

Hydrolysis, redox, metathetical, and halide abstraction reactions are covered here. Some of these reactions lead to specific complexes with Ti-O, Ti-N, and Ti-C bonds which are described in subsequent sections. Comments on the applications of the mono-Cp trihalo titanium complexes as olefin polymerization pre-catalysts have been mentioned in Section 4.05.3.1.1 and some recent advances in this field are also considered here. (See Chapter 4.09 of this work.)... 

Mono-Cp titanium derivatives show reactivity as catalyst precursors for olefin polymerizations, particularly for the polymerization of styrene and functionalized monomers. A review highlighting the developments in the design and applications of non-metallocene complexes, including mono-Cp derivatives, as catalyst systems for a-olefin polymerization has appeared.440 Titanium complexes bearing Cp in addition to chloro ligands and activated by aluminum... 

The intramolecular coordination chemistry of substituted bis-Cp titanium complexes, dealing with the syntheses, reactions, structures, and some applications in homogeneous and catalytic reactions, has been summarized.1134... 

The extensive use of Cp -titanium complexes in the development of organic synthetic methodology is beyond the scope of this chapter. We include here some examples of the various reaction types in which titanium(iv) derivatives are used as reagents in organic synthesis, although no attempt has been made to cover this subject comprehensively. Section 4.05.8 covers additionally an overview of the applications of titanium complexes in stoichiometric and catalytic organic reactions. 

One of the most remarkable aspect on the bis-Cp titanium derivative chemistry has been the production of new and unprecedented variety of polyolefins. The use of this type of complexes as Ziegler-Natta pre-catalyts for the olefin polymerization has opened new possibilities to produce polyolefins with different properties, and significant effort has been devoted to the design of new bis-Cp catalyst structures. This section summarizes simple aspects related to the polymerization of ct-olefins catalyzed by bis-Cp titanium complexes containing a cr-Ti-C bond. A more comprehensive review of the catalytic applications of titanium complexes in the a-olefin polymerization processes is covered in Chapter 4.09. 

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