SUBJECTS titanium complexes

Acetylenes are well known to undergo facile trimerizations to derivatives of benzene in the presence of various transition metal catalysts 23). A number of mechanisms for this process have been considered including the intervention of metal-cyclobutadiene complexes 24). This chemistry, however, was subjected to close examination by Whitesides and Ehmann, who found no evidence for species with cyclobutadiene symmetry 25). Cyclotrimeri-zation of 2-butyne-l,l,l-d3 was studied using chromium(III), cobalt(II), cobalt(O), nickel(O), and titanium complexes. The absence of 1,2,3-trimethyl-4,5,6-tri(methyl-d3) benzene in the benzene products ruled out the intermediacy of cyclobutadiene-metal complexes in the formation of the benzene derivatives. The unusual stability of cyclobutadiene-metal complexes, however, makes them dubious candidates for intermediates in this chemistry. Once formed, it is doubtful that they would undergo sufficiently facile cycloaddition with acetylenes to constitute intermediates along a catalytic route to trimers. 

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. 

Heferogeneous olefin epoxidation over solid titania-silica catalysts has been the subject of numerous publications in the open literature. The general picture that emerges is that isolated titanium (IV) species on a silica surface or in a zeolife mafrix are responsible for the high epoxidation activity [2]. This picture is supported by model catalyst work on titanium silasesquioxane complexes [3,4] that form active homogeneous epoxidation catalysts [5] and by various successful atfempfs fo prepare well-defined, site-isolated titanium complexes by grafting molecular precursors on mesoporous silica [6-9]. These site-isolated titanium complexes have been shown to possess catalytic activity in olefin epoxidation. 

The addition of cyanide to an aldehyde or ketone (hydrocyanation) is an old reaction, but it has been the subject of renewed interest since Reetz s discovery that a chiral Lewis acid could be used to catalyze the asymmetric addition of trimethyl-silylcyanide to isobutyraldehyde ([91] reviews [59,92]). The general process, illustrated in Scheme 4.7, usually employs trimethylsilylcyanide because hydrogen cyanide itself catalyzes the addition as well (nonselectively). Most of the catalysts are chiral titanium complexes some of the more selective examples are shown in Table 4.6. A clear mechanistic picture of the titanium catalyzed additions nis not yet emerged. ... 

An efficient stereoselective synthesis of (-l-)-241D (13) using reductivea-mination was reported by Rao and coworkers [22]. Decanal (70) was subjected to an enantioselective Maruoka allylation using titanium complex of (S,S)-BINOL and furnished 71 with 98% ee. Tosylation of the hydroxyl group of 71 followed by azidation gave 72. The azide 72 was reduced to amine followed by protection with CbzCl to give 73. Sharpless AD of 73 with AD mix... 

The efficient syntheses of IVa allows an effective access to the organometallic chemistry of the Tbf moieties. Using the remarkable r -coordinated lithium salts y and VI, monosubstituted titanium(IV) alkoxides and aryloxides become available. The Tbf moiety is coordinated as a sterically demanding Cp derivative in an ii fashion to the titanium centers. The formal benzannula-tion of the fluorenyl system, as present in the Tbf titanium complexes, leads to large mono Cp complexes of high thermal stability compared to ii -fluorenyl titanium derivatives. Due to its heUcal twist, the T -coordinated Tbf ligands show Cl symmetry in the solid state but seem to interconverse in solution. Additionally, the titanium complexes were subjected to polymerization experiments of styrene to some extent. 

For further conversions via the titanium derivative see Section D.l. 3.3.3.8.2.3. The complex of the trimethylsilyl derivative was subjected to an X-ray structure analysis6. 

Complexation of an amino acid derivative with a transition metal to provide a cyanation catalyst has been the subject of investigation for some years. It has been shown that the complex formed on reaction of titanium(IV) ethoxide with the imine (40) produces a catalyst which adds the elements of HCN to a variety of aldehydes to furnish the ( R)-cyanohydrins with high enantioselectivity[117]. Other imines of this general type provide the enantiomeric cyanohydrins from the same range of substrates11171. 

Jeong and co-workers utilized a cobalt-alkyne complex to enhance enantioselectivity of the addition of bis (homoallyl)zinc to propargyl aldehydes 68 by the exaggeration of steric environment. The reaction provided optically enriched propargyl alcohol 69 in the presence of a chiral ligand and titanium tetra(isopropoxide) in excess. Adduct 69 was subjected to PKR to yield optically enriched bicyclic compounds 70 (Equation (39)). ... 

The detail of the structure of the polymerisation centre present in suppported Ziegler-Natta catalysts for a-olefin polymerisation has been the subject of much research effort (e.g./-/2) The catalyst consists of a solid catalyst MgC /TiC /electron donor and a co-catalyst, an aluminium alkyl complexed with an electron donor. Proposed mechanisms for the polymerisation involve a titanium species attached to magnesium chloride with the olefin coordinated to titanium. The detail of the site at which the titanium species is attached is an important area of study in understanding the mechanism of catalysis and several recent papers 10-12) have investigated the surface structure of magnesium chloride and the attachment of TiCl4, in particular the interaction of titanium species with the 100 and 110 planes of a and (3- magnesium chloride. 

Buchwald and coworkers studied Cp2Ti complexes and developed a titanium(II)-catalyzed Pauson-Khand-type reaction (Fig. 14) [133]. When a number of ortho-allylacetophenones 42 was subjected to 5-20 mol% of Cp2Ti(PMe3)2 in the... 

Ti(0 Bu)4 (100 mM) and carbobenzyloxy-L-alanine (Cbz-L-Ala, 25 mM) were mixed in toluene/ethanol and stirred at room temperature for more than 12 h. After addition of water and aging for several hours, the stock solution was diluted with toluene, and subjected to the surface sol-gel process. Uniform adsorption was observed up to 15 cycles with frequency shifts of 140-160 Hz per cycle. The template molecule, Cbz-L-Ala, was removed from the gel film by dipping in 1 wt % aqueous ammonia, as confirmed from the disappearance of characteristic peaks of the titanium-carboxylate complex and carbamate in reflection FT-IR spectra. 

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