Titanium complexes hydrolysis

Olefin epoxidation is an important industrial domain. The general approach of SOMC in this large area was to understand better the elementary steps of this reaction catalyzed by silica-supported titanium complexes, to identify precisely reaction intermediates and to explain catalyst deachvahon and titanium lixiviation that take place in the industrial Shell SMPO (styrene monomer propylene oxide) process [73]. (=SiO) Ti(OCap)4 (OCap=OR, OSiRs, OR R = hydrocarbyl) supported on MCM-41 have been evaluated as catalysts for 1-octene epoxidation by tert-butyl hydroperoxide (TBHP). Initial activity, selechvity and chemical evolution have been followed. In all cases the major product is 1,2-epoxyoctane, the diol corresponding to hydrolysis never being detected. 

Titanium alkoxide hydrolysis, however, is more complex. Titanium differs from aluminum in that it does not have multiple forms of oxides [3]. The variability of oxide content in the polymerized alkoxide is described by the following equation ... 

The titanium complex is diamagnetic which, if the titanium is present as Tin, indicates a strong exchange interaction. It reacts with C02 forming THF.Ti(OOCH)2MgCl15, which yields formic acid on hydrolysis and ethyl formate with C2H5I. This indicates that the hydrogen is bound to the carbon atom of the co-ordinated C02 molecule as in (7) or (8). The formation of C—H bonds implies the presence of reactive metal-hydride intermediates.76... 

The structure of titanium complexes affects the formation of hydrated titanium dioxide structure, since rutile and anatase lattices are composed of TiO octahedrons connected in definite manner. The formation of anatase structure occurs when two octahedral complexes form a common vertex. When two octahedrons are united via their edges, rutile structure is formed. Based on this assumption, it is considered that if titanium (IV) complexes with one reactive centre are formed during hydrolysis, anatase structure is formed if there are two reactive centres, then rutile structure is formed. 

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. ... 

The reactivity of the dialkyl complexes TiR2(LL)2 (LL = N,N -dimethylaminotroponiminato) has been widely studied. Reactions with CO and aldehydes or ketones afford unsymmetrical diolato complexes that convert to the corresponding vicinal diols after hydrolysis. CO and acetylene react to form the oxametallacyclopentene complex. Treatment with RNC yields the free imine and low-valent titanium species (Scheme 131). In the reaction with BucNC, free ButN=CMe2 is formed and the addition of benzaldehyde or benzyl reagents affords titanium diolato or enediolato complexes. Thiolato-alkoxo or amido-alkoxo titanium complexes can also be similarly prepared (Scheme 132).123-125... 

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.)... 

Previously, Pasini [27] and Colonna [28] had described the use chiral titani-um-Schiff base complexes in asymmetric sulfide oxidations, but only low selec-tivities were observed. Fujita then employed a related chiral salen-titanium complex and was more successful. Starting from titanium tetrachloride, reaction with the optically active C2-symmetrical salen 15 led to a (salen)titani-um(IV) dichloride complex which underwent partial hydrolysis to generate the t]-0x0-bridged bis[(salen)titanium(IV)] catalyst 16 whose structure was confirmed by X-ray analysis. Oxidation of phenyl methyl sulfide with trityl hydroperoxide in the presence of 4 mol % of 16 gave the corresponding sulfoxide with 53% ee [29]. 

Thus, it is clear why water cannot be used as a solvent Most titanium esters or halides are too reactive and would react with the water solvent rather than the silica surface. One procedure that permits titanation from an aqueous solution is to use a water-soluble titanium complex that resists hydrolysis until after silica dehydration temperatures are reached, when only silanol groups remain. Titanate complexes of triethanolamine [569], acetylacetonate [570], lactate [569], or even peroxide [571] can be used in this way to perform aqueous titanation. 

A-alkynylsulfonamides 174 are useful intermediates for diastereoselective synthesis <04OL727>. An efficient copper-promoted alkynylation of sulfonamide 173 has been developed to afford 174 with completely retained enantiomeric purity. The acetylene-titanium complexes 175, obtained from 174 upon treatment with titanium(II) alkoxide, react with aldehydes 176 to give alcohol 178, after hydrolysis, with virtually complete regio- and /Z-diastereoselectivity and also with high 1,5-diastereoselectivity (up to de = 98 2). The N-... 

Titanium is one of the most important transition metals used in catalytic enantioselective reactions. Whereas rhodium, palladium, copper and ruthenium are rather rare in Nature, and the depletion of natural resources is evoked for these, titanium does not suffer from lack of availability. In fact, it is the 9th most abundant element on Earth and one of the cheapest transition metals. The products resulting from the hydrolysis of titanium complexes are nontoxic and do not cause any environmental problems. This low toxicity has allowed titanium to be used for multiple applications, including medical uses (prostheses, sun screens, etc.). 

Regarding the compounds XLI-XLIII as representatives of di-, tetra- and polynuclear cyclopentadienyl titanium complexes, the optimum cure rates ranged between 38 and 70%, whereby as long as bis(cyclopentadienyl) titanium moieties were present, the antitumor activity was still more pronounced than in the case of XLII containing four mono-(cyclopentadienyl) titanium units. The oxobridged complexes XLI and XLII are typical examples of products formed by hydrolytic reactions after dissolution of I and XXXVIII, respectively, in water. The clearly reduced strength of their antitumor potency in comparison to the parent compoimds underlines that hydrolysis does not seem to be a step producing the intrinsically active species. 

Duthaler and co vorkers used carbohydrate-titanium complexes for synthesis of optically active syn-/i-hydroxy-a-amino acids [51]. These syn-a-aminoaldols vere obtained in moderate yield and excellent syn diaster-eoselectivity, as shosvn in Table 2.25. Transmetalation of the lithium enolate of glycine ester derivative 145 svith chiral titanium complex 146 provided a titanium enolate svhich upon reaction svith a svide variety of aldehydes provided syn-j5-hydroxy-a-amino esters 148. Subsequent hydrolysis and N-protection gave a-aminoaldols 149. 

In 1995, Sato and co-workers reported that low-valent titanium alkoxide prepared from Ti(0 -Pr)4 and f-PrMgCl (1 2) can readily incorporate alkynes to give a titanacyclopropene complex, hydrolysis of which then leads to Z-alkenes with high efficiency and excellent stereoselectivity (Fig. 13) [35]. 

---