Titanium complexes hydrolysis reactions

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. 

The complexes TiXMe(salen) (Scheme 24) are obtained when the dichloro parent compound TiCl2(salen) reacts with AlMe3 by further addition of OEt2 with additional formation of titanium(m) species (salen = N,N -bis(salicyli-dene)ethylenediamine). The reactivity of these compounds has been studied toward magnesium reduction, halide abstraction with SbCl5 and AgBF4, S02 insertion and hydrolysis reactions.73,74... 

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]. 

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. 

This intramolecular nucleophilic acyl substitution reaction of acetylenic carbonates proceeds similarly to afford lactones or a,/3-unsaturated esters after hydrolysis of the resulting titanium complexes (Scheme 12.62) [81]. This sp organotitanium species 94 can also be trapped with an aldehyde, which easily undergoes recycliza-tion to give a substituted butenolide after acidic workup. 

Gagliardi, C. D. Dunuwila, D. Van Vlierberge-Torgerson, B. A. Berglund, K. A. 1992. Reaction kinetics for the hydrolysis of titanium isopropoxide carboxylate complexes. In Better Ceramics Through Chemistry V, edited by Hampden-Smith, M. J. Klemperer, W. G. Brinker, C. J. Mat. Res. Soc. Symp. Proc. 271 257-262. 

Since the hybridization and structure of the nitrile group resemble those of alkynes, titanium carbene complexes react with nitriles in a similar fashion. Titanocene-methylidene generated from titanacyclobutane or dimethyltitanocene reacts with two equivalents of a nitrile to form a 1,3-diazatitanacyclohexadiene 81. Hydrolysis of 81 affords p-ketoena-mines 82 or 4-amino-l-azadienes 83 (Scheme 14.35) [65,78]. The formation of the azati-tanacyclobutene by the reaction of methylidene/zinc halide complex with benzonitrile has also been studied [44]. 

In other examples, also involving propargyl carbonates, the parent derivative 86 was first coupled with 87 - obtained by reaction of 5-octyne with the titanium diiso-propoxide - propene complex at -50 °C, providing the titanated vinylallene 88, which on hydrolysis furnished the vinylallenes 89 in good yield [29]. Carbonate 90 in the presence of a Pd° catalyst readily decarboxylated and yielded the allenylpalladium intermediate 91, which could be coupled with various vinyl derivatives to afford the vinylallenes 92. Since X represents a functional group (ester, acetyl), functionalized vinylallenes are available by this route [30]. 

Unsymmetrical vicinal diols can be prepared from a three-component reaction of aldehydes, CO, and aminotroponiminate-ligated titanium dialkyl complexes. Solutions of Me2TiL,2 (L = N -dimethylaminolroponiminalc) react rapidly with CO at room temperature. Double methyl migration to CO produces an 2-acclonc complex which inserts the aldehyde to afford a titana-dioxolane and releases the unsymmetrical diol upon hydrolysis [65]. 

Reaction of tris(neopentyl) complexes of titanium, zirconium and hafnium with molecular oxygen furnishes the corresponding tris(neopentoxy) complexes [42, 43, 51]. A peroxo complex is an intermediate in this reaction, being relatively stable in the case of titanium [42]. The alkoxide species can also be formed upon reaction with alcohols under mild conditions [42, 52]. The alcoholysis reaction is fast, with a low dependence on the steric hindrance of the alkyl chain [42]. Hydrolysis leads to ](=SiO)M(OH)3] or ](=SiO)2M(OH)2], depending on the precursor species. Deu-... 

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