Titanium complexes hydroxy

In 1977, an article from the authors laboratories [9] reported an TiCV mediated coupling reaction of 1-alkoxy-l-siloxy-cyclopropane with aldehydes (Scheme 1), in which the intermediate formation of a titanium homoenolate (path b) was postulated instead of a then-more-likely Friedel-Crafts-like mechanism (path a). This finding some years later led to the isolation of the first stable metal homoenolate [10] that exhibits considerable nucleophilic reactivity toward (external) electrophiles. Although the metal-carbon bond in this titanium complex is essentially covalent, such titanium species underwent ready nucleophilic addition onto carbonyl compounds to give 4-hydroxy esters in good yield. Since then a number of characterizable metal homoenolates have been prepared from siloxycyclopropanes [11], The repertoire of metal homoenolate reactions now covers most of the standard reaction types ranging from simple... 

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... 

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... 

The synthesis of mono-Gp triphenoxo titanium complexes with the chelating tris(2-hydroxyphenyl)amine and tris(2-hydroxy-3,5-dimethylbenzyl)amine has been reported (Scheme 378). Electrochemical experiments provide useful information on the reduction potentials of the compounds, from which it is clear that tris(2-hydroxy-3,5-dimethylbenzyl)amine is a stronger donor than tris(2-hydroxyphenyl)amine. The chelate ring size is also important while the reduction of complex containing tris(2-hydroxyphenyl)amine is largely reversible, the reduction of the tris(hydroxybenzyl)amine derivative is irreversible. In the presence of MAO these compounds show high activity and appreciable selectivity for the preparation of syndiotactic polystyrene.909... 

A novel titanium(iv)-catalysed substitution of a carbon xygen bond by a carbon arbon bond, which relied on a DKR process and led to products resulting from a highly enantioselective carbon allylation, was described in 2004 by Braun and Kotter. By means of the chiral titanium complex depicted in Scheme 2.90, the substitution of a hydroxy, silyloxy, or alkoxy group by an allylic residue was possible, for the first time, in a DKR transformation. 

In 2004, Walsh and coworkers demonstrated that titanium complexes of tra s-l,2-bis(hydroxy-camphorsulfonylamino) cyclohexane were excellent catEilysts for asymmetric ZnPh additions to ketones. The reactions showed excellent enantioselectivities (Scheme 7.49) [79]. The reaction employs the readily available bis(sulfonamide) diol ligand (Scheme 7.49), a sub-stoichiometric amount of titanium tetraisopropoxide, and commercially available diphenylzinc. The reactions were clean, affording high yields of tertiary alcohol in less than 24 h at room temperature. This, in fact, was an improvement over the results reported previously by Dosa and Fu [78]. Additionally, there was no need for methanol as an additive. 

The reaction of diketone with aldehydes in the presence of a stoichiometric amount of a titanium complex, prepared from the tridentate ligands and Ti(O Pr)4, provided 5-hydroxy-p-ketone esters in good yield and high optical purity [118] (Scheme 14.41). 

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. 

The first practical method for asymmetric epoxidation of primary and secondary allylic alcohols was developed by K.B. Sharpless in 1980 (T. Katsuki, 1980 K.B. Sharpless, 1983 A, B, 1986 see also D. Hoppe, 1982). Tartaric esters, e.g., DET and DIPT" ( = diethyl and diisopropyl ( + )- or (— )-tartrates), are applied as chiral auxiliaries, titanium tetrakis(2-pro-panolate) as a catalyst and tert-butyl hydroperoxide (= TBHP, Bu OOH) as the oxidant. If the reaction mixture is kept absolutely dry, catalytic amounts of the dialkyl tartrate-titanium(IV) complex are suflicient, which largely facilitates work-up procedures (Y. Gao, 1987). Depending on the tartrate enantiomer used, either one of the 2,3-epoxy alcohols may be obtained with high enantioselectivity. The titanium probably binds to the diol grouping of one tartrate molecule and to the hydroxy groups of the bulky hydroperoxide and of the allylic alcohol... 

The pharmaceutical interest in the tricyclic structure of dibenz[6,/]oxepins with various side chains in position 10(11) stimulated a search for a convenient method for the introduction of functional groups into this position. It has been shown that nucleophilic attack at the carbonyl group in the 10-position of the dibenzoxepin structure renders the system susceptible to water elimination. Formally, the hydroxy group in the enol form is replaced by nucleophiles such as amines or thiols. The Lewis acids boron trifluoride-diethyl ether complex and titanium(IV) chloride have been used as catalysts. 

The reagents prepared by lithiation (see Section 1.3.3.3.1.2.) and titanium exchange of (S)-(Z)-l-methyl-2-butenyl diisopropylcarbamate106 show a diminished reactivity when compared with those derived from the ( -isomer, indicating that in both metalation steps the doublebond geometry is retained16. After treatment of the lithium -TMEDA complex with chlorotris-(diethylamino)titanium and 2-methylpropanal, the homoaldol adduct (3S,47f)-(Z)-4-hydroxy-1,3,5-trimethyl-l-hexenyl diisopropylcarbamate [( + )-4], is formed with 88% ee16. 

Covalently bonded chiral auxiliaries readily induce high stereoselectivity for propionate enolates, while the case of acetate enolates has proved to be difficult. Alkylation of carbonyl compound with a novel cyclopentadienyl titanium carbohydrate complex has been found to give high stereoselectivity,44 and a variety of ft-hydroxyl carboxylic acids are accessible with 90-95% optical yields. This compound was also tested in enantioselective aldol reactions. Transmetalation of the relatively stable lithium enolate of t-butyl acetate with chloro(cyclopentadienyl)-bis(l,2 5,6-di-<9-isopropylidene-a-D-glucofuranose-3-0-yl)titanate provided the titanium enolate 66. Reaction of 66 with aldehydes gave -hydroxy esters in high ee (Scheme 3-23). 

The aldol condensation of benzaldehyde with the thioacetamide carbanion (RCHCSNRV), derived from the desilylation of the silyl-thioether with tetra-/i-buty-lammonium fluoride, is stereoselective at—80°C producing the erythro isomer of the p-hydroxy thioamide preferentially (Scheme 6.18, R = Me, erythro threo 95 5) via a conformationally mobile intermediate. However, when R is bulky, a greater amount of the threo isomer is formed. Predictably, as the reaction temperature is raised, so the stereoselectively decreases. This procedure contrasts with the corresponding condensation catalysed by titanium salts, where the complexed intermediate produces the threo isomer [47, 48],... 

The films, ca. 50 nm thick, comprised small (2-4 nm) nanocrystals of anatase Ti02, possibly in an amorphous matrix, and were uniform, adherent, and pore free. In contrast, only a small amount of irregular deposit was formed on bare Si. The role of the sulphonate endgroups was believed to promote nucleation of the nanocrystals and/or facilitate attachment of TiOi clusters in solution to the substrate. Hydrolysis of TiCU proceeds through various titanium hydroxy and chloro-hydroxy complex cations. The anionic sulphonate groups could thus promote attachment and nucleation of these cationic complexes. 

Reduction of alkenes to alcohols.3 The combination of TiCI4 and NaBH4 in DMF produces a low-valent titanium-borane complex that converts alkcncs to alcohols in which the hydroxy group is introduced by an anti-Markovnikoff addition. 

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