Ti tetraisopropoxide

Efficient homocoupling of the aryl iodonium salt 827 using Zn is catalyzed by Pd(acac)2[708], Homocoupling of the arylsulfonyl chloride 828 as a pseudohalide takes place in the presence of 2 equiv. of Ti tetraisopropoxide[709]. 

Its nominal compn is ultra-fine particle PETN, 20—50 60% AN soln, 0—80 diethyleneglycol, 0—25 guar gum or polyacrylamide, 0.1 —0.3 and w to make 100%. A cross-linking agent (Ti tetraisopropoxide) is added as required... 

Ti02 loaded on AC by adding AC to Ti02 colloid from Ti tetraisopropoxide... 

Ti02 loaded on MWCNT by a modified sol-gel method from Ti tetraisopropoxide with MWCNT as additive. 

Startg. chiral 2,3-oxidoalcohol and benzoic acid in methylene chloride treated with Ti-tetraisopropoxide at room temp, for 0.5 h - intermediate 1,2,3-triol 1-monoester, treated with 4.2 eqs. NaI04 and a little 2% RuCl3 5H20 in 2 2 3 acetonitrile/carbon tetrachloride/water at room temp, for 2 h - product (Y 88%). F.e.s. V.S. Martin et al., Tetrahedron Letters 29, 2701-2 (1988). 

Cyclohexane/TritonX-35/water] - - [Cyclohexane/TiitonX-35/ Ti-tetraisopropoxide ]. 

MOCVD of Titania. Ti02 is often produced by the pyrolysis of a titanium alkoxide, such as titanium ethoxide, Ti(OC2H5)4, in an oxygen and helium atmosphere at 450°C. Another reaction is based on titanium tetraisopropoxide, Ti(OC3H-7)4, with oxygen at 300°C and at low pressure (< 1 Torr).P ]P ] These precursors have low boiling point (ca. 120°C). 

Allylic alcohols can be converted to epoxy-alcohols with tert-butylhydroperoxide on molecular sieves, or with peroxy acids. Epoxidation of allylic alcohols can also be done with high enantioselectivity. In the Sharpless asymmetric epoxidation,allylic alcohols are converted to optically active epoxides in better than 90% ee, by treatment with r-BuOOH, titanium tetraisopropoxide and optically active diethyl tartrate. The Ti(OCHMe2)4 and diethyl tartrate can be present in catalytic amounts (15-lOmol %) if molecular sieves are present. Polymer-supported catalysts have also been reported. Since both (-t-) and ( —) diethyl tartrate are readily available, and the reaction is stereospecific, either enantiomer of the product can be prepared. The method has been successful for a wide range of primary allylic alcohols, where the double bond is mono-, di-, tri-, and tetrasubstituted. This procedure, in which an optically active catalyst is used to induce asymmetry, has proved to be one of the most important methods of asymmetric synthesis, and has been used to prepare a large number of optically active natural products and other compounds. The mechanism of the Sharpless epoxidation is believed to involve attack on the substrate by a compound formed from the titanium alkoxide and the diethyl tartrate to produce a complex that also contains the substrate and the r-BuOOH. ... 

Sharpless epoxidation involves treating an allylic alcohol with titanium(IV) tetraisopropoxide [Ti(0-/Pr)4], tert-butyl hydroperoxide [t-BuOOH], and a specific enantiomer of a tartrate ester. 

The 4 A Molecular Sieves System. The initial procedure for the Sharpless reaction required a stoichiometric amount of the tartrate Ti complex promoter. In the presence of 4 A molecular sieves, the asymmetric reaction can be achieved with a catalytic amount of titanium tetraisopropoxide and DET (Table 4-2).15 This can be explained by the fact that the molecular sieves may remove the co-existing water in the reaction system and thus avoid catalyst deactivation. Similar results may be observed in kinetic resolution (Table 4-3).15... 

The first synthetically useful reaction of titanium complexes of type 4, leading to the formation of two new carbon—carbon bonds, was developed by Kulinkovich et al. [55]. They found that treatment of a carboxylic acid ester with a mixture of one equivalent of titanium tetraisopropoxide and an excess of ethylmagnesium bromide at —78 to —40 °C affords 1-alkylcyclopropanols 9 in good to excellent yields (Scheme 11.2) [55,56], This efficient transformation can also be carried out with sub-stoichiometric amounts of Ti(OiPr)4 (5—10 mol%) [57,58]. In this case, an ethereal solution of two equivalents of EtMgBr is added at room temperature to a solution containing the ester and Ti(OiPr)4. Selected examples of this transformation are presented in Table 11.1 (for more examples, see ref. [26a]). 

Table 11.4. 1,2-Disubstituted cyclopropanols 22 from carboxylic acid esters 8 and 2-substituted ethyl-magnesium halides in the presence of titanium tetraisopropoxide or chlorotitanium triisopropoxide. Entry Starting Product Conditions Yield Ref. Ester R1 R3 [mol% (%) R2 Ti(OR)4] (d. r. Z/Eb) ...

Substituted cyclopropanols were also obtained, albeit in moderate yields, upon reaction of esters such as methyl pentanoate with l,4-bis(bromomagnesium)butane (38) in the presence of titanium tetraisopropoxide. This corroborates the formation of a titanacy-clopropane—ethylene complex 40 from an initially formed titanacyclopentane derivative 39 (Scheme 11.12) [103], Apparently, an ester molecule readily displaces the ethylene ligand from 40, and a subsequent insertion of the carbonyl group into the Ti—C bond, a formal [2S + 2J cycloaddition, leads to the oxatitanacyclopentane 42, the precursor to 1-butylcyclopropanol (43). 

Catalytic Reactions. Certain catalytic reactions show a considerable departure from the linear relationship between the ee of a chiral source and the extent of the asymmetric induction (Scheme 42) 68). The Sharpless epoxidation of geraniol using Ti(IV) tetraisopropoxide modified by enantiomerically pure diethyl tartrate (DET) (Ti DET =1 1) gives the epoxide in 94% ee. Kagan and Agami first found that, by using the tartrate auxiliary in 50% ee, the optical yield varied to 70% ee. This optical yield is considerably higher than the expected value, 47 % ee... 

Cyclic derivatives of 1,2- and 1,3-amino alcohols have been trialled as chiral catalysts in the addition of diethylzinc to benzaldehyde.229 Enantioselective addition of diethylzinc to benzaldehyde is the subject of other reports,230,231 including the use of triazinyl-BINOLs as enantioselective catalysts of addition to araldehydes, using Ti(IV) tetraisopropoxide.232 Two optically active amino thiocyanate derivatives (60) of (-)-norephedrine act as aprotic ligands for enantioselective addition of diethylzinc to aldehydes in up to 96% ee.233 The ee drops drastically if the -SCN group is changed to -SR. 

Chiral bis-sulfonamides have been employed as catalysts of enantioselective addition of a range of organozincs to simple aryl ketones, in ees up to 99%, using Ti(IV) tetraisopropoxide methodology.237... 

Hie first of Sharpless s reactions is an oxidation of alkenes by asymmetric epoxidation. You met vanadium as a transition-metal catalyst for epoxidation with r-butyl hydroperoxide in Chapter 33, and this new reaction makes use of titanium, as titanium tetraisopropoxide, Ti(OiPr)4, to do the same thing. Sharpless surmised that, by adding a chiral ligand to the titanium catalyst, he might be able to make the reaction asymmetric. The ligand that works best is diethyl tartrate, and the reaction shown below is just one of many that demonstrate that this is a remarkably good reaction. 

Sharpless asymmetric epoxidation ° is an enantioselective epoxidation of an allylic alcohol with ferf-butyl hydroperoxide (f-BuOOH), titanium tetraisopropoxide [Ti(0-fPr)4] and (-b)- or (—)-diethyl tartrate [(-b)- or (—)-DET] to produce optically active epoxide from achiral allylic alcohol. The reaction is diastereoselective for a-substituted allylic alcohols. Formation of chiral epoxides is an important step in the synthesis of natural products because epoxides can be easily converted into diols and ethers. 

The SAE is arguably one of the most important reactions discovered in the last 30 years. The SAE converts the double bond of allyl alcohols into epoxides with high enantioselective purity using a titanium tetraisopropoxide catalyst, Ti(0-iPr)4, chiral additive, either L-(+)-diethyl tartrate [(+)-DET, 7.45] or D-(—)-diethyl tartrate [(—)-DET, 7.46], and tert-butyl peroxide (t-BuOOH, TBHP (f-butylhydroperoxide)) as the source of the oxidant in stoichiometric amounts (see section 1.5, references 28-30 of Chapter 1). 

Selected results employing 0.05 mol% loading of each ligand and titanium tetraisopropoxide are illustrated in Table 2. It is noteworthy that the enantioselectivities obtained with the L5/L6/Ti combination are 0.1-20% higher than the L5/L5/Ti combination. These results indicate that the most enantioselective catalyst contains both ligands. 

Diisopropoxy Ti-TADIX)Lates (la) and (Id) are conveniently made from (2) and Titanium Tetraisopropoxide with removal of i-PrOH by evaporation under reduced pressure or by azeotropic distillation (eq 3).i >.g,2a,3e... 

Additions to Aldehydes. Alkylation of aromatic and aliphatic aldehydes with a combination of titanium tetraisopropoxide, Ti(0-/-Pr)4, and diethy Izinc, ZnEt2, in the presence of a catalytic amount of the bis-sulfonamide la leads to formation of (S)-l-phenyl-1-propanol 4 with high enantioselectivity (eq 2, Table 1). Use of the (R,7 )-l,2-(trifluoromethanesulfonamido)-cyclohexane lb [CAS 122833-60-7] allows for an equally selective reaction, but at exceptionally low catalyst loadings. In the case of aromatic aldehydes, these reactions are fairly rapid, requiring at most 2 hours to reach full conversion. 

The reverse regiocontrol, giving 1,2-diols, is observed with DIBAL-H (diisobutylaluminum hydride). The remarkable effect of titanium tetraisopropoxide as an additive to lithium borohydride has also been reported. In this reaction benzene is a better solvent than THF, probably because a Ti complex using both oxygens in epoxy alcohols is formed in benzene before the hydride attack. Other metal hydrides used include sodium hydrogen telluride (NaHTe) and an ate complex derived from DIBAL-H and butyllithium, both of which reduce epoxides to alcohols, although they have been tested with only a small number of examples. In the former case the reaction may proceed via a 2-hydroxyalkyltellurol intermediate. 

Allyltitanium complexes (22) readily add to carbonyl compounds with high regio- and stereo-selection. They are prepared by reaction of a chlorotitanium complex (21) with an allyl-magnesium or -lithium derivative (equation 13). Some of these unsaturated Ti complexes, like (23)-(25) in Scheme 2, obtained from allylmagnesium halides or allyllithium by reaction with titanium tetraisopropoxide or titanium tetramides, are known as titanium ate complexes . The structure of these ate complexes, at least from a formal point of view, can be written with a pentacoordinate Ti atom. Some ate complexes have synthetic interest, as is the case of (allyl)Ti(OPr )4MgBr which shows sharply enhanced selectivity towards aldehydes in comparison with the simple (allyl)Ti(OPr )3. ... 

The same conversion is successfully catalyzed by using in-situ generated complexes of Ti(OPr )4 and tridentate Schiff bases (Stmcture 54), which are derived from substituted salicylaldehydes with chiral aminoalcohols [85]. Another similar chiral reagent is derived from reaction of titanium tetraisopropoxide and the Schiff base of 3,5-di-tert-butylsalicylaldehyde and (5)-valinol. The mechanism and stereoselectivity of these chiral Lewis acids are discussed by Corey and co-workers. Other chiral Ti Schiff base complexes have been employed in asymmetric TMSCN addition to benzaldehyde [85]. 

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