Catalysis titanium

In 2009, Gau and coworkers demonstrated the easy preparation of a series of ALArEt fTHF) reagents from reactions of AlEt2Br(THF) with ArMgBr. These workers proved that these compounds 

---

The most active d metal peroxo complexes toward nucleophilic substrates, like amines, phosphines, thioethers, double bonds etc., are molybdenum, tungsten and rhenium derivatives vanadium and titanium catalysis is also important, in particular when... 

Imines and their derivatives could be used in an analogous way to aldehydes, ketones, or their derivatives this subject has been reviewed [79]. A competition experiment between an aldimine and the corresponding aldehyde in the addition to an enol silyl ether under titanium catalysis revealed that the former is less reactive than the latter (Eq. 14) [80]. In other words, TiCU works as a selective aldehyde activator, enabling chemoselective aldol reaction in the presence of the corresponding imine. (A,0)-Acetals could be considered as the equivalent of imines, because they react with enol silyl ethers in the presence of a titanium salt to give /5-amino carbonyl compounds, as shown in Eqs (15) [81] and (16) [79,82]. 

Alkenylsilanes and -stannanes, and arylsilanes and -stannanes are useful reagents for transfer of an sp -carbon unit to electrophiles under titanium catalysis. Epoxides are opened by TiCE to generate cationic carbon, which is successfully trapped with bis(trimethylsilyl)propene as an aUcenylsilane (Eq. 122) [305]. Other Lewis acids, for example ZnCla, SnCU, and BF3 OEt2, proved less satisfactory. Cyclic epoxides such as cyclopentene and cyclohexene oxides gave poorer yields. An intramolecular version of this reaction proceeded differently (Eq. 123) [305]. Eqs (124) and (125) illustrate diastereoselective alkenylation and arylation of (A,0)-acetals that take advantage of the intramolecular delivery of alkenyl and aryl groups [306], Cyclic ethers... 

Titanium alkoxide is quite effective, presumably as an acid/base catalyst, at facilitating transesterification between esters and alcohols [511-513]. The reaction conditions are mild and relatively hindered alcohols can be used. Methyl phenylacetate has been transformed to other esters of relatively hindered alcohols under the influence of Ti(OEt)4 (Eq. 216) [514]. Ethyl (or methyl) esters of a variety of functionalized carboxylic acids could be converted into menthyl esters in good yields under titanium catalysis (Eq. 217) [514]. 

In contrast to the epoxidation of allyl alcohols and their derivatives described above, that of simple olefins under titanium catalysis seems to be undeveloped [604,605]. 

Several researchers have reported synthetic approaches based on asymmetric Diels-Alder reactions catalyzed by TADDOL-Ti complexes [117-120]. Dendritic [121] and polymer-supported TADDOL-Ti complexes [122] have also been employed as recoverable and reusable catalysts to give comparatively high enantioselectivity. Transition-state models have been proposed independently by several groups for TADDOL-type titanium catalysis [121,123]. 

In contrast with the impracticality of hydroaluminating open-chain internal alkenes, all the unsubstituted cycloalkenes can be usefully hydroaluminated, with titanium catalysis if need be, since isomerization of the C=C bond is of no significance. 

Scheme 3 Enantioselective chlorination and bromination through TADDOLate-titanium catalysis...

The catalytic cycle resembles the one from the titanium catalysis. Again substrate coordination to the catalyst induces enolate formation. Interaction with the fluorinating reagent ultimately leads to the stereochemical installment of the new C-F bond and generates a dicationic catalyst state. Exchange of the product for a... 

The search for atom-economical epoxidation of olefins led to the recent discovery of efficient titanium catalysis using aqueous hydrogen peroxide as the oxidant.From the viewpoint of green chemistry, aqueous hydrogen peroxide is the oxidant of choice, since it is inexpensive, with a high active hydrogen content (47%) and the only byproduct is water. 

Most applications of sulfide oxidations by alkyl hydroperoxides have involved titanium catalysis together with chiral ligands for enantioselective transformations. The groups of Kagan in Orsay [61] and Modena in Padova [62] reported independently on the use of chiral titanium complexes for the asymmetric sulfoxidation by the use of BuOOH as the oxidant. A modification of the Sharpless reagent with the use of Ti(0 Pr)4 and (J ,J )-diethyl tartrate (J ,J )-DET) afforded chiral sulfoxides with up to 90% ee (Eq. (8.17)). 

Scheme 7.14 Annulation reaction catalysed by chiral JV-heterocyclic carbene catalysis and titanium catalysis.

Scheme 7.17 Domino acylqtanation reaction catalysed by Lewis base catalysis and chiral titanium catalysis.

Scheme 7.19 Domino aldol-cyclisation reaction catalysed by chiral cinchona catatysis and titanium catalysis.

Scheme 8.1 Domino nucleophiKc addition-kinetic resolution reaction catatysed by biocatal3 is and chiral titanium catalysis.

Godet, T., Bonvin, Y, Vincent, G., Merle, D., Thozet, A., Ciufolini, M. A. (2004). Titanium catalysis in the Ugi reaction of a-amino acids with aromatic aldehydes. Organic Letters, 6,3281-3284. 

Titanium catalysis is suitable not only for combining of compounds with the C-C multiple bonds but also for combining C-heteroatom double bonds. Such an example is reductive cyclization of a,Cc)-enones in the presence of silanes (Ph2SiH2, Si(OEt)3H) and a catalytic amount of the complex 119 into the corresponding silylated cycloalkanols 138, which after deprotection afford the alcohols 139 (Scheme 61). Some representative examples are given in Table 28 [76]. 

---