Catalytic using titanium

The sulfoxidation of organosulfur compounds by tert-butyl hydroperoxide has been effected catalytically using titanium alkoxide-based catalyst. A robust titanium alkoxide catalyst was developed from the reaction of a homochiral trialkanolamine N CH2CHR(0H) 3 (R = H, MePh) with Ti(OPr )4 to form a mononuclear isopropoxy titanatrane (E 0)Ti 0CHRCH2 3N containing five-coordinated titanium. 

Excessive anodic overvoltage at the catalytic coating must also be tackled (Fig. 23.6). This may lead to excessive titanium dioxide growth, eventually leading to breakdown. Such overpotentials can be countered by using titanium stray current dumpers. 

Catalytic use of these compounds revealed an activity comparable to the titanium species. The most important points of this work are that under the experimental conditions there is no metal loss in solution (a rare cases in heterogeneous catalysis with such reactive media) and that the reaction is completely heterogeneous (Figure 3.29). 

The following combinations have also been reported of titanium-mediated reductive coupling of imines TiCU-Mg in DME (or THF), TiCU-Li in DME (or THF), TiCU-EtjN in Catalytic use of... 

Catalysts of such small dimensions can hardly be handled in practice. A procedure for binding 0.1-0.3-//m particles into agglomerates of 20-30 pm that can be easily handled while maintaining the same catalytic properties has been a key step in the development of industrial processes using titanium silicates (Bellussi et al, 1986). 

For redox catalysis, efforts have been spent on preparing transition metal modified mesoporous materials. These materials are capable of extending the catalytic oxidation chemistry to large molecules. The selective catalytic activity has also been demonstrated, for example, in the oxidation of aromatic compounds by using titanium-containing mesoporous silica (Ti-MCM-41 and Ti-HMS). ... 

Reactions of simple silanediols and disilanols with titanium orthoesters, titanium halides and titanium amides proceed to give cyclic titanasiloxanes [30]. On the other hand, the silanetriols with three functional OH groups would prove appropriate synthons for constructing three-dimensional titanasiloxanes which would in turn serve as model compounds for catalytically useful Ti-doped zeolites [32]. The synthesis of cubic titanasiloxanes has been achieved in two ways. 

Furstner, A., Hupperts, A. Carbonyl Coupling Reactions Catalytic in Titanium and the Use of Commercial Titanium Powder for Organic Synthesis. J. Am. Chem. Soc. 1995, 117, 4468-4475. 

Kagan has reported NLE as an indicator for distinction of closely related chiral catalysts (Eq. (7.6)) [20J. In asymmetric oxidation of sulfides with hydroperoxides promoted by chiral DET-Ti complexes, a wide diversity of titanium species is observed just by minor modifications of the catalyst preparation step. Stoichiometric use of a 1 4 mixture of Ti(Ot Pr)4 and DET exhibits (-)-NLE. An addition of /PrOH to this mixture, e.g. a 1 4 4 mixture of Ti(0/Pr)4, DET, and iPrOH, provides (-f)-NLE, while catalytic use of this ternary system leads to the disappearance of NLE. 

An actual breakthrough in the field of asymmetric epoxidation was the discovery of a new method for the preparation of homochiral epoxy alcohols by Katsuki and Sharpless in 1980 [13]. The main idea was to change the catalytic system and to use titanium tetraisopropoxide and l-( + )- or d-( — )-diethyl tartrate (DET). With water-free solutions of TBHP various allylic alcohols can be epoxidized with high enantioselectivity. 

Among the reactions catalyzed by titanium complexes, the asymmetric epoxidation of allylic alcohols developed by Sharpless and coworkers [752, 807-810] has found numerous synthetic applications. Epoxidation of allylic alcohols 3.16 by ferf-BuOOH under anhydrous conditions takes place with an excellent enantioselectivity (ee > 95%) when promoted by titanium complexes generated in situ from Ti(0/ -Pr)4 and a slight excess of diethyl or diisopropyl (R,R)- or (iS, 5)-tartrates 2.69. The chiral complex formed in this way can be used in stoichiometric or in catalytic amounts. For catalytic use, molecular sieves must be added. Because both (RJ )- and (5,5)-tartrates are available, it is posable to obtain either enantiomeric epoxide from a single allylic alcohol. Cumene hydroperoxide (PhCMe20OH) can also be used in place of ferf-BuOOH. This method has been applied to industrial synthesis of enantiomeric glycidols [811, 812]. 

Addition of the A1—H bond to C=C double bonds in alkenes is greatly accelerated by catalytic quantities of certain titanium compounds [e.g., TiCU, Ti(OC4H9)4]. For example, when propene is passed into dipropyl-aluminum hydride at room temperature no reaction is observed, whereas small amounts of TiCl4 (R2A1H TiCl4, 100 1) lead to rapid addition with the evolution of heat. Small amounts of polypropylene are formed simultaneously, but this can be avoided by using titanium (IV) butylate (256). 

Posner has used titanium complexes of BINOL to promote inverse electron demand Diels-Alder reactions between the same diene (8.120) and vinyl ethers. Most of this work involved the use of stoichiometric Lewis acid to give 95-98% ee. Curiously, catalytic conditions (10 mol%) afforded the opposite enantiomer in 50—60% ee. 

In conclusion, the epoxidation of propylene with bulky oxidants (such as cumene or TBHP) can be successfully achieved using titanium-containing mesoporous materials as catalysts. The catalytic chemistry of the active sites can be controlled via the synthesis conditions and postsynthesis modifications. The hydrophobicity of the catalyst is of great importance to achieve a highly selective catalyst. The Ti-MCM-41-based heterogeneous catalyst has demonstrated excellent performance in the commercial process for PO manufacture. 

Lemaire et al. developed a catalytic system for the reduction of secondary phosphine oxides to obtain secondary phosphine boranes using titanium(iv) isopropoxide as eatalyst and tetramethyldisiloxane as hydride source, giving excellent yields. ... 

As follow-ups of the original reaction procedure, numerous improvements have been made, including the lowering of the reaction temperature, use of other transition metals, and the catalytic use of various metal species. In the 1990s, cobalt, titanium, and ruthenium complexes were found to serve as catalysts for the Pauson-Khand reaction, but these catalytic systems often need to employ medium to high pressures of... 

Bradley, C. A. McMurdy, M. J. Tilley, T. D., Selective Catalytic Cyclohexene Oxidation Using Titanium-Functionalized Silicone Nanospheres. J. Phys. 

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