Titanium catalysts, olefin epoxidation

J. Jarupatrakom, T. D. Tilley, Silica-supported, single-site titanium catalysts for olefin epoxidation. A molecular precusor strategy for control of catalyst structure, /. Am. Chem. Soc. 124 (2002) 8380. 

T.D. Tilley et al. - Silica-Supported, Single-Site Titanium Catalysts for Olefin Epoxidation. A Molecular Precursor Strategy for Control of Catalyst Structure,... 

A conveniently prepared amorphous silica-supported titanium catalyst exhibits activity similar to that of Ti-substituted zeolites in the epoxidation of terminal linear and bulky alkenes such as cyclohexene (22) <00CC855>. An unusual example of copper-catalyzed epoxidation has also been reported, in which olefins are treated with substoichiometric amounts of soluble Cu(II) compounds in methylene chloride, using MCPBA as a terminal oxidant. Yields are variable, but can be quite high. For example, cis-stilbene 24 was epoxidized in 90% yield. In this case, a mixture of cis- and /rans-epoxides was obtained, suggesting a step-wise radical mechanism <00TL1013>. 

The catalytic oxidation of hydrocarbons with peroxides, especially the epoxidation of olefins, in liquid phase by titanium catalysts is one of the most actively investigated reactions (60). The active species for this epoxidation reaction is usually assumed to be titanium peroxo moieties, derived from four-coordinate titanium and peroxides. However, the isolation of the active intermediate remains a challenge owing to the inherent instability of such species. We have been able to synthesize and stabilize the related cubic p-oxo-silicon-titanium complex (35) by reacting a bulky... 

Similarly the alkali metal-exchanged titanium beta was shown to be an effective catalyst for olefin epoxidations with TBHP [47]. Titanium has also been incorporated into mesoporous molecular sieves, such as MCM-41, by framework substitution [48] or by grafting Ti(IV) species to the internal surface by reaction... 

For the titanium catalyzed epoxidation, only 1 mol% of catalyst and 1.05 equiv. of H2O2 are required to obtain high yields and selectivities. Not only activated olefins gave excellent results, but also for simple styrene a 93% ee can be achieved by using this chiral titanium complex." It was speculated that this peroxotitanium species is activated by an intramolecular hydrogen bond with the amine proton (Scheme 19). 

This simple picfure can only be partly correct, as it has been shown that both unsupported ultrafine titania nanoparticles [10] and silica-supported monodis-persed subnanometric titania particles [11] are active catalysts for olefin epoxidation with hydroperoxides. In these small titania particles, the titanium atoms possess coordination states between four and six. Quanfum chemical calculations derived similar activation energies for oxygen transfer on mononuclear four- and five-coordinafed fifanium sifes [12,13]. 

Titanium—Vanadium Mixed Metal Alkoxides. Titanium—vanadium mixed metal alkoxides, VO(OTi(OR)2)2, are prepared by reaction of titanates, eg, TYZOR TBT, with vanadium acetate ia a high boiling hydrocarbon solvent. The by-product butyl acetate is distilled off to yield a product useful as a catalyst for polymeri2iag olefins, dienes, styrenics, vinyl chloride, acrylate esters, and epoxides (159,160). 

Titanium containing hexagonal mesoporous materials were synthesized by the modified hydrothermal synthesis method. The synthesized Ti-MCM-41 has hi y ordered hexa rud structure. Ti-MCM-41 was transformed into TS-l/MCM-41 by using the dry gel conversion process. For the synthesis of Ti-MCM-41 with TS-1(TS-1/MCM-41) structure TPAOH was used as the template. The synthesized TS-l/MCM-41 has hexagonal mesopores when the DGC process was carried out for less than 3 6 h. The catalytic activity of synthesized TS-l/MCM-41 catalysts was measured by the epoxidation of 1-hexene and cyclohexene. For the comparison of the catalytic activity, TS-1 and Ti-MCM-41 samples were also applied to the epoxidation reaction under the same reaction conditions. Both the conversion of olefins and selectivity to epoxide over TS-l/MCM-41 are found hi er flian those of other catalysts. 

The zirconium alkoxides Zr(OR)4 are reported as inactive for the epoxidation of olefins under the conditions recommended with the titanium analogs. When synthesized by reaction of Zr(CH2CMe3)4 with silica, followed by hydrolysis or calcination, a solid as active as the related Ti-based catalyst is obtained. The low selectivity for the formation of the epoxide is related to the fact that the same Zr centers catalyze both the formation and the decomposition of the epoxide.46... 

Following the success with the titanium-mediated asymmetric epoxidation reactions of allylic alcohols, work was intensified to seek a similar general method that does not rely on allylic alcohols for substrate recognition. A particularly interesting challenge was the development of catalysts for enantioselective oxidation of unfunctionalized olefins. These alkenes cannot form conformationally restricted chelate complexes, and consequently the differentiation of the enan-tiotropic sides of the substrate is considerably more difficult. 

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. 

Sheldon et al. [2] have shown that in the epoxidation of olefins with TBH, compounds like titanium acetylacetonate or tetra-n-butyl titanate containing Ti(IV) produce epoxides with extremely high selectivities (98%), even though the rate of reaction is generally lower with respect to Mo(VI) or V(V) catalysts. 

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