Ti-based catalytic systems

Prior to the usage of the Ti-based catalytic system , the Sharpless group had reported their first asymmetric epoxidation of allylic alcohols using a combination of VO(acac)2/ TBHP and the chiral hydroxamic acid 67 (ee < 50%) or derivatives (ee 80%) ". In 1999, Yamamoto and coworkers described an improvement of this oxidation protocol, ee values up to 94%, by using hydroxamic acids derived from binaphthol, 68 being the... 

Table 6.2 Comparison of catalytic performance with EBHP versus different heterogeneous Ti-based catalytic systems in propene epoxidation.

At -20°C, Ti-tartarate plus the hydroperoxide-based catalytic system give near-identical enantioselectivities for a variety of allyl alcohols with different substituents on C2 and C3. What could be concluded from this observation ... 

The second reason is not so evident for example, the data were obtained for a soluble catalytic system based on biscyclopentandienyltitanium-dichloride (178, 179) for which case the propagation center formed contains a Ti(IV) ion. 

Recently, bis(imino)pyridyl Fe(II)-based catalysts have been reported to afford isospecific chain-end controlled propene polymerization occurring through secondary monomer insertion.138 139 Even more recently, catalytic systems based on the octahedral bis(salicylaldiminato)Ti complex have been reported to afford syndiospecific chain-end controlled propene polymerization,140 which possibly occurs through secondary monomer insertion.141... 

The situation for the hydrosilylation of C = N functions with regard to ecology and economy is somewhat similar as for the hydride reduction, except that fewer effective catalytic systems have been developed [91]. Despite some recent progress with highly selective Ti-based [92] and Cu-based [93] catalysts using cheap polymethylhydrosiloxane as reducing agent, hydrosilylation will see its major applications in small-scale laboratory synthesis. 

A solid-phase sulfur oxidation catalyst has been described in which the chiral ligand is structurally related to Schiff-base type compounds (see also below). A 72% ee was found using Ti(OPr-i)4, aqueous H2O2 and solid-supported hgand 91 . More recently, a heterogeneous catalytic system based on WO3, 30% H2O2 and cinchona alkaloids has been reported for the asymmetric oxidation of sulfides to sulfoxides and kinetic resolution of racemic sulfoxides. In this latter case 90% ee was obtained in the presence of 92 as chiral mediator. ... 

It is well known that in conventional catalyst systems a chemical interaction between the catalyst and the metal-alkyl takes place, which essentially leads to a variation of the transition metal oxidation state. This is likewise true with MgCl2 catalysts however, in this case there are many more possible reactions, given the greater complexity of the system. Thus, besides modifying the Ti valence, the metal-alkyl may interact with the Lewis base incorporated in the catalyst. The Lewis base added to the cocatalyst can, in turn, interact both with the support and with the TiCl4, as can the byproducts originating from the reaction between Al-alkyl and Lewis base. The situation appears to be quite complex. However, detailed knowledge about these processes is absolutely necessary for any attempt to rationalize the polymerization behavior of these catalytic systems. 

Atactic polypropene has been synthesized with homogeneous catalytic systems based on mono-Cp trialkoxo titanium complexes activated by MAO.951 Syndiotactic polystyrene has been synthesized with different mono-Gp trialkoxo titanium derivatives activated by MAO and AlMe3, and the catalytic efficiency has been compared with bis-Cp titanium catalysts.952 The titanium ligands affect both catalytic activity and stereoregularity of the polypropylene obtained. For the CpTi(OPrn)3/MAO system, factors influencing the propylene polymerization, such as temperature, Al/Ti molar ratio, and monomer pressure, have been studied. 

Reetz et al. reported that a chiral Ti complex prepared from TiCL). and the dilithium salt of (S)-BINOL promoted the aldol reaction of 3-mefhylbutanal with KSA 48 with only poor enantioselectivity (60%, 8% ee) [115 b]. After this pioneering work, the titanium-based catalyst system has been intensively improved to attain an efficient catalytic cycle and high stereoselectivity [147-155]. 

The catalytic system is applicable to the synthesis of yS-aminonitriles from aziridines [624b]. A Ti(Oi-Pr)4-Schifr base complex also is useful in the ring-opening reaction of epoxides leading to yS-siloxynitriles [625]. 

A vast scientific literature exists on propene oxidation with O2 [32]. Catalysts based on supported Ag have been investigated by several groups [32b,d,i]. Moreover, other types of catalytic systems have been studied, for example, Ti/HSZ [32e], Ti/Si02 [32f], Mo03/Si02 [32g], Ag-Mo03/Zr02 [32h] andTi-Al-HMS [32i,p]. Table 6.6 compares some of the results reported in the scientific literature. 

A composite catalytic system will consist of a redox transition metal catalyst and an electron mediator. The redox catalyst is selected from metal ions such as Ti ", Mn, Fe etc based on their ability to form the oxo species. The electron mediator is selected from Ru, Rh and Pd based on their well established ability to remove and transport electrons from hydrogen and secondary alcohols. 

In a review on non-Cp type homogeneous catalytic systems for olefin polymerization, complexes based on Tpx-Sc,-Hf,-Ti,-Zr,-Y have been also described.107... 

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