Titanium-salen catalyst

Scheme 7.8 Enantioselective oxidation of sulfide using titanium-salen catalyst 12.

As mentioned before (Section 7.2.1), aqueous hydrogen peroxide is an ecofriendly and versatile reagent. Thus, as shown in Scheme 7.8, Katsuki used chiral titanium-salen catalyst 12 for the oxidation of sulfides with hydrogen peroxide or urea-hydrogen peroxide adduct (UHP). Under these mild reaction conditions, chiral sulfoxides were obtained in good yields with high enantioselectivities. 

The addition of trimethylsilyl (TMS) cyanide to aldehydes produces TMS-protected cyanohydrins. In a recent investigation a titanium salen-type catalyst has been employed to catalyse trimethylsilylcyanide addition to benzaldehyde at ambient temperature1118]. Several other protocols have been published which also lead to optically active products. One of the more successful has been described by Abiko et al. employing a yttrium complex derived from the chiral 1,3-diketone (41)[119] as the catalyst, while Shibasaki has used BINOL, modified so as to incorporate Lewis base units adjacent to the phenol moieties, as the chiral complexing agent11201. 

Group 4 metals have also been used widely in conjunction with salen-type ligands (Figure 25). In 2006 Gregson et reported several chiral and achiral titanium salen alkoxide complexes for the ROP of lactide. All catalysts reported were modestly active and heteroselective (P 0.51-0.57). Several achiral Ti and Zr salan catalysts were reported by Gendler et for melt polymerization of lactide. While no stereoselectivity has been reported for either system, the Zr complexes were more active towards lactide ROP than the Ti analogs. 

Previously, Pasini [27] and Colonna [28] had described the use chiral titani-um-Schiff base complexes in asymmetric sulfide oxidations, but only low selec-tivities were observed. Fujita then employed a related chiral salen-titanium complex and was more successful. Starting from titanium tetrachloride, reaction with the optically active C2-symmetrical salen 15 led to a (salen)titani-um(IV) dichloride complex which underwent partial hydrolysis to generate the t]-0x0-bridged bis[(salen)titanium(IV)] catalyst 16 whose structure was confirmed by X-ray analysis. Oxidation of phenyl methyl sulfide with trityl hydroperoxide in the presence of 4 mol % of 16 gave the corresponding sulfoxide with 53% ee [29]. 

The efficiency of new unsymmetrical chiral salen ligands was examined in the asymmetric trimethylsilylcyanation of benzaldehyde. A very high level of enantioselectivity was attainable over chiral Ti(IV) salen complexes prepared from salicylaldehyde and 3,5-Di-/ert-butylsalicylaldehyde derivative as compared to the conventional salen catalyst. Enantiomeric excess of the corresponding reaction product was generally more than 70% over unsymmetric chiral salen catalysts. The chiral Titanium(IV) salen complexes immobilized on a mesoporous MCM-41 by multi grafting method showed a relatively high enantioselectivity for the addition of trimethylsilyl cyanide to the benzaldehyde. 

Avecia, a former part of the Zeneca Group, developed a range of cheap and highly active titanium- or vanadium-based salen catalysts called CACHy catalysts (Scheme 23). They are based on Jacobsen s salen technology, but they are much more reactive and can, for example, be used in concentrations as low as 0.1 mol% in the cyanation of aryl aldehydes. The catalysts show similar reactivity with alkyl aldehydes and ketones and are applicable to the synthesis of the commercially important mandelic acid derivatives. 

Jacobsen and Katsuki have both reported the use ofmanganese(III) (salen) catalysts for sulfide oxidation. These catalysts can be effective for the enantiose-lective oxidation of several aryhnethylsulfides using iodosylbenzene as the stoichiometric oxidant. Additionally, titanium(salen) complexes function as efficient catalysts in this procedure, providing up to 94% ee in the oxidation of methyl phenyl sulfide using the more economical urea hydrogen peroxide as oxidant. ... 

Titanium complexes have been shown to be active for the synthesis of cyclic carbonates or either di- or trithiocarbonates from epoxides and either carbon dioxide or carbon disulfide (Scheme 5.6). Titanium-catalysed synthesis of cyclic carbonates has been recently reviewed by North and coworkers. Titanium-salen complexes find application as catalysts, in combination with tetrabutylammonium bromide or tributylamine, for the synthesis of di- or trithiocarbonates from epoxides and carbon disulfide. It is worth highlighting that the catalyst loading can be reduced to 0.5 mol%, although 1 mol% of catalyst was required in order to achieve quantitative yields. The catalyst system showed a preference for dithiocarbonate formation for most of the epoxides studied. 

Another way to prepare a reusable catalyst was proposed by Venkataraman. The ligand 25, bearing a linear polyethyleneglycol (PEG) moiety (MW 5 kDa) was used in the titanium-catalysed addition of tri-methylsilyl cyanide to benzaldehyde (Figure 7.1). The silylated cyanohydrin was obtained in more than 95% yield and 86% enantiomeric excess after 24 h at room temperature, with only 0.1 mol% of catalyst. The titanium-salen complex was separated from the reaction mixture by dialysis by means of a Soxhlet apparatus. 

An unsymmetrical salen ligand bearing a Lewis base catalyses Ti(OPr-i)4-promoted addition of TMSCN to benzaldehyde with as little as 0.05 mol% loading, quantitative conversion is achieved in 10 min at ambient temperature. Another salen catalyst - a bifunctional salen-phosphine oxide-Ti(IV) combination - promotes enantioselective cyanosilylation of aldehydes. Fine tweaking of the structure of another series of bifunctional chiral salen-Ti(IV) complexes allows the enantioselectivity to be reversed. Biaryl-bridged salen-titanium complexes are also highly efficient catalysts, one example giving 87% ee at room temperature. ... 

Imido and 0x0 compounds are intermediates in many of the transfers of oxygen atoms and nitrene units to olefins to form epoxides and aziridines, and they are intermediates in many of the insertions of oxygen atoms and nitrene units into the C-H bonds of hydrocarbons to form alcohols and amine derivatives. The enantioselective epoxidation of allylic alcohols (Scheme 13.22) " is the most widely used epoxida-tion process, and the discovery and development of this process was one of the sets of chemistry that led K. Barry Sharpless to receive the Nobel Prize in Chemistry in 2001. The mechanism of this process is not well established, despite the long time since its discovery and development. Nevertheless, most people accept that transfer of the oxygen atom occurs from a titanium-peroxo complex - rather than from an 0x0 complex. Jacobsen s and Katsuki s - manganese-salen catalysts for the enantioselective epoxidations of unfunctionalized olefins, which were based on Kochi s achiral chromium- and manganese-salen complexes, are a second set of... 

Hou reported the use of a chiral (salen)titanium catalyst for the desymmetriza-tion of meso-epoxides with thiols (Scheme 7.14). The complex, fonned in situ... 

A titanium catalyst 20 that incorporates binaphthyl chirality along with imine and phenolic (salen) donors is highly active in addition of silyl ketene acetals to aldehydes.160... 

Dialkylzincs are much less reactive than phenyl or alkynylzincs. In 2002, Kozlowski et al. developed a chiral salen-based catalyst 62 that can promote the diethylzinc addition to a-ketoesters in high yield, [Eq. (13.38)]. In their catalysis, titanium acts as a Lewis acid, and amine nitrogen acts as a Lewis base (63). The enantioselectivity was up to 78% ee ... 

New catalysts were also discovered by combinatorial methods for epoxidation, specifically, diamine Mn complexes, highly active Fe and salen complexes, and titanium-silsesquioxane.223... 

The Direct Enantioselective Synthesis of Diols from Olefins using Hybrid Catalysts of Chiral Salen Cobalt Complexes Immobilized on MCM-41 and Titanium-containing Mesoporous Zeolite... 

Very few methods developed recently are applicable to the cyanation of not only electron-rich but also electron-deficient aromatic, as well as aliphatic aldehydes. Titanium(IV)-derived complex 1 (Fig. 1), reported by Uang, catalyzes the hydro cyanation of aromatic, a, 3-unsaturated, and aliphatic aldehydes with high enantioselectivity (>88% ee for all substrates reported) [15]. Similarly, Ti(IV)-catalyst 2 developed by Choi has proved to be highly enantioselec-tive for the cyanation of various classes of aldehydes (>90% ee for most substrates) [16]. Belokon and co-workers reported two chiral salen-based systems (salen)VO catalyst 3a and [(salen)TiO]2 3b, both of which provided moderate levels of enantioselection when applied to the asymmetric cyanation of diverse... 

A titanium complex (1) with a salen ligand is an efficient catalyst for the enan-tioselective epoxidation of alkenes with hydrogen peroxide as the terminal oxidant. The participation of a titanium-peroxo species, activated by hydrogen bonding, in the reaction, has been postulated.73... 

During recent years, the homogeneous Lewis acid-catalyzed asymmetric Diels-Alder reactions and hetero-Diels-Alder (HDA) reactions have each undergone extensive study. Various chiral Lewis acids including aluminum, titanium or boron, and chiral ligands such as chiral amino alcohols, diols, salen, bisoxazoline or N-sulfonylamino acids have been used as the catalysts [84]. Much efforts have also been made in the investigation of heterogeneous diastereoselective Diels-Alder reactions. 

A titanium (IV) complex with the identical (salen) figand was reported recently to effect the addition of thiophenol with moderate enantioselectivity [18]. The complex was formed in situ from 5 mol % Ti(0-z-Pr)4 and 5.5 mol % of the chiral ligand 9, and this catalyst promoted the ring opening of cyclohexene oxide in 93% yield and 63% ee at -40 to -25 °C (Scheme 6). 

Titanium chelates of semi-salen 59 and salen 60 are used in asymmetric synthesis of a-cyanoalkyl ethyl carbonates from aldehydes and ethyl cyanoformate. By changing the metal atom to aluminum for complexing 60 a catalyst for elaborating a-acetoxy amides (Passerini reaction) is obtained (but enantioselectivity varies)/ ... 

Because of this catalyst degradation, organometallic catalysts are currently the best synthetic reagents for enantioselective epoxidation of olefins. Chiral Mn(III)-salen complexes yield up to 99% ee for cw-disubstituted, tri- and tetra-substituted alkenes [62], but the best results require less desirable oxidants - iodosyl benzene or hypochlorite. Other catalysts accept a more limited substrate range the Sharpless-Katsuki titanium-tartrate ester [65] for allylic alcohols and the JuUa-Colonna epoxidation for a,P-unsaturated ketones [66]. 

Whilst the Sharpless epoxidation with titanium catalysts and the Jacobsen-Katsuki epoxidation with manganese(salen) complexes are at the forefront of enantioselec-tive epoxidation with metal catalysts, there are alternative systems available. Ruthenium pyridinebisoxazoline (PYBOX) complexes have been independently reported, using either phenyliodinium diacetate or sodium periodate as... 

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