Titanium catalysts sulfoxidation reactions

The use of ionic liquids to perform asymmetric sulfoxidation reactions was proposed by Halligudi and coworkers. A chiral titanium-BINOL complex was immobilised onto ionic liquid-modified mesoporous silica (SBA-15) support 14 and the resulting heterogeneous catalyst was successfully employed in asymmetric sulfoxidation of thioanisole using TBHP as... 

The patent literature contains several references to the use of sulfoxide complexes, usually generated in situ, as catalyst precursors in oligomerization and polymerization reactions. Thus, a system based upon bis(acrylonitrile)nickel(0> with added Me2SO or EtgSO is an effective cyclotrimerization catalyst for the conversion of butadiene to cyclo-1,5,-9-dodecatriene (44). A similar system based on titanium has also been reported (407). Nickel(II) sulfoxide complexes, again generated in situ, have been patented as catalyst precursors for the dimerization of pro-pene (151) and the higher olefins (152) in the presence of added alkyl aluminum compounds. 

Complexes based on titanium excess tartrate combination (the Padova system). In 1984, the same year the Orsay group developed their system, a group in Padova, Italy, headed by Modena,47 developed a different system, able to oxidize sulfides to sulfoxides with high selectivity, also based on a modification of the Sharpless catalyst. The Padova group used TBHP in the presence of 1 mol equiv of Ti(0-/-Pr)4/(/ ,/ )-DET, 1/4 combination. The reactions were performed at -20... 

Phenylthioalkylation of silyl enol ethers. Silyl enol ethers of ketones, aldehydes, esters, and lactones can be alkylated regiospecifically by a -chloroalkyl phenyl sulfides in fhe presence of a Lewis acid. Zinc bromide and titanium(IV) chloride are the most effective catalysts. The former is more satisfactory for enol ethers derived from esters and lactongs. ZnBr2 and TiCL are about equally satisfactory for enol ethers of ketones. The combination of TiCL and Ti(0-f-Pr)4 is more satisfactory for enol ethers of aldehydes. Since the products can be desulfurized by Raney nickel, this reaction also provides a method for alkylation of carbonyl compounds. Of more interest, sulfoxide elimination provides a useful route to a,B-unsaturated carbonyl compounds. 

Uemura described use of a Ti(OiPr)4/(i )-BINOL complex for the oxidation of alkyl aryl sulfides with aqueous ferf-butyl hydroperoxide as stoichiometric oxidant [22]. At room temperature p-tolyl methyl sulfide was converted into the corresponding sulfoxide with 96% ee in 44% yield with as little as 5 mol % of the chiral ligand. The reaction is insensitive to air, while the presence of water seems to be essential for the formation of the catalytically active species, long catalyst lifetime, and high asymmetric induction. The authors observed a large positive non-linear effect which indicates that the actual catalyst consists of a titanium species with more than one (K)-BINOL ligand (11) coordinated to the metal. 

A new modification was recently described by Imamoto who employed a combination of (S,S)-2,2,5,5-tetramethyl-3,4-hexane diol (13) and Ti(OiPr)4 as catalyst. The active species was proposed to be monomeric with two diols and one cumyl hydroperoxide ligand leading to an octahedral coordination sphere around titanium. Under conditions similar to those reported by Kagan, p-tolyl methyl sulfoxide was obtained with 95% ee in 42% yield. Sulfone formation was a dominant, albeit beneficial side reaction giving in a kinetic resolution process (s=... 

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]. 

Titanium-pillared montmorillonites (Ti-PILC) modified with tartrates were described as heterogeneous Sharpless epoxidation catalysts [33] as well as for the oxidation of aromatic sulfides [34]. Metal oxides modified with histamine showed modest efficiencies for the kinetic resolution of activated amino acid esters (kj /k5 2) [35]. Silica or alumina treated with diethylaluminium chloride and menthol catalyzed the Diels-Alder reaction between cylopentadiene and methacrolein with modest enantioselectivities of up to 31% ee [36]. ZeoHte HY, modified with chiral sulfoxides had remarkable selectivities for the kinetic resolution of 2-butanol (k /kj =39) but unfortunately the catalyst is not very stable... 

Kagan and Pitchen ° and Modena and coworkers independently reported the oxidation of sulfides to sulfoxides using modified Sharpless epoxidation catalyst (titanium/diethyl tartrate). By 1987, Kagan had already reported a catalytic variation of the reaction and an improved catalytic system allows for the use of lower (10 mol%) loading of catalyst. For example, sulfide (5.143) undergoes sulfoxidation with good enantioselectivity. An alternative catalyst based on Ti(0 Pr)4 and BINOL is also effective for sulfoxidation, providing up to 96% ee. ... 

According to this concept, the catalyst residual in PBT can have an important role to catalyze the degradation of hydroperoxides (step 4 of scheme 6). In fact it is well-known the capability of titanium(IV) to catalyze important oxidation reactions, such as epoxidation of allylic alcohols and sulfoxidation, in which hydroperoxides are involved as oxidants. [44,45] The role of titanimn center in these reactions is the coordination and activation of the hydroperoxides to favor oxygen atom transfer. 

Jimenez et al. have developed tridentate cyclopentadienyl-silesquioxanate titanium complexes for the epoxidation of cyclic and linear alkenes with aqueous hydrogen peroxide under mild reaction conditions with excellent reactivity and selectivity. The authors extended the studies of these complexes as catalysts for the oxidation of sulfides to sulfoxides or sulfones under mild reaction conditions. The catalysts showed high chemoselec-tivity and proved to be very stable as no loss of activity or selectivity was observed after 14 cycles. 

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. 

Correia et al. used titanium-salan eatalysts in ionie liquids as solvents. Unfortunately, despite good catalytie aetivity, only modest enantioselec-tivities were detected (around 20% enantiomerie exeess). Oxidation of sulfides was also performed with silica-immobilised eatalysts. Titanium-tartaric acid catalysts were grafted onto amorphous and MCM-41 silica by reaction of the metal with the silanol groups on the surface, and used in the sulfide oxidation. In both cases, sulfoxides with low enantiomeric excesses were obtained. 

Fujita et al. used a catalytic amount of a binuclear titanium(IV) complex in an attempt to find an efficient system to oxidize sulfides with high enantioselectivity [102]. Prior to this study, they investigated other systems with several transition metals. A similar asymmetric sulfoxidation was discovered [105] using a catalytic amount of nonracemic Schiff base oxovanadium complex (Table 1.4) under atmospheric conditions at room temperature in dichloromethane. With 0.1 mol% of catalyst and cumene hydroperoxide as oxidant, oxidation produces sulfoxides in excellent yields. However, the reaction is limited to alkyl aryl sulfide substrates, and the best enantioselectivity obtained was 40% ee, for (S)-methyl p-methoxy phenyl sulfoxide. 

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. 

A one-pot titanium-catalyzed tandem sulfoxidation-kinetic resolution process was developed by Chan using TBHP as the oxidant This process combines asymmetric sulfoxidation (at 0°C) and kinetic resolution (at room temperature). Excellent enantiomeric excesses (up to >99.9%) and moderate to high chemical yields of sulfoxides were obtained [270] (Scheme 14.113). The effect of fluorine substitution at the backbone of BINOL on the catalytic activity in titanium-catalyzed sulfide oxidation with TBHP or cumyl hydrc en peroxide (CHP) was studied by Yudin [271]. Introduction of fluorines into the BINOL scaffold was found to increase the electrophilic character of the Lewis acidic titanium center of the catalyst The most intriguing difference between the FsBINOL and BINOL systems is the reversal in the sense of chiral induction upon fluorine substitution. A steroid-derived BINOL ligand has also been used for the same reaction [272]. 

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