Asymmetric sulfides, oxidation

Extensive studies of stereoselective polymerization of epoxides were carried out by Tsuruta et al.21 s. Copolymerization of a racemic mixture of propylene oxide with a diethylzinc-methanol catalyst yielded a crystalline polymer, which was resolved into optically active polymers216 217. Asymmetric selective polymerization of d-propylene oxide from a racemic mixture occurs with asymmetric catalysts such as diethyzinc- (+) bomeol218. This reaction is explained by the asymmetric adsorption of monomers onto the enantiomorphic catalyst site219. Furukawa220 compared the selectivities of asymmetric catalysts composed of diethylzinc amino acid combinations and attributed the selectivity to the bulkiness of the substituents in the amino acid. With propylene sulfide, excellent asymmetric selective polymerization was observed with a catalyst consisting of diethylzinc and a tertiary-butyl substituted a-glycol221,222. ... 

The oxidation of heteroatoms and, in particular, the conversion of sulfides to asymmetric sulfoxides has continued to be a highly active field in biocatalysis. In particular, the diverse biotransformations at sulfur have received the majority of attention in the area of enzyme-mediated heteroatom oxidation. This is particularly due to the versatile applicability of sulfoxides as chiral auxiliaries in a variety of transformations coupled with facile protocols for the ultimate removal [187]. 

Other asymmetric sulfide oxidation methods (a) Bolm, C. and Bienewald, F. (1995) Angew. Chem. Int. Ed., 34, 2640 ... 

Figure 9. Schematic illustration of the growth process of hie encapsulated sulfide—oxide nanowhisker, which leads eventually to die formation of hollow WS2 nanotubes when die sulfidization process is completed (a) initialization of die sulfidization process of die asymmetric oxide nanoparticle (b) growth of a long sulfide—oxide encapsulated nanowhisker (c) enlargment of the 001 R CS planes exhibiting within one CS plane regions of edge-sharing octahedra (8a).

A further catalytic method for asymmetric sulfoxidation of aryl alkyl sulfides was reported by Adam s group, who utilized secondary hydroperoxides 16a, 161 and 191b as oxidants and asymmetric inductors (Scheme 114) . This titanium-catalyzed oxidation reaction by (S)-l-phenylethyl hydroperoxide 16a at —20°C in CCI4 afforded good to high enantiomeric excesses for methyl phenyl and p-tolyl alkyl sulfides ee up to 80%). Detailed mechanistic studies showed that the enantioselectivity of the sulfide oxidation results from a combination of a rather low asymmetric induction in the sulfoxidation ee <20%) followed by a kinetic resolution of the sulfoxide by further oxidation to the sulfone... 

Recently, Feng and co-workers reported an asymmetric sulfide oxidation" catalyzed by titanium complexes bearing HydrOx ligands, for example, 576 (Scheme 8.199). ° Enantioselectivities approached a level of synthetic utility for oxidation of aryl alkyl sulfides 632 although the yields of the sulfoxide 633 were poor due to overoxidation to the sulfone 634. The overoxidation is especially significant for reactions with high enantioselectivity. 

Federsel, H.-J. and Larsson, M. An Innovative Asymmetric Sulfide Oxidation The Process Development History behind the New Antiulcer Agent Esomeprazole in Asymmetric Catalysis on Industrial Scale, Blaser, H.U. and Schmidt, E. (Eds). Wiley-VCH New York, 2004, 413 36. 

Electrochemistry (Continued) purely organic compounds, 342 sulfide oxidation, 361 Electrode materials, 342 Electrophilic allylation, 192 attractive interaction, 196 mechanism, 192, 197 turnover-limiting step, 197 Electroreaction, asymmetric, 342 Electrostatic interaction, 328 Elimination and insertion, 3 Enamide reactions ... 

The first example of iron-catalyzed asymmetric oxidation of sulfides was described by Fontecave and coworkers in 1997 [163]. An oxo-bridged diiron complex, which contained (—)-4,5-pinenebipyridine as chiral ligand, was reported to catalyze sulfide oxidations with H202 in acetonitrile, having the potential to transfer an oxygen atom directly to the substrates. However, the enantioselectivity of this process remained rather low (<40% ee, Scheme 3.53). 

The major breakthrough in this field was achieved in 2003 by Legros and Bolm [164], who reported a highly enantioselective iron-catalyzed asymmetric sulfide oxidation. Optically active sulfoxides were obtained with up to 96% ee in good yields under very simple reaction conditions using Fe(acac)3 as precatalyst in combination with a Schiff base-type ligand (Table 3.9). Furthermore, inexpensive and safe 35% aqueous hydrogen peroxide served as terminal oxidant. 

Shortly thereafter, Bryliakov and Talsi described chiral [iron(salen)Cl] catalysts for the asymmetric sulfide oxidation using PhIO as terminal oxidant (Table 3.10) [166]. Whereas good selectivities in the formation of sulfoxides vs. sulfones were achieved, only poor to moderate enantiomeric excesses were obtained with this system (22-62% ee). 

The large-scale production of esomeprazole is now successfully achieved by asymmetric oxidation of the same sulfide intermediate as is used in the production of omeprazole (Scheme 2.5). Using the titanium-based catalyst originally developed by K. Barry Sharpless for allyl alcohol oxidation [56] and by H.B. Kagan for certain sulfide oxidations [57], a process was developed that could achieve initial enantiomeric excesses of about 94% [53]. During the production process, the optical purity is further enhanced by the preparation of esomeprazole magnesium salt, with subsequent re-crystallization. 

Asymmetric Epoxidation, Dihydroxylation and Sulfide Oxidation New Routes to Chiral Agrochemicals and Pharmaceuticals... 

Legros,)., Dehli, J.R. and Bolm, C. (2005) Applications of catalytic asymmetric sulfide oxidations to the syntheses of biologically active sulfoxides. Advanced Synthesis Catalysis, 347,19-31. 

Asymmetric sulfide oxidations are reported using oxaziridines other than A -sulfonyloxaziridines, but it is necessary to use a protic acid or Lewis acid to increase their reactivity. For example, -tolyl methyl sulfide 152 with bicyclic oxaziridine 153 in the presence of TFA gave the (A)-sulfoxide 154 in 50% yield and 42% ee in 24 h <1999T155>. It is interesting to note that use of MsOH resulted in much faster reaction with the oxidation complete in less than a minute. Similarly, sulfide 155 with chiral oxaziridine 156 in the presence of zinc chloride afforded sulfoxide 157 in 30% yield and 55% ee <2005JOC301>. 

Federsel H-J, Larsson M. An innovative asymmetric sulfide oxidation the process development history behind the new antiulcer agent esomeprazole. In Asymmetric Catalysis on Industrial Scale. Blaser H-U, Schmidt E, eds. 2004. Wiley-VCH, Wein-heim, Germany, pp. 413-436. 

An innovative asymmetric sulfide oxidation the process development history behind the new antiulcer agent Esomepra-zole... 

An Innovative Asymmetric Sulfide Oxidation The Process Development History Behind the New Antiulcer Agent Esomeprazole1 ... 

This then concluded our efforts aimed at designing an oxidative asymmetric transformation applied to our sulfide substrate. The understanding of the reaction had reached a much more advanced level during this investigation, and given the extraordinary positive results the decision was an easy one to make to go ahead with all available resources devoted to this approach ... 

Fig. 7 Performance (expressed as % ee) of asymmetric sulfide oxidation as function of time and presence/absence of base during the equilibration.

The asymmetric sulfide oxidation described in Scheme 2 has a (-)-NLE until eeaux=70% [5]. The (S)-proline catalyzed cyclization of a triketone shows a weak asymmetric depletion [5], as does allylic oxidation of cyclohexene in the presence of a catalyst prepared from Cu(OAc)2 and (S)-proline [41]. 

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]. 

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