Asymmetric sulfoxidation reaction

Scheme 18.5 Oxidation method developed by Kagan [56, 57] for catalyzing asymmetric sulfoxidation reactions.

Table 13.4 Selected results from the asymmetric sulfoxidation reactions based on VOS04 loaded streptavidin...

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... 

Figure 3.7 Stabilizing CH...JI (blue) and CH...O (red) interactions found in the transition states for the sulfoxidation of thioanisole catalyzed by imidodiphos-phonic acid 21. (From Jindal, G. and Simoj, R. B. Axially Chiral Imidodiphosphoric Acid Catalyst for Asymmetric Sulfoxidation Reaction Insights on Asymmetric Induction. Angew. Chem. bit. Ed. 2014. 53, 4432-4436. Copyright Wiley-VCH Verlag GmbH Co. KGa A. Adapted with permission.)...

Jindal, G. Sunoj, R. B. Axially Chiral Imidodiphosphoric Acid Catalyst for Asymmetric Sulfoxidation Reaction Insights on Asymmetric Induction. Angew. Chem. Int. Ed. 2014,53,4432-4436. 

SCHEME 48.11. Asymmetric sulfoxidation reaction developed by Bolm and based on the employment of the Fe(in) metal center with Schiff base 26 chiral hgand in the presence of carhoxyhc acid 27 or salt additives with H2O2 as the terminal oxidant. 

Self-Supported BINOLate/Ti(IV)-Catalyzed Asymmetric Sulfoxidation Reaction... 

A new enantiopure, bidentate ligand (35, 45 )-2,2,5,5-tetramethyl-3,4-hexanediol [(35, 45)-186] was developed by Yamanoi and Imamoto and investigated as asymmetric inductor in the titanium-catalyzed sulfoxidation reaction with various hydroperoxides as oxygen donors (Scheme 107). The catalytically active species was then prepared in situ from Ti(OPr-/)4 and ligand (35,45)-186. The most efficient hydroperoxide in terms of enantio-selectivity turned out to be cumyl hydroperoxide (95% ee compared to 30% ee in the case of methyl p-tolyl hydroperoxide), and molecular sieves 4 A had a beneficial effect on the... 

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... 

A similar asymmetric nitroolefination reaction has been described that uses an optically active / -nitro-a,/ -unsaturated sulfoxide, e.g., 2-nitro-1-[(/ )-2-phenylpropylsulfmyl]cyclohexene, where the chiral sulfoxide moiety functions as a leaving group. Condensation oflactam enolates with this sulfoxide affords substituted lactams with high enantiomeric excesses and good yields27. 

After the first discovery of the asymmetric sulfoxidation by Kobayashi et al. [226], it could be shown that a large number of aryl alkyl sulfides are oxygenated with enantiomeric excesses higher than 98% [227-229]. Other peroxidases also catalyze this reaction. Interestingly, the plant peroxidase HRP [230] yields the (S)-sulfoxide, whereas mammalian myeloperoxidase [223] and lactoperoxidase [231] catalyze the formation of the R-enantiomers. The stereospecific sulfoxidation of aryl alkyl sulfides by purified toluene dioxygenase (TDO) from P. putida was also studied in this context [232] and showed that sulfoxidation yielded the (S)-sulfoxides in 60-70% yield, whereas CPO under the same conditions yielded 98% (R)-sulfoxides (Scheme 2.15). CPO is thus again an exception from the rule in that it produces R-enantiomeric sulfoxides, besides its bacterial origin [227]. The reason for this behavior lies in the... 

Proline was among the first compounds to be tested in asymmetric conjugated reactions, both as a chiral ligand [8] and also as an organic catalyst [3]. The earliest asymmetric intermolecular Michael-type addition, in which proline catalyzed the reaction (arguably via enamine formation) was reported by Barbas and colleagues [9, 10] and by List and co-workers [11]. The reaction, which proceeded in high chemical yield (85-97%) and diastereoselectivity, albeit afforded near-racemic products in dimethyl sulfoxide (DMSO) [11] (Scheme 2.37). The enantio-selectivity of the addition was later ameliorated by Enders, who demonstrated that a small amount of methanol rather than DMSO was beneficial to the enantiose-lectivity of the addition reaction [12]. 

The influence of solvents was extensively studied [38, 40b, 42], with reactions shown capable of being performed in neat, or, virtually in any polar medium. Whilst high dielectric constant oxygenated solvents such as tetrahydrofuran (THF), 1,4-dioxane, acetone (Et20), dimethyl sulfoxide (DMSO), and dimethyl-formamide (DMF) are used in non-asymmetric MBH reactions, dichloroethane (CH2C12) or acetonitrile are preferred for asymmetric transformations. MBH re-... 

In an interesting example of the first asymmetric Heck reaction,6 using sulfoxides as chiral auxiliaries, Carretero et al.145 have recently used a new chiral sulfoxide, obtained by the DAG methodology. The palladium-catalyzed arylations of 4-arylsulfinyl-2,3-dihydrofurans 102 have shown that the stereochemical outcome of the reaction is highly dependent on the substitution of the sulfoxide. Thus, independently of electronic substitution of the aryl iodide, different aryl sulfoxides... 

Remarkably, there is a noticeable lack of general methods for the asymmetric preparation of chiral sulfoxides from sulfides. The most satisfactory method would be a generally applicable enantioselective sulfoxidation reaction which would allow the preparation of sulfoxides from any prochiral sulfide with high ee s and in which the sulfoxide would be amenable to enantioselective preparation in both senses. 

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