Methyl aryl sulfoxides

Since these adducts undergo reductive desulfuration with Raney nickel, optically active aryl methyl sulfoxides are versatile reagents for the conversion of imines to optically active amines. 

Two more recent studies of the nature of the sulfur-carbon bond cleavage in sulfoxide photolyses have been conducted, using ESR5 and CIDNP6 detection methods, respectively. In the latter case there is some evidence for a triplet process being involved in the photocleavage of aryl methyl sulfoxides. 

Carbanions derived from optically active sulfoxides react with esters, affording generally optically active )S-ketoesters ° . Kunieda and coworkers revealed that treatment of (-t-)-(R)-methyl p-tolyl sulfoxide 107 with n-butyllithium or dimethy-lamine afforded the corresponding carbanion, which upon further reaction with ethyl benzoate gave (-l-)-(R)-a-(p-tolylsulfinyl)acetophenone 108. They also found that the reaction between chiral esters of carboxylic acids (R COOR ) and a-lithio aryl methyl sulfoxides gave optically active 3-ketosulfoxides The stereoselectivity was found to be markedly influenced by the size of the R group of the esters and the optical purity reached to 70.3% when R was a t-butyl group. 

Kinetic studies of the oxidation of diaryl sulfoxides by Cr led to the proposition of a single electron transfer as in the case of aryl methyl sulfoxides . These reactions were performed in aqueous acetic acid/perchloric acid medium, HCrOs" being the active... 

The Orsay group discovered that addition of 4 mol equiv. of isopropanol to the Padua complex (Ti(0-i-Pr)4/(i ,7 )-DET = 1 4) gave a complex that can be used in catalytic quantities (10 mol % with respect to sulfide) in the sulfoxidations with cumene hydroperoxide [39,40], With this catalyst system, which needs the presence of 4A molecular sieves, enantioselectivities in the range of 90% ee have been achieved for the preparation of various aryl methyl sulfoxides. Even benzyl methyl sulfoxide was obtained with 90% ee. 

Andersen, K. K., Bujnicki, B., Drabowicz, J., Mikolajczyk, M., and O Brien, J. B. (1984) Synthesis of enantiomerically pure alkyl and aryl methyl sulfoxides from cholesteryl methanesulfinates, J. Org. Chem. 49, 4070-4072. 

The main methodologies developed until now for enantioselective oxidation of sulfides are effective only in the oxidation of alkyl aryl sulfoxides. Dialkyl sulfoxides on the other hand are generally oxidized with only poor selectivity. In an attempt to solve this problem, Schenk s group69 recently reported a stereoselective oxidation of metal-coordinated thioethers with DMD. The prochiral thioether is first coordinated to a chiral ruthenium complex by reaction with the chloride complexes [CpRu[(S,S)-chiraphos]Cl], 36. Diastereoselective oxygen transfer from DMD produces the corresponding sulfoxides in high yield and selectivity. The chiral sulfoxides 37 are liberated from the complexes by treatment with sodium iodide. Several o.p. aryl methyl sulfoxides have been obtained by this method in moderate to high ee (Scheme 12). 

A number of other oxidants which produce sulfones from sulfoxides are known, such as N02BF4, ° oxygen with Ir or Rh catalysts, ozone,KHSOs (Oxone) and K2S20s. Oxone is a highly chemoselective oxidant for the conversion of sulfides to sulfones without affecting hydroxy or al-kenic groups (equation 37). Similarly flavin (14) oxidizes aryl methyl sulfoxides to sulfones fairly selectively. ... 

Since these adducts undergo reductive desulfurization with Raney Ni, the optically active aryl methyl sulfoxide is a versatile reagent for the synthesis of optically active amines from imines. 

Chiral sulfoxides. These have been prepared by a Sharplcss-type asymmetric oxidation of sulfides (13,53), but this route is only efficient for preparation of aryl methyl sulfoxides (ee up to 92-96%). A more general route involves reaction of a pure chiral... 

The second mechanism in the literature was suggested several years later on the basis of CIDNP investigation of the photochemistry of aryl methyl sulfoxides [31,45]. It will be referred to as the sulfinyl mechanism. The key step in the sulfinyl mechanism is the reaction of a free sulfinyl radical (formed by a-cleavage) with another radical to effect O atom transfer. 

Uemura et al. found that the combination Ti(OPr%/binaphthol/water in ratio 1 2 >10 acts as a catalyst for oxidation of aryl methyl sulfides into the corresponding sulfoxides by Bu OOH (see also Section 1.4.1) [159]. A mechanistic study showed that the titanium complex was a sulfoxidation catalyst (initial ee -50%) as well as a catalyst for the overoxidation into sulfones, with an enhancement of the ee of the residual sulfoxides (because the minor sulfoxide enantiomer is preferentially oxidized). In a subsequent paper, the authors reported the kinetic resolution of racemic aryl methyl sulfoxides by the same catalyst [160]. A stereoselectivity factor s of 2.6 was calculated for the kinetic resolution of racemic methyl p-tolyl sulfoxide. For example, methyl p-tolyl sulfoxide (<99% ee) could be recovered from oxidation at about 75% conversion. Using partially resolved l,l -binaphthol, a positive nonlinear effect was established. 

An improved catalytic reaction with the use of 10 mol% of titanium using a ratio Ti(0 Pr)4/(J ,J )-DET/ PrOH = 1 4 4 in the presence of molecular sieves gave an efficient sulfoxidation vfith ees up to 95% with various aryl methyl sulfoxides [64]. The asymmetric Ti-catalyzed sulfoxidations with alkyl hydroperoxides have been reviewed by Kagan [65[. 

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