Percamphoric acid

Optically active peracids such as percamphoric acid have been used to oxidize selectively one sulphoxide enantiomer in a racemic mixture. These reactions involve the use of 0.5 molar equivalents of the peracid in either ether47 or chloroform48 as solvent. The presence of nitro groups causes the oxidant to be consumed without oxidation of the sulphoxide functionality. This method is usually used to obtain an optically active sulphoxide by recovery of the unreacted material after oxidation. 

According to this correlation model, in which the principles of steric control of asymmetric induction at carbon (40) are applied, the stereoselectivity of oxidation should depend on the balance between one transition state [Scheme 1(a)] and a more hindered transition state [Scheme 1(6)] in which the groups and R at sulfur face the moderately and least hindered regions of the peroxy acid, respectively. Based on this model and on the known absolute configuration of (+)-percamphoric acid and (+)-l-phenylperpropionic acid, the correct chirality at sulfur (+)-/ and (-)-5 was predicted for alkyl aryl sulfoxides, provided asymmetric oxidation is performed in chloroform or carbon tetrachloride solution. Although the correlation model for asymmetric oxidation of sulfides to sulfoxides is oversimplified and has been questioned by Mislow (41), it may be used in a tentative way for predicting the chirality at sulfur in simple sulfoxides. 

Optically active aromatic thiosulfinates were first prepared by asymmetric oxidation of diaryl disulfides with (+)-percamphoric acid (105,112). Apart from the fact that the optical purity of diaryl... 

Optically active iV-p-toluenesulfinyl piperidine 75 prepared in low optical purity by oxidation of iV-p-toluenesulfenyl piperidine with (+)-percamphoric acid represents the first example of a chiral sul-flnamide (105). As in the case of asymmetric oxidation of sulfides and sulfenates, the synthetic utility of this method is strongly limited by its low stereoselectivity. 

Several peroxy acids are need in the conversion of olefins into epoxides. Their properties and preparations have been described by Swem.1<78>1878 Included among them are performio acid, peracetic arid, parbenzoio acid, monoperphthalic acid, and percamphoric acid, Mere recently trifluoroperacetic acid has attained some promi-nenee.S01>1I4fl-1778 Certain desirable features have been discovered in y-nitroperbenzoic arid as well.1 ... 

Reactions on natural rubber or cis-l,4-polyisoprene, in the presence of various OA catalysts such as c -camphorsulfonic acid, barium active-isoamyl-alcoholate were reported by Minoura [189]. Optically active polymers, after hydrolysis of the optically active group, were only obtained with barium alcoholate. The rotatory powers are very small, even for the best reported value of the addition (20%). With other catalysts, there was no asymmetric induction. The mechanism producing the active adduct polymer was thought by the author to be as follows (Scheme LXXI) ... 

More recently, an asymmetric oxidation was reported by Minoura et al [191 ], more precisely the asymmetric oxidation of polyphenylvinylsulfide and poly-f-butylvinylsulfide, with optically active percamphoric acid. Optically active copoly sulfoxides were obtained (% oxidation max.=56). The specific rotation of the oxidized polymers increased linearly with the sulfoxide group content of polymers. The dispersion curve of these copolymers fit the simple Drude equation. The authors also report on bio-asymmetric oxidation by Aspergillus niger and Penicillum notatum. The oxidation did not occur so easily (% oxidation max.=5) and of course the specific rotations are very low. 

Asymmetric oxidation of polyphenylvinyl sulfide and poly- er. -butylvinyl sulfide with percamphoric acid, Aspergillus niger and Penicillium notatum are also described [27]. 

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