Oxidation of sulfenamides

Mikolajczyk and coworkers have summarized other methods which lead to the desired sulfmate esters These are asymmetric oxidation of sulfenamides, kinetic resolution of racemic sulfmates in transesterification with chiral alcohols, kinetic resolution of racemic sulfinates upon treatment with chiral Grignard reagents, optical resolution via cyclodextrin complexes, and esterification of sulfinyl chlorides with chiral alcohols in the presence of optically active amines. None of these methods is very satisfactory since the esters produced are of low enantiomeric purity. However, the reaction of dialkyl sulfites (33) with t-butylmagnesium chloride in the presence of quinine gave the corresponding methyl, ethyl, n-propyl, isopropyl and n-butyl 2,2-dimethylpropane-l-yl sulfinates (34) of 43 to 73% enantiomeric purity in 50 to 84% yield. This made available sulfinate esters for the synthesis of t-butyl sulfoxides (35). 

The anodic oxidation of sulfenamides has been the subject of a review in the literature [5]. 

Sulfenamides arc usually oxidized to sulfonamides tns(tnfluorotnetliane-sulfenyl) and bis(tnfluoromethanesulfenyl)arnineare converted to the corresponding sulfonamides by sodium hypochlorite at 20 tor 3 h m 61 and 92% yield, respecbvely [111] Oxidation of pentafluorobenzenesulfendimde by manganese dioxide yields a sulfinamide intermediate that can be trapped [772] (equabon 102)... 

A range of other sulfur(IV) and (VI) derivatives formally obtained by the oxidation of thioglycosides have been prepared, but they are apparently not employed to date in O-glycosylation reactions. These include glycosyl sulfenamides and sulfonamides, as well as sulfinates and sulfonates [289]. S-Glycosyl sulfenic acids have been prepared as transients by the syn-elimination of S-(2-cyanoethyl) glycosyl sulfoxides [311]. 

Early reports of an Movl-Cys complex have been shown to be erroneous because redox occurs to give the Mov complex [Mo2 03 (CysO)4]4 and cystine further electrochemical reduction to Mom is also possible. Oxidation of Cys coordinated to Co111 can give a sulfinic acid or a sulfenamide, according to the reagents and conditions used. 

In the case of the oxidation of methyl alkyl sulfides, enantioselectivity remains in the range of 50-60% ee (Table 6C.2). Disufides R-S-S-R, sulfenamides R-S-NR 2, and sulfenates R-S-O-R were oxidized to the corresponding chiral thiosulfinates, sulfinamides, and sulfinates, respectively (<52% ee, Table 6C.3) [21]. 

The hydrolysis of a series of spiro-/4-sulfanes such as (330)-(332) leads to sulfoxides in dioxane-water solutions. A mechanism involving slow nucleophilic attack of water on the positively polarized sulfur atom and simultaneous 0—H and S—N bond cleavage is proposed.298 The photo-oxidation of the sulfenamides (333 R = Me, Et, Ph, Bn, etc.) has been reported.299... 

In a broad program of using chiral oxazolidinones in asymmetric synthesis,100 Evans s group published a paper in 1992 on the synthesis and utilization of fV-sulfinyl oxazolidinones as new sulfinylating agent.87 Two chiral auxiliaries were used in the study oxazolidinones derived from (4R, 5S)-norephedrine 74101 and (45)-phenylalanine 75.102 The corresponding fV-sulfinyl oxazolidinones 77 and 78 were obtained either by sulfmylation of the metallated oxazolidinone or by oxidation of the derived N-sulfenamides (Table 15). 

In the oxidation of the N-sulfenamides 79, the w-chloroperbenzoic acid proved to be the best oxidant, yielding the /V-sulfinyl oxazolidinones 78b-d in good yields (72-96%) as 1.4-2.5 1.0 mixtures of diastereoisomers, which were readily purified by chromatography (Scheme 23). 

Yang J, Groen A, Lemeer S et al (2007) Reversible oxidation of the membrane distal domain of receptor PTPalpha is mediated by a cyclic sulfenamide. Biochemistry 46 709-719... 

Direct oxidation of sulphenamides or sulphinamides to the sulphonamide have been performed with a variety of oxidants439-443. For example, potassium permanganate oxidation of 2,4-dimethoxy-6-pyrimidine sulphenamide gave the sulphonamide in 64% yield, as shown in equation 108444. In another example, meta-chloroperbenzoic acid was used to oxidize sulfenamides to the sulphonamide442. However, Chiang and collaborators have reported that they were unable to isolate any sulphonamide product after one such attempted oxidation445. 

Nitrobenzene and many of its 2-, 3-, and 4-substituted derivatives are converted into nitroaniline derivatives by treatment with sulfenamides in the presence of t-BuOK (eq 90). In this conversion, termed vicarious nucleophilic substitution (VNS), the base presumably promotes both the formation of the nucleophilic sulfenamide anion and the -elimination of the thiocar-bamoyl group from the other examples of t-BuOK-promoted VNS reactions of nitrobenzenes have appeared in recent years. An interesting example of this process involves the synthesis of dithianylated nitrobenzenes which are hydrolyzable to aldehydes (eq 91). The treatment of mixtures of m-nitroaniUne and enoUzable ketones with t-BuOK in DMSO leads to nitroindoles by oxidative nucleophilic substitution of hydrogen (eq 92). The proposed mechanism for this transformation involves attack of the potassium enolate of the ketone on the ring, spontaneous oxidation of the a-adduct, and imine formation and tautomerization. 

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