Sulfene intermediates

On the basis of these findings, the reaction of acyl imines with methanesulfony 1 chloride-triethylamine is not expected to proceed via a sulfene intermediate as previously proposed [99]. Again, a carbanion intermediate accounts nicely for the experimental facts. The electrophihcity of the hetero-l,3-diene is exdemely high, therefore the carbanion, formed on reaction of triethylamme with methanesulfonyl chloride, should undergo nucleophilic attack at C-4 of the hetero-1,3-diene faster than sulfene formabon by chloride elimination. 

Route b involves the formation of one carbon-carbon bond and one carbon-sulfur bond. It belongs to the category of sulfene chemistry143. Sulfene intermediates react readily with diazoalkanes to produce, after the loss of nitrogen, thiirane dioxides. So far, this appears to be the method of choice for the preparation of thiirane dioxides of all types. 

The photolysis of various substituted thiete dioxides under similar conditions resulted in the formation of the unsaturated ketones (255)264, most probably via a vinyl sulfene intermediate followed by a loss of sulfur monoxide as shown in equation 96. The same results were obtained in the thermolysis of 6e (R1 = R3 = Ph R2 = R4 = H)231, which further demonstrates that similar mechanisms are operative in thermolyses and pho-tolyses of thietane dioxides and thiete dioxides. 

One of the first uses of the allylic sulfoxide-sulfenate interconversion was made by Jones and coworkers64, who reported exclusive suprafacial rearrangement of the allyl group in the steroidal sulfoxide 17 shown in equation 13. Two other examples are shown in equations 1465 and 1566. Evans and coworkers have demonstrated the utility of the suprafacial allylic sulfoxide-sulfenate rearrangement in a new synthesis of the tetracyclic alcohol 24 (equation 16)67, as well as in a synthesis of prostaglandin intermediates as shown in equation 1768. The stereospecific rearrangement of the unstable sulfenate intermediate obtained from the cis diol 25 indicates the suprafacial nature of this process. 

When the ring system containing the sulfone moiety is unsaturated, a quite different set of products has been observed on photolysis, apparently arising from an initial cycloreversion to a short-lived sulfene intermediate. For example, irradiation of 2H-1-benzothiopyran 1,1-dioxide (41) in dichloromethane or methanol gave the cis- and trans-sultines (42) and the ring-expanded sultine (43) in roughly equal amounts, accompanied by minor amounts ( 5%) of the products of formal loss of S02, i.e. indene, and SO, i.e. 4472... 

Scheme 3-6 Triflate and sulfenate intermediates in the sulfoxide glycosylation.

The first evidence that an elimination-addition mechanism could be important in nucleophilic substitution reactions of alkanesulfonyl derivatives was provided by the observation (Truce et al., 1964 Truce and Campbell, 1966 King and Durst, 1964, 1965) that when alkanesulfonyl chlorides RCH2S02C1 were treated in the presence of an alcohol R OD with a tertiary amine (usually Et3N) the product was a sulfonate ester RCHDS020R with exactly one atom of deuterium on the carbon alpha to the sulfonyl group. Had the ester been formed by a base-catalysed direct substitution reaction of R OD with the sulfonyl chloride there would have been no deuterium at the er-position. Had the deuterium been incorporated by a separate exchange reaction, either of the sulfonyl chloride before its reaction to form the ester, or of the ester subsequent to its formation, then the amount of deuterium incorporated would not have been uniformly one atom of D per molecule. The observed results are only consistent with the elimination-addition mechanism involving a sulfene intermediate shown in (201). Subsequent kinetic studies... 

In certain cases in which the substrate carries an a hydrogen, there is strong evidence1728 that at least some of the reaction takes place by an elimination-addition mechanism (ElcB, similar to the one shown on p. 382), going through a sulfene intermediate,1729 e.g., the reaction between methanesulfonyl chloride and aniline. 

Barton et a/.618-818 have shown that the sulfenic intermediates produced thermally from penicillin sulfoxides can be added across acetylenic esters to produce conjugated sulfoxides in good yields. For example, the methyl ester 95 with DMAD afforded the isomeric... 

Related compounds such as alkanesulfonyl chlorides by treatment with base afford sulfene intermediates, which undergo attack by nucleophiles (H2O, alcohols, amines,. ..) present in the reaction mixture. Although sulfenes have been the subject of intense studies345,346 it has been impossible so far to succeed in the isolation of even one of them. 

A diastereoselective synthesis of /3-sultams by 1,3-asymmetric induction in [2+2] cycloaddition of a sulfene intermediate and a chiral imine has been described (Scheme 48), and it was found that jV-alkylirnines give better diastereo-selectivity than iV-arylimines. The best selectivity was found in the case ofiV-(l-/-butylethyl)imine (67% yield, >95% de), and the diastereoselectivity was independent of the size and conformation of the N-substituents in the imines. Diastereoisomers bearing an /V-aryl group were separable by silica gel column chromatography <1998JOC8355>. 

Sultams are accessible using a [2+2] cycloaddition reaction between an alkylsulfonyl chloride and an aryl or /-butylimine, better yields being obtained in the latter case. A diastereoselective synthesis of these /3-sultams has been described where 1,3-asymmetric induction occurs in a [2+2] cycloaddition between a sulfene intermediate and... 

Thermolysis of thiete 1,1-dioxide (116), either in the vapor phase or in benzene solution, gives the oxathiole (117) in over 80% yield. The most likely mechanism involves a sulfene intermediate as shown in Scheme 38 (70CJC3704). Not surprisingly, the flash thermolysis of thietane 1,1-dioxide, the saturated analog of (116), gives only cyclopropane and propene. 

Mutarotation at S is also reported to be catalyzed by light,1041 possibly via an O-bound sulfenate intermediate (equation 162). Adamson and coworkers report that photodecomposition to Co j is the final outcome.1049 For sulfinate complexes donor S to O isomerization occurs1020,1049 and red O-bound (N,0)[Co(cyst02)(en)2](C104)2-H20 has been isolated.1029 This process (equation 163 hv 350-450 nm) is thermally reversible (r1/2 600h, r.t.) but which O atom is involved, axial or equatorial, is not known. The O-bound form is now asymmetric, and some specificity might be expected. 

The proposed mechanism is in accordance with the observed solvent dependence of the reaction. Whereas the dipolar sulfoxide is expected to be more strongly solvated with an increase in solvent polarity, the less dipolar sulfenate should be relatively insensitive to such a solvent change. Stabilization of the sulfoxide, relative to the less dipolar activated complex (which should be similar to the sulfenate intermediate), increases the enthalpy of activation, AH. This is refiected in the necessity of breaking increasingly strong solute-solvent interactions. On the other hand, because desolvation on activation is expected to increase the degrees of freedom in the system, a more positive AS is expected to work in the opposite direction and effect a compensating increase in k with... 

As discussed previously, treatment of thiolates generated from thiols and MeLi with sterically hindered A -sulfonyl-oxaziridine 116 gave the corresponding sulfenate intermediates which were alkylated to afford sulfoxides 118 (Equation 4) <1997JOC8626, 1999PS(153)381, 2004JOC6916>. Oxidation of thiolates generated from aryl thiols... 

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