Sulfenylation alkenes

MO calculations for the gas phase indicate that sulfurane intermediate (17) is more stable than the ion (18) by about 380 kJ moP, which suggests that sulfuranes may be important in the reaction of sulfenyl halides with alkenes in non-polar solvents (77JCS(P2)1019). 

Alkenes and alkynes react with sulfur dichloride (SC12), giving 2-chloroethyl(or vinyl)sulfenyl chlorides. The reaction is an electrophilic addition to the multiple bond, and consequently the possible intermediacy of thiiranes, or thiiranium ions analogous to bromonium ions, has been... 

Sulfur dichloride is difficult to handle it disproportionates readily, has an unpleasant odor, and tends to introduce extra, unwanted chlorine atoms. Several reagents have been developed, especially by Harpp and co-workers, in which these undesirable features have been modified or eliminated. Succinimide-N-sulfenyl chloride (28) and phthalimide-N-sulfenyl chloride (29) are both stable crystalline compounds which undergo many of the reactions of the sulfur dichloride itself. They can, for example, be used in a facile, high yield synthesis of thiiranes from alkenes.33... 

Insertion of carbon monoxide into Csp2—Zr bonds occurs readily at ambient temperatures or below to produce a,(5-unsaturated, reactive acyl zirconocene derivatives [27—29]. Early work by Schwartz demonstrated the potential of such intermediates in synthesis [5d], as they are highly susceptible to further conversions to a variety of carbonyl compounds depending upon manipulation. More recently, Huang has shown that HC1 converts 16 to an enal, that addition of a diaryl diselenide leads to selenoesters, and that exposure to a sulfenyl chloride gives thioesters (Scheme 4.11) [27,28]. All are obtained with (F)-stereochemistry, indicative of CO insertion with the expected retention of alkene geometry. 

The addition of sulfenyl halides to both alkenes and alkynes had been the subject of extensive mechanistic study. The salient results up to about 1969 have been reviewed by Mueller (1969) and Modena and Scorrano (1968). Considerable additional work on various aspects of the detailed mechanisms has been done since that time by Schmid and his co-workers (Schmid and Csizmadia, 1972 Schmid et al., 1972 Schmid and Nowlan, 1976 Schmid et al., 1976b Cerksus et al., 1978). A recent short review by Smit et al. (1979) criticizes some aspects of earlier interpretations of mechanism. 

Sulfenyl chlorides and most of the other sulfenyl derivatives react with alkenes to give generally anti addition products with a high stereoselectivity. Although the mechanism of these reactions is still under study, it is usually accepted that sulfenyl transfer from the carrier to nucleophilic double bonds is consistent with the multistep mechanism reported in equation 907b. 

The formation of the bridged intermediate has been represented as an SN2-like displacement of the leaving group from the sulfenyl sulphur of 85116, or alternatively, as reported in equation 90 in agreement with the addition of other electrophiles to alkenes, it has been proposed that the reaction involves the initial formation of jr-complex 86 in a rapid equilibrium with the reagents76. 

In agreement with the mechanism reported in equation 90, the reaction generally follows a second-order rate law (equation 92), first order in the sulfenyl halide and in the alkene, respectively. 

Simple 1,2-additions to this compound have been observed123131132 also in other sulfenylation reactions, and in other electrophilic additions involving strongly bridged intermediates. Although these results have been interpreted as evidence that additions of sulfenyl halides to symmetrical alkenes do not involve open carbenium ions before the product-determining step, the different behavior observed in the case of 49 suggests123 that close proximity is necessary to have transannular participation of 7r-bonds, at least in additions of sulfenyl derivatives and of some other electrophiles carried out in the presence of efficient nucleophiles. 

Mechanistic studies have been most thorough with the sulfenyl halides.63 The reactions show moderate sensitivity to alkene structure, with electron-releasing groups on the alkene accelerating the reaction. The addition can occur in either the Markownikoff or anti-Markownikoff sense.64 The variation in regioselectivity can be understood by... 

Di-tert-butyl methylenemalonate was originally prepared by phenyl-sulfenylation of di-tert-butyl methylmalonate and thermal elimination of the related sulfoxide.8 Because methylenemalonate esters are customarily prepared by Knoevenagel-type condensation of malonic esters with formaldehyde equivalents, the considerably more convenient procedure described herein was subsequently adapted from Bachman and Tanner s study using paraformaldehyde under metal ion catalysis.39 The approximately 6% di-tert-butyl malonate accompanying the product has presented no interference in the aforementioned reactions with nucleophilic alkenes under neutral or acidic conditions, but its presence should be taken into consideration in other applications. 

Treatment of bis(trimethylsilyl)sulfide (8) with bromine in anhydrous dichloromethane at - 78°C followed by addition of 1,2-disubstituted alkenes (9) to the reaction mixture at the same temperature gave the thiiranes (10) in about 30% yields (Scheme 5) (88TL4177). This synthesis is (i) highly selective, since only 1,2-disubstituted alkenes undergo the transformation to thiiranes and (ii) stereospecific since the stereochemistry of the alkene is retained in the thiirane. The nature of the reagents suggests that the silyl subsituted sulfenyl bromide (11), formed by attack of bromine on (8), is an intermediate. 

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