Unsaturated sulfenes

Cyclization of y, -unsaturated sulfenic acids to give thietan 1-oxide derivatives. Crystal structure of rei-(l/ ,2S,3/ ,4/ )-3-hexyl-2-hydroxy-methy 1-4-methylthietan 1 -oxide. ... 

The unsaturated sulfene, generated from 1-propenesulfonyl chloride and triethylamine, reacts with ketene diethylacetal to give the [2+2] cycloadduct 18 and a [4+2] cycloadduct 19. 

Besides radical additions to unsaturated C—C bonds (Section III.B.l) and sulfene reactions (see above), sulfonyl halides are able to furnish sulfones by nucleophilic substitution of halide by appropriate C-nucleophiles. Undesired radical reactions are suppressed by avoiding heat, irradiation, radical initiators, transition-element ion catalysis, and unsuitable halogens. However, a second type of undesired reaction can occur by transfer of halogen instead of sulfonyl groups283-286 (which becomes the main reaction, e.g. with sulfuryl chloride). Normally, both types of undesired side-reaction can be avoided by utilizing sulfonyl fluorides. 

Practically speaking, almost all syntheses of these systems are based on the enamine-sulfene cycloaddition reaction143 250. The thietane sulfone thus obtained yields, by elimination of R2NH, the desired unsaturated, four-membered sulfone system187 189 231 250 251 (equation 87). 

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. 

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... 

The methanol adduct may arise by photoprotonation of the double bond. A similar photoaddition of methanol has been reported on photolysis of 4973. In a closely related study, King and coworkers74 have shown that photolysis in methanol of the monocyclic unsaturated sulfone (50) gives the methanol adduct (51) and the acyclic sulfonate, CH2= C(Ph)CH—CHCH2S020Me, in 43 and 30% yield, respectively. Formation of the sulfonate ester in this case strongly supports the intermediacy of the sulfene (52) produced on cycloreversion. 

Direct elimination of toluene-/>-sulfenic acid (/>-TolSOH) from 210 occurred during its purification using column chromatography on silica gel, to give the unsaturated lactam 211 (Scheme 26) <1990CL1547>. 

If an unsaturated trapping agent is not present, however, things get really complicated. First, the sulfenic acid RSOH formed in (57) can form thiolsulfinate RS(0)SR and water (24). If the initial thiolsulfinate is an un-symmetrical one, RS(0)SR, this will mean that the symmetrical thiolsulfinate RS(0)SR will begin to appear in the reaction mixture. Second, in some way, perhaps aided by reaction of some of the water formed in (24) with thiolsulfinate, traces of acid become present in the solution. This acid catalyzes a complex disproportionation of alkyl thiolsulfinates that parallels in a number of important aspects the mechanism of the acid- and sulfide-catalyzed disproportionation of aryl thiolsulfinates already discussed. The steps that are important in this disproportionation are shown in (61)—(66). 

The group name is intended to cover esters derived from sulfenic, sulfinic and sulfonic acids, some of which are thermally unstable. This is especially so for esters of unsaturated alcohols, which are also liable to polymerise, catalysed by the liberated acids. Individually indexed compounds are ... 

One of the most widely applied cycloaddition techniques for the preparation of thietanes is the reaction of sulfenes with enamines. The stereochemistry of these reactions has been extensively investigated by Truce and Rach. Whether the mechanism is a two-step or a concerted process, both in accordance with the stereoselective formation of the cis form in Scheme 1, is still unresolved. The special orientation of the 1,4-dipolar intermediate 64, in which the charged phenyl and dimethylamino moieties are in proximity, enforces the cis geometry of the resulting thietane dioxide. In the concerted mode of reaction, formation of the orthogonal oriented unsaturated system, 65 should also yield the cis cycloadduct. 

Simple unsaturated sulfides cannot be used in place of enamines in cycloaddition reactions with sulfines leading to thietane dioxide derivatives. " Alkyl, vinyl, and cycloalkylvinyl sulfides, which carry a C=C double bond, are considerably less nucleophilic than the enamines and thus do not partake in cycloadditions to sulfene. But when the more electrophilic methylsulfonyl sulfene is used in association with an unsaturated sulfide substituted with a strong electron donating alkylamino group, the formation of thietane dioxides 72 is successful. 

Application of the enamine-sulfene cycloaddition allowed the synthesis of 78 and 79 from the corresponding a,/l-unsaturated amines and sulfenes. Similarly, using enamine 80, the crystalline morpholino thietane dioxides (81) were produced. Elimination via the N-oxide gave the corresponding thiete dioxide, whereas reduction by LiAlH4 yielded the thietane 82. 

Cycloaddition of sulfenes to a,)8-unsaturated sulfides that contain electron-donor substituents, such as alkyl and arylthio groups, has proved to be a valuable synthetic pathway for the production of thietane 1,1-dioxides. Methylsulfonylsulfene (84) is more reactive than the regular sulfenes because of the activating effect by the strong electron-acceptor properties of the sulfonyl group. If, in addition, a strong electron-donor group, such as the alkylamino function, is provided by the unsaturated sulfide molecule, the cycloaddition proceeds much more readily. 

An interesting procedure has been developed by Vat le to obtain C-5 branched-chain sugars [189]. It also takes advantage of the stereospecificity of the Claisen rearrangement of allyl (vinylsulfinyl) ether 158 derived from a 4,5-unsaturated sugar. An acrylic derivative 159 is formed in the process by elimination of sulfenic acid (Scheme 53). 

Sulfeny I chlondes react with allyl alcohols to yield allyl sulfenates, which are in equilibrium with the allyl sulfoxides [12] (equation 9a) These products can be oxidized to the corresponding sulfones (equation 9b) Pyrolysis of the sulfoxides gives sulfines or evidence for the presence of sulfmes Pyrolysis of sulfones leads to unsaturated compounds by extrusion of sulfur dioxide [12] (equation 9c)... 

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