Alkenyl sulfides, formation

A detailed study of the halocyclisation of unsaturated benzyl sulfides has shown that tetrahydrothiopyran formation is favoured from alkenyl sulfides through 6-endo-trig and 6-exo-trig modes. However, for alkynyl sulfides only the 6-exo-dig cyclisation is predominant, leading to dihydrothiopyrans (95JOC6468). 

The treatment of 7V-tosyl-/>-anisidine (75) and several of its derivatives with BTIB in the presence of activated alkenes provides an interesting example of an intermolecular cyclodehydrogenation sequence (99H1785). Arylpropenes are converted to 7V-tosylindolines 76 under these conditions, while alkenyl sulfides lead to the 7V-tosylindoles 77 (Scheme 22). The intermediate formation of 7V-tosylnitrenium ions in these reactions was suggested. 

The regiochemical outcomes, thus, are strong indicatives for the reaction pathways the formation of linear alkenyl sulfides via a radical-mediated reaction pathway and the branched alkenyl sulfides via an insertion of alkyne into an M S bond. An interesting complementary selectivity between Tp Ph(PPh3)2 and Wilkinson s catalysts has been observed in the hydrothiolation of alkyl thiols, where a migratory Rh-H insertion pathway 17 was proposed (Scheme 46.4). 

Treatment of l-chloro-2-methylalkenyl p-tolyl sulfoxides with AMithio-2-pipcrid-one in THF at room temperature resulted in the formation of l-chloro-2-(hydroxy-methyl)alkenyl p-lolyl sulfides in good yields. The reaction is the first example of the Mislow-Braverman-Evans rearrangement retaining the sulfur atom on the original carbon.29... 

Fig. 15. Rates of products formation as a function of cyclopentene pressure in the COS-cyclo-CjHio system. ( ) CO (O) cyclopentene sulfide (V) alkenyl mercaptans ( ) vinylic mercaptan ( ) sum of sulfur-containing products.

Alkyl and allyl sulfides and selenides can be transformed readily into the corresponding alkyl and alkenyl halides respectively. This reaction takes advantage of the easy formation of the corresponding sul-fonium and selenonium salts on reaction with alkyl halides, alkyl bromoacetates or bromine - (Scheme S3). A related process involves the intermediary formation of a selenoxide and its further reaction with hydrochloric or hydrobromic acids. ... 

The iron-catalysed reaction of heteroarenes, including indoles, pyrroles, thiophene, and furan, with 3-methyl-2-quinonyl boronic acids allows the formation of alkylated products, such as (68), rather than the more usual alkenylated products. The unusual alkylation, at the 5-position, of oxindoles to give products such as (69) has been reported using the acid-catalysed reaction with benzylic alcohols in nitromethane. Silylation of indole, at the 3-position, to give (70) has been achieved using a cationic ruthenium(II) sulfide complex as a catalyst. A sulfur-stabilized silicon electrophile is formed resulting in a Wheland intermediate which is deprotonated by sulfur atom. ... 

Cross-coupling of the silyl derivatives of thiols R3SiSH led to the formation of vinyl (or aryl) silylsubstituted sulfides (Scheme 3.4) [10]. Further transformation of the products in alkylation or Pd-catalyzed alkenylation reactions was reported as a route to unsymmetric RSR sulfides as final products. This stepwise synthetic approach allowed avoidance of the use of some scarcely available Ar(Het)SH, which otherwise would be required to prepare the same final products in a single step utilizing crosscoupling. 

However, the thermal stability of the adducts depends on the nature of the sulfenyl chloride and on the structure of the alkenyl group. For instance, 1-chlorosilatrane (38) and alkenyl phenyl sulfides are formed by treatment of 20 or 76 with phenylsulfenyl chloride (equation 122). It is likely that the initial adducts (86 and 87), which are unstable at — 10°C, undergo elimination reaction via the formation of an episulfonium intermediate (88), followed by nucleophilic attack of Cl at silicon. In contrast, the adducts from phenylsulfenyl chloride and trialkylalkenylsilanes are thermally stable and decompose by strong bases . The larger electron-donating ability of the silatranyl group compared with the trialkylsilyl group is most probably responsible for these diffeences in stability. 

---