The rates and products of addition of 4-chlorobenzenesulfenyl chloride to a series of trisubstituted alkenes

Abstract

. Can. J. Chem. 65, 1172Chem. 65, (1987. The rates and products of addition of 4-chlorobenzenesulfenyl chloride to E-and Z-3-methyl-2-pentene, E-and Z-3,4-dimethyl-2-pentene, E-and Z-3,4,4-trimethyl-2-pentene, and 2,4,4-trimethyl-2-pentene were studied to determine if the mechanism involved a tertiary P arylthio carbocation. It was found that the rates of addition are not as fast as one would predict based on the polar effects of the substituents on the rate of addition to mono-and disubstituted alkenes. The products are formed by anti-stereospecific addition as in the case of addition to 1,2-disubstituted alkenes. As a result it is concluded that the reaction occurs by means of the usual mechanism involving a bridged thiiranium ion. Dans le but de dtterminer si le mtcanisme de la reaction implique un carbocation P-arylthio tertiaire, on a dttermint les vitesses de rtaction ainsi que la nature des produits obtenus lors de l'addition du chlorure de chloro-4 benzknesulfknyle sur les mtthyl-3 pentknes-2 (E ou Z), sur les dimCthy1-3,4 pentknes-2 (E ou Z), sur les trimtthyl-3,4,4 pentknes-2 (E ou Z) et sur le trimtthyl-2,4,4 pentkne. On a trouvt que les vitesses d'addition ne sont pas aussi rapides que celles que l'on pourrait prtvoir en se basant sur les effets polaires des substituants sur les vitesses d'addition sur des alcknes mono-ou disubstituts. Les produits se foment par une addition sttrtosptcifique anti, comme dans les cas des additions sur des alcknes disubstituts en positions 1,2. Sur la base de ces rtsultats, on en conclut que la rtaction se produit par le biais du mtcanisme usuel impliquant un ion thiiranium pontt.[Traduit par la revue]The systematic study of the addition of arenesulfenyl chlorides to alkenes carried out in this laboratory has established many of the mechanistic features of this reaction (1). It is possible to conclude that in this addition reaction the ratedetermining transition state is bridged and there is little charge on either carbon or sulfur (2). Furthermore, in the productdetermining step, attack by chloride ion occurs at the least hindered carbon of the thiiranium ion (3). These results are in contrast to the reactions of stable thiiranium ions, which undergo reactions with nucleophiles at the most hindered carbon atom (4). A possible explanation (5) for this difference in product regiochemistry is that the addition reaction, carried out in relatively nonpolar solvents, occurs by means of a contact ion pair. The reaction of stable thiiranium ions, carried out in highly polar solvents, occurs by means of a dissociated ion.The importance of ion pairs in the mechanism of many reactions in solution is well documented (6). Electrophilic additions are no exception. A number of people have proposed that the mechanism of an electrophilic addition reaction could involve ion-pair intermediates (7). Subsequent work by Rolston and Yates (8) and Gamier and Dubois (9) established the importance of ion pairs in the bromination of alkenes. In 1972, we provided evidence that the mechanism of...