Alkenes with thiols, free-radical

When thiols are added to substrates susceptible to nucleophilic attack, bases catalyze the reaction and the mechanism is nucleophilic. These substrates may be of the Michael type or may be polyhalo alkenes or alkynes. As with the free-radical mechanism, alkynes can give either vinylic thioethers or dithioacetals ... 

Thiols may be converted into alkenes with one extra carbon atom by successive chloromethylation, oxidation at sulfur, and Ramberg-BScklund rearrangement. If the original thiol is prepared by free radical addition of H2S to an alkene, the sequence constitutes a one-carbon homologation of alkenes. Although these transformations were at one time important applications of the Ramberg-Backlund reaction, they have hardly been used in recent years. 

It has long been known that unsymmetrical ketones can be prepared by the reaction of aldehydes with alkenes under free-radical reaction conditions. Recently the revision of this chemistry has been reported by the Roberts group [42], They introduced thiols as a polarity reversal catalyst for the addition of aldehydes to alkenes. Thiyl radicals are electrophilic, and therefore a polar Sh2 type transition state for the hydrogen transfer step from an aldehyde would be ideal in this situation. Indeed, the addition of aldehydes to a variety of alkenes can be effected by... 

Thio-ene addition is another attractive method for the attachment of organic thioether groups to polysiloxane chains. It is based on the free-radical addition of a thiol group to a vinyl-substituted siloxane or the addition of a terminal alkene to a thiol-substituted siloxane (Eq. 2). " Boileau et al. used this reaction to graft different amino acids to the polymer backbone to induce potential interactions with polar surfaces. 

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