Radical reactions hydrothiolation

In a simpler NiCl2/Et3N catalytic system, high selectivity and good yields in the hydrothiolation reaction were achieved in the presence of a radical trap, which suppressed the side-reaction [149]. For the activated alkynes with R= Ph and COOMe significant amounts of anti-Markovnikov products were obtained. Regioselectivity of the reaction depended on the alkyne RZH ratio and much better yields were achieved using Ni(acac)2 as a catalyst precursor [151]. 

The Diels-Alder [4 + 2] reaction (Fig. 12.4c), which is a cycloaddition between a conjugated diene (a four r-electron system) and a dienophile (a two r-electron system), is a elick reaction that has attracted much attention in macromolecular chemistry, particularly in providing new materials (Jones et al., 1999 Imai et ah, 2000 Gheneim et al., 2002 Vargas et ah, 2002 Durmaz et al., 2006 and Gacal et ah, 2006). Thiol-ene reactions (Fig. 12.4d, e), which also fall within the realm of click chemistry, refer to hydrothiolation of virtually any alkene C=C bond oeeurring by either radical or nucleophilic mechanisms. These have proven to be practieally useful for polymer synthesis imder extremely mild conditions, often with no solvent and little or no by-produet formation. Some of these reactions will be discussed more elaborately in later sections. 

Based on the premise established for click reactions, the interaction of thiols with reactive C—C double bonds, or simply enes, is indicated as another method able to be implemented and exploited for synthetic and materials development. This reaction, already well known in the early 1900s, involves the hydrothiolation of the C—C double bond and presents some advantages that make it particularly attractive for researchers it is a facile and versatile process. It can proceed under a variety of conditions, including a radical pathway via catalytic processes mediated by nucleophiles, acids. 

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). 

Even in the absence of catalyst, thiols add to alkynes under neutral conditions to afford a f -Markovnikov-type vinyhc sulfides with excellent regioselectivity usually as a stereoisomeric mixture. Indeed, the reaction of benzenethiol with 1-octyne in the absence of transition metal catalyst provides a t/-Markovnikov adduct 4a regioselectively with the E Z ratio of ca. 1 1. This hydrothiolation takes place, most probably via the radical process induced by trace amounts of oxygen existed in the reaction system. The radical addition of thiols to alkynes sometimes seems to proceed even in the presence of transition metal catalysts. Accordingly, when the a f -Markovnikov adducts are obtained with approximately equal amounts of E- and Z-isomers, the following possibility is present the anti-Markovnikov adducts are formed by the radical process, regardless of the presence of transition metal catalysts. 

A variety of alkynes undergo regio- and stereoselective hydrothiolation with arenethiols in the presence of RhCl(PPh3)3. Ethanol and dichlorometane is suitable solvents for this hydrothiolation. This hydrothiolation proceeds well, even when galvinoxyl as a radical inhibitor is added to the reaction system. Recently, Wilkinson s catalyst is reported to be an excellent catalyst for alkyne hydrothiolation with alkanethiols to provide -isomers of anri-Markovnikov-type addition products 3 selectively [54]. 

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