Thiolative Lactonization Alkynes

Recently, cobalt carbonyl has been used in thiolative lactonization of alkynes with thiols with incorporation of two molecules of carbon monoxide which affords a,(3-xmsaturated y-thio y-lactones regioselectively in 50-70% yield, depending upon the type of thiol and acetylene used [44]. This reaction becomes relevant as sulphur compoimds are generally believed to be catalyst poison (Scheme 10.5). 

Scheme 10.5 Cobalt carbonyl catalyzed thiolative lactonization of alkynes (partially reproduced from Ref. [44]).

Hydrocarbonylation of alkynes. 2-Substituted acrylic thioesters are obtained from 1-alkynes under CO in the presence of the Pt complex and thiols, in contrast to the generation of the regioisomeric products when using (Ph3P)jRhH(CO). The Pt-catalyzed reaction on 5-hydroxy-l-pentyne yields a-thiomethyl-8-lactones. ... 

The free radical addition of a thiol to carbon-carbon double or triple bonds is a well-established reaction. It represents one of the most useful methods of synthesizing sulfides under mild conditions. Since its discovery [5] and its much later formulation as a free-radical chain reaction (Scheme 1) [6], the anti-Markovnikov addition of thiols to unsaturated compounds has been the subject of many reviews [8, 9]. These reactions were originally initiated by thermal decomposition of peroxides or azocompounds, by UV irradiation or by radiolysis [10]. (An example of addition of 1-thiosugar to alkenes initiated by 2,2 -azobisisobutyronitrile (AIBN) [11] is reported in equation (1)). More recently, organoboranes have been used as initiators and two examples (Et3B and 9-bora-bicyclo [3.3.1.] nonane) are reported in equations (2) and (3) [12,13]. Troyansky and co-workers [14a] achieved the synthesis of macrocycles like 12- and 13-membered sulfur-containing lactones by the double addition of thiyl radical to alkynes. An example is depicted in equation (4). The same approach has also been applied to the construction of 9- and 18-membered crown thioethers [14b]. The radical chain addition of thiyl radicals to differently substituted allenes has been considered in detail by Paste and co-workers [15], who found that preferential attack occurs at the central allenic carbon and gives rise to a resonance-stabilized ally radical. The addition of benzenethiol to allenic esters has been reported and the product formation has been similarly inferred (equation (5)) [16]. 

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