Transition-metal hydrides thiols

As the rate of cyclization becomes slower, the reactivity of the precursor becomes more important. To ensure that the radical generation step does not break the chain, it is important to use the most reactive precursor available. For very slow cyclizations, the advice is simple use iodides whenever possible. The purity of the precursor is also critical for slow cyclizations because tin hydride can sometimes react with impurities to generate hydrogen atom sources that are much more reactive than itself. Any impurities that might generate thiols or selenols may cause undue amounts of reduction (thus, the purity of phenyl sulfides and selenides is especially important). Metal impurities, which may form transition metal hydrides, can be devastating, even for fast cyclizations.41 Empirically, it seems that breaking of the chain is less of... 

RS-M" L c-i)- The other is the oxidative addition of thiols to low-valent transition metals (M"L ) to give the corresponding transition metal complexes bearing both hydride and thiolate ligands (RS-M" L c 2-H). The reaction of the former complexes (RS-M" L c-i) with carbon-carbon unsaturated compounds such as alkynes may proceed via thiometallation, in which relatively more bulky is bonded at the terminal carbon of alkynes. On the other hand, in the reaction of the latter complexes (RS-M" L c-2 H) with alkynes both hydrometallation and thiometallation processes are possible. These processes proceed via sy -addition. Alternative pathway for the addition of thiols to alkynes involves coordination of alkynes to transition metals and then nucleophilic addition of thiols (or thiolate anions) to the alkynes. These processes take place via a f -addition. By controlling these pathways, regio- and stereoselective hydrothiolation of alkynes is expected to be attained successfully. 

Although thiol and hydride/thiolate complexes are known, there are few studies which show that the hydrogen can move between metal and sulfur [35-38]. The transfer of a proton from sulfur to metal has been proposed before. For example, the so-called oxidative-addition of RSH to [IrCl(CO)(PPh3)2] has been suggested to involve a three-centre, Ir-H-SR transition state in which the lengthening of the S-H bond is synchronous with the binding of these two atoms to the iridium centre [39]. More recently [40] an agostic Os—H-S interaction has... 

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