Hydrothiolation double

Abstract The use of A-heterocyclic carbene (NHC) complexes as homogeneous catalysts in addition reactions across carbon-carbon double and triple bonds and carbon-heteroatom double bonds is described. The discussion is focused on the description of the catalytic systems, their current mechanistic understanding and occasionally the relevant organometallic chemistry. The reaction types covered include hydrogenation, transfer hydrogenation, hydrosilylation, hydroboration and diboration, hydroamination, hydrothiolation, hydration, hydroarylation, allylic substitution, addition, chloroesterification and chloroacylation. 

Hydrothiolation of 1-alkynes with thiols gives vinyl sulfides with high regio- and stereoselectivities. Reaction of 1-octyne with benzenethiol catalyzed by Pd(OAc>2 gave the Markonikov-type product 110. Isomerization of the double bond occurred and the isomer 111 was obtained when PdCl2(PhCN)2 was used [34]. 

The thiol-ene reaction, which has been used for 110 years [1], corresponds to the simple hydrothiolation of a C=C double bond (Scheme 6.1). However, there are some peculiarities of this reaction that make it remarkably attractive today. It is a tool for researchers in all areas of organic synthesis due to recognition of its click characteristics. 

It was reported that the pyrazolyl-borate complex of rhodium (Tp Rh(PPh3)2, Tp =hydrotris(3,5-dimethylpyrazolyl)borate) is active not only in the hydrothiolation with ArS H, but also with AlkS H in good to high yields of Markovnikov isomer at room temperature (Scheme 3.91) [162,163]. However the reaction of 1-octyne gave a mixture of isomers in 70% yield. In addition to the double bond isomerization, a Markovnikov/anti-Markovnikov ratio of 12 1 was found in the case of 1-octyne. 

Hydrothiolation of Conjugated C-C Double Bonds. This reaction reported by He was best catalyzed by Ph3PAuBF4 in most cases and showed excellent regioselectivity. Complex 1 was shown in one case as an effective catalyst (eq 17). 

The stoichiometric reaction of PdCl2(PhCN)2 with PhSH (2 equiv) provides PdCl(SPh)(PhSH), which shows a high catalytic activity toward both Markovnikov-type hydrothiolation and double-bond isomerization reactions. 

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. 

In the transition-metal-catalyzed addition reactions of thiols to terminal alkynes, several addition products, i.e., Markovnikov-type adduct 1, Markovnikov addition and then double-bond-isomerization product 2, a n -Markovnikov adduct 3, double hydrothiolation product 4, and bisthiolation product 5, may be formed (Scheme 2). Controlling the product selectivity can be attained by the selection of transition metal complexes as catalysts, the use of additives, and/or the optimization of the reaction conditions (solvent, temperature, molar ratios of the starting materials, and so on). 

In the transition-metal-catalyzed hydrothiolation reactions, dithioketals 4 as double thiolation products are sometimes formed as byproducts. Since dithioketals 4 are synthetically useful as carbonyl equivalents, selective synthesis of 4 fi om alkynes and thiols is attractive. Indium bromide (InBr3) as a water-tolerant green Lewis acid catalyst is found to catalyze efficiently the hydrothiolation of alkynes with arene-and alkanethiols to produce the corresponding dithioketals in excellent yields (Scheme 20) [64]. 

Scheme 20 InBrs-catalyzed double hydrothiolation leading to dithioketals...

In the Sect. 2.1, a variety of transition-metal-catalyzed hydrothiolation reactiOTis of alkynes with thiols have been described. Transition metal catalysts are clearly useful for the regio- and/or stereoselective addition of thiols to carbon-carbon triple bonds. In contrast, the transition-metal-catalyzed addition of thiols to carbon-carbon double bonds such as alkenes has not been developed hitherto, except for Lewis acid-catalyzed hydrothiolation [8, 67-72]. This is most probably due to the lower coordination ability of alkenes compared with alkynes, which may permit the polymerization of metal sulfide complexes, resulting in inactivation of them as key catalysts for hydrothiolation. 

The thiol additions to conjugated dienes are catalyzed by gold complexes, Ph3PAuBp4, with excellent yields at room temperature (Scheme 25) [81]. This hydrothiolation occurs regioselectively at the less electron-rich double bond of conjugated dienes. 

The palladium-catalyzed intramolecular hydrothiolation of (Z)-2-en-4-yne-1-thiols, followed by double-bond-isomerization successfully provides substituted thiophenes in good yields. The hydrothiolation step may involve the nucleophilic attack of the sulfanyl group to the carbon-carbon triple bond coordinated by palladium (Scheme 31) [103]. 

Mitamura T, Daitou M, Nomoto A, Ogawa A (2011) Highly regioselective double hydrothiolation of terminal acetylenes with thiols catalyzed by palladium diacetate. Bull Chem Soc Jpn 84 413 15... 

---