Allenes hydrothiolation

Equation 27 Pd(PPh3)4 allene hydrothiolation carbonylation sequence [183]... 

When the reaction of 1,1-dimethyl allene with o-BrC6H4SH was carried out in the presence of Pd(OAc)2/dppf/i-Pr2HN/CO in benzene, hydrothiolation of the allene took place (Eq. 7.26) [37]. However, the regiochemistry of the adduct 36 was different from that obtained by the Pd(OAc)2-catalyzed hydrothiolation of mono-substituted allenes (cf. Eq. 7.24), showing that the regiochemistry of the hydrothiolation of allenes can be controlled by the reaction conditions even when the same metal(Pd) catalyst is used. 

Hydrothiolation of Allenes The first example of a gold-catalyzed carbon-sulfur bond formation was published by Kraus et al. who synthesized 2,5-dihydrothiophenes by allenyl thiocarbinols [38]. The best results were obtained with AuCI in CH2C12, providing an impressive diastereoselectivity. In the same study, other coin-metal precatalysts were tested but only gold afforded the cydization product. 

For sulfur nucleophiles, the intramolecular gold-catalyzed hydrothiolation of allenes was described in 2006. a-Thio-allenes led to the formation of 2,5-dihydrothiophenes in moderate to good yields and with complete stereoselectivity. ... 

A number of similar transformations of allenes have been reported. As with alkyne hydrothiolation, the key to the utility of this process is the ability to control regio-and stereoselectivity. A few key recent discoveries are highlighted below. For related reactions, see the following references [165, 168, 197, 236-243]. 

Building from Krause s study with allenes,it was discovered that Au(l) catalysts can effect intermolecular hydrothiolation of unactivated olefins (32) [12]. 2-Mer-captoethanol reacts exclusively with sulfur, demonstrating chemoselectivity and functional group compatibility. As with the other systems, both aliphatic and aromatic thiols work well. 

Menggenbateer M, Narsireddy M, Ferrara G, Nisbna N, Jin T, Yamamoto Y. Gold-catalyzed regiospecific intermolecular hydrothiolation of allenes. Tetrahedron Lett. 2010 51 4627-4629. 

In 1996, Ogawa et al. reported the hydrothiolation of an allene catalyzed by Pd(OAc)2 to provide 32, whose formation can be explained as follows (1) insertion of the allene into the Pd-S bond of Pd(SPh)2 to give a it-allyl palladium thiolate 33, and (2) protolysis by PhSH to form 32 and Pd(SPh)2 (Eq. 7.24) [35]. The authors proposed the direct reaction of a a-aUyl palladium with PhSH before the formation of 33. 

Abstract This chapter deals with the transition-metal-catalyzed hydrothiolation and hydroselenation of alkynes and allenes and related imsaturated compounds with thiols and selenols. In these reactions, the regio- and/or stereoselectivities of the addition products can be controlled by switching the transition metal catalysts. Metal sulfides and selenides (RE-ML , E = S, Se, M = Ni, Pd, Rh, Zr, Sm, etc.) play an important role as key catalyst species in these hydrothiolation and hydroselenation. The introduction of carbon monoxide into these hydrothiolation and hydroselenation systems leads to novel carbonylation with simultaneous addition of thio and seleno groups to unsaturated bonds. 

Reaction of dialkyl disulfide with allenes is catalyzed by a rhodium-phosphine complex and trifluoromethanesulfonic acid giving ( )-2-alkylthio-l,3-dienes [78]. AuBr3-catalyzed regioselective hydrothiolation of aromatic allenes with arenethiols affords the corresponding dithioketals in good yields under mild conditions [79]. Intramolecular hydrothiolation of a-thioallenes to 2,5-dihydrothiophenes is successfully catalyzed by AuCl (Scheme 24) [80]. 

Benzeneselenol as a representative selenol is a colorless liquid of greater acidity than benzenethiol (p a = 5.9 (PhSeH) 6.5 (PhSH)). The bond energy of Se-H is 73 kcal/mol, is smaller than S-H (87 kcal/mol) [82]. These properties may contribute to the efficiency in the oxidative addition of selenols to low-valent transition metals, ligand-exchange reaction between high-valent transition metal complexes and selenols, and protonation process of carbon-metal bonds. Indeed, several transition metal complexes catalyze the highly selective hydrothiolation of alkynes and allenes. 

When the transition-metal-catalyzed hydrothiolation of unsaturated compounds is performed in the presence of carbon monoxide, carbonylation reactions may proceed with the introduction of sulfanyl groups. In fact, a series of carbonylative thiolation reactions of alkynes and allenes are reported. These reactions provide useful tools to synthetically important organosulfur compounds [104, 105]. For example, the rhodium-catalyzed reaction of alkynes with thiols and CO provides the corresponding thioformylation products regioselectively [106, 107]. Switching the catalyst from rhodium complex to platinum complex leads to a sharp reversal of regioselectivity of CO introduction [108, 109] (Scheme 32). 

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