Anti-Markovnikov Addition of Thiols Across Double Bonds Catalyzed by H-Rho-Zeolite

Abstract

A variety of olefins react with thiols in the presence of a catalytic amount of H-Rho-zeolite to afford the corresponding antiMarkovnikov addition products in good to excellent yields.The last few years have witnessed a considerable upsurge of interest in the area of zeolite induced organic transformations. 1 In a series of publications from our group, 2 we have exploited the catalytic potential of zeolites for various organic synthetic transformations, e.g. thioacetalization of carbonyl compounds, sulfoxidation of thioethers, deketalization, tetrahydropyranylation of alcohols, oxidative cleavage of tosylhydrazones, methoxymethylation of alcohols, chemo-and stereoselective epoxidation and acetylation of aldehydes etc. In continuation, we wish to report a mild, convenient and heterogeneous catalytic methodology for the synthesis of anti-Markovnikov addition products by the reaction of thiols and olefins using Rho-zeolite.In general, the protic acid 3 or Lewis acid 4 catalyzed addition of thiols across double bonds is known to give thioethers having structures which are in accordance with Markovnikov's rule. However, in the presence of freeradical initiator, thiols have been reported to add to double or triple bonds in anti-Markovnikov fashion by a freeradical mechanism. 5 A variety of hydroboration reagents developed for this reaction are also reported to give the anti-Markovnikov product. 6 However, the use of conventional protic or Lewis acid catalysts entails the problem of corrosivity, work-up and effluent pollution. Consequently, there is genuine need for an efficient and heterogeneous catalytic method for this reaction using inexpensive and nonpolluting reagents. Now we report that H-Rhozeolite serves as an efficient catalyst for the thiol addition to olefins (Scheme). SchemeThus, when the olefins were treated with thiols in the presence of catalytic amount of H-Rho-zeolite, the corresponding anti-Markovnikov products were obtained in good to excellent yields (Table). In a typical reaction procedure, to a stirred solution of olefin 1 (20 mmol) in nhexane (25 ml) were added H-Rho-zeolite (0.5 equivalent by wt. of olefin) and thiol 2 (25 mmol) and the mixture was refluxed with stirring for the indicated length of time (Table). The reaction was monitored by TLC and G. C. After the completion of reaction, zeolite was filtered off and washed with n-hexane; the filtrate was then washed with 10% aq. NaOH solution to remove excess thiol and then further washed with water, brine and dried over Na 2 SO 4 . Removal of solvent and subsequent silica gel column chromatography afforded the pure product 3 which was analyzed by 1 H NMR. Much to our surprise, an anti-Markovnikov addition product was obtained which otherwise formed in the presence of peroxide. Due to steric constraints on the diffusional path of the molecules imposed by the different structural feature of the zeolite, the more bulkier Markovnikov adduct could not be retained in the zeolite pore and presumably for this reason only the anti-Markovnikov product w...

The Emergence of Transition‐Metal‐Mediated Hydrothiolation of Unsaturated Carbon–Carbon Bonds: A Mechanistic Outlook

“…However, this strategy is limited to aromatic‐substituted unsaturated substrates or those activated for Michael‐type additions, and sometimes requires a stoichiometric amount of base 31. More recently, several methods, including the use of zeolites,32 nanoparticles,33 ionic liquids,34 and other supports have been developed, especially for alkynes, in an attempt to improve the recyclability of the reaction 35. In this respect, special attention has been paid to the use of water as a green solvent in hydrothiolation 36.…”

Übergangsmetallvermittelte Hydrothiolierung ungesättigter Kohlenstoff‐Kohlenstoff‐Bindungen: ein mechanistischer Ausblick

“…Allerdings ist diese Strategie auf ungesättigte Substrate mit aromatischen Substituenten oder für Michael‐Additionen aktivierte Substrate beschränkt und erfordert manchmal stöchiometrische Mengen der Base 31. Vor kurzem wurden besonders für Alkine einige Methoden entwickelt, einschließlich der Verwendung von Zeolithen,32 Nanopartikeln,33 ionischen Flüssigkeiten34 und anderen Trägermaterialien, um die Wiederverwertbarkeit des Reaktionssystems zu verbessern 35. Dabei wurde besonders auf die Verwendung von Wasser als umweltfreundlichem Lösungsmittel für die Hydrothiolierung geachtet 36.…”

Heterogeneous Hydrothiolation of Alkynes with Thiols Catalyzed by Diphosphino-Functionalized MCM-41 Anchored Rhodium Complex

“…In all cases listed in Table 1, the addition reaction proceeded with excellent regio-and stereoselectivity to provide only the E isomer of the anti-Markovnikov adduct 3. Although the solid supports such as H-Rho-Zeolite and Montmorillonite K 10 are known to catalyze regioselectively the addition of thiols to olefins in high yields in an anti-Markovnikov or a Markovnikov manner [35,36], no the addition of thiols to alkynes catalyzed by the solid supports has been reported until now. We also found that the addition reaction of thiols to alkynes did not occur in the presence of MCM-41 or MCM-41-2P support without rhodium.…”