Rhodium-catalyzed Transannulations Of 1,2,3-thiadiazoles With Cyanoepoxides And Isothiocyanates

Transition-metal-catalyzed [3 + n] transannulation of 1,2,3-thiadiazoles with unsaturated compounds is one of the most important and favoured methods for the highly efficient construction of multisubstituted thiacycles.There are still three urgent issues to address:(1)Exploring new partners that could transannulate with 1,2,3-thiadiazoles to construct new thiacycles;(2)Solving possible potential stereochemical issues in such transannulations;(3)Verifying the existence of key intermediates in order to explain and predict the results of the reactions.The following research results have been obtained:(1)Synthesized epoxy isothiazoles via highly chemoselective,regioselective and diastereoselective transannulations of 1,2,3-thiadiazoles with cyanoexpoxides.It is revealed that:(i)the large steric hindrance of epoxy group guarantees excellent chemoselectivity,and the electron-withdrawing of epoxy group determines the regioselectivity and improves the reactivity of adjacent cyano group.(ii)Biphosphines not only act as bidentate ligands together with Rh(I)catalysts to catalyze the transannulations of 1,2,3-thiadiazoles with cyanos,but also act as catalysts to catalyze the isomerization of cis-epoxides to its trans-isomers.(iii)The intermediate was detected for the first time,and is proposed to resonance between α-thiavinyl Rh(I)-carbenoid and four-membered cyclometalated Rh(III)complex.(2)Realizing the fact that the sulfur atom of α-thiavinyl Rh(I)-carbenes is electrophilic while the carbenic center is nucleophilic,we have strategically synthesized 3H-1,2-dithiol-3-imines or 3H-1,2-dithiole-3-thione via efficient transannulations of 1,2,3-thiadiazoles with isothiocyanates or carbon disulfide.This study overcomes the drawbacks such as low yields,poor functional group tolerance,tedious steps and poor atom economy encountered in previous synthetic methods.It further verifies the rationality of our previous speculation on reaction intermediates and reaction mechanism.