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Selective Formation of 1,4-Disubstituted Triazoles from Ruthenium-Catalyzed Cycloaddition of Terminal Alkynes and Organic Azides: Scope and Reaction Mechanism

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posted on 2016-02-20, 17:40 authored by Pei Nian Liu, Juan Li, Fu Hai Su, Kun Dong Ju, Li Zhang, Chuan Shi, Herman H. Y. Sung, Ian D. Williams, Valery V. Fokin, Zhenyang Lin, Guochen Jia
The catalytic activity of a series of ruthenium complexes lacking cyclopentadienyl ligands has been evaluated for the cycloaddition of terminal alkynes and azides to give selectively 1,4-disubstituted 1,2,3-triazoles. The complex RuH­(η2-BH4)­(CO)­(PCy3)2 was found to be an effective catalyst for the cycloaddition reactions. In the presence of RuH­(η2-BH4)­(CO)­(PCy3)2, primary and secondary azides reacted with a range of terminal alkynes containing various functionalities to selectively produce 1,4-disubstituted 1,2,3-triazoles. The ruthenium-catalyzed azide–alkyne cycloaddition appears to proceed via a Ru–acetylide species as the key intermediate, which undergoes formal cycloaddition with an azide to give a ruthenium triazolide complex. The 1,4-disubstituted 1,2,3-triazole product is generated by metathesis of the triazolide complex with a terminal alkyne. In support of the reaction mechanism, the acetylide complex Ru­(CCCMe3)2(CO)­(PPh3)3 reacts cleanly with benzyl azide to give a ruthenium triazolide complex, which reacts with excess tert-butylacetylene in the presence of PPh3 to give 4-tert-butyl-1-benzyl-1,2,3-triazole and the diacetylide complex Ru­(CCCMe3)2(CO)­(PPh3)3. The mechanism is also supported by DFT calculations.