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Download fileSelective 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 JiaThe 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(CCCMe3)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(CCCMe3)2(CO)(PPh3)3. The mechanism
is also supported by DFT calculations.