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

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.