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Scheme 3 Proposed mechanism.
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Scheme 4 Trapping of the triazolate samarium intermediate by MeI.
Since amines can act as proton sources to abstract the resulting
triazolate ligand and as suitable ligands to activate the catalyst
in the catalytic cycle,16b,17 the presence of amine additives
would be favorable to the cycloaddition.
Consistent with this, the treatment of a 1 : 1 mixture of 1a and
2a with one equivalent of Sm[N(SiMe3)2]3 followed by reacting
with MeI gave the expected product 5 in 23% yield (Scheme 4).
In conclusion, the Ln[N(SiMe3)2]3-catalyzed cycloaddition of
terminal alkynes with azides is an ideal addition to the family of
click reactions. The process exhibits broad scope, readily avail-
able catalyst and mild conditions, and provides 1,5-disubstituted
1,2,3-triazoles in moderate to excellent yields. Furthermore, the
present catalytic system not only differentiates between internal
and terminal alkynes but also shows unprecedented mechanistic
features involving a tandem anionic cascade cyclization reaction
and anti-addition across the CRC triple bond. Quite generally,
organolanthanide-catalyzed intramolecular hydroelementation
reactions produce exclusively the exocyclic hydroamination pro-
ducts resulting from syn-addition.15 We believe that the present
5-endo cyclization pattern seems to indicate the untapped
potential of organolanthanide catalysis, and would have a great
impact on the regioselectivity control of lanthanide-mediated
intramolecular additions. Further investigations regarding the
mechanism and synthetic applications are in progress.
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We thank the National Natural Science Foundation of China,
973 program (2009CB825300).
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c
This journal is The Royal Society of Chemistry 2013
Chem. Commun.