Communication
Organic & Biomolecular Chemistry
and i-PrOH for the facile and mild synthesis of a variety of
indole derivatives. This method would also be an effective tool
for the formation of 2,3-disubstituted indoles.
Acknowledgements
This work was financially supported in part by a Strategic
Research Foundation Grant-Aided Project for Private Univer-
sities from the Ministry of Education, Culture, Sports, Science
and Technology of Japan (MEXT) and Kyoto Pharmaceutical
University Fund for the Promotion of Scientific Research.
Notes and references
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T. Torroba, Org. Biomol. Chem., 2006, 4, 757; G. Zeni and
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Scheme 1 Plausible reaction mechanism.
the proximal position of the allene group afforded 2,3-disubsti-
tuted indole 6c bearing a silicon functional group at the C2
position, which served as a potential precursor for further
functionalization by palladium(0)-catalysed transformation, in
high yield (entry 3).7,19 Furthermore, the reaction of 5d with
the benzyl group at the proximal position of the allene group
gave 6d with a benzyl group at the C2 position in excellent
yield (entry 4).
A plausible reaction mechanism for the samarium(II)-
mediated aryl radical coupling reaction with an allene group is
shown in Scheme 1. The SmI2-mediated SET to the iodide of 1
generates an aryl radical that would undergo a 5-exo-type intra-
molecular cyclisation by attacking the aryl radical on the
center carbon of the allene group to produce radical intermedi-
ate A. The following SET generates anion B, and protonation of
B in the presence of i-PrOH would promote the equilibrium of
SET between A and B to afford preferentially B. However, the
compound produced by isomerization of B into C followed by
protonation was not observed at all. The fact that the reactions
in the absence of i-PrOH gave indole products in lower yields
than those in the presence of i-PrOH indicates that i-PrOH
facilitates the equilibrium of SET between A and B, by tapping
this anionic intermediate.12 According to the results of
Inanaga, Curran, and Reiβig,13,20 under the reaction con-
ditions in the absence of a proton source, the direct conver-
sion of intermediate A into indole 2 by abstracting a hydrogen
from the solvent THF or the additive HMPA has to be taken
into account as an alternative pathway.
11 For recent reviews of SmI2-mediated reactions: H. B. Kagan
and J. L. Namy, Top. Organomet. Chem., 1999, 2, 155; A. Krief
and A. M. Laval, Chem. Rev., 1999, 99, 745; F. G. Steel,
J. Chem. Soc., Perkin Trans. 1, 2001, 2727; A. Hölemann,
Synlett, 2002, 1497; B. K. Banik, Eur. J. Org. Chem., 2002,
2431; H. B. Kagan, Tetrahedron, 2003, 59, 10351.
Conclusions
We have demonstrated a SmI2-mediated cyclisation reaction of
an aryl radical with an allene group in the presence of HMPA
6814 | Org. Biomol. Chem., 2014, 12, 6812–6815
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