Table 2 (continued )
reaction showed its unique superiority over traditional liquid-
phase reaction.
To shed light on the reaction mechanism, control experi-
ments were carried out. Dihydrobenzofurans might involve
Michael intermediate 4aa formed from 1a and 2a after hydro-
lysis. Nevertheless, treatment of 4aa with Mn(OAc)3ꢀ2H2O
(4.0 equiv.) and DMAP (1.0 equiv.) in the presence of ammonium
acetate (1.7 equiv.) for 2 h afforded only tetrahydrobenzofuran
derivative 5aa (Scheme 2). Furthermore, treatment of 5aa with
Mn(OAc)3ꢀ2H2O (2.0 equiv.) and DMAP (0.5 equiv.) for 2 h
resulted in complete recovery of 5aa. Therefore, the possibility that
3aa was formed via the intermediate 4aa and 5aa can be ruled out.
Our previous study showed that the Mn(OAc)3-mediated reaction
of 1,3-cyclohexanediones with 1-(pyridin-2-yl)-enones afforded
tetrahydrobenzofurans.4c Although the attempt to isolate the
Michael intermediate of 1a and 2a failed and the detailed
pathway remains to be elucidated, the amino group in 1a must
play a crucial role in the formation of 3aa.
a
Properly characterized by FT-IR, 1H NMR, 13C NMR, and HRMS
spectral data (see ESI).
In conclusion, we have described that the solvent-free
Mn(OAc)3-mediated reactions of b-enamino carbonyl compounds
with 1-(pyridin-2-yl)-enones unexpectedly afford dihydrobenzo-
furan derivatives instead of the 2-acyl-3-aryl-indole derivatives
under mechanical milling conditions. Efforts towards under-
standing the reaction mechanism and further development of
this kind of novel cyclization reactions are underway.
We are grateful for financial support from the Knowledge
Innovation Project of the Chinese Academy of Sciences
(KJCX2.YW.H16) and the Key Project of Science and Tech-
nology of the Department of Education, Anhui Province,
China (KJ2010ZD05).
Scheme 1 Reaction of 1a with 2a promoted by Mn(OAc)3 in toluene
at 100 1C for 24 h.
Notes and references
1 (a) A. Loupy, Top. Curr. Chem., 1999, 206, 153; (b) K. Tanaka and
F. Toda, Chem. Rev., 2000, 100, 1025.
2 For reviews, see: (a) G.-W. Wang, Fullerene Mechanochemistry. in
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American Scientific Publishers, Stevenson Ranch, 2004, vol. 3, p. 557;
(b) G. Kaupp, Top. Curr. Chem., 2005, 254, 95; (c) K. Komatsu, Top.
Curr. Chem., 2005, 254, 185; (d) B. Rodrıguez, A. Bruckmann,
´
Scheme 2 Conversion of 4aa to 5aa in the presence of Mn(OAc)3.
T. Rantanen and C. Bolm, Adv. Synth. Catal., 2007, 349, 2213;
(e) A. Bruckmann, A. Krebs and C. Bolm, Green Chem., 2008,
10, 1131; (f) A. Stolle, T. Szuppa, S. E. S. Leonhardt and
B. Ondruschka, Chem. Soc. Rev., 2011, 40, 2317; (g) S. L. James,
Reaction of 3-aminocyclohex-2-enone (1h) with 1-(pyridin-2-yl)-
enones containing an electron-donating or electron-withdrawing
group worked as well, and afforded 3ha, 3hd, 3hl and 3hm in
68–70% yields. However, the reaction of 1a with (E)-1-phenyl-3-
p-tolylprop-2-en-1-one or (E)-4-(4-methoxy-phenyl)-but-3-en-
2-one in place of (E)-1-(pyridin-2-yl)-3-p-tolylprop-2-en-1-one
failed, indicating that the nitrogen-containing aromatic moiety
was very important in the current reaction.
C. J. Adams, C. Bolm, D. Braga, P. Collier, T. Friscic, F. Grepioni,
´
K. D. M. Harris, G. Hyett, W. Jones, A. Krebs, J. Mack, L. Maini,
A. G. Orpen, I. P. Parkin, W. C. Shearouse, J. W. Steedk and
D. C. Waddelli, Chem. Soc. Rev., 2012, 41, 413; (h) R. B. N. Baig and
R. S. Varma, Chem. Soc. Rev., 2012, 41, 1559; (i) T. Friscic
Soc. Rev., 2012, 41, 3493.
´
, Chem.
3 For reviews, see: (a) J. Iqbal, B. Bhatia and N. K. Nayyar, Chem. Rev.,
1994, 94, 519; (b) B. B. Snider, Chem. Rev., 1996, 96, 339; (c) G.-W. Wang
and F.-B. Li, J. Nanosci. Nanotechnol., 2007, 7, 1162; (d) A. S. Demir and
M. Emrullahoglu, Curr. Org. Synth., 2007, 4, 321; (e) G.-W. Wang and
F.-B. Li, Curr. Org. Chem., 2012, 16, 1109.
4 (a) Z. Zhang, G.-W. Wang, C.-B. Miao, Y.-W. Dong and
Y.-B. Shen, Chem. Commun., 2004, 1832; (b) X. Cheng,
G.-W. Wang, Y. Murata and K. Komatsu, Chin. Chem. Lett.,
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and Y.-B. Shen, J. Org. Chem., 2008, 73, 7088.
The reaction was examined in organic solvents as well, but the
results were inferior to those obtained by employing the mechanical
milling technique. For example, the reaction system became
complicated and a low yield of 38% was obtained when
the reaction of 1a (3.0 equiv.) with 2a (1.0 equiv.) promoted
by Mn(OAc)3ꢀ2H2O (4.0 equiv.) in the presence of DMAP
(1.0 equiv.) was performed in toluene at 100 1C for 24 h
(Scheme 1). Therefore, the solvent-free mechanochemical
5 G.-W. Wang, H.-T. Yang, C.-B. Miao, Y. Xu and F. Liu, Org.
Biomol. Chem., 2006, 4, 2595.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 11665–11667 11667