K. Yu. Koltunov et al. / Tetrahedron Letters 45 (2004) 3547–3549
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4. (a) Olah, G. A.; White, A. M.; O’Brien, D. H. In
Carbonium Ions; Olah, G. A., Schleyer, P. R., Eds.;
Wiley-Interscience: New York, 1973; Vol. 4, pp 1697–
1781; (b) Bagno, A.; Scorrano, G. J. Phys. Chem. 1996,
100, 1536–1544.
5. Bagno, A.; Kantlehner, W.; Scherr, O.; Vetter, J.; Ziegler,
G. Eur. J. Org. Chem. 2001, 16, 2947–2954, and references
cited therein.
6. Koltunov, K. Yu.; Prakash, G. K. S.; Rasul, G.; Olah, G.
A. Heterocycles 2004, 62, 757–772.
7. (a) Manimaran, T.; Thiruvengadam, T. K.; Ramakrish-
nan, V. T. Synthesis 1975, 739–741; (b) Johnston, K. M.
Tetrahedron 1968, 24, 5595–5598, and references cited
therein.
shows the signal of the hydrogen bound to carbonyl
oxygen at d 10.6 ppm and the signal of the amino
hydrogens at d 8.7 ppm (HSO3F, )80 ꢁC), that is similar
to chemical shifts for O-protonated amides.4a The
chemical shifts of signals assigned to the benzylic site of
the dication are in accord with those reported for
analogous benzylic dicationic systems.16 Similar O,C-
diprotonation of a,b-unsaturated ketones17 and nitro
compounds18 in superacids have been described earlier.
The involvement of such dications (or analogous pro-
tonated complexes with aluminum halides) as plausible
reaction intermediates also have been discussed.17–19
Recently, alkylation of aromatics with a,b-unsaturated
carboxylic acids was performed in triflic acid and
intermediacy of dicationic species suggested.20
8. Wang, T.-C.; Chen, Y.-L.; Lee, K.-H.; Tzeng, C.-C.
Synthesis 1997, 87–90.
9. Koncos, R.; Friedman, B. S. In Friedel–Crafts and Related
Reactions; Olah, G. A., Ed.; Interscience: New York, 1964;
Vol. 2, Part 1, pp 289–412; Thomas, C. H. Anhydrous
Aluminum Chloride in Organic Chemistry; Reinhold: New
York, 1941.
It is worth to note, that the utility of AlCl3 as cheap and
convenient alternative to strong protic (super)acids is
probably underestimated. Whereas the presence of small
amounts of base (water) decreases drastically the acidity
of superacid,21 traces of humidity are rather beneficial to
the reactivity of aluminum halides, which may be used
without special dryness of starting materials or solvent
and do not require inert gas atmosphere.
10. b-Arylcarboxylic acids (acid chlorides) have mostly been
prepared by different ways as precursors and subsequently
worked up with ammonia or amines. Another approach
includes nucleophilic 1,4-addition of arylsiloxanes or
Grignard reagents to a,b-unsaturated amides. See, for
example: (a) Elz, S.; Kramer, K.; Pertz, H. H.; Detert, H.;
Laak, A. M.; Kuhne, R.; Schunack, W. J. Med. Chem.
2000, 43, 1071–1084; (b) Froimowitz, M.; Wu, K.-M.;
Moussa, A.; Haidar, R. M.; Jurayj, J.; George, C.;
Dardner, E. L. J. Med. Chem. 2000, 43, 4981–4992; (c)
Oi, S.; Taira, A.; Honma, Y.; Inoue, Y. Org. Lett. 2003, 5,
97–99; (d) Quallich, G. J. U.S. Patent 5196607, 1993.
11. Typical experimental procedure: To a stirred suspension
of AlCl3 in benzene 7 was added and the resulting mixture
was stirred at 25 ꢁC for 0.5–5 h, followed by pouring over
ice and extraction with CH2Cl2. The organic phase was
dried with anhydrous Na2SO4 and concentrated in vacuo
to give 6 as pure individual compound.
In conclusion, we have demonstrated that the reaction
of a,b-unsaturated amides with aromatics initiated by
strong Lewis or Bronsted acid can be a convenient
alternative to known preparations of 3-arylpropion-
amides. We suggest that dicationic electrophiles, such as
those directly observable by low-temperature NMR
when a,b-unsaturated amides are dissolved in liquid
superacids, are a convenient rationale in accord with the
reaction mechanism.
12. Olah, G. A.; Prakash, G. K. S.; Sommer, J. Superacids;
Wiley: New York, 1985; pp 51–52.
Acknowledgements
13. Olah, G. A.; Molnar, A. Hydrocarbon Chemistry; Wiley-
Interscience: New York, 1995; pp 107–112.
14. Taylor, R. Electrophilic Aromatic Substitution; Wiley: New
York, 1990; Chapter 2.
Financial support of the work by the Loker Hydrocar-
bon Institute, U.S.C., Los-Angeles, and the ‘CNRS’ is
gratefully acknowledged.
15. Hindered rotation about the C–C6H4–OCH3 bonds results
in the observation of two ‘frozen’ conformers (cis–trans
isomers) of the dication in ꢀ1:2 ratio (HSO3F, )80 ꢁC).
1
The spectra of the predominant isomer: H NMR d 4.52
(s, 3H), 4.57 (s, 2H), 7.36 (d, J 7 Hz, 1H), 7.53 (d, J 7 Hz,
1H), 8.31 (s, 1H), 8.32 (d, J 7 Hz, 1H), 8.4 (d, J 7 Hz, 1H),
8.66 (br s, 2H), 10.6 (br s, 1H); 13C NMR d 35.7, 62.2,
118.7, 127, 136.3, 136.3, 143.2, 158.1, 177.2, 188.7.
Quenching this acidic solution with ice provided back
the starting material.
References and notes
1. The mechanism of these reactions involves O-protonation
(coordination to a Lewis acid) of b-phenethylamides
followed by electrophilic aromatic cyclialkylation. See:
Barclay, L. R. C. In Friedel–Crafts and Related Reactions;
Olah, G. A., Ed.; Interscience: New York, 1964; Vol. 2,
Part 2, pp 873–937.
16. Klumpp, D. A.; Aguirre, S. L.; Sanchez, G. V.; Leon, S. J.
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