Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.chem.msu.ru/eng/misc/babaev/papers/017e.pdf Дата изменения: Fri Jan 21 12:57:06 2011 Дата индексирования: Sat Feb 12 03:04:27 2011 Кодировка:

Bull. Soc. Chim . Belg. vol . 101 / n° 9 / 1992 SHORT COMMUNICATIO N

0037-9646 / 92 / $ 2.00 + 0.00 ©
19g2

Comit e va n Beheer va n het Bulletin v.z.w.

p -AMPHOTERI C

HETEROCYCLES

Eugeny V. Babaev Chemical Department, State University Moscow, 119899, Russia
Received : 24/08/1992 - Accepte d : 08/09/1992

The formation of the Wheland or Meisenheimer cr-complexes - the wellknown phenomena in chemistry of aromatics and heteroaromatics [1 , 2] - is a key step in the both classical SE2Ar and SN2Ar substitution. The ability of arenes and hetarenes to form predominantly the cationic or the anionic type of s-complexes is, in general, determined by the p-excessive or p-deficient character of aromatic nucleus (cf. known C-protonation of azulenes, indoles, donor-substituted benzenes or vice versa addition of nucleophiles to nitroazines and polynitroarenes). To our knowledge the simple aromatics, capable of undergoing easy transformation in the both directions - into the anionic, as well as the cationic s-complexes - are still unknown. The simplest possible aromatics of this type would be expected to contain two condensed rings together with a high asymmetry of the p -electro n density. With the indolizines la,b, we found the first example of the bicyclic arenes, which easily form cationic and anionic s-complexes on carbon atoms with a simple variation in the pH of the solvent system.

Thus, indolizines la,b [3] are reversibly transformed in the strong acids (CF3COOH, HCI, H2SO4) into the cations lla,b and in the alkaline solution (NaOH/aq EtOH, MeOH/MeONa) into the anions llla,b. The corresponding spectral changes in acidic and alkaline media for indolizine la are illustrated in the Table; the picture observed for Ib is quite analogous. The typical red color ( l m a x 440-450 nm, EtOH) of the indolizines la,b disappeared during basification or acidification due to the formation of pyridinium-like cations II or pyrrole-like anions III. In PMR spectra of the cations II (in CFgCOOH) the strong downfield shift of the pyridinium proton H-5 and the appearance of CH2-signal in the region, expected for 3H-indolizinium cations (i.e., at 5.3 - 6.3 ppm, according to [4]), confirms that protonation takes place at C-3. In anions III (CD3ONa/CD3OD, 20°C) all the proton signals (including H-5) undergo the upfield shift. TABLE

-823 -


Our CNDO/2-calculations [3a] of the molecules Ic and 2-methyl indolizine show that a 6-NO2-group does not change neither the topology of HOMO, no the highest ir-charge on C-3 but strongly decreases both the p- and the (p+s)-charges on C-5; the LUMO of Ic is very similar to that of nitroethylene. The resulted electron density in indolizines I turns out to be highly polarized: the peri-positions C-3 and C-5 in the molecule Ic have the highest charges of the opposite signs. Since the known addition of dienophiles to indolizines occurs just across the C-3 and C-5 positions (usually treated as the rare [8 + 2] cycloaddition type [5]), one would expect a rather unusual mechanism of Diels-Alder reaction for nitroindolizines I, their derivatives and analogs. The unique " p -amphoteric " chemical behavior of indolizines la,b is analogous to the chemistry of peri-condensedtricyclic perimidine [6], while the reactivity of simplest possible carbocyclic analog - the known 5-nitroazulene [7] (p-isoelectronic to the indolizines I) - have been still not studied. REFERENCES AND NOTES 1. Recent works : Organic Reaction Mechanisms - 1988, A.C. Knipe, W.E. Watts, eds., Interscience, Chichester, 1990, pp. 305, 323. 2. For numerous examples see : Comprehensive Heterocyclic Chemistry, A.R. Katritzky, C.W. Rees eds., Pergamon Press, Oxford, 1984, Vol. 1-8. 3. For the synthesis of la,b see a) E.V. Babaev, Diss. Moscow University, 1987; b) S.I. Bobrovskii, E.V. Babaev, Yu.O. Vasiljev, Yu.G. Bundel, Moscow Univ. Chem. Bull. 42, 93 (1987) (Engl. transl.). 4. W.LF. Armarego, J. Chem. Soc. (B), 191 (1966). 5. a) F.J. Swinborne, J.H. Hunt, G. Klinkert, Adv. Heteroc. Chem., 23, 443 (1987); b) W. Flitch, Ref. [2], Vol. 4, p. 443. 6. A.F. Pozharskii, V.V. Dal'nikovskaya, Russ. Chem. Rev., 50(9), 816 (1981) (Engl. transl.). 7. T. Severin, I. Ipach, Synthesis, 12, 981 (1979).