Novel Polycyclic Dianions: Metal Reduction of Nitrogen Heterocycles

Fused benzenoid heterocyclic systems, i.e., benzo[c]cinnoline (3), dibenzo[a,c]phenazine (4), and quinoxaline derivatives 5 and 6, were reduced with lithium and sodium in ether solvents to the corresponding antiaromatic doubly charged species. These dianions represent a new class of charged species, Le., nitrogen-containing polycyclic anions. The 'H and I3C NMR spectra as well as theoretical calculations are reported and discussed in terms of antiaromatic contributions as well as the modes of interaction between the organic moiety and the cations. In view of the strong dependency of the 'H NMR spectra on the countercation and temperature, an equilibrium between ionic and covalent structure is suggested. Experimental and theoretical studies of neutral and charged *-conjugated polycyclic hydrocarbons are quite extensive.' Parameters such as degree of charging, different modes of charge delocalization and its distribution over the *-framework, states of solvation of the counterions, and the aromatic (or antiaromatic) contributions sustained as a result of the charging processes attracted much interest and are well documented.*+ The factors which govern these parameters were studied with the aid of various chemical and physical methods including quenching experiments, UV, ESR, and N M R spectros~opies.~ Theoretical approaches, aimed to interpret these experimental findings such as Platt's peripheral method? the conjugated circuits theory suggested by RandiE,6b*c the Hess and Schaad semiempirical mode16d*e as well as theories concerned with ion-solvation equilibria are generally found to nicely correlate with the experimental results. In view of these considerations, the scarcity of information related to heteropolycyclic anions seems quite startling. In fact, the only reported *-conjugated dianions which contain heteroatoms in their path of conjugation are those derived from 2,4,6-triphenyl-sym-triazine (la), its p-tolyl analogue (lb), and the substituted azocine system, Le., 2. The dianions, l a and lb, are characterized by a singlet ground state and a low-lying thermally excited triplet state,' and detection of their NMR characteristics is therefore precluded. This result presents a major drawback, (1) For general reviews see: (a) Garratt, P. J. 'Aromaticity"; McGrawHill: London, 1971. (b) Snyder, J. P., Ed. 'Nonbenzenoid Aromatics"; Academic Press: New York, 1971; Vol. 11. (c) Bergmann, E. D., Pullman, B., Eds. 'Aromaticity, Pseudoaromaticity and Antiaromaticity"; Jerusalem Academic Press: Jerusalem, 1971. (d) Agranat, I.; Barak, A. "MTP International Review of Science"; Butterworth: London, 1976; Vol. 3. (e) Breslow, R. Acc. Chem. Res. 1973,6, 393. (f) Breslow, R. Chem. Eng. News 1965, 43,90. (g) Haddon, R. C.; Haddon, V. R.; Jackman, L. M. Fortschr. Chem. Forsch. 1970/1971, 16, 103. (h) Rabinovitz, M., Ed. "Non-Benzenoid and Homoaromatic Systems"; Isr. J . Chem. 1980, 20, 213-321. (i) GraovaE, A,; TrinajstiE, N. Pure Appl. Chem. 1980,52, 1397-1667. 6 ) Agranat, I. Pure Appl. Chem. 1982, 54, 927-1 155. (2) (a) Streitwieser, A.; Suzuki, S. Tetrahedron 1961, 26, 153. (b) Lawler, R. G.; Ristagno, C. V. J . Am. Chem. SOC. 1969, 91, 1534. (c) Becker, B.; Huber, W.; Miillen, K. J . Am. Chem. SOC. 1980,202,7803. (d) Minsky, A,; Klein, J.; Rabinovitz, M. J . Am. Chem. SOC. 1981, 103, 4596. (e) Young, R. N. Prog. Nucl. Magn. Reson. Spectrosc. 1978, 12, 261. (3) (a) Hogen-Esch, T. E.; Smid, J. J . Am. Chem. Soc. 1966,88,307. (b) Hogen-Esch, T. E. Adu. Phys. Org. Chem. 1977, 15, 154. (c) Grutzner, J. B.; Lawler, J. M.; Jackman, L. M. J . Am. Chem. Soo. 1972,94, 2306. (d) O'Brien, D. H.; Russell, C. R. Ibid. 1979,101, 633. ( e ) Miillen, K.; Huber, W.; Meul, T.; Nakagawa, M.; Iyoda, M. Tetrahedron 1983, 39, 1575. (4) (a) Miillen, K. Helu. Chim. Acta 1977,61, 1296. (b) Rabinovitz, M.; Willner, 1.; Minsky, A. Acc. Chem. Res. 1983, 6, 298. (c) Nakasazi, K.; Yoshida, K.; Murata, I. J . Am. Chem. SOC. 1983, 105, 5136. ( 5 ) Buncel, E., Durst, T., Eds. "Comprehensive Carbanion Chemistry"; Elsevier Scientific Publishing Co.: Amsterdam, 1980. (6) (a) Platt, J. R. J. Chem. Phys. 1952, 22, 1448. (b) RandiE, M. Chem. Phys. Lett. 1976, 38, 68. (c) RandiE, M. J. Am. Chem. SOC. 1977, 99, 444. (d) Hess, B. A., Jr.; Schaad, L. J. Ibid. 1971, 93, 305. (e) Schaad, L. J.; Hem, B. A., Jr. Ibid. 1972, 94, 3068. (7) Van Broekhoven, J. A. M.; Sommerdijk, J. T.; De Boer, E. Mol. Phys. 1971, 20, 993. 0002-7863/85/1507-1501$01.50/0 r 1 -2 r 1 -2