Cyclic mesoionic compounds. Part 14. The electric dipole moments of mesoionic heterocycles

摘要

J.C.S. Perkin I Cyclic Mesoionic Compounds. Part 14.l The Electric Dipole Moments of Mesoionic Heterocycles By Robert N. Hanley. W. David Ollis.' and Christopher A. Ramsden, Department of Chemistry, The Uni- versity, Sheffield S3 7HF Geoffrey Rowlands and Leslie E. Sutton, Physical Chemistry Laboratory, South Parks Road, Oxford OX1 3QZ The electric dipole moments of derivatives of fourteen classes of mesoionic compounds have been measured. Vector analysis gives heterocyclic ring moments whose magnitude and direction provide support for the meso- ionic formulations. DIPOLE moment studies provided excellent support for the formulation of the sydnones (1) as mesoionic compounds because, by the choice of suitable derivatives, it was possible to determine not only the magnitude Similar studies have also been carried out on iso-sydnones (3), (4) and on the mesoionic 1,3,4-thia-diazoles (S), (6),6and the results support their forniu- lation as niesoionic compounds.but also the direction of the heterocyclic ring m~rnent.~.~ We now report upon the measurement of electric N-Phenylsydnone (1; R1= Ph, R2 = H) has a dipole moment of 6.48 D (in benzene), and results from the vectorial analysis of the dipole moments of a large R* R2 (3 (5) number of sydnones (1) were consistent with a sydnone ring moment of 5.5 D inclined at the indicated angle of 20" (2) to the R1-N bond direction. t Details of the experimental results leading to the dipole moment values listed in the Table are available as Supplementary Publication No.SIJP 22117 (6 pp.). For details of Supplemen-tary Publications see Notice to Authors No. 7, J.C.S. Perkin I, 1977, Index issue. Part 13, C. A. Ramsden and W. D. Ollis, J.C.S. Perkin I, 1974, 645. W.Baker, W. I). Ollis, and V. I). Poole, J. Chem. Soc., 1949, 307; 1950, 1542; W. Baker and W. D. Ollis, Chem. and Ind., 1955, 910; Quart. Rev., 1957, 11, 15; I:. H. C. Stewart, Chem. Rev., 1964, 64, 129; M. Ohta and H. Kato in ' Nonbenzenoid Aromatics,' ed. J. P. Snyder, Academic Press, New York, 1969, pp. 117-248; C. A. Ramsden and W. D. Ollis, Adv. Heterocyclic Chem., 1976, 19, 1. dipole moments of representatives of fourteen additional mesoionic systems (Table) .t EXPERIMENTAL The dipole moments were determined by measurement of the dielectric constants of dilute solutions of the hetero- cycles in pure benzene or pure dioxan at 28 "C in a shielded concentric condenser using a Wayne-Kerr I3331 Auto-balance Precision Bridge.The solubilities of the niesoionic 1,2,4-triazol-3-imines (7),1,2,3,4-tetrazol-5-yliclenenialonon~tr~les(23), and 1,2,3,4- thia triazol-5-ylidenemalononitriles (28) in benzene at 28" were low (weight fractions 0.000 12--0.002), and the meso- ionic 1,2,4-triazol-3-ones (8) and 1,2,4-triazole-3-thiones(9) were virtually insoluble in benzene, so their dipole moments were measured in dioxan solution. The remaining coin- pounds were fairly readily soluble in benzene, but only in the cases of the niesoionic 1,2,3,4-oxatriazoIe-5-thiones (16) and 1,2,3,4-thiatriazol-5-ones(27) were the solubilities sufficiently high to permit the measurement of refractive indices.Consequently, the molar polarisations of the remaining compounds were calculated from bond re-fractivity tables.' A value of 30.7 cm3 was calculated for the unsubstituted mesoionic 1,2,4-triazol-3-imine system (7), using the R, value of 102.2 cm3 for nitron (7; l