Azulene‐substituted pyranylium salts. Syntheses and products characterization

Abstract

Azulene-substituted Pyranylium Salts. Syntheses and Products Characterization 965Structure of azulen-1-yl pyranylium salts. A biphenyllike isomerism of azulenyl pyranylium salts can be predicted due to the short distance, (C Az -C Py ), between the two moieties mainly when the system has bulky groups substituted in the proper positions. A useful tool for structure assignment was considered the computation (with MOPAC and PCMODEL method) [16] of the dihedral angle, ( ), and (C Az -C Py ) bond length and also of the energy barrier to rotation (racemization); the calculated values are given in Table 1 and 2. Assuming that better conjugation results from shorter bond lengths, the calculated values for C Az -C Py length indicate that the conjugation between 4-pyranylium and 1-azulene is stronger than that with benzene or 1-naphthalene. The increase of the angle in order azulene < phenyl < naphthyl results from the greater electron donating character of the azulenyl moiety, which stabilizes the positive charge of the 4-pyranylium moiety.

4-(azulen-1-yl)-2,6-diphenylchalcogenopyrylium Perchlorates; Synthesis and Characterization

“…Pure salts were obtained by the chromatography of the reaction mixture on silica gel column. The good yields obtained for the products 7-12, reported in Table 1, are compared with those already found for the 4-(azulen-1-yl)-2,6-diphenylpyrylium salts 4-6 [1]. The results obtained so far, for tellurium-containing compounds are not conclusive and the research in this direction is underway.…”

“…The generation of these pyrylium salts includes the obtaining of 2,6-disubstituted pyran-4-ones, their transformation in the corresponding 4-chloropyrylium salts and finally the electrophilic reaction with azulenes. However, first step, the known synthesis of 2,6-diphenylpyran-4-one shown in Scheme 1 (route (a) [8] or route (b) [1]) do not work for the chalcogenopyrones while the last two steps can be used to the achieving chalcogenopyrylium salts.…”

“…As result of the relationship between the structure of pyranylium salt and their NMR spectra [1] it was established that two effects govern the chemical shifts of azulene protons. One is the azulene conjugation with good electron withdrawing pyrylium moiety that strongly deshielded all azulene protons comparing with those from 1-phenylazulene [14] (Table 4) or 4-(azulen-1-yl)-2,6-diphenylpyridine [1]. The second effect consists in the different position of azulene protons towards pyrylium magnetic field.…”

Azulene Moiety as Electron Reservoir in Positively Charged Systems; A Short Survey

“…The obtained products were characterized by melting points, elemental analysis and sometimes infrared spectra. In the desire to obtain a large and varied number of pyranylium salts with azulene moieties as substitute and to start from these salts in the synthesis of pyridines, pyridinium salts or bis azulene, Razus et al varied both the hetero-cycle and azulene substituents [37]. The reaction route and the obtained 4-(azulen-1-yl)pyranylium salts are described in Scheme 11.…”

“…The beneficial effect on the pyranylium ring salt stability can also be exerted by the azulen-1-yl moiety placed in position 2 of the ring as in products 8, 9. The reaction started from 2H-pyran-2-one and occurred as shown in Scheme 12 with good yields when R = Ph (36-45%) and only with 17% for R = Me [37,38]. The pyranylium salts were then used as valuable synthons for obtaining a series of pyridines and pyridinium salts [34].…”