2‐(Azulen‐1‐yl)‐4,6‐diphenyl substituted six‐membered heteroaromatics

Abstract

2-(Azulen-1-yl)-4,6-diphenyl substituted pyranylium salts, pyridinium salts and pyridines were efficiently synthesized and the new obtained compounds were completely characterized. Comparative structural analysis between these compounds and their corresponding isomers that contain the azulen-1-yl moiety in the 4-position of the heterocycle, were carried out. These studies are based on calculated dihedral angles formed between central heterocycle and the aromatic substituents and on the obtained electronic and NMR spectra. Due to the restriction in the rotation around azulenyl-pyridinium bond produced by the quaternary nitrogen substituent, in the herein reported pyridinium salts, the substitution groups of the quaternary nitrogen atom are prochiral. This property leads to the non-equivalence of gem-protons or gemmethyl groups of N-substituents in the 1 H nmr spectra of the synthesized pyridinium salts.

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

“…With the decrease of push-pull efficiency exerted by the π-electronic system at the increase in dihedral angle, a bathochromic effect results, observed in UV-Vis spectra in the order: pyridines, pyridinium salts, pyranylium salts [37,38,40,41]. The values of λ max (in methanol) for these compounds with R az = R 3 = H; R 2 = Ph (R py = Me) are 370, 434 and 530 nm, respectively.…”

“…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].…”

“…The pyranylium salts were then used as valuable synthons for obtaining a series of pyridines and pyridinium salts [34]. The general pathway for the synthesis of pyridinium salts in two steps [39] was applied to the generation of salts substituted with azulen-1-yl moieties (Scheme 13) [38,40,41]. In the first step the pyranylium ring is opened by amine in the presence of triethylamine followed by the ring closure of the intermediate enamine.…”

“…Research has also been pursued on oxygen replacement from 2,4-diphenylpyranylium salts, 8, with N + Rpy, just as for 2.6-pyranilium salts (Scheme 14). The yields of products 15 were very good except when Rpy = Ph (yields: 26-34%) [38]. Research has also been pursued on oxygen replacement from 2,4-diphenylpyranylium salts, 8, with N + R py, just as for 2.6-pyranilium salts (Scheme 14).…”

“…Research has also been pursued on oxygen replacement from 2,4-diphenylpyranylium salts, 8, with N + R py, just as for 2.6-pyranilium salts (Scheme 14). The yields of products 15 were very good except when R py = Ph (yields: 26-34%) [38]. Symmetry 2021, 13 In addition to the research on pyranylium and pyridinium salts with phenyl or methyl at positions 2 and 6, these positions were also replaced by 5-membered heterocycles, thiophen-2-yl and furan-2-yl as shown for compounds 12 and 13 in Scheme 11 [42].…”