Five-membered ring heterocycles, relative aromaticity

Relative Aromaticity of Five-Membered Ring Heterocycles ... 

Diazaphospholes constitute the most widely investigated class of heterophospholes, the 67t-aromatic phosphorus heterocycles [1,2]. Diazaphospholes are unique in the manner that the five-membered ring incorporates one phosphorus atom. First diazaphosphole representative, i.e. 2//-[l,2,3 diazaphospholc was obtained as early as 1967 [3] and until 1980s the interest of organophosphorus chemists remained in the development of different synthetic routes and in investigating their varied reactivity due to the structural diversity within the class [4], On the basis of the relative positions of the three heteroatoms in the five-membered ring, six monocyclic diazaphosphole systems (A-F) are possible and all of them have been reported (Structure 1). 

Four chapters in the present volume bring older reviews up to date. Those on the quinoxalines (Cheeseman and Werstiuk) and on hetero-aromatic quaternization (Zoltewicz and Deady) carry forward contributions on the same subjects in Volumes 2 (1963) and 3 (1964), respectively, of this Series, while those on the phenanthrolines (L. A. Summers) and the isatogens and indolones (Hiremath and Hooper) deal with topics previously covered elsewhere. The cyclazines, a relatively new field of chemistry, are reviewed by Flitsch and Kramer, and the azapentalenes— the wide variety of nitrogen-containing heterocycles formed by the fusion of two aromatic five-membered rings—are collected into one chapter by Elguero, Claramunt, and A. J. H. Summers. 

While it is clear that 1,2,5-thiadiazoles are clearly aromatic in nature, efforts have been made to quantify the degree of aromaticity. Three detailed comparative studies of relative aromaticity in five membered heterocyclic rings have been carried out by Bird <85T1409>, Katritzky <90JPR885>, and... 

To confirm this finding based on the degree of the benzene ring C-C distortion, the magnetic susceptibility anisotropies and the relative energies for the benzene-fused five-membered heterocycles have been computed (Table 68). A similar approach was used by Schleyer et al. <1995AGE337> to evaluate the aromaticity of five-membered heterocycles using the ab initio methods. 

Many heterocycles containing two or more heteroatoms can exist in more than one tautomeric form. Both external (polarity of the solvent, intramolecular hydrogen bonding) and internal (relative resonance stabilization of the conjugated system, stabilities of the functionalities) factors influence such tautomeric equilibria. For five- and six-membered rings, frequently more than one tautomer preserves the aromatic character. Comparisons of tautomeric equi-... 

In five-membered aromatic heterocyclic systems like fiiran or pyrrole, the system mechanics requires two pairs of ring stretching vibrations in this region of the spectrum. The modes, however, are somewhat perturbed relative to the six-membered series. For example, in furan the values are found at ... 

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