Heteroaromatics five-membered ring systems

The present review deals with condensed systems of heteroaromatic five-membered rings—thienothiophenes, di enothiophenes, seleno-phenoselenophenes, and selenophenothiophenes. The number of publications devoted to such compounds has grown rapidly in recent years. This is indicative of the mounting interest in various aspects of this field of heterocyclic chemistry, which, however, has not been reviewed since the 1954 monograph of Hartough and Meisel. ... 

Heteroaromatic compounds do not undergo the same variety of photocycloadditions with alkenes as do their carbocyclic counterparts. There are very few reports of this type of reaction for six-membered ring compounds such as pyridines, but five-membered ring systems such as furans do give 1,2-cycloadducts with a range of alkenes (e.g. 357). 

Another theoretical criterion applied to estimation of aromaticity of homo- and heteroaromatic ring system is aromatic stabilization energy (ASE). Based on this approach, the aromatic sequence of five-membered ring systems (ASE in kcal mol-1) is pyrrole (20.6) > thiophene (18.6) > selenophene (16.7) > phosphole (3.2) [29], According to geometric criterion HOMA, based on the harmonic oscillator model [30-33], thiophene is more aromatic than pyrrole and the decreasing order of aromaticity is thiophene (0.891) > pyrrole (0.879) > selenophene (0.877) > furan (0.298) > phosphole (0.236) [29],... 

All polycyclic pigments, with the exception of triphenylmethyl derivatives, comprise anellated aromatic and/or heteroaromatic moieties. In commercial pigments, these may range from systems such as diketopyrrolo-pyrrol derivatives, which feature two five-membered heteroaromatic fused rings (DPP pigments) to such eight-membered ring systems as flavanthrone or pyranthrone. The phthalo-cyanine skeleton with its polycylic metal complex is somewhat unique in this respect. 

In condensed heteroaromatic systems with a bridge-head pyrrolic nitrogen atom, tr-electron density is always shifted from the electron-rich six-membered ring (formally contains 7 Tr-electrons) towards the five-membered ring (formally has 6 Tr-electrons). As a result electrophiles are directed to carbon atoms of the latter. Thus, imidazo[ 1,2-a]pyridines (140) unsubstituted at C-3 almost always react with electrophiles at that position. 

All these frequencies are in the region of other heteroaromatic compounds and of azulenes. Infrared absorption spectra for several derivatives of the following pseudoazulene systems have been reported 26,56 28,77 2985 86 33 96.uio 35,165 39,"3114 42, 23 49,135 136 and 56.143144-146 The key frequencies for substituents at positions 1 or 3 in the five-membered ring are shifted to lower wavelengths in a typical manner. This is especially pronounced in the case of carbonyl groups. 

This group constitutes a virtually infinite class of heteroaromatic ring systems. However, relatively few of the parent compounds have yet been made, and fewer still have had their quantitative (or indeed qualitative) reactivities measured. The compounds described in this chapter are subdivided as follows Section 2, compounds with one five- and one six-membered ring Section 3, compounds with one five and two six-mem-bered rings Section 4, compounds with two five- and one six-membered rings Section 5, compounds with two five-membered rings and Section 6, compounds with three or more five-membered rings. 

The earlier reviews described a number of synthetic approaches to this system. The most generally useful, however, were observed to be those starting with A -aminopyridinium derivatives and constructing the five-membered ring. The chemistry of A -aminoazonium salts, and heteroaromatic N-imines in general, has been reviewed <81AHC(29)73>. Syntheses reported since the publication of the first edition continue to reflect the above-mentioned observation. 

In general, five-membered heteroaromatic ring systems with one heteroatom all undergo preferential a rather than /3 electrophilic substitution. This is rationalized in terms of the more effective delocalization of charge in the intermediate (36) leading to a substitution than in the intermediate (37) leading to /3 substitution. 

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