Conjugated heterocycles, structures

Benzo[c]phenanthridine alkaloids are widespread in Papaveraceae, Fumariaceae, and Rutaceae. Fagaridine (118), the structure of which had to be revised, is a derivative of the unknown 5-methyl-benzo[c]phenan-thridine-8-olate (119) which is isoconjugate with the 2-methyl-chrysene anion (Scheme 43). Thus, Fagaridine is a member of class 1 of conjugated heterocyclic mesomeric betaines, which are isoconjugate with odd alternant hydrocarbon anions. 

Fluorescence spectroscopy is reported for a benzo[4,5]thieno[2,3- / thiazolium iodide salt <2004SAA129>. A wide variety of structural variants of the nonconjugated heterocycles have reported X-ray crystallographic data however, perhaps surprisingly, only very limited X-ray structural information has been reported for the fully conjugated heterocycles. 

There are relatively few fully conjugated heterocycles within the scope of this chapter. Compound 7 is one of a number of similar structures which undergo an intramolecular aza-Diels-Alder reaction followed by extrusion of molecular nitrogen to give fused pyridine derivatives such as 8 (Equation 1) <2003T8489>. 

Very recently a theoretical study of the tautomerism between 1,2-dithiins and dithiones was published139. Contrary to 1,2-dithiete, 1,2-dithiin (34a) is a nonplanar heterocycle with an energy barrier of inversion via the fully conjugated planar structure of 8.3 kcal mol 1, estimated attheMP2/6-31G(d) level. Similarly to what was found for 1,2-dithiete with respect to 1,2-dithioglyoxal, the cyclic unsubstituted 1,2-dithiin is predicted to be about 15 kcal mol 1 more stable than the open-chain dithiones (34b-34d). 

As utilized in the synthesis of PPV-based polymers, soluble precursor routes have been developed for the synthesis of various heterocyclic Jt-conjugated polymers. The two most widely employed of these methods for heterocycle formation, shown in Scheme 66, center around ring closure of pre-polymers containing diacetylene 65 or 1,4-diketone units 66 [335-339]. The synthesis of heterocyclic structures from 1,4-diketones has been a known transformation in organic chemistry for decades. While once mainly used for monomer preparation through various cyclizations, it is now being employed to make heterocycle-containing polymers and copolymers [340-342]. 

Q Show how to construct the molecular orbitals of a conjugated cyclic system similar to benzene and cyclobutadiene. Predict whether a given heterocyclic structure will be aromatic. For heterocycles containing nitrogen, determine... 

Let us bear in mind that for compounds of the 3-ionone series we can consider two possibilities of reaction through six-center transition states leading respectively to heterocyclic rings (lactone or oxide) and to isomers with exocyclic conjugated diene structures. The two mechanistic t3rpes are shown in Figure 1. 

Using benzene-like aromatic systems and pericyclic reactions with an even number of centers, the principles of graph-theoretical structure theory are described and extended to conjugated heterocycles and cyclic systems with an odd number of centres. With topological analysis of the graphs of these systems as a foundation, a graph-theoretical definition of the idea of aromaticity in regard to monocyclic compounds is presented. 

The fortunate fact that a number of the biochemical units are conjugated heterocycles has permitted the first theoretical unraveling of problems connected with their electronic structure in the framework of the 7r-electron approximation. However simple the procedures utilized, a careful analysis of the results has allowed the interpretation of a considerable body of experimental facts as well as a number of predictions later confirmed by subsequent experimentation 0. Although these results have survived the test of the successive refinements of the 7r-electron theories and have been complemented by the introduction of a simple representation of the a-framework, the possibilities of treating the a and 7r electrons simultaneously on an equal footing had to be explored in order to establish the theory on a firmer basis and also to gain further insight into some fine features of electronic properties which are otherwise inaccessible. Thus, the first outcome of the penetration of all-valence and all-electrons methods into biochemistry has been to deepen and refine previous studies. 

Five-membered conjugated heterocycles which contain nitrogen atoms (azoles) have been thoroughly investigated by nitrogen NMR spectroscopy (ref. Id, pp. 204-218). Some additional data are now available (Table XVIII). There are two types of nitrogen atoms in such heterocyclic structures, e.g. the case of imidazole derivatives [24]. The... 

An important exception to this rule is provided by the magnesium complexes of tetrapyrrole systems, the parent compound of which is porphine (7-II). These conjugated heterocycles provide a rigid planar environment for Mg2+ (and similar) ions. The most important of such derivatives are the chlorophylls and related compounds,28 which are of transcendental importance in photosynthesis in plants.29 The structure of chlorophyll-a, one of the many chlorophylls, is (7-III). 

The pK associated for protonation of piperidine, a typical secondary amine, is about 13. The equivalent pK for protonation of pyridine—a compound with a similar heterocyclic structure, but with its lone pair in an sp rather than an sp orbital, is only 5.5 pyridine is a weaker base than piperidine (its conjugate acid is a stronger acid). Nitriles, whose lone pair is sp hybridized, are not basic at all. Lone pairs with more p character (sp orbitals are 3/4 p, while sp orbitals are 1/2 p) are higher in energy—they spend more time further from the nucleus—and are therefore more basic. 

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