Heterocyclic aromatic species

CYP1A2. CYP1A2 has been implicated in the activation of procarcinogenic species such as aflatoxin Bl, 2-acetylaminofluorene, and other arylamines. It tends to favor aromatic substrates, both heterocyclic aromatic substrates like caffeine and aromatic substrates like phenacetin (10). In the case of caffeine, 1A2 is the major isoform catalyzing the... 

Redox shuttles based on aromatic species were also tested. Halpert et al. reported the use of tetracyano-ethylene and tetramethylphenylenediamine as shuttle additives to prevent overcharge in TiS2-based lithium cells and stated that the concept of these built-in overcharge prevention mechanisms was feasible. Richardson and Ross investigated a series of substituted aromatic or heterocyclic compounds as redox shuttle additives (Table 11) for polymer electrolytes that operated on a Li2Mn40g cathode at elevated temperatures (85 The redox potentials of these... 

However, measured data is missing for numerous cases of putative aromatic species with multiple heteroatoms, a situation only worse for the aforementioned approaches to thermochemical understanding of the aromaticity of these more general heterocycles. We are not ready to relinquish this study to computational theorists and so we will present yet other models in this chapter. While these models are not universal, i.e., not all comparisons can be made for all heterocycles, they interleave in that we may use more than one comparison for some heterocycles and derive inequalities and bounds for their varying degrees of aromaticity. 

Our aromaticity-determining reactions for the three heterocycles are endothermic by 70, 40, and 48 kJ mol-1, to be compared with the exothermicity for the corresponding carbocycle, indene, of nearly 9 kJ mol-1. Indole, benzofuran, and benzothiophene are truly aromatic species in terms of thermochemical stabilization. 

Consider now the enthalpies of formation of our diverse indane heterocycles and corresponding one-ring species formed by de-benzoannelation (Table 1). De-benzoannelation resulting in aromatic one-ring species is generally more favorable than processes involving non- or antiaromatic species. The enthalpy of formation difference for aromatic species is typically 50-60 kJ mol-1, for nonaromatic species typically ca. 30 kJ mol-1, and for antiaromatic species ca. 24 kJ mol-1. The difference of the enthalpies of formation between the benzoannelated and one-ring dithiol-2-thiones is —11 kJ mol-1. No explanation for this discrepancy is apparent. 

Quinones have also been used to dehydrogenate adjacent to C—N in a variety of substituted nitrogen heterocycles with the uitimate generation of aromatic species (equation 14). In some instances DDQ has been claimed to be preferable to other reagents generally used for this purpose s but the eventual choice must be determine by the substituents present and the intrinsic stability of both product and starting material. 

Those heterocyclic systems attacked by complex metal hydrides are required to be relatively electron deficient. Nitrogen heterocycles in which the heteroatom contributes a single electron to the n system are considered electron deficient (e.g., 1). However, systems where the nitrogen atom contributes two electrons are considered electron rich (e.g., 2) and are not normally attacked by metal hydrides. Aromatic species that contain both a pyridine-like (1) and a pyrrole-like (2) heteroatom (e.g., 3) exhibit be-... 

The chapter related to this one in the first edition of Comprehensive Heterocyclic Chemistry (CHEC-I) <84CHEC-I(2)525> dealt mainly with quinolizinium ion, and quinolizine or quinolizidine were mentioned only as reaction products of the aromatic species. Since that time, new developments in the chemistry of quinolizinium compounds have been relatively scarce, and a number of reviews covering them, together with the earlier work, have appeared <81H(16)803,82AHC(3l)l, 91HOU(E7a)759, 92HOU(E7b)239, 92AHC(55)261>. For these reasons, this chapter is devoted mainly to nonaromatic compounds. 

This technique is almost routinely applied for unambiguous characterization of this class of heterocycles. Thus the base peak for the compound (19) (Table 2) is the (M-1)" peak due to the formation of aromatic species (75) <9lOMS(26)667>. 

Although the OH initiated oxidation of aromatics is generally more important under atmospheric conditions than the reaction with NO3 the latter type of reaction becomes more important with increasing alkyl-substitution and predominates for phenolic and heterocyclic aromatics [12]. Thus a basic understanding of the mechanism of these reactions is needed to investigate the role of aromatic species in tropospheric photochemistry. 

Oz, F., Kaban, G., and Kaya, M. 2007. Effects of cooking methods on the formation of heterocyclic aromatic amines of two different species trout. Food Chem. 104 67-72. 

The results from the one-dimensional Py-GC-MS of Murchison meteorite are shown in Figure 18.1 the results are consistent with previous studies of Murchison and other CM2 chondrites [11]. As well as a number of well-resolved species such as alkylbenzenes, sulfur heterocyclic compounds, and naphthalenes in the chromatogram, there is a significant portion of products that are present as an unresolved complex mixture (UCM), which can be seen as a broad rising baseline or hump. From the main fragment ions present, the UCM appears to be dominated by saturated, unsaturated, and cyclic hydrocarbons and not aromatic species which dominate the resolved components. Compared with typical terrestrial samples, the main difference is the lack of n-alkanes/alkenes in the pyrolyzate [35]. 

Matejka, P., Stavek, J., Volka, K., and Schrader, B. (1996). Near-infrared surface-enhanced Raman scattering spectra of heterocyclic and aromatic species adsorbed on TLC plates activated with silver. Appl. Spectrosc. 50 409-414. 

It is frequently very attractive, rather than building a saturated heterocycle from acyclic precursors, to reduce a suitably substituted aromatic species. Several reductions of pyridines have appeared. Hydrogenation was, of course, the most common method employed.A complete study of selectivity in the hydrogenation of quinoline was also published, and the authors claimed to be able to reduce out either ring at will thus a platinum catalyst at 3 atmospheres hydrogen pressure cleanly gave the 1,2,3,4-tetrahydroquinoline, whereas... 

Additional evidence has been presented in support of hydro-aromatic species, e.g. (82), as intermediates in the thermolysis of aryl 2-azidophenyl sulphides, leading to phenothiazines (Vol. 2, p. 780). A successful synthesis of 117-phenothiazin-l-one (83), a member of a novel class of heterocyclic o-quinone-imines, by oxidation of 1-hydroxyphenothiazine in an inert solvent under carefully controlled conditions has been reported. A dominant feature of the chemical behaviour of (83) is its tendency to undergo dimerization to give a rather intractable product, identified tentatively as (84). A multi-step synthesis of l-methyl-2-aminophenothiazin-3-one has also been described. ... 

Typical nucleophiles known to react with coordinated alkenes are water, alcohols, carboxylic acids, ammonia, amines, enamines, and active methylene compounds 11.12]. The intramolecular version is particularly useful for syntheses of various heterocyclic compounds[l 3,14]. CO and aromatics also react with alkenes. The oxidation reactions of alkenes can be classified further based on these attacking species. Under certain conditions, especially in the presence of bases, the rr-alkene complex 4 is converted into the 7r-allylic complex 5. Various stoichiometric reactions of alkenes via 7r-allylic complex 5 are treated in Section 4. 

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