Benzo pyridines, properties

Table 11 summarizes the main results on the tautomerism of mono-hydroxy-, -mercapto-, -amino- and -methyl-azines and their benzo derivatives, in water. At first sight the equilibrium between 2-hydroxypyridine (71) and pyridin-2-one (72) is one between a benzenoid and a non-benzenoid molecule respectively (71a 72a). However, the pyridinone evidently has a continuous cyclic p- orbital system, containing six it- electrons, the usual aromatic count, if the carbonyl group contributes none. This assumption implies the formula (72b), from which by redistribution of electrons we arrive at (72c), which has the same benzenoid system as (71a). Further canonical forms (71b, 71c) can be drawn of (71) which correspond to the non-benzenoid forms of (72). The elusive property of aromaticity is therefore possessed by both tautomers, although not necessarily by both equally. When the carbonyl oxygen of (72) is replaced by less electronegative atoms, as in the imine tautomers of amino heterocycles, or the methylene tautomers of methyl derivatives, the tendency towards polarization in forms corresponding to (72b) and (72c) is considerably less, and the amino and methyl tautomers are therefore favoured in most instances. 

Since the chemical nature of a heterocyclic system is determined by the valent state of the heteroatom, on the basis of data described previously in this review, one could not consider the names used for a lot of peri-heterocycle series to be very suitable. Thus, indole is a 7r-excess system, and so-called benzo[o/]indole II (X = N) is a 7r-deficient one whose properties more resemble pyridine and quinoline. But by analogy, there are some common features in the chemical behavior of azepine and naph-tho[6c]azepine XII (X = N) (as one example). 

This agrees with the relative ease of polarographic reduction of the same heterocycles. However, there is no strict dependence between 7r-deficiency and -acceptor strength of the compounds. -Acceptor properties of azines also increases on transition to benzo derivatives, e.g., acridine > quinoline > pyridine. 

It should be noted that a number of SPC polymers which contain other heterocycles have been prepared, motivated by their promising optical or electrical properties. Examples include pyridine (65) [123], pyrrole (66) [124], oxadiazole (67) [125], selenophene (68) [126], benzo[2,l,3]thiadiazole (69) [127], benzo[2,l,3]selenadiazole (70) [126], perylene bisimide (71) [128], 1,4-diketo- pyrrolo[3,4-c]pyrrole-l,4-dione (72 and 73) [127,129], and triphenyleamine (74) [127] as part of the polymer backbone by SPC (Figure 19c). Specifically for metal complexation, porphyrin [130], difluoroboraindacene [131], bipyridine [132], phenanthroHne [113], terpyridine [133, 134], and the like [123] were embedded in the backbone. In this context, an interesting report was submitted by Rehahn et al., in which l,l -ferrocenyl units were incorporated into a PPP (Figure 22.20). Due to a low-energy barrier for rotation around the Cp-Fe-Cp axes (Cp = cyclopentadienyl), the obtained polymer 75 was assumed to take randomly coiled conformations [135]. 

While the importance and the production of pyridines has been well studied, a number of new pyridine-containing natural products and materials with promising biochemical, physical, and chemical properties have been reported in 2013. This chapter will provide an overview and summary of some of the advances in the field of pyridine chemistry. Some attention will be paid to key areas of utility for materials with a pyridine scaffold this review will focus primarily on the preparations and reactions of pyridines and their benzo derivatives. 

It is well known that 2,6-diheteroaryl-containing pyridines display important biological properties such as antitumor and cyclin-dependent kinase inhibitory activities. Also, the introduction of ferf-butyl group into some heterocycles can greatly enhance their biological activity. Recently, Vang et al. [99] described a facile ultrasound-assisted synthesis of new 2,6-bis(5-fcrf-butyl-benzo[fc]furan-2-ylcarbonyl)pyridines from 2,6-bis(bromoacetyl)pyridine with f-butyl-substituted salicylaldehydes in acetonitrile in the presence of nontoxic and inexpensive catalyst as PEG-400. The new heterocyclic compounds were synthesized in short reaction times under mild reaction conditions with satisfactory yields. 

We tried to improve the donor-acceptor properties obtained for previously reported compounds [26,162] by combining a strong electron-acceptor moiety such as benzo-TCNQ or benzo-DCNQI with the 1,4-benzoxazine system as the donor fragment. The synthesis of compounds 92 and 94 was carried out according to Scheme 1.17 by reaction of 2,3-dichloro-1,4-naphthoquinone with 2-methylaminophenol in pyridine yr) at 100°C to yield the novel A(-methylben-zo[6]phenoxazin-6,11-dione (91) in good yield. 

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