Azines chemical reactivity

All these methods demonstrate that the 2-positions of pyridine, pyrimidine, and other azines are the most electron deficient in the ground state. However, considerably greater chemical reactivity toward nucleophiles at the 4-position is often observed in syntheses and is supported by kinetic studies. Electron deficiency in the ground state is related to the ability to stabilize the pair of electrons donated by the nucleophile in the transition state. However, it is not so directly related that it can explain the relative reactivity at different ring-positions. Certain factors which appear to affect positional selectivity are discussed in Section II, B. 

In terms of chemical reactivity, cycl[3,2,2]azine shows the normal behavior of a stable aromatic system, undergoing substitution reactions smoothly and in good yields. From MO calculations it was predicted that electrophilic substitution should occur most readily at the 1-position.lb Reaction at positions 2, 5, and 6 have not yet been observed. 

Theoretical Studies of Physico-chemical Properties and Reactivity of Azines... 

Theoretical Studies of Physico-chemical Properties and Reactivity of Azines R. Zahradnik and J. Koutecky, Adv. Heterocycl. Chem., 1965, 5, 69-118. 

The fact that none of these reports has emphasized the physical aspects of electrophilic substitution in the series reflects the paucity of quantitative studies, and the low reactivity of these compounds in the presence of electrophiles. Few kinetic studies have been reported and the regio-chemical effects of substituents have seldom been quoted in quantitative form. The present chapter brings together those quantitative results that are available, and collates data on substituent effects. One worthwhile field of study would appear to be the application to the azines of Taylor s method involving thermolysis of esters [75JCS(P2)277, 75JCS(P2) 1783]. 

Benzannulation (Figure 11) significantly influences both -electron distribution and reactivity of azoles. Compared to imidazole 51, benzimidazole 54 has an increased positive charge at position 2 and this is consistent with its tendency to undergo reaction at this position with hard nucleophiles. This behaviour is analogous to the chemistry of azines. Purine is also notable for its high -deficiency, 7/7-purine 56 being more -deficient than the 9//-tautomer 55. Possibly this is responsible for the natural occurrence of the less -deficient and therefore chemically more stable derivatives of 9/7-purine. 

According to their chemical structures and the Cl system, dyes can be classified into 17 groups nitro dyes, triphenylmethane derivatives, xanthenes, acridine derivatives, quinoline derivatives, azines, ant-hraquinones, indigoid dyes, phthalocyanines dyes, oxydation bases, insoluble azo dye precursors, and azo dyes (classes XII-XVII). In practice, dyes are classified into different application classes disperse, acid, basic, direct, vat, fiber-reactive, sulfur, preme-tallic, solvent dyes, and naphthols. 

Zahradnik, R., Electronic Structure of Heterocyclic Sulfur Compounds, 5, 1. Zahradnik, R., Koutecky, J., Theoretical Studies of Physico-chemical Properties and Reactivity of Azines, 5, 69, Zahradnik, R., see Mayer, R., 8, 219. Zoltewicz, J. A., Deady, L. W.,... 

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