Theoretical calculations azines

In summary, theoretical calculations have not been a reliable guide to relative nucleophilic reactivity at different positions in the same azine or at comparable positions in different ring systems, but they may become so in the future. 

Jug and co-workers investigated the mechanism of cycloaddition reactions of indolizines to give substituted cycl[3,2,2]azines <1998JPO201>. Intermediates in this reaction are not isolated, giving evidence for a concerted [8+2] cycloaddition, which was consistent with results of previous theoretical calculations <1984CHEC(4)443>. Calculations were performed for a number of substituted ethenes <1998JPO201>. For methyl acrylate, acrylonitrile, and ethene, the concerted [8+2] mechanism seems favored. However, from both ab initio and semi-empirical calculations of transition states they concluded that reaction with nitroethene proceeded via a two-step intermolecular electrophilic addition/cyclization route, and dimethylaminoethene via an unprecedented two-step nucleophilic addition/cyclization mechanism (Equation 1). 

The other two chapters deal with the application of molecular orbital theory to heterocyclic chemistry. The groups surveyed are (a) sulfur heterocycles and (6) azines. The chapters, which are authored by R. Zahradnik and J. Koutecky, discuss the relevance of theoretical calculation to reactivity, electronic structure, and other physicochemical properties of the compounds. The new techniques of theoretical chemistry have been applied with great success to carbocyclic compounds their significance in heterocyclic chemistry will surely increase. 

Theoretical calculations of proton affinities, relative basicities, and preferred protonation sites of azines have been carried out by two groups <87JST(35)135, 88Mi 622-01). As expected, all of the methods predict that the introduction of additional nitrogens decreases the overall absolute basicity. Furthermore, comparison of the relative basicities of isomers, and of the preferred protonation site for each isomer, indicates that nitrogen atoms with only one a-N, and without a y-N, are more basic than any others. The results, using different methods, illustrate the necessity for using fully optimised structures to obtain reliable proton affinities. 

The parent azine systems discussed in this chapter, compounds 1-4, have not been prepared experimentally, but there has been continuing interest in their theoretical analysis. The past 10 years have seen the incorporation of computational chemistry into the mainstream of chemical research, facilitated by the advancement of computer hardware, and computational software and methods. Hence, not surprisingly, recent studies have been performed using advanced methods, such as MP2, CCSD, and a considerable number of density functional theory (DFT) calculations. Azines have been investigated in terms of structural features, aromaticity, kinetic and thermodynamic stability, and decomposition reactions. 

Therefore in the literature there are some N NMR data of ring fluorinated diazines. The N NMR of perfluorinated diazines are summarized in Table 7 [144], Also recently few works devoted to theoretical calculations of couphng constants in fluorinated azines were published [145,146],... 

The tautomerism of functional derivatives of azines (e.g., pyridones) and other more complex systems (guanines, porphyrins, etc.) has been much studied both experimental and theoretically [40, 76-82], Our contribution to this field has been modest using a combination of 13C and 15N NMR in solution and in the solid state, together with GIAO//B3LYP/6-311++G calculations, the tautomerism of 2-pyri-done, 4(37/)-pyrimidone, uracil, and cytosine were determined [83],... 

Structural parameters of azines, calculated at the MP2(fc)/6-31G" and B3LYP/6-31G levels, were reported in a recent comprehensive theoretical study <2004CJC50>, with results for relevant azines shown in Figure 1. Both B3LYP and MP2 calculations showed that hexazine is a hilltop, whereas hexazine (Z>2) is a minimum, in... 

---