Structure of pyridazines

Several calculations on the electronic structure of pyridazine are avail-able. ° ° Calculations of the polarizability and rt-bond orders were... 

The X-ray structure of pyridazine itself at 100 K showed that the determined valence angles agreed closely to those obtained by combined analysis of electron diffraction, microwave, and liquid-crystal NMR data. Significant differences were, however, observed in bond lengths, which were attributed to the crystal packing effect [91 AX(C)1933]. An X-ray... 

Z. Gamba,/ Chem. P/ys., 83, 5892 (1985). Crystal Structure of Pyridazine and Azabenzene Crystals A Test for an Intermolecular Potential. 

Pyridazine-3(2//)-thiones exist in the thione form (14), as is evident from an X-ray structure analysis of pyridazine-3(2//)-thione. 6-Mercaptopyridazine-3(2//)-thione is predominantly in the monothiolmonothione form (15) in aqueous solution and in the solid state, 6-hydroxypyridazine-3(2//)-thiones are in the hydroxythione form (16) and 6-aminopyridazine-3(2//)-thiones exist in the aminothione form (17) for further details see (73HC(28)755). Cinnoline-4(l//)-thiones and phthalazine-l(2//)-thione have been shown on the basis of UV data and ionization constants to exist in the thione forms. 

N-protonation the absolute magnitude of the Ad values is larger than for Af-methylation <770MR(9)53>. Nuclear relaxation rates of and have been measured as a function of temperature for neat liquid pyridazine, and nuclear Overhauser enhancement has been used to separate the dipolar and spin rotational contributions to relaxation. Dipolar relaxation rates have been combined with quadrupole relaxation rates to determine rotational correlation times for motion about each principal molecular axis (78MI21200). NMR analysis has been used to determine the structure of phenyllithium-pyridazine adducts and of the corresponding dihydropyridazines obtained by hydrolysis of the adducts <78RTC116>. 

A variety of 3,6-disubstituted pyridazines have been quatemized and the structures of the salts determined by unambiguous synthesis, degradation, or reactivity, On the basis of these data the following... 

The reaction of several substituted imidazo[4,5-c/]-, pyrazolo[3,4-r/]- and triazolo[4,5-zf]pyrid-azines 3 with ynamines, in competition with [4 + 2] cycloaddition, leads to [2 + 2] derivatives 4, which rearrange to l,2-diazocines5.7 8 The reaction seems to be sensitive to the substituents, as replacement of the electron-withdrawing group R on the pyridazine ring of the pyrazolo compound (A = N, B = CH) by chlorine completely inhibits both the [4 + 2] and [2 + 2] cycloaddition reactions. The X-ray structure of the imidazo derivative 5 (R = Ms, A = CH, B = N) reveals a tub conformation of the eight-membered ring. 

Corsaro and co-workers studied the reaction of pyridazine, pyrimidine, and pyrazine with benzonitrile oxide and utilized H NMR spectral analysis to determine the exact structure of all the cyclized products obtained from these reactions <1996T6421>, the results of which are outlined in Table 1. The structure of the bis-adduct product 21 of reaction of pyridazine with benzonitrile oxide was determined from the chemical shifts of the 4- and 5-isoxazolinic protons at 3.76 and 4.78 ppm and coupled with the azomethine H at 6.85 ppm and with the 5-oxadiazolinic H at 5.07 ppm, respectively. They determined that the bis-adduct possessed /(-stereochemistry as a result of the large vicinal coupling constant (9.1 Hz). Similarly, the relative stereochemistry of the bis-adducts of the pyrimidine products 22-25 and pyrazine products 26, 27 was determined from the vicinal coupling constants. 

A 1,2-diazetidine has been proposed as an intermediate in the reaction of pyridazine-3,6-dione (12) with styrene.87 The observed product was thought to arise from addition of water to the 1,2-diazetidine, although the alternative more likely explanation involving a dipolar intermediate (cf. Scheme 5) was apparently not considered. In the photochemical reaction of styrene with DEAZD, a 1,2-diazetidine structure was tentatively assigned to a minor product.88 Attempted photochemical [2 + 2] cycloaddition of DEAZD to other olefins failed to give any 1,2-diazetidines.88... 

Cycloaddition reactions of nitrile oxides with 5-unsubstituted 1,4-dihydro-pyridine derivatives produced isoxazolo[5,4-Z>]pyridines in moderate to good yield. In each case examined, the reaction produced only a single isomer, the structure of which was assigned by NMR spectra and confirmed by X-ray diffraction analysis of 102 (270). A study of the cycloaddition behavior of substituted pyridazin-3-ones with aromatic nitrile oxides was carried out (271). Nitrile oxides undergo position and regioselective 1,3-dipolar cycloaddition to the 4,5-double bond of pyridazinone to afford 3a,7a-diliydroisoxazolo 4,5-<7]pyridazin-4-ones, for example, 103. 

In Japan, a variety of heterocyclylaminophenylpyridazinones as represented by the general formula (29) have been prepared and investigated [89-92]. Structure-activity relationships have been discussed [92] additional classes of compounds in which the heterocyclic substituent is separated from the aminophenyl ring by a carbon chain have been patented [93,94]. From these studies, a novel class of pyridazine-derived cardiotonics and vasodilators has emerged. 

Most of the cardiotonic pyridazine derivatives discussed in the preceding chapter also exhibit vasodilator effects. In addition, various dihydropyrida-zine derivatives like (52), structurally closely related to calcium-antagonistic Hantzsch-type dihydroypyridines, have been patented as coronary vasodilators [169-171]. Another type of pyridazine analogue of the previously mentioned dihydropyridines, namely compounds of type (53), in which the aryl substituent at C-4 is represented by a pyridazine nucleus, has been claimed in a patent [172]. Quite recently, pyridazinyldihydropyridine-3,5-dicarboxyl-ates (54,55) have been prepared in Austria [173]. 

With this in mind, the coordination chemistry of 52 with different diazine structural isomers was investigated. There were no detectable changes in the H NMR spectrum of 52 in a THF-Jg solution when either pyrazine or pyrimidine were added in 1 1 or 1 2 molar ratios, which suggested that only weak interactions might occur between 52 and these bases. In contrast, incremental addition of pyridazine or phthalazine to a THF-Jg solution of 52 at 25 °C resulted in an upheld shift of the aromatic NMR resonances of the diindacycle 52 thus reflecting the formation of complexes between 52 and the 1,2-diazines. Analysis of the tritration data clearly indicated the formation of 1 1 Lewis acid-diazine complexes 52-pyridazine-(THF)2 and 52-phthalazine-(THF)2 whose stability constants are equal to 80 ( 10) and 1000 ( 150) M respectively (Scheme 29). These data, as a whole, indicate that 52 is a selective receptor for 1,2-diazines. 

Of the six possible conformers containing chair rings, two tra/j5-fused conformers 18 and 19, with a slight excess of 18, could be identified as major conformers, with one a>-fused conformer as a minor constituent in the C NMR spectrum of l-methylperhydropyrido[l,2-h]pyridazine in acetone-dfi in the temperature range -75 to -89°C (78JA4012). The low intensity of the signal of the c -fused conformer did not allow determination of the exact structure of this component. The results of low-temperature cyclic voltammetry experiments supported the NMR findings. 

In CHEC(1984) <1984CHEC(2)1> electron diffraction, microwave spectroscopy, and X-ray analysis of pyridazine and simple derivatives were included. All data are consistent with a planar structure and significant N-N single bond character. CHEC-II(1996) <1996CHEC-II(6)1> contained some additional structural parameters derived from X-ray... 

The pyrazine ring structure warrants the use of methodology analogous to that of pyridazines for their preparation. Condensation of diaminoethane with 1,2-dicarbonyl compounds 155 provides non-symmetrical pyrazines 156 after aromatization <99SL1203>. 

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