Properties of Pyrazine

The molecular susceptibility (— Xm x 10 /cm mol) of pyrazine has been determined as 37.7 (576) or 38.0 (577) these show large differences from those of pyrimidine (43.1) and pyridazine (40.5). The nature of these differences suggests that they result from delocalization of the lone-pair orbitals of the nitrogen atoms in these compounds (577). 

Pyrazine forms an azeotrope with water [60% pyrazine-40% water, b.p. 95.5° (uncorr.) (760mmHg) /ip 1.4510] (578). A method of assay for pyrazine and some common impurities has been developed (579). The dipole moment (Debye units) of pyrazine has been determined in dioxane, cyclohexane, and benzene as zero (580, cf. 581) and it has also been calculated as zero (133, 582). The e.s.r. spectrum (583) and the polarized single-crystal absorption spectra of pyrazine (and tetramethylpyrazine) (584) have been recorded. The photoelectron spectra of pyrazine and tetramethylpyrazine have been determined and suggest a different behavior towards electrophilic attack in the two cases (585). 

Polarographic studies have been made on pyrazine and methylpyrazines they indicate that 1,4-dihydropyrazines are produced, and that substitution (by methyl groups) makes the reduction more difficult. The reduction of the parent pyrazine proceeds reversibly (125, 586-588). The experimental half-wave reduction potentials [pyrazine (2.17 eV), methylpyrazine (2.23) 2,6-dimethylpyrazine (2.28), tetramethylpyrazine (2.50), pyridine (2.76), and quinoxaline (1.80)] also revealed that pyrazine was more easily reduced than pyridine but less easily reduced than quanoxaline (589). 

Kinetic studies of hydrogen-deuterium exchange in pyrazine (590, 591) and 

Electron diffraction photography of benzene, pyridine, and pyrazine are closely similar (120). 

Recently reported physical data for pyrazine (and its salts or simple derivatives) are collected with references under pyrazine in the Appendix (Table of Simple Pyrazines). More extensive studies on such aspects of pyrazine (and some hydro or putative dehydro derivatives) are here indicated briefly with references. 

Aromaticity. An aromaticity index, based on deviation of peripheral bond orders,1691 has been applied to pyrazine (89% of that for benzene) and some derivatives.257,376,379,383 The aromaticity of l,s4-dihydropyrazines has been studied.565,1734 

Conformations. Calculations have been made of the preferred conformations for 1,4-dihydropyrazine,456, 1080 1,2,3,4-tetrahydropyrazine,100 piperazine (and several alkyl derivatives),1079 and the (reduced) pyrazine ring in several biologically important di- and tetrahydropteridines.100 

Crystal phases. The measured heat capacities for crystalline pyrazine in the range 20-40°C suggest that, in each of the phases involved, —50% of the molecules must be disordered.556 

Electron distribution. The tt- and cr-electron distributions in pyrazine and other azines have been studied theoretically 458,562 there appears to be a reasonable correlation between the net charges on nitrogen atoms and the measured 15N NMR shifts.562 

Conformations. Calculations have been made of the preferred conformations for 

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Table 3 lists some of the basic physical properties of pyrazine, quinoxaline and phenazine (references are given in the main text). 

Table 3 Physical Properties of Pyrazine, Quinoxaline and Phenazine...

Our understanding of the physicochemical properties of pyrazines has deepened. The internal rotation and IR spectrum of 2,5-pyrazinedicarboxamide were studied by quantum chemical calculations <05TC73>, and ab initio MO calculations at the MP2/6-31++G( ) level were used to explain the electronic and vibrational properties of complexes of pyrazine and HX linear acids <05JMS2822>. MM and MO calculations were used to investigate the conformational and electronic properties of odor-active pyrazines <05JMS169>, and NMR, IR, X-ray, and DFT methods were used to examine the structures of pyridol l,2-a pyrazinium bromide <05JMS7>. 

Theoretical calculations have been a successful approach for the description of structures and thermochemistry for a wide variety of chemical substances in fact, the theoretical methods have greatly increased in importance for investigating various properties of pyrazines, quinoxalines, and phenazines <1996CHEC-II(6)233>. In most cases, the calculated results have been discussed in comparison with spectroscopic properties accordingly, in this chapter, such examples are shown in each section on spectroscopy. 

Silver(I) complexes of pyrazine were first reported in 1895." Since then there have been numerous reports on the preparation and properties of pyrazine complexes of silver(I).85 Thermodynamic data for some of these complexes are given in Table 18. 

B. Properties of Pyrazine A-Oxides and their C-Alkyl Derivatives... 

The absence of mesomorphism in these compounds was explained on the basis of space-filling requirements. Thus, the intercalation of pyrazine between the binuclear units creates free volume which needs to be filled to obtain a stable, condensed phase when the carboxylates bear only one chain, the interdimeric space is likely filled by the aliphatic chains belonging to a different polymeric chain, giving rise to a crossed structure which prevents the formation of a columnar mesophase. However, as will be seen later, liquid-crystalline behavior was induced in the case of mixed-valence diruthenium(II,III) carboxylate complexes with bulky equatorial Kgands bearing two and three aliphatic chains as with such ligands, it was possible to fill the interdimeric space and thus to induce a thermotropic columnar mesophase. Very recently, the synthesis, characterization, and mesomorphic properties of pyrazine-polymerized divalent rhodium benzoates have also been reported (99). " Most of these compounds exhibit columnar (Colh, Coir, CoIn) and cubic mesophases with melting transition temperatures close to, or even below, room temperature. 

Ring substituents show enhanced reactivity towards nucleophilic substitution, relative to the unoxidized systems, with substituents a to the fV-oxide showing greater reactivity than those in the /3-position. In the case of quinoxalines and phenazines the degree of labilization of a given substituent is dependent on whether the intermediate addition complex is stabilized by mesomeric interactions and this is easily predicted from valence bond considerations. 2-Chloropyrazine 1-oxide is readily converted into 2-hydroxypyrazine 1-oxide (l-hydroxy-2(l//)-pyrazinone) (55) on treatment with dilute aqueous sodium hydroxide (63G339), whereas both 2,3-dichloropyrazine and 3-chloropyrazine 1-oxide are stable under these conditions. This reaction is of particular importance in the preparation of pyrazine-based hydroxamic acids which have antibiotic properties. 

We wish to thank Professor F. W. Bergstrom for the sample of pyrazine, Dr. W. E. Vaughan for that of tetrabromobutane, Dr. K. J. Palmer for his help with experimental work, Dr. Sidney Weinbaum for assistance in the preparation of the figures, and Dr. E. R. Buchman for discussion of the chemical properties of the substances. We are especially indebted to Professor L. O. 

The adverse properties of red beet preparations are without doubt, their high nitrate content, their unpleasant smell due to pyrazine derivatives and geosmin with a low odor threshold, and finally the risk of carry-over of earth-bound germs. - " Since nitrate has been associated with nitrite and nitrosamine formation, the latter exhibiting cancer-inducing properties, procedures have been developed to reduce the nitrate content of red beet preparations. This has been achieved by microbial denitrification. - ... 

The quinoxaline 100, with self-contained donor-acceptor properties, has potential in optoelectronic <06JACS10992>. Electroactive dendrimeric bis-quatemary salts have been prepared by direct quatemisation of pyrazine using dendrimeric benzyl bromides <06TL4711>. 

An efficient one-pot synthesis of some novel azolo[l,5-a]pyrimidines, via enaminonitriles, has been described <00SC1985>. The utility of 3-aminocinnamonitrile in the synthesis of new pyrazolo[l,5-a]pyrimidines has been reported <00ZN(B)321>. The synthesis of novel arylaminopyrazolo[3,4-d]pyrimidin-4-ones has been described <99IJC(B)1075>. The synthesis and properties of a-Thiagra, a substituted 5-(2-thienyl)pyrazolo[4,3-d]pyrimidin-7-one bioisostere of Viagra, have been described <00H(53)2643>. Imidazo[4,5-b]pyrazines are obtained by the reaction of 4-amino- 5-imino-imidazole derivatives with acetophenone dimethylacetal . 

Second-order nonlinear optical properties of noncentrosymmetric LB films composed of pyrazine derivatives... 

The versatility of the quinolyl derivative (295) is exemplified in the complex [Ru(295-A,A )(295-A,A, A")C1]. The coordination behavior of (295) has been compared with those of (296) and (297) and factors influencing didentate vs. tridentate preferences have been examined. Perchlorate salts of [Ru(tpy)L)] " and [RUL2] where L = (298)-(300) have been synthesized and characterized, and their properties compared with those of [Ru(tpy)2] ". Complexation of Ru with 2,6-bis(4 -phenyl-2 -quinolyl)pyridine (L) affords [RUL2] , spectrosccmic properties of which have been discussed. " The bis(7 -methyl)-derivative of this ligand, L, has also been prepared and incorporated into the complexes [M(L )2f, [ (L CIs] (M = Ru, Os), and [Ru(L )(tppz)] + where tppz = 2,3,5,6-tetra(2 -pyridyl)pyrazine. Related cyclometallated complexes have also been studied. All the complexes luminesce at 77 K and [M(L )2] (M = Ru or Os), [Ru(L )(L )] , and two of the cyclometallated species are luminescent at room... 

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