Quinoxaline protonation

Scheme 2.39 Quinoxaline protonation can access other resonance forms including thiolate, thione chelates on Mo.

Inductive and resonance stabilization of carbanions derived by proton abstraction from alkyl substituents a to the ring nitrogen in pyrazines and quinoxalines confers a degree of stability on these species comparable with that observed with enolate anions. The resultant carbanions undergo typical condensation reactions with a variety of electrophilic reagents such as aldehydes, ketones, nitriles, diazonium salts, etc., which makes them of considerable preparative importance. 

Protonation of pyrazine A-oxides takes place at the unsubstituted ring nitrogen as revealed by examination of their UV spectra and ionization constants in water. The same holds for unsubstituted quinoxaline A-oxide and the 3-amino derivative. Pyrazine and quinoxaline di-A-oxides are protonated at one A-oxide oxygen atom (74KGS1554). 

Alkyl-Alkylidene Tautomerism. Some 2- or 3-(substituted alkyl)quinoxalines, like 3-ethoxycarbonylmethyl-2(177)-quinoxalinone (133), have long been known to exist in equilibrium with their (substituted methylene) tautomers, for example 3-ethoxycarbonylmethylene-3,4-dihydro-2( 1 /7)-quinoxalinone (133a).The effects of solvent change, protonation, and the like on such tautomeric systems have been examined as well as the kinetics thereof. In... 

Square planar Ni11 complexes (50a) and (50b) of the quinoxaline-2,3-dithiolate ligand are oxidizable in chemically reversible, electrochemically quasi-reversible processes to yield Ni111 species, also featuring the (dxy)1 configuration.198 Interestingly, the difference in protonation state makes for a 0.20V difference in oxidation potential ((50a) +0.12V (50b) +0.32V vs. SCE), consistent with the less basic S-donors in the thione form. 

The UV irradiation of quinoxaline in methanol yields radicals, not by hydrogen abstraction, but by protonation of the first singlet excited state, followed by exiplex formation.82 Irradiation of quinoxaline in acidified methanol furnishes 2-methylquinoxaline, and the reaction is suggested to go through a pathway involving electron-transfer from the solvent to an excited state of the protonated quinoxaline (Scheme 3).83... 

Methylquinoxaline also undergoes a novel cycloaddition reaction with two equivalents of tetrachloro-l,2-benzoquinone129 (see Section III,A) with formation of a quinoxaline-orange type product. The reaction probably proceeds by initial formation of the quaternary salt 231 which by elimination of HC1, followed by protonation and dehydration, leads to the cyclized product 232. This on reaction with a further equivalent of tetrachloro-l,2-benzoquinone at the C=N function gives the final yellow compound 117. 

Analysis248 of the UV spectra of the monoprotonated 2-substituted quinoxalines, and the Hammett correlation of the pKa shifts with the substituent constants, give two straight lines, corresponding to two sets of substituents, and so reflecting a change in the position of protonation. Thus, 2-methoxyquinoxaline was found to protonate at N-4, and 2-aminoquinoxaline at N-1. However, the site of protonation of 2-chloro-quinox aline was ambiguous. 

Acyl radicals obtained by the oxidation of aldehydes or the oxidative decarboxylation of a-keto acids react selectively at the a- or y-position of the protonated heterocyclic nitrogen. Pyridines, quinolines, pyrazines and quinoxalines all react as expected yields are typically 40 to 70%. Similarly, pyridines can be carbamoylated in acid media at C-2 (Scheme 38). 

Andronati and co-workers reported a detailed study of the A-basicity of compounds 203 (X = H, Me R1 = H R2 = H, Br, Cl, Me) which contain amide, imine, and amine nitrogens in one molecule. The basicities, obtained from the half-neutralization potentials in potentiometric titrations, showed 203 to be monoacidic bases the basicity varied predictably with substitution and fell between the stronger quinoxaline-2-ones and the weaker l,4-benzodiazepine-2-ones. Ultraviolet spectroscopic studies demonstrated that the amino, not the imino, nitrogen was N-protonated (83CHE337). 

Photoluminescence from ambient temperature solutions of metal bis(l,2-dithiolenes) is rarely observed, and there have been only a few reports on luminescence of any type from these compounds. In addition to the weak emission (< ) = 10-5) seen for [Pt(mnt)2]2- (A.max = 775 nm), a similarly weak emission is observed for [Pt(qdt)2]2- (qdt = quinoxaline-2,3-dithiolate, 4) but at significantly higher energy (7,max = 606 nm) (58). Both absorption and emission spectra for the latter complex are highly dependent on solution pH, with protonation of one of the qdt nitrogen atoms leading to the shifts shown in... 

Fig. 2. The isosbestic points at 446 and 556 nm in the absorption spectra are matched by an isoemissive point at 685 nm indicating only two species present in solution, both of which are emissive. The shift in emission maximum from 606 nm in neutral solutions to 728 nm upon addition of acid may have interesting sensor applications. The results for 4 stand in contrast with results from dppz-containing Ru(II) tris diimine complexes, where dppz = dipyrido-ipyridophenazine, in which reversible protonation of quinoxaline N atoms leads to quenching of emission. Luminescence in frozen solvent glasses for 4 at 77 K is much stronger ( = 0.044 for the qdt complex), but still broad and without resolved structure.

The redox properties of tris(quinoxaline-2,3-dithiolato)molybdate(IV), [Mo(qdt)3]2, in the presence of protons provides a clear demonstration of the chemical versatility that is possible for a redox-active metal dithiolene center that involves a pyrazine ring linked to the dithiolene group. In an aprotic solvent, two reversible, Nernstian, waves are observed that (formally) correspond to the Mo(V)/Mo(IV) and Mo(IV)/Mo(III) couples. However, on addition of trifluoroacetic acid (Htfa), the Mo(V)/Mo(IV) couple slightly shifts to a higher potential and becomes non-Nernstian and a new three-electron, quasir-eversible, couple occurs some 900 mV less negative than the original Mo(IV)/ Mo(HI) couple. The latter is attributed to the addition of one electron and one... 

A further significant consequence of variation in the extent of conjugation in aromatic radicals is demonstrated by a comparison of proton hyperfine splittings in pyrazine and quinoxaline anion-radicals (22,23) and also in the... 

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