6- Substituted quinoxalines

Quinoxalines substituted in the 5- or 6-position generally follow the pattern of reactions expected for substituted benzene derivatives, although recently there have been reports of interesting and unexpected reactions with nucleophiles (see Section III, A,2 and references 85-90). 6-Methylquinoxaline is brominated in the side chain when treated with N-bromosuccinimide in carbon tetrachloride in the presence of azobisiso-butyronitrile, to form 6-bromomethylquinoxaline.182... 

Newer synthetic methods have allowed pterin and quinoxaline substituted 1,2-enedithiolate complexes to be prepared. The complexes Cp2Mo S2C2(6-N(2)-pivaloylpterin)(C(0)Me) and Cp2Mo S2C2(2-quinoxaline)(C(0)Me) have been prepared with both natural abundance and >80% 34S enrichment (Eq. 9 and 10) [170,171],... 

In addition to providing spectroscopic models, the oxidation of quinoxaline-substituted molybdenum and tungsten 1,2-enedithiolates has yielded the corresponding 3-sulfido-thienoquinoxaline (thiophene-containing) derivatives (Figure 19) [172,173], This transformation models the oxidative conversion of MPT to form B and the metabolic conversion of MPT to urothione [37,49],... 

Figure 19 Conversions of a quinoxaline-substituted metallo-1,2-enedithiolate to a sul-fido-thienoquinoxaline, analogous to the conversion of MPT to urothione and Form B [37,49],...

Both the C and N chemical shifts of a number of quinoxalines substituted at position 2 with the n-electron excess 2 -benzo[ ]furanyl substituent, which has at position 3 a hydroxy or amino group, could be satisfactorily calculated by the GIAO method on the basis of HF and DFT ab initio structures. Quantum mechanical calculations using the 3-21 G(d) basis-set were performed on some p-substituted diaryl tellurides and aryl Me tellurides, and the corresponding cationic radicals of these compounds. Calculated relative radical stabilization energies were shown to correlate with experimental data, and the peak oxidation potentials and Te chemical shifts were... 

The acidity constant of the unsubstituted heterocycle is 3.94 in 50% aqueous ethanol, indicating the relatively weak basic nature of the ring system. Nevertheless pyrrolo[l,2-a]quinoxalines are appreciably stronger bases than quinoxalines. Substitution of the ring system by methyl groups results in a base-strengthening effect dependent on the site of substitution. The 4-methyl compound has a pK<, of 4.58 in 50% alcohol, which is consistent with protonation at the 5-position, as the increase in pKa (0.64) is characteristic of the effect of methyl substitution in six-membered rings a to the site of protonation. The much smaller increase in pKa between the 4-methyl compound (4.58) and 2,4-dimethyl compound (4.89) is also consistent with protonation at the 5-position. 

Routes via o-aminophenylpyrroles present the most convenient syntheses of a wide variety of pyrrolo[l,2-a]quinoxalines. Thus reaction of the amino compound 6 with acetic anhydride in acetic acid gave the acetamido derivative which was cyclized with phosphoryl chloride to give the 4-methyl compound 7 (R = Me) in 56% yield. The 4-phenyl compound 7 (R = Ph) has been prepared similarly. An even more convenient synthesis of 4-aryl compounds is achieved by reaction of compound 6 with aromatic aldehydes to give the 4,5-dihydro derivatives These are readily oxidized to 4-arylpyrrolo[l,2-a]quinoxalines 9 with manganese dioxide. This approach may be carried out in one step by reaction of compound 6 with aromatic aldehydes (e.g., benzaldehyde) in the presence of cupric acetate. Reaction of the aminophenylpyrrole 6 with 90% formic acid gave pyrrolo[l,2-a]quinoxaline (7, R = H) directly in 98% yield. Pyrrolo[l,2-a]quinoxalines substituted in the l-position and the 7-position have also been prepared from appropriately substituted... 

Cyclic voltammetry (CV) is useful to probe the electronic effect of pterin and quinoxaline groups on the Mo reduction potential and this method was applied to a series of I MoO(dithiolene) complexes. The results are graphically summarized in Figure 2.16. Within a series of T MoO(dithio-lenes), it is clear that pterin (or quinoxaline) substitution causes a significant shift in the Mo redox potential to more positive values compared to simpler dithiolenes like benzenedithiolate (bdt) or ethanedithiolate (edt). This conclusion seems to contradict the results from EPR and MCD studies of Tp MoO(pterin-dithiolene vs. T MoO(bdt), which, as noted above, failed to reveal any differences among the dithiolene complexes. 

The luminescence and excited state electron transfer reactions of (dppe)Pt S2C2(2-pyridine(ium))(H) and (dppe)Pt S2C2(4-pyridine(ium))(H) are dependent on the protonation state of the pyridine [30-35]. The switching on of the luminescence in these compounds results from a change in the ordering of the electronic transitions in the pyridine and pyridinium substituted complexes. Unlike the quinoxaline-substituted complexes, the neutral pyridine complexes have a lowest lying d-to-d transition, which leads to rapid nonradiative decay of the ILCT excited states. However, upon protonation the ILCT becomes the low-lying transition. The pyridinium complexes are room temperature lumiphores with emission from ILCT and ILCT excited states (see Table Ic). 

Cyanogen-di-N-oxide appeared to be a convenient reagent for the synthesis of quinoxaline-substituted (E, )-dioxime with a dioxadithiadi-azamacrobicycle 14 when interacting with the 2,3-DAB attached to the mixed-donor-macrobicycle 13 (2002POL1865). 

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