Quinoxaline reactions with radicals

The regioselectivity observed in Eq. (6) with diprotonated quinoxaline (reaction performed in 96% H2S04) is of particular interest. Whereas in monoprotonated quinoxaline (Eq. 1) the C-2 carbon atom has the lowest electron density and substitution occurs at C-2 position only, in diprotonated quinoxaline (Eq. 6) the electron density of the equivalent C-2 and C-3 is as low as that of the equivalent C-6 and C-7 carbon atoms (NMR and IN DO calculations) [9] and substitution occurs at both C-2 and C-6 positions. To minimize polysubstitution, the conversions in Eq. (6) were limited to about 50 % (selectivity, based on the reacted bases, is >90 % in any case). Another feature of Eq. (6) is the exceptional selectivity of hydrogen abstraction from the C-5 position of w-hexyl derivatives by the aminium radical i-Bu2NH+, generated from i-Bu2NHCl+ and Fe(II) [2]. 

It is worth noting that, unlike the analogous reactions with alkynes, all of these annulations involving the cyano group always led to a unique quinoxaline derivative, since the final iminyl radical cyclizes onto the aromatic ring of the isonitrile in an exclusive 1,6-fashion. 

The effect of the heavy-atom substituents, bromine and iodine, on the electron-donor aniline in the electron-transfer reaction with thiopyronine triplet has been investigated by flash spectroscopy in solvents of different viscosity and polarity. Triplet quenching and radical yields are presented in Table 30. The results are analysed in terms of decay constants of an intermediate triplet exciplex. The influence of an external heavy-atom effect on the phosphorescence spectra of quinoxaline and 2,3-dichloroquinoxaline has also been reported. The influence of chloride ion on the decay rate of Methylene Blue triplet in 0.01 M acid in the... 

Lund and coworkers [131] pioneered the use of aromatic anion radicals as mediators in a study of the catalytic reduction of bromobenzene by the electrogenerated anion radical of chrysene. Other early investigations involved the catalytic reduction of 1-bromo- and 1-chlorobutane by the anion radicals of trans-stilhene and anthracene [132], of 1-chlorohexane and 6-chloro-l-hexene by the naphthalene anion radical [133], and of 1-chlorooctane by the phenanthrene anion radical [134]. Simonet and coworkers [135] pointed out that a catalytically formed alkyl radical can react with an aromatic anion radical to form an alkylated aromatic hydrocarbon. Additional, comparatively recent work has centered on electron transfer between aromatic anion radicals and l,2-dichloro-l,2-diphenylethane [136], on reductive coupling of tert-butyl bromide with azobenzene, quinoxaline, and anthracene [137], and on the reactions of aromatic anion radicals with substituted benzyl chlorides [138], with... 

The ethyl radical directly attacks the heteroaromatic base, while the acetaldehyde acts as a source of acetyl radical. Photochemical oxy-alkylation has also been tried with ethers. The reaction has been successfully carried out with pyridines, quinolines, isoquinolines,cinno-lines, and quinoxalines. Particularly good yields were obtained with caffeine (16) (Scheme 14). ... 

In aprotic medium (acetonitrile), quinoxalino[2,3-b]quinoxaline (231) undergoes two reversible reductions to an anion-radical and further to a dianion.367 In 1 1 H20-DMF 231 gives a two-electron wave followed by a four-electron wave. The first reduction leads to 5,12-dihydroquinoxa-lino[2,3-h]quinoxaline (232), whereas the second reduction in a four-electron reaction leads to the 5,5a,6,11,1 la,12-hexahydro derivative (233). On heating with acetic anhydride a triacetyl derivative (234) is obtained367 [Eq. (128)]. 

Quinoxaline is alkylated with selenuranes, which are formed by the reaction of diphenyl selenoxide and carboxylic anhydrides, under irradiation. This reaction is presumed to proceed by a radical pathway accompanying decarboxylation. 

The phenylation of pyridazine and quinoxaline has been carried out using dibenzoyl peroxide, iV-nitrosoacetanilide, and benzenedia-zonium hydroxide as the sources of phenyl radical, the first two methods giving very much better yields than the third.63 The most reactive positions in these ring systems are the 4-position in pyridazine and the 2-position in quinoxaline. Phthalazine has been phenylated with iV-nitrosoacetanilide, giving a low yield of 5-phenylphthalazine, but the main product from cinnoline in this reaction was 4,4 -bicin-nolyl, although a small quantity of 4-phenylcinnoline was obtained.63 Pyrimidine has been arylated only with the 4-nitrophenyl radical, substitution occurring at the 2- and 4-positions.12... 

Kaim has shown that the single electron transfer (SET) reactions of AIH3 and AID3 in THE with pyrazine (1), quinoxaline (2), and phenazine (3) can be studied by ESR spectroscopy, which fully characterizes persistent radical complexes formed as escape products besides the conventional diamagnetic reduction products <84JA1712>. The same author was able to isolate unusually stable... 

Radical reactions have become a powerful pathway to provide functionalization to the diazine rings. Provide a pathway to produce a carbamoyl radical using formamide, cerium ammonium nitrate, and N-hydroxyphthalimide and then treat that radical with quinoxaline to provide the functionalized product. ... 

In 2008, Itami reported a transition-metal-free C-H arylation of simple azines such as pyrazines, pyrimidines, pyridazines, and quinoxalines with aryl iodides under the influence of KOt-Bu. Unfortunately, this reaction showed no regios-electivity due to its radical reaction pathway (e.g., the ratio for the arylation of... 

This somophihc isocyanide insertion reaction is also suitable for use with hydrazines [48], arylsulfonyl chlorides [49], diaryliodonium salts [50], and Umemoto s reagent [50] (Scheme 13.22). Polycyclic quinoxalines can also be synthesized by the visible-light-induced decarboxylative radical cyclization of... 

---