Quinoxaline ring, metalation

A number of quinoxalines carrying substituents in the benzene ring base have been quaternized, including 5-ethoxy,6-methyl, 6-chloro, and some 2-phenyl derivatives, but in none of these cases has the position of quatemization been ascertained. 5-Hydroxy-quinoxaline gives a methiodide which can still form metal complexes, indicating that salt formation occurred on N-1. ... 

This type of dinuclear unit has been reported elsewhere and has been characterized with bis-2-pyridyl-pyrazine or quinoxaline both in Cu [31] and in Ag [32] compounds. However, until now such units have not been used as building blocks (loops) in the construction of coordination polymers, an aspect that is discussed below. Each ligand acts in a bidentate manner with one metal center and as a monodentate system with the other. The two rings that are coordinated to the same metal center are nearly coplanar but the other deviates clearly from this plane. In this way, the structure if this dinuclear unit is very well adapted to the preferred conformation of the free ligand. The relative disposition of the ligands allows the existence of face-to-face n-jt-stacking interactions between the quinoxaline and... 

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... 

Formation of pyrazines and quinoxalines has been a focal point of many years of study. The most common method to make these rings relies on the condensation of 1,2-diamines with 1,2-dicarbonyls in ethanol or acetic acid in 35-85% yields. The mechanism proceeds through a double imine formation. Recently improved methods have been reported, employing transition-metal catalysis and microwaves. ... 

The metallation of diazines, in particular pyrazine and quinoxalines, is extremely difficult due to nucleophilic addition reactions related to the low LUMO energy levels of the rings. The functionalization of these rings was an important synthetic goal due to the many uses of the molecules. 

Reports describing the use of Minisci-type reaction conditions to functionalize pyrazine ring systems emerged in 2014. Siddaraju and coworkers devised a metal-free Minisci reaction, which they applied to the acylation of quinoxaline (Scheme 54) (14JOC3856). Reaction of quinoxaline (0.24 mmol) with the respective benzaldehyde in the presence of tetrabutyl-ammonium bromide (30mol%) and ammonium persulfate (2 equivalents) in 1,2-dichloroethane at 110°C provided (4-methoxyphenyl)(quinoxalin-2-yl)methanone in 66% yield and quinoxalin-2-yl(p-tolyl)methanone in 44% yield. 

Different methods for the introduction of either halides or heteroatoms onto the pyrazine, quinoxaline, and phenazine rings were explored. Hwang and collaborators devised a metal-catalyzed regioselective way of placing heteroatoms at the 8-position of quinoxaline-N-oxides and the 4-position... 

It has been reportedthat a C-C bond in an aromatic ring is cleaved with a stoichiometric amount of a transition metal complex. When quinoxaline was treated with the tungsten complex 90, dihydrogen was released and the tungsten was inserted into the C-C bond to furnish the di(isocyanide) tungsten complex 92 (Scheme 7.37) [52]. The authors proposed the mechanism to be through sequential C-H... 

The transition-metal-catalyzed formation of pyridine-type six-membered rings was reported by Mukhopadhyay and Kundu in 2000 [94], Bis-tosylamides of 1,2-diaminobenzene derivatives 193 undergo an unusual pyrazine formation by 6-exo-dig cyclization to give l-(quinoxalin-2-yl)ketones 194 in moderate yields by treatment with 10 mol % Cul in the presence of K2CO3 (Scheme 19.49). A reaction mechanism was not proposed. 

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