Metalation quinoxalines

Quinoxaline 1-oxide gave complexes with the chlorides of Cr, Mn, Fe, Co, Ni, Cu, and Zn also with the perchlorates of the same metals. Quinoxaline 1,4-dioxide gave complexes with the chlorides of Cr, Mn, Fe, Co,... 

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. ... 

Note Only a few simple examples of the metal complexes formed by quinoxaline oxides and dioxides are given here. 

Note Many of the (substituted hydrazino)quinoxalines covered in this section have been converted into metal complexes, randomly exemplified here. 2-(Pyridin-2-ylhydrazonomethyl)quinoxaline (246) Fe(ll), Ni(ll), and Cu(ll)... 

The diazines pyridazine, pyrimidine, pyrazine, and their benzo derivatives cinnoline, phthalazine, quinazoline, quinoxaline, and phenazine once again played a central role in many investigations. Progress was made on the syntheses and reactions of these heterocycles, and their use as intermediates toward broader goals. Some studies relied on solid-phase, microwave irradiation, or metal-assisted synthetic approaches, while others focused attention more on the X-ray, computational, spectroscopic, and natural product and other biological aspects of these heterocycles. Reports with a common flavor have been grouped together whenever possible. 

Hemoglobin, myoglobin, cytochrome C, and cytochrome oxidase are all electrochemically reduced in the presence of quinoxaline. Quinoxaline is preferentially reduced to the anion radical that can release its electron. Metalloproteins can serve as the electron acceptors in which the metal ion is reduced [424]. 

Bis(2-pyridyl)pyrazine (2,3-dpp = (88)), 2,3-bis(2-pyridyl)quinoxaline and 2,3-bis(2-pyridyl) benzoquinoxaline (91) find numerous applieations as bridging ligands in homo- and heterome-tallic complexes. At this point, we consider some dinuelear species complexes eontaining >3 metal centers are covered in Section 5.5.3.1.5. By using the complexes in the series [(bpy)2M-(/u-L)M(bpy)2]" " (M-M = Ru-Ru, Os-Os, Ru-Os L = (88), 2,3-bis(2-pyridyl)quinoxaline or (91))... 

Very little attention has been given to the metalation of the quinoxaline system, although lithiation of the 2-methoxy and 2-methylthio derivatives has recently been achieved (Scheme 118) (91JHC765). Reaction with N-methoxy-/V-methylbenzamide occurred at -78°C to give the analogous phenyl ketones in moderate yield. Highly colored insoluble solids were also produced, and these were the sole products when metalation of 2-chloroquinoxaline was attempted. 

Directed ort o-metalations are also applied to quinoxalines possessing 2-chloro, 2-methoxy, and pivaloylamino substituents <1993JHC1491>, but successful syntheses via the lithio intermediates are far fewer compared to pyrazines. In fact, no example of metalation on aromatic carbons can be found in the literature since 1994. However, lithiation on benzene carbons of 6-chloro-2,3-dimethoxyquinoxaline was reported <1999T5389>. 

Due to the significant biological interest of quinoxalines, several methodologies for derivatization of these heterocyclic skeleta have been reported. The majority of the reactions involving quinoxalines are analogous to those of the other heterocycles discussed previously including SnAt substitution, lithiation, and transition metal catalyzed cross coupling reactions. 

Polymers ofthe loop and chain type (A) can be obtained after crystallization by slow diffusion of the metallic salts and the quinoxaline or pyrazine derivatives in two different phases in the presence of organic spacers such as pyrazine (prz) or 4,4 -bipyridine (bpy) [see Eq. (8)]. 

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... 

Quinoxaline-2,3-dithione, as reported earlier,1 is useful for its coordinating properties with transition metals. The metal complexes of the dithione with Cu, Ni, Zn, Pd, and Pt have been prepared,171 and the spectral properties of the Ni and Pd complexes examined.171,172 UV data indicate that quinoxaline-2,3-dithione (153) is present as such, rather than as 2,3-dimercaptoquinoxaline the highly colored nature of its complexes is attributed to charge transfer.171... 

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