Pyrazine-carboxylate ligand

Efficient Pd-catalyzed carbonylation of 2-chloropyrazine afforded pyrazine carboxylic acid esters (177) in 82% yield in the presence of dppf and dppb ligands [85]. 

Initial investigations showed that the direct hydrogenation of pyrazine carboxylic acid derivatives 14—16 was possible [15] (Fig. 13). This is one of the few examples known today for a homogeneous heteroarene hydrogenation [16]. With [Rh(NBD)Cl]2 and a variety of standard ligands, for example Chiraphos, Norphos, Duphos, Deguphos, PPm, BDPP, BINAP, BPPFOAc no reaction was observed. Similarly no reaction could be observed with Ir catalysts. The positive results are given in Tab. 6. 

Scheme 2.6 Examples of transition states (TSs) involved in a proton-transfer step from a ligated HjOj to an oxo ligand on the way to generate the hydroxyl radical (a) six-membered TS (water-assisted H+-transfer) at a PCA-V catalyst (PCA=pyrazine carboxylate) [38] (b) five- or four-membered TSs (PCA-assisted H+-transfer, robot s arm mechanism) at a PCA-V catalyst [41-43] and (c) six-membered oxo-divanadium TS at a divanadate-type model [40]. (See insert for color representation of the figure.)...

Carboxamides containing alkylamine arms like (34), synthesized from dialkylmalonate and ethylene-diamine, act as chelating dianionic N4-donors,44,45 whereas the geometric isomer (35) was prepared from 1,3-diaminopropane and chloroacetyl chloride.46 The hexadentate bis(pyrazine)-carboxamides (36) and (37) can be obtained by the addition of 1,3-propanediamine to the appropriate carboxylic acid.47 Compound (36), in the dianionic form, can be used to obtain homo- and hetero-metal bridged complexes, whereas (37) has been shown to yield tetranuclear copper(II) complexes involving opened-up dianionic ligand molecules.47... 

Pyrazine forms pink insoluble [CoX2(pyz)2] and [CoX2(pyz)] complexes (X = Cl, Br, I), which are presumably polymeric and octahedral,but monomeric tetrahedral units are possible with increased methyl substitution. No direct confirmation of bridging pyrazine has been found in these Co" systems. Some orange-red 2-substituted carboxylate and carboxamide pyrazine complexes have been prepared " and a distorted octahedral geometry has been confirmed by O Connor and Sinn, with chelated ligands (Table 22). No well-characterized pyridazine or pyrimidine complexes have been reported. 

The absence of mesomorphism in these compounds was explained on the basis of space-filling requirements. Thus, the intercalation of pyrazine between the binuclear units creates free volume which needs to be filled to obtain a stable, condensed phase when the carboxylates bear only one chain, the interdimeric space is likely filled by the aliphatic chains belonging to a different polymeric chain, giving rise to a crossed structure which prevents the formation of a columnar mesophase. However, as will be seen later, liquid-crystalline behavior was induced in the case of mixed-valence diruthenium(II,III) carboxylate complexes with bulky equatorial Kgands bearing two and three aliphatic chains as with such ligands, it was possible to fill the interdimeric space and thus to induce a thermotropic columnar mesophase. Very recently, the synthesis, characterization, and mesomorphic properties of pyrazine-polymerized divalent rhodium benzoates have also been reported (99). " Most of these compounds exhibit columnar (Colh, Coir, CoIn) and cubic mesophases with melting transition temperatures close to, or even below, room temperature. 

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