Pyridines pyrrole formation from

Many pyrroles have been identified in roasted coffee. Regarding their formation in coffee pyridine, pyrrole and N-methyl-2-formylpyrrole are trigonellin derivatives 3 and 4 typical pyrroles from primary amino acids and 6 to 8 hydroxyproline derived Maillard products ( 26 ). ... 

Interestingly, this Heck-type palladium-catalyzed oxidative addition/insertion manifold can also be applied to the actual formation of the carbon-heteroatom bond. This was illustrated by Narasaka in the reaction of olefin-tethered oxime derivatives. This chemistry can be considered to arise from oxidative addition of the N—O bond to palladium (30) followed by the more classical olefin insertion and (3-hydride elimination, ultimately allowing the assembly of pyrroles (Scheme 6.58) [79]. The nature of the OR unit was found to be critical in pyrrole formation, with the pentafluorobenzoylimine leading to selective cyclization and rearrangement to the aromatic product. An analogous approach has also been applied to pyridines and imidazoles [80]. 

SCHEME 1.96 Formation of a complex of 2,6-di(pyrrol-2-yl)pyridine with DMSO from 2,6-diacetylpyridine dioxime and acetylene in the LiOH/DMSO system. 

Ciamician reported the formation of 3-halogenopyridines in low yield from the reaction of pyrryl potassium with chloroform, or bromo-form, in ether. Similar reactions of pyrrole with benzal chloride and methylene iodide gave 3-phenylpyridine and traces of pyridine, respectively. These reactions were later reinvestigated by Alexander... 

Formation of a central pyridine ring can also be effected by reaction of 2-(pyrrol-2-yl)imidazoles with ethyl bromoacetate <1999TL8157>. Kandeel et al. also synthesized angular systems in this manner from the thioxo-pyranopyrazole precursor <2002H(57)1121>. 

Carbon and nitrogen are the most common elements from the first row of the periodic table to form aromatic compounds, characterized by cyclic electron delocalization. The bonding of these elements in the conjugated systems shows a large variety. Carbon can be a divalent (carbene), sp carbon with one jT-electron, but also sp carbon can be part of hyperconjugate aromatic systems, provided that it is properly substituted. The pyrrole- and pyridine-type nitrogens also allow the formation of cyclic electron delocalization in a large variety of aromatic systems. 

This near-thermoneutrality gives confidence in both values of the oxime enthalpies of formation, the salicylaldoxime some 50 years old and the pyridine-2-carboxaldoxime within a year from when the chapter was submitted. Consider now the formal solid phase reaction 25 involving the some decades older pyrrole-2-carbaldoxime. ... 

We can now make sensible guesses as to the order of rate constant for water replacement from coordination complexes of the metals tabulated. (With the formation of fused rings these relationships may no longer apply. Consider, for example, the slow reactions of metal ions with porphyrine derivatives (20) or with tetrasulfonated phthalocyanine, where the rate determining step in the incorporation of metal ion is the dissociation of the pyrrole N-H bond (164).) The reason for many earlier (mostly qualitative) observations on the behavior of complex ions can now be understood. The relative reaction rates of cations with the anion of thenoyltrifluoroacetone (113) and metal-aqua water exchange data from NMR studies (69) are much as expected. The rapid exchange of CN " with Hg(CN)4 2 or Zn(CN)4-2 or the very slow Hg(CN)+, Hg+2 isotopic exchange can be understood, when the dissociative rate constants are estimated. Reactions of the type M+a + L b = ML+(a "b) can be justifiably assumed rapid in the proposed mechanisms for the redox reactions of iron(III) with iodide (47) or thiosulfate (93) ions or when copper(II) reacts with cyanide ions (9). Finally relations between kinetic and thermodynamic parameters are shown by a variety of complex ions since the dissociation rate constant dominates the thermodynamic stability constant of the complex (127). A recently observed linear relation between the rate constant for dissociation of nickel complexes with a variety of pyridine bases and the acidity constant of the base arises from the constancy of the formation rate constant for these complexes (87). 

This order of base activity corresponds almost exactly to that observed in the formation of pyrroles from ketoximes and acetylene, evidently for the same causes. The failure of trimethylbenzylammonium hydroxide to catalyze the reaction of vinylation is believed (59MI1 66MI1) to be caused by its lack of coordination. Along with inhibition of the reaction with water, pyridine, o-phenanthroline, and diketones, this indicates the reaction occurs by complex ionic mechanisms in which the participation of the complex ion as an intermediate is possible. 

Figure 2.8. N Is XANES spectra of (a) fulvic acid isolated from a glucose-glycine-8-Mn02 system and (b) the lyophilized solid phase. The peaks are assigned to pyridinic (398.6eV), pyridone (400.7 eV), amide (401.3 eV), and pyrrolic (402.0 eV) moieties. Reprinted from Jokic, A., Schulten, H.-R., Cutler, J. N., et al. (2004). A significant abiotic pathway for the formation of unknown nitrogen in nature. Geophys. Res. Lett. 31, L05502, with permission from the American Geophysical Union.

---