Heterocyclic detection

Some of the aminoacids and, to a greater extent, some of the nitrogen heterocycles detected as free molecules in the chondritic matter might be fragments of the polymer. This fragmentation may have been caused by hydrolysis during extraction procedures 35). 

IR spectroscopy has been particularly helpful in detecting the presence of keto tautomers of the hydroxy heterocycles discussed in Section 3.01.6. Some typical frequencies for such compounds are indicated in Figure 4. Here again the doublets observed for some of the carbonyl stretching frequencies have been ascribed to Fermi resonance. 

In the genuine low-temperature chemical conversion, which implies the incoherent tunneling regime, the time dependence of the reactant and product concentrations is detected in one way or another. From these kinetic data the rate constant is inferred. An example of such a case is the important in biology tautomerization of free-base porphyrines (H2P) and phtalocyanins (H2PC), involving transfer of two hydrogen atoms between equivalent positions in the square formed by four N atoms inside a planar 16-member heterocycle (fig. 42). 

Once formed, 7 and 8 undergo a Michael reaction that gives rise to ketoenamine 9. Ring closure, to form 10, and loss of water then afforded 1,4-dihydropyridine 11. The presence of 9 and 10 could not be detected thus ring closure and dehydration were deduced to proceed faster than the Michael addition. This has the result of making the Michael addition the rate-determining step in this sequence. Conversely, if the reaction is run in the presence of a small amount of diethylamine, compounds related to 10 could be isolated. Diol 20 has been isolated in an unique case (R = CFb). Attempts to dehydrate this compound under a variety of conditions were unsuccessful. Stereoelectronic effects related to the dehydration may be the cause. In related heterocyclic ring formations, it has been determined that dehydration (20 —> 10) is about 10 times slower than diol formation (19 —> 20). Therefore, one would expect 20 to... 

Isoindole (1) and its analogs, isobenzofuran (2) and isothianaphthene (3), have attracted considerable theoretical and synthetic interest. -Of the jjarent heterocycles, only isothianaphthene (3) has proved sufficiently stable for isolation, - although both (1) and (2) have been detected as transient species, and it seems likely that a more complete characterization will be forthcoming with improved experimental techniques. This review is concerned with the chemistry of... 

Derivatization of heterocycles for their detection by HPLC, GLC and GLC-MS 97YZ647. 

In 1956 it was found that when pyridine is refluxed with a modified Raney-nickel catalyst, 2,2 -bipyridine (1) is formed in satisfactory yield. The isomeric bipyridines could not be detected, and the product was readily purified. Similar heterocyclic biaryls have been formed in the same way from substituted pyridines and from some related compounds, the yield being dependent on the nature of the compound. The reaction has become the method of choice for the preparation of 2,2 -bipyridine, and it is now used on an industrial scale. Bipyridyls are of particular importance as chelating agents. 

There are also some couplings in which hydrazones are formed but for which the azo tautomer is not detectable and probably does not exist. This is the case in some coupling reactions involving methyl groups of aromatic heterocycles (see, for example, 12.48 and 12.49 in Sec. 12.5). Replacement of a methyl proton by an arylazo group (Scheme 12-3) would result in an azo compound containing an sp3-hybridized — CH2 — group (12.1). The latter is less stable than the tautomeric hydrazone (12.2), in which there is a n-n orbital overlap from the heteroaromatic to the aromatic system. 

For most simple phenols this equilibrium lies well to the side of the phenol, since only on that side is there aromaticity. For phenol itself, there is no evidence for the existence of the keto form. However, the keto form becomes important and may predominate (1) where certain groups, such as a second OH group or an N=0 group, are present (2) in systems of fused aromatic rings and (3) in heterocyclic systems. In many heterocyclic compounds in the liquid phase or in solution, the keto form is more stable, although in the vapor phase the positions of many of these equilibria are reversed. For example, in the equilibrium between 4-pyridone (118) and 4-hydroxypyridine (119), 118 is the only form detectable in ethanolic solution, while 119 predominates in the vapor phase. " In other heterocycles, the hydroxy-form predominates. 2-Hydroxypyridone (120) and pyridone-2-thiol (122) are in equilibrium with their tautomers, 121 and 123, respectively. In both cases, the most stable form is the hydroxy tautomer, 120 and 122. ... 

Coupling reactions with diazonium salts to yield intensely colored azo derivatives have often been used for the detection of phenols, primary aromatic amines and electron-rich heterocyclics. 

The ZwKKER reaction involving Co salts is frequently used for the detection of barbituric acid derivatives [31-35], but some purine, pyridine and piperidine derivatives and heterocyclic sulfonamides also yield colored derivatives. The Zwkker reaction is particularly sensitive when it is possible to form a tetrahedral complex [Co(Barb)2 Xj] (X = donor ligand, e.g. amine) [4]. 

The availability of aniline has made possible a direct study of the reactions of aniline with humic and fulvic acids (Thom et al. 1996), and the detection of resonances attributed to anilinoquinone, imines, and N-heterocyclic compounds are fully consistent with reactions involving quinone and ketone groups. 

OH elimination from ortho substituted aldoximes 179 (X = CH2, NH, O) may be at least partially the result of a hydrogen migration/cyclization/elimination process, whereby the heterocycles 182 are formed72 (46). A metastable peak shape analysis, the investigation of 2H-labelled derivatives and the study of positional isomers indicate that in addition to 182 the protonated isocyanide 183 is formed via a mechanism which is not fully understood. However, it is known that the generation of 183 occurs without any detectable interaction with the XH ortho substituent. 

---