Imine-enamine system

The reported 114) formation of acetic acid on ozonolysis of conkurchine was confirmed by later reinvestigation 79). Obviously, ozonolysis is unsuitable for structure determination of pyrrolines. This is in accord with Witkop s conclusion 133) that ozonolysis cannot be applied for structure determinations in imine-enamine systems. 

Tautomer Ratio in the imine-Enamine System Substitution at Nitrogen... 

S Tautomer Ratio in the Imine-Enamine System Substitution at Carbon 1105 5.5... 

The A-acylimine-enamide tautomerism of methyl 2-acetamidoacrylate has been studied641 by means of ab initio calculations. A 13C NMR investigation has been undertaken to study the tautomerism between the hydrazone imine and diazenylen-amine forms of 3-(arylhydrazono)methyl-2-oxo-l,2-dihydroquinoxalines,642 and the effects of temperature and side-chain on the imine-enamine tautomerism in quinoxalinone and pyridopyrazinone systems have been studied.643 A detailed... 

For several tautomeric systems ketones/enols, imines/enamin and others) a distinct reversal of the stability order is observed when going from the neutral compounds to the radical cations, the first use of which in a new preparative a-Umpolung reaction has been documented for keto/enol systems. The present review provides a critical evaluation of the chemistry of enol radical cations in solution with a special emphasis on the Umpolung reaction and the intermediates thereof. Other enol type of radical cations are discussed with respect to their potential to provide a-carbonyl radical and a-carbonyl cation intermediates. Hence, this article does not constitute a comprehensive summary on all enol type of radical cation reactions. All potentials in this review are referenced versus SCE, unless noted otherwise. Potentials measured against the ferrocene/ferrocenium couple were converted to SCE by adding 0.334 V. 

An important, but not widely recognized effect in electron transfer chemistry is the phenomenon that the thermochemical stability order of several tautomeric systems can be inverted upon one-electron oxidation. Hitherto, most of the data stem from gas-phase measurements (photoelectron and mass spectrometry data) or from calculations at several levels of theory (Table 2). While the explicit numbers still differ for each method they all agree on the inversion of the stability order for keto/enol, alkine/allene, imine/enamine, nitrile/isonitrile and aldimine/aminocarbene pairs. One-electron reduction, on the other hand, does not necessarily lead to a thermochemical stability inversion, as demonstrated in the case of acetaldehyde 7cthenol [41]. 

There are other types of proton shift tautomers, such as phenol/dienone, nitroso/oxime and imine/enamine, but these are less often encountered. There are also valence tautomers that exhibit fluxional structures, which undergo rapid sigmatropic rearrangements. Molecules that exhibit this type of isomerism are very interesting, but are not often encountered in undergraduate courses. An example of a valence tautomer is illustrated by the Cope system. 

The structure of the amine acrylates used in this study after curing is likely to be very complex and this makes it difficult, if not impossible, to evaluate clearly the nature and mechanism of the photooxidation and photoyellowing observed in these systems. The infra-red data clearly indicates the loss of amine alkyl functionality resulting in amine or amide formation. The carbonyl absorption reported previously (14,15) at 1680 cm indicates the latter. The band situated at 1612 cm l, also reported previously (14,15), may be due to imine, enamine, vinyl ether, amine or an, -unsaturated carbonyl species. If it was due to imine or enamine it would be easy to explain its loss due to hydrolysis during irradiation. 

The above-described epimerization system is entirely different from those involved in the biosynthesis of bacterial BLAs. Thus, the prokaryotic IPN epimerase, CefD, catalyzes the reversible epimerization of the L-a-aminoadipoyl side chain in IPN to the D-a-aminoadipoyl side chain in penicillin N in a PLP-dependent manner, probably through a mechanism involving imine/enamine type intermediates, as outlined in Figure 4.35 (B) [176]. The conversion of ACV analogs to cephalosporins using partially purified S. clavuligerus extracts provided evidence that the prokaryotic epimerase may tolerate modifications in the penam nucleus [177]. 

Martin and Beller [77] conducted the reaction either in supercritical NHj or close to the supercritical point of NHj (,T = 132.35 °C,p = 133.5 bar) by employing a dual catalytic system consisting of [lrCl(COD)]2/Rh(acac)(CO)2 in the ratio 5 1 (CO/H2 =1 3,190-240 bar 140 C). Noteworthy, the position of the critical point is dependent on the ratio NHj/olefin and water, the latter being a product of the imine/enamine formation in the reaction system [78]. 

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