Nitroarenes stability

A special example is the replacement of a nitro group on [(Tj6-nitrobenzene)FeCp]+.78,79 Reaction with O-, S-, N- and stabilized C-nucleophiles gives overall addition-elimination (equation 25). The nitroarene complexes are prepared by oxidation of the corresponding aniline complexes and are readily available.78,79... 

R. L. Bolduc, P. R. Thermal stability studies on a homologous series of nitroarenes. /. Phys. Chem. 1995, 99, 9593-9602. 

The nitroarenes, picric acid and TNT, have been used as explosives since the late 1800 s (Fig. 3). At temperatures over which the thermal stability... 

With our nitroarene tip in host 5, we tried to establish additional n-n interactions with electron-rich residues in aromatic amino acids. To this end, we titrated Ac-Phe-Ala-OMe first with reference compound 3 and then with host 5 and obtained l l-association constants of <40 m-1 and 350 m-1, respectively. Again, a ninefold increase from the steric repulsion by the reference compound and the re-stacking attraction by host 5 was found. Substantial upfield shifts in the aromatic regions of host and guest 1H NMR spectra confirmed the additional stabilizing interaction. 

The loss of OH from the protonated molecule appears to be a general feature of nitroarenes see Scheme 36. However, this process is particularly dominant for compounds with electron-donating substituents ortho or para to the nitro function. Thus, this fragmentation reaction can be rationalized in terms of protonation of the nitro group, the [MH —OH]+ fragment being stabilized by electron donation through the mesomeric effect of the substituent . 

Also the reactions of nitroaromatic compounds with various nucleophiles have been established to give anionic o -adducts, as evidenced from NMR spectra, providing a good diagnostic basis for elucidation of their structure, stability, and chemical transformations. Reactivity of nitroarenes, rate, and equilibrium constants for the formation of anionic o -adducts, as well as the features of the o -adducts, have been discussed in detail in a number of reviews, monographs [1, 2, 5,10,11, 15-18, 198], and original papers, for instance [199], dedicated to mechanistic aspects of VNS of hydrogen (for more details [213]). 

Equilibrium of the addition of nucleophiles to nitroarenes is a function of many factors, such as their nucleophilicity, electron deficiency of arenes, and their ability to stabilize adducts, as well as the reaction conditions. Thus, all these parameters are responsible for the feasibility of ONSH with nucleophiles sensitive to oxidation. Of substantial importance is temperature, since, due to the entropy factor, the equilibrium is shifted toward the adducts at a low temperature. For instance, addition of highly nucleophilic carbanion of 2-phenylpropionitrile to moderately active m-chloro nitrobenzene at —70°C in liquid ammonia or DMF/THF proceeds to completion, selectively in the para-position. Further oxidation of the formed adducts with... 

Direct incorporation of arylamino fragments into heteroaromatic compounds has some features. Unfortunately, the oxidative amination and alkylami-nation procedures cannot be applied for arylamination, because of a reduced A-nucleophilic character, a high sensitivity of aromatic amines towards oxidation, and a low stability of arylamino-arylamination reactions are still rare. A vast majority of the known examples are intramolecular reactions, which are performed in the presence of mild oxidants, such as sulfur, chloranU, or nitrobenzene (for review, see [34]). In some cases, air oxidation of the intermediate 7 -complexes has been observed. In the series of nitroarenes the nitro group often acts as the hydride ion acceptor. Similarly, in arylamination of azaaromatics the cyclic C=N bmid can intercept the hydride ion. 

Nucleophilic Substitutions. 7V,7V-Dimethyldithiocarbamoyl-acetonitrile (1) serves as an active methylene compound, because its carbanion is stabilized by sulfur and cyano groups. It can be alkylated stepwise in aqueous sodium hydroxide under phase transfer catalysis (eq 1). The anion undergoes nucleophilic aromatic substitution to give nitroarenes, with elimination of the dithiocarbamate group (eq 2). Nucleophilic addition of (1) to phenyl isothiocyanate also occurs readily. ... 

Certainly, the most popular application of [(NHC)Pt] complexes to date is in hydrosilylation reactions. These reactions are covered in Chapter 13, together with homogeneous hydrogenation reactions, their second major application in catalysis. As an interesting example, Lara and co-workers evidenced the great functional tolerance of Pt nanoparticles stabilized with NHC ligands (NHC-Pt NPs) [eqn (10.42)], which reacted under mild conditions for chemoselective hydrogenation of nitroarenes. " ... 

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