17O NMR spectra of some Meisenheimer adducts

Abstract

The 17O NMR spectra of 17O isotope‐enriched 1,3,5‐trinitrobenzene and its Meisenheimer adducts with hydride ion (1), acetone anion (2) and methoxide ion (3) were measured. The adducts 1–3 show 17O signals shifted upfield compared with those of 1,3,5‐trinitrobenzene. A higher upfield shift is exhibited by the oxygen atoms of the 4‐nitro group (∼90 ppm) and a lower shift by those of the 2‐ and 6‐nitro groups (∼40 ppm). The shift is practically independent of the structure or bulkiness of the nucleophile bound at the tetrahedral centre C1 in the adduct and also, within an experimental error, of the counter ion and solvent (acetonitrile or a mixture of DMSO‐d6 and methanol). The measured δ(17O) values indicate that in the Meisenheimer adducts the negative charge at the oxygen atoms is larger in 4‐NO2 than in 2‐ and 6‐NO2 groups. Copyright © 2000 John Wiley & Sons, Ltd.

Straightforward access to oxazaborines, diazaborinones and triazaborines by reactions of β-enaminoamides with 4-methylbenzenediazonium tetraphenylborate

“…1 (top); the values of chemical shifts of nitrogen atoms are given in Table 2. The positive values of d( 15 N) 12.2 and 86.9 ppm indicate the presence of an azo group [23,24], which is not present in the suggested structures D and E. Hence, the compound 2a has structure A.…”

“…The primary product of the azo coupling reaction can exist in several forms stabilised by the formation of intramolecular hydrogen bonds. These forms were observed spectroscopically in the case of azo coupling reaction products obtained from b-enaminones [23]. Theoretically, the coordination between boron atom and the atoms originally linked by means of the intramolecular hydrogen bond can lead to five structures with six-membered rings: A-E (Scheme 2).…”

Study of hydrazone-Hydrazoimine tautomerism in α-Azo-6-ketomethylphenanthridines

“…The azo-coupling reaction with ȕ-enaminones produces stable azo compounds which were the *Corresponding author. E-mail: h.log119@sci.ui.ac.ir first case of this kind in aliphatic series [10]. Also, azohydrazoimine prototropic equilibrium has been reported in the azo-coupling reaction with ȕ-methylaminonaphthalene [11].…”

“…The reactions of ȕ-diketones and activated ketones with diazonium salts produce pure mono hydrazone of diketones [8,9]. Replacement of C=O in aromatic conjugated ketones by C=NR group in the substrates results in a shift towards the azo forms as it is in the case aromatic series [10]. The azo-coupling reaction with ȕ-enaminones produces stable azo compounds which were the *Corresponding author.…”

NMR Spectroscopic Study of Tautomerism in Solution and in the Solid State

“…Both δ( 15 N) and 1 J N-H in the two tautomers are characteristically different and can be employed to study the equilibria [-N(1)=N-(δ N-1 = 119.4 ppm) -N(1)H-N= (δ N-1 = −42.6 ppm); δ N-2 = 219.8 ppm; 1 J N-1,NH = 96 Hz] [6] but model values are needed of the certain tautomers 66a,b (therefore reference compounds have to be included that exhibit no temperature dependence of the implemented NMR parameters [6,129], or the pure tautomers are computed theoretically) [130,131]. In addition, δ( 13 C) of a characteristically different aromatic ring carbon atom was used [74], the β-isotope effect [132] was employed, and the chemical exchange of the N-H protons was followed by nuclear Overhauser effect spectroscopy by NOESY experiments adopting various mixing times [133]. …”