Vladimír Macháček

Tautomerism and hydrogen bonding in reaction products from 4-aminopent-3-en-2-one and diazonium ions

The azo-coupling products have been prepared from 4-aminopent-3-en-2-one and 4-chloro- or 4-nitrobenzenediazonium ion. Structure of the products obtained was studied by means of multinuclear NMR spectroscopy in CDCl 3 solutions and X-ray diffraction in solid state. Both products are predominantly present as azo forms in both the solution and the solid state, the azo form content being higher in solid state. The substituent effect on azo-hydrazone tautomeric equilibrium of the substances is more significant in solution than in solid state.

Structure and tautomerism of azo coupling products from N-alkylenaminones derived from acetylacetone and benzoylacetone in solid phase and in solution

A series of azo coupling products have been prepared by reaction of substituted benzenediazonium tetrafluoroborates with N-alkyl 4-aminopent-3-en-2-ones or 3-amino-1-phenylbut-2-en-1-ones. The structure and tautomerism of the reaction products were studied by means of single-crystal X-ray study and by NMR spectroscopy in deuteriochloroform solution. The azo coupling products obtained from 4-methylaminopent-3-en-2-ones (3a–i) exist in CDCl3 solution as E/Z isomer mixtures with the Z isomer strongly predominating. The major isomer is a mixture of enamino–azo and imino–hydrazo tautomers with the former predominating. The proportion of the azo form depends on substitution of the benzene ring of the diazonium salt and decreases in the order of MeO > Me > Br > NO2. The position of tautomeric equilibrium is practically unaffected by switching from 4-methylaminopent-3-en-2-ones to 3-methylamino-1-phenylbut-2-en-1-ones. In the solid phase, the azo form always predominates; substitution of diazonium salt and at N3 nitrogen does not significantly affect the position of the tautomeric equilibrium. The azo coupling products always exist in the form of a single Z isomer. All determined structures, in the solid state, consist of a mixture of the two tautomeric forms, amino–diazenyl and imino–hydrazone, in ratios ranging from 82/18 to 91/9%. The weighed superimposition of both the hydrogen-bonded N1N2–C1C2–N3H/ HN1–N2C1–C2N3 heterodienic fragments, however, do not allow to clarify the effects of the para-substituents at the N1-phenyl ring both on the N1⋯N3 hydrogen-bond distances and on the bond lengths in the heterodienic systems within the series 3a–c,f and 4a–c,e,f.

An NMR and X-ray study of the structure of the azo coupling product of 4-dimethylaminopent-3-en-2-one and benzenediazonium-tetrafluoroborate

4-Dimethylaminopent-3-en-2-one reacts with two molecules of benzenediazonium-tetrafluoroborate to give compound 1. The structure of this compound was determined by means of X-ray analysis of its crystal and 1H, 13C and 15N NMR spectra of its solution in CDCl3. The molecule of this compound contains one azo group and one hydrazone group. The substance exists, both in crystal form and in solutions of concentrations above 0.1 mol l(-1), in the form of a dimer, in which the pair of molecules are bound by two hydrogen bonds N-H...N. On diluting the solution, the dimers decompose, the two forms being in an equilibrium that is rapid on the NMR time scale.

Solution and solid state structure and tautomerism of azo coupled enaminone derivatives of benzoylacetone

The reaction of 4-substituted benzenediazonium tetrafluoroborates with 3-amino-1-phenylbut-2-en-1-one, 4-amino-4-phenylbut-3-en-2-one and their N-aryl derivatives 1a-1g has been used to prepare the respective azo coupling products i.e. compounds 2-5 from enaminone 1a, compounds 6-9 from enaminone 1c, compound 10 from enaminone 1d, compound 11 from enaminone 1e, compounds 12, 13 from enaminone 1f, compounds 14, 15 from enaminone 1b and compound 16 from enaminone 1g. Tautomerism of the azo coupling products prepared has been investigated in CDCl3 solutions by means of 1H, 13C and 15N NMR spectra. Crystal structures of selected products have also been investigated by means of X-ray diffraction.

13 C and 15N nuclear magnetic resonance spectra of Meisenheimer complexes of 1,3,5-trinitrobenzene

13 C and 15N n.m.r. spectra (at the natural abundance level of 15N) have been measured and interpreted for the Meisenheimer complexes (I)–(XII) of 1,3,5-trinitrobenzene (TNB) with anions of cyclopentanone, cyclohexanone, cycloheptanone, acetone, dimethyl malonate, methyl cyanoacetate, methyl acetoacetate, pentane-2,4-dione, 3-methylpentane-2,4-dione, phenol, and methanol. 13C N.m.r. spectra of the corresponding carbon-acids have been measured in [2H6]DMSO. The complexes derived from ketones and esters are present in the keto-form, that of (IX) exists in the enol form. Nitrogen atoms of nitro-groups at the 2- and 6-positions of complexes derived from carbon-acids of the type CH2XY are anisochronous as also are C(2) and C(6), C(3) and C(5), and H(3) and H(5).

Structure and base catalysed cyclization of methyl (2,6-disubstituted-4-nitrophenylsulphanyl)ethanoates

Abstract The conformation of side chain –SCH2COOCH3 in title compounds in crystal agrees with the reactivity of these compounds in base catalysed ring closure in solution. In the 2,4-dinitro derivative, the side chain is oriented towards the unsubstituted ortho-position of benzene ring, in the case of the 2-methoxycarbonyl derivative towards this substituent.

Preparation of Substituted Methyl o-Nitrophenyl Sulfides

The nucleophilic substitution of substituted o-nitrochlorobenzenes with substituted methanethiolates, catalysed with triethylamine or pyridine, has been used to prepare a series of appropriately substituted methyl-o-nitrophenylsulfides. The prepared compounds were identified by their 1H- and 13C-NMR spectra. The base catalysed ring closure of methyl 2-(methoxycarbonylmethylsulfanyl)-3,5-dinitrobenzenecarboxylate only results in an attack of carbanion on the ester group, not on a nitro group as with the other compounds prepared. The cyclisation product is methyl 3-hydroxy-5,7-dinitro-benzo[b]thiophene-2-carboxylate (11).

3-Butyl-1-(5-nitrobenzo(c)(1,2)thiazol-3-yl)-3-phenyltriazene

The molecule of the title compound, C(17)H(17)N(5)O(2)S, consists of three pi systems, viz. two aromatic rings and the triazene moiety, which are mutually deconjugated although coplanar. The n-butyl chain is roughly perpendicular to the molecular plane, with the terminal methylene and methyl groups disordered between two equally populated positions. The molecules in the crystal associate in an antiparallel fashion, forming dimers across the centre of symmetry, the principal intradimer interaction being stacking of the pi-electron portions of the molecules.

17O NMR spectra of some Meisenheimer adducts

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

Aromatic nucleophilic substitution of hydrogen: mechanism of reaction of 6-nitroquinoline with cyanide ions, with and without participation of methyl cyanoacetate

The mechanism of nucleophilic substitution of hydrogen (SNArH) in 6-nitroquinoline 1 by action of cyanide ion in the presence or in the absence of methyl cyanoacetate in dimethyl sulfoxide has been studied by means of 1H NMR. The main reaction products and some side products have been identified and their time–concentration dependences have been determined. An experiment with 13C enriched potassium cyanide proved that the cyano group replaces hydrogen at the 5-position of the starting compound 1. The two main products obtained from the reaction mixture of 6-nitroquinoline 1 and potassium cyanide are 6-hydroxyquinoline-5-carbonitrile 11 and 6,6′-azoquinoline-5,5′-dicarbonitrile 13. On the basis of the data obtained, a mechanism has been suggested for aromatic nucleophilic substitution of hydrogen by cyanide ion in 6-nitroquinoline 1 involving a Meisenheimer adduct of 6-nitroquinoline 1 with cyanide ion and 6-nitrosoquinoline-5-carbonitrile 14 as unstable intermediates.