Jaromír Kaválek

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.

The base-catalysed cyclisation of phenyl N-(2-hydroxybenzyl)-N-methylcarbamates is concerted

The kinetics of the cyclisation in aqueous solution of phenyl-(2-hydroxybenzyl)-N-methylcarbamates to 3-methyl- 3,4-dihydrobenzo[e][1,3]oxazin-2-ones and phenolate ions fit the rate law: kobs = kc/(1 + [H3O+]/Ka) The values of kc and pKa fit Brønsted equations against the pKas of the corresponding free phenols but the system does not conform to the reactivity-selectivity hypothesis. The values of the Brønsted parameters beta Y and beta X vary as a function of Y and X according to the equations: beta X = -0.179pKaHY + 0.87 beta Y = -0.179pKaHX + 2.30 The magnitude and sign of the Cordes-Thornton cross-interaction coefficient pXY (-0.179) rule out a stepwise mechanism involving a tetrahedral intermediate and is consistent with a concerted displacement mechanism. A similar concerted mechanism is proposed for the base-catalysed cyclisation of phenyl-N-(2-hydroxyphenyl)-N-methylcarbamate esters to benzoxazol-2-ones.

Kinetics and mechanism of the reaction of substituted O‐benzoylbenzamidoximes with sodium methoxide in methanol

The kinetics of reaction of substituted O-benzoylbenzamidoximes with sodium methoxide in methanol were studied at 25 °C. The only reaction products are substituted benzamidoximes and methyl benzoates. The slope of the dependence of rate constant on sodium methoxide concentration gradually increases, but in the presence of C18 crown ether the dependence becomes linear and the rate constant is lower than in the absence of the crown ether, which means that the reaction is catalysed by sodium cation. The rate constants of reactions with the ion pair and with methoxide ion were determined with the presumption that the rate-limited step of the catalysed reaction is the reaction of substituted O-benzoylamidoximes with the ion pair of sodium methoxide. The rate constants of the reaction with the ion pair are about 20 times higher than those of the non-catalysed process. The slopes of the dependence of log k of the non-catalysed and catalysed reactions on the pKa of substituted benzamidoximes are 1.05 and 0.94, respectively. These high values indicate the rate-limiting step involving the splitting off of substituted benzamidoxime from the tetrahedral intermediate. On the basis of the relatively high ρ constant of methanolysis at the benzoyl group substituted derivatives (2.17 and 1.98 for the non-catalysed and catalysed reactions, respectively), it can be presumed that the transition state structure will be close to the tetrahedral intermediate. Copyright

Synthesis of Substituted Phenyl N-(2-hydroxybenzyl)-N-Methylcarbamates

Thirteen previously unreported substituted phenyl N-(2-hydroxybenzyl)-N-methylcarbamates were prepared by the reaction of substituted 2-hydroxybenzyl-N-methylamines with phenyl chlorocarbonates. They were identified by their 1H- and 13C-NMR spectra.