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Kinetics and mechanism of base-catalysed degradations of substituted aryl-N-hydroxycarbamates, their N-methyl and N-phenyl analogues
Authors: Beier Petr | Mindl Jaromír | Štěrba Vojeslav | Hanusek Jiří
Year: 2004
Type of publication: článek v odborném periodiku
Name of source: Organic & Biomolecular Chemistry
Publisher name: Royal Society of Chemistry
Place: Cambridge
Page from-to: 562-569
Titles:
Language Name Abstract Keywords
cze Kinetika a mechanismus bazicky katalyzované degradace substituovaných aryl-N-hydroxykarbamátů, jejich N-methyl a N-fenyl analog Byla studována kinetika, mechanismus a produkty následných reakcí hydrolýzy substituovaných aryl-N-hydroxykarbamátů, jejich N-methyl a N-fenylanalog ve vodných roztocích pufrů a hydroxidu sodného při 20 a 60 °C. Hydrolýza substituovaných fenyl-N-hydroxykarbamátů probíhá mechanismem E1cB za vzniku fenolů resp. fenolátů a N-hydroxykarbamátu, který se dále v reakčním prostředí rozkládá na alkalický uhličitan, amoniak a dusík. Reakce N-methyl a N-fenylanalog ve stejném prostředí probíhá concerted mechanismem a konečnými produkty hydrolýzy těchto látek jsou fenoly resp. fenoláty, alkalický uhličitan, methylamin resp. anilin a azoxybenzen.. Pro posouzení mechanismů bylo užito Brønstedova kriteria. 4-Nitrophenyl-N-methylkarbamát podléhá za podmínek hydrolýzy v rozsahu při pH 7-14 převážně Smilesovu přesmyku na natrium-N-methyl-(4-nitrofenoxy)karbamát
eng Kinetics and mechanism of base-catalysed degradations of substituted aryl-N-hydroxycarbamates, their N-methyl and N-phenyl analogues The kinetics and mechanism of the degradation reactions of substituted phenyl N-hydroxycarbamates and their N-methyl and N-phenyl analogues have been studied at pseudo-first-order reaction conditions in aqueous buffers and sodium hydroxide solutions at 20 ˘C and 60 ˘C and at I = 1. The dependence of log kobs on pH for phenyl N-hydroxycarbamates at pH < 9 and pH > 13 is linear with the unit slope; at pH 10-12 log kobs is pH independent. The Bronsted coefficient beta(lg) is about 1 (pH 7-13) and ˇV1.53 (pH > 13) indicating that the degradation reaction of phenyl N-hydroxycarbamates follows an E1cB mechanism giving the corresponding phenol/phenolate and HOˇVN=C=O. The latter species undergoes further decomposition to give carbonate, nitrogen and ammonia as final products. In contrast to the phenyl N-hydroxycarbamates the N-methyl derivatives at pH 7-9 undergo degradation to the corresponding phenol/phenolate, carbonate and methylamine via a concerted mechanism (?Ňlg is about ˇV0.75). The only exception is 4-nitrophenyl N-hydroxy-N-methylcarbamate in the which the predominant break down pathway proceeds via the Smiles rearrangement to give sodium N-methyl-(4-nitrophenoxy)carbamate. At pH > 9 the reaction of N-hydroxy-N-methylcarbamates is kinetically complex: the dependence of absorbance on time is not exponential and it proceeds as a consecutive two-step reaction. N-Hydroxy-N-phenylcarbamate under the same conditions undergoes degradation to phenol, carbonate, aniline and azoxybenzene. Kinetics, mechanism, substituted aryl-N-hydroxycarbamates

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