The cyclization of 2-substituted imidazolines in sulfuric acid

Cox, Robin A.; Moore, David; McDonald, Robert

doi:10.1139/v94-242

Cited by 9 publications

(3 citation statements)

References 20 publications

(35 reference statements)

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“…The technique used to analyse the rate constant data is the excess acidity method, 18 which has been used to determine the mechanisms of a variety of reactions in aqueous sulfuric and perchloric acids, among them the hydrations of fluoroalkylsubstituted vinyl ethers, 19 the cyclisations of substituted imidazolines, 20 alkene hydrations, 21 thioacetal hydrolyses, 22 methyl azophenyl ether cleavages 23 and aromatic hydrogen exchange processes. 24 Most recently it has been used to show that the mechanism of decomposition of aliphatic N-nitroamines to N 2 O and alcohols in aqueous sulfuric acid is an S N 2 displacement by water or hydrogensulfate ion at the alkyl group, 25 and that the parent molecule, nitramide (NH 2 NO 2 ), decomposes similarly in an acid-catalysed process in relatively strong acid media but that the decomposition mechanism involves neutral water molecules without acid catalysis in more dilute acid.…”

Section: Introductionmentioning

confidence: 99%

The mechanisms of the hydrolyses of N-nitrobenzenesulfonamides, N-nitrobenzamides and some other N-nitro amides in aqueous sulfuric acid 1

Cox¹

1997

J. Chem. Soc., Perkin Trans. 2

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The mechanisms of the hydrolysis reactions of some N-nitrobenzenesulfonamides (YC 6 H 4 SO 2 NHNO 2 ), N-nitrobenzamides (YC 6 H 4 CONHNO 2 ) and N-methyl-N-nitrobenzamides (YC 6 H 4 CON(CH 3 )NO 2 ) have been determined in aqueous sulfuric acid using the excess acidity method. Also studied were N-methyl-Nnitroacetamide and nitrourea, with N,N-dinitromethylamine for comparison. N-Nitrobenzenesulfonamides give either YC 6 H 4 SO 2 ϩ and NH 2 NO 2 (electron-donating Y) or YC 6 H 4 SO 2 NH 2 and NO 2 ϩ (electron-withdrawing Y) in A1 processes; the change in product is reflected in the different ϩ values found for the two modes of cleavage. N-Nitrobenzamides behave similarly in strong acid, with an A1 reaction following presumed O-protonation, but in more moderate acid they exhibit a neutral watercatalysed hydrolysis mechanism, and in dilute acid the parent N-nitrobenzamides actually show hydroxide catalysis. N-Methyl-N-nitroacetamide shows only the neutral water-catalysed process. Nitrourea has an A1 acid-catalysed hydrolysis reaction in acid, analogous to the known B1 mechanism in base (also visible in dilute sulfuric acid), but has no water reaction; the pH-rate profile for the hydrolysis of this substance is here extended into the non-ideal acid region. N,N-Dinitromethylamine loses NO 2 ϩ in an A1 process following initial nitro-group protonation, giving N-nitromethylamine which is identifiable by its known hydrolysis rate. Activation parameters, m*m ‡ slopes and ϩ values given by the excess acidity analysis are shown to be compatible with the postulated mechanisms.

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Section: Introductionmentioning

confidence: 99%

The mechanisms of the hydrolyses of N-nitrobenzenesulfonamides, N-nitrobenzamides and some other N-nitro amides in aqueous sulfuric acid 1

Cox¹

1997

J. Chem. Soc., Perkin Trans. 2

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“…The results are analyzed using the excess acidity method (l), which has been widely applied to organic reactions in sulfuric acid in recent years, most recently to aromatic hydrogen exchange (12), phenylazo ether cleavage (13), imidazole cyclizations (14), and the hydrations of fluorinated alkyl vinyl ethers (15).…”

Section: F 'mentioning

confidence: 99%

The decomposition of aliphatic N-nitro amines in aqueous sulfuric acid. Bisulfate as a nucleophile

Cox¹

1996

Can. J. Chem.

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Abstract:In aqueous sulfuric acid, aliphatic N-nitro amines decompose to N,O and alcohols. An excess acidity analysis of the observed rate constants for the reaction shows that free carbocations are not formed. The reaction is an acid-catalyzed SN2 displacement from the protonated aci-nitro tautomer, the nucleophile being a water molecule at acidities below 82-85% H,S04, and a bisulfate ion at higher acidities. Bisulfate is the poorer nucleophile by a factor of about 1000. Twelve compounds were studied, of which results obtained for nine at several different temperatures enabled calculation of activation parameters for both nucleophiles. The reaction appears to be mainly enthalpy controlled. The intercept standard-state rate constants are well correlated by the u* values for the alkyl groups; the slopes are negative, with a more negative value for the slower bisulfate reaction. Interestingly the m'm* slopes also correlate with u*, although the scatter is bad.Key words: N-nitro amines, excess acidity, bisulfate, nucleophiles, acid-catalyzed, kinetics.

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“…7, as transient intermediates in the disproportionation. The technique used to analyse the observed rate constants is the excess acidity method, 18 which has seen extensive use in recent years for the determination of reaction mechanisms in aqueous strong acid media; for instance, aromatic hydrogen exchange processes, 19 imidazoline cyclizations, 20 alkene hydrations, 21 ether hydrolyses, 13,22 thioacetal hydrolyses 23 and many other reactions have been studied. Most recently it has been used to determine the acid hydrolysis mechanisms of alkylnitramines 24 and nitramide, 25 several N-nitro amides 26 and acylimidazoles.…”

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confidence: 99%

Acid-catalysed aryl hydroxylation of phenylazopyridines: reaction intermediates, kinetics and mechanism 1

Cheon¹,

Cox²,

Keum³

et al. 1998

J. Chem. Soc., Perkin Trans. 2

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A kinetic and product analysis study of the reactions of the three isomeric phenylazopyridines (PAPys) in aqueous sulfuric acid media (30-97 wt% H 2 SO 4 ) is reported. The final products obtained from the reaction of 4-(phenylazo)pyridine (4-PAPy) are the hydroxylated product 4-(4-hydroxyphenylazo)pyridine, the reduction products 4-aminophenol and 4-aminopyridine, and a small amount of a dimerized product. 3-(Phenylazo)pyridine is unreactive, but 2-(phenylazo)pyridine gives the equivalent 2-(4-hydroxyphenylazo)pyridine, 4-aminophenol and 2-aminopyridine products. This product pattern, an oxidized azo-compound and two reduced amines, is similar to that found in the disproportionation of di-psubstituted hydrazinobenzenes observed in benzidine rearrangement studies. Consequently it has been proposed that the corresponding [NЈ-(4-hydroxyphenylhydrazino)]pyridines were formed as reaction intermediates in the present system; this is confirmed by showing that [NЈ-4-(4-hydroxyphenylhydrazino)pyridine synthesized independently gave the same products as 4-PAPy under the same conditions. The kinetic study shows that the 4-isomer reacted faster than the 2-isomer at all the acid concentrations investigated (the 3-isomer being inert). Rate maxima are observed, at ~72 wt% H 2 SO 4 for 4-PAPy and ~86 wt% H 2 SO 4 for 2-PAPy. To facilitate the kinetic analysis, values of pK BH 2 2؉ for the protonation of the substrates and the possible hydroxy products at the azo-group were determined, using the excess acidity method; the first protonation occurs on the pyridine nitrogen in the pH region. An excess acidity analysis of the observed pseudo-first-order rate constants as a function of acidity indicate an A2 mechanism, with the diprotonated substrate and either one HSO 4 ؊ ion or one H 2 O molecule in the activated complex. The proposed mechanism thus involves nucleophilic attack of HSO 4 ؊ or H 2 O at an aryl carbon of the diprotonated substrate in the slow step, resulting in an intermediate hydrazo species which gives the observed products in a subsequent fast step (cf. benzidine rearrangement).

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The cyclization of 2-substituted imidazolines in sulfuric acid

Cited by 9 publications

References 20 publications

The mechanisms of the hydrolyses of N-nitrobenzenesulfonamides, N-nitrobenzamides and some other N-nitro amides in aqueous sulfuric acid 1

The mechanisms of the hydrolyses of N-nitrobenzenesulfonamides, N-nitrobenzamides and some other N-nitro amides in aqueous sulfuric acid 1

The decomposition of aliphatic N-nitro amines in aqueous sulfuric acid. Bisulfate as a nucleophile

Acid-catalysed aryl hydroxylation of phenylazopyridines: reaction intermediates, kinetics and mechanism 1

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