F. L. Scott

A free-radical analogue of the Chapman rearrangement; conversion of arylhydrazonates into N′N′-diarylhydrazides

Aryl N-(aryl)benzohydrazonates with substituents in the C-, N-, and O-aryl rings have been prepared from the corresponding hydrazonyl halides and substituted sodium phenoxide in benzene. The hydrazonates were shown to undergo rearrangement to N′N′-diarylbenzohydrazides on heating either alone or in the presence of a solvent. Radical initiators and oxidizing agents (e.g. manganese dioxide) catalyse the rearrangement. The intramolecularity of the rearrangement was demonstrated by the absence of cross-over products and the observation that the configuration of the migrating group is retained. Kinetic experiments have shown that electron-withdrawing substituents in each of the aromatic rings reduce the rate of rearrangement, the effect being greatest in the N-aryl ring where ρ=–2·1. A mechanism involving radical initiated hydrogen abstraction followed by a 1,4-group migration to give a more stable hydrazyl free radical is proposed. O-Alkyl hydrazonates failed to isomerise under these conditions as did the S-aryl thio-analogues and the N-aryl nitrogen analogues. The N′N′-diarylhydrazides were hydrolysed in concentrated acid to the corresponding NN-diarylhydrazines, providing a route to these materials which are otherwise difficult to obtain, particularly when the aryl groups are dissimilar.

The mechanism of bromination of heterocyclic hydrazones. syn–anti-Isomerisation of 5-(arymethylenehydrazino)-1- and -2-benzyltetrazoles

The rates of bromination (in 70% acetic acid at 25°) of a series of 5-(arylmethylenehydrazino)tetrazoles, benzylated in the heteroaromatic ring, are independent of the bromine concentration. The slow step is a syn–anti-isomerisation of the hydrazone. A mechanism involving rotation about the CN bond is favoured in the light of data on the effect of substituents on the rate of isomerisation. The products formed on bromination are hydrazonyl bromides and the replacement of the labile bromine by external and internal nucleophiles is reported.

The reactivity of phenyl isocyanate in aqueous solution

The rate of hydrolysis of 1-phenylcarbamoylimidazole (3) in water at 30° is markedly depressed by the addition of imidazole in the pH region 6–10. This is rationalised in terms of the formation of a small equilibrium concentration of phenyl isocyanate in solution. Phenyl isocyanate, besides reacting with imidazole to regenerate (3), can also be competitively trapped by H2O, HO–, and other nucleophiles; the relative rates of reaction of HO– and H2O with phenyl isocyanate are 7 × 105: 1. Phenyl isocyanate also shows a low sensitivity to the nature of primary amine nucleophiles with a Bronsted β value of 0.30. Secondary and α-effect amines lie on the same correlation line as primary amines, consistent with a transition state in which there is little bond formation. In all cases, urea formation shows simple second-order kinetics (first order each in amine and phenyl isocyanate) and no catalyiss by general acids or bases (or by products formed) is obseved; this contrasts with the results of previous studies which have been carried out in non-aqueous solution.

The kinetics of bromination of hydrazones

Kinetic measurements on the rate of bromination of a series of arylidene arylhydrazines have been made in 70% aqueous acetic acid at 20°, by following the diffusion current of bromine (initially ca. 10–5M) at a rotating platinum electrode. Substituents in the benzylidene ring have a smaller effect (ρ=–0·62) than those in the other ring (ρ=–2·2). This has been rationalised in terms of an intermediate stabilised by considerable charge delocalisation.

Ambident neighbouring groups. Part V. Mechanism of cyclization of 2-halogenoethylsulphonamides to aziridines

N-(2-Bromo- and 2-chloroethyl)sulphonamides are smoothly cyclized in basic solution to N-(arylsulphonyl)-aziridines. Kinetic studies [in water at 25°, µ= 1·0 (KCl)] imply that the sulphonamidate anion is the reactive species. Electron-withdrawing substituents in the aryl ring aid formation of the anion (ρ=+0·94, pKa0= 10·59), while retarding the rate of cyclization of the anion (ρ=–0·58, log k0=–1·30), with Br– as leaving group. The chlorides behave similarly, but are ca. 50-fold less reactive than the corresponding bromides. In ethanol containing an excess of ethoxide ion, particularly at higher substrate concentrations, other products including N-2-(ethoxyethyl)arylsulphonamides and dimeric materials such as piperazines are also formed. In aqueous solution the N-arylsulphonylaziridines undero slow ring cleavage [to give N-(2-hydroxyethyl)arylsulphonamides], but only in strongly acidic or basic media.

A change from rate-determining bromination to geometric isomerisation of pyridylhydrazones

The rate-determining step in the bromination of aldehydic 4-pyridylhydrazones has been identified as isomerisation of the predominant E-isomer (in which the large groups are trans) to the more reactive Z-isomer. Substituent and temperature effects favour a transition state with considerable rotational character. The Z-isomer was isolated in one case (pyridine-2-carbaldehyde 4-pyridylhydrazone) and shown to brominate rapidly as expected. The change in rate-determining step from bromination to isomerisation in closely related hydrazones is discussed. Representative novel 4-pyridylhydrozonyl bromides, which were formed on bromination, were isolated and characterised.

Kinetics of hydrolysis of NN′-diarylsulphamides

The kinetics of hydrolysis of five para-substituted NN′-diarylsulphamides, p-XC6H4NHSO2NHC6H4X-p(X = H, Me, MeO, Cl, or NO2) in 9 : 1 v/v water–acetone containing added hydrochloric acid have been measured at 50 and 75°. The Hammett ρ for the hydrolysis reaction is +1·03. The effect on rate of varying the pH from 3·05 to 1·05 has been studied for both NN′-diphenylsulphamide and N-phenylsulphamic acid. The rate of hydrolysis of phenylsulphamic acid is ca. 100 times faster than the rate of hydrolysis of diphenylsulphamide under identical conditions. Thus the intermediacy of phenylsulphamate in the hydrolysis of diphenylsulphamide does not complicate the kinetics and first-order kinetics have been observed. The major hydrolytic pathway involves bimolecular water attack on unprotonated diarylsulphamide.

Mechanism of 1,3-dipolar ion formation

Kinetic studies on hydrazidic halides in 70% dioxan at 25° show a base-catalysed process (in some cases even at pH 3·0); structural effects on this reaction, which was identified as nitrilimine formation, are reported.

Mechanism of cyclisation of N-(1,2,4-triazol-3-yl)hydrazonyl bromides to mixtures of isomeric triazolotriazoles

The reaction of N-(5-phenyl-1,2,4-triazol-3-yl)arenohydrazonyl bromides (2) in mixed aqueous, organic solvents may yield three products, dependent on the acidity of the medium. In strongly acidic solution, N-(5-phenyl-1,2,4-triazol-3-yl)arenohydrazides (4) are formed as major products; at pH 3–6, 3-aryl-5-phenyl-1H-s-triazolo-[3,4-c]-s-triazoles (5) were formed exclusively while in basic media the isomeric 3-aryl-6-phenyl-5H-s-triazolo-[4,3-b]-s-triazoles (6)[which are also formed by the action of lead tetra-acetate on the hydrazone (1)] predominate. Kinetic studies (in 85 : 15 dioxan–water at 25°) indicate that in neutral solution (5) is formed by intramolecular cyclisation of the neutral triazole on an azocarbonium ion centre (Hammett ρ for the variation of substituents in Ar is –1·32); when the triazole ring is protonated intermolecular attack by water [formation of (4) occurs]. The isomer (6) is formed by [1,5] dipolar cycloaddition involving the triazolyl anion as nucleophile. The isomer (6) was synthesised by an independent route. Under appropriate conditions intermolecular reactions of (2) with chloride ion, aniline, and morpholine yielded products in which bromide ion was replaced by the nucleophile.

Competition between carbon, nitrogen, and oxygen nucleophilic centres for sulphur(VI) attack in NN′-disubstituted sulphamides

Reactions of five kinds, namely (i) the hydrolysis of substituted sulphamides to sulphamic acids, (ii) the hydrolysis of the substituted sulphamic acids to arylamine sulphates, (iii) the trans-sulphonation reactions of the sulphamic acids, (iv) the trans-sulphamoylation reactions of the substituted sulphamides, and (v) the amine interchange processes of the NN′-disubstituted sulphamides all appear to show certain common critical characteristics. Electron donation to the reaction site, the hexavalent sulphur atom, hinders all these processes, whereas electron withdrawal facilitates them. Because of this consistency we regard the key process around which this present work was constructed (the trans-sulphamoylation reaction) as being best interpreted in terms of direct nucleophilic attack at sulphur (VI) in the protonated sulphamide, most probably by the para-aryl carbon site in the aromatic nucleophilic substrate.