Ennio Ciuffarin

Pummerer-like reaction of sulphinamides

Abstract Treatment of secondary sulphinamides ( 1 ), bearing one hydrogen atom at α-carbon to nitrogen, with electrophilic reagents leads in some cases to formation of N-sulphenylimines ( 2 ), via a Pummerer-like reaction.

Nucleophilic substitution at four-co-ordinate sulphur. Mobility of the leaving group

The leaving group effect has been measured for the reactions of benzenesulphonyl halides with aniline, n-butylamine, and hydroxide ion. The specific rate constants of displacement at 25° with I, Br, Cl, and F, respectively, as leaving groups, are the following: with aniline, 3·55 × 10–2, 31·2 × 10–2, 4·27 × 10–2, and 2·6 × 10–7; with n-butylamine, 21·9, 103, 42·6, and 1·01 × 10–2; and with hydroxide ion, 43·6, 28·9, 15·5, and 3·40. The almost identical leaving group mobility of I, Br, and Cl for each nucleophile and the enormous change in relative group mobility of fluorine on changing the pKa of the nucleophile, point to a mechanism involving an intermediate complex with bond forming or bond breaking as the rate-limiting step according to the substrate. The activation parameters are also reported and agree with this interpretation.

Neutral and alkaline hydrolysis of 2,4,6-trimethylbenzenesulphonyl chloride

The kinetics of the alkaline hydrolysis of 2,4,6-trimethylbenzenesulphonyl chloride have been re-studied. Contrary to a previous report, the reaction does depend on the concentration of nucleophile. This eliminates the only hitherto undisputed example of an SN1 mechanism in nucleophilic substitution at sulphur.

CATALYSIS IN APROTIC-SOLVENTS - EFFECT OF THE POLARITY OF THE CATALYST

Abstract The catalytic constants for the n -butylaminolysis of sultones in aprotic-apolar solvents are linearly correlated with a combination of the hydrogen-bonding parameter and the polarity parameter of the catalyst.

Chemistry of sulphenates in acidic media

Methyl toluene-p-sulphenate is hydrolysed rapidly in moist solvents (dioxan, benzene, chloroform, or nitrobenzene) yielding methyl toluene-p-sulphinate, di-p-tolyl disulphide, and methanol. The reaction is acid catalysed. At higher concentration of water (>1%) the products are: p-tolyl toluene-p-thiolsulphonate, di-p-tolyl disulphide, and methanol. A mechanism is suggested where thiolsulphinate, which is usually suggested as the intermediate in the hydrolysis of sulphenyl derivatives, combines with unchanged sulphenate ester to yield an intermediate sulphonium salt. The sulphonium slat then rapidly reacts with either methanol or water to yield the products. Some implications for the chemistry of sulphenic acids are also discussed.

Nucleophilic substitution at sulphur. Reaction of p-nitrophenyl triphenylmethanesulphenate with n-butylamine and benzamidine

The reaction of n-butylamine with p-nitrophenyl triphenylmethanesulphenate is of the second order in nucleophile, but the reaction of benzamidine with the same substrate in the same solvent is of the first order in nucleophile. The same pattern is found when n-butylamine and benzamidine react with carboxylic esters and aromatic derivatives. It is suggested that this pattern has a common origin and that the first-order behaviour of benzamidine can be explained without resorting to bifunctional catalysis.

Reactions of thiolsulphonates with amines

The equilibrium constants for the reaction of morpholine with methyl or t-butyl toluene-p-thiolsulphonates have been measured in acetonitrile–water (7 : 1,v/v) at 25 °C. Contrary to previous data they are 2·5 × 10–3 and 3·4 for the methyl and t-butyl substrates respectively. In the same solvent medium have been measured the reaction rates and the activation parameters of the nucleophilic displacement reaction of morpholine with the same substrates. The overall mechanism is discussed.

Nucleophilic substitution at two-co-ordinate sulphur. Brønsted relationships with nitrogen nucleophiles

The rate of reaction of p-nitrophenyl triphenylmethanesulphenate with a variety of amines has been measured in 45% dioxan–water. The kinetic data, plotted in a Bronsted fashion against the pKa of the conjugate acid of the nucleophile, yield linear relationships characterized by β= 1·5 for anilines, 0·84 for pyridines, 0·75 for secondary heterocyclic amines, and 0·58 for primary aliphatic amines. The large effect of the basicity of the nucleophile on the rate of reaction is interpreted in terms of large bond formation in the transition state and the much larger slope found for anilines is discussed.

Nucleophilic substitution at two-co-ordinate sulphur. Effect of the leaving group

The effect of the basicity of the leaving group has been measured in the reaction of pyridine and butylamine with para-substituted phenyl triphenylmethanesulphenates. The kinetic data plotted in a Bronsted fashion against the pKa of the conjugate acid of the leaving group indicate a large sensitivity to the basicity of the leaving group; β–1·06 and –1·09 for pyridine and butylamine, respectively. The element effect has been measured on a number of triphenylmethanesulphenyl derivatives with n-butylamine and hydroxide. The specific rate constants of displacement with I, SCN, Br, and Cl, respectively, as leaving groups, are the following: (a) with n-butylamine, 0·44, 0·41, 85, 147; (b) with OH–, 3·6, 1·03, 21, 28. These data indicate small sensitivity to the nature of the element displaced. The results are discussed in terms of an intermediate complex mechanism with bond formation as the rate-limiting step.

Nucleophilic substitution at two-co-ordinate sulphur. Effect of the basicity of the nucleophile and structural effects on the sulphur substrate

The rate of reaction of p-nitrobenzenesulphenyl chloride with substituted anilines has been measured in benzene at 20 °C with a stopped-flow spectrophotometer. The kinetic data, plotted in a Bronsted fashion against the pKa of the conjugated acid of the nucleophile, yield good linear relationships characterized by β= 1·250 in absence of additives and by β= 1·256 in the presence of tetra-n-butylammonium perchlorate. The large effect of the basicity of the nucleophile is interpreted in terms of almost complete bond forming in the transition state and d-orbital participation is suggested. In contrast, the effect of para-substitution on the sulphur substrate is almost nil.