Mauro Isola

A Self‐Assembled Dinuclear Double‐Stranded Helicate Based on a Square‐Planar Coordination Geometry

The nickel(II) complex of a new bis-N,O-bidentate Schiff base, in which two salicylaldiminate units are connected by an α,α′-dioxo-ortho-xylidene spacer via the 3-position of salicyl moieties, was structurally characterised in the solid state and in solution as a dinuclear double helicate based on a square-planar coordination geometry. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

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.

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 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.