Neil S. Isaacs

Cycloadditions of olefins and acetylenes to dimethylketen: evidence for the (π2s+π2a) concerted mechanism

The products and relative rates of cycloaddition of a series of olefins and acetylenes to dimethylketen have been examined. Steric and electronic factors are in accord with a concerted reaction in which the keten adds antarafacially, with differing degrees of bond formation, at the reaction termini in the transition state.

Reaction studies at high pressure. Part I. Activation volumes of some[2 + 2] and dipolar cycloadditions

Activation volumes have been determined over the pressure range 0–2 kbar for several [2 + 2] cycloadditions both with concerted and non-concerted mechanisms and also for a 1,3-dipolar cycloaddition. Values of ΔV are found to be large and negative there being little distinction between mechanistic types for the [2 + 2] reactions. The 1,3-dipolar cycloaddition and the dimerisation of dimethylketen show values comparable with those of Diels–Alder reactions. The implications of these findings are discussed.

Solvent effects upon the rates of acid-induced decomposition of 3-methyl-1-p-tolyltriazene and diphenyldiazomethane

Rates of reaction of 3-methyl-1-p-tolyltriazene with benzoic acid to form methyl benzoate, p-toluidine, and nitrogen have been measured in a wide variety of aprotic solvents. Similar studies are also reported on the reactions of diphenyldiazomethane with benzoic acid and with p-nitrophenol. Large solvent effects upon all reactions are observed; those of the triazene correlate well with the donor character of the solvent as expressed by several empirical parameters. The reactions of diphenyldiazomethane appear to be better interpreted by a multiple correlation with both donor and polar (electrostatic) contributions to the solvent effect.

Kinetics and mechanism of the decomposition of 3-alkyl-1-aryltriazenes by carboxylic acids

A kinetic study of reactions between a series of 3-alkyl-1-aryltriazenes with benzoic acids to yield alkyl benzoates supports a mechanism in which proton transfer and departure of an alkyl cation are rate-determining and synchronous. The resulting alkyl cation–carboxylate ion-pair may suffer nucleophilic attack or Wagner–Meerwein rearrangement before collapsing to product.

The thermal decomposition of cis- and trans-2-methoxy-4-methyl-3,4-dihydro-2H-pyran

The thermal decomposition of both cis-and trans-2-methoxy-4-methyl-3,4-dihydro-2H-pyran has been investigated in the gas phase from 287 to 345°. In this temperature range both isomers decomposed to give crotonaldehyde and methyl vinyl ether by a homogeneous process that obeyed first-order kinetics. The data for the trans-isomer yileded the Arrhenius equation (i). Similarly the cis-isomer gave equation (ii). These results log k1/s–1= 14·246 ± 0·106 –(201·46 + 1·20 kJ mol–1)RTIn 10 (i), log k2/s–1= 13·958 ± 0·062 –(196·00 ± 0·70 kJ mol–1)RTIn 10 (ii) are discussed in terms of a unimolecular decomposition occurring by a concerted mechanism.

Volumes of reaction for the formation of some analogues of tetrahedral intermediates

Two reactions which comprise equilibrium addition of a protic species to a carbonyl group, the hydroxyketone–hemiacetal interconversion (1a)⇌(1b), and the thiol addition of acetyl cyanide (2a)⇌(2b), have been studied by n.m.r. and their spectra thoroughly analysed. Equilibrium constants have been measured as a function of pressure to obtain volumes of reaction which relate to the formation of the ‘tetrahedral intermediate’ of ester hydrolysis. The intramolecular reaction is shown to have a much less negative volume change than the intermolecular analogue, pointing to the important contribution to ΔV and to ΔV‡ of molecular association prior to bond formation.

The kinetics and mechanisms of aromatic halogen substitution. Part XXXIII. Kinetics and products of chlorination of some aryl acetates

The rates and products of reaction of 4-methylphenyl acetate, 3,4- and 2,6-dimethylphenyl acetate, and 2,3,5,6-tetramethylphenyl acetate with molecular chlorine in acetic acid have been studied. ‘Abnormal’ reaction paths play a large part in the determination of the products of the chlorination of the first two of these compounds, probably through electrophilic attack on an activated ‘ipso’-position. The observed rates of reaction are compared with those calculated on the basis of additivity of substituent effects; they allow an estimate to be made of the effect of steric inhibition of resonance on the directing power of the acetoxy-group. The product of chlorination of the corresponding phenols are recorded also.