Transmission of polar effects. Part 15. Ionisation and esterification with diazodiphenylmethane of [2.2]paracyclophane-4-carboxylic acid and its pseudo-bromo-substituted derivatives, and the alkaline hydrolysis of their methyl esters

Abstract

lchester C04 3S0, EssexThe pK, values of [2.2] paracyclophane-4-carboxylic acid and its four pseudo-bromo-su bstituted derivatives have been determined in 80% (w/w) 2-methoxyethanol-water at 25 "C. The rate coefficients for the esterification of these acids with diazodiphenylmethane have been measured in 2methoxyethanol at 30 "C. The rate coefficients for the alkaline hydrolysis of the corresponding methyl esters have been determined in 70% (v/v) dimethyl sulphoxide-water at both 56.3 and 76.3 "C. In the ionisation and esterification reactions both normal and reversed su bstituent polar effects were detected. Kirkwood-Westheimer calculations were carried out for the ionisation reaction and account for ApK, values both qualitatively and (for the pseudo-ortho-, -meta-, and -para-isomers) quantitatively. In the alkaline hydrolysis of the esters, significant steric retardation was observed for the pseudo-gern-isomer.From previous work on inductive and electrostatic-field effect model^,^-^ a field-effect model appears to give the more satisfactory description of the substituent polar effect. A reversal of the normal polar effect has been detected in some of these system^^-^ and such behaviour has been accounted for in terms of the angular dependence of the polar effect. Thus either the normal or the reverse of the normal substituent polar effect would be expected on the basis of the geometrical disposition of the substituent with regard to the reaction centre.We describe here the ionisation of [2.2]paracyclophane-4carboxylic acid and its four pseudo-bromo derivatives (l), their esterification with diazodiphenylmethane (DDM), and the alkaline hydrolysis of the corresponding methyl esters.' The results are related to Kirkwood-Westheimer calculations of the electrostatic field effect.

Perfluoropolyether alkyl diesters: Structure effects of the alkyl group on the kinetics of the hydrolysis reactions

“…Tentatively, steric inhibition of solvation could be considered. Actually, it has been demonstrated that the solvation inhibition goes hand in hand with the steric effect increases and appears to be very pronounced for tert ‐butyl and 2,6‐dimethyl substituents 30, 37–39. Esters, when protonated, form bulky carbocations having high steric inhibition to solvation, whereas the flat tert ‐butyl carbocation, formed by alkyl cleavage, can be easier to solvate, thanks to its planar configuration.…”