Journal of the Chemical Society, Dalton Transactions p. 1283 - 1288 (1981)
Update date:2022-07-29
Topics:
Goldschmidt, Jacob M. E.
Goldstein, Rebecca
With the aim of elucidating the mechanistic origins of trans preference as observed in the amination reactions of chlorocyclophosphanzenes, a kinetic study of the reactions of pentachloro(dimethylamino)cyclotriphosphazene, N3P3Cl5(NMe2), with NMe2H in tetrahydrofuran produce cis-and trans-N3P3Cl4(NMe2)2 has been undertaken.Gas chromatograpic analysis was employed to follow the changes in the concentrations of the components of the product mixture with time, from which rate constants for both cis and the trans reactions at various temperatures, as well as the values of the enthalpies and the entropies of activation of both reactions, have been evaluated.The data show that the trans prefence is due solely to the fact that ΔS(exit)trans>ΔS(exit)cis.To explain the origin of trans preference the operation of modified substituent-solvating effect is postulated, according to which the substituent, after protonation, functions as a neighbouring group that by intramolecular acid catalysis facilitates departure of the chloride ion.This mechanism specifically favours the formation of the trans isomer as for steric reasons it cannot aid the formation of the cis isomer.
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