Studies in cyclophosphazenes. Part 10. The mechanism for trans-isomer preference in the non-geminal diamination of hexachlorocyclotriphosphazene

Abstract

With the aim of elucidating the mechanistic origins of trans preference as observed in the amination reactions of chlorocyclophosphazenes, a kinetic study of the reactions of pentachloro(dimethylamino)cyclotriphosphazene, N₃P₃Cl₅(NMe₂), with NMe₂H in tetrahydrofuran to produce cis- and trans-N₃P₃Cl₄(NMe₂)₂ has been undertaken. Gas chromatographic 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 the cis and the trans reactions at various temperatures, as well as the values of the enthalpies and entropies of activation of both reactions, have been evaluated. The data show that the trans preference is due solely to the fact that ΔS^‡_trans > ΔS^‡_cis. To explain the origin of trans preference the operation of a 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.