HETEROLYTIC AND HOMOLYTIC BOND-DISSOCIATION ENERGIES OF THE N-H BONDSIN YLIDE PRECURSORS AND PROPERTIES OF THE CORRESPONDING NITROGEN-CENTERED RADICALS

It is well known that the reaction pattern and reactivity of Ylides ar e related to the stabilities of the corresponding organic anions([1,2] ) and therefore to the pK(a) values of their precursor molecules. Howe ver, water, the most commonly used medium for deriving the acidities f or most inorganic acids, is not a good solvent for determining the pK( a)'s of Ylide precursors because of the differential solvation effect( [3]) and sometimes of unexpected side-reactions of water with the subs trates. It was established only in recent years that dimethyl sulphoxi de (DMSO) is an excellent medium for precisely determining the intrins ic free-ion pK(a) values, which covers a much wider pK range (0 to 33) than that in water (0 to 14)([3]). In this note we report, for the fi rst time, the equilibrium acidities (eq.(1)) of a group of nitrogen ac ids in DMSO solution, where the nitrogen atom is adjacent to triphenyl phosphonium moiety. No pK(a)'s of the nitrogen-centered Ylide precurso rs of the type R(3)P(+)-NHR (R=alkyl) appeared to be available in lite rature. The pK(a) values determined in this work therefore not only se rve as the first collection of the data for this general type of compo unds, but also provide a base for investigating stabilities of the cor responding Ylide anions. [Ph(3)P - NHR](+) + DMSO reversible arrow(Ka) Ph(3)P(+) - N(-)R + DMSOH(+) The homolytic bond dissociation energies (BDE) of the N-H bonds in the Ylide precursors are also a much desire d quantity and can easily be obtained using the method (eg. (2)) repor ted in an early paper where the pK(a) value is utilized([4]): BDE(kJ/m ol) = 5.707pK(a) + 96.48E(ox)(N-) + 308.4. The thermodynamic stabiliti es of the corresponding nitrogen-centered radicals are discussed accor dingly based on the relative BDE values.