2-Aryl-1,3-dithianes and -dithiolanes: A nearly ideal series for relating the energies for bond breaking to electron transfer

Abstract

By virtue of the stabilizing effect of the 1,3-sulfur atoms on the carbocations, radicals, and carbanions generated from the title compounds, it has been possible to measure a variety of bond-making and bond-breaking processes in the two very similar solvents, DMSO and sulfolane, and relate them to electron-transfer energies obtained by electrochemical techniques. Important properties reported from this and previously published work are as follows: heats of hydride transfer to the cations from cyanoborohydride ion, pKR+. in aqueous acid, heats of deprotonation by K+DMSYL/DMSO, pKHA, redox potentials for the cations, and carbanions, which relate their energies to their conjugate radicals and to each other. The results support our previous assertion that the electron-transfer energy between the three trivalent oxidation states of carbon and the Parr-Pearson absolute hardness, η, derived from it are the fundamental properties that determine energies for making and breaking two-electron bonds and thus detennine most of organic chemistry. Excellent correlations are found for the substituent effects on energy changes associated with the various processes for making and breaking bonds to the cations, radicals, and carbanions and the electrontransfer energies for inter converting them. Many comparisons can be made with the corresponding 2-aryl-1,3-dioxo systems. Careful "bookkeeping" of these energies through appropriate thermochemical cycles shows excellent consistency despite a small solvent effect for transferring the ions from sulfolane to DMSO. Direct reaction of the carbocation with the carbanion of 2-phenyl-1,3-dithiane produced a clean formation of the dimer from which the heat of heterolysis (40.6 kcal/mol) and homolysis (19.1 kcal/mol) could be calculated. AM1 structures and heats of formation of two neutral species and two cations, a radical and an anion, have been computed and are generally consistent with stabilizing interactions of the gem sulfurs with the reactive center. The present study is the first, to our knowledge, to provide a coordinated view of the energies for generating the carbocations, radicals, and carbanions from a series of heterocycles. These energies are related to each other and to the electron-transfer energies for interconverting these reactive trivalent forms of carbon. © 1996 John Wiley & Sons, Inc.