Electrophilic Substitution at Saturated Carbon. XV. Asymmetric Carbanions, Asymmetric Solvation and d-Orbitals of Sulfur1,2

The stereochemical course of the base-catalyzed hydrogen--deuterium exchange reaction was studied through rate comparisons of racemization and exchange of 2-octyl phenyl sulfone and 2-octyl-2-d phenyl sulfone in deuterated and non-deuterated solvents. The exchange rates exceeded the racemization rates by factors that ranged from a low of 10 to a high of 1980, depending on the solvent, and in one solvent on the character of the base. In dissociating solvents such as ethylene glycol and dimethyl sulfoxide-- methanol mixtures, and in the nondissociating solvent t-butyl alcohol with tetramethylammonium hydroxide as base, the smaller factors and lower degrees of retention of configuration were observed. In t-butyl alcohol with potassium t-butoxide as base, the larger facters were obtained. These facts coupled with similar experiments carried out previously on other systems indicate that in dissociating solvents, most of the stereospecificity arises from the asymmetry of the carbanion intermediate, and some from asymmetric solvation. In associating solvents, asymmetric solvation plays an important role in the high retention observed for the reaction. The configurational stability of carbanions attached to the sulfone group is attributed to overlap between the dorbitals of sulfur and the orbital occupied by the two electrons of the anion. Hydrogen-- deuterium isotope effectsmore » for both the exchange and racemization reactions were measured, and found to vary between extremes of 0.3 and 1.9. These low values are interpreted as evidence that both the exchange and racemization rates are governed by mechanisms that involve kinetically distinguishable, discretel stages in which covalent bonds between carbon and hydrogen, or oxygen and hydrogen are neither made nor broken. (auth)« less