Nucleophilicity and Nucleofugality of Phenylsulfinate (PhSO2−): A Key to Understanding its Ambident Reactivity

摘要

Second-order rate constants for the reactions of the phenylsulfinate ion PhSO2n- with benzhydryliumnions Ar2CH+ have been determined in DMSO, acetonitrile, and aqueous acetonitrile solution usingnlaser-flash and stopped-flow techniques. The rate constants follow the correlation equation log k (20 °C)n) s(N + E), which allows the determination of the nucleophile-specific parameters N and s for PhSO2n- inndifferent solvents. With N ) 19.60, PhSO2n- is a slightly weaker nucleophile than malonate and azide ionsnin DMSO. While PhSO2n- reacts with highly stabilized benzhydrylium ions to give benzhydryl phenyl sulfonesnexclusively, highly reactive benzhydrylium ions give mixtures of sulfones Ar2CH-SO2Ph and sulfinatesnAr2CH-OS(O)Ph; the latter rearrange to the thermodynamically more stable sulfones through an ionizationnrecombination sequence. Sulfones generated from PhSO2n- and stabilized amino-substituted benzhydryliumnions undergo heterolysis in aqueous acetonitrile and the rate of formation of the colored benzhydryliumnions was followed spectrophotometrically by stopped-flow techniques. The ranking of the electrofugalitiesnof the benzhydrylium ions (i.e., the relative ionization rates of Ar2CH-SO2Ph) was not the inverse of thenranking of their electrophilicities (i.e., the relative reactivities of Ar2CH+ with nucleophiles), which wasnexplained by differences in Marcus intrinsic barriers. While sulfones are thermodynamically more stablenthan the isomeric sulfinates, the intrinsic barriers for the attack of benzhydrylium ions at the oxygen ofnPhSO2n- are significantly lower than the intrinsic barriers for S-attack, and the activation energies for thenattack of carbocations at sulfur are only slightly smaller than those for attack at oxygen. Because reactionsnof PhSO2n- with carbocations of an electrophilicity E > -2 (i.e., carbocations which are more reactive thannPh3C+) are diffusion-controlled, the regioselectivities of the reactions of PhSO2n- with “ordinary” carbocationsndo not reflect relative activation energies.