Eberlin reaction of arenesulfenylium cations with cyclic acetals and ketals: ring contraction and cycloreversion

Arenesulfenylium ions (ArS+), elusive species in the condensed phase, are readily generated in the gas phase by electron ionization of sulfur-containing compounds and here their reactions with cyclic acetals and ketals are studied in a pentaquadrupole mass spectrometer. As a strong electrophile, the arenesulfenylium ion (ArS+) reacts with five-membered cyclic acetals and ketals by hydride abstraction and electrophilic addition followed by ketone or aldehyde elimination. This latter process is ascribed to an oxygen-assisted ring-opening process in the adduct, followed by intramolecular nucleophilic substitution at carbon by the neutral arylthio group (ArS). This latter recyclization step generates a four-membered 2-aryl-1,2-oxathietan-2-ium ion. Ab initio calculations at the Becke3LYP/6-31G(d) level are performed to obtain the optimized structure of the ring contraction product and they predict the overall reaction to be highly exothermic. Collision-induced dissociation of the elimination product is consistent with its being the four-membered ring contraction product and this provides evidence for the gas-phase cycloreversion. Similarly, reaction with six-membered cyclic acetals and ketals forms a five-membered 2-aryl-1,2-oxathiolan-2-ium product ion. The overall reaction is of the Eberlin reaction type, the prototype of which is the transacetalization of acylium ions (M. N. Eberlin and R. G. Cooks, Org. Mass. Spectrom., 1993, 28, 679). Substituents at the para-position of the arenesulfenylium cation have a significant influence on reactivity; the p-fluorobenzenesulfenylium cation displays similar reactivity to the unsubstituted arenesulfenylium cation, while the Eberlin product is not observed for the electron-donating amino (NH2) or methoxy (CH3O) substituted ions.