cis-Bromination of a non-conjugated cyclic alkene. Unprecedented electrophilic stereoselection by means of a remote SO2 group

Abstract

Contrary to expectation, addition of molecular bromine to 3-thiabicyclo[3.2.0]hept-6-ene 3,3-dioxide (1) yields substantial amounts of the cis-1,2-dibromide (confirmed by X-ray diffraction), suggesting that the remote SO₂ group exerts an extraordinary directive influence by means of a long-range Coulomb interaction that stabilises an open carbocation intermediate at the expense of the usually favoured bridged bromonium ion.