The Endocyclic Restriction Test: The Geometries of Nucleophilic Substitutions at Sulfur(VI) and Sulfur(II)

The trajectories for nucleophilic substitutions at sulfur(VI) and sulfur(II) have been investigated by the endocyclic restriction test. On the basis of double-labeling experiments, the sulfur(VI) transfer in the conversion of 1 to 2 is found to be intramolecular, while the sulfur(VI) transfer in the conversion of 3 to 4 and the sulfur(II) transfer in the conversion of 5 to 6 are found to be intermolecular. These results are taken to be consistent with transition structures for these sulfur transfer reactions which require a large angle between the entering and leaving group, a geometry analogous to apical group positions in trigonal bipyramidal transition states.