Sulfuration of 9,9′-bibenzonorbornenylidenes by elemental sulfur: exclusive episulfide formation with retention of the configuration of alkenes

Sulfuration of syn-bibenzonorbornenylidene (5a) by elemental sulfur in refluxing o-dichlorobenzene furnished the episulfide (6a) exclusively in 83% yield with retention of the configuration of 5a, while that of anti-9,9′-bibenzonorbornenylidene (5b) under the same conditions resulted in the exclusive formation of the episulfide (6c) in 72% yield also with retention of the configuration of 5b. Non-stereoselective sulfuration of 5a and 5b by S2Cl2 is also described.