A new method for radical generation: reductive C–Se or C–S bond cleavage of cyclic onium salts

2-Methyl-3,4-dihydro-1 H-2-benzoselenopyranium salt (1) was reduced by some metallic reagents or magnesium metal via the single-electron transfer (SET) process to give 2-[2-(methylseleno)ethyl]-toluene (2). Magnesium metal was a good SET reducing agent. Some other selenonium or sulphonium salts (7), (14). (22), (27) and (32) were similarly reduced by magnesium. Particularly, sonication accelerated the reductions. ε-Eneselenonium salt (43) was treated with activated magnesium to give a cyclised pyrrolidine derivative (44).