Reactions of 2‐(Organylseleno)propenenitriles with Dimethyl Ethynedicarboxylate

( 2‐tert ‐Butylseleno)propenenitrile ( 12a ) has been prepared and treated with dimethyl ethynedicarboxylate ( 3 ) and methyl propiolate ( 18 ) to give the 2,3‐dihydroselenophenes 16 and 21 , respectively. In contrast, 2‐(ethylseleno)‐, 2‐(methylseleno)‐ and 2‐(phenylseleno)propenenitrile ( 12b–d ) tend to form polysubstituted butadienes 17b–d with 3 by net organoylseleno group transfer from 12b–d to C‐2 of 3 following attachment of C‐3 of 17 to C‐3 of 3 . Five‐membered selenonium ylides are proposed as intermediates.