Osmium-mediated electrophilic addition reactions with selenophene and activation of the Se-C bond

The complex [Os(NH3)(5)(eta(2)-selenophene)](OTf)(2) (I) has been prepared and treated with several electrophilic reagents to afford Se-alkyl and 2H-s elenophenium complexes in, high yield. In many aspects, the reactivity obse rved for 1 directly parallels that seen, for the thiophene analogue. Proton ation (HOTf/MeCN) and alkylation (acetal/TBSOTf) occur at C2, affording (1, 5-eta(2))-2H-selenophenium complexes (2 and 3, respectively). Methylation ( MeOTf/MeCN) occurs at the heteroatom, affording the 1-methylselenophenium c omplex 4, which, when treated with a nucleophile, undergoes C-Se cleavage t o form an. eta(2)-4-(alkylseleno)-1,3-butadiene complex (5).