Platinum-Catalyzed Synthesis of Pyrrolo[3,2-c]azepin-4-ones

Significance

Aiming to synthesize new pyrrolo[2,3-c]azepin-8-one derivatives 4 from the ­Sonogashira product 2 of the acetylenic pyrrole 1 under gold or platinum catalysis, Beller and co-workers unexpectedly obtained the isomeric pyrrolo[2,3-c]azepin-4-ones. Thus, instead of the expected endo-dig cyclization of 2 to give 4, the presumed initial acetylenic gold(III) or platinum(IV) complex undergoes C2 to C3 carbonyl migration prior to the alternative C2 endo-dig cyclization to yield the isomer 3. The reaction proceeds in the presence of AuCl₃ or H₂PtCl₆˙6H₂O, with H₂PtCl₆˙6H₂O giving the best results. Alkynes with R = Ar gave moderate to good yields of products, whereas heteroaromatic-substituted alkynes led to poorer yields.