Selective 6-endo-dig and ring-expansion cycloisomerizations of ortho-disubstituted thiophenes bearing 1-en-3-yne moieties

Abstract

Most metal-catalyzed 6-endo-dig cyclizations are restricted to ortho-unsubstituted aromatic substrates. When the two ortho-positions are substituted, it is generally difficult to conduct a 6-endo-dig cyclization reaction on such an ortho-disubstituted aromatic ring. Herein, with the assistance of PtCl₂, unexpected 6-endo-dig and ring-expansion cycloisomerizations have been discovered for ortho-disubstituted thiophene derivatives bearing 1-en-3-yne moieties, although the two ortho-positions on the thiophene ring are occupied. The scope investigation suggests that the cycloisomerization pathway is determined by the substrate structure. When the CC double bond in the 1-en-3-yne moiety belongs to an aromatic ring, a benzo[b]thiophene derivative is obtained via a 6-endo-dig cyclization along with a [1,2]-alkyl shift. In contrast, if the double bond comes from an aliphatic alkene, a ring-expansion cyclization reaction occurs, which produces a cyclopenta[c]thiapyran analogue. The mechanisms of both the 6-endo-dig and ring-expansion cycloisomerizations are proposed and verified by density functional theory calculations.