Gold(I)-Catalyzed Regiodivergent Rearrangements: 1,2- and 1,2′-Alkyl Migration in Skipped Alkynyl Ketones

A series of 2-alkynyl carbonyl compounds that contain a cyclopentene ring or a heterocycle can be transformed into various fused dihydrobenzofurans and tetrahydrofuro[2,3-c]pyridines by means of a 1,2-alkyl migration process. Both of these reactions proceed with excellent regioselectivity and stereospecificity when using a cationic gold(I) catalyst. Treatment of 4-styrylcyclopent-1-enecarboxylates under different conditions affords a range of highly functionalized dihydrobenzofurans and dihydroisobenzofurans. A divergence in product selectivity, which depends on the anion of the silver salts used, was observed. Interestingly, ring-fused tetrahydroquinolines undergo only 1,2'-alkyl migration reaction by means of a C?C cleavage/cyclization sequence to provide tetrahydroazepine derivatives. Mechanistic studies suggest that the gold complexes catalyze 1,2-alkyl migration reactions through a concerted reaction pathway and 1,2'-alkyl migration reactions through a stepwise reaction pathway.