Metal-free graphene oxide-catalyzed aza-semipinacol rearrangement to prepare 2-(indol-2-yl)phenols and benzofuro[3,2-b]indolines containing quaternary carbon centers†
We report a metal-free, heterogeneous, inexpensive, and environmentally friendly graphene oxide-catalyzed aza-semipinacol rearrangement of 2-(3-methyleneindolin-2-yl)phenols to prepare 2-(indol-2-yl)phenols and benzofuro[3,2-b]indolines containing quaternary carbon centers in good to excellent yields under mild conditions. The detailed studies demonstrate that graphene oxide can function as a heterogeneous catalyst and provide hydrogen bonding and ion pair interactions on the surface to promote the rearrangement process. The substituent effect on the C2-position of indoline is two-fold, affording 2-(indol-2-yl)phenols by aza-semipinacol rearrangement and benzofuro[3,2-b]indolines by 5-exo-trig cyclization. The stereochemistry of the C3-quaternary carbon center generated from the C2-quaternary carbon center of indoline is retained during the rearrangement. More importantly, the graphene oxide can be easily recovered by washing and recycled seven times without loss of catalytic activity.