A synthesis of 3-fluoroindoles and 3,3-difluoroindolines by reduction of 3,3-difluoro-2-oxindoles using a borane tetrahydrofuran complex

Abstract

A borane tetrahydrofuran complex has been used to study the reduction of 3,3-difluoro-2-oxindoles and been found to yield either 3-fluoroindoles or 3,3-difluoroindolines. The latter have been found to be reasonably stable when the aromatic nucleus is substituted with an electron withdrawing group and are in these cases the predominant product. The efficient synthesis of the former occurs by climination of HF in the presence of silica from the latter. The 3,3-difluoro-2-oxindoles were prepared by the reaction of appropriately substituted isatin derivatives with DAST.