One-pot Modular Synthesis of Hydroquinolines, Hydrofluorenones, and Aryl Fluorenones

Many natural products and bioactive molecules are built on hydroquinolines, hydrofluorenones, and fluorenones. In addition, aryl fluorenones and their derivatives, such as spirobifluorene, are widely used as a key molecular framework for the development of organic electronic materials. The synthesis of aryl fluorenones relies on multiple steps involving at least two transition metal-catalyzed C(sp²)-C(sp²) coupling reactions to build the fluorenone ring, which impede the easy accessibility of these materials for academic research and follow-up practical applications. Herein, we report a fundamentally unique method to build the fluorenone skeleton via a one-pot reaction under metal-free conditions. The method relies on the formation of hydroquinolines and hydrofluorenones via a diastereo- and regio-selective multi-bond forming reaction of feedstock chemicals. Up to 8 new (C-C, C-N, and C-O) bonds, 3 cycles, and up to 4 stereocenters were built in a single operation with exclusive diastereo and regioselectivity. DFT studies provided insights into the diastereo- and regioselectivity of this reaction. The aryl groups of aryl fluorenones, which are generally installed via transition metal-catalyzed coupling reaction of pre-functionalized arene moiety, are incorporated from readily available aryl aldehyde under metal-free conditions. The spirobifluorenes derived from these arylated fluorenones showed to have appropriate triplet energy, HOMO/LUMO levels, and thermal stability which indicate their potential as a host material for PhOLED.