Synthesis Of Fluorenone Derivatives By Intramolecular Dehydrogenation Coupling Reaction Of O-arylbenzaldehyde

Fluorenone and its derivatives are important chemical raw materials and these scaffolds are found as core structures in many biologically and pharmacologically important compounds.They are also widely used in functional polymers,optoelectronic materials,dyes,and pesticides,etc.Traditional methods for the synthesis of fluorenone and its derivatives include the Friedel-Crafts ring-closure reaction of 2-phenylbenzoic acid,the oxidation of fluorene,and noble metal catalyzed coupling reactions,etc.Although these protocols provide efficient ways for the synthesis of fluorenone derivatives in both laboratory and industry,however,they are usually limited to electron-rich aromatic substrates or require multi-step reactions.At the same time,these approaches also suffer from some other drawbacks like the complicated sequences for the preparation of the starting materials,the requirement of expensive catalysts and harsh reaction conditions,etc.Over the past decade,C-H bond activation reaction has become an attractive method,because it allows to constructing new carbon-carbon bonds or carbon-heteroatom bonds between organic components in an atom-economy manner.Especially,the direct coupling reaction involving two C-H bonds has received extensive attention,and has been used in the synthesis of fluorenone derivatives.But so far,there are only a few reports of such reactions have been reported in the literature,and the dangerous peroxides are usually employed as oxidants in this transformations.In this paper,we have developed a new method for the synthesis of fluorenone compounds,which uses 2-arylbenzaldehydes or 2-arylcyclohexenals as the raw materials through a Fe（OTf）₃or Fe（OTf）₃/DDQ promoted intramolecular dehydrogenation coupling reaction.The reaction has the advantages of cheap and easily available raw materials,simple operation,high atom-economy,and no need of peroxide as oxidants,etc.In this paper,the cross-dehydrogenation coupling reaction（CDC）as well as the application and the methods for the synthesis of fluorenone compounds are firstly introduced.In the research content,our experimental results of synthesis of fluorenones by intramolecular CDC reaction are discussed,using arylbenzaldehydes or arylcyclohexenals as raw materials under the DDQ-promoted or DDQ-free conditions.We first use 2-phenylbenzaldehyde as the template substrate to optimize the reaction conditions,and the best conditions for this transformation were obtained:Fe（OTf）₃（120 mol%,or Fe（OTf）₃ 10 mol%/DDQ 200 mol%）,reaction temperature 140℃,solvent DCE.Under this two reaction conditions,the yield of fluorenone can reach more than 90%.Then,the limitation and scopes of the reaction were explored under the optimal conditions.Finally,several control experiments were conducted and two reaction pathways were proposed.The results show that our method has good substrate universality and provides an efficient protocol for the synthesis of fluorenone and benzofluorenone compounds.The method has a good application prospect due to its some advantages such as easily available starting materials,simple operation,and high atom-economy,etc.