Fluorenes As New Molecular Scaffolds For Carbon-carbon σ-bond Cleavage Reaction: Acylfluorenylation Of Arynes

摘要

Acyl and fluorenyl moieties are facilely installed into neighbouring positions of aromatic skeletons via C-C bond cleavage reaction using arynes.rnDirect installation of two carbon units (C~1 and C~2) into carbon-carbon unsaturated compounds via C~1-C~2 bond cleavage has gained much attention, because it provides new organic transformations for constructing complex carbon frameworks in a straightforward manner. In this context, Stoltz's and our groups have already disclosed insertion reactions of arynes into a C-C bond of β-dicarbonyl or α-cyanocarbonyl compounds, which offer facile approach to highly functionalized arenes, being hardly available by conventional methods, however, applicability of the reaction is totally restricted to active methylene compounds bearing two electron-withdrawing groups (EWG), to the best of our knowledge. The key for the reaction to proceed would be smooth formation of a carboanionic species ((EWG)_2C~-) via rapid deprotonation induced by two EWGs, and thus we envisaged that the use of another molecular scaffold having a C-H moiety of suitable acidity would result in a new C-C bond cleavage reaction. We report herein on acylfluorenylation of arynes using fluorene derivatives, where formation of a key carboanionic species is facilitated by its aromatic stabilization, allowing two carbon-carbon bond-forming processes (aryl-acyl and aryl-fluorenyl bonds) to occur at neighbouring positions of an aromatic skeleton all at once (Scheme 1).