Cyclization of terphenyl-bisfluorenol a mechanistic study of the regioselectvity

Abstract : This works reports the mechanistic principles governing the synthesis of a series of positional isomers of DiSpiroFluorene-IndenoFluorene with different geometry profiles (Syn or Anti). These isomers possess either a dihydro[1,2-b]indenofluorenyl (Anti) or a dihydro[2,1-a]indenofluorenyl (Syn) backbone. The present work explains the apparent paradox that increased bulkiness of the substituents borne by the fluorene moieties leads to the more encumbered and less energetically stable positional isomer. Through an experimental and theoretical study, we show that low polarity and weakly donor solvents, low temperature and small substituents on the fluorene moieties all favour the formation of anti isomers. We show that the most important feature, which drives the regioselectivity of the reaction is linked to the size and the position of the substituents borne by the fluorene units. A small substituent induces the formation of anti isomers whereas a bulky one leads to the syn isomers. In the light of the steric hindrance of the transition states, the origin of the regioselectivity has been unraveled. As dihydroindenofluorene positional isomers are nowadays important building units in organic electronics, this work allows to better understand one of their synthetic access described in the last years and provides rational ways of favoring the formation of one of the two isomers.
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Cyril Poriel, Frédéric Barrière, Joëlle Rault-Berthelot, Damien Thirion. Cyclization of terphenyl-bisfluorenol a mechanistic study of the regioselectvity. Chemistry - A European Journal, Wiley-VCH Verlag, In press, ⟨10.1002/chem.201901457⟩. ⟨hal-02150454⟩

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