Construction of Substituted [4]Acene Frameworks Based on Double Cationic Cyclizations of Fluoroalkenes

Abstract

In this study, 5-substituted and 5,6-disubstituted [4]acenes were synthesized by the double cationic cyclization of fluoroalkenes. (a) After being treated with Me₂AlCl (1.2 equiv), 2-trifluoromethyl-1-alkenes bearing two aryl groups underwent domino Friedel–Crafts-type cyclization (two-ring construction) followed by dehydrogenation to generate 5-fluorinated [4]acenes. The same (trifluoromethyl)alkenes were treated with both Me₂AlCl (1.2 equiv) and Me₃Al (1.0 equiv), resulting in selective one-ring construction and the creation of bicyclic 1,1-difluoro-1-alkenes. (b) When treated with triflic acid, the bicyclic difluoroalkenes underwent regioselective protonation to generate CF₂ cations; Friedel–Crafts-type cyclization of these cations provided tetracyclic ketones. The obtained ketones act as an appropriate platform for the introduction of substituents at the 5-position of [4]acenes. (c) When treated with DDQ/H⁺, the bicyclic difluoroalkenes underwent oxidative generation of allylic CF₂ cations; Friedel–Crafts-type cyclization of these cations produced tetracyclic enones. The enones were subjected to double addition of carbanions to facilitate the introduction of two substituents at the 5- and 6-positions of dihydro[4]acenes.

Key words

acenes - cations - fluorine - cyclization - domino reactions

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