This research was financially supported by Japan Society for the Promotion of Science (JSPS) KAKENHI grant number JP19H02707 (J.I.) in a Grant-in-Aid for Scientific Research (B), JSPS KAKENHI grant number 20K21186 (J.I.) in a Grant-in-Aid for Challenging Research (Exploratory), and JSPS KAKENHI grant number 20K05486 (K.F.) in a Grant-in-Aid for Scientific Research (C).
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.
16 DFT calculations indicated that trans-4a is more stable than cis-4a by 1.7 kcal/mol.
17 The generation of ketones from difluoromethylene compounds can be explained by the substitution of OTf for F, followed by O–S and C–F bond cleavage. However, this mechanism is unlikely in our case because SbF5 did not afford difluoromethylene compound 11a but rather ketone 4a. See:
Kethe A,
Tracy AF,
Klumpp DA.
Org. Biomol. Chem. 2011; 9: 4545
18 Two equivalents of TfOH in relation to 2 were used based on the 2/10 ratio determined by 19F NMR analysis.
19 Partially saturated structures were proposed for 14 based on GC-MS analysis, but the saturated positions were not determined.
20 Under microwave irradiation, partial dehydrogenation products 14 were obtained as a mixture of two isomers.
21 Using a cannular in place of a glass filter resulted in the formation of 14. Rapid gas flow (120 mL/min) shortened the reaction time but the solvent tended to be lost.
