Expansion of the pentafluorobenzene ring and other skeletal transformations in the reaction of perfluoro(1,2-diethyl-1-phenyl-1,2-dihydrocyclobutabenzene) with antimony pentafluoride

Abstract

The reaction of perfluoro(1-phenyl-1,2-diethyl-1,2-dihydrocuclobutabenzene) with SbF 5 at 20°C, followed by treatment of the reaction mixture with water gave perfluoro{4-[1-(2-propylphenyl)propylidene]-2,5-cyclohexadien-1-one} together with perfluoro[4b,10-diethylbenzo[a]azulen-7(4bH)-one] resulting from unusual expansion of the pentafluorobenzene ring to seven-membered ring. Analogous reaction at 90°C, apart from the above compounds, afforded perfluorinated 10-ethyl-and 3,10-diethylbenzo[a]azulen-6(10H)-ones via elimination of C 2 F 5 group from the seven-membered ring or its migration to the benzene ring. * For preliminary communication, see [1].Reactions of perfluorinated 1-alkyl-and 1,1-and 1,2-dialkyl-1,2-dihydrocyclobutabenzenes with SbF 5 involve expansion or opening of the four-membered ring with formation of polyfluorinated indans, o-alkylstyrenes, or o-dialkylbenzenes [2]. Perfluorinated 1-(2-or 4-ethylphenyl)-1,2-dihydrocyclobutabenzenes [3] and 1-or 2-alkyl-1-phenyl-1,2-dihydrocyclobutabenzenes [4,5] in SbF 5 undergo opening of the fourmembered ring, and the subsequent cyclization yields indan, fluorene, and anthracene derivatives. On the other hand, perfluoro(1-phenyl-1,2-dihydrocyclobutabenzene) having no perfluoroalkyl group, did not undergo skeletal transformations under analogous conditions [3]. In the present work we examined the reaction of perfluoro(1,2-diethyl-1-phenyl-1,2-dihydrocyclobutabenzene) (I) with antimony pentafluoride with a view to elucidate how accumulation of perfluoroalkyl and perfluoroaryl groups in the four-membered ring of 1,2-dihydrocyclobutabenzene affects the direction of skeletal transformations of polyfluorinated 1,2-dihydrocyclobutabenzenes.We previously showed that compound I in SbF 5 generates perfluoro(1,2-diethyl-2-phenyl-1,2-dihydrocyclobutabenzen-1-yl) cation (II) [6]. When a solution of compound I in excess SbF 5 (1 : 57) with addition of SO 2 ClF was kept for 1 month at 20°C a mixture of salts of perfluorinated 1-(2-propylphenyl)-1-phenylprop-1-yl and 4b,10-diethyl-4b,7-dihydrobenzo[a]-azulen-7-yl cations III and IV was formed ( 19 F NMR data), the former considerably prevailing (90 : 10). The reaction performed under analogous conditions with cyclobutabenzene I obtained as a 1 : 1 mixture with HF from perfluoro(1,2-diethyl-1,2-dihydrocyclobutabenzene) (V) and C 6 HF 5 in SbF 5 (1 : 1 : 42) gave cations III and IV at a ratio of 75 : 25. Reduction of the amount of SbF 5 to 6 mol per mole of the substrate and increase of the reaction time to two months changed the ratio of cations III and IV in favor of the latter (20 : 80). Cation III is not converted into IV. Hydrolysis of the reaction mixture containing cations III and IV afforded perfluoro{4-[1-(2-propylphenyl)propylidene]cyclohexa-2,5-dien-1-one} (VI) and perfluoro{4b,10-diethylbenzo-[a]azulen-7(4bH)-one} (VII), respectively (Scheme 1).Raising the temperature to 50°C appreciably accelerated the reaction. Hydrolysis of the reaction mixture gave ketones VI and VII, perfluoro(4b,10-diethyl4b,5,6,7-tetrahydroben...