Reaction of 5-phenylpenta-2,4-dienoic acid with benzene in trifluoromethanesulfonic acid
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Russian Chemical Bulletin, International Edition, Vol. 69, No. 10, pp. 1928—1932, October, 2020
Reaction of 5-phenylpenta-2,4-dienoic acid with benzene in trifluoromethanesulfonic acid A. R. Ismagilova,а D. N. Zakusilo,а L. V. Osetrova,b and A. V. Vasilyevа,c аSt.
Petersburg State Forest Technical University, 5 Institutskiy per., 194021 St. Petersburg, Russian Federation. Fax: +7 (812) 670 9390. Е-mail: [email protected] bResearch Institute of Synthetic Rubber, 1 Gapsal´skaya ul., 198035 St.-Petersburg, Russian Federation. Fax: +7 (812) 251 4813 cSt. Petersburg State University, Institute of Chemistry, 7/9 Universitetskaya nab., 199034 St. Petersburg, Russian Federation. Fax: +7 (812) 428 6733 The reaction of 5-phenylpenta-2,4-dienoic acid with benzene in CF3SO3H, depending on the reaction conditions, gives three products, namely, 5,5-diphenylpent-2-enoic acid and tetralone and indanone derivatives. These carbocyclic compounds are formed through the addition of two benzene molecules to the starting diene acid and subsequent intramolecular acylation. Key words: dienoic acids, 5-phenylpenta-2,4-dienoic acid, indanones, 3,4-dihydronaphthalen-1(2H)-ones, 5,5-diphenylpent-2-enoic acid, trifluoromethanesulfonic acid, carbocations, superelectrophilic activation.
Superelectrophilic activation of organic compounds under the action of various strong Brønsted and Lewis acids or acidic zeolites is one of the most efficient organic methods.1,2 This activation promotes the generation of multiply charged cations in low nucleophilic media, in which they can react with poor nucleophiles, for example, with arenes.2 Some of the best studied objects in this area are alkenes, the protonation of which in superacids or coordination with strong Lewis acids gives alkyl cations, which would undergo further transformations.3—14 Despite the large amount of data on the superelectrophilic activation of alkenes, the transformations of conjugated dienes under these conditions remain poorly studied. The 1,3-butadiene systems conjugated with various acceptor substituents contain several basic centers, such as acceptor heteroatoms and carbon atoms of multiple bonds. Their protonation gives cationic species with several electrophilic sites capable of reacting in different directions. The purpose of the present work is to study the reactions of 5-phenylpenta-2,4-dienoic acid 1 with benzene in Brønsted superacids (CF3SO3H, FSO3H) or under the action of strong Lewis acids AlX3 (X = Cl, Br). We found that in trifluoromethanesulfonic acid (TfOH), depending on the conditions, the reaction of phenyl diene 1 with benzene gives diphenylalkene 2, tetralone 3, and indanone 4 (Scheme 1, Table 1). The structures of compounds 3 and 4 was confirmed by X-ray diffraction analysis (Fig. 1). Carrying out this
reaction in the TfOH—CH2Cl2 system (Tf = F3CSO2) at 0 C for 1 h gives products 2—4 in a 55% total yield with the incomplete conversion of substrate 1 (see Table 1, entry 1). Raising the temperature to ambient and prolongation the reaction to 1—5 h leads to a sig
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