Attempted synthesis of ortho-phenylene phosphino-tritylium cations

With the synthesis of ortho-phenylene phosphino-tritylium cations as an objective, we generated (2-lithiophenyl)diphenylphosphine and (2-lithiophenyl)di-isopropylphosphine and allowed these organolithium reagents to react with benzophenone. The resulting phosphino-triarylcarbinols were allowed to react with HBF₄ in the presence of trifluoroacetic anhydride in order to generate the corresponding cations. Instead of the targeted ortho-phenylene phosphino-tritylium, the cations formed in these reactions were identified as the four-membered phosphonium species 7,7-bis(phenyl)-8,8-bis(phenyl)-7-phosphoniabicyclo[4.2.0]octa-1,3,5-triene (3⁺) and 7,7-bis(phenyl)-8,8-bis(isopropyl)-7-phosphoniabicyclo[4.2.0]octa-1,3,5-triene (4⁺), which were both isolated as tetrafluoroborate salts. The structure of these compounds has been confirmed by X-ray analysis. These new cations are thermally unstable and isomerize into the corresponding 5,10-dihydro-5,5-diphenyl-10-phenyl-acridophosphinium (5⁺) and 5,10-dihydro-5,5-di(isopropyl)-10-phenyl-acridophosphinium (6⁺) as tetrafluoroborate salts. These reactions suggest the involvement of ortho-phenylene phosphino-tritylium cations, which would be obtained by dissociation of the R₃P⁺-CAr₃ bonds in 4⁺ and 5⁺This article is part of the themed issue 'Frustrated Lewis pair chemistry'.