P=C bonds as building blocks for three- and four-membered heterocyclic cations: synthesis, structures and mechanistic studies

The activation of the P=C bond of phosphaalkenes with electrophiles is investigated as a means to prepare and characterize unusual organophosphorus compounds. Treatment of RP=CHtBu (1a: R=tBu; 1b: R=1-adamantyl) with HOTf (0.5 equiv) affords diphosphiranium salts RP-CHtBu-PR (CH(2)tBu)]OTf (2a]OTf and 2b]OTf), each containing a three-membered P(2)C ring. In contrast, the addition of MeOTf (0.5 equiv) to either 1a or 1b affords diphosphetanium salts RP-CHtBu-P(Me)R-CHtBu]OTf (3a]OTf and 3b]OTf) containing four-membered P(2)C(2) heterocycles. The phosphenium triflate tBuP(CH(2)tBu)]OTf (5a]OTf) and methylenephosphonium triflate tBu(Me)P=CHtBu]OTf (7a]OTf) are identified spectroscopically as intermediates in the formation of 2a](+) and 3a](+), respectively. The phosphenium triflate intermediate can be trapped with 2-butyne to afford phosphirenium salt MeC=CMe-tBuPCH(2)tBu]OTf (6a]OTf). Treatment of diphosphetanium 3a]OTf with an excess MeOTf affords Me(2)P-CHtBu-PMetBu-CHtBu](OTf)(2) (4a](OTf)(2)), a compound containing a diphosphetanium dication. The molecular structures are reported for 2a]OTf, 2b]H(OTf)(2)], 3a]I, 3b]I, 4a](OTf)(2), and 6a]OTf.