PC Bonds as Building Blocks for Three‐ and Four‐Membered Heterocyclic Cations: Synthesis, Structures and Mechanistic Studies

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