Chemoselectivity in the cationic Phospha-Wittig reaction: accessing phosphorus heterocycles, phosphaalkenes, and their annulated [4 + 2] dimers

Triflate salts of phosphito-phosphanides [L_CP-P(OR)₃]⁺ (1[OTf], R = alkyl, L_C = <i>N</i>-heterocyclic carbene) were obtained <i>via</i> nucleophilic fragmentation of the tetraphosphetane [(L_C)₄P₄][OTf]₄ (3[OTf]₄) with organophosphites P(OR)₃. The salts 1[OTf] act as versatile reagents in the cationic Phospha-Wittig reaction, converting aldehydes into imidazoliumyl-substituted phosphaalkenes 2[OTf] and, <i>via</i> a competing pathway, into diphosphiranes 4[OTf]₂. The product distribution is governed by the aldehyde substituent, enabling selective access to isolable derivatives of both compound classes. The resulting phosphaalkenes 2[OTf] serve as precursors to diverse phosphorus heterocycles, undergoing expected [2 + 2] dimerisation to 1,3-diphosphetanes <i>syn</i>/<i>anti</i>-(2)₂[OTf]₂ and trapping reactions with 1,3-dienes to yield the tetrahydrophosphinine 8[OTf] and bicyclic derivative 9[OTf]. Most notably, an unprecedented annulative [4 + 2] dimerisation pathway for cationic <i>C</i>-aryl phosphaalkenes is uncovered that furnishes benzannulated tetrahydro-1,2-diphosphinines 7[OTf]₂. Computational studies reveal that the operative mechanism of this transformation involves a Phospha-Diels-Alder step followed by an acid-base-catalytic proton transfer, which is calculated to be energetically more accessible than the classical [2 + 2] dimerisation.