Chemoselectivity in the Cationic Phospha-Wittig reaction: Accessing Phosphorus-Heterocycles, Phosphaalkenes, and their annulated [4+2] Dimers

Abstract

Triflate salts of phosphito-phosphanides [L_CP–P(OR)₃]⁺ (1[OTf], R = alkyl, L_C = N-heterocyclic carbene) were obtained via 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, via 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 syn/anti-(2)₂[OTf]₂ and trapping reactions with 1,3-dienes to yield the tetrahydrophosphinine 7[OTf] and bicyclic derivative 8[OTf]. Most notably, an unprecedented annulative [4+2] dimerisation pathway for cationic C-aryl phosphaalkenes is uncovered that furnishes benzannulated tetrahydro-1,2-diphosphinines 6[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.