λ5‐Phosphetes, Benzo‐λ5‐Phosphetes, Naphtho‐λ5‐Phosphetes: Four‐π‐, Eight‐π‐, and Twelve‐π‐Electron Systems

A number of possible strategies for the preparation of λ5‐phosphetes were tested as follows: cyclopropenium 6 was treated with the lithium salt of diphenylphosphine to give phosphinocyclopropene 7, but 7 did not undergo ring expansion upon photolysis or thermolysis. P‐chloro‐Ctrimethylsilyl‐substituted ylide 8b reacted with two equivalents of dimethyl acetylenedicarboxylate to afford phosphinine 13 via a transient λ5‐phosphete 12. Addition of aluminum trichloride to P‐halogenated ylides 17a–b led to dihydrophosphetium salts 19a–b, which, upon treatment with pyridine, isomerized into the 1, 2‐dihydrophosphet‐2‐ium salts 20a–b. Hydrolysis of derivatives 20a–b cleanly afforded phosphoniums 21 a–b, which reacted with NaN(SiMe3)2 to give rise to the corresponding λ5‐phosphetes 22a–b. The benzo‐λ5‐phosphete 22a underwent ring expansion reactions with dimethyl acetylenedicarboxylate and acetonitrile, leading to benzo‐λ5‐phosphinine and benzo‐1, 4λ5‐azaphosphinine in good yields. Derivative 22b was characterized by X‐ray crystal structure analysis. Ab initio SCF calculations, IGLO‐13C chemical shifts and Δχ for various benzannulated derivatives and phosphorus heterocycles are presented.