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

Abstract

Abstract A number of possible strategies for the preparation of λ 5 ‐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 λ 5 ‐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(SiMe 3 ) 2 to give rise to the corresponding λ 5 ‐phosphetes 22a–b . The benzo‐λ 5 ‐phosphete 22a underwent ring expansion reactions with dimethyl acetylenedicarboxylate and acetonitrile, leading to benzo‐λ 5 ‐phosphinine and benzo‐1, 4λ 5 ‐azaphosphinine in good yields. Derivative 22b was characterized by X‐ray crystal structure analysis. Ab initio SCF calculations, IGLO‐ 13 C chemical shifts and Δχ for various benzannulated derivatives and phosphorus heterocycles are presented.