Synthesis and structures of non-cyclic and cyclic mono- and bisphosphonium salts derived from 1,8-bis(diphenylphosphino)naphthalene

A monoalkylated 1,8-bis(diphenylphosphino)naphthalene (dppn) was prepared by treating diphosphine with 1,8-bis(bromomethyl)naphthalene. Unprecedented cyclizations of monophosphonium salts in polar solvents, such as DMF or acetonitrile were elucidated. The mechanistic pathway of the cyclization reaction was postulated and X-ray crystal structure analyses of the resulting 2,2-diphenyl-2,3-dihydro-1H-2-phosphoniaphenalene bromide was performed. 1,8-Bis(diphenylphosphino)naphthalene and α,α′-dibromo-o-xylene afforded—despite unfavorable steric strain—the first dialkylation product of 1,8-bis(phosphino)naphthalene, namely corresponding cyclic bisphosphonium salt. The use of acetonitrile as the solvent was the key for this synthesis. The bromide anions were exchanged in metathesis reaction with hexafluorophosphate anions and the new compound was fully characterized including single crystal X-ray diffraction. The large distance between phosphorus atoms of 3.974 Ǻ clearly demonstrate a strong proximity effect in this hindered bisphosphonium salt.