Carbon-phosphorus heterocycles. New route to tetrahydrophosphinolines, tetrahydroisophosphinolines, and related systems via cyclization of alkenyl-substituted phosphonium salts with 115% polyphosphoric acid

Abstract

A convenient method of wide scope for the synthesis of the rare tetrahydroisophosphinolinium and tetrahydrophosphinolinium salts has been developed from readily available starting materials. Treatment of a variety of tertiary phosphines (all containing an aryl group and/or an arylmethyl group) with allylic type halides gave phosphonium salts containing a 3-alkenyl substitutent. Cyclization of these salts occurred in the presence of 115% polyphosphoric acid (PPA) at 160°for 30 min to give the C-P heterocyclic systems in modest to good yields (24-82%). Work-up of the reaction mixtures simply involved addition to ice water. The resulting homogeneous solution was treated with KPFe (saturated aqueous solution) which caused the precipitation of the PF6-salt of the respective isophosphinolinium or phosphinolinium system.NMR, 31P NMR, infrared and mass spectral and elemental analyses support the structures of these phosphorus analogs of the corresponding tetrahydroisoquinoline and tetrahydroquinoline heterocycles. Benzyldiphenylvinylphosphonium bromide (3e) cyclized at 300°with 115% PPA after 1.25 hr to give, after work-up, l,2,3,4-tetrahydro-2,2-diphenylisophosphinolinium hexafluorosphate (4e, 51%). Thus, formation of the six-membered ring was favored over formation of the phospholane ring system. A mechanism is tentatively put forth to involve a rather classic electrophilic substitution in the cyclization process. The role of the anion(s) of PPA is unknown but probably involves direct association with the phosphonium cation prior to addition of KPFe.

Polyphosphoric acid catalyzed cyclization of aralkenyl-substituted quaternary ammonium salts

“…Apparently, protonation of the olefinic bond must have occurred so as to produce a cation at the y carbon (in relation to >N+<) rather than a cation at the alternate ß position. The results imply that (1) a cation intermediate is probably more stable at the y position than a cation at the ß position (hyperconjugative effect2); or (2) protonation is favored at the y position in order to place the positive centers at the most remote positions. Unfortunately, lack of kinetic data with such ammonium salts or with related systems containing sulfur or phosphorus makes predictions extremely difficult.13 An additional consideration must include the >N+< group which, although insulated from the aryl ring by the adjacent methylene, has been reported to deactivate the aromatic ring in electrophilic substitution reactions.14 Nevertheless, the ring closure occurs in our systems.…”

Polycyclic carbon-phosphorus heterocycles

“…A summary of phosphasteroid systems prepared to date was released recently. 90 Intermolecular quaternization of diphosphines with alkyl or aryl dihalides, in certain cases, has provided quite novel cyclic phosphonium salts. By the simple interaction of 2,2-biphen-OHO) + Br<CH2>nBr ylenebis(diethylphosphine) (17) with , -dibromoalkanes containing a trace of methanol in a sealed tube at 100 °C (10-20 h), Mann and co-workers9•11 obtained a number of such salts 18…”