Megatherium (Stranger)
12-16-02 23:54
No 390079
      Hydrogenation of cyclic unsaturated oxyphosphorane  Bookmark   

This thread involves the synthesis of P2P, starting from propiophenone.

According to Org. React.(1949), vol 5, p 331 propiophenone can be oxidized by SeO2, yielding 1-phenyl-1,2-propanedione in 60 % yield.

The alpha diketone can be converted to its corresponding cyclic unsaturated oxyphosphorane by mixing equimolar amounts of trimethyl phosphite (which is distilled before use) and 1-phenyl-1,2-propanedione at room temperature.
Ref.: JACS (1960) vol 82 p 2652

The oxyphosphorane can be reduced in quantitative yield to P2P (steric & electronic factors prevent the formation of P1P).

Procedure:

Hydrogenation was accomplished at 1 atm H2 over reduced PtO2.  Cyclohexane, ethyl acetate or benzene, 5-10 % in oxyphosphorane, were used as solvents.  The samples typically absorb 1 molar equivalent of H2 within 2 - 6 hours, at which time H2 uptake had slowed substantially.  Hydrogenation was terminated at this point.  The reaction mixture was filtered to remove the catalyst.  The yield was quantitative with no other products detectable.  The product could be isolated by column chromatography over silica gel with ether eluent.

Ref.: JOC (1976) vol 41 n 17 p 2928