A Study on P-C Bond Activation Reaction Catalyzed by a Transition-Metal Complex

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Various alpha-ketophosphonates, RC(O)P(O)(OR')_2, are catalytically decarbonylated to give phosphonates, RP(O)(OR')_2, on treatment with PdR_2L_2(R = Me, Et ; L = tertiary phosphine), Pd(PPh_3)_4, or Pd(PCy_3)_2. Crossover experiments of two kinds of alpha-ketophosphonates revealed that a metathesis reaction at a P-C bond of an alpha-ketophosphonate takes place prior to the decarbonylation in the presence of a catalytic amount of palladium complex(R^1C(O)P(O)(OR^2)_2 + R^3C(O)p(O)(OR^4)_2 <double arrow> R^1C(O)P(O)(OR^4)_2 + R^3C(O)P(O)(OR^2)_2. The reaction of trans-PdEt_2(PMe_3)_2 with RC(O)P(O)(OR')_2 for a several minutes in a toluene reflux condition affords transPd(PMe_3)_2(C(O)R)(P(O)(OR')_2) as yellow crystals. The isolated palladium complexes yield the corresponding phosphonate in toluene at the reflux temperature and also serve as a catalyst of the decarbonylation and metathesis reactions of alpha-ketophosphonates. The overall catalytic decarbonylation reaction is proposed to consist of oxidative addition of RC(O)P(O)(OR')_2 at a P-C bond to a "PdL_2" species generated from PdR_2L_2 or PdL_4 to give trans-PdL_2(C(O)R)(P(O)(OR')_2), followed by decarbonylation of the C(O)R ligand to give PdL_2R(P(O)(OR')_2) and the subsequent reductive elimination to give RP(O)(OR')_2 with regeneration of a "PdL_2" species. The mechanism of the alpha-ketophosphonate metathesis reaction is also discussed.