New aspects of the carbonylation of allylpalladium complexes

The carbonylation of (η3-allyl)palladium(II) chloride dimer in the presence of an excess of ylide, such as Ph3PC(H)COR (R = Me or Ph) (Pd:ylide = 1:5) in MeOH or EtOH, at a CO pressure of 4 atm at room temperature occurs with reduction of the palladium(II) complex to palladium metal and with formation of the corresponding alkyl 3- butenoate with a high yield. The ylide does not give rise to any carbonylation product. When the carbonylation is carried out in the presence of PPh3 (Pd : PPh3 = 1 : 2–3), there is also formation of the unsaturated ester, although in lesser amount, together with [Pd3(PPh3)n(CO)3] (n = 3 or 4) or [Pd(PPh3)3(CO)] and trans-[Pd(PPh3)2(COOR)Cl] (R = Me or Et). These products also form when the carbonylation is carried out in the presence of NEt3 or PrCOONa, in place of the ylide, and of PPh3. It has also been found that [Pd(PPh3)2Cl2] reacts in MeOH or EtOH at a CO pressure of 4 atm at ambient temperature in the presence of an excess of ylide to give the corresponding carbalkoxy complextrans-[Pd(PPh3)2(COOR)Cl]. These findings suggest that the ylide probably promotes formation of carbalkoxy species, as do NEt3 or PrCOONa because the ylide can behave as a base (pKa ⋍7). They are strong support for the suggestion that the carbonylation of (allyl)palladium complexes occurs via a (carbalkoxy)palladium species.