Room-Temperature Isolation of Palladium(II) Organocarbonyl Intermediates in the Synthesis of Eight-Membered Lactams after Alkyne/CO Sequential Insertions

The dinuclear complexes [Pd(C boolean AND N)(mu-X)](2) (C boolean AND N = C,N-C(Ph)=C(R ')ArCH2CR2NH2-2; Ar = C6H4, C6H2(OMe)(2)-4,5; R = H, Me; R' = Ph, CO2Me, Me; X = Cl, Br), arising from the monoinsertion of internal alkynes into the Pd-C bond of ortho-palladated phentermine or homoveratrylamine, react with CO at room temperature to afford the neutral mononuclear organocarbonyl complexes [Pd(C boolean AND N)X(CO)]. When the reactions with CO are carried out in the presence of TlOTf, the triflato complexes [Pd(C boolean AND N)(OTf)(CO)] are obtained. These organo-carbonyl complexes show an unexpected stability, in spite of containing CO and a sigma-alkenyl ligand in mutually cis positions, and represent real intermediates in the insertion reactions of CO into the Pd-C bond. Indeed, they undergo decomposition under the proper conditions, behaving as CO-releasing molecules or yielding Pd(0) and the corresponding dihydro-3-benzazocinones, resulting from a C-N coupling process. Crystal structures of each type of compound have been determined by X-ray diffraction studies.