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

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The dinuclear complexes [Pd(C∧N)(μ-X)]2 (C∧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∧N)X(CO)]. When the reactions with CO are carried out in the presence of TlOTf, the triflato complexes [Pd(C∧N)(OTf)(CO)] are obtained. These organo-carbonyl complexes show an unexpected stability, in spite of containing CO and a σ-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 ...