Divergent Behavior in the Cyclopalladation of Phosphorus Ylides and Iminophosphoranes

The cyclopalladation of the stabilized iminophosphoranes Ph₃PNC(O)C₆H₄R (R = H 1a, 4-OMe 1b, 3-OMe 1c, 2-Me 1d, 3-Me 1e) results in the regioselective activation of the ortho CH bond of the benzamide ring, giving exo-[Pd(μ-Cl){C,N-C₆H₃(R){C(O)NPPh₃-2}}]₂ (R = H 2a, 5-OMe 2b, 4-OMe 2c, 3-Me 2d, 4-Me 2e). The palladated ligand behaves as a strong C,N-chelating group and cannot be easily displaced by other chelating ligands. This is clear from the reaction of 2c with Tl(acac), py, or AgClO₄/L∧L, which gives [Pd(acac){C,N-C₆H₃(MeO-4){C(O)NPPh₃-2}}] (3c), [PdCl{C,N-C₆H₃(MeO-4){C(O)NPPh₃-2}}(py)] (4c), or [Pd{C,N-C₆H₃(MeO-4){C(O)NPPh₃-2}}(L∧L)]ClO₄ (L∧L = dppe 5c, bipy 6c, phen 7c). However, Pd(OAc)₂ reacts with the ylides Ph₃PCHC(O)C₆H₄R (R = H 8a, 3-OMe 8b, 2,5-(OMe)₂ 8c) to give the C,C-orthometalated complexes [Pd(μ-Cl){C,C-[C₆H₄(PPh₂CHC(O)C₆H₄R)-2]}] (R = H 9a, 3-OMe 9b, 2,5-(OMe)₂ 9c), which are also regioselectively obtained. The C,C-metalated chelate is very stable, as shown by the reactions of 9b with Tl(acac), PPh₃, and AgClO₄/L∧L. The X-ray structures of 2d and 9b have been determined. Unexpectedly, the reaction of Pd(OAc)₂ with the ylide [Ph₃PCHC(O)C₆H₃-2,4-(OMe)₂] (16) gives the polymer [(μ-Cl)Pd{C₆H₄(PPh₂CH−C(O)C₆H₂-3,5-(OMe)₂)-2}-κ-C,C,C,O)Pd(μ-Cl)]_n (17) as a result of a double palladation, giving two types of metalacycles:  in one of them, the Pd atom is bonded to the ylidic Cα atom and has activated an ortho C(Ph)−H bond of the PPh₃ group; in the other one, the Pd atom is bonded to the carbonyl oxygen and has activated an ortho C−H bond of the C₆H₃(OMe)₂ unit. This tetradentate ylide ligand is remarkably stable.