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Divergent Behavior in the Cyclopalladation of Phosphorus Ylides and Iminophosphoranes

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posted on 03.04.2020, 15:11 by David Aguilar, Miguel Angel Aragüés, Raquel Bielsa, Elena Serrano, Rafael Navarro, Esteban P. Urriolabeitia
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.