Divergent Behavior in the Cyclopalladation of Phosphorus Ylides and Iminophosphoranes
作者:David Aguilar、Miguel Angel Aragüés、Raquel Bielsa、Elena Serrano、Rafael Navarro、Esteban P. Urriolabeitia
DOI:10.1021/om7002877
日期:2007.7.1
The cyclopalladation of the stabilized iminophosphoranes Ph3PNC(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(mu-Cl)C,N-C6H3(R)C(O)NPPh3-2}}](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/LandL, which gives [Pd(acac)C,N-C6H3(MeO-4)C(O)NPPh3-2}}] (3c), [PdClC,N-C6H3(MeO-4)C(O)NPPh3-2}}(py)] (4c), or [PdC,N-C6H3(MeO-4)C(O)NPPh3-2}}(LandL)]ClO4 (LandL = dppe 5c, bipy 6c, phen 7c). However, Pd(OAc)(2) reacts with the ylides Ph3PCHC(O)C6H4R (R = H 8a, 3-OMe 8b, 2,5-(OMe)(2) 8c) to give the C,C-orthometalated complexes [Pd(mu-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/LandL. The X-ray structures of 2d and 9b have been determined. Unexpectedly, the reaction of Pd(OAc)(2) with the ylide [Ph3PCHC(O)C6H3-2,4-(OMe)(2)] (16) gives the polymer [(mu-Cl)PdC6H4(PPh2CH-C(O)C6H2-3,5-(OMe)(2))-2}-kappa-C,C,C,O)Pd(mu-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 C alpha 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.