Divergent Behavior in the Cyclopalladation of Phosphorus Ylides and Iminophosphoranes

The cyclopalladation of the stabilized iminophosphoranes Ph₃PNC(O)C₆H₄R (R = H <b>1a</b>, 4-OMe <b>1b</b>, 3-OMe <b>1c</b>, 2-Me <b>1d</b>, 3-Me <b>1e</b>) results in the regioselective activation of the ortho CH bond of the benzamide ring, giving <i>exo</i>-[Pd(μ-Cl){<i>C,N</i>-C₆H₃(R){C(O)NPPh₃-2}}]₂ (R = H <b>2a</b>, 5-OMe <b>2b</b>, 4-OMe <b>2c</b>, 3-Me <b>2d</b>, 4-Me <b>2e</b>). 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 <b>2c</b> with Tl(acac), py, or AgClO₄/L∧L, which gives [Pd(acac){<i>C,N</i>-C₆H₃(MeO-4){C(O)NPPh₃-2}}] (<b>3c</b>), [PdCl{<i>C,N</i>-C₆H₃(MeO-4){C(O)NPPh₃-2}}(py)] (<b>4c</b>), or [Pd{<i>C,N</i>-C₆H₃(MeO-4){C(O)NPPh₃-2}}(L∧L)]ClO₄ (L∧L = dppe <b>5c</b>, bipy <b>6c</b>, phen <b>7c</b>). However, Pd(OAc)₂ reacts with the ylides Ph₃PCHC(O)C₆H₄R (R = H <b>8a</b>, 3-OMe <b>8b</b>, 2,5-(OMe)₂ <b>8c</b>) to give the C,C-orthometalated complexes [Pd(μ-Cl){<i>C,C</i>-[C₆H₄(PPh₂CHC(O)C₆H₄R)-2]}] (R = H <b>9a</b>, 3-OMe <b>9b</b>, 2,5-(OMe)₂ <b>9c</b>), which are also regioselectively obtained. The C,C-metalated chelate is very stable, as shown by the reactions of <b>9b</b> with Tl(acac), PPh₃, and AgClO₄/L∧L. The X-ray structures of <b>2d</b> and <b>9b</b> have been determined. Unexpectedly, the reaction of Pd(OAc)₂ with the ylide [Ph₃PCHC(O)C₆H₃-2,4-(OMe)₂] (<b>16</b>) gives the polymer [(μ-Cl)Pd{<i>C</i>₆H₄(PPh₂<i>C</i>H−C(<i>O</i>)<i>C</i>₆H₂-3,5-(OMe)₂)-2}-κ-C,C,C,O)Pd(μ-Cl)]<i>_n</i> (<b>17</b>) 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.