Cyclopalladation of 3-methoxyimino-2-phenyl-3H-indoles

The direct cyclopalladation of 3-methoxyimino-2-(4-chlorophenyl)-3H-indole (1a) and 3-methoxyimino-2-phenyl-3H-indole (1b) results in the regioselective activation of the ortho σ[C(sp2, phenyl)-H] bond affording (μ-OAc)2[Pd{κ2-C,N-C6H3-4R-1-(C8H4N-3′-NOMe)}]2 (2) {R = Cl (2a) or H (2b)} that contain a central “Pd(μ-OAc)2Pd” core. Compounds 2a and 2b reacted with triphenylphosphine (in a molar ratio PPh3:2 = 2) giving [Pd{κ2-C,N-C6H3-4R-1-(C8H4N-3′-NOMe)}(OAc)(PPh3)] (3) {R = Cl (3a) or H (3b)}. Treatment of 2a or 2b with a slight excess of LiCl in acetone produced the metathesis of the bridging ligands and the formation of (μ-Cl)2[Pd{κ2-C,N-C6H3-4R-1-(C8H4N-3′-NOMe)}]2 (4) {R = Cl (4a) or H (4b)} with a central “Pd(μ-Cl)2Pd” moiety. The reactions of 4a or 4b with deuterated pyridine (py-d5) or triphenylphosphine gave the monomeric derivatives [Pd{κ2-C,N-C6H3-4R-1-(C8H4N-3′-NOMe)}Cl(L)] with R = Cl or H and L = py-d5 (5) or PPh3 (6). The crystal structure of 6b·1/2CH2Cl2 confirmed the mode of binding of the ligand, the nature of the metallated carbon atom and a trans-arrangement of the phosphine ligand and the heterocyclic nitrogen. Theoretical calculations on the free ligands are also reported and have allowed the rationalization of the regioselectivity of the cyclopalladation process.