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.

Graphical abstractA study of the cyclopalladation of the 3-methoxyimino-2-phenyl-3H-indole compounds: C6H4-4R-1-(C8H4N-3′-NOMe) {R = Cl (1a) or H (1b)} is reported. The experimental results reveal that the reaction is regioselective giving five-membered palladacycles where 1a or 1b act as bidentate [C(sp2, phenyl), Nindole]− ligands. Theoretical calculations undertaken have allowed us to rationalize the preferential formation of this type of metallacycles.Figure optionsDownload full-size imageDownload as PowerPoint slide