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

Abstract

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(sp 2, phenyl)-H] bond affording (μ-OAc) 2[Pd{κ 2- C,N-C 6H 3-4R-1-(C 8H 4N-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 PPh 3: 2 = 2) giving [Pd{κ 2- C,N-C 6H 3-4R-1-(C 8H 4N-3′-NOMe)}(OAc)(PPh 3)] ( 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{κ 2- C,N-C 6H 3-4R-1-(C 8H 4N-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- d 5 ) or triphenylphosphine gave the monomeric derivatives [Pd{κ 2- C,N-C 6H 3-4R-1-(C 8H 4N-3′-NOMe)}Cl(L)] with R = Cl or H and L = py- d 5 ( 5) or PPh 3 ( 6). The crystal structure of 6b·1/2CH 2Cl 2 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.