The cyclopalladation reaction of 2-phenylaniline revisited

Abstract

2-Phenylaniline reacted with Pd(OAc) 2 in toluene at room temperature for 24 h in a one-to-one molar ratio and with the system PdCl 2, NaCl and NaOAc in a 1 (2-phenylaniline):1 (PdCl 2):2 (NaCl):1 (NaOAc) molar ratio in methanol at room temperature for one week to give the dinuclear cyclopalladated compounds (μ-X) 2[Pd{κ 2- N2 ′, C1 -2-(2′-NH 2C 6H 4)C 6H 4}] 2 [ 1a (X = OAc) and 1b (X = Cl)] in high yield. Moreover, the reaction between 2-phenylaniline and Pd(OAc) 2 in one-to-one molar ratio in acid acetic at 60 °C for 4 h, followed by a metathesis reaction with LiBr, allowed isolation of the dinuclear cyclopalladated compound (μ-Br) 2[Pd{κ 2- N2 ′, C1 -2-(2′-NH 2C 6H 4)C 6H 4}] 2 ( 1c) in moderate yield. A parallel treatment, but using monodeuterated acetic acid (DOAc) as solvent in the cyclopalladation reaction, allowed isolation of a mixture of compounds 1c, 1c d1 [Pd{κ 2- N2 ′, C1 -2-(2′-NH 2C 6H 4)C 6H 4](μ-Br) 2[Pd{κ 2- N2 ′, C1 -2-(2′-NH 2C 6H 4)-3- d-C 6H 3] and 1c d2 (μ-Br) 2[Pd{κ 2- N2 ′, C1 -2-(2′-NH 2C 6H 4)-3- d-C 6H 3}] 2 in moderate yield and with a deuterium content of ca. 60%. 1a and 1b reacted with pyridine and PPh 3 affording the mononuclear cyclopalladated compounds [Pd{κ 2- N2 ′, C1 -2-(2′-NH 2C 6H 4)C 6H 4}(X)(L)] [ 2a (X = OAc, L = py), 2b (X = Cl, L = py), 3a (X = OAc, L = PPh 3) and 3b (X = Cl, L = PPh 3)] in a yield from moderate to high. Furthermore, 1a reacted with Na(acac) · H 2O to give the mononuclear cyclopalladated compound 4 [Pd{κ 2- N2 ′, C1 -2-(2′-NH 2C 6H 4)C 6H 4}(acac)] in moderate yield. 1H NMR studies in CDCl 3 solution of 2a, 2b, 3a, 3b and 4 showed that 2a and 3a presented an intramolecular hydrogen bond between the acetato ligand and the amino group, and were involved in a dynamic equilibrium with water present in the CDCl 3 solvent; and that the enantiomeric molecules of 2b and 4 were in a fast exchange at room temperature, while they were in a slow exchange for 2a, 3a and 3b. The X-ray crystal structures of 3b and 4 were determined. 3b crystallized in the triclinic space group P 1 ¯ with a = 9.9170(10), b = 10.4750(10), c = 12.0890(10) Å, α = 98.610(10)°, β = 94.034(10)° and γ = 99.000(10)° and 4 in the monoclinic space group P2 1/ a with a = 11.5900(10), b = 11.2730(10), c = 12.2150(10) Å, α = 90°, β = 107.6560(10)° and γ = 90°.