Palladium/Norbornene-Cocatalyzed Ortho-Amination of Aryl Iodides: A Systematic Elucidation of the Mechanisms and Roles of Cs2CO3 Base via Computational and Mass Spectrometric Approaches

The Catellani-type palladium/norbornene-cocatalyzed ortho-amination of aryl halides has been widely applied. Although a Pd(0)/Pd(II)/Pd(IV) mechanism is generally adopted, some key issues remain to be addressed. Combined mass spectrometry (MS) and theoretical study have been carried out, and the following observations and conclusions are obtained: (1) MS studies detected a Cs-containing PdIV complex (m/z = 752.0947) to confirm the existence of PdIV intermediate; (2) density functional theory (DFT) calculations indicate that 2-iodotoluene is more reactive than morpholine–OBz in Pd0(PPh3)2 oxidative addition to form PdII intermediate; (3) for the PdII aryl-NBE-palladacycle (ANP) species, Cs2CO3 as a ligand coordinates better than PPh3 by computation; (4) with the involvement of Cs2CO3, PdII oxidative addition into morpholine–OBz to form a PdIV intermediate becomes favored over PdII oxidative addition into 2-iodotoluene and C(sp2)–C(sp3) reductive elimination; and (5) the base effects of Cs2CO3 and K2CO3 are also compared, which are in agreement with the experimental findings that the latter results in a much lower yield.