Reevaluation of the Mechanism of the Amination of Aryl Halides Catalyzed by BINAP-Ligated Palladium Complexes

Two previous mechanistic studies of the amination of aryl halides catalyzed by palladium
complexes of 1,1‘-binaphthalene-2,2‘-diylbis(diphenylphosphine) (BINAP) are reexamined by the authors
of both studies. This current work includes a detailed study of the identity of the BINAP-ligated palladium
complexes present in reactions of amines with aryl halides and rate measurements of these catalytic
reactions initiated with pure precatalysts and precatalysts generated in situ from [Pd2(dba)3] and BINAP.
This work reveals errors in both previous studies, and we describe our current state of understanding of
the mechanism of this synthetically important transformation. 31P NMR spectroscopy shows that several
palladium(0) species are present in the catalytic system when the catalyst is generated in situ from [Pd2(dba)3] and BINAP, and that at least two of these complexes generate catalytic intermediates. Further,
these spectroscopic studies and accompanying kinetic data demonstrate that an apparent positive order
in the concentration of amine during reactions of secondary amines is best attributed to catalyst
decomposition. Kinetic studies with isolated precatalysts show that the rates of the catalytic reactions are
independent of the identity and the concentration of amine, and studies with catalysts generated in situ
show that the rates of these reactions are independent of the concentration of amine. Further, reactions
catalyzed by [Pd(BINAP)2] with added BINAP are found to be first-order in bromoarene and inverse first-order in ligand, in contrast to previous work indicating zero-order kinetics in both. These data, as well as
a correlation between the decay of bromobenzene in the catalytic reaction and the predicted decay of
bromobenzene from rate constants of studies on stoichiometric oxidative addition, are consistent with a
catalytic process in which oxidative addition of the bromoarene occurs to [Pd(BINAP)] prior to coordination
of amine and in which [Pd(BINAP)2], which generates [Pd(BINAP)] by dissociation of BINAP, lies off the
cycle. By this mechanism, the amine and base react with [Pd(BINAP)(Ar)(Br)] to form an arylpalladium
amido complex, and reductive elimination from this amido complex forms the arylamine.