Mechanistic studies of the palladium-catalyzed amination of aryl halides and the oxidative addition of aryl bromides to Pd(BINAP)(2) and Pd(DPPF)(2):An unusual case of zero-order kinetic behavior and product inhibition

Mechanistic studies of the amination of aryl bromides catalyzed by palladiu m complexes containing the chelating phosphines BINAP and DPPF are reported . The coupling of primary alkyl- and arylamines, secondary cyclic alkylamin es, and secondary arylalkylamines with bromoarenes in the presence of stoic hiometric base and Pd(BINAP)2 (1a) as catalyst, and the reaction of aniline with 4-Br-C6H4-t-Bu in the presence of base catalyzed by Pd(DPPF)(2) (2), were studied. The stoichiometric oxidative additions of PhBr to la and to 2 were turnover limiting, and kinetic studies were also conducted on this in dividual step. The stoichiometric oxidative addition of PhBr to la showed a n inverse first-order dependence on added ligand when the PhBr concentratio n was low but depended solely on the rate of chelating Ligand dissociation at high [PhBr]. There was no measurable solvent effect. In addition, the ra tes were indistinguishable in the presence and in the absence of amines and salts that are present in the catalytic amination reactions. Similar quali tative data for the oxidative addition of PhBr to 2 was obtained by H-1 NMR spectroscopy. The observed rate constants for the overall amination reacti ons catalyzed by la were shown to be zero order in aryl halide, amine, base , and added ligand, while they were first order in catalyst. These data ind icated that the kinetic behavior of the overall reaction was dictated solel y by the rate of ligand dissociation from la, as observed for the oxidative addition. When secondary amines were used deviation from this behavior was observed. This anomalous behavior resulted from decay of catalyst rather t han a change in the turnover-limiting step. A catalyst decomposition pathwa y that involves backbone P-C bond cleavage of the chelating bisphosphine Li gands was revealed by the stoichiometric oxidative addition studies. Quanti tative rate data were also obtained for reaction of 4-Br-C6H4-t-Bu with ani line in the presence of base catalyzed by 2. The observed rate constants we re zero order in amine and base, inverse first order in added ligand, and f irst order in aryl bromide. At low concentration of added ligand, the react ion appeared to be first order in amine. However, this deviation from the e xpected behavior was due to reversible reaction of the catalyst with produc t.