Mechanistic Study of the Palladium-Phenanthroline Catalyzed Carbonylation of Nitroarenes and Amines: Palladium-Carbonyl Intermediates and Bifunctional Effects

Palladium-phenanthroline complexes catalyze both the nitroarene carbonylation reaction and the amine oxidative carbonylation reaction to give, depending on the conditions, carbamates and ureas. There is evidence that the key step in both processes is the amine carbonylation. Here, we show that when the reaction is run in methanol key intermediate compds. have the general formula [Pd(RPhen)(COOMe)2] (1) (RPhen=1,10-phenanthroline or one of its substituted derivs.). The kinetics of the reaction of 1 with toluidine in the presence of a carboxylic or phosphorus acid is first-order with respect to complex, acid, and toluidine. A CO atm. is also required for the reaction to proceed. Acid dimerization was shown not to be influential under the concn. conditions examd., but reaction between the acid and toluidine is not negligible and a correction has to be applied. Diphenylphosphinic acid is more effective than any carboxylic acid in promoting this reaction, as also obsd. under catalytic conditions. A series of equil. and an irreversible acid-assisted proton transfer explain the obsd. data. Formation of an adduct between complexes of the kind 1 and CO was spectroscopically obsd. when RPhen=2,9-Me2Phen. Several analogous complexes were also spectroscopically characterized and the X-ray structure of [Pd(2,9-Me2Phen)Cl2(CO)] was solved. This shows an asym. coordination of the nitrogen ligand. Kinetic measurements were also conducted under catalytic conditions. An Eyring plot shows that the effect of the acidic promoter is to decrease the Delta S value, whereas no pos. effect is obsd. on Delta H. A temp.-dependent correction for the reaction between the acid and aniline and phenanthroline present under the reaction conditions has to be applied. Comparison of the results obtained under stoichiometric and catalytic conditions strongly supports the view that 1 is involved even in the latter and that the acid is acting as a bifunctional promoter.