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

Abstract

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 compounds have the general formula [Pd(RPhen)(COOMe)(2)] (1) (RPhen = 1,10-phenanthroline or one of its substituted derivatives). 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 atmosphere is also required for the reaction to proceed. Acid dimerization was shown not to be influential under the concentration conditions examined, 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 observed under catalytic conditions. A series of equilibria and an irreversible acid-assisted proton transfer explain the observed data. Formation of an adduct between complexes of the kind 1 and CO was spectroscopically observed when RPhen = 2,9-Me(2)Phen. Several analogous complexes were also spectroscopically characterized and the X-ray structure of [Pd(2,9-Me(2)Phen)Cl(2)(CO)] was solved. This shows an asymmetric 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 DeltaS(double dagger) value, whereas no positive effect is observed on DeltaH(double dagger). A temperature-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.

Transition Metal Catalyzed Reductive Cyclization Reactions of Nitroarenes and Nitroalkenes

“…In the latter case, contrary to the reactions described in this review, the reaction proceed through the intermediate formation of anilines and the role of the acidic promoter appears to play at least two different roles. One is to act as a bifunctional promoter, accelerating a proton transfer in the rate‐determining carbonylation of aniline, and is well understood . The other is the increase in the selectivity into carbonylated products by a corresponding decrease of the amounts of azo‐ and azoxyarenes formed.…”

“…It is also worth of note that non‐symmetric phenanthrolines, having inequivalent pyridinic rings, have been found to be better ligands than their corresponding symmetric analogues when the carbonylation reaction of nitrobenzene to methyl phenylcarbamate was investigated . This appears to be due to the enhanced hemilability of the non‐symmetric ligands, which is important in the carbonylation reaction . However, no improvement was observed by using non‐symmetric phenanthroline when they were employed as ligands in a reductive cyclization reaction, indicating that hemilability is not important for these reactions.…”

“…This and a second byproduct may derive from an intermediately formed isocyanate. Carbonylation of simple nitroarenes by a palladium/phenanthroline system is known to give isocyanates catalytically and these are also formed as intermediates under conditions where the finally observed product is a diarylurea or a carbamate …”

Phosgene-free synthesis of 1,3-diphenylurea via catalyzed reductive carbonylation of nitrobenzene

“…The difference in catalytic behavior induced by the various P-P ligands can be largely explained by steric reasoning, assuming that the stabilization of the Pd(0) intermediates and the reoxidation to Pd(II) compounds are crucial points in the mechanism generally accepted, based on the steps depicted in Scheme 1 [29,[46][47][48][49][50][51][59][60][61][62][63]. Initially, in the presence of CO and of the catalyst, the nitro compound generates a metallacycle 1 [26][27][28][29][30]64].…”

One pot synthesis of ureas and carbamates via oxidative carbonylation of aniline-type substrates by CO/O2 mixture catalyzed by Pd-complexes

“…Carbonylation of nitrocompounds [1][2][3][4][5] and amines [6][7][8][9][10] is particularly important in polymer industry -during production of carbamates, isocyanates and ureas. Carbonylation of amines is efficiently catalyzed by square planar palladium(II) complexes with nitrogen donor ligands [11][12][13][14][15][16][17][18][19][20][21] and two directions of this process are possible -the process carried out in alcohols leads to carbamates (Eq.…”