Michela Gasperini

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 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.

Reduction of nitrobenzene to aniline by CO/H2O, catalysed by Ru3(CO)12/chelating diimines

Abstract Chelating diimines of the kind bis(arylimino)acenaphtene (Ar-BIAN) and bis(phenylimino)phenanthrene (Ph-BIP) are very effective promoters for the Ru 3 (CO) 12 catalysed reduction of nitroarenes to anilines by CO/H 2 O. Their promoting efficiency for this catalytic system is higher than the one of any previously reported ligand. The corresponding quinones are also promoters for the same reaction and a comparison between several ligands under the same experimental conditions is reported. The reaction can be performed without any other solvent except water, but yields are better if ethanol is also added. The reduction is chemoselective for the nitro group with respect to olefins and keto groups.

Carbonylation of nitrobenzene to N-methyl phenylcarbamate catalyzed by palladium-phenanthroline complexes Bifunctional activation by anthranilic acid

The palladium–phenanthroline catalyzed carbonylation reaction of nitrobenzene to methyl phenylcarbamate is known to be accelerated by both the addition of aniline and a carboxylic acid. Here, we report that combining the acidic and amino function in the same molecule, 2-NH2C6H4COOH, anthranilic acid, an higher activity is observed with respect to the use of simple benzoic acid. The 4-amino isomer does not show the same increased activity.

Stability-inducing strain: application to the synthesis of alkyl-BIAN ligands (alkyl-BIAN = bis(alkyl)acenaphthenequinonediimine)

Ring strain is normally associated with increased reactivity and decreased stability of the strained molecule. However, we report here some examples in which the presence of a strained ring causes a stabilization of the molecule, allowing the isolation of some members of a class of otherwise unstable compounds. Alkyl-BIAN (alkyl-BIAN = bis(alkyl)acenaphthenequinonediimine) ligands have been elusive for 70 years. We have investigated the reason for earlier failures and identified it as an isomerization of the initially formed CN double bond. This isomerization is driven by a release of ring strain in the five-membered ring of the acenaphthene moiety. The use of amines in which the –NH2 group is bound to a quaternary carbon atom cannot be employed to avoid the isomerization because these amines are too sterically encumbered to react at all. However, the use of amines in which the amino group is bound to a strained ring solves the problem, because the isomerization would cause an even larger strain than the one that is released. Cyclopropylamine (Cypr-NH2) is the ideal amine, no isomerization being observed at all. Cyclobutylamine (Cybu-NH2) can also be employed, as well as amines in which the strain derives from the presence of a bi- o tri-cyclic system: 2-amino-exo-norbornane (Norb-NH2) and 2-aminoadamantane (Ad-NH2). The best synthetic procedure involves a transimination reaction from a [ZnCl2(Ar-BIAN)] complex, where Ar contains electron-withdrawing groups, but the direct synthesis from acenaphthenequinone and the amine is also possible in the case of Cypr-BIAN. The structure of [Pd(Cypr-BIAN)(η3-CH2C(CH3)CH2)][PF6], [ZnCl2(Cybu-BIAN)], [ZnCl2(Norb-BIAN)] and [NiBr2(Ad-BIAN)], has been determined by X-ray diffraction. Preliminary data indicate that Cypr-BIAN is a much stronger ligand than any Ar-BIAN compound.

Using ring strain to inhibit a decomposition path: first synthesis of an Alkyl-BIAN ligand (Alkyl-BIAN = bis(alkyl)acenaphthenequinonediimine).

N-Alkyl imines of acenaphthenequinone are not stable because an isomerization occurs that releases part of the ring strain of the initially formed imine by changing the hybridization of one of the ring carbon atoms from sp(2) to sp(3); however, if an even more strained ring is present in the alkyl group, the isomerization becomes unfavorable and the compound is stable.