Fabio Ragaini

Catalytic reductive carbonylation of organic nitro compounds

Preface. List of Abbreviations. 1. Introduction. 2. Synthesis of Isocyanates. 3. Synthesis of Carbamates and Ureas. 4. Synthesis of Other Non-Cyclic Compounds. 5. Synthesis of Heterocycles. 6. Mechanistic Studies. Index.

Carbonylation of nitrobenzene to phenyl isocyanate and methyl carbamate catalyzed by palladium and rhodium activated by chelating nitrogen donor ligands

Abstract During our studies on the reductive carbonylation of nitrobenzene, we have found that palladium metal supported on alumina is an effective catalyst for the formation of phenyl isocyanate, PhNCO, when activated by ortho-phenanthroline or its derivatives, and by 2,4,6-trimethylbenzoic acid (TMBA) (T = 180 °C, P(CO) = 40 atm, in dry benzene). In similar conditions (T = 200 °C, P(CO) = 100 atm in toluene) rhodium metal showed very poor activity. However, when methanol was also present in the reaction medium, N-phenyl methyl carbamate was obtained in good yields and selectivities. The presence of TMBA was not required in the latter case. In both cases, at the end of the reaction, the catalytic solution showed the presence of metal complexes, suggesting the formation in situ of homogeneous catalytic systems. In fact, preformed Pd(O2CC6H2-2,4,6-Me3)2 and Rh4(CO)12 gave even better results when used as catalysts in the same experimental conditions.

Mechanistic studies of palladium-catalysed carbonylation reactions of nitro compounds to isocyanates, carbamates and ureas

Abstract Many different palladium-based catalytic systems have been reported for the carbonylation reactions of organic nitro compounds to isocyanates, carbamates and ureas. Almost all of these can be roughly divided into three groups: (i) those containing a second (or even a third) metal (usually a Lewis acid or a metal oxo compound or both), (ii) those in which phenanthroline or similar chelating nitrogen ligands are used and (iii) those in which monodentate phosphines are employed as ligands. The systems in which chelating phosphines are used as ligands lie in between the last two groups. The reaction mechanisms for the catalytic systems in each group appear to be related. Most of the information available does not derive from strictly mechanistic studies, but rather from synthetic studies and it is here critically analysed and compared with the information obtained from other related fields.

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.

[Ru(TPP)CO]‐Catalysed Intramolecular Benzylic CH Bond Amination, Affording Phenanthridine and Dihydrophenanthridine Derivatives

Shedding light on azides: [Ru(TPP)CO] (TPP=tetraphenyl porphyrin dianion), white light and O(2) were found to be a suitable catalyst combination to perform the annulation of several biaryl azides. The high chemoselectivity of the process allows the synthesis of phenanthridines and dihydrophenanthridines in good yield and purity.

Rearrangement of N-aryl-2-vinylaziridines to benzoazepines and dihydropyrroles: a synthetic and theoretical study.

Herein we report the one-pot synthesis of several N-heterocyclic compounds by rearrangement reactions of N-aryl-2-vinylaziridines. The optimization of the synthetic methodology employed allowed us to obtain differently substituted 2,5-dihydro-1H-benzo[b]azepines in good yields and purities. The relationship between the nature of the starting N-aryl-2-vinylaziridine and the obtained N-heterocycle was also investigated. Finally, to rationalize all the experimental results reported in this paper a theoretical study was performed that casts light on the reaction mechanism.

Investigation of the reactivity of palladium(0) complexes with nitroso compounds: relevance to the palladiumphenanthroline-catalysed carbonylation reactions of nitroarenes

Abstract The electron transfer reaction between palladium(0) complexes and RNO compounds afforded different palladium species depending on the aromatic or aliphatic nature of R. When R=Ph a paramagnetic palladium complex 1 was isolated, whereas if R=Bu t the palladium enolate complex 2 was the unexpected reaction product. Complex 1 reacted with methanol and CO to yield Pd(phen){C(O)OCH 3 } 2 3 , which was characterised by single-crystal X-ray structure determination. Compound 3 is a probable intermediate in the reductive carbonylation reaction of organic nitro compounds catalysed by palladium complexes. Nitrobenzene is in fact carbonylated to PhNHCO 2 Me, by using 3 as a very efficient catalyst.

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.

Mechanistic study of the Ru3(CO)12/chloride catalyzed carbonylation reactions of nitroarenes to carbamates and ureas; the role of the alkylammonium cation

Abstract The effect of the chloride countercation on the mechanism of the Ru 3 (CO) 12 /chloride catalyzed carbonylation of nitroarenes to carbamates has been investigated. The reason for the higher activity and selectivity obtained with tetraethylammonium chloride with respect to [PPN][Cl] is due to the higher igroscopicity of the former (only when no aniline is added) and to its ability do decompose to yield triethylamine. The role of this last compound is twofold. On one hand, it accelerates the alcoholysis of the intermediately formed diarylurea. On the other, it favors a reaction pathway that consumes aniline together with nitrobenzene, thus converting a by-product into the desired product.