Corrado Crotti

Synthesis of heterocycles via palladium-catalyzed carbonylation of ortho-substituted organic nitro compounds in relatively mild conditions

Abstract Carbonylation of o-nitroaniline in the presence of aldehydes catalyzed by the Pd (TMB)2/TMPhen/TMBH catalytic system (TMBH = 2,4,6-trimethyl benzoic acid; TMPhen = 3,4,7,8-tetramethyl-1,10-phenanthroline) at 140–180°C and 20–40 atm of carbon monoxide, gives the corresponding benzimidazoles, with a selectivity ranging from 10 to 80%, strongly depending on the aldehyde used. The main by-product was shown to be the 2(1H) benzimidazolone, derived by direct carbonylation of o-nitroaniline. The same catalytic system has been used in the carbonylation of o-nitrostyrenes at 120–180°C and 20–60 atm of carbon monoxide, obtaining the corresponding indoles with a selectivity generally ranging from 70 to 100%. Interestingly, in the latter case the presence of TMBH was not necessary in order to obtain good conversions and selectivities. On the contrary, the presence of excess TMBH is indispensable when nitrobenzene is catalytically converted to phenyl isocyanate. Moreover, in the catalytic synthesis of indoles no carbonylated products are formed. These observations suggest a catalytic cycle different from what has been proposed in the synthesis of phenyl isocyanate from nitrobenzene, catalyzed by palladium.

[Rh(CO)4]−, [Rh5(CO)15]−, and bimetallic clusters as catalysts for the carbonylation of nitrobenzene to methyl phenylcarbamate

Bimetallic clusters of the general formula [PPN]2[MRh4(CO)15] (M  Fe, Ru, or Os; PPN+ = (PPh3)2N+) are active catalysts in the presence of methanol for the carbonylation of nitrobenzene to methyl phenylcarbamate. The clusters are active even without co-catalyst, but the addition of bipy (2,2′-bipyridine) greatly enhances both rates and selectivities. In the absence of methanol the conversion was much lower, with a low selectivity in phenyl isocyanate. Interchanging Fe, Ru, and Os has a negligible effect on the activity and a moderate effect on the selectivity, with ruthenium being best. However, the monometallic rhodium cluster [Rh5(CO)15]− proved to be both more active and selective, and the monomer [Rh(CO)4]− was found to be even better. This last catalytic system has been optimized with respect to temperature, CO pressure and the bipy/Rh ratio. The influence of solvent was also examined.

Ruthenium carbonyl-catalyzed deoxygenation by carbon monoidde of o-substituted nitrobenzenes. Synthesis of benzimidazoles

Abstract Ru3(CO)12 is a very efficient catalyst for the deoxygeiiation of 2-nitro-N-(phenylmethylene) benzeneamine derivatives to give the corresponding 2-substituted benzinmidazoles, at 220 °C and 50 bar of carbon monoxide. Main byproducts are the corresponding amines. The same benzimidazoles are also obtained starting from o-nitroaniline and the corresponding aldehyde. When N-(2-nitrophenyl)methylene benzeneamine derivatives are used as substrates, the reaction changes dramatically and only traces of the corresponding heterocyclic compounds are obtained, although complete conversion of the starting nitro compounds is achieved.

Synthesis of carbazole by Ru3(CO)12-catalyzed reductive carbonylation of 2-nitrobiphenyl: the crystal and molecular structure of Ru3(μ3-NC6H4-o-C6H5)2(CO)9

Abstract The Ru 3 (CO) 12 -catalyzed reductive carbonylation of 2-nitrobiphenyl 1 in acetonitrile gives carbazole 2 and 2-aminobiphenyl 3 . Compound 3 is the major reaction product in tetrahydrofuran, toluene, toluene-methanol, thiophene and cis -cyclooctene. Some products deriving from intermolecular insertion into the solvent are also found with acetonitrile, thiophene and cis -cyclooctene. By reaction of 2-nitrosobiphenyl 7 with Ru 3 (CO) 12 , the nitrene complex Ru 3 (μ 3 -NC 6 H 4 - o -C 6 H 5 ) 2 (CO) 9 11 has been isolated and its crystal structure has been determined. Compound 11 is orthorhombic, space group P2 1 2 1 2 1 with a = 11.160(4), b = 11.472(2), c = 25.478(6) A, Z = 4, R = 0.019, R w = 0.028, for 3161 independent reflections with I >3σ( I ). The ortho -substituent phenyl ring of the nitrene ligands in compound 11 is free to rotate around the CC bond and thus can assume the proper arrangement to allow insertion of nitrogen into the aromatic CH bond to give carbazole 2 . This possibility has been confirmed by treating 11 with CO (50 bar) at 220°C to obtain 2 and Ru 3 (CO) 12 .

Metal carbonyl catalyzed reductive carbonylation of substituted nttrobenzenes in presence of alkenes as solvents

Abstract Several nitrobenzene derivatives were submitted to reaction with carbon monoxide in alkene solvents in the presence of catalytic amounts of Ru3(CO)12. Ureas and amines were formed in this reaction, and the secondary amine derived from insertion of the nitrene intermediate at the allylic position of the alkene solvent was a secondary product. The influence of the electronic factor of the substituents on the urea vs. amine ratio is discussed.

Chemometric optimization of the ruthenium carbonyl catalysed cyclization of 2-nitrostilbene to 2-phenylindole

A chemometric optimization of the Ru3(CO)12 catalysed deoxygenation of 2-nitrostilbene to 2-phenylindole was carried out. The effects of temperature, CO pressure, amounts of catalyst and substrate on conversion and selectivity were examined by factorial design/response surface methods. The conversion was found to increase on increasing the temperature and decreasing the CO pressure, it assumed a minimum value for medium amounts of catalyst and was almost independent of the amount of substrate. These results were also confirmed using a learning system and were used to develop a mechanism for the reaction. The data suggest two different mechanisms: one based on a Ru(CO)5 catalysed process and another one based on a Ru3(CO)12 catalysed process, which are first and zero order with respect to the substrate, respectively.

Metal Catalysed Deoxygenation Reactions by Carbon Monoxide of Nitroso and Nitro Compounds

In recent years the catalytic carbpnylation of aromatic nitro compounds, ArNO2, has become a very intense field of research. This is due to the fact that a series of industrially important compounds can be obtained from nitro compounds and carbon monoxide in a single step. Probable intermediates in these carbonylation reactions are the corresponding nitroso compounds, ArNO. The interaction of nitroso and nitro compounds with the metal centre has been reviewed and their metal-assisted stoichiometric reactions have been discussed. Their catalytic reactions, with particular emphasis on nitro compounds as substrates, have also been reviewed. Different sections have been thus-devoted to the synthesis of: amines, azo derivatives, Schiff bases, heterocycles (pyridines, quinolines and indoles), isocyanates, carbamates, ureas, N-substituted amides and imides. Some reactions involving aliphatic nitro compounds have also been considered. Particular attention has been paid to the catalytic reactions leading to isocyanates and carbamates, being these derivatives important final products and/or intermediates in the synthesis of pesticides and polyurethanes. The action of homogeneous, and of heterogeneous catalysts as well, has been discussed.

Metal catalyzed deoxygenation by carbon monoxide of o-substituted nitrobenzenes. Synthesis of 1,4-dihydro-2H-3,1-benzoxazin- 2-one derivatives

The reductive carbonylation of o-nitrobenzylalcohols, o-NO2C6H4CR1R2OH [R1  R2  H (1); R1  R2  CH3 (2); R1  H, R2  CH3 (3); R1  H, R2  C6 H5 (4)], catalyzed by ruthenium and palladium-based catalytic systems gives the corresponding 1,4-dihydro-2H-3,1-benzoxazin-2-one derivatives, 1a-4a. Reaction conditions used were 20–60 atm of carbon monoxide and 100–170°C. The palladium catalyst has been shown to be by far superior to the ruthenium catalyst in this reaction as far as selectivity is concerned. By carbonylation of o-nitrophenethylalcohol (5) with the palladium system as catalyst a mixture of the monomeric 5a and dimeric 5b cyclic carbamates has been obtained.

Insertion of carbon dioxide, of Co2-like molecules, and of other unsaturated compounds into the platinum–nitrogen bond of [Pt(PPh3)2(PhNO)]

The compound [Pt(PPh3)2(PhNO)](1) reacts reversibly with carbon dioxide to give [[graphic omitted]}(PPh3)2](2). Other CO2-like molecules readily and irreversibly insert into the platinum–nitrogen bond of (1); with CS2, PhNCO, and PhNCS the corresponding 1 : 1 adducts [[graphic omitted]}(PPh3)2](3), [[graphic omitted]}(PPh3)2](4), and [[graphic omitted]}(PPh3)2](5), respectively have been obtained. Compound (2) is an isomer of the insertion product (6), [[graphic omitted]}(PPh3)2], obtained from the reaction of [Pt(PPh3)2(O2)] with PhNCO. A labile insertion product, [[graphic omitted]}(PPh3)2], (7), was obtained by reaction of (1) with PhCHO, while no reaction was observed between (1) and acetone or diethyl carbonate. However, compound (1) reacts with EtO2C–NN–CO2Et to give the substitution product (8), [[graphic omitted]}(PPh3)2]. The reactions of compound (2) towards mineral acids, organic acids, benzoyl chloride, and sulphonyl chlorides have been studied. The products are identical to those obtained from the corresponding reactions of (1). l.r., 1H, 31P, and 195Pt n.m.r. spectra of the new compounds are reported and discussed.

The reaction of Ru3(μ3-NPh)(μ-DPPM)(CO)8 (DPPM = diphenylphosphinomethane) with alkynes. Crystal structure of Ru2(μ-DPPM)(CO)4− (μ2-η3-HCCPhC(O)NPh)

Abstract The μ3-nitrene cluster Ru3(μ3-NPh)(μ-DPPM)(CO)8 (1) (DPPM = diphenyl-phosphinomethane) reacts with PhCCR (R  H, Ph) to give the binuclear metallapyrrolidone complexes Ru2(μ-DPPM)(CO)4(μ2-η3-RCCPhC(O)NPh) (2, R  H; 3, R  Ph)), which are produced by combination of the alkyne with CO and the nitrene ligand. The structure of 2 has been determined crystallographically (2 is monoclinic, space group P21/n (no. 14) with a 11.749(4), b 28.428(5), c 12.586(3) A, β 109.13(2)°, V 3972(3) A3, Z = 4, R = 0.025 and Rω = 0.027 for 5150 absorption-corrected reflections having I ⩾ 3σ(I)). Compounds 2 and 3 are compared with the analogous derivatives obtained by reaction of Ru3(μ3-NPh)(CO)10 with alkynes.