Nitro compounds catalysts, rhodium complexes

Secondary amines can be added to certain nonactivated alkenes if palladium(II) complexes are used as catalysts The complexation lowers the electron density of the double bond, facilitating nucleophilic attack. Markovnikov orientation is observed and the addition is anti An intramolecular addition to an alkyne unit in the presence of a palladium compound, generated a tetrahydropyridine, and a related addition to an allene is known.Amines add to allenes in the presence of a catalytic amount of CuBr " or palladium compounds.Molybdenum complexes have also been used in the addition of aniline to alkenes. Reduction of nitro compounds in the presence of rhodium catalysts, in the presence of alkenes, CO and H2, leads to an amine unit adding to the alkene moiety. An intramolecular addition of an amine unit to an alkene to form a pyrrolidine was reported using a lanthanide reagent. 

It is clear from the examples reported that carbon monoxide, when coordinated to a metal in a neutral complex, is not sufficiently activated to react with organic nitro compounds under mild conditions. More precisely, the first act of this reaction is the electron transfer from the metal to the nitro group to give a radical couple and this requires a very basic metal. This explains why basic ligands usually activate transition metal carbonyls in these catalytic reactions. Moreover, basic ligands such as Bipy favor the in-situ formation of the [Rh(CO)4] species from rhodium clusters. The effect of co-catalysts such as halide anions is more subtle, but even the action of these might, at least in part, be directed toward an increase of the electron density of the metal. 

Rhodium-Phosphine Complexes as Homogeneous Catalysts. Hydrogenation of Aromatic Nitro Compounds ... 

The conversion o f the nitro compound with R=R =Me to the isocyanate using a variety o-f palladium halide complexes has also been studied. In a comparative study o-f the rhodiumwith Lewis acids as co-catalysts, the activity and selectivity have been -found to -follow the order (Bu N) fPdCl33-U or VO chlorides > [Rh Rh... 

The palladium catalysed reaction -follows a rate law which is independent on the substrate concentration, but dependent on the CO pressure. In a later work, a AS =-233 J mol " "K, instead o-f -414, has been reported -for this react i on [1843, tor which it has been confirmed a zero order in substrate and first order in each metallic component and in CO pressure. The carbonyl ation of an intermediate complex forming the isocyanate is considered the rate determinin step in the palladium-catalysed reaction. In this work[183j, the oxidative addition of the nitro compound to the catalyst was considered a more likely rate determining step in the case of the rhodium-catalysed reaction. 

A deoxygenation reaction o-f the nitro compound by the metal is also probably involved when tetraphenylporphyrinate and phtalocyaninate derivatives o-f metals such as palladium or rhodium are used as catalysts [169,170]jpossibly -forming dimeric complexes having oxo and imido groups as bridging ligands. 

At 120 °C and 1 atm, AH =18 Kcal mol and AS = -30 cal mol K have been calculated [112]. In nonpolar solvents such as decane the reaction does not proceed. The deoxygenation of the nitro group to nitroso is considered to be the rate determining step of the reaction, since the subsequent carbonylation of the nitroso compounds to give the isocyanate derivative is about ten times faster than the one of the nitro compound in the presence of [Rh(CO)2Cl]2 as catalyst [114, 115], a reaction which is accelerated by M0CI5. The mechanism of this reactions is discussed in more detail in Chapter 6 we mention here that side-on rhodium nitrosoarene complexes were proposed as intermediates. However, which really are the coordination modes of the nitrosoarene to rhodium(I) in this system is still a question to be solved. As a matter of fact, the known arylnitroso complexes of mononuclear rhodium(I) complexes have a nitrogen-o-bonded nitroso group [116], and some of them can catalyse the carbonylation of nitrosobenzene to PhNCO[117]. 

Catalytic systems based on rhodium clusters or [Rh(CO)4] for the carbonylation of nitro compounds to carbamates have been described in Chapter 3. The application of the same or similar systems to the reduction of nitro compounds to anilines have been described in Chapter 4. Initial work has shown that rhodium clusters Rh4(CO)i2 and Rh<5(CO)i6, but even several mononuclear compounds such as Rh(CO)2(acac) (acac = acetylacetonate), are active catalyst precursor for the carbonylation of nitrobenzene to carbamates, when promoted by an heterocyclic nitrogen base [56, 140, 187, 188]. Later, Liu and Cheng and us independently reported that even higher catalytic activities could be obtained by the use of preformed [PPN][Rh(CO)4] (PPN = (PPh3)2N ) [189, 190]. We have also conducted a mechanistic study of the catalytic cycle using this last complex [1, 192] and the initial stage of the reaction has also been reinvestigated by Liu et al. [193]. Since no mechanistic study has been yet reported on the cluster-based systems, we will first discuss the [PPN][Rh(CO)4] system and then draw a comparison with the other systems. 

For nitromesitylene in dichlorobenzene at 120 °C and 1 atm, k = 1.3 10, AH = 18 kcal mol and AS = - 30 cal K. Large substituents in the or//io position of the nitro compound accelerate the reaction, a situation already described in the case of some palladium and ruthenium systems [26, 38,43] (see paragraph 6.3.1. for a discussion of this effect). If other rhodium complexes were used as catalyst precursors, [Rh(CO)2(Cl)]2 was anyway formed by ligand exchange with the molybdenum chloride [207]. 

There are rather few water-soluble hydrogenation catalysts. A number of rhodium complexes derived from ligands such as (PH2PCH2CH2)2NC0C6H4S03" are active in this way. " Catalysts have been developed for hydrogenation of ketonescarboxylic acids/ esters/ nitriles/ and nitro compounds. 

The carbonylation of nitro arenes can lead to amines, isocyanates, carbamates, azo compounds, or ureas.80 Iron, ruthenium, rhodium, or palladium complexes have been used as catalysts. The carbonylation to give isocyanates, for example, involves the steps... 

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