Nitro ligands metal complex catalysts

Transition metal complexes having phosphorous donor ligands have also been used as catalysts in the carbonyl ation of aromatic nitro compounds to carbamates. 

Asymmetric synthesis of tricyclic nitro ergoline synthon (up to 70% ee) is accomplished by intramolecular cyclization of nitro compound Pd(0)-catalyzed complexes with classical C2 symmetry diphosphanes.94 Palladium complexes of 4,5-dihydrooxazoles are better chiral ligands to promote asymmetric allylic alkylation than classical catalysts. For example, allylic substitution with nitromethane gives enantioselectivity exceeding 99% ee (Eq. 5.62).95 Phosphi-noxazolines can induce very high enatioselectivity in other transition metal-catalyzed reactions.96 Diastereo- and enantioselective allylation of substituted nitroalkanes has also been reported.9513... 

The hydroxylation reaction of phenol with hydrogen peroxide and zeolite encapsulated MePc has received considerable attention. With the perchlorinated phthalocyanine (ClnPc) and tetra-nitro ((N02)4Pc) substituted ligands, catalysts with superior activity have been obtained [32], Such catalysts have been prepared via the zeolite synthesis method around the individual complexes. With the former more bulky complex only the slimmer hydroquinone (HQ) has been obtained, while with the encapsulated perchloroPc equal ratios of catechol (CAT) and the para-isomer have been obtained (see table). The unsubstituted Pc in zeolite Y both with Co and Cu as metallating ion, show an excess of the ortho-isomer (CA T) [32J, corresponding to the approximate thermodynamic ratio. This points to the critical importance of the available space close to the encapsulated Pc as selectivity determining parameter when there is more space, the catalyst yields more catechol. 

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. 

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. 

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