Aromatic chemoselective hydrogenation

The. selective hydrogenation of a nitro group in the presence of other reactive functionalities is a frequently encountered problem in fine chemicals manufacture. Ciba-Geigy (Novartis). scientists developed, in collaboration with a catalyst manufacturer, a new Pt/Pb on CaCO. catalyst that allows the chemoselective hydrogenation of an aromatic nitro group in the presence of C=C, C=0, C=N as well as Cl or Br substituents in selectivities > 95% (even C C groups react very slowly) (Bader et al., 1996). Eqn. (3) shows an example (Bader eJ a/., 1996). 

CHEMOSELECTIVE HYDROGENATION OF AROMATIC CHLORONITRO COMPOUNDS WITH AMIDINE MODIFIED NICKEL CATALYSTS. 

Amidine derivatives are effective dehalogenation inhibitors for the chemoselective hydrogenation of aromatic halonitro compounds with Raney nickel catalysts. The best modifiers are unsubstituted or N-alkyl substituted formamidine acetates and dicyandiamide which are able to prevent dehalogenation even of very sensitive substrates. Our results indicate that the dehalogenation occurs after the nitro group has been completely reduced i.e. as a consecutive reaction from the halogenated aniline. A possible explanation for these observations is the competitive adsorption between haloaniline, nitro compound, reaction intermediates and/or modifier. The measurement of the catalyst potential can be used to determine the endpoint of the desired nitro reduction very accurately. 

Electrocatalytic hydrogenation has the advantage of milder reaction conditions compared to catalytic hydrogenation. The development of various electrode materials (e.g., massive electrodes, powder cathodes, polymer film electrodes) and the optimization of reaction conditions have led to highly selective electrocatalytic hydrogenations. These are very suitable for the conversion of aliphatic and aromatic nitro compounds to amines and a, fi-unsaturated ketones to saturated ketones. The field is reviewed with 173 references in [158]. While the reduction of conjugated enones does not always proceed chemoselectively at a Hg cathode, the use of a carbon felt electrode coated with polyviologen/Pd particles provided saturated ketones exclusively (Fig. 34) [159]. 

The highly efficient catalytic system for the chemoselective transfer hydrogenation of aldehydes was reported by Xiao et al. [52]. This system consisted of [Cp IrCl2]2 (1), a diamine and HCOONa, and worked on water and in air. A wide range of aromatic aldehydes were reduced to the corresponding primary alcohols in a highly chemoselective manner some representative examples are summarized in Table 5.9. 

In 2003, Velusamy and Punniyamurthy reported on a copper(II)-catalyzed C—H oxidation of alkylbenzenes and cyclohexane to the corresponding ketones with 30% hydrogen peroxide (Scheme 131). The reaction was catalyzed by the copper complex 192a depicted in Scheme 131 and yields were high in the case of alkylbenzenes (82-89%) whereas cyclohexanone was obtained with a low yield of 18%. Chemoselectivity was very high in every case neither aromatic oxidation nor oxidation at another position of the alkyl chain was observed. 

The photolytic cleavage of alkyl aryl sulfoxides has been shown to occur via initial C—S bond homolysis, in accordance with the common mechanistic assumption. Secondary and tertiary alkyl groups show high chemoselectivity for alkyl C—S cleavage. Uniquely, alkene products have been isolated, formed by disproportionation of the initial alkyl radical, with the formation of benzaldehyde and racemization of primary alkyl compounds. An investigation into the photochemical conversion of N-propylsulfobenzoic imides into amides in various solvents revealed a solvent dependence of the observed mechanism. In ethanol, sulfur dioxide extension forms a biradical which abstracts a hydrogen atom from the solvent, whereas in aromatic solvents biradical formation by a single electron transfer is implicated. The photolysis and thermolysis of l,9-bis(alkylthio)dibenzothiophenes and /7-aminophenyl disulfide have been studied. 

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