Chemoselective hydrogenation catalyst

Chauhan BPS, Rathore, J S, Bandoo T. (2004) Polysiloxane-Pd Nanocomposites as recyclable chemoselective hydrogenation catalysts. J Am Chem Soc 126 8493-8500. 

Fig. 1.5 Polysiloxane-PdNPs, generated by reduction of Pd(0Ac)2 with polymethylhydrosiloxane, as recyclable chemoselective hydrogenation catalysts selective reduction of styrene (a) and polysiloxane-PdNP catalyzed reduction of alkenes (b). (Ref [14e] Caudhan group,J. Am. Chem. Soc. 2004, 726, 8493).

Active heterogeneous catalysts have been obtained. Examples include titania-, vanadia-, silica-, and ceria-based catalysts. A survey of catalytic materials prepared in flames can be found in [20]. Recent advances include nanocrystalline Ti02 [24], one-step synthesis of noble metal Ti02 [25], Ru-doped cobalt-zirconia [26], vanadia-titania [27], Rh-Al203 for chemoselective hydrogenations [28], and alumina-supported noble metal particles via high-throughput experimentation [29]. 

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). 

In this chapter we describe a novel, safe and efficient large-scale synthetic approach to tricycle thienobenzazepines. The key steps in the synthesis include a chemoselective hydrogenation of an aryl-nitro functionality in the presence of a 3-bromo thiophene and a subsequent palladium-catalyzed intramolecular aminocarbonylation telescoped sequentially after simple catalyst and solvent exchange. 

Geraniol can be converted into citronellol and menthol over Cu/A1203 under catalytic hydrogenation conditions owing to chemoselective hydrogenation and a three-functional process taking place on the catalyst surface. 

This is a quite remarkable result, as the chemoselective hydrogenation of geraniol over a heterogeneous catalyst has rarely been reported. It can be carried out over platinum containing zeolite (9), over Pt/Al203 modified with carboxylic acids (10), over Ni/diatomaceous earth and alkali hydroxides or carbonates (11) or NiRaney and alkali or alkaline earth metal hydroxides (12), yields never exceeding 85%. 

The synthesis of 2,3,5-trialkylpyrroles can be easily achieved by conjugate addition of nitroalkanes to 2-alken-l,4-dione (prepared by oxidative cleavage of 2,5-dialkylfuran) with DBU in acetonitrile, followed by chemoselective hydrogenation (10% Pd/C as catalyst) of the C-C- double bond of the enones obtained by elimination of HN02 from the Michael adduct. The Paal-Knorr reaction (Chapter 10) gives 2,3,5-trialkylpyrroles (Eq. 4.124).171... 

In summary, the research effort aimed towards active, chemoselective hydrogenations of certain C=0 and C=N bonds have delivered several catalysts that approach the level of activity required for use in the synthesis of alcohols and amines. However, other classes of substrate require considerable additional investigations to be conducted before homogeneous catalysts may be considered for this purpose. 

Ir/tppts catalysts exhibit almost the same selectivity as Ru/tppts in the hydrogenation of a,p-unsaturated aldehydes albeit with approximately 70 times lower rates.485 In sharp contrast to the ruthenium and iridium based tppts catalysts, RhJ tppts complexes catalyse the chemoselective hydrogenation of a,fl-unsaturated aldehydes to the corresponding saturated aldehydes (Figure 14, III).54-485... 

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. 

New conditions for the Baeyer-Villiger oxidation continue to be explored including selenium-catalysed oxidation with aqueous hydrogen peroxide (e.g. 115 to the oxepanone 117 in 95% yield) [01JOC2429] and tin-zeolite as a chemoselective heterogeneous catalyst [01NAT423]. 

Rh and Ir complexes stabilized by tertiary (chiral) phosphorus ligands are the most active and the most versatile catalysts. Although standard hydrogenations of olefins, ketones and reductive aminations are best performed using heterogeneous catalysts (see above), homogeneous catalysis becomes the method of choice once selectivity is called for. An example is the chemoselective hydrogenation of a,/ -unsaturated aldehydes which is a severe test for the selectivity of catalysts. 

Other oxidation/reduction related reactions are also explored with ceria based catalysts. For example, Murugan and Ramaswamy (2007) reported the oxidative dehydrogenation of ethylbenzene on nanocrystal-line ceria using N2O as the oxidant Concepcion et al. (2004) reported the chemoselective hydrogenation of crotonaldehyde catalyzed by Ft on mesostructured Ce02 NPs embedded within layers of Si02 binder. 

Asyimnetric hydrogenation of prochiral ketones is an important method for the preparation of chiral secondary alcohols. Until recently, however, such reactions were limited to substrates with pendent metal binding sites, like /3-keto esters. Many of the catalysts that efficiently hydrogenate C-C double bonds exhibit little or no reactivity with isolated ketones. This discrepancy may be ascribed to the different binding modes of alkenes and ketones, and the chemoselectivity of catalysts for these groups. While substrates with C-C double bonds can form metal... 

Copper chromite (CuCr204) has historically been widely used as a hydrogenation catalyst. Generally because of its low catalytic activity its chemoselectivity is useful, although it does require high temperature and autoclave pressure conditions. It is effective for cleavage of benzylic alcohols, primary and secondary benzylic esters and ethers. Efficient cleavage of benzylamines has also been utilized (equation 29). Other copper salts and copper alloys have found infrequent use. 

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