Hydrogenation, catalytic heteroaromatic compounds

Kuwano R, Sato K, Kurokawa T, Karube D, Ito Y (2000) Catalytic asymmetric hydrogenation of heteroaromatic compounds, indoles. J Am Chem Soc 122 7614-7615... 

Because a comprehensive review on the catalytic performance of Josiphos ligands has been published,20 we restrict ourselves to a short overview on the most important fields of applications. Up to now, only the (7 )-(S)-family (and its enantiomers) but not the (R)-(R) diastereoisomers have led to high enantioselectivities (the first descriptor stands for the stereogenic center, and the second stands for the planar chirality). The most important application is undoubtedly the hydrogenation of C = N functions, where the effects of varying R and R1 have been extensively studied (for the most pertinent results see Table 15.5, Entries I—4). Outstanding performances are also observed for tetrasubstituted C = C bonds (Entry 5) and itaconic and dehydroamino acid derivatives (Entries 6 and 7). A rare example of an asymmetric hydrogenation of a heteroaromatic compound 36 with a respectable ee is depicted in Scheme 15.6.10b... 

The use of rhodium-amino acid complexes in catalytic hydrogenation has been reported by Rajca to be capable of reducing a wide variety of aromatic and heteroaromatic compounds under mild conditions (1 atm, 22 °C, DMF 1 atm = 101 kPa). Thus, the rhodium-anthranilic acid catalyzed hydrogenation of pyrrole under these mild conditions yields a 2 1 mixture of pyrrolidine and 2,5-dihydropyrrole (53% conversion after 8 h)." Recently, Lunn found that the hydrogenation of pyrrole can be carried out with a nickel-aluminum alloy, as generated with aqueous KOH, to give pyrrolidine in 58% yield, albeit relatively slowly (4 d, r.t.)."... 

Dehydrogenation is a key reaction in the production of commodity chemicals such as butadiene, styrene and formaldehyde and in the catalytic reforming of petroleum naphtha [1-3], In the fine chemical industry, however, dehydrogenation is used less than the numerous hydrogenation reactions which are available. Dehydrogenation is usually an endothermic reaction which requires high temperatures. For such conditions the chemical stability of many fine chemicals is often insufficient. Most of the dehydrogenation reactions used in fine chemistry yield aromatic or heteroaromatic compounds and aldehydes or ketones. 

This cycle presents some similarities with the monohydride RhH(CO)(PPli3)3 catalyst. Both these catalytic systems are very selective for the hydrogenation of terminal alkenes versus internal or cyclic alkenes, because the secondary alkyl intermediates are formed less readily than the 1-alkyl intermediates because of the steric hindrance of the PPh3 ligands. The RuHCl(PPh3)3 complex is also active for the hydrogenation of alkynes, polynuclear heteroaromatic compounds, and nitro compounds (73). In the latter case, basic reaction conditions increase the reduction rate by deprotonation of the nitro compound to its anionic form according to equation (23). 

Besson M, Pinel C. Diastereoselective heterogeneous catalytic hydrogenation of aromatic or heteroaromatic compounds. Top. CataL 2003 25 43-61. 

With aromatic and heteroaromatic aldehydes,193 4-oxo-6,7,8,9-tetra-hydro-4T/-pyrido[l,2-a]pyrimidines yielded the 9-arylmethylene derivatives,133,266 whereas with glyoxylic acid the 9-carboxymethylene derivative was formed.266,267 The primary addition product (237) could be isolated from the reaction mixture of ethyl 6-methyl-4-oxo-6,7,8,9-tetrahydro-4//-pyrido[l,2-a]pyrimidine-3-carboxylate and benzaldehyde. On dehydration the addition product (237) gave the 9-benzylidene compound.266 The 9-carboxymethylene derivatives may be transformed by catalytic hydrogenation to the 9-acetic acids, which can be esterified.266,267... 

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