Hydrogenation of heterocycles

Catalytic hydrogenation of furan can be effected by Sabatier s method with nickel at 170°. Cloke and Ayers236 effected it at 50° with Raney nickel. Starr and Hixon237 hydrogenated 120 g of furan in absence of solvent in about 20 h at 7 atm, obtaining a 96% yield by using palladium oxide as catalyst. 

Hydrogenation of pyrrole is very difficult. Platinum in glacial acetic acid,240 and rhodium or palladium on asbestos,241 should be considered. Andrews and McElvain used platinum oxide.69 

In general, pyrroles are appreciably more easily reduced when substituted on nitrogen than when unsubstituted. For instance, 1-phenylpyrrole can be smoothly reduced to 1-cyclohexylpyrrolidine 242 Rainey and Adkins243 used Raney nickel for hydrogenation of ethyl 1-pyrrolecarboxylate. 

Schroter, personal communication (see Houben-Weyl, Methoden der organischen Chemie, Georg Thieme Verlag, Stuttgart, 1955, Vol. 4, Part 2, p. 307). 

---

A recent development is the transfer hydrogenation of heterocyclic systems such as pyrrole, pyridinium and quinoline systems. Whilst at present the yields and enantioselectivities are modest, further development may improve this situation. For example, 1-methyl-isoquinoline has been reduced to the tetrahydro species and 1-picoline has been reduced to 1-methylpiperidine [86]. Interestingly, these reductions involve alkene as well as imine reduction. 

Percent cis Product Obtained on Hydrogenation of Heterocycles XXX, XXXI, and XXXII in Neutral and Acidic Media ... 

Hydrogenation of heterocyclic enamines of the alkaloid series can be accomplished either in acetic acid using Adams catalyst185 or, better, with amalgamated Zinc186 (Scheme 129). 

Other conditions for the reduction of the aromatic nucleus are discussed in method 4. The hydrogenation of heterocyclic nuclei is treated in method 554. 

Catalytic Hydrogenation of Heterocyclic Sulfur and Nitrogen Compounds in Raw Oils... 

---