Ionic hydrogenation indoles

Imidates, rearrangement of, 14, 1 Imines, additions of allyl, allenyl, propargyl stannanes, 64, 1 additions of cyanide, 70, 1 as dienophiles, 65, 2 synthesis, 70, 1 Iminium ions, 39, 2 65, 2 Imino Diels-Alder reactions, 65, 2 Indoles, by Nenitzescu reaction, 20, 3 by reaction with TosMIC, 57, 3 Ionic hydrogenation, 71, 1 Isocyanides, in the Passerini reaction, 65, 1... 

Ionic hydrogenation is also described for indole, carbazole and furan derivatives (79MI1). However, it is most important for thiophenes since the latter are well known as poisons for most hydrogenation catalysts. 

The ionic hydrogenation of indoles to indolines using EtsSiH/TFA has been described by Fames et al. and utilized by McKenzie, Magnus,Hlasta and Ward, and their coworkers in several different situations (equations 85-89). For the most part, the chemoselectivity and stereoselectivity that are observed with this reagent combination parallel that which was observed with NaBH4/TFA vide supra). 

Pyrroles and indoles can be reduced by ionic hydrogenation1 19. Reduction of indole 1 with various reducing agents showed that ionic hydrogenation is the method of choice for the exclusive diastereoselective formation of the c/v-product 220. This excellent selectivity is surprising, especially since only modest diastcreosclcctivitics arc obtained in the reduction of the corresponding 2.3-dimethylindole [(cisjtrcms) 42 58]21. 

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