Hydrogenation of Nitrogen-containing Aromatic Ring Systems

5 Hydrogenation of Nitrogen-containing Aromatic Ring Systems 

Multiply substituted pyridines can be hydrogenated under a variety of reaction conditions with high levels of stereocontrol, to give mainly the cis product [32]. This is illustrated by the preparation of enantiomerically pure sedrine and conhydrine by use of platinum catalysts [33]. 

The hydrogenation of indoles is rather difficult and is normally achieved by use of Raney nickel at high temperatures and pressures. Either of the two rings may be saturated, depending on the conditions employed, and on the substitution pattern of the indole. The presence of a substituent on one of the two rings promotes reduction of the other ring. Stereochemical control of this type of reaction is not observed except for tricyclic systems [37]. 

The presence of a stereogenic center influences the stereochemical outcome of the hydrogenation. This effect has been used to prepare a series of enantiomerically pure, naturally occurring pyrrolidizine and indolidizine alkaloids [38]. It seems that a substituent on the non-aromatic ring affects the adsorption side of the pyrrole ring thereby directing the stereochemical outcome of the hydrogenation. 

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