Hydrogenation of polynuclear N-heterocycles

Several homogeneous systems have been reported to catalyze the hydrogenation of polynuclear N-heterocycles efficiently the regioselective reduction of the N-containing ring in quinoline, isoquinoline, indole, benzoquinolines, acridine and other related molecules can be achieved with relative ease and under moderate conditions of temperature and pressure. A number of metals are known to be active (Cr, Mo, W, Mn, Fe, Ru, Os, Co, Rh, Ir) but most of the published work has concentrated on ruthenium and rhodium precursors [95-100] as shown in Table 3.3. 

The RhCl2(DMF)(py)2(BH4) system reported by Jardine and McQuillin [94] and described in the previous section, is also capable of hydrogenating quinoline under mild conditions. Surprisingly, this catalyst, discovered 30 years ago has not been further exploited or studied since then. Metal carbonyls were also found to display some hydrogenation activity for a variety of polynuclear N-heterocycles under Hj, H2/CO or CO/HjO [49, 50, 61] the heterocyclic ring was regioselectively hydrogenated in all cases were activity was observed, but conditions were rather harsh and turnover frequencies relatively low. This makes this type of catalyst unattractive for either practical uses or mechanistic studies. 

Other ligand environments (e.g. cyclopentadienyl or phosphine metal complexes) have proved more adequate for detailed investigations of the hydrogenation of N-heterocycles. The groups of Fish [91], Sanchez-Delgado [99] and Rosales [100] have provided extensive studies including kinetic data, deuteration experiments, detailed NMR studies, isolation of important intermediates, and some theoretical calculations. 

The mechanism of quinoline hydrogenation by use of [Cp Rh(NCMe),] (Adapted from ref 91). 

The mechanism of Fig. 3.10 is easily adapted to explain the hydrogenation of pyridine mentioned in Section 3.3.2.1. as well as that of other polynuclear N-heterocycles. that are reduced by this catalyst at rates that decTease with increasing basicity and steric hindrance at the nitrogen atom  

---