Rhodium aminopyridines

To extend the concept to combinatorial asymmetric catalysis a new library of chiral aminopyridine and isoquinolone systems equipped with phosphine and phospho-nite donors was prepared and applied to the asymmetric rhodium-catalyzed hydrogenation (Scheme 2.8) [21],... 

The reaction of [Rh2Cl2(CgHi4)4] with 2-aminopyridine led to a species believed to be a cationic solvated rhodium(I)-aminopyridine complex. This was more active than [RhCl(PPh3)3] or [RuH(Cl)(PPh3)3] for the hydrogenation of cyclohexene. The mechanism was thought to involve oxidative addition of hydrogen to rhodium(I) prior to alkene coordination.143... 

Catalytic hydroacylation. Aldimines of 3-methyl-2-aminopyridine and aromatic aldehydes react with chlorotris(triphenylphosphine)rhodium(I) (1) in THF at 55 to afford products of imine C—H insertion (3). Aminals of 2-aminopyridine and aldehydes with a-hydrogens (4) similarly react with 1 to give 5 presumably, the aminals are in equilibrium with the corresponding imines under these conditions. These complexes undergo hydroacylation reactions with ethylene as illustrated for 2. The overall reaction can be performed with catalytic quantities of 1, as indicated for the reaction of 4. 

While Wheland (91) suggests that 4-aminopyridine is an aromatic amine little success has been achieved in hydrogenation. Orthner (92) cites failure with platinum catalysts under a variety of conditions. Very low yield (16.5%) resulted from reduction of the hydrochloride salt with platinum catalyst under 80 atm pressure (93). In the reduction of A7-(4-pyridyl)morpholine where the 4-amino nitrogen atom is tertiary, hydrogenation in alcohol was successful with ruthenium (5), but unsuccessful with rhodium on alumina or platinum oxide in acetic acid. It is possible that the pressure conditions used for reduction with ruthenium catalysts may be conducive to conversion of 4-aminopyridine, since these catalysts are less inhibited by strong nitrogen bases. 

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