Hydrogen cationic amino acids

The dynamic behavior of the model intermediate rhodium-phosphine 99, for the asymmetric hydrogenation of dimethyl itaconate by cationic rhodium complexes, has been studied by variable temperature NMR LSA [167]. The line shape analysis provides rates of exchange and activation parameters in favor of an intermo-lecular process, in agreement with the mechanism already described for bis(pho-sphinite) chelates by Brown and coworkers [168], These authors describe a dynamic behavior where two diastereoisomeric enamide complexes exchange via olefin dissociation, subsequent rotation about the N-C(olefinic) bond and recoordination. These studies provide insight into the electronic and steric factors that affect the activity and stereoselectivity for the asymmetric hydrogenation of amino acid precursors. 

Based on these preliminary findings, related couplings to pyruvates and iminoacetates were explored as a means of accessing a-hydroxy acids and a-amino acids, respectively. It was found that hydrogenation of 1,3-enynes in the presence of pyruvates using chirally modified cationic rhodium catalysts delivers optically enriched a-hydroxy esters [102]. However, chemical yields were found to improve upon aging of the solvent 1,2-dichloroethane (DCE), which led to the hypothesis that adventitious HC1 may promote re-... 

The formation of intramolecular hydrogen bonds can also anchor specific conformations of the amino acid cation-radicals. Using a combination of resonant two-photon laser ionization and... 

---