Two systems exist for the asymmetric hydrogenation of 2-substituted quinolines with isolated yields generally greater than 80% and ee values generally greater than 90%. The first is an iridium(I)/chiral phosphine/I system, first reported by Zhou et al.. While the first chiral phosphine used in this system was MeOBiPhep, newer iterations have focused on improving the performance of this ligand. To this end, systems use phosphines (or related ligands) with improved air stability, recyclability, ease of preparation, lower catalyst loading and the potential role of achiral phosphine additives. As of October 2012 no mechanism appears to have been proposed, although both the necessity of I or a halogen surrogate and the possible role of the heteroaromatic N in assisting reactivity have been documented.
The second is an organocatalytic transfer hydrogenation system based on Hantzsch esters and a chiral Brønsted acid. In this case, the authors envision a mechanism where the isoquinoline is alternately protonated in an activating step, then reduced by conjugate addition of hydride from the Hantzsch ester.
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Much of the asymmetric hydrogenation chemistry of quinoxalines is closely related to that of the structurally similar quinolines. Effective (and efficient) results can be obtained with an Ir(I)/phophinite/I system and a Hantzsh ester-based organocatalytic system, both of which are similar to the systems discussed earlier with regards to quinolines.