“Backdoor Induction” of Chirality: Asymmetric Hydrogenation with Rhodium(I) Complexes of Triphenylphosphane-Substituted β‑Turn Mimetics

Bioconjugate bidentate ligands 2–10 were obtained by tethering triphenylphosphanecarboxylic acid to amino acid substituted spacers with different flexibility, ranging from a rigid enediyne-based β-turn inducer to flexible linear aliphatic chains with up to eight carbon atoms. The 21 synthesized ligands revealed up to 81% ee selectivity in rhodium-catalyzed asymmetric hydrogenation of α,β-unsaturated amino acids. The key feature of the catalysts is the prochiral coordination sphere of the catalytic metal while the chirality is transmitted by “backdoor induction” from distant hydrogen-bonded amino acids. DFT calculations were applied to study the structure and relative stability of the precatalytic organometallic Rh(I) complexes, with particular emphasis on hydrogen-bonded secondary structures.