Abstract
AbstractA novel dihydroazepine‐bridged BIPHEP phosphoramidite ligand with an amino acid moiety in the backbone was synthesized and evaluated in the Rh‐catalyzed asymmetric hydrogenation. The scorpion tail‐like amino acid backbone is capable of hydrogen bond formation and able to shift the rotamer composition of the biphenyl axis with the two scissor‐like arms. Pivaloyl‐l‐valine was studied as chiral selector unit and compared with pivaloylglycine as the achiral reference substance. The enantiomerization barrier of the pivaloylglycine‐modified biphenylamide was determined to be ΔG≠=110 kJ/mol. In the case of pivaloyl‐l‐valine, the (Sax) isomer is thermodynamically favored. Due to the relatively high barrier, the ligand is atropisomeric at room temperature and allows the preparative separation of the stereoisomers. The obtained phosphoramidite ligands were separated by chiral HPLC. For the first eluting rotamer, Rh complex ([Rh(cod)(L)2]BF4) was generated in situ and examined in the enantioselective hydrogenation of 2‐acetamidoacrylate and methyl 2‐acetamido‐3‐phenylacrylate, achieving enantiomeric excesses of up to 94 %.
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