Tandem 1,4-Addition/Enantioselective Protonation Catalyzed by Rhodium Complexes: Efficient Access to α-Amino Acids

Conjugate addition of potassium trifluoro(organo)borates 2 (R=aryl, alkenyl) to 2‐acetamidoacrylate 1 mediated by a chiral rhodium catalyst and with in situ enantioselective protonation afforded a variety of α‐amino acids 3 with high yields and ee values up to 90 % (see scheme, cod=cycloocta‐1,5‐diene, binap=2,2′‐bis(diphenylphosphanyl)‐1,1′‐binaphthyl). Among the tested chiral ligands and proton sources, binap in combination with 2‐methoxyphenol (guaiacol), an inexpensive and nontoxic phenol, afforded the highest asymmetric inductions.