Conjugate addition of potassium trifluoro(organo)borates 2 to dehydroalanine derivatives 1, mediated by a chiral rhodium catalyst and in situ enantioselective protonation, afforded straightforward access to a variety of protected alpha-amino esters 3 with high yields and enantiomeric excesses up to 95%. Among the tested chiral ligands and proton sources, Binap, in combination with guaiacol (2-methoxyphenol), an inexpensive and nontoxic phenol, afforded the highest asymmetric inductions. Organostannanes have also shown to participate in this reaction. By a fine-tuning of the ester moiety, and using Difluorophos as chiral ligand, increased levels of enantioselectivity, generally close to 95%, were achieved. Deuterium labeling experiments revealed, and DFT calculation supported, an unusual mechanism involving a hydride transfer from the amido substituent to the alpha carbon explaining the high levels of enantioselectivity attained in controlling this alpha chiral center.
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.
In the presence of a rhodium catalyst, unactivated Baylis-Hillman adducts reacted with arylboronic acids to afford trisubstituted alkenes with good yields. This highly efficient reaction (aerobic conditions, low temperature) is believed to proceeds via an unexpected mechanism involving 1,4-addition/beta-hydroxy elimination steps and not pi-allyl type rhodium intermediates.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.