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Access to Enantioenriched α-Amino Esters via Rhodium-Catalyzed 1,4-Addition/Enantioselective Protonation
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Abstract
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 α-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 α carbon explaining the high levels of enantioselectivity attained in controlling this α chiral center.
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History
- Published In Issue May 14, 2008
- Article ASAPApril 09, 2008
- Received: November 29, 2007
Accepted: February 21, 2008
ACS
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