Fluorine... and pi...alkali metal interactions control in the stereoselective amide enolate alkylation with fluorinated oxazolidines (Fox) as a chiral auxiliary: an experimental and theoretical study.
The alpha-alkylation of amide enolates by using a pseudo-C(2) symmetry trans 4-phenyl-2-trifluoromethyloxazolidine (trans-Fox) as a chiral auxiliary occurs with an extremely high diastereoselectivity (>99 % de). The origin of this excellent stereocontrol was investigated by an experimental and theoretical (DFT) study. With this trans chiral auxiliary, both F...metal and pi...metal interactions compete to give the same diastereomer through Re face alkylation of the enolate. A 5.5 kcal mol(-1) energy difference found between the Re face and the Si face attack transition states is consistent with the complete diastereoselectivity that has been experimentally achieved. On the other hand, in the case of the cis chiral auxiliary (cis-Fox) the competition between the F...metal and pi...metal interactions is unfavourable to the diastereoselectivity. In this case, the Re face and the Si face attack transition states were found to be nearly isoenergetic (0.3 kcal mol(-1) difference), which is in good agreement with the very low diastereoselectivity observed.
Sini
Gjergji
G
Laboratoire LPPI, Université de Cergy-Pontoise, 5 Mail Gay-Lussac, Neuville sur Oise, Cergy-Pontoise cedex, France. Gjergji.Sini@u-cergy.fr
Tessier
Arnaud
A
Pytkowicz
Julien
J
Brigaud
Thierry
T
D000478:Alkylation; D005461:Fluorine / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy; D013058:Mass Spectrometry; D008672:Metals, Alkali / Q000737:chemistry*; D008962:Models, Theoretical; D010080:Oxazoles / Q000737:chemistry*; D013237:Stereoisomerism
D008672:Metals, Alkali; D010080:Oxazoles; C064210:oxazolidine; D005461:Fluorine