Kinetic and computational studies of the composition and structure of activated complexes in the asymmetric deprotonation of cyclohexene oxide by a norephedrine-derived chiral lithium amide†
2001; Royal Society of Chemistry; Issue: 9 Linguagem: Inglês
10.1039/b101657l
ISSN1472-779X
AutoresDaniel Pettersen, Mohamed Amedjkouh, Sten O. Nilsson Lill, Kristian Dahlén, Per Ahlberg,
Tópico(s)Asymmetric Hydrogenation and Catalysis
ResumoRational design of efficient chiral lithium amides for enantioselective deprotonations demands understanding of the origin of the selectivity. The mechanism of deprotonation of cyclohexene oxide 1 by lithium (1R,2S)-N-methyl-1-phenyl-2-pyrrolidinylpropanamide 3, which yields (S)-cyclohex-2-en-1-ol (S)-5 in 93% enantiomeric excess in tetrahydrofuran (THF), has been investigated. Kinetics have been used to show that the reaction is first order with respect to the reagents 1 and 3, respectively. NMR investigations of a 6Li and 15N labelled isotopologue of 3 have previously shown that 3 is mainly a dimer of the lithium amide monomer in THF in the initial state. On the basis of these results it is concluded that the rate-limiting activated complexes for the epoxide deprotonation are composed of two molecules of monomer of lithium amide 3 and one molecule of epoxide. Structures and energies of unsolvated and specific THF-solvated reagents and activated complexes have been calculated using PM3 and B3LYP/6-31+G(d). The results are currently being explored for the rational design of chiral lithium amides with improved stereoselectivities.
Referência(s)