1,3-HYDRON TRANSFER IN SOME 5- AND 7-SUBSTITUTED 1-METHYLINDENES. ENANTIOSELECTIVITIES AND ENANTIOMER-DEPENDENT KINETIC ISOTOPE EFFECTS

摘要

Base-catalysed substrate-enantioselective 1,3-hydron transfer (kinetic resolution) was studied in the indene system. A series of 1-methylindenes substituted in the aromatic ring [5-methoxy- (2), 5-fluoro- (3), 5-nitro- (4) and 7-nitro (5)] and 1-methylindene (1) were employed as substrates. The rate constants, the enantioselectivities and the kinetic isotope effects (KIEs) for the enantioselective reactions [(k_H/k_D)~+ and (k_H/k_D)~-] were determined at 20℃ using (+)-(8R,9S)-dihydroquinidine as chiral catalyst in the solvent c-dichlorobenzene. The rate constants vary according to the electronic effects of the substituents. The primary deuterium KIE, ranging from 4.73 [for (+)-(S)-2] to 11.3 [for (―)-(R)-5], is correlated with the rate constants as expected on the basis of the Melander-Westheimer postulate. The introduction of a substituent in the aromatic ring decreases the enantioselectivity. All compounds except 5 show the same sense of the enantioselectivity [k~+/k~- > 1; all substrates used are (+)-(S)/(―)-(R)]. The enantiomer dependence of the KIE is most pronounced for 1 [(k_H/k_D)]~+ = 5.71 and (k_H/k_D)~- = 6.46] and vanishes for the most acidic substrates.