close

Вход

Забыли?

вход по аккаунту

?

2307986

код для вставки
Bull. SOC. Chim. Belg. vol.91/ne
5/1982
ISrnPE E
F
F
E
m AND ASyPwE3rRIC INDUCTION
Ann Charlotte Malmvik and (XiranBergson
Institute of Chemistry, University of Uppsala,
P. 0.Box 531, 5-751 21 Uppsala, Sweden
In recent years, the asymnetric reduction of carbonyl compwnds by chiral Lithium Aluminum
hydride-amino alcohol reagents has been the subject of much study.
Al’Uough a nurkr of different aminmlcohols have been uscd and very high asymwtric indnctions have been reported, little is known about the structure of the reacting romplex and
of the diasteremeric transition states involved. In order to gain further knowledge, we
have undertaken a study of isotope effects on the reduction of prochiral carbonyl compounds
with a complex of
(-)
quinine and LiALM4 and LiAlD4 respectively. (1:l).
From the fact that we yet different m u n t s of asynmtric induction with WI and LAD, we
assume that there is an isotope effect mainly due to the diastereomrism in the transition
states associated with the reaction.
Depending on the choice of solvent and the use of LAH (W) as a “slurry” or as a clear,
standardized solution, different degrees of asymetric induction is obtained. In diethylether,
where
(-)
quinine is not very soluble, the asymnetric induction as well as the deuterium
isotope effect is high and the reproducibility is very good. In ‘IliF vhere quinine is readily
soluble, the asymnetric induction and the isotope effect is less and there is very pcor
reproducibility. This ray be due, in part, to the homogeneity versus heterogeneity of the
media, but it m y also be due to the 1,l-complex initially formed disproportionating, giving
free WI
(LN))and
higher complexes with different chiralities. This has been suggested by
0. Cervinka, who proposed a mechanism for the disproportionation of the 1,l-complex.
We have made a thorough study of the reacting complex mixture using
(-)
quinine and (-)N-
methylephedrine by 27Al and ’Li M. We can dearly establish that disproportionation is
ccurring, to a varying degree dewriding on the alkaloid used. We have shown that the 1:lreaction mixture contains at least three different reducing species, one of them being free
(m).
-
492
-
Документ
Категория
Без категории
Просмотров
2
Размер файла
51 Кб
Теги
1/--страниц
Пожаловаться на содержимое документа