The New Exploration for Asymmetric Cyclopropanation of Olefins
| MS Chen Li(1997-2000) | Directed by Prof. Xiuwen Han |
| Abstract | Present address: Department of Pharmacy University of Connecticut USA |
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It is very
important for the asymmetric cyclopropanation to develop new catalytic
system and to study the matching of central metals with substrates and
catalytic the mechanism. The asymmetric cyclopropanation of olefins catalyzed
by metal complexes containing N,O- or N,P- ligands was preliminarily investigated.
The effects of different additives on asymmetric cyclopropanation of olefins
were studied. The coordination process of 1R, 2R-cyclohexyl-N2P2-RuCl2
and the roles of OTf were investigated by variable temperature in situ
31P or 19F NMR, molecular mechanics and dynamics.
The contents of the thesis are as follow: |
(1). A new C2-symmetric Schiff base with
large chiral cavity was synthesized from L-(+)-tartric acid over four steps.
Due to its large chiral cavity, it would have different chiral environments
for different substrates. Its copper complex was preliminarily investigated
for asymmetric cyclopropanation. The results showed that it had different catalytic
activities and enantioselectivities to styrene and 2,5-dimethyl-2,4-hexadiene.
The observed results were explained through the discussion of its catalytic
mechanism.
(2). The 1R, 2R-cyclohexyl-N2P2 was synthesized according to the references. Its ruthenium complex was investigated for asymmetric cyclopropanation of styrene. In the presence of Et3N, the best enantiomeric excess (e.e %) was 65.8 %(trans-isomer). By investigating its dynamics behavior with variable temperature in situ 31P NMR and analyzing the experimental results, we proposed that the Et3N probably had two roles in this catalytic system.
First, Et3N might assist the chloride ion to dissociate from the central metal. Second, Et3N might have a role of steric hindrance in the reaction process, which could improve the enantioselectivity.
Based on the above information, the mechanism of asymmetric cyclopropanation of styrene catalyzed by 1R, 2R-cyclohexyl-N2P2-RuCl2 in the presence of Et3Nwas elucidated.
(3). In order to improve the catalytic activity of 1R, 2R-cyclohexyl-N2P2-RuCl2 for asymmetric cyclopropanation of styrene, weakly coordinated anions as an additive was investigated. The yield of cyclopropanation was improved to 73.7 % at 25 °C using the AgOTf as an additive. The best enantiomeric excess (e.e %) was 59.2 % (cis-isomer) and 43.4 % (trans-isomer). But more interesting was that the ratio of trans and cis of products was near 1:1, and this is unusual in asymmetric cyclopropanation.
(2). The 1R, 2R-cyclohexyl-N2P2 was synthesized according to the references. Its ruthenium complex was investigated for asymmetric cyclopropanation of styrene. In the presence of Et3N, the best enantiomeric excess (e.e %) was 65.8 %(trans-isomer). By investigating its dynamics behavior with variable temperature in situ 31P NMR and analyzing the experimental results, we proposed that the Et3N probably had two roles in this catalytic system.
First, Et3N might assist the chloride ion to dissociate from the central metal. Second, Et3N might have a role of steric hindrance in the reaction process, which could improve the enantioselectivity.
Based on the above information, the mechanism of asymmetric cyclopropanation of styrene catalyzed by 1R, 2R-cyclohexyl-N2P2-RuCl2 in the presence of Et3Nwas elucidated.
(3). In order to improve the catalytic activity of 1R, 2R-cyclohexyl-N2P2-RuCl2 for asymmetric cyclopropanation of styrene, weakly coordinated anions as an additive was investigated. The yield of cyclopropanation was improved to 73.7 % at 25 °C using the AgOTf as an additive. The best enantiomeric excess (e.e %) was 59.2 % (cis-isomer) and 43.4 % (trans-isomer). But more interesting was that the ratio of trans and cis of products was near 1:1, and this is unusual in asymmetric cyclopropanation.
(4). The coordination
process of 1R,2R-cyclohexyl-N2P2-RuCl2
was investigated by variable temperature in situ 31P
NMR and modeled by dynamics and molecular mechanics. We found
that it probably underwent via four-coordinated and five-coordinated
intermediates during its coordination process. The role of OTf
- was also studied by variable temperature in situ 19F
NMR. The experimental results and catalytic mechanism of asymmetric
cyclopropanation of styrene catalyzed by 1R,2R-cyclohexyl-N2P2-RuCl2
in the presence of AgOTf were discussed by using molecular mechanics
and dynamics. |
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| The preponderant conformation optimized by dynamics and molecular mechanics |