Theoretical investigation on the chiral diamine-catalyzed epoxidation of cyclic enones: mechanism and effects of cocatalyst.
J Phys Chem A
; 116(4): 1251-60, 2012 Feb 02.
Article
em En
| MEDLINE
| ID: mdl-22201253
ABSTRACT
The asymmetric epoxidation of 2-cyclohexen-1-one with aqueous H(2)O(2) as oxidant, 1,2-diaminocyclohexane as catalyst, and a Brønsted acid trifluoroacetic acid (TFA) as cocatalyst has been studied by performing density functional theory calculations. It is confirmed that the catalyzed epoxidation proceeds via sequential nucleophilic addition and ring-closure processes involving a ketiminium intermediate. Four possible pathways associated with two Z isomers and two E isomers of ketiminium have been explored in detail. Our calculation indicates that these four pathways have high barriers and a small energy gap between two more favorable R and S pathways. We have analyzed the effects of the TFA anion and H(2)O on the activity and enantioselectivity of catalytic epoxidation. It is found that the TFA anion acts as a counterion to stabilize the transition states of the catalytic epoxidation by hydrogen-bond acceptance, leading to decreases in the barriers of the nucleophilic addition and ring-closure processes. The most significant decrease occurred in the ring-closure step of the Z-R-pathway, resulting in H-bond-induced enantioselectivity. Our calculations also show that water cooperates with TFA to further increase the reaction rate significantly.
Texto completo:
1
Coleções:
01-internacional
Base de dados:
MEDLINE
Assunto principal:
Teoria Quântica
/
Diaminas
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Compostos de Epóxi
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Cetonas
Idioma:
En
Ano de publicação:
2012
Tipo de documento:
Article