Theoretical investigation on the chiral diamine-catalyzed epoxidation of cyclic enones: mechanism and effects of cocatalyst.
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.
Key Lab of Colloid and Interface Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, People's Republic of China.
Ping-Li
PL
Lv
Rong-Xiu
RX
Zhu
Dong-Ju
DJ
Zhang
Chong-Gang
CG
Duan
Cheng-Bu
CB
Liu
D002384:Catalysis; D003959:Diamines / Q000737:chemistry*; D004852:Epoxy Compounds / Q000138:chemical synthesis* / Q000737:chemistry; D007659:Ketones / Q000737:chemistry*; D015394:Molecular Structure; D011789:Quantum Theory
D003959:Diamines; D004852:Epoxy Compounds; D007659:Ketones