Sharpless Asymmetric Epoxidation - McMaster University
[Pages:34]Sharpless Asymmetric
Epoxidation
R2
R3
R1
R4
HO
"Magic"
O R2
R1
HO
R3 R4
Karl Barry Sharpless
? Born in Philadephia in 1941
? Ph.D from Stanford University in 1968
? Postdoc at Harvard and at Stanford
? Research on chiral synthesis and catalysts at the Scripps Institute
? Received Nobel Prize in 2001 for his work on stereoselective oxidation reactions
Sharpless Asymmetric Epoxidation
(SAE)
R3
R1
OH
O R1
R3 OH
R2
R4
R2
R4
- Converts primary and secondary allylic alcohols
into 2,3 epoxyalcohols
-The reaction is enantioselective (only one enantiomer produced)
-Enantiomer formed depends on stereochemistry of catalyst
The Reaction
R2
R1
R2
R3
Ti(OiPr)4 - (+) - DET
OH
tBuOOH
O
R4
CH2Cl2, -20?C
R3
R2
R1
R2
Ti(OiPr)4 - (-) - DET
R3
OH
tBuOOH
O
R4
CH2Cl2, -20?C
R3
? The catalyst is titanium tetra(isopropoxide) with
diethyltartrate.
R1
OH R4 R1
OH R4
? The use of + or ? tartrate will yield different enantiomers
? Tertbutylperoxide is used as the oxidizing agent
? Dichloromethane solvent and -20?C temperature
The Catalyst
H3C
CH3
H3C H3C
O
O Ti O
O
CH3 CH3
CH3 H3C
Ti(OiPr)4 catalyst
OH O
H3C
O
O
CH3
EtO
O iPrO
O CO2Et
O Ti OiPr O
O OiPr OEt
Ti
OiPr O
O
OH
Diethyl Tartrate (DET)
Chirally controls reaction
EtO2C
? Via rapid ligand exchange of OiPr and diethyl tartrate
The Mechanism
EtO
O iPrO
O CO2Et
O Ti OiPr O
EtO2C
O OiPr OEt
Ti
+ OiPr O
O
HO
HO
CH3
+ CH3
CH3
CH2
OH
OEt
O
EtOOC
O Ti
O H2C
tBu O
O
+
EtOOC EtOOC
OiPr
O
OiPr
Ti
O
OiPr
OiPr
Transition State
OEt EtOOC
OH O
O Ti
O
tBu O O
H2C
OH
OEt
O
EtOOC
O Ti
O
tBu O
CH O
CH2
Products
EtOOC EtOOC
OiPr
O
OiPr
Ti
O
OiPr
OiPr
OH
OEt
O
EtOOC
O Ti
O
tBu O
CH O
CH2
EtO
O iPrO
O CO2Et
O Ti OiPr O
O OiPr OEt
Ti
OiPr O
EtO2C
+
tBuOH
+
O H
OH
HH
................
................
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