16
Alkene Epoxidations
ā¢ A huge topic which we will only skim the surface of
ā¢ References will be include for those who want more detail
Peracids: The Prilezhaev (Prileschajew) Reaction
Reagent:
Transformation:
General Mechanism
R O
O
OH
O
ā¢ for the Caddick group with
their love of named reactions
R'
RR" O
O
OH
O
O
O
H
R'
R
R"
R
R'O
R" O
O
H
Use in Synthesis
ā¢ Peracids much weaker acids than carboxylic acids (pKa 8.2 vs 4.8)
ā¢ But carboxylic acid is a by-product so buffer with NaHCO3
ā¢ Peracids are electrophilic so electron withdrawing groups on R good (mCPBA)
ā¢ Electron-rich alkenes more reactive
ā¢Hydrogen-bonding can direct epoxidations
OH ArCO3HOH
O
Hydroperoxides
ā¢ H2O2 & alkyl hydroperoxides require the presence of a transition metal to initiate epoxidation
ā¢tBuO2H (TBHP) favoured as safe, soluble and stable in anhydrous solvents and cheap
M O
O
R
+
M O
O
R
O M
O
R
M
O
O
R
OM OR+
H
O
H
O
O
Ar
OH
