Enantioselective epoxidation of olefins is a challenging and
useful synthetic transformation.
[1]
Since
its introduction in the early nineties, Jacobsen’s catalyst
has proved reliable for highly enantioselective epoxidation of prochiral
olefins.
[2]
Belonging to the
salen-ligand class,
[3]
the
catalyst has a manganese core in a square planar geometry,
stabilised with a chlorine atom in the axial position.
[2]
Several stoichiometric
oxidants have been used successfully,
[4]
with
NaOCl the most frequently involved.
[1a]
[b]
[5]
Easy
to prepare, the catalyst has become commercially available in both
enantiomeric forms.
[6]
Applied also
for the enantioselective oxidation of sulfides to sulfoxides,
[4e]
and for the oxidation of hydrocarbons
at the benzylic position,
[4c]
the
catalyst has even been synthesised in a dimeric form for the epoxidation
of olefins under heterogeneous conditions.
[7]
Recently,
it has provided the first example of didehydrogenation of secondary
hydroxylamines to the corresponding nitrones.
[8]
