Leading Question!
A short review on Lewis‐acid mediation, stereospecific rearrangement of an epoxide to the corresponding ketone.
The
Lewis
acid
used
in
the
experiment
is
BF3.
Because
of
its
open
shell,
one
of
the
lone
pairs
on
the
epoxide
oxygen
attaches
to
BF3,
leading
the
epoxide
oxygen
to
gain
a
positive
charge
and
the
boron
attached
to
it
to
gain
a
negative
charge.
The
adjacent
epoxide
oxygen
donates
a
lone
pair
to
the
bond
with
an α
carbon
that
is
also
bonded
to
the
epoxide
that
the
BF3
is
attached
to.
The
epoxide
with
the
Lewis
acid
detaches
from
the
α
carbon
and
the
hydrogen
on
the
β
carbon
shifts
over
to
α
carbon
to
neutralize
the
positive
charge
on
the
electron-donating
oxygen.
The
hydrogen
shifts
into
an
axial
position
on
the
α
carbon
because
it
is
highly
unlikely
to
attack
the
carbon
from
the
bottom
in
the
equatorial
position
as
it
would
cause
serious
steric
hindrance
problems
with
the
oxygen
group and
cyclohexane
attached
to
it.
Lastly,
BF3
detaches
from
the
oxygen
donating
its
bond
electron
pair
to
the
bond
between
the
oxygen
and
β
carbon
to
form
a
double
bond.
