Chemistry Reference
In-Depth Information
Ph
Ph
Ru-catalyst A
O
O
O
O
O
O
8.354
8.388
76% e.e.
Scheme 8.107
AcO
OAc
TIPSO
TIPSO
Ru-catalyst B
OAc
8.389
8.390
52% e.e.
Scheme 8.108
a strategy was used in a synthesis of africanol
8.387
involving ring opening of a substituted norbornene
8.381
with the molybdenum catalyst
8.382
.
117
The vinyl group of the metathesis product
8.383
was converted
to a methyl group by conversion to an aldehyde and rhodium-catalysed decarbonylation (see Section 4.7).
An additional carbon was installed by an unregioselective hydroformylation reaction (Section 4.4), followed
by a sequence to introduce an alkene and isomerize it into an
endo
-cyclic isomer
8.386
. Deprotection and
cyclopropanation
118
then yielded africanol
8.387
.
Chiral ruthenium complexes,
8.391
and
8.392
, have been used in a variety of asymmetric metathesis
reactions, including ring-openingmetathesis (Scheme 8.107, comparewith Scheme 8.98) and cross-metathesis
(Scheme 8.108).
119
The chirality of the complexes resides both in the backbone of the carbene ligand
by the placement of substituents, and also in the conformation of the aryl substituents on the carbene
nitrogen atoms.
Ph
Ph
Ph
Ph
t
-Bu
MeO
i
-Pr
i
-Pr
i
-Pr
NN
OMe
t
-Bu
NN
i
-Pr
i
-Pr
i
-Pr
Ru
Cy
3
P
Ph
Ru catalyst A
8.391
Ru
Cy
3
P
Ph
Ru catalyst B
8.392
8.3.7 Ene-Yne Metathesis
Alkynes can also participate in alkene metathesis chemistry. The reaction between an alkene and an alkyne is
known as ene-ynemetathesis.
120
Themechanism is slightlymore complicated then for alkenemetathesis and it
is unclear whether it starts with the alkene or the alkyne (Scheme 8.109). In any event, the same product, a con-
jugated diene
8.394
, is obtained. If the catalytically active carbene species ([Ru]
=
) reacts first with the alkyne,
