Handbook of Functionalized Organometallics Applications in S

476
11 Polyfunctional Metal Carbenes for Organic Synthesis
O O
[(t BuO) 3W CCMe3] 153
(10 mol%)
65 %
O
H 2,
Lindlar catalyst
94 %
152 154 155
Scheme 11.37 Synthesis of civetone 155 by an alkyne metathesis/syn-hydrogenation sequence.
Mes
Cl
Cl
N
Ru
PCy 3
Mes
Ph
Mes
Cl
Cl
N
Ru
O
Mes
Mes
N Cl
Mes
N Ru
Cl N
Ph
Br
156 157 158
Scheme 11.38 Second- and third generation Grubbs catalysts.
Recent advances focus on the chiral modification of molybdenum-based catalysts
and their application in enantioselective olefin metathesis. Both reactivity
and selectivity can be controlled by structural variation of the alkoxide and imido
coligands. Alkoxide ligands offer a straightforward option for the incorporation of
chiral information via readily available enantiomerically pure bishydroxy ligands
such as biphenyl and binaphthyl derivatives. Representative examples are molybdenum
alkylidene complexes 159±161 (Scheme 11.39) showing high levels of
induction in kinetic asymmetric ring-closing (ARCM) and ring-opening (AROM)
metathesis [80].
So far, enantioselective olefin metathesis has the largest impact on organic synthesis
in the desymmetrization of achiral polyenes [81]; an illustrative example
refers to the total synthesis of endo-brevicomin [82]. The catalytic asymmetric cyclization
of achiral trienes and meso-tetraenes via ARCM proceeds in excellent enantioselection
(e.e. > 99 %) as demonstrated for dihydrofuran 163 (Scheme 11.40).
Molybdenum-catalyzed ARCM has been applied to the enantioselective synthesis
of 7- or 8-membered heterocycles [83] as well as for midsized cyclic tertiary
amines [84]. Chiral molybdenum catalysts allow for efficient kinetic resolution of
1,6-dienes and diallyl ethers.
Br