Chemistry Reference
In-Depth Information
an enantioselective synthesis of quebrachamine—the Mo-catalyzed enantioselective
RCM delivers the desired product in 84% yield and establishes the all-carbon quaternary
center in 96% ee.
Several additional points regarding this most recent development are noteworthy: (1)
This study presents the fi rst example of the use of a stereogenic-at-metal complex,
bearing only monodentate ligands, as an effi cient and selective catalyst for enantioselec-
tive synthesis. (2) The stereogenic-at-Mo complexes constitute a rare example of an
alkoxide or aryloxide as a chiral ligand. (3)
In situ
- generated catalysts often display
identical reactivity and selectivity to isolated, diastereomerically pure complexes, and
the olefi n metatheses studied can be performed in a fume hood on gram scale.
8E.3. CATALYTIC ENANTIOSELECTIVE RING - OPENING/
RING - CLOSING METATHESIS ( RORCM ) AND RING - OPENING/
CROSS - METATHESIS ( ROCM )
8E.3.1. General Considerations
Whereas RCM reactions are driven by entropy (e.g., formation of ethylene), RCM
processes are enthalpy driven, often through release of a ring strain. Subsequent to ring
opening, the resulting alkylidene/carbene may be trapped intramolecularly by a pendant
olefi n (RORCM, path A, Scheme 8E.9) or intermolecularly with a cross partner (ROCM,
path B). In some cases, ROCM may be accompanied by RCM. Reaction pathways can
O
Me
Me
O
H
M
M
R
H
R
H
Me
Me
O
O
O
Path A
intramolecular
trap
Me
M
R
R
R
M
RORCM (R = H )
R
O
Me
O
Path B
intermolecular
trap
R
Me
R
M
R
ROCM (R = Ph)
Scheme 8E.9.
Catalytic pathways resulting in the formation of enantioselective RORCM versus
ROCM products.
