Chemistry Reference
In-Depth Information
6.5.2 Cyclopentadienes (Heterobimetallic)
Here we include an example based on the heterobimetallic approach. 18 The isolobal sub-
stitution of a Co(CO) 3 vertex of the alkyne 16 by a molybdenum fragment bearing a chiral
cyclopentadiene ligand (Cp c ) affords a mixture of diastereomeric complexes with moder-
ate selectivity (Scheme 6.12). If the diastereomers can be separated, then each one will
yield opposite enantiomers of the corresponding PK adduct in excellent ee. This reaction
is particular in that it furnishes endo fused tricyclic structures, an unusual feature for inter-
molecular PKRs; indeed, only certain substrates such as ynamides give significant amounts
of the endo isomer. 19
Cp c
Cp c
(OC) 3 Co
Co(CO) 3
(OC) 3 Co
Mo(CO) 2
Na[Cp c Mo(CO) 3 ]
(OC) 2 Mo
Co(CO) 3
+
THF
Me
Me
Me
R
R
R
16
dr up to 2:1
O
O
N
R =
O
90 °C
90 °C
H
Cp c
Me
O
Me
O
O
H
H
i-Pr
Me
O
R
R
H
H
H
H
Me
Me
Me
O
O
O
Me
17
Me
17'
60-75% yield
< 95% ee
Me
O
Me
O
Me
Me
Scheme 6.12 Desymmetrization through isolobal replacement of a Co(CO) 3 fragment by a
molybdenum bearing a chiral cyclopentadiene ligand (Cp c ).
6.5.3 C-Chiral and Axially Chiral Phosphines
6.5.3.1 Intramolecular PKR
In 2000 the Hiroi group pioneered studies on asymmetric cobalt-catalyzed PKRs through
a chiral ligand approach. 20 After screening a structurally diverse set of readily available
chiral phosphines, they found that BINAP offered unparalleled results in the cyclization
of 1,6-enynes (Scheme 6.13, Table 6.1). Other atropoisomeric binaphthyl derivatives that
are weaker ligands (e.g. BINOL and binaphthyl diamine) gave poor enantioselectivities.
Extending this work, Consiglio achieved similar enantiomeric induction levels by exploiting
the closely related atropoisomeric bis(phosphine) MeO-BIPHEMP. 21 A later study by
Gibson 22
completed a thorough compendium of atropoisomeric chiral bis(phosphines),
 
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