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diastereomeric complexes that can be separated by column chromatography (Scheme 6.4).
PK reaction of the individual complexes with norbornene (NBN) or norbornadiene (NBD)
yields the diastereomeric adducts
2/2'
or
3/3'
, respectively, which have opposite stere-
ochemistry around the cyclopentenone core (Scheme 6.4). Although these are formally
diastereoselective reactions, the results indicate that the chirality of the product is dic-
tated by the chirality around the cobalt cluster. This was corroborated when the hexacar-
bonyldicobalt complexes of the same chiral alkynes were subjected to the PKR: very little
or no diastereoselectivity was observed. Importantly, the
N
-oxide protocol is required, as
thermal activation would most likely lead to equilibration of the diastereomeric complexes
through ligand exchange, which would in turn lead to an enantiomeric excess as low as the
thermodynamic ratio of the diastereomeric pair in question.
PR'
3
PR'
3
(OC)
3
Co
Co(CO)
3
(OC)
3
Co
Co(CO)
2
(OC)
2
Co
Co(CO)
3
PR'
3
+
R' = Ph, OMe
*R
*R
*R
1a
R* =
N
-oxide
N
-oxide
O
NBN or NBD
OH
O
O
H
H
1b
R* =
Ph
R*
*R
H
H
2a/2b
(NBN)
3a/3b
(NBD)
2a'/2b'
(NBN)
3a'/3b'
(NBD)
Opposite configuration at the cyclopentenone core
Scheme 6.4
Desymmetrization of dicobalt-alkyne clusters driven by a chiral auxiliary.
6.3.2
Synthesis of Chiral Monophosphine Complexes Using Chiral
N
-Oxides
Chiral resolution can also be accomplished by using a chiral
N
-oxide without chiral auxil-
iaries, as demonstrated by Nicholas in the work on his namesake reaction.
9
Kerr exploited
this approach for the PKR
10
(Scheme 6.5). His results demonstrate that the chirality of the
final product is dictated solely by olefin coordination to, and subsequent insertion at, either
of the enantiotopic cobalt vertices, away from the phosphine ligand.
6.3.3 Heterobimetallic Complexes
A third approach to the preparation of intrinsically chiral complexes entails a hetero-
bimetallic PKR.
11
Desymmetrization of dicobalt-alkyne through iso-lobular replacement
