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H
(OC) 3 Co
Co(CO) 3
Amines
(OC) 3 Co
Co(CO) 3
O
11a-c
BF 3 ·OEt 2
H
N
Ts
TsHN
N
MeO
Ts
12
(racemic)
9
10
NH 2
NH 2
NH 2
11b
11a
11c
Scheme 6.10 Non enantioselective PKRs promoted by chiral amines.
6.5 Chiral Ligands
Here we describe chiral molecules that have been specifically designed to coordinate to
dicobalt clusters, as well as common Lewis bases for which evidence of stable coordinated
cobalt clusters exists.
6.5.1 Sulfides
The promising results obtained with thioether-containing chelating auxiliaries 16 prompted
the group of Pericas and Riera to explore sulfides as ligands. However, the intrinsically
weak coordination properties of thioethers preclude the isolation of stable complexes,
except for the case of alkynes substituted with strong electron-withdrawing groups. A stable
complex 15 from bis( tert -butyl sulfonyl)acetylene dicobalt hexacarbonyl complex ( 13 ) and
the camphor-derived chiral thioether 14 could be prepared 17 (Scheme 6.11). Unfortunately,
when this complex was subjected to PKR with norbornadiene, no PK adducts could be
isolated—presumably, due to the severe steric congestion imposed by the two tert -butyl
sulfonyl groups.
Cl
O
S
S
(OC) 3 Co
Co(CO) 3
(OC) 3 Co
Co(CO) 2
14
SO 2 t Bu
SO 2 t Bu
t -BuO 2 S
t -BuSO 2
13
15
Scheme 6.11 Failed attempt at using a chiral sulfide as a PKR ligand.
 
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