Chemistry Reference
In-Depth Information
Tab l e 6 . 1 Cobalt-catalyzed intramolecular asymmetric PKRs with
bis(phosphine) ligands.
R 1
CO
R 1
Z
Z
O
R 2
Co 2 (CO) 4 Ligand)
R 2
6, 19
7, 20
R', R'' = H, Me
Z = C(CO 2 Me) 2 , NTs, NBn, CMe 2
R', R'' = H, Me
Z = C(CO 2 Me) 2 , NTs, NBn, CMe 2
Substrate Z
R 1
R 2
Ligand
Product yield (%) ee (%)
6a
C(COOEt) 2 HH BINAP
7a
55
90
6a
C(COOEt) 2 HH DIOP
7a
76
5
6b
C(COOEt) 2 Me H
BINAP
7b
31
63
6b
C(COOEt) 2 Me H
DIOP
7b
54
40
6c
C(COOEt) 2 Me Me
BINAP
7c
90
0
6c
C(COOEt) 2 Me Me
DIOP
7c
47
31
19a
N-Ts
H H
BINAP
20a
60
93
19b
N-Ts
Me H
BINAP
20b
13
62
19c
N-Bn
H H
BINAP
20a
36
82
6a
C(COOEt) 2 HH BIPHEMP
7a
90
61
6a
C(COOEt) 2 HH MeO-BIPHEMP
7a
86
91
6a
C(COOEt) 2 HH 18
7a
87
22
6d
C(COOEt) 2 H h 18
7d
75
75
6e
C(COOEt) 2 HMe 18
7e
97
7
alkyne terminus often results in a substantial decrease in enantioselectivity as well as
unpredictable changes in relative reactivity (see Table 6.1).
6.5.3.2
Intermolecular PKR
The first attempts to develop a chiral ligand based approach of the PKR date back to the
late 1980s and were focused on the intermolecular version. However, despite three decades
of continued research in this direction, practical methodologies have only just been devised
over the past few years. Indeed, a catalytic protocol with reasonable scope and induction
level remains elusive.
The first example stemmed from a collaboration between the groups of Brunner and
Pauson himself, who harnessed the glycerol-derived monophosphine GLYPHOS as desym-
metrizing element for the dicobalt-alkyne cluster. 3 , 27 Ligand exchange and thermodynamic
equilibration of the phenylacetylene-derived complex 1h yielded the diastereomers 21 and
21' in a 6:4 ratio (Scheme 6.15). After fractional crystallization and/or chromatographic
separation, followed by reaction with norbornene, each diastereomer furnished opposite
enantiomers of the corresponding cyclopentenone. High induction levels (albeit with low
yields) were obtained by thermal activation at 45 C. However, the use of temperatures
above the 60 C required for the equilibration of the diasteromeric mixture of carbonylic
complexes ( t 1
170 min in toluene), resulted in some loss in enantiomeric excess. Amore
/ 2
 
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