Chemistry Reference
In-Depth Information
Ph
Rh(COD)
2
OTf
Chiral ligand
R
1
Ph
R
1
O
Ph
O
O
and/or
O
+
OR
2
OR
2
OR
2
H
2
(1 atm)
Benzene, 25°C
40
45
47
46
OH
OH
R
1
R
1
R
2
Entry
Ligand
Temp (°C)
46:47
Yield (%)
% ee
1(
S
)-BINAP
Ph
Et
25
46a
=
47a
51
69 (+),
46a
2(
S
)-tol-BINAP
Ph
Et
25
46a
=
47a
50
57 (+),
46a
3(
S
)-xylyl-BINAP
Ph
Et
25
46a
=
47a
73
49 (+),
46a
4(
R
)-DIFLUORPHOS
Ph
Et
25
46a
=
47a
76
79 (-),
46a
5(
R,R
)-xyxly-WALPHOS
Ph
Et
25
46a
=
47a
17
72 (-),
46a
6(
R
)-Cl-OMe-BIPHEP
Ph
Et
25
46a
=
47a
78
82 (-),
46a
7(
R
)-Cl-OMe-BIPHEP
Ph
Et
45
46a
=
47a
83
78 (-),
46a
8(
R
)-Cl-OMe-BIPHEP
CH
3
Et
25
3.3:1
78
89 (-),
47b
9(
R
)-Cl-OMe-BIPHEP
CH
3
t-
Bu
25
3.0:1
52
87 (-),
47b
10
(
R
)-Cl-OMe-BIPHEP
t-
Bu
Et
40
>99:1
78
88 (+),
46c
11
(
R
)-Cl-OMe-BIPHEP
TMS
Et
25
1:>99
74
93(+),
47d
12
(
R
)-Cl-OMe-BIPHEP
TMS
Et
40
1:>99
84
91(+),
47d
F
O
Ar
2
P
F
O
PAr
2
PAr
2
PPh
2
PAr
2
PPh
2
Fe
O
F
F
Me
O
(
S
)-BINAP: Ar= Ph
(
S
)-tol-BINAP: Ar= 4-Me-C
6
H
4
-
(
S
)-xylyl-BINAP: Ar= 3,5-Me
2
C
6
H
3
-
(R,R)
-xylyl-WALPHOS:
Ar = 3,5-Me
2
C
6
H
3
-
(
R
)-DIFLUORPHOS
Scheme 8C.18.
Following up this fi nding, the reactions of several other trialkylsilyldiynes
48
with
45
were studied under the optimal conditions mentioned above, which confi rmed that
hydrometallation and C-C bond formation took place exclusively on the acetylene
moiety bearing a trialkylsilyl group to give
49
(Scheme 8C.19 ) [27,28] . The remarkable
directing effect of trialkylsilyl group in this reaction can be accommodated by taking
into account the enhanced
- back - bonding ability of trialkylsi-
lylacetylene moiety of the 1,3-diyne system as well as oxarhodacycle
50
formation upon
insertion of the aldehyde carbonyl moiety of glyoxalate (Scheme 8C.19) [27,28].
The enantioselective reaction of 1,3-enynes
51
in place of 1,3-diynes with glyoxalate
45
is also catalyzed by cationic chiral Rh-diphosphine complexes to give the correspond-
ing dienyl -
π
- acidity and hence
π
- hydroxyesters
52
(Scheme 8C.20) [28,29]. Naturally, there is no regioselec-
tivity issue in this reaction, and the C-C bond formation takes place at the acetylene
moiety exclusively. A number of commercially available chiral diphosphine ligands were
screened for the reaction of 1-phenylbutenyne (
51a
) with ethyl glyoxalate (
45a
). Among
the chiral ligands examined, (
R
)-OMe-BIPHEP (75% ee, entry 3) and (
R
) - Cl - OMe -
BIPHEP (81% ee, entry 4) gave the best results. Optimization of OMe-BIPHEP ligands
by introducing a very bulky substituent to the phosphine moiety led to a highly effi cient
α
