Chemistry Reference
In-Depth Information
TABLE 8E.3. Comparison of the Activity of Chiral R u Catalyst in Enantioselective
ROCM of Norbornenes
Ru-based
chiral catalyst
Ph
Ph
O
O
O
O
O
O
Ph
Chiral catalysts:
Ph
i-Pr
N
N
t-Bu
i-Pr
MeO
L
OMe
t-Bu
Ru
Ph
L
PCy
3
10a
L = Cl
10b
L = I
Ph
Ph
MesN
N
MesN
N
L
I
Ru
Ru
O
Oi-Pr
O
Oi-Pr
Ph
Ph
11a
L = Cl
11b
L = I
12b
Catalyst;
mol %
Equiv
Ph
Time (h);
Temp ( ° C)
Yield (%);
E
/
Z
Entry
Product
ee (%)
1
10a
; 1
10
1; 22
95; 1:1
76
Ph
2
10b
; 3
10
1; 22
96; 1:1
80
3
11a
;5
5
0.4; 22
82; > 98:2
78
O
O
O
4
11b
; 5
5
2; 22
72; > 98:2
> 98
5
12b
; 5
5
2; 22
50; > 98:2
90
6
10a
; 1
10
1; 22
30; 1.1:1
33
Ph
7
11a
; 5
5
0.25; 22
40; > 98:2
10
HO
OH
Although full conversion of
exo
-substituted oxabicycles can be obtained with Ru catalyst
11b
, reactions are relatively sluggish (entry 4), requiring 5 mol % catalyst loading and
extended reaction times (36 h at 22°C) to deliver products in good yield and with high
enantioselectivity. Development of Ru complex
12b
has led to substantially improved
reactivity: transformations can be performed at lower temperatures and with reduced
catalyst amounts (see entry 5: at
15 ° C with 2 mol %
12b
), allowing access to the desired
products with greater enantioselectivity and requiring shorter reaction times (
−
98% ee
in 15 h) [37]. Mo catalyst
2
offers the highest level of activity for this transformation,
>
