Chemistry Reference
In-Depth Information
TABLE 4.12. Dihydrobenzofuran Formation
R
R'
O
O
R
R'
CO
2
Me
Rh(II)-cat.
CO
2
Me
N
2
90
91
Compound
Catalyst
R =
R
′
=
Conditions
Yield (%)
de (%)
ee (%)
a
Rh
2
(
S
- DOSP)
4
Me
Me
Hexane/ − 50 ° C
98
—
94
b
Rh
2
(
S
- DOSP)
4
c
- C
4
H
8
Hexane/ − 50 ° C
93
—
90
c
Rh
2
(
S
- PTTL)
4
H
Me
Toluene/ − 78 ° C
91
72
a
97
d
Rh
2
(
S
- PTTL)
4
H
c
- Hex
Toluene/ − 78 ° C
63
92
96
e
Rh
2
(
S
- PTTL)
4
H
Ph
Toluene/ − 78 ° C
86
> 98
94
f
Rh
2
(
S
- PTAD)
4
H
Me
Toluene/ − 60 ° C
79
> 98
95
a
anti
- Diasteromer major.
OBn
OBn
H
O
Rh
2
(
S
-DOSP)
4
CH
2
Cl
2
Me
O
Br
O
Me
H
N
N
O
O
Br
O
O
63% yield
86% de
N
2
O
9
9
OH
H
O
H
O
H
O
HN
N
HN
N
H
94
(-)-Ephedradine A
Scheme 4.18.
Synthetic application of benzofuran formation.
4.2.4. Intermolecular C - H Insertion
For a long time, intermolecular C-H insertion via carbenoid intermediates was consid-
ered to have little or no synthetic utility, primarily because carbene dimerization was a
major side reaction and selectivity was poor [42,47]. The traditionally used carbenoid,
derived from ethyl diazoacetate, is a very reactive and hence quite unselective species.
Although this has been addressed in recent years by the development of copper and
silver scorpionate complexes and other catalysts [103,198-202], the major breakthrough
