Chemistry Reference
In-Depth Information
R =
R' =
a
b
c
d
e
f
g
Ph
4-(Me)Ph
4-(Me)Ph
3,5-(Me)
2
Ph
H
R'
O
4-Me
Rh
F
3
C
PR
2
O
3-Me
3,5-(Me)
2
PR
2
O
O
F
3
C
Rh
4-(
t-
Bu)
4-(
t-
Bu)Ph
7
4-(F)Ph
4-F
H
R'
Me
Figure 4.6.
Dirhodium(II) orthometallated phosphines.
4.2.3. Intramolecular C - H Insertion
Since highly electrophilic catalysts are required and selectivities are low for intermolecu-
lar C-H insertions with acceptor and acceptor/acceptor carbenoids, intramolecular ver-
sions were the fi rst to be developed [3,45,49]. High selectivity for the intramolecular
reaction, rather than an intermolecular process, can be achieved to form four- and fi ve-
membered rings, but also three-, six-, and higher-membered ring sizes have been gener-
ated [3,45,48,49] . Intramolecular C -H insertions of diazocarbonyl compounds occur
effectively in a very selective manner, even with relatively electron-rich catalyst systems.
A variety of heterocyclic and carbocyclic structures can be generated with high regio-
and stereocontrol [8,49,59,61,139,148,149]. The most important results will be discussed
in this section.
4.2.3.1. Synthesis of Carbocycles
The fi rst report of enantioinduction in an intramo-
lecular C-H insertion was by McKervey and others, who employed the chiral
dirhodium(II) prolinate complex Rh
2
(
S
- BSP)
4
[150] . The cyclopentanone
9
was formed
in good yield but with only about 12% ee (Scheme 4.2). However, this discovery paved
the way for further developments in the fi eld, and Ikegami, Hashimoto, and others
subsequently reported cyclopentanone formation with diazocarbonyl compound
10
(Table 4.1 ) with enantioselectivities in the 33 - 80% ee range [123,151] .
O
O
Rh
2
(
S
-BSP)
4
SO
2
Ph
SO
2
Ph
CH
2
Cl
2
N
2
Me
>90% yield,
12% ee
8
9
Scheme 4.2.
First example of enantioselective C- H insertion.
Dirhodium(II) carboxylates are the most effective catalysts for reactions of acceptor/
acceptor diazo compounds since they can be made suffi ciently electron defi cient to be
kinetically active for the decomposition of the relatively stable carbenoid precursors
[42,152]. The more electron-rich dirhodium(II) carboxamidates are unable to form the
