Chemistry Reference
In-Depth Information
TABLE 4.1. Intramolecular C - H Insertion with Rh
2
( S - PTPA )
4
O
O
Rh
2
(
S
-PTPA)
4
CO
2
CH(
i-
Pr)
2
CO
2
CH(
i-
Pr)
2
CH
2
Cl
2
, 0°C
N
2
R
R
1
1
1
Compound
R =
Yield (%)
ee (%)
a
Ph
86
76
b
4 - (MeO)Ph
86
57
c
4 - (CF
3
SO
2
)Ph
84
80
H
H
R
O
O
O
R
CO
2
(
i-
Pr)
CO
2
(
i-
Pr
)
H
III
H
CO
2
(
i-
Pr)
IV
R
II
I
Figure 4.7.
Model for cyclopentanone formation with Hashimoto's catalysts [62,155].
carbenoid complex under ambient conditions [72,118]. The phthaloyl-protected amino
acid-derived dirhodium complexes are the most effective catalysts for intramolecular
C-H insertions of
- ketoesters (Table 4.1 ) [121,123,151,153,154] . The enantio-
control was found to be very dependent on ester group size as well as substituents
adjacent to the insertion site. The highest enantioselectivities (57-80% ee) were obtained
with relatively large ester groups. Aryl substituents at the insertion site also increased
enantiocontrol relative to vinyl or alkyl substituents. Electron-withdrawing groups on
the aryl ring signifi cantly enhanced the level of enantioinduction [3].
Figure 4.7 shows a schematic model that rationalizes the outcome of the intramolecu-
lar C-H insertion process to form cyclopentanones with Hashimoto's catalyst systems
[3,122,155] . The O - Rh -O plane is represented by a disk that is divided into four quad-
rants [62]. The phthaloyl groups are represented by rods, which sterically restrict the
space above the O- Rh -O plane. Since these catalysts are proposed to be
C
2
- symmetric,
two pthaloyl groups will be present in adjacent quadrants II and III [62,122]. The bulky
ester substituent on the carbenoid will minimize steric interactions with the blocking
groups and therefore align along the
y
-axis, pointing away from quadrants III/IV. Fur-
thermore, the ketone substituent will also minimize steric interactions such that the ring
closure will mainly occur via quadrant IV to result in
Si
-face attack on the carbenoid
carbon through a half-chair-type transition state [122,155]. This model successfully pre-
dicts the major products of these reactions.
Taber and Malcolm reported a total synthesis of the marine secosteroid (− ) - astrogor-
giadiol, in which they employed chiral dirhodium(II) catalysts in the intramolecular C- H
α
- diazo -
β
