Chemistry Reference
In-Depth Information
O
OCH
3
P
P
PPh
2
H
3
CO
PPh
2
OCH
3
O
P
(
S
)-CAMP
(
S,S
)-DIPAMP
(
S,S
)-DIOP
PPh
2
PPh
2
Ph
NHPPh
2
PPh
2
PPh
2
PPh
2
Ph
NHPPh
2
PPh
2
(
S,S
)-CHIRAPHOS
(
S
)-PROPHOS
(
R,R
)-CBD
(
S,S
)-PNNP
Ph
2
P
PPh
2
X
Fe
PPh
2
PPh
2
N
COOtBu
(
R,S
)-BPPFA: X = NMe
2
(
R,S
)-BPPOH: X = OH
(
S,S
)-BPPM
Figure 7.1.
Early development in chiral phosphorus ligands.
of enol acetate [23b], while the C3-TunePhos was more effective for cyclic
β
-
dehydroamino acids and
-phthalimide ketones [23c,d]. Recently, another variation of
this ligand family was prepared by Chan, Zhang, and others possessing a chiral linker.
The recently developed analogous C3*-TunePhos not only improved the enantioselec-
tivity, but the synthetic route was also signifi cantly improved [24] .
It seems that the great inspiration emerges every 10 years. In the early 1990s, Burk
and coworkers made another great contribution to asymmetric hydrogenation by intro-
ducing their fi rst
trans
- 2,5 - dialkyl - substituted phospholane ligand BPE [25] , followed
by the more famous 1,2 - phenyl - linked DuPhos family (Fig. 7.6 ) [26,27] . These dialkyl -
or trialkyl-substituted phosphines are considered to be more electron rich compared
with the triaryl-substituted BINAP family. The cyclic structures also provide great
rigidity. These advantages have entitled BPE and DuPhos as one of the most effi cient
ligands for a wide variety of substrates such as
α
- dehydroamino acid derivatives,
enol acetates,
N
-acylhydrazones, enamides, enol esters, itaconic acids and
α
- and
β
- keto ester
derivatives [26,27,28]. The successful early application and modifi cation of these two
ligands have soon promoted the intensive studies on the phospholane structure motif.
A large number of ligands were developed for various purposes. Some of the good
β
