Chemistry Reference
In-Depth Information
TABLE 8B.46. R h - Catalyzed Allylic Alkylations of Chelated Enolates
OR'
N
O
OPO(OEt)
2
PG
PGHN
CO
2
R'
Zn
CO
2
R'
+
R
R
[RhCl(PPh
3
)
3
]/P(OR)
3
,
THF,
R
NHPG
−
78°C rt
b
l
Entry
R
R
′
PG
P(OR)
3
Yield (%)
b
:
l
d r
1
Me
t
- Bu
TFA
P(OMe)
3
84
87:13
92:8
2
Me
t
- Bu
TFA
P(OEt)
3
85
89:11
91:9
3
Me
t
- Bu
TFA
P(O
i
- Pr)
3
98
99:1
94:6
4
n
- P r
t
- Bu
TFA
P(OEt)
3
97
71:29
92:8
5
i
- B u
t
- Bu
TFA
P(OEt)
3
95
7:93
88:12
6
n
- Pr
Me
Cbz
P(OEt)
3
65
97:3
90:10
dr, diastereomeric ratio.
(Table 8B.46, entries 1-3), while the regio- and diastereoselectivity increased with the
steric demand of the phosphite. Increasing the size of the substituent R at the allyl
moiety shifted the product ratio toward the linear product (entries 4 and 5). While most
reactions were carried out with trifl uoroacetyl (TFA)-protected glycine enolates, other
standard protecting groups can be used as well. The Cbz group gave the highest branched
selectivity although in lower yield (entry 6).
8B.6.1.1. Asymmetric Rh-Catalyzed Allylations
The stereochemical outcome of the
reaction was investigated by Evans and Nelson [272]. The reactions of nonracemic
2-butenyl carbonate with soft nucleophiles proceeded with nearly complete retention of
confi guration (Scheme 8B.66). This is in strong contrast to results of the Pd-catalyzed
reactions.
OCO
2
Me
CH(CO
2
Me)
2
NaCH(CO
2
Me)
2
RhCl(PPh
3
)
3
/P(OMe)
3
,
THF, 30°C
97% ee
95% ee (86%)
Scheme 8B.66.
Rh-catalyzed allylic alkylation with retention of confi guration.
As intermediate, a confi gurationally stable enyl complex
A
(Scheme 8B.67 ) was
postulated. Such complexes could be characterized by IR and NMR [273] and also by
an X-ray structure analysis [274]. According to this model, the retention [275] of the
regio- and stereochemical information can be explained by a hindered rotation around
the
-bond based on the coordination of the double bond toward the Rh. The nucleo-
philic attack is probably much faster than this isomerization (k
2
σ
>>
k
2
), otherwise loss of
the chiral information is expected.
