Chemistry Reference
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Ph
Ph
Ph
N
N
Ph
Ph
N
Ph
N
N
+
+
SO
3
H
N
Mes
Cl
−
Mes
Cl
−
+
Cl
−
Mes
HO
HO
L81
L80
L82
Figure 8B.42.
Chiral carbene ligands used in allylic alkylations.
TABLE 8B.63. Allylic Alkylations Using Chiral Carbene Complexes
R
1
R
1
R
R
2
Zn
γ
R
2
R
2
OPO(OEt)
2
CuX/
L
THF, -15°C
R
1
R
2
Entry
R
CuX
Ligand
Yield (%)
ee (%)
1
Me
H
Ph
CuCl
2
(Ag -
L80
)
2
58
71
2
Et
H
Ph
CuCl
2
(Ag -
L80
)
2
68
86
3
Et
Me
Ph
Cu(OTf)
2
(Ag -
L80
)
2
88
91
4
Et
H
1 - Naphth
Cu(OTf)
2
(Ag -
L80
)
2
80
89
5
E t
H
c
- Hex
Cu(OTf)
2
(Ag -
L80
)
2
53
94
6
Et
H
PhMe
2
Si
Cu(OTf)
2
(Ag -
L80
)
2
72
98
7
Et
Me
PhMe
2
Si
Cu(OTf)
2
(Ag -
L80
)
2
75
91
8
i
- Pr
H
PhMe
2
Si
Cu(OTf)
2
(Ag -
L80
)
2
75
96
9
Ph
H
PhMe
2
Si
Cu(OTf)
2
(Ag -
L80
)
2
91
92
10
Me
H
Ph
CuCl
2
(Ag -
L81
)
2
68
90
11
Et
H
Ph
CuCl
2
(Ag -
L81
)
2
80
90
12
Et
H
c
- Hex
CuCl
2
(Ag -
L81
)
2
80
95
13
Et
Me
Ph
CuCl
2
(Ag -
L81
)
2
84
97
14
Et
Me
c
- Hex
CuCl
2
(Ag -
L81
)
2
76
97
a dimeric Ag complex [(Ag-
L80
)
2
], which on addition of copper salts effectively gener-
ated the catalytically active copper-carbene complex (Table 8B.63, entries 1-5). This
protocol is also suitable for the highly stereoselective addition of various zinc reagents
toward silylated allylic substrates (entries 6-9), giving easy access to chiral allyl silanes
[369] .
The results could be further improved by using the new ligand
L81
, respectively its
dimeric silver complex (Ag-
L81
)
2
[370]. This NHC ligand bears a chiral diamine back-
bone and an achiral biphenol group. Upon coordination to the metal (Ag or Cu), the
diamine subunit controls the coordination mode of the biphenol such that the complex
is formed as a single atropisomer. Because the ligand does not require optically pure
biaryl amino alcohol, its synthesis is much easier compared with the binaphthol ligand
