Chemistry Reference
In-Depth Information
O
O
(
R
)-TolBINAP-AgOTf
177
NHPh
THF
OSnR
3
O
-nitroso aldol
175
(95%, 97% ee from
174a
)
O
+
N
Ph
O
OH
N
174a
: R = Me
174b
: R = Bu
*
(
R
)-BINAP-2AgOTf
178
Ph
EtOCH
2
CH
2
OEt
176
(94%, >99% ee from
174b
)
N
-nitroso aldol
Ar
Ph
Ar
Ph
P
P
P
P
AgOTf
AgOTf
AgOTf
Ar
Ar
Ph
Ph
Ar =
(
R
)-TolBINAP-AgOTf
177
(
R
)-BINAP-2AgOTf
178
Scheme 5.54.
OSiMe
2
TMS
O
Ligand
183
, AgBF
3
O
O
(1:1 ratio)
NHPh
+
N
Ph
CsF, THF, MeOH
Ph
Ph
Ph
dr >99:1
179
O
180
(91%, 99% ee)
POPh
O
OSiMe
2
TMS
O
Ligand
183
, AgBF
3
O
O
(1:1 ratio)
NHPh
Ph
+
N
CsF, THF, MeOH
Ph
Ligand
183
Ph
Ph
dr 91:9
181
182
(70%, 99% ee)
Scheme 5.55.
The enantioselective hydroxylation of silyl enol ethers with nitrosobenzene was also
studied in the presence of a chiral silver catalyst by Yamamoto and others (Scheme 5.55)
[72e]. The disilanyl enol ethers having an Si-Si bond were excellent nucleophiles in the
presence of a fl uoride source. Because the electron-defi cient ligands gave the
O
- adduct
preferentially, the chiral phosphite ligands were tested and then the ligand
183
was found
to be effective for asymmetric hydroxylation. The utility of a chiral silver catalyst,
derived from phosphite ligand
183
and AgBF
3
in 1:1 ratio, was examined in diastereose-
lective reaction of the chiral disilanyl enol ethers
179
and
181
. The reaction of
179
afforded (2
R
,3
R
) -
180
as a single diastereomer in 91% yield. The enantiomeric substrate
181
reacted with nitrosobenzene to give (2
R
,3
S
) -
182
with high diastereoselectivity.
These results show that the stereochemical outcome of the reaction can be controlled
by the catalyst regardless of the confi guration of the disilanyl enol ethers at C3.
