Chemistry Reference
In-Depth Information
group [34b,c]. Oxygen functionality including free hydroxyl group, ester, acetal, ketone,
and silyl ether as well as nitrogen functionality such as carbamate and tertiary amine
were well tolerated. In these reactions, the catalysts activate the C-C double bond for
attack by the internal nitrogen nucleophile, with the trichloroacetimidate functional
group eventually serving as a leaving group [34d].
The highly active ferrocenyl-imidazoline palladacycles FIP-Cl
89a
and
89b
have been
developed as catalysts for the asymmetric rearrangement of
N
-
para
- methoxyphenyl tri-
fl uoroacetimidate
84
by Peters and others (Scheme 5.27) [36a]. Catalysts have an
N
-
sulfonyl group and an imidazoline constituent that is synthesized from an enantiomerically
pure
C
2
-symmetric diamine. The rearrangements were performed in the presence of PS
1,8 - bis(dimethylamino)naphthalene. The FIP - Cl
89a
shows better reactivity and selectiv-
ity than
86a,b
, giving the desired product in 95% yield with 98% ee. Furthermore, amide
(
R
) -
85
was formed in excellent yield even with 0.05 mol % catalyst loading. In contrast,
catalyst
89a
did not provide useful ee values for the rearrangement of (
Z
) -
84
. Good
enantioselectivity was observed in the rearrangement of (
Z
) -
84
by using FIP-Cl
89b
to
give opposite isomer (
S
) -
85
. In these reactions, the silver salts presumably not only lead
to an exchange of Cl in the active catalyst species, but also oxidize the ferrocene moiety
to provide a ferrocenium cation since the rearrangement proceeds extremely slowly with
only two equivalents of silver salts per dimmer catalysts. This method using FIP-Cl
89a
was extended to the rearrangement of 3,3-disubstituted allylic trifl uoroacetimidates to
form allylic amines with quaternary
N
- substituted stereocenters [36b] . Highly active
bispalladacycle catalyst
90
has also been developed for the rearrangement of
Z
- confi g-
ured trifl uoroacetimidate (
Z
) -
84
[7c]. In this reaction, the use of AgOTs led to increased
catalyst activity and an improved enantioselectivity, providing (
S
) -
85
with 95% ee.
Ph
FIP-Cl
89a
FIP-Cl
89b
Ph
Ts
N
2
Ph
N
Pd
Cl
AgTFA, PS
Pd
2
N
(
S
/
R
)-
85
(
E
/
Z
)-
84
Ph
Cl
CH
2
Cl
2
N
Ts
F
e
R
F
e
R
FIP-Cl (mol %) Yield (%)
ee (%)
(
E
/
Z
)-
84
Cl
Pd
2
R
R
(
E
)-
84
(
E
)-
84
(
Z
)-
84
89a
(0.5)
89a
(0.05)
89b
(5.0)
95
95
72
98 (
R
)
95 (
R
)
93 (
S
)
N
N
Ts
Ph
R
FIP-Cl
89a
: R=Ph
FIP-Cl
89b
: R=Me
Ph
Bispalladacycle
90
Scheme 5.27.
The study on aza-Claisen rearrangement was extended to the catalytic asymmetric
allylic esterifi cation reaction by Kirsch and Overman (Scheme 5.28) [37a]. Although
(
Z
) - allylic trichloroacetimidates (
Z
) -
87
showed low reactivity toward aza-Claisen rear-
rangement [34b], the imidates (
Z
) -
87
reacted effi ciently with carboxylic acids to give
chiral allylic esters
91
. When COP - OAc
86c
was employed as a catalyst, the allylic esters
91
are produced in high enantiopurity (99% ee). However, the same treatment of the
E
stereoisomer (
E
) -
87
with acetic acid provided
91
in low yield and enantioselectivity. In
this reaction, the COP-OAc
86c
activates the C-C double bond for attack by the external
oxygen nucleophile, with the trichloroacetimidate functional group both templating the
