Chemistry Reference
In-Depth Information
synthesized chiral allylic amine derivatives starting from achiral allylic imidates [31,32].
In 1997, the fi rst catalytic asymmetric rearrangement of prochiral allylic imidates to
chiral allylic amides was described [31a]. Two years thereafter, ferrocenyloxazoline pal-
ladacycle, FOP - TFA
83
, was introduced for the asymmetric rearrangement of prochiral
N
- arylbenzimidates
81
to give chiral
N
- arylbenzamides
82
by Donde and Overman
(Scheme 5.25) [31d]. In this section, recent progress in this area is described [33].
Overman and others reported that
N
- (4 - methoxyphenyl)trifl uoroacetimidate
84
is an
excellent substrate for the asymmetric rearrangement and its product
85
can be depro-
tected in two steps (Scheme 5.26) [34]. The palladacycles COP-Cl
86a
and COP - TFA
86b
have been developed as catalysts for the rearrangement of trifl uoroacetimidate
84
[34a] . Particularly, COP - Cl
86a
is a superior catalyst because it does not require activa-
tion with silver salts. In general, enantioselection for rearrangement of
E
- confi gured
imidate (
E
) -
84
was higher using COP-Cl
86a
, whereas higher enantioselection in the
rearrangement of
Z
- confi gured imidate (
Z
) -
84
was realized with COP-TFA
86b
. The
rearrangement catalyzed by COP-TFA
86b
was also described by Kang and others [35].
The palladium-catalyzed rearrangements are frequently plagued by competing elimina-
tion reactions and slow reaction rates due to coordination of an imidate nitrogen to a
neutral palladium center. Overman and others reported that COP-Cl
86a
is an excellent
catalyst not only for the rearrangement of
N
- (
p
- methoxyphenyl)trifl uoroacetimidate
84
but also for the rearrangement of trichloroacetimidates
87
without an
N
- aryl protecting
SiMe
3
Ph
Ph
O
Ar
Ar
N
FOP-TFA
83
N
O
N
O
Pd
F
e
X
R
R
*
2
81
82
FOP-TFA
83
: X = OCOCF
3
Scheme 5.25.
MeO
MeO
2
X
CF
3
CF
3
O
COP-Cl
86a
COP-TFA
86b
Pd
N
O
N
N
CH
2
Cl
2
n
-Pr
n
-Pr
*
O
Ph
C
o
Ph
(
E
/
Z
)-
84
(
S
/
R
)-
85
(
S
)-
85
(92%, 92% ee from (
E
)-
84
by using
86a
)
(
R
)-
85
(78%, 89% ee from (
Z
)-
84
by using
86b
)
Ph
Ph
COP-Cl
86a
: X = Cl
COP-TFA
86b
: X = OCOCF
3
CCl
3
O
CCl
3
COP-Cl
86a
HN
HN
O
R = alkyl, Ph, CH
2
OH,
(CH
2
)
3
NBn(Boc), (CH
2
)
9
NBn
2
CH
2
Cl
2
R
R
(
E
)-
87
(
S
)-
88
(up to 98% ee)
Scheme 5.26.
