Chemistry Reference
In-Depth Information
alkenes can be used in benzotrifl uoride (BTF) as the best solvent. The Tol-BINAP-Cu(I)
complexes with ClO
4
or PF
6
provide high yield and enantioslectivity.
The asymmetric catalysis of imine-ene reactions is reported by dicationic SYNPHOS-
Pd complex
5d
(92% ee) rather than the BINAP-Pd complex
5b
(89% ee) (Scheme
8D.34) [65]. Regardless of the high catalyst loading (20 mol %), it should be noted that
the use of dicationic Pd complex
5
in the presence of MS 4Å leads to good yield and
enantioselectivity. Additionally, (
S
) - Tol - BINAP - Pd/(
±
) - DABN complex
29
signifi cantly
enhances the catalytic activity and enantioselectivity.
Pd cat
NTs
N
HTs
+
CH
2
Cl
2
, r.t., 20 h
H
2
Et
CO
2
Et
89% ee (66%)
92% ee (65%)
92% ee (51%)
5b
(20 mol %, MS 4A)
5d
(20 mol %, MS 4A)
29
(5 mol %)
O
Ar
2
P
H
2
N
Ph
2
P
O
O
Pd
2+
Pd
2+
Ar
2
H
2
2SbF
-
Ph
2
2SbF
-
O
5d
29
(Ar = 4-Me-C
6
H
4
)
Scheme 8D.34.
While enamides can be easily prepared, handled, and stored at room temperature,
their use as nucleophiles is limited in asymmetric synthesis. Recently, Kobayashi et al.
reported the fi rst example of Cu(II)-catalyzed enantioselective addition of enamides to
imines (Scheme 8D.35 ) [66] . The Cu(II) complex
30
bearing chiral diamine ligand by
the treatment of
N
-carbamate-protected imine and several enamides facilitates the aza-
ene-type reaction to provide the adducts in high yields and enantioselectivities. The
diamine product obtained by the one-pot treatment [67] can be converted into lactam
with two chiral centers in high yield (Scheme 8D.36).
This asymmetric aza-ene-type reaction of enamides to imines using chiral Cu com-
plexes is applied to chiral Brønsted acid catalysis (Scheme 8D.37) [68]. The reaction
using chiral phosphoric acid
26b
smoothly proceeds to afford excellent yield and enan-
tioselectivity without any detrimental effect even on a gram scale.
Terada et al. extended their study to chiral Brønsted acid-catalyzed tandem aza-ene-
type reaction-cyclization cascade for the asymmetric synthesis of piperidine derivatives
(Scheme 8D.38 ) [69] . The reaction of
N
-Boc aldimines with monosubstituted enecarba-
mates by chiral phosphoric acid
26c
occurs to give the cyclic products piperidines in
high diastereo- and enantioselectivity. The generation of more reactive intermediate by
using the monosubstituted enecarbamates induces the second aza-ene-type reaction
(Scheme 8D.39 ).
