Chemistry Reference
In-Depth Information
Buchwald mentioned that the Ni-mediated PK reaction was a failure in a catalytic
manner, because of the formation of nickel-isocyanide complexes, which prevent further
coordination, and hence cyclization of enyne. Upon using other isocyanide sources, such
as trialkylsilyl cyanides, the coupling transformation was effectively accomplished catalyt-
ically. The bicyclic cyclopentenones (after acid hydrolysis) were obtained in up to 85%
isolated yield (Scheme 7.39).
70
They hypothesized that trialkysilyl cyanides generate a
sufficient yet relatively low equilibrium concentration of the trialkylsilyl isocyanide isomer
(
K
0.01) to effect the PK coupling reaction.
71
≈
Ph
Ph
(
i
-Pr)
3
SiCN
L=
N
N
R
Ph
Ph
+
5-20 mol% [Ni(cod)
2
/L]
Z
O
R
H
+
DMF, 8-36 h
110-135 °C
Z
Up to 85% yield
R=Ph,alkyl
Z= O,C(CO
2
Et)
2
,N-Ph
Scheme 7.39
Ni-catalyzed cyclization of 1,6-enynes.
In 2003, Grigg
et al.
reported the Pd(PPh
3
)
2
-catalyzed intramolecular cycloaddition of
1,6-heptadiynes with CO (Scheme 7.40).
72
The corresponding bicyclic octadienones were
furnished in up to 74% yield
10 mol% Pd(PPh
3
)
2
R
R'
R
R'
O
DMF, 70
°
C, 8-48 h
K
2
CO
3
,CO(1atm)
30-74% yield
R, R' = H, CN, CO
2
Me, CO
2
Et
Scheme 7.40
Catalytic Pd-catalyzed PKR of 1,6-diynes with CO.
In 2005, the first successful Pd-catalyzed intramolecular Pauson-Khand-type cyclization
was achieved by Chen/Yang and co-workers.
73
They showed that the Pd-complexes derived
from the
N,N
-tetrasubstituted thiourea ligands with PdCl
2
in 1:1 ratio was critical for the
success of this Pd-catalyzed PKR. Ligand tmtu was the best ligand of choice to give the
desired products in up to 96% yield (Scheme 7.41). They pointed out that
C
- and
O
-tethered
1,6-enynes as substrates were ineffective in this reaction, while the
N
-tethered enynes were
efficiently transformed to corresponding bicyclic adducts in good yields. In addition, the
N
-tethered substrates containing an electron-deficient group on an alkyne terminus gave
better results than other substrates.
Later the same research group mentioned that the addition of LiCl as an additive into
the Pd-catalyzed intramolecular PK reaction could significantly enhance the corresponding
product yields (Scheme 7.42).
74
This catalytic system was found to be more general than
the previously developed Pd catalyst system.
73
