Chemistry Reference
In-Depth Information
O
CO
2
Me
O
OMe
[Rh( cod) ]SbF
5
CH
2
Cl
2
, 25 °C
R
2
CO
2
Me
R
1
(OC)
5
Cr
R
1
R
1
R
2
66
65
R
2
CO
2
Me
67
68
R
1
=Ph R
2
=H
67a
75%yield
R
1
=2-furyl R
2
=H
67b
88%yield
R
1
=2-furyl R
2
=Ph
68a
75%yield
R
1
=2-furyl R
2
=1-cyclohexyl
68b
81%yield
Scheme 10.18
Rhodium catalyzed [3
+
2] cycloaddition.
using Nickel(0) complexes as catalysts. The reaction tolerates unactivated alkynes, boron
or tin substituted alkynes and electron rich alkynes furnishing the final cyclopentenones
with moderate to good yields (43-75%) and with total regioselectivity as shown in
Scheme 10.19.
O
OEt
OMe
1) Ni(cod)2, ACN
R
2
R
1
R
1
(OC)
5
Cr
2) SiO
2
70a-d
69
OEt
65
R
2
70a
R
1
=Ph R
2
=Bn 48%yield
70b
R
1
=2-furyl R
2
=Bn 43%yield
70c
R
1
=2-furyl R
2
=TMS 45%yield
70d
R
1
=Ph R
2
=TMS 49%yield
Scheme 10.19
Nickel catalyzed [3
+
2] cycloaddition.
The proposed mechanism (Scheme 10.20) involves Cr-Ni exchange, followed by regios-
elective insertion of a disubstituted alkyne unit into the nickel carbine complex leading to
carbine
71
; this species undergoes ring closure rendering the corresponding cyclopentadi-
ene
74
, possibly via the nickelacyclohexadiene species
73
. The origin of the regioselectivity
lies in the alkyne insertion step and is consistent with the trend found in the insertion of these
types of alkynes into Fischer complexes of chromium. Remarkably, when terminal alkynes
were used a [3
+
2
+
2] cyclization took place affording the corresponding 7-membered
carbocycle.
322
10.4 Nickel(0) and Palladium(0) Synthesis of Cyclopentenones
A transformation reaction using Ni and Pd complexes, which is similar to the Pauson-
Khand reaction, has also been reported. The main difference between the techniques is that
the nickel and palladium route requires the use of allyl halides and alkynes to furnish the
