Chemistry Reference
In-Depth Information
10.3.2
Synthesis of Cyclopentenones via [3
2] Cycloadditions
+
of Fisher Alkenyl Complexes
One important strategy for the synthesis of cyclopentenones is based on the [3
2]
cycloaddition between chromium-alkenyl complexes and alkynes. The reaction was
developed by Barluenga and co-workers using a Rh(I) complex as catalyst.
30
Ini-
tially, chromium methoxycarbene complexes react with rhodium(I) complexes to afford
rhodium(I) alkenyl(methoxy)carbine complexes in high yields. Remarkably, the simulta-
neous transfer of two ligands (carbine and CO) represents a rare process. Next, a metalla-
Diels-Alder reaction takes place to render a rhodium cyclohexadiene species that produces
the desired cyclopentadiene. The regioselectivity of the reaction is strongly dependent on
the nature of the alkyne. In the case of terminal alkynes the regiochemistry is simply dic-
tated by steric discrimination, the larger alkyne substituent being placed away from the
rhodium. In the case of internal electron-poor alkynes, the steric differentiation is not as
pronounced and the electronic factors, which favor the interaction between the electrophilic
metal center and the nucleophilic C-alkyne, dominate leading to cycloadducts with opposite
regiochemistry, as illustrated in Scheme 10.17.
+
O
O
L
Rh
MeO
RhL
n
H
MeO
H
R
1
R
1
R
2
R
2
R
2
R
1
R
2
R
1
60
59
56
57
58
O
L
Rh
O
MeO
RhL
n
EWG
MeO
EWG
EWG
EWG
R
1
R
2
R
1
R
1
R
2
R
2
R
2
R
1
64
63
56
61
62
Scheme 10.17
Regioselectivity in the Rhodium catalyzed [3
+
2] cycloaddition of Fisher
alkenyl complexes.
It should be noted that most of these types of cyclopentenones are not easily accessible
by the intermolecular Pauson-Khand reaction. As shown in Scheme 10.18, the rhodium-
catalyzed [3
2] cycloaddition affords the corresponding cyclopentenones in good yields
and with total regioselectivity.
An alternative [3
+
2] strategy for the synthesis of cyclopentenones is the nickel-catalyzed
cyclization of alkenyl Fisher carbene complexes and internal alkynes.
31
As was highlighted
in the foregoing discussion, alkenyl Fisher carbenes react with terminal alkynes or electron-
poor alkynes under Rh(I) catalysis to render cyclopentenones. In order to circumvent the
limitations of that method, Barluenga and co-workers developed a powerful methodology
+
