Chemistry Reference
In-Depth Information
R
1
O
O
OR
1
R
2
Ru
3
(CO)
12
,PEt
3
,THF
R
2
O
CO, 160 °C
O
124
121
127a-d
127a
R
1
=
i-
Pr R
2
=
n
-Bu 75%yield
127b
R
1
=
n
-Bu R
2
=
n
-Bu 60%yield
127c
R
1
=Et R
2
=
n
-Bu 47%yield
127d
R
1
=
n-
Bu R
2
=O
n
-Bu 46%yield
Scheme 10.36
Ru catalyzed synthesis of cyclopentenones.
Cyclopentenones may also be synthesized by the rhodium-catalyzed hydroacylation of
alkynals.
It has been reported that Rhodium(I) complexes catalyze the intramolecular hydroacy-
lation of a variety of 4-alkynals
128
to generate cyclopentenones
131
.
75, 76
The accepted
mechanism indicates a
trans
addition of the rhodium hydride to the alkyne to generate
the six-membered rhodium metallacyclohexene
130
. Finally, reductive elimination of com-
plex
130
renders the desired cyclopentenone and regenerates the rhodium(I) catalyst as
illustrated in Scheme 10.37.
O
O
H
R
R
R
Rh
+
Rh
+
CHO
Rh
+
O
R
Rh
+
131
128
129
130
Scheme 10.37
Proposed mechanism for the Rh catalyzed hydroacylation.
The reaction is simply catalyzed by [Rh(dppe)]
2
(BF
4
)
2
affording the final cyclopen-
tenones in good yields as shown in Scheme 10.38.
O
Ph
Ph
10 mol% [Rh(dppe)]
2
(BF
4
)
2
Acetone, ACN, 100 °C
CHO
88% yield
Me
Me
132
133
Scheme 10.38
Rh catalyzed hydroacylation.
The scope of the reaction was subsequently expanded by the use of 5-alkynals. Cy-
clopentenones were obtained via a tandem hydroacylation/double bond migration that
takes place at elevated temperatures. This methodology gives access to
α
,
β
-disubstituted
cyclopentenones such as dihydrojasmone (Scheme 10.39).
77