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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
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