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the reaction conditions. These two cyclic enones are easily separated by chromatography
and show characteristic
1
H NMR patterns. The allenes formally serve both as an excel-
lent alkene- and alkyne-type moiety in the PK-type [2
1] cycloaddition with carbon
monoxide. The optimum reaction conditions were established as follows: 1.0 mmol allene,
5mol%
Co
2
Rh
2
catalyst, 5 mL toluene, 130
◦
C, CO (2 atm), and 6 h. For reference pur-
poses, they also studied the same reaction in the presence of [Rh(CO)
2
Cl]
2
, which showed
a lower yield (57%). They tested other allenes under the optimized reaction conditions
(Scheme 9.20). The overall yields from carbonylative cycloaddition reactions were rela-
tively high (76-92%). Thus this process provides a rapid and atom-economical method for
the synthesis of a variety of cyclopentenones with an exocyclic double bond. This study
widens the scope of a substrate in the PK-type reaction.
+
2
+
O
O
Co
2
Rh
2
, 2 atm CO
130 °C, 6 h, toluene
•
2
R
+
R
R
R
R
A
B
R
Yield(A / B) (%)
4-tolyl
1-naphthyl
4-anisyl
4-acetylphenyl
hexyl
70/14
72/20
61/0
64/20
52/7
Scheme 9.20
9.4.2.7
Co
2
Rh
2
-catalyzed PK-type reaction of bisallenes
51
Compared to the advances in the chemistry of allenes and related compounds, those of
bisallenes lags far behind,
52
presumably because of their high instability and the difficulty
of their preparation.
Chung reported
51
the first use of bisallenes in a catalytic PK-type cycloaddition in the
presence of carbon monoxide using
Co
2
Rh
2
as a catalyst (Scheme 9.21).
•
Co
2
Rh
2
(5 mol%)
TsN
TsN
O
100 °C, CO, toluene
•
CO (atm)
Time (h)
Yield (%)
1
2
3
2
12
12
4
4
33
66
70
68
Scheme 9.21
