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7.5 Ruthenium-Catalyzed Pauson-Khand-Type Cyclizations
In 1997, Mitsudo 65 and Murai 66 independently disclosed Ru-complexes that were effective
in promoting intramolecular Pauson-Khand-type cyclizations. Under a high atmosphere of
carbon monoxide and at a high temperature, the Ru-catalyzed PK reaction was successfully
achieved and gave the desired bicyclic products in high yields (Scheme 7.35). In con-
trast, the complex of [RuCl(cod)Cp ], which was generally an effective catalyst for [2
+
2]
cycloaddition of norbornenes with alkynes, 67
was totally ineffective in this carbonylative
[2
+
2
+
1] reaction.
M i tsudo - p r o t ocol
R=alkyl,TMS
Z=C(CO 2 Me) 2
DMAc, 140
C
CO (15 atm), 8 h
°
Up to 89% yield
R
R
Z
2 mol% Ru 3 (CO) 12
Z
O
R = Ar, alkyl, TMS
Z = C(CO 2 Et) 2 ,O,NTs
Dioxane, 160 ° C
CO (10 atm), 20 h
Up to 95% yield
Mu r a i- p r o t oco l
Scheme 7.35 Ru-catalyzed PKR of 1,6-enynes.
1] co-
cyclization of isocyanates, alkyne and CO affording the corresponding maleimides in up to
98% isolated yield. 68 This process allows a rapid and atom-economical protocol to generate
a series of unsymmetrical and polysubstituted maleimides in one-pot (Scheme 7.36).
In 2006, Kondo e. al. demonstrated the first Ru-catalyzed intermolecular [2
+
2
+
O
R 1
3.3 mol% Ru 3 (CO) 1 2
Mesitylene, 130
RNC
O
R 1
R 2
R
N
°
C
R 2
R 1 ,R 2 =Ph,alkyl
3-42 h
CO (1 atm)
R=Ar,alkyl
O
82-98% yield
Scheme 7.36 Ru-catalyzed intermolecular [2
+
2
+
1] co-cyclication.
Kondo and co-workers also postulated a mechanism for this newly developed co-
cyclization (Scheme 7.37). With the evidence of 13 C-labelled experiments to hand, they
suggested the formation of azaruthenacyclopentenones was as a mixture of regioisomers.
The formation of
-phenylsubstituted intermediate was verified to be more favourable than
-methylsubstituted intermediate, as judged by 13 C NNE spectra. The insertion of CO into
 
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