Chemistry Reference
In-Depth Information
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