Chemistry Reference
In-Depth Information
In a further example with Co
2
(CO)
8
as catalyst, Hashimoto and Saigo have shown that the
introduction of tributylphosphine sulfide allows the use of only atmospheric pressure CO
to deliver efficient cyclisations in a series of intramolecular PK reactions (Scheme 1.23).
48
Notably with this method, an intermolecular reaction was also reported, albeit with the
strained and reactive olefin, norbornene.
EtO
2
C
EtO
2
C
Co
2
(CO)
8
(3 mol%), Bu
3
P=S (18 mol%)
O
CO (1 atm), C
6
H
6
, 70 °C, 5 h
EtO
2
CCO
2
Et
90%
Ph
Co
2
(CO)
8
(3 mol%), Bu
3
P=S (18 mol%)
+
Ph
H
Co
2
(CO)
6
CO (1 atm), C
6
H
6
, 70 °C, 7 h
O
92%
H
H
Scheme 1.23
Heterogenous cobalt-based catalysis has also been applied, in particular with the use
of Co loaded onto a range of supports, or by the employment of recyclable colloidal Co
nanoparticles, as divulged in an extensive series of reports by Chung, Hyeon, and co-
workers.
49
As illustrated in Scheme 1.24, cobalt on charcoal can be employed effectively in
intramolecular PK annulations, albeit at relatively elevated temperature and CO pressure.
Interestingly, the Co on C catalyst can be recycled extensively without loss in efficiency
and with
0.1 ppm of Co being bled from the support. This specific technique has also
been shown to be successful in intermolecular examples, again, with the reactive alkene,
norbornadiene.
EtO
2
C
EtO
2
C
Co/C (12 wt%),
CO (20 atm), THF, 130 °C, 7 h
O
EtO
2
CC
O
2
Et
91%
Scheme 1.24
From this brief summary of cobalt-based catalytic methods, it is clear that systems have
yet to be developed that would allow less reactive alkenes to be applied in an intermolecular
sense and, more generally, the employment of milder reaction conditions to be adopted.
Accordingly, catalytic PK protocols continue to be a focus of the preparative chemistry
community. In relation to this, a series of additional and complementary catalytic techniques
using other metal species are adding extensively to the growing repertoire of PK methods,
and these will be detailed in Chapter 7.
1.6 Concluding Remarks
From relatively modest beginnings at the University of Strathclyde, due to the outstanding
contributions of an extensive spectrum of international researchers, the Pauson-Khand
