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of the developments in this domain. In relation to this, the preparative interconversion
now known as the Pauson-Khand reaction is a cyclopentenone-forming [2
1] annu-
lation process involving an alkyne, an alkene, and a unit of carbon monoxide, originally
supplied as a ligand in the starting hexacarbonylalkynedicobalt complex, as illustrated in
Scheme 1.1. Despite many citations to this process having been divulged initially in 1973,
the first account of this cobalt-mediated cyclisation was noted within a very short Chem-
ical Communications article from Pauson's laboratory, submitted in November, 1970 and
appearing in press in 1971, with the following line, β€œ
+
2
+
the reaction of norbornadiene
with complexes (I) [hexacarbonylalkynedicobalt complexes] yields hydrocarbon and ke-
tonic products derived from norbornadiene, acetylene, and carbon monoxide.” 1 Pauson and
co-workers went on to describe this process in appreciably more detail in 1973. 2
...
O
R !
R 2
Co 2 (CO) 6
+
R !
R 2
R 1 > R 2
Scheme 1.1
As detailed by Pauson and Khand, the formation of cyclopentenones in a single step
was a serendipitous discovery. 3 Within Pauson's laboratories at the University of Strath-
clyde, a programme of work had been aiming to establish the preparation of a series of
cobalt-containing compounds from accessible cobalt complexes and alkynes, as a method
by which to probe, for example, the reaction pathway involved in alkyne trimerisation pro-
cesses. Believing that sufficiently reactive alkenes could be employed in a similar fashion
to alkynes, norbornadiene was reacted with hexacarbonylalkynedicobalt complexes. Sur-
prisingly, the main organic products from these reactions were ketonic in nature and, more
specifically, possessed the general cyclopentenone structure, as shown in Scheme 1.2. 2a
This seminal discovery established the foundations for the study and enhancement of the
synthetic transformation that is the subject of this monograph.
R 2
R 1 = R 2 = H: 43%
R 1 = Me; R 2 = H: 33%
R 1 = Ph; R 2 = H: 45%
R 1 = R 2 = Ph: 28%
R 1 = R 2 = Et: 23%
R !
Benzene or toluene
R !
R 2
Co 2 (CO) 6
+
Ξ”
O
H
H
Scheme 1.2
Throughout the remainder of the 1970s, the initial scope and generality of this cyclopen-
tannulation process was established through an extensive series of studies by Pauson and
his co-workers. In relation to this, these reactions were conveniently performed by typically
heating a mixture of the alkyne Co-complex and the alkene in hydrocarbon (or ethereal)
solvents. Additionally, even from the initial series of reactions, for example those shown
in Scheme 1.2, good levels of regio- and stereoselectivity were achievable within these
 
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