Chemistry Reference
In-Depth Information
[2
1] carbonylative cyclization reaction. This type of reaction was pioneered by Pauson
and Khand in 1971.
2
This transition metal-mediated reaction is a cycloaddition of three
components, an alkyne, alkene and a carbon monoxide moiety, for generating a variety of
synthetically useful cyclopentenones (Scheme 7.1).
3
+
2
+
O
O
R
R
+
R''
R'''
R''
R
R'
R'
CO
R'''
R''
Pauson-Khand
(-type) reaction
R'
O
O
R'''
R'
R'
+
R''
R'''
R
R
R'''
R''
Scheme 7.1
Pauson-Khand type reaction.
In this pioneering study of an intermolecular-type reaction, only activated and symmet-
rical alkenes such as ethylene and norbornene were used, since regioisomers are expected
to be formed when unsymmetrical alkyne and alkene are applied. Nevertheless, previous
research studies showed that the regiochemistry from the alkyne partner is predictable and
does not give a mixture of products in general. In addition to the intermolecular reaction, the
intramolecular version of this reaction is expected to avoid the formation of regioisomeric
mixture and thus only gives single bicyclic cyclopentenone (Scheme 7.2). In this context,
we briefly summarize the early developments on the catalytic Pauson-Khand reaction. Thus
we focus on the evolution of the catalytic systems and their potential applications from the
late 1990s to early 2011.
R
R
CO
O
Pauson-Khand
(-type) reaction
R'
R'
(intramolecular)
Scheme 7.2
Intramolecular Pauson-Khand reaction.
7.2 Rhodium-Catalyzed Pauson-Khand-Type Cyclizations
The first successful Rh-catalyzed asymmetric intramolecular Pauson-Khand-type reac-
tion was reported by Jeong and co-workers in 2000.
4
They disclosed a complex of
[RhCl(CO)
2
]
2
/(
S
)-BINAP which was highly effective for the enantioselective transforma-
tion of tethered 1,6-enynes substrates to give corresponding cyclopentenones (Scheme 7.3).
Excellent yields and enantioselectivities (up to 96% ee) were obtained from an array of
C
-,
O
- and
N
-tethered 1,6-enynes.
