Chemistry Reference
In-Depth Information
where remarkable increases in efficiency and, indeed, selectivity for the cyclopentenone
products have been achieved using rhodium-catalysed techniques.
10
1.2.1 Regioselectivity of Alkyne Insertion
As detailed previously in relation to Pauson's original findings, alkyne regiochemistry is
most often based on substituent sterics, with the larger of the C
C triple bond components
residing ultimately in the 2-position of the PK cyclopentenone product. The reasons for
this appear to be embedded in key steps within the perceived reaction pathway (as will
be detailed in Chapter 2). In most instances, the regioselectivity of alkyne insertion is
complete;
5c
that shown in Scheme 1.4 effectively illustrates this point, as well as providing
an example of the typical conditions traditionally employed for the use of a gaseous olefin
such as ethylene (
vide supra
).
7d
≡
O
n-C
5
H
11
Ethylene (120 atm),
Tol., 85 °C, 36 h
n-C
5
H
11
H
Co
2
(CO)
6
55%
Scheme 1.4
In relation to internal alkynes where the substituents are similar in size, mixtures of
products can result. One such example from Billington and Pauson, where both alkyne
insertion regioisomers are observed, is provided in Scheme 1.5;
11
despite a mixture of
products resulting, an 8:1 ratio of cyclopentenones (
5
:
6
) is obtained, with the less prevalent
regioisomer
6
being formed in only 3% yield.
O
O
Et
Me
Ethylene (35 atm),
Tol., 110 °C, 36 h
+
Et
Me
Co
2
(CO)
6
5
24%
6
3%
Me
Et
Scheme 1.5
Further examples have appeared in the literature where the mode of alkyne insertion
cannot be explained by steric factors. In this regard, it has been shown by Krafft that, in the
case of internal alkynes bearing electron-withdrawing components, electronic considera-
tions are required to be invoked in attempts to explain the regiochemical outcome of the P-K
process. More specifically, with alkynes conjugated with an electron-withdrawing group
(EWG), the cyclisation product formed is that with the EWG installed at the cyclopentenone
3-position.
12
This alkyne polarisation effect is further illustrated by Gimbert and Greene,
with the example of an internal alkyne such as
7
possessing electronically distinct yet
sterically rather similar substituents. In this instance, the sole cyclopentenone product
8
is
that with the more electron-deficient arene positioned in the
-position (Scheme 1.6).
13
In
attempts to explain these observations, in relation to the anticipated cyclisation mechanism,
Krafft has proposed an argument based on preferential alkene insertion into one Co-C bond
