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within a polarised alkyne complex intermediate. In contrast, following theoretical studies,
Gimbert and Greene have used electronic differences between the alkyne termini in order
to support the loss of a specific CO ligand (via a
trans
-effect exerted by the alkyne), in
turn, leading to selective olefin insertion and, ultimately, delivering the cyclopentenone
regiochemistry observed.
EtO
2
C
Tol., 80 °C, 6 h
+
CO
2
Et
O
Co
2
(CO)
6
H
8
65%
7
H
Scheme 1.6
1.2.2 Regioselectivity of Alkene Insertion
From the broad array of examples performed as the scope of the PK annulation process was
being established, it became clear that the selectivity associated with the alkene reaction
partner was significantly less predictable.
5c,9
In particular, as well as delivering only modest
yields of cyclopentenones, simple acyclic alkene substrates are incorporated with no re-
gioselection when reacted with terminal alkynes (although the elevated levels of selectivity
associated with the alkyne component are retained). This is illustrated in Scheme 1.7 by the
reaction of 1-octene
9
with the phenylacetylene complex
1
to give a 1:1 mixture of the PK
cyclisation products.
14
In contrast, when internal alkynes (e.g.
10
) were employed with the
same class of aliphatic alkene substrates (e.g.
9
) significantly enhanced regioselectivity in
relation to alkene insertion is achieved, albeit at the cost of appreciably lowered chemical
yields in most instances.
14
O
O
Tol., 95 °C
24 h
49%
Ph
C
6
H
13
-n
Ph
+
+
Ph
H
Co
2
(CO)
6
n-C
6
H
13
1
9
1 : 1
C
6
H
13
-n
O
O
Tol., 97-98 °C
24 h
Ph
C
6
H
13
-n
Ph
+
+
Ph
Me
Co
2
(CO)
6
n-C
6
H
13
10
9
18%
Me
Me
19 : 1
C
6
H
13
-n
Scheme 1.7
It is worth noting that incorporation of aryl-substituted and heteroatom-containing
alkenes can offer good to excellent levels of regioselectivity with terminal alkyne partners.
5c
Indeed, the same can be said, in a general sense, for unsymmetrically-substituted cyclic
alkenes. Returning to the issue of employing more simple alkenes with terminal alkynes
