Chemistry Reference
In-Depth Information
MX
n
MX
n
MX
n
R
R
R
6.233
6.234
6.235
MX
n
X
n
M
H
+
attack at
carbon a
R
R
R
X
n
M
H
R'O
R'O
6.237
6.239
6.238
b
R
X
n
M
a
MX
n
H
H
+
R
attack at
carbon b
HOR'
R
R
6.236
6.242
H
H
OR'
OR'
6.241
6.240
R
6.243
Scheme 6.99
6.2.5 Ene-Yne Cyclization
The metal-catalysed ene-yne cylization involves an electron-poor
2
-alkyne complex, with metals including
gallium(III),
109
platinum(II), platinum(IV),
110
gold(I)
111
and gold(III), but the mechanism is not as simple as
nucleophilic attack. A proposed mechanism (Scheme 6.99), supported by calculation work, involves formation
of a
2
-alkyne complex
6.234
, alkene cyclopropanation by the complex, with the complex itself converting into
a carbene
6.236
112
(for a simpler alkyne to carbene conversion see Section 8.2).
113
The carbene intermediate
may also be viewed as metal-stabilized carbocation
6.237
. The carbene and carbocation representations of
these intermediates are “mesomeric extremes of a generic picture”.
114
The ene-yne cyclization reaction may
operate in either an
exo-
or an
endo-
mode. The cyclopropyl carbene
6.236
cannot be viewed as two isolated
functional groups as the orbitals of the two are interlaced. An anionic
1
-complex
6.242
conjugated through
a migrated cyclopropane to a carbocation has been put forward as a canonical form. Both forms are useful
for understanding the subsequent reactions. In the
exo-
mode, a nucleophile, such as an alcohol may attack at
either of two carbons (a or b) of the cyclopropane
6.236
to deliver either
1
-vinyl complex
6.238
or
6.240
,
which then undergo protio-demetallation to give the cyclization-addition products,
6.239
or
6.241
.
115
These
nucleophilic reactions proceed with stereochemical integrity. Alternatively, demetallation may occur to give
a diene
6.243
,
116
similar to the products of ene-yne metathesis (Section 8.3.7). Related cycloisomerization
products can be obtained using palladium, but these are via metallacyclic intermediates (Section 11.5).
Related pathways can operate in the
endo
-mode, yielding the same functionality, but in a different carbon
skeleton (Scheme 6.100). Other reactions may also occur, such as demetallation of the carbene to give an
alkene
6.248
.
