Chemistry Reference
In-Depth Information
6
Electrophilic Alkene and Alkyne Complexes
Simple, isolated alkenes are relatively unreactive. They do react with radicals or powerful electrophiles, such
as strong acids, halogens, ozone and carbocations, but do not typically react with weak electrophiles, such
as alkyl halides. They do not react with nucleophiles, unless they bear an electron-withdrawing group. If an
alkene is coordinated to a transition metal in a high oxidation state, however, then ligand to metal electron
donation makes the alkene electrophilic and susceptible to nucleophilic attack. Extensive stoichiometric
studies have been carried out with iron complexes (Section 6.3); both catalytic and stoichiometric work with
palladium (Section 6.1). Recently, other metals, such as gold, silver and platinum, have gained in prominence
(Section 6.2). In some cases, isolable
2
-complexes, with the metal coordinated to one face of the alkene
-system are used (Figure 6.1). More often, the alkene complexes are intermediates in catalytic processes.
The product of nucleophilic attack on an
1
-complex (Scheme 6.1). Both alkynes
(Scheme 6.2) and allenes (Scheme 6.3) can be activated towards nucleophilic attack in a similar way. In the
case of allenes, the mechanism may not be a straightforward attack on a
2
-alkene complex is an
2
-complex: a
1
-coordinated allyl
1
-vinyl intermediate. Such
intermediates, in rare cases, have been isolated.
1
The same types of products may also be formed through
another mechanism involving insertion of the alkene, alkyne or allene into a metal-nucleophile bond.
The usefulness of these processes in organic synthesis depends upon what is made of the resulting
cation has been proposed. Nevertheless, both pathways would lead to the same
1
-complexes. In some cases, they proceed with typical organometallic reactions such as alkene insertion,
thereby creating a tandem process, or
-hydride elimination. In other cases, the C-M bond is cleaved by
protonolysis, or reaction with another electrophile.
6.1 Electrophilic Palladium Complexes
Nucleophilic attack on alkene-palladium(II) complexes became commercially important when the Wacker
process was developed.
2
In this process, ethylene is converted to acetaldehyde (Scheme 6.4). This involves
coordination of ethylene to palladium.
3
The oxygen atom, which comes from water, may then become attached
to carbon in one of two different ways. Direct nucleophilic attack on one carbon atom of the
2
-ethylene
1
-complex
6.3
. Alternatively, ligand exchange at palladium, with water
replacing chloride, can be followed by insertion of the coordinated ethylene into the palladium-oxygen bond
to give the same
complex
6.1
by water can form an
1
-complex
6.3
, via the hydroxy complex
6.2
. Which of these two variants of the mechanism
