Chemistry Reference
In-Depth Information
9
3
-or
-Allyl Complexes
3
-or
In an
-allyl complex, all three carbons of the allyl unit are coordinated to the metal and are in a plane.
This can be seen in the X-ray structure of the ferrilactone complex
9.1
(Figure 9.1), which also contains
three coordinated carbon monoxide molecules and an
1
-acyl unit. The chemistry of these iron complexes is
discussed further in Section 4.5.1.
9.1 Stoichiometric Reactions of
-Allyl Complexes
These complexes can be made using many different transition metals and are often stable. Cationic iron
complexes can be made from allylic alcohols or dienes via their
4
-complexes by treatment with acid
(Schemes 9.1 and 9.2). The non-coordinating BF
4
−
counter ion is often used.
2
-or
-Allyl complexes of many other elements can also be easily made. One way is to use a nucleophilic
metal complex. Substitution of bromide or chloride from an allylic halide gives an
1
-allyl complex,
9.9
or
3
-complex, with loss of one ligand (Scheme 9.3), in
9.12
. While these often spontaneously convert to the
1
-complex may be sufficiently stable to be isolated and requires heating to convert it to the
other cases the
3
-complex (Scheme 9.4).
1
3
-Allyl complexes may also be formed by the insertion of dienes into carbon-metal bonds (Scheme 9.5).
The easily prepared acyl cobalt complex
9.14
reacts with 1,3-dienes to give the
3
-allyl cobalt complex
9.16
.
2
The CO ligands are electron withdrawing and these allyl complexes are electrophilic. The iron complexes,
being cationic, are the more reactive.
3
Very weak nucleophiles such as enol ethers, allyl silanes and electron-
rich aromatics may be used (Scheme 9.6). Malonate anions, amines and organozinc reagents have also been
used. The resulting
2
-complexes are usually quite unstable
9.18
and decomplex on exposure to air to give
the allylation product
9.19
.
In the case of aromatics, this is a mild way of carrying out a Friedel-Crafts reaction. The cobalt complexes
are also electrophilic, but, being neutral, less so.
4
The intramolecular reactions are particularly effective
(Scheme 9.7).
5
The iron complexes have found a particular value in rendering allyl cations chiral.
6
An ordinary allyl
cation is planar and, therefore, achiral. Coordination to iron can retain the chirality as coordination of the
metal distinguishes the two faces. It has been shown that the
2
-sulfonyl alkene complex
9.23
can be formed
in good d.e. and recrystallized to complete purity (Scheme 9.8). Acid treatment yields the desired cationic
3
-allyl complex
9.24
, and treatment of this with suitable nucleophiles yields the allylated products, after
