metal centre (μ
2
Àη
1 :η
1 ). A 1,2-sigmatropic shift of an alkyne substituent (R in
structure B) can also yield end-on coordinated vinylidene species (structure C,
μ
2
Àη
2 ). For metal acetylide moieties the proximity of a second metal can often
yield (μ
2
Àη
1 :η
2 ) hybrid structures D, where the second metal coordinates in a
π-fashion to the σ-bonded metal acetylide. Finally an alkyne may coordinate to
two metal centres via its two orthogonal π-bonds, resulting in a tetrahedral C 2 M 2
structure (E), where the C C axis lies across the metal–metal axis (μÀη
2 :η
2 ).
This review is not intended to be fully comprehensive but instead should serve to
highlight current understanding of bimetallic cooperative catalysis as it applies to
the activation of the alkyne triple bond. We have divided the review into four
sections, separated by reaction type, which emphasise different aspects of the
bimetallic alkyne activation mechanism. These four sections are as follows:
1. Cyclotrimerisation, Dimerisation and Nucleophilic Substitution. These three
reactions are promoted by a class of catalysts where the bimetallic mechanism
is generally very well understood. In this section the diversity of alkyne activation processes is highlighted through the characterisation of intermediate structures or their analogues.
2. Cycloaddition with Azides, Alkynes, Alkenes and Allenes. The copper-catalysed
azide–alkyne cycloaddition reaction is typically catalysed by simple, monometallic Cu(I) salts. However, the mechanism of catalysis was recently determined
to involve a bimetallic process. Similar bimetallic mechanisms have also been
discovered in the cycloaddition of alkynes with alkenes, allenes and other
alkynes using Au catalysts. This reaction is discussed for its broad application
to many areas of chemistry and for the potential of bimetallic catalyst design to
enhance the reaction.
3. The Pauson–Khand Reaction and Silylformylation. Perhaps one of the earliest
and most widely studied bimetallic alkyne activation processes is the Pauson–
Khand Reaction (PKR), which involves the [2 + 2 + 1] cycloaddition of an
alkyne, alkene and CO to yield a cyclopentenone product. The key intermediate
in both the PKR and the related silylformylation reaction is a (μÀη
2 :η
2 )-bonded
intermediate of structure E (Fig. 2). Extensive DFT modelling of the catalytic
cycles has provided an excellent insight into the electronic changes that occur
within the bimetallic unit during the reaction.
4. Hydroelementation with Silanes, Alcohols, Carboxylic Acids and Amines. This
final class of reaction, involving the addition of Si–H, O–H or N–H bonds across
an unsaturated C–C bond, has only recently been explored using bimetallic
catalysts. The catalysts used also represent a distinct type of bimetallic design;
where two discreet complex fragments are linked by a shared ligand scaffold, but
typically do not contain any prearranged metal–metal bonding or bridging atom
interactions. Such catalysts can easily be compared to monometallic catalysts of
similar structure, thereby allowing a quantitative evaluation of the bimetallic
cooperativity.
106
M.J. Page et al.
2
Àη
1 :η
1 ). A 1,2-sigmatropic shift of an alkyne substituent (R in
structure B) can also yield end-on coordinated vinylidene species (structure C,
μ
2
Àη
2 ). For metal acetylide moieties the proximity of a second metal can often
yield (μ
2
Àη
1 :η
2 ) hybrid structures D, where the second metal coordinates in a
π-fashion to the σ-bonded metal acetylide. Finally an alkyne may coordinate to
two metal centres via its two orthogonal π-bonds, resulting in a tetrahedral C 2 M 2
structure (E), where the C C axis lies across the metal–metal axis (μÀη
2 :η
2 ).
This review is not intended to be fully comprehensive but instead should serve to
highlight current understanding of bimetallic cooperative catalysis as it applies to
the activation of the alkyne triple bond. We have divided the review into four
sections, separated by reaction type, which emphasise different aspects of the
bimetallic alkyne activation mechanism. These four sections are as follows:
1. Cyclotrimerisation, Dimerisation and Nucleophilic Substitution. These three
reactions are promoted by a class of catalysts where the bimetallic mechanism
is generally very well understood. In this section the diversity of alkyne activation processes is highlighted through the characterisation of intermediate structures or their analogues.
2. Cycloaddition with Azides, Alkynes, Alkenes and Allenes. The copper-catalysed
azide–alkyne cycloaddition reaction is typically catalysed by simple, monometallic Cu(I) salts. However, the mechanism of catalysis was recently determined
to involve a bimetallic process. Similar bimetallic mechanisms have also been
discovered in the cycloaddition of alkynes with alkenes, allenes and other
alkynes using Au catalysts. This reaction is discussed for its broad application
to many areas of chemistry and for the potential of bimetallic catalyst design to
enhance the reaction.
3. The Pauson–Khand Reaction and Silylformylation. Perhaps one of the earliest
and most widely studied bimetallic alkyne activation processes is the Pauson–
Khand Reaction (PKR), which involves the [2 + 2 + 1] cycloaddition of an
alkyne, alkene and CO to yield a cyclopentenone product. The key intermediate
in both the PKR and the related silylformylation reaction is a (μÀη
2 :η
2 )-bonded
intermediate of structure E (Fig. 2). Extensive DFT modelling of the catalytic
cycles has provided an excellent insight into the electronic changes that occur
within the bimetallic unit during the reaction.
4. Hydroelementation with Silanes, Alcohols, Carboxylic Acids and Amines. This
final class of reaction, involving the addition of Si–H, O–H or N–H bonds across
an unsaturated C–C bond, has only recently been explored using bimetallic
catalysts. The catalysts used also represent a distinct type of bimetallic design;
where two discreet complex fragments are linked by a shared ligand scaffold, but
typically do not contain any prearranged metal–metal bonding or bridging atom
interactions. Such catalysts can easily be compared to monometallic catalysts of
similar structure, thereby allowing a quantitative evaluation of the bimetallic
cooperativity.
106
M.J. Page et al.