contains two triazolyl moieties bridging the two Cu centres, was then shown to be a
highly efficient catalyst for the click reaction. Another example of bimetallic
catalyst design leading to cooperative rate enhancement in the Cu-catalysed click
reaction was reported by Straub and coworkers [52]. The bis-triazolylidene
dicopper(I) complex 23 was synthesised, and its catalytic activity for the coupling
of benzyl azide with phenylacetylide was compared to the monometallic N-heterocyclic carbene complex [Cu(ICy)]PF 6 (ICy ¼ 1,3-bis(cyclohexyl)imidazol-2ylidene). The cycloaddition of benzyl azide and phenylacetylene catalysed by the
bimetallic catalyst 23 was complete within 1.7 h, whereas the monometallic catalyst
[Cu(ICy)]PF 6 required approximately 4.5 h to reach 60% conversion. It should be
noted that this difference in reactivity between 23 and [Cu(ICy)]PF 6 is likely to also
be affected by the dissimilar carbene ligand properties and the presence of an
acetate coligand in 23.
3.2 Cycloaddition of Alkynes with Alkynes, Alkenes
and Allenes
The use of homogeneous gold catalysts in synthetic organic chemistry has grown
rapidly in recent decades and has proven to be a versatile tool in the total synthesis
of natural products among other applications [53, 54]. In the gold-catalysed cycloaddition of alkynes, the participation of bimetallic gold complexes that contain
μ
2 Àη
1 :η
2 -acetylide or gem-diaurated moieties has been increasingly observed
[55]. For example, Corma et al. have observed that μ
2
Àη
1 :η
2 -acetylide catalysts
such as 24 are superior to their monometallic analogues for the intermolecular [2
+ 2] alkyne–alkene cycloaddition reaction (Scheme 11a) [56]. Hashmi and
coworkers have identified the gem-diaurated complex 25 as an effective catalyst
precursor for the cycloaddition of dialkynes (Scheme 11b) [57]. Toste and
coworkers have reported the cycloisomerisation of allenynes using the gold catalyst
26 and determined the involvement of bimetallic μ
2
Àη
1 :η
2 -acetylide and gemdigold intermediates (Scheme 11c) [58]. In each of these examples however, the
dinuclear catalyst structure is not retained throughout the reaction due to the
absence of spectator bridging ligands that can immobilise the two Au centres in
close proximity. Although a number of related cycloaddition reactions using chiral
bimetallic Au catalysts have been reported [59], the two gold centres are not
considered to act through a bimetallic interaction with the substrate as described
here. The chemistry of such catalysts is discussed in detail below for the hydrofunctionalisation reactions.
Alkyne Activation Using Bimetallic Catalysts
115
highly efficient catalyst for the click reaction. Another example of bimetallic
catalyst design leading to cooperative rate enhancement in the Cu-catalysed click
reaction was reported by Straub and coworkers [52]. The bis-triazolylidene
dicopper(I) complex 23 was synthesised, and its catalytic activity for the coupling
of benzyl azide with phenylacetylide was compared to the monometallic N-heterocyclic carbene complex [Cu(ICy)]PF 6 (ICy ¼ 1,3-bis(cyclohexyl)imidazol-2ylidene). The cycloaddition of benzyl azide and phenylacetylene catalysed by the
bimetallic catalyst 23 was complete within 1.7 h, whereas the monometallic catalyst
[Cu(ICy)]PF 6 required approximately 4.5 h to reach 60% conversion. It should be
noted that this difference in reactivity between 23 and [Cu(ICy)]PF 6 is likely to also
be affected by the dissimilar carbene ligand properties and the presence of an
acetate coligand in 23.
3.2 Cycloaddition of Alkynes with Alkynes, Alkenes
and Allenes
The use of homogeneous gold catalysts in synthetic organic chemistry has grown
rapidly in recent decades and has proven to be a versatile tool in the total synthesis
of natural products among other applications [53, 54]. In the gold-catalysed cycloaddition of alkynes, the participation of bimetallic gold complexes that contain
μ
2 Àη
1 :η
2 -acetylide or gem-diaurated moieties has been increasingly observed
[55]. For example, Corma et al. have observed that μ
2
Àη
1 :η
2 -acetylide catalysts
such as 24 are superior to their monometallic analogues for the intermolecular [2
+ 2] alkyne–alkene cycloaddition reaction (Scheme 11a) [56]. Hashmi and
coworkers have identified the gem-diaurated complex 25 as an effective catalyst
precursor for the cycloaddition of dialkynes (Scheme 11b) [57]. Toste and
coworkers have reported the cycloisomerisation of allenynes using the gold catalyst
26 and determined the involvement of bimetallic μ
2
Àη
1 :η
2 -acetylide and gemdigold intermediates (Scheme 11c) [58]. In each of these examples however, the
dinuclear catalyst structure is not retained throughout the reaction due to the
absence of spectator bridging ligands that can immobilise the two Au centres in
close proximity. Although a number of related cycloaddition reactions using chiral
bimetallic Au catalysts have been reported [59], the two gold centres are not
considered to act through a bimetallic interaction with the substrate as described
here. The chemistry of such catalysts is discussed in detail below for the hydrofunctionalisation reactions.
Alkyne Activation Using Bimetallic Catalysts
115