18
J. Oliver–Meseguer and A. Leyva–Pérez
the easy coordination of arene derivatives to this particular type of quasi-linear Pd 4
cluster, [46] the benzene molecule is also activated by the Pd 4 cluster to promote
the final coupling. This recoverable Pd 4 –MOF solid catalyst, with an estimated price
10–100 times lower than the Rh catalysts, constitutes a paradigmatic shift in the
Buchner reaction, opening new avenues in the use of this reaction at larger scales
under flow conditions.
1.3.1.3 Homocoupling of Alkynes
The homocoupling of alkynes is a classical organic reaction performed during years
with Cu salts, in the Glaser, [24] Eglinton, [15] or Hay [27] versions. However,
no significant reports on the use of the other two group XI metals, Ag and Au, as
catalysts for the reaction have been shown, despite the electronic resemblance of these
metals. When one examines the different accepted mechanisms for the Cu-catalyzed
homocoupling of alkynes, it can be seen that a key step during the coupling is the
oxidation of Cu(I) to Cu(III), a step intrinsically difficult for Ag but not so much for
Au. Indeed, a second key feature during the homocoupling reaction is the plausible
dimerization of the Cu atoms through coordinated alkyne bounds. Following this
mechanistic rationale, it has been recently found that mixed-valence Au(I, III) clusters
with ligands are very active species for the homocoupling of alkynes [36]. The
particular structure of the cluster sterically discriminates between linear carbon chain
alkynes with 10 or 12 atoms during the oxidative homocoupling of alkynes: the
former is fully reactive, and the latter is practically unreactive. A distal size selectivity
occurs by the impossibility of trans-metalating two long alkyl chains in an A-framed,
mixed-valence di–Au (I, III) acetylide complex, as shown in Fig. 1.17. The reductive
elimination of two alkyne molecules from a single Au(III) atom occurs extremely
fast, in < 1 min at −78° C (turnover frequency 40.016 s
−1 ). Notice that the extremely
high catalytic activity of Au(III) and the stability of mixed Au(I, III) clusters for
the homocoupling of alkynes is somewhat related to the relativistic effects present
in the gold because is a lateheavy metal and absent in Cu and Ag [35]. The subtle
steric and electronic discrimination of alkynes by this Au-catalyzed system allows
the heterocoupling of two different alkynes in equimolecular amounts regardless of
the nature of the terminal triple bond (Fig. 1.17).
It is not necessary to ligate the Au atoms through ligands to promote the homocoupling of alkynes since very small Au nanoparticles catalyze the aerobic coupling
of alkynes [5]. For the latter, O 2 is dissociated as catalyzed by the air-tolerant Au
nanoparticles, without significant oxidation of the metal. In contrast, ligand-free
soluble sub-nanometer metal clusters do not dissociate O 2 in the presence of the
alkyne and, thus, do not catalyze the aerobic coupling, [6] which makes sense considering the higher affinity of cationic Au atoms for alkynes than for O 2 . These results
exemplify the dramatic catalytic differences that can be found for a given metal in
different aggregation forms, in this case Au, and when different ligands and supports
are employed.
J. Oliver–Meseguer and A. Leyva–Pérez
the easy coordination of arene derivatives to this particular type of quasi-linear Pd 4
cluster, [46] the benzene molecule is also activated by the Pd 4 cluster to promote
the final coupling. This recoverable Pd 4 –MOF solid catalyst, with an estimated price
10–100 times lower than the Rh catalysts, constitutes a paradigmatic shift in the
Buchner reaction, opening new avenues in the use of this reaction at larger scales
under flow conditions.
1.3.1.3 Homocoupling of Alkynes
The homocoupling of alkynes is a classical organic reaction performed during years
with Cu salts, in the Glaser, [24] Eglinton, [15] or Hay [27] versions. However,
no significant reports on the use of the other two group XI metals, Ag and Au, as
catalysts for the reaction have been shown, despite the electronic resemblance of these
metals. When one examines the different accepted mechanisms for the Cu-catalyzed
homocoupling of alkynes, it can be seen that a key step during the coupling is the
oxidation of Cu(I) to Cu(III), a step intrinsically difficult for Ag but not so much for
Au. Indeed, a second key feature during the homocoupling reaction is the plausible
dimerization of the Cu atoms through coordinated alkyne bounds. Following this
mechanistic rationale, it has been recently found that mixed-valence Au(I, III) clusters
with ligands are very active species for the homocoupling of alkynes [36]. The
particular structure of the cluster sterically discriminates between linear carbon chain
alkynes with 10 or 12 atoms during the oxidative homocoupling of alkynes: the
former is fully reactive, and the latter is practically unreactive. A distal size selectivity
occurs by the impossibility of trans-metalating two long alkyl chains in an A-framed,
mixed-valence di–Au (I, III) acetylide complex, as shown in Fig. 1.17. The reductive
elimination of two alkyne molecules from a single Au(III) atom occurs extremely
fast, in < 1 min at −78° C (turnover frequency 40.016 s
−1 ). Notice that the extremely
high catalytic activity of Au(III) and the stability of mixed Au(I, III) clusters for
the homocoupling of alkynes is somewhat related to the relativistic effects present
in the gold because is a lateheavy metal and absent in Cu and Ag [35]. The subtle
steric and electronic discrimination of alkynes by this Au-catalyzed system allows
the heterocoupling of two different alkynes in equimolecular amounts regardless of
the nature of the terminal triple bond (Fig. 1.17).
It is not necessary to ligate the Au atoms through ligands to promote the homocoupling of alkynes since very small Au nanoparticles catalyze the aerobic coupling
of alkynes [5]. For the latter, O 2 is dissociated as catalyzed by the air-tolerant Au
nanoparticles, without significant oxidation of the metal. In contrast, ligand-free
soluble sub-nanometer metal clusters do not dissociate O 2 in the presence of the
alkyne and, thus, do not catalyze the aerobic coupling, [6] which makes sense considering the higher affinity of cationic Au atoms for alkynes than for O 2 . These results
exemplify the dramatic catalytic differences that can be found for a given metal in
different aggregation forms, in this case Au, and when different ligands and supports
are employed.