A Switchable Gold Catalyst by Encapsulation in a Self-Assembled Cage

Article abstract of DOI:10.1002/chem.201603162

Dinuclear gold complexes have the ability to interact with one or more substrates in a dual-activation mode, leading to different reactivity and selectivity than their mononuclear relatives. In this contribution, this difference was used to control the catalytic properties of a gold-based catalytic system by site-isolation of mononuclear gold complexes by selective encapsulation. The typical dual-activation mode is prohibited by this catalyst encapsulation, leading to typical behavior as a result of mononuclear activation. This strategy can be used as a switch (on/off) for a catalytic reaction and also permits reversible control over the product distribution during the course of a reaction.

Full text of DOI:10.1002/chem.201603162

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Title: A Switchable Gold Catalyst by Encapsulation in a Self-Assembled Cage
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Authors: Anne Jans; Adrian Gomez-Suarez; Steven Nolan; Joost Reek
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A Switchable Gold Catalyst by Encapsulation in a Self-Assembled
Cage
Anne C. H. Jans,[a] Adrián Gómez-Suárez,[b] Steven P. Nolan,[c,d] Joost N. H. Reek*[a]
Abstract: Dinuclear gold complexes have the ability to interact with
one or more substrates in a dual activation mode, leading to different
reactivity and selectivity than their mononuclear relatives. In this
contribution, we use this difference to control the catalytic properties
of a gold-based catalytic system by site-isolation of mononuclear
gold complexes by selective encapsulation. The typical dual
activation mode is prohibited by this catalyst encapsulation, leading
to typical behavior as a result of mononuclear activation. This
strategy can be used as a switch (on/off) for a catalytic reaction and
also permits reversible control over the product distribution during
the course of a reaction.
cage 16 and showed that the encapsulated catalyst gives a
different product distribution than the free complex.[13] This
supramolecular complex is formed in apolar, water-saturated
solvents (Scheme 1).[14,15] In the current contribution, we report
how we change the active gold complex from dinuclear to
mononuclear by reversible encapsulation and demonstrate that
this can be used for both switching on/off a reaction, as well as
for controlling its selectivity during the course of a reaction.
Changing the reactivity or selectivity of a gold catalyst has been
shown before by changing the ligands[16] or Brønsted acid/base
effects[17] and even by guest binding by a rotaxane,[18] but
reversibly changing the active species of a gold-catalyzed
reaction as a result of encapsulation has, to the best of our
knowledge, not been shown before.
There is a growing interest in transition metal catalysis in
confined spaces as the approach provides an additional tool to
control selectivity and activity in catalysis.[1] For example, it has
been demonstrated that the encapsulation of rhodium
complexes in a hemispherical porphyrin assembly results in
catalysts with increased activity and unprecedented branched
selectivity in the hydroformylation of terminal and internal
alkenes.[2] In addition, the encapsulation of transition metal
complexes in preformed cavities via weak interactions can lead
to unexpected reactivity: M4L6 anionic tetrahedral capsules, for
instance, can host cationic organometallic catalysts in their
hydrophobic cavities,[3] thereby inducing substrate selectivity.[4]
Moreover, reaction rates[5] and product distribution[6] can be
greatly affected by catalyst encapsulation. Interestingly, many
catalytic reactions operate via a dinuclear mechanism[7] or
deactivate via a dinuclear pathway;[8,9] by encapsulation of a
transition metal complex, such decomposition pathways can be
suppressed leading to higher catalytic turnover numbers.[10]
In principle, an encapsulation event could also change the
catalytic pathway of a reaction, and as such can be used as a
switch for a catalytic transformation. Switchable catalysis is an
interesting upcoming field of research as it provides new tools to
control the reaction process with external stimuli such as light,
pH or metal coordination, a concept that is important to control
reactions in Nature.[11] In a recent study, encapsulation of a
photoredox catalyst was shown to be a feasible stimulus to steer
reactivity.[12] In a previous paper we reported the encapsulation
of an N-heterocyclic carbene (NHC) mononuclear gold(I)
complex inside a self-assembled hexameric resorcin[4]arene
Scheme 1. Formation of the self-assembled hexameric resorcin[4]arene cage.
In this study we use a dinuclear hydroxyl-bridged gold complex
[{Au(NHC)}2(-OH)][X] instead of
a
mononuclear gold
complex.[19] This complex reacts via different mechanisms than
mononuclear gold complexes[20] and is a privileged catalyst for
dual activation reactions.[21] It, however, is too large to fit inside
[22]
the cage 16, and should be split into mononuclear complexes
upon encapsulation. The NHC-Au-X fragment displays the
traditional Lewis acidic character of mononuclear cationic gold
catalysts, activating unsaturated substrates such as alkynes via
-coordination, making them more electrophilic and susceptible
towards nucleophilic attack.[23] Meanwhile, the NHC-Au-OH
species behaves as a Brønsted base and is known to -activate
substrates such as terminal alkynes or phenols.[24] Together the
two species, NHC-Au-X and NHC-Au-OH, can dually activate
substrates via both - and -activation (Scheme 2). This dual
activation mode was originally proposed by Toste and Houk[25]
and is well established and employed nowadays. In addition, this
dual activation has elegantly been explored by Hashmi[26] and
Zhang[27] for the formation of gold acetylides, which can react as
a nucleophile and attack the -activated bond of the substrate,
and as such represents a common strategy in gold-mediated
synthesis.
[a]
A.C.H. Jans, Prof. Dr. J.N.H. Reek*
Homogeneous, Bioinspired and Supramolecular Catalysis,
van ‘t Hoff Institute for Molecular Sciences
University of Amsterdam
Science Park 904, 1098 XH Amsterdam, the Netherlands
E-mail: j.n.h.reek@uva.nl
[b]
[c]
Dr Adrian Gomez-Suarez
School of Chemistry, University of St Andrews
St Andrews, KY16 9ST, UK
Prof. Dr. Steven P. Nolan
Department of Inorganic and Physical Chemistry
Ghent University
Krijgslaan 281 - S3, 9000 Gent, Belgium
E-mail: steven.nolan@ugent.be
Prof. Dr. Steven P. Nolan
[d]
Chemistry Department
College of Science, King Saud university
PO Box 2455, Riyadh, 11451, Saudi Arabia
Supporting information for this article is given via a link at the end of
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COMMUNICATION
-
Et4N+BF4 (7) to expel the gold catalyst from the cage, the
catalytic activity was restored and product 6 was obtained in
89% yield after 1 hour!
Scheme 2. Dinuclear complexes [{Au(NHC)}2(-OH)][X] can dually activate
substrates.
We envisioned that encapsulation of the two fragments of the
dinuclear gold catalyst [{Au(NHC)}2(-OH)][X] in separate cages
would alter its typical reactivity as they undergo transformations
in a site isolated fashion. We anticipated that by complex
encapsulation we could reversibly switch between the dinuclear
and mononuclear catalyst and, therefore, have a tool to shift
from dual gold catalysis to a mononuclear reaction mechanism.
This forms the basis for an on/off switchable system, but can
also alter the product distribution during a gold-catalyzed
transformation.
Scheme 4. Switching the gold catalyzed hydrophenoxylation off and on.
Reaction conditions: solvent = toluene, T = 80°C, [4] = 550 mM, [5] = 500 mM,
[2] = 2.5 mM (0.025 mol%), [1] = 50 mM.
Next we explored the cage effect on changing the product
distribution of a reaction. For this purpose we explored the
conversion of 4-phenyl-1-butyne (8). Depending on the available
reaction pathways, the transformation can yield up to four
products (Table 1). In the absence of the cage, 2 can activate
two substrates in a - and -activation mode and, therefore,
dimerization of 8 takes place via a dual activation mechanism,
yielding the branched and linear products, 11 and 12
respectively (Table 1, entry 1; Scheme 5).[26b,30] As anticipated,
this dual activation reaction pathway is completely blocked upon
encapsulation of the catalyst and the production of 11 and 12
stopped. Instead, when in the cage, the catalyst produced the
intramolecularly cyclized product 10, which is known to form via
a mononuclear pathway inside the cage (Table 1, entry 2).[13] In
the presence of the cage and the competing guest 7, product 10
was not formed and dimerization products 11 and 12 were
obtained (Table 1, entry 3), indicating that under these
conditions the catalysis proceeds via a dual gold mechanism
and thus takes place outside the cage.
Due to the presence of water molecules to facilitate the self-
assembly of the capsule, the well documented hydration
product 9 was formed in all cases via -activation of the triple
bond,[31,32] but only in minor amounts. We observed an increased
rate of formation of this product in the presence of both cage
and competing guest 7, which is interesting but difficult to
explain. The addition of only the competing guest 7, in the
absence of cage but with the same amount of water present,
does not lead to increased formation of product 9 (Table 1,
entry 4).
Scheme 3. Encapsulation of [{Au(IPr)}2(-OH)][BF4] in capsule 16.
Indeed, when complex [{Au(IPr)}2(µ-OH)][BF4] 2 (Scheme 3) and
the capsule were mixed together, encapsulation of
a
mononuclear gold complex was confirmed by 1H NMR and
1H 2D DOSY NMR (see supporting information). To demonstrate
the principle of switching catalyst activity by selective
encapsulation, the dual gold-catalyzed hydrophenoxylation
reaction was studied.[21,28] This reaction requires -activation of a
phenol by the NHC-Au-OH moiety and -activation of an alkyne
by the cationic NHC-Au-X fragment (Scheme 4). Indeed, it was
observed that the standard reaction between phenol (4) and
diphenylacetylene (5) readily takes place when using 2 as
catalyst in the absence of the cage (full conversion within
60 minutes). However, in the presence of the cage, the dinuclear
complex 2 is broken and encapsulated as mononuclear species.
As the dual reaction pathway is no longer available, no
conversion to vinyl ether 6 is obtained, even after 24 hours.[29]
We wondered if the reaction could be switched on again by
adding a competing guest that would bind more strongly to the
cage than the gold catalyst. For this purpose we selected
tetraalkylammonium salts, as they are known to bind very
strongly inside the hexamer.[15] Gratifyingly, upon addition of
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COMMUNICATION
Table 1. Gold catalyzed conversion of 4-phenyl-1-butyne.[a]
Entry
16
7
Yield 9 (%)
Yield 10
Yield 11
Yield 12
(%)
(%)
(%)
Figure 1. Formation of products from the reaction of substrate 8 9 and 10 in
presence of stepwise addition of cage and competing guest. Reaction
conditions: [8] = 66 mM, [2] = 1.7 mM (2.5 mol%), [H2O] = 44 mM, [1] = 33 mM,
[7] = 33 mM. Yields are averaged from two measurements.
1
2
3
4
-
+
+
-
-
-
7
3
0
16
0
38
0
8
0
4
8
+
+
47
6
28
47
formation of this product could not be switched; the faster
formation of this product after addition of both cage and
0
-
Et4N+BF4 (7) is in correspondence with the observations in
Table 1. (see supporting information).
[a] Reaction conditions: [8] = 66 mM, [2] = 1.7 mM (2.5 mol%), [H2O] =
44 mM, [1] = 33 mM, [7] = 33 mM. Yields are averaged from two
measurements.
In conclusion, we have demonstrated that it is possible to control
the pathway by which gold complexes convert substrate
molecules by complex encapsulation events. In the presence of
a self-assembled hexameric resorcin[4]arene cage the dinuclear
complex [{Au(NHC)}2(µ-OH)][X] breaks into mononuclear units
that are encapsulated as the dinuclear gold complex 2 is too
large to fit inside the cage. By doing so, the dual activation
reactivity typical for complex 2 is switched into (re)activity that is
typical for mononuclear complexes. By using this strategy on a
reaction that can only proceed via a dual activation pathway, we
have demonstrated on/off switching of a gold-catalyzed reaction.
Moreover, this approach also provides tools to change the
product distribution during the course of a reaction, which is
demonstrated in the activation of 4-phenyl-1-butyne.
In
Scheme 5. Dimerization of terminal alkynes via ,-activation.
summary, this strategy of switching the active species of a gold-
catalyzed reaction catalysis by means of host-guest interactions
provides a new approach to controlling catalytic transformations,
even during the course of the reaction.
Next we investigated whether it would be possible to switch the
product selectivity during the course of the reaction (Figure 1).
We decided to monitor the formation of products 10 and 11 as
their formation depends on the availability of the mononuclear or
dinuclear pathways (Figure 1). The reaction was initiated using
only the dinuclear gold catalyst 2. Under these reaction
conditions, the branched dimer 11 was the main product formed,
indicating that the dual activation pathway is dominating. After
3 hours, an excess of cage 16 was added, thereby removing the
dinuclear gold complex from the reaction mixture by
encapsulation of the cationic mononuclear gold fragment. The
formation of 11 immediately stopped, while production of
compound 10 started. After another 3 hours, compound 7 was
added as a competing guest for 16, releasing the catalyst from
the cage. As compound 10 can only be formed while the cationic
gold species is encapsulated in 16, its production stopped,
whereas formation of 11 started again. The formation of product
12 followed the same trend as 11, but it was less clear as this
compound was formed in much smaller amounts. As ketone 9
can be formed both inside and outside the cage (Table 1), the
Experimental Section
Hydrophenoxylation experiments: The catalyst 2 (0.025 mol%) was
added to a solution of 4 (550 mM) and 5 (500 mM) and in selected
experiments cage 16 (50 mM) and/or competing guest 7 (50 mM) in
toluene-d8. The mixture was heated to 80°C and yields were monitored
using GC and 1H NMR of the crude mixture.
Catalytic experiments with substrate 8: The catalyst 2 (2.5 mol%), H2O
(44 mM), substrate 8 (66 mM) and in selected experiments cage 16
(33 mM) and/or competing guest 7 (33 mM) were mixed in benzene-d6
and heated to 70°C for 48 hours. Yields were monitored using GC and
1H NMR of the crude mixture and were determined as the average of two
experiments.
Acknowledgements
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Layout 2:
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Author(s), Corresponding Author(s)*
Page No. – Page No.
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Title
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