Au-Cavitands: Size governed arene-alkyne cycloisomerization

Article abstract of DOI:10.1016/j.tetlet.2020.152333

With an inwardly directed reactive center and a well-defined binding pocket, Au(I) functionalized resorcin[4]arene cavitands have been shown to catalyze molecular transformations. The reactivity profiles that emerge differ from other Au(I) catalysts. The added constraint of a binding pocket gives rise to the possibility that the substrates might have to fit into the resorcinarene pocket; our hypothesis is that substrates that match the available space have different reaction outcomes than those that do not. Herein we report on the intramolecular cyclization of alkyne-aromatic substrates with variable alkynes and aromatic composition. We see that scaffold size most drastically dictates reactivity, especially when the substrate's features are particularly designed. The results of these experiments add to the veritable goldmine of information about the selectivity in catalysis that cavitands offer.

Full text of DOI:10.1016/j.tetlet.2020.152333

Tetrahedron Letters 61 (2020) 152333
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Au-Cavitands: Size governed arene-alkyne cycloisomerization
⇑
Lisa E. Rusali, Michael P. Schramm
Department of Chemistry and Biochemistry, California State University Long Beach, 1250 Bellflower Blvd., Long Beach, CA 90840, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
With an inwardly directed reactive center and a well-defined binding pocket, Au(I) functionalized resor-
cin[4]arene cavitands have been shown to catalyze molecular transformations. The reactivity profiles that
emerge differ from other Au(I) catalysts. The added constraint of a binding pocket gives rise to the pos-
sibility that the substrates might have to fit into the resorcinarene pocket; our hypothesis is that sub-
strates that match the available space have different reaction outcomes than those that do not. Herein
we report on the intramolecular cyclization of alkyne-aromatic substrates with variable alkynes and aro-
matic composition. We see that scaffold size most drastically dictates reactivity, especially when the sub-
strate’s features are particularly designed. The results of these experiments add to the veritable goldmine
of information about the selectivity in catalysis that cavitands offer.
Received 23 April 2020
Revised 28 July 2020
Accepted 2 August 2020
Available online 16 August 2020
Keywords:
Supramolecular
Catalysis
Gold
Friedel-Crafts
Host-Guest
Ó 2020 Published by Elsevier Ltd.
Introduction
and mild reaction conditions, a variety of alkyne-acids underwent
cyclization with 75–97% conversion [20]. The preparation of 3 is
Gold cavitands [1] are an emerging catalytic species that have
expanded on fundamental discoveries in the arena of C-Heteroa-
tom and C-aryl bond forming reactions that originate from alkyne
and alkene centers [2–5]. Cavitands are molecular cavities [6–8]
and under certain circumstances fold into a ‘vase’ conformation.
This state provides a binding pocket for supramolecular scientists
to explore. Cavitands have a rich history of host–guest chemistry
and now, with an inwardly directed coupled Au atom (1) [9], we
have proposed that their potential to behave like biological cata-
lysts will come to be [10–13].
Gold cavitands have a defined pocket that comes from their
resorcinarene cavitand component. This pocket has been shown
to select for certain sized guests, stabilize intermediates [14], bias
the outcome of chemical transformations [15], or be inhibited by
certain sized entities [11]. Under current study is cavitand 1 [9], that
has three walls (Scheme 1) that create a small, defined pocket. The
second salient feature relevant to biologically inspired catalysis is
an inwardly directed reactive center, in our case Au. Other varia-
tions of 1 are easy to prepare including bis-Au two-walled analogs
[16]. We are also learning that the walls are not benign in yne-yne
coupling reactions, shortening the walls results in low or no reac-
tivity [17,18].
typical (Scheme 2). Work has been continued on this reaction [21].
When we applied Au-cavitand 1 to a variety of alkyne-acid sub-
strates we demonstrated that the R and R’ groups play an impor-
tant role [11]. We reported that the R’ group, when matched to
fit the cavitand, were potent in slowing down the reaction. More-
over, when R’ is benzyl, lactone 3 can serve to inhibit catalysis of
other alkyne acids. Given these observations, the walls should provide
an environment that can be used to select substrates or influence reac-
tion outcomes.
The results encouraged us to explore other transformations
where Au and alkynes react and where substrate variability could
result in cavitand interaction. A series of reports that explored the
cycloisomerization of alkyne 4 caught our attention [22,23]. When
R = OMe and X = H, a variety of metals could affect the transforma-
tion of 4, most often to six-membered 6, but the 5-membered flu-
orene 5 could also be isolated (Scheme 3). For example, when PtCl2
or AuCl3 were employed (toluene 80 °C), 6 was the dominant spe-
cies (5:6 5:95, in 75% or higher yield). Using InCl3, the yield plum-
meted to 44%, but the ratio of 5:6 inverted to 56:44. RuCl2 species
were similar. Revisiting PtCl2, where R = OMe and X = COOMe
resulted in favoring 5 over 6 (95:5), or with X = p-methoxyphenyl
(40:60). A variety of X groups as well as R groups, including addi-
tional rings are well tolerated. In the case of X = halide they
behaved as expected with a variety of metals (e.g. InCl3), but inter-
estingly when AuCl was used 1,2 migration of the halide occurred
[24].
Most recently we have explored the Au catalyzed cyclization of
alkyne-acids, which already is an important method to prepare
functionalized lactones 3 [19]. In an earlier work utilizing AuCl
Several alkaloid syntheses [22 24,25] have been facilitated by
this methodology, where the metal directed isomerization takes
⇑
Corresponding author.
E-mail address: michael.schramm@csulb.edu (M.P. Schramm).
https://doi.org/10.1016/j.tetlet.2020.152333
0040-4039/Ó 2020 Published by Elsevier Ltd.

2
L.E. Rusali, M.P. Schramm / Tetrahedron Letters 61 (2020) 152333
potential to behave like biological catalysts will come to be.10-13
Scheme 4. Preparation of alkyne functionalized biphenyl 12.
Scheme 1. Inwardly directed Au cavitand 1 and electronically similar complex 2
that are used in this study.
tion partners provided aldehyde 10 in acceptable yield. We found
that modifying the procedure using microwave irradiation [32]
improved the outcome in our hands (see ESI). Continuing with
the Corey-Fuchs protocol, we obtained the target alkyne 12 after
isolation of dibromo compound 11. Replacing triphenylphosphine
with triisopropyl phosphite gave an easier purification and higher
yield of 11. After several repetitions, we also explored the conver-
sion of aldehyde 10 to alkyne 12 using a variation of the Bestmann-
Ohiray [33 34] reagent. When using azides or diazo compounds
appropriate care should be taken. Azide 13 was prepared on a
1 g scale from the corresponding sulfonyl chloride and used within
48 h. Diazo 14 was prepared in situ following known protocols and
was reacted with 10. These two variants (13 and 14) of many pos-
sible permutations were chosen specifically for their larger molec-
ular mass as well as electronic composition (e.g. 13) to provide
added measures of safety.
Scheme 2. AuCl lactonization of alkyne-acids [20].
We next prepared a variety of substrates (Scheme 5) with which
to compare the effects of 1 vs. 2. Halides 15, 16, 17 were prepared
by treatment of 12 with the corresponding N-halidesuccinimide
[35]. Internal alkynes (18, 19, 20) were prepared from reaction of
dibromoalkyne 11 with n-butyllithium, followed by reaction with
an appropriate electrophile [23]. Finally, variations on the aromatic
scaffold (21, 22, 23) were obtained by altering the Suzuki-coupling
reaction partners (see ESI).
With substrates in hand, we explored the Au catalyzed reaction
(Scheme 6). Alkyne 12 was subjected to a variety of AuCl catalytic
species in the presence of AgOTf. We found that without Ag, the
reactions were unsuccessful; indeed with cavitand 1 (Table 1, entry
1), no reaction occurred even after extended heating as monitored
Scheme 3. Metal catalyzed cycloisomerization of alkyne-arenes (4) and late-stage
ring formation products (7–9) bolded bond indicates reaction locant (the right side
of the bond originates from a functionalized alkyne).
place at a late stage (7 [22], 8 [26], 9 [24]). This arene-alkyne
cycloisomerization has also played
a central role in several
reviews, where focused discussion on the mechanism has taken
place and variable outcomes, say comparing AuCl3 vs AuCl, give
mechanistic insight into plausible Au intermediates. [25,27–28]
Variations where the alkyne is linked via ether [29] or ester [30]
groups also exist, expanding on the application of this ring closing
strategy. These reactions are putative Friedel-Crafts in nature and
furans [31] (as well as other heterocycles) can replace phenyl as
the nucleophilic partner. These three latter mentioned approaches
use phosphorus ligated Au(I) as a mild and relatively stable cata-
lyst. These substrates and this reaction were of great interest to
us, given our experience activating alkynes with Au-1. The aro-
matic rings found in a variety of substrates provide a handle for
us to work with in attempts to match size of guest with volume
of cavity. We were attracted to the variable outcomes of the reac-
tion of 4 to give different isomeric ratios of 5 and 6. Additionally,
we wanted to know if halide rearrangement when 4 is converted
to 6 (where X = halide) might change with 1. We believed Au-1
would indeed offer different reactivity compared to 2 and thus
began a screening effort.
Scheme 5. Alkyne variations on 12: 15–20, and aromatic scaffold variations on 12:
21–23.
Results and discussion
In this work, we contrast the results of cavitand 1 with electron-
ically similar complex 2 (Scheme 1). We began with the prepara-
tion of a simple alkyne functionalized biphenyl (Scheme 4).
Following literature procedures [23], the Suzuki coupling of reac-
Scheme 6. Catalytic screening of substrate 12.

L.E. Rusali, M.P. Schramm / Tetrahedron Letters 61 (2020) 152333
3
Table 1
Cycloisomerization of 12 under a variety of conditions.
Entry
1
Conditionsa
Time
Conversionb
1, 24 °C
1 h
0%
1, 70 °C
1 h
0%
1, 70 °C
16 h
1 h
16 h
1 h
1 h
16 h
1 h
0%
0%
0%
0%
48%
99%
0%
2
3
AgOTf, 70 °C
AgOTf, 70 °C
1, + AgOTf, 24 °C
1, + AgOTf, 70 °C
1, + AgOTf, 70 °C
2, 70 °C
happened.
Scheme 8. Catalytic screening of substrates 18–20, resulting in 26.
4
5
2, 70 °C
16 h
1 h
1 h
16 h
16 h
16 h
0%
0%
7%
99%
11%
8%c
Table 2
Cycloisomerization of 18–20: variation of R under a variety of conditions.
2 + AgOTf, 24 °C
2 + AgOTf, 70 °C
2 + AgOTf, 70 °C
AuCl, 70 °C
Entry
1
Substrate
Conditionsa
Time
Conversionb
6
7
18
1, + AgOTf, 70 °C
1, + AgOTf, 70 °C
2 + AgOTf, 70 °C
2 + AgOTf, 70 °C
1, + AgOTf, 70 °C
1, + AgOTf, 70 °C
2 + AgOTf, 70 °C
2 + AgOTf, 70 °C
1, + AgOTf, 70 °C
1 h
15%
99%
7%
AuCl, + AgOTf, 70 °C
16 h
1 h
16 h
1 h
16 h
1 h
16 h
16 h
a
[12] = 0.04 mM, [Au] = 0.002 mM (5 mol%), [additive] = 0.004 mM (10 mol%),
reaction volume 0.60 mL.
As determined by NMR integration, all species were cleanly resolved and except
for entry 7, the only observable compounds were starting 12 or product 24.
2
3
4
5
18
19
19
20
99%
0%
50%
0%
37%
0%
b
c
6% of fluorene 5 was detected.
by NMR. The same unresponsiveness resulted with AgOTf alone
(Entry 2). The combination of the two gave appreciable turnover
after 1 h with heating (48% conversion, entry 3). After 16 h, clean,
quantitative conversion was achieved. Using a non-cavitand AuCl
surrogate, namely (di-t-butylphenylO)3PAuCl 2, which is nearly
isoelectronic at P and thus at Au compared to 1, we see similar
results; no conversion on its own (Entry 4) and complete conver-
sion after 16 h heating in the presence of AgOTf (Entry 5). Interest-
ingly, AuCl was mildly reactive without an additive (Entry 6) and
with AgOTf, produced trace amounts of 5 (Entry 7). We assume
for now that Au/Ag synergistic/dependent effects are not at work
[36,37], and that Ag is simply playing the role of Au activation
through ligand replacement. We will explore this matter in a
future report, and for the time being, the effect of substrate shape
with cavitand will be our point of focus.
Knowing that 12 is compatible with 1 and that this reaction is
comparable with chloro[tris(2,4-di-tert-butylphenyl)phosphite] 2,
we continued our inquiry looking for differences in reactivity.
We continued with chloro (15), bromo (16) and iodo (17) termi-
nated alkynes that gave 25 (Scheme 7). The cycloisomerization of
16 and 17 involving 1,2-migration of the halogen has been
reported using AuCl [24]. Chloro alkyne (15) was resistant to
cyclization in our hands with AuCl, AgOTf, or a combination, but
cyclized readily when treated with 1 and AgOTf, giving 74% conver-
sion after 16 h at 70 °C. Bromo alkyne (16), was completely con-
verted to product 25 after 16 h, again with 1 + AgOTf, and iodo
alkyne (17) was completely converted after only 1 h.
a
[Substrate] = 0.04 mM, [Au] = 0.002 mM (0.05 mol%) [additive] = 0.004 mM
(0.1 mol%), reaction volume 0.60 mL.
b
As determined by NMR integration, all species were cleanly resolved, the only
observable compounds were starting [Substrate] or product 26.
with 1 vs. 2, resulting in 50% and 37% conversion, respectively.
The ester-functionalized alkyne 20 was unreactive, whereas it
was previously cyclized with PtCl2 to give fluorene 5 as the major
product [24]. This sequence of experiments aimed to probe the
effect of short vs. long alkyl groups on the alkyne in the reaction
with 1; the hope was to find a permutation of cavitand volume
and guest size that would alter reactivity. This was not found in
experiments where the alkyne was modified, but when the aro-
matic scaffold was changed, something different happened.
We prepared modified scaffold 21 where the xylyl group of 12
was replaced with tolyl (Scheme 9). Resorcinarene cavitands admit
phenyl, benzyl, tolyl, and cyclohexyl sized groups with ease, but
larger o-, m- xylyl or mesityl (1,3,5-trisubstituted) become too
wide to access the interior. Alkynes 22 and 23 are further
variations.
Immediately,
a reactivity difference was noted between
dimethyl substituted 12 and tolyl 21. Cavitand 1 was ineffective
at producing measurable amounts of 29 (Table 3, Entry 1), even
after multiple replicates. Using AuCl 2, 29 was produced with
We then explored substrates 18–20 (Scheme 8, Table 2) with
terminal methyl, propyl, and ester groups. Methyl-terminated
alkyne 18 readily converted to cycloadduct 26 under Au catalysis
in the presence of AgOTf, with either 1 (Entry 1) or gold 2 (Entry
2), with complete conversion achieved after 16 h. The elongated
propyl-terminated alkyne 19, had marginally higher reactivity
Scheme 7. Catalytic screening of substrates 15–17. 1,2-Migration of
observed in the product 25 (as previously reported).
X
was
Scheme 9. Catalytic screening of substrates 21–23: variation in the aromatic
scaffold.

4
L.E. Rusali, M.P. Schramm / Tetrahedron Letters 61 (2020) 152333
Table 3
the cyclization of alkyne acids to give lactones 3 (Scheme 2) [11].
In that work, auxiliary R’ = benzyl and p-tolyl groups resulted in
sluggish reactivity, when compared to the wider R’ = 3,5-dimethyl,
or naphthyl substituted substrates. For now, guests that can fit
inside the cavitand behave differently, than those that can not –
even in cases where larger size does not seem to interfere with
access to the gold center (namely, 12).
Substrate 23 reinforces these observations – now two flanking
phenyl groups which do not play a large role with catalyst 2, but
with cavitand 1, conversion to 30 is abysmal. Taken together these
examples of scaffold variation point to a size-effect that takes place
when AuCl is mounted inwardly using a size-restrictive cavitand.
The electronically similar 2 is much more promiscuous – taking
almost all substrates in its stride, whereas 1 results in a restriction
of sorts. Further investigation into the interactions of a substrate’s
scaffold with a binding pocket will hopefully give rise to a better
picture of these limits, directing us towards challenges which
require size selectivity, perhaps with multiple reactivity centers.
For now, the scaffold size of the substrate somehow interferes with
the Au center of 1 from finding its otherwise nimble reaction part-
ner, the alkyne.
Cycloisomerization of 21–23: variation of aromatic scaffolds under
conditions.
a variety of
Entry
1
Substrate
Conditionsa
Time
Conversionb
21
1, + AgOTf, 70 °C
1, + AgOTf, 70 °C
2 + AgOTf, 70 °C
2 + AgOTf, 70 °C
1, + AgOTf, 70 °C
1, + AgOTf, 70 °C
2 + AgOTf, 70 °C
2 + AgOTf, 70 °C
1, + AgOTf, 70 °C
1, + AgOTf, 70 °C
2 + AgOTf, 70 °C
2 + AgOTf, 70 °C
1 h
16 h
1 h
16 h
1 h
16 h
1 h
16 h
1 h
16 h
1 h
0%
0%
0%
2
3
4
5
6
21
22
22
23
23
76%
1%
19%
13%
53%
0%
5%
20%
75%
16 h
a
[Substrate] = 0.04 mM, [Au] = 0.002 mM (0.05 mol%) [additive] = 0.004 mM
(0.01 mol%), reaction volume 0.60 mL.
b
As determined by NMR integration, all species were cleanly resolved, the only
observable compounds were starting [Substrate] or product 26.
76% conversion (Table 3, Entry 2). This is less than when 12 was
reacted with AuCl 2 (Table 1, Entry 5, 99% conversion after 16 h),
thus the difference with 2 could be electronic, as substrate 12
has a methyl group para to the cyclization center, while it is meta
in 21. An electronic effect should be small enough in this case, that
1 should still give some turnover. None was observed, which we
then interpret to be a size effect. Tentatively, the alkyne cannot
effectively approach Au in 1; could this be caused by the tolyl
group having a predilection for the cavity interior?
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Acknowledgments
Thus far, binding studies have proven inconclusive with multi-
ple substrates – even with bulky solvents that are excluded from
the cavity (i.e. mesitylene-d12), we have not observed host–guest
interactions with 1. These types of solvents have been used with
great success to bias guest binding by removing competition with
solvents (such as chloroform, benzene and toluene which all fit).
For now we propose that some modicum of complementary bind-
ing is at work. This issue could be resolved at a later date.
The project described was supported by the generous support
of the National Science Foundation (NSF RUI CHE-1708937). NMR
instrumentation was provided by the National Science Foundation
(MRI CHE-1337559). This project is supported in part by RISE (L.E.
R.) (NIH 2R25GM071638-09A1). The content is solely the responsi-
bility of the authors and does not necessarily represent the views
of the National Science Foundation.
With substrate 22 we expanded on this idea by giving the cav-
itand another ‘handle’ with which to bind in the form of an addi-
tional phenyl ring. With catalyst 1, we see very poor conversion
to 30; with 2, modest conversion (Table 3, Entries 3–4). Again,
there is a difference between 1 and 2, which could be ascribed sim-
ply to steric bulk of Au-10s ‘ligand’; we think the elongated biphe-
nyl has some degree of complementarity for 1, significant enough
to prevent the alkyne from finding the reactive Au(I) center.
In our last substrate 23, the alkyne is positioned between two
phenyl groups, and the reactivity difference between cavitand 1
and complex 2 (Table 3, Entries 5–6) was stark. Complex 2 resulted
in 75% conversion of 23 to 30 after overnight heating, whereas 1
gave a minimally measurable amount of product.
Appendix A. Supplementary data
The electronic supplementary material contains all procedures
for the synthesis of new compounds including their characteriza-
tion. Representative 1H NMR stack plots from which conversions
were calculated are included. Supplementary data to this article
can be found online at https://doi.org/10.1016/j.tetlet.2020.
152333.
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