Tropolonate salts as acyl-transfer catalysts under thermal and photochemical conditions: Reaction scope and mechanistic insights

Article abstract of DOI:10.1021/acscatal.0c03702

Acyl-transfer catalysis is a frequently used tool to promote the formation of carboxylic acid derivatives, which are important synthetic precursors and target compounds in organic synthesis. However, there have been only a few structural motifs known to efficiently catalyze the acyl-transfer reaction. Herein, we introduce a different acyl-transfer catalytic paradigm based on the tropolone framework. We show that tropolonate salts, due to their strong nucleophilicity and photochemical activity, can promote the coupling reaction between alcohols and carboxylic acid anhydrides or chlorides to give products under thermal or blue light photochemical conditions. Kinetic studies and density functional theory calculations suggest interesting mechanistic insights for reactions promoted by this acyl-transfer catalytic system.

Full text of DOI:10.1021/acscatal.0c03702

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Research Article
Tropolonate Salts as Acyl-Transfer Catalysts under Thermal and
Photochemical Conditions: Reaction Scope and Mechanistic Insights
Demelza J. M. Lyons, Claire Empel, Domenic P. Pace, An H. Dinh, Binh Khanh Mai,*
Rene M. Koenigs,* and Thanh Vinh Nguyen*
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ABSTRACT: Acyl-transfer catalysis is a frequently used tool to
promote the formation of carboxylic acid derivatives, which are
important synthetic precursors and target compounds in organic
synthesis. However, there have been only a few structural motifs
known to efficiently catalyze the acyl-transfer reaction. Herein, we
introduce a different acyl-transfer catalytic paradigm based on the
tropolone framework. We show that tropolonate salts, due to their
strong nucleophilicity and photochemical activity, can promote the
coupling reaction between alcohols and carboxylic acid anhydrides
or chlorides to give products under thermal or blue light photochemical conditions. Kinetic studies and density functional theory
calculations suggest interesting mechanistic insights for reactions promoted by this acyl-transfer catalytic system.
KEYWORDS: acyl-transfer, acylation, tropolone, esterification, catalysis
and amidation reactions of carboxylic acids.7 Presumably,
INTRODUCTION
■
tropone possesses these types of reactivities because of its
Esters of carboxylic acids are ubiquitous not only as useful
ability to partially aromatize7,8 to the tropylium oxide form
synthetic precursors in organic synthesis but also as important
(Figure 1), which displays Lewis base characteristics.9 Its 2-
building blocks in pharmaceuticals and agrochemicals.1 The
hydroxy derivative, tropolone, has been frequently used as a
synthesis of these derivatives directly from carboxylic acids and
model system to study intramolecular proton transfer and
alcohols in Fischer esterification processes normally suffers
tunneling processes.10 The tautomerism between the carbonyl
from incomplete conversion due to condensation/hydrolysis
moiety and the adjacent hydroxyl group through the seven-
equilibrium.1 Thus, reactive acylation reagents such as acid
membered nonbenzenoid aromatic ring renders it relatively
chlorides or acid anhydrides are typically the reagents of choice
Brønsted acidic with a pKa value of ∼6.7 (Figure 1).11 In its
to couple with alcohols or phenols in esterification reactions.2
deprotonated tropolonate form, this species has been used as a
Nevertheless, when the alcohol or phenol substrates are
powerful bidentate chelating agent for d-block and f-block
nonreactive due to electronic or steric hindrance, these
organometallic complexes.12 Many of these are interesting
reactions become sluggish and inefficient.2a,b A number of
photoactive materials, thanks to the chromophoric contribu-
acyl-transfer catalysts have therefore been used to promote
tions from tropolone chelation.12,13 Furthermore, tropolone
these reactions.3 Most of these acyl-transfer systems include a
has also been used as a luminogen to enhance fluorescence and
4-aminopyridine motif,2a,b,4 developed based on the well-
for recognition of biological systems such as amino acids,
known catalytic activity of 4-dimethylaminopyridine (DMAP)5
proteins, or DNAs.14
in the Steglich esterification reaction discovered by Steglich,
Although tropolone is a commonly found framework in
Hassner, and other researchers.6 The inherent Brønsted
biologically valuable natural products,15 little has been
basicity of 4-aminopyridine system leads to some limitations
explored on its synthetic applications except as precursors in
in its catalytic scope, such as in coupling reactions of easily
the synthesis of azulenes in the seminal works by Nozoe.16
racemized substrates.1−3 Therefore, it is of significant
importance to develop other structural motifs to broaden
acyl-transfer catalysis. Herein, we introduce a new acyl-transfer
paradigm based on the tropolone scaffold, which can promote
the coupling reaction between alcohols and carboxylic acid
With such interesting structural features discussed above, we
Received: August 24, 2020
Revised: October 1, 2020
anhydrides or chlorides to give ester products with excellent
efficiency under thermal or photochemical conditions.
We had previously developed several methods to use
tropone as a stoichiometric or catalytic system in esterification
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Figure 1. Tropylium, tropone, and tropolones.
envisioned enormous potential for tropolone in organic
synthesis. There are common electronic properties and
reactivity between tropolone and DMAP, as depicted in Figure
1, that triggered our interest in exploring the acyl-transfer
catalytic activity of tropolone.
This study provides a promising theoretical standpoint for our
hypothesis on the acyl-transfer catalytic activity of tropolone. It
also suggests that the tropolonate salt might be better for this
purpose, which is in line with typical catalytic acylation
reactions, where a basic additive that can easily convert
tropolone to tropolonate would normally be used to facilitate
the reaction.
We started the investigations using an unreactive and slightly
sterically challenging phenol 2,4,6-trichlorophenol (1a) as the
model substrate in the reaction with acetic anhydride (2a)
using tropolone or tropolonate salts (3) as reaction promoters.
Interestingly, all tropolonate salts outperformed the parent
tropolone and gave much better outcomes than the control
experiments without catalysts (Table 1).19 Contrary to DMAP
and other pyridine-based acyl-transfer agents,1b,2a,4c,5 we found
that the use of a stoichiometric amount of base such as NEt3 is
not necessary for our reactions with tropolonate. The presence
of a base slightly sped up the reaction, but we set forth without
it to optimize the atom economy. A range of commonly used
organic solvents were tested for the reaction and acetonitrile
seemed to be optimal, presumably with the best tropolonate
solubility. In most solvents, potassium tropolonate (3-K) gave
higher efficiency than the lithium, sodium, or tetrabutylammo-
nium analogues.
RESULTS AND DISCUSSION
In order to establish if tropolone would be a suitable motif for
■
acyl-transfer catalysis, we investigated its nucleophilicity index
17
(N(Nu)
)
using density functional theory (DFT) calcula-
tions.18 The nucleophilicity index values of tropone,
tropolonate ion, and DMAP are calculated to be 2.74, 4.29,
and 3.22 eV, respectively (Figure 2).19 While tropolone can be
Catalyst loading was optimal at 5 mol % (entries 23−25),
and increasing the temperature reduced the reaction time from
2 days to just 4 h (entry 26). Without the catalyst, the
background reaction was sluggish and inefficient (entries 1−3
and 27, Table 1, also see Figure 3d), with some starting
materials remaining unreacted and some degraded. With
potassium benzoate or acetate as weak Brønsted base catalysts,
the reaction outcomes were not much better (entries 28−29).
Similarly, the use of potassium phenoxides with a range of
aromatic substituents did not lead to any satisfactory results
either (entries 30−31). It should be noted that the parent
carboxylic acids or phenols of the potassium salts in entries
28−31 (Table 1) have pKa ranges similar to those of
tropolones. Salicylaldehyde (the last entry of 31, Table 1)
can also be considered a structural analogue of tropolone. The
negative results of these control experiments further highlight
Figure 2. Calculated nucleophilicity at M06-2X/6-311+G(2d,2p)-
SMD(MeCN)//M06-2X/6-31+G(d,p)-SMD(MeCN).
considered a weak nucleophile, the nucleophilicity of
tropolonate (estimated Nparameter ∼ 17.9 from N(Nu) = 4.29
eV) is much higher than that of DMAP (∼15.5 according to
Mayr’s nucleophilicity scale).20
In addition, the nucleus-independent chemical shifts
[NICS(1)zz] values21 of tropolonate ion and phenyl
tropolonate ester are −9.1 and −8.3 ppm, respectively, which
indicate that the aromaticity of tropolonate ion is higher than
that of its acyl form and, therefore, tropolonate can be
considered as a good leaving group for acyl-transfer reactions.
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a
Table 1. Optimization of Tropolonate-Promoted Acylation reaction
a
Conditions: phenol 1a (0.5 mmol), acetic anhydride 2a (0.55 mmol), and catalyst 3 in solvent (0.5 mL) under normal atmospheric conditions.
b
c
Reaction time for total consumption of 1a. Yield of the isolated product.
the intriguing catalytic activity of tropolonate salts in acyl-
demanding and less electrophilic nature of the benzoyl group.
The most obvious examples to demonstrate this difference was
with 2,4-di(tert-butyl)phenol, which showed a significant drop
in yield when it reacted with benzoic anhydride (46% of 5e) in
comparison to the reaction with acetic anhydride (89% of 4e).
1-Adamantol was acylated with poor yields of ester products
4m and 5m, perhaps due to the sterically hindered position of
the alcohol group. Biologically relevant alcohols and phenols
such as menthol, cinchonine, quinidine, and estrogen reacted
smoothly with acid anhydrides under the optimal conditions to
transfer reactions and hint that the tropolonate salt did not
merely act as a Brønsted base catalyst in these reactions.
Having the optimal conditions in hand, a range of
unactivated alcohols and phenols were subjected to the
reaction to produce their corresponding esters with acetic
and benzoic anhydride (Scheme 1). High-to-excellent yields
were obtained for a majority of the acylated substrates, with
the benzoic anhydride generally yielding slightly lower than the
acetic anhydride, probably due to the somewhat more sterically
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Figure 3. (a) First mechanistic hypothesis; (b) control studies; (c) ESI−MS analysis of metal tropolonate salts; (d) kinetic studies of the reaction
between 1a and 2a with different catalyst loadings.
produce the corresponding products in good-to-high yields
(4n−4q and 5n−5p). Several mercaptans could also be
subjected to the same tropolonate-catalyzed acyl-transfer
reactions with similar good efficiency (4s, 4t, 5x). Comparison
of our tropolonate-promoted reactions to those catalyzed by
DMAP (also 5 mol % under the same conditions) was
investigated on some challenging substrates (4a, 4e, 4n−p,
Scheme 1), which showed that our catalyst has an efficiency
similar to that of DMAP.
A similar optimization study was carried out for the
tropolonate salt-catalyzed reaction between alcohols or
phenols and acid chlorides.19 It identified a parallel set of
optimal reaction conditions to the reaction with acid
anhydrides, except that the best solvent was dichloromethane
instead of acetonitrile. It should be noted that control reactions
without a catalyst revealed significantly worse reaction
outcomes, confirming the essential role of the tropolonate
catalyst for esterification reactions with acid chlorides. Again, it
was observed that a stoichiometric amount of base such as
NEt3 sped up the reaction but was not essential for it to be
effective.
Using the optimal conditions, high-to-excellent yields were
generally obtained for the acylation reactions of a range of
alcohols and phenols with pivaloyl and benzoyl chlorides
(Scheme 2). The reaction also worked well for the
benzoylation of cholesterol (8d, Scheme 2) and hexafluor-
oisopropanol (8e). The latter is a useful precursor in organic
synthesis as its ester functionality is known to be activated by
the perfluorinated alkyl moiety to undergo interesting
nucleophilic substitution reactions.22 A few other acid
chlorides also proved to work smoothly under the reaction
conditions (8f−8h, Scheme 2).
Thus, we have developed a robust synthetic method to use
tropolonate salts as an acyl-transfer catalyst for esterification
reactions of inactive alcohols and phenols with acid anhydrides
or chlorides. At this stage in the project, it is clear that the
tropolone framework brought the 2-hydroxyl group within the
suitable pKa and nucleophilicity ranges to promote the
reaction. However, the contribution of the aromatic cyclo-
heptatrienone moiety in the tropolonate salts and its role in the
catalytic cycle remain unknown to us. The mechanism of the
tropolonate-promoted acyl-transfer reaction is especially
interesting, given that such reactivity is not observed with
analogous carboxylate or phenolate Brønsted bases, as
discussed earlier in Table 1.
Mechanistic Studies. We initially hypothesized that the
acyl-transfer reaction with tropolonate salts occurred via a
mechanistic pathway similar to what has been widely accepted
for DMAP (Figure 3a). In that, the tropolonate ion 3 acted as a
nucleophile to react with acid anhydrides or chlorides 2 to
form acyl tropolonate ester intermediates 9. These inter-
mediates subsequently underwent nucleophilic acyl-transfer
reactions with alcohols or phenols 1 via 11 to afford the
products and release the tropolonate ion back to the catalytic
cycle. We indeed observed evidence of H-bonding complex-
ation, hinting at 11, between preformed benzoyl tropolonate
ester 9b19 and phenol 1a in a preliminary 1H nuclear magnetic
resonance (NMR) study (Figure 3a and also page S46 in the
experimental Supporting Information). However, attempts to
detect the acyl tropolonate ester intermediates 9 by gas
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Scheme 1. Substrate Scope with Acid Anhydrides
chromatography−mass spectrometry (MS) or NMR spectros-
copy during the course of the reaction were unsuccessful. On
the other hand, control experiments with stoichiometric
amounts of preformed acyl tropolonate ester intermediates
919 and phenol 1a did lead to the formation of products
(Figure 3b), confirming the “acyl transfer” role of the
tropolonate moiety. However, the efficiency of such direct
reactions was unsatisfactory compared to the tropolonate-
promoted acylation reactions (Schemes 1 and 2). These
experimental results suggest that the simple hypothesis in
Figure 3a cannot accurately describe the mechanism of the
reaction under investigation.
A deeper analysis of our experimental results revealed that
the tropolonate ion likely did not operate alone in the reaction
mixture. As mentioned in the introduction, tropolonate is a
strong bidentate chelating agent. Our negative electron spray
ionization (−ESI) high-resolution mass spectrometric analyses
of tropolonate salts used in this study showed obvious evidence
for cluster formation between tropolonate and all alkali-metal
counterions (Li+, Na+, and K+, Figure 3c and also page S6 in
the experimental Supporting Information). These results are
consistent with literature reports on the complexation of
tropolonate with metal ions in different stoichiometric
ratios.12,13,23 Given the concentrated nature of our catalytic
reaction mixtures and the low solubility of tropolonate salts, in
the optimal setting being potassium salt (3-K), it is most likely
that the catalyst exists and promotes the reaction in cluster
form. The analysis with different catalyst loadings indicated
that the reaction rate is not first order with respect to
tropolonate salt 3-K (reaction order ∼ 1.5, Figure 3d and also
page S47 in the experimental Supporting Information),19
which further makes us believe that more than one tropolonate
ion was involved in the promotion of the reaction. It should
also be noted here that the outcomes of the direct acyl-transfer
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Scheme 2. Substrate Scope with Acid Chlorides
Figure 4. Calculated free-energy profile [kcal/mol, M06-2X/6-311+G(2d,2p)-SMD(MeCN)//M06-2X/6-31+G(d,p)-SMD(MeCN)] and
optimized structures of transition states for the tropolonate-catalyzed acylation of benzoic anhydride.
reactions between tropolonate ester intermediates 9 and
phenol 1a improved significantly in the presence of a base
such as Et3N (Figure 3b). Similar results were observed with
the potassium carboxylate salt as an additive, despite the fact
that the carboxylate salt is less Brønsted basic than Et3N. This
hinted that we probably should not too soon be dismissive of
the role of the carboxylate salt byproduct (10 in Figure 3a) in
this acyl-transfer reaction.
We therefore turned our focus to DFT calculations to seek
better insights into the tropolonate-catalyzed acylation of
benzoic anhydride (2b) and benzoyl chloride (6b) with
cyclohexanol (1l) as model substrates. After exploring several
possible reaction pathways, we came up with a possible
explanation. The calculated free-energy profile for the acylation
of benzoic anhydride 2b is shown in Figure 4. The reaction
starts with the nucleophilic attack of solvated potassium
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Figure 5. Calculated free-energy profile [kcal/mol, M06-2X/6-311+G(2d,2p)-SMD(CH2Cl2)//M06-2X/6-31+G(d,p)-SMD(CH2Cl2)] and
optimized structures of transition states for the tropolonate-catalyzed acylation of benzoyl chloride.
tropolonate 3-K-MeCN to benzoic anhydride via TS-1 to form
intermediate 12, which is a chelating potassium complex. The
activation barrier for the first step is calculated to be 20.2 kcal/
mol relative to 3-K-MeCN. The nucleophilic attack of
cyclohexanol 1l to the benzoyl moiety of 12 takes place via
TS-2, which involves another potassium tropolonate, to
generate product 5l and potassium complexes 13 and 14.
TS-2 is calculated to be 15.3 kcal/mol higher in energy than 3-
K-MeCN. Finally, a proton transfer step from 14 to 13 occurs
to produce benzoic acid, regenerate 3-K-MeCN, and close the
catalytic cycle. It is important to mention that in the second
step, the participation of another potassium tropolonate as a
base catalyst is necessary in order to abstract proton from
cyclohexanol and, thus, enhance the nucleophilicity of
cyclohexanol. We have also considered the possibility that
the benzoate ion acts as a base catalyst instead of potassium
tropolonate (TS-2′). This reaction pathway is also viable,
however, the activation barrier of TS-2′ is calculated to be 4.3
kcal/mol higher in energy than TS-2. Thus, it agrees well with
our earlier belief that the presence of benzoate ion (Figures
3a,b) assists the reaction but is not the driving factor for it to
happen like the tropolonate ion.
Our DFT calculations suggest that, in this reaction,
potassium tropolonate acts as nucleophilic and base catalysts
and that the nucleophilic attack of potassium tropolonate to
benzoic anhydride is the rate-determining step (Figure 4). Our
proposed mechanism is consistent with previous calculations
for the acylation of DMAP derivatives.2a,4d Overall, this
mechanism also agrees well with the experimental results we
collected, as depicted and discussed earlier in Figure 3.
Obviously, this acylation reaction can also take place via a
single-step mechanism with the direct nucleophilic attack of
cyclohexanol to benzoic anhydride (see Figure S1 page S5 in
the computational Supporting Information for optimized
structure).19 In this mechanistic pathway, potassium tropolo-
nate only participates as a base catalyst. However, the
activation barrier for this step is calculated to be 7.7 kcal/
mol higher in energy than TS-1 and can, therefore, be ruled
out as a feasible or competitive mechanism.
The calculated free-energy profile for the acylation of
benzoyl chloride 6b is shown in Figure 5. It is likely that this
reaction also occurs via a two-step mechanism, that is, the first
step is the nucleophilic attack of potassium tropolonate,
solvated by cyclohexanol instead of the non-coordinating
dichloromethane, to benzoyl chloride, which is followed by the
nucleophilic attack of cyclohexanol to generate the product. It
is interesting to note that the potassium complex cluster (16 +
17) is calculated to be the most stable species. This result
suggests that (16 + 17) is a possible resting state of the
reaction, that is, after the first cycle, the reaction starts from
(16 + 17), which is followed by a proton transfer from 17 to
16 and a ligand exchange giving 3-K-CyOH; after that, two
subsequent nucleophilic additions can then take place giving
the product. The nucleophilic attack of potassium tropolonate
to benzoyl chloride is the rate-determining step with an overall
activation barrier of 22.3 kcal/mol [from (16 + 17) to TS-3,
see Figure 5]. This barrier, although higher than that of the
reaction with benzoic anhydride, is still quite feasible and
consistent with the experimental conditions of this reaction. It
should also be noted here that computational exploration of a
possible role of HCl, such as protonating the carbonyl of 6a′
hence activating it, did not lead to any feasible pathways that
need to be taken into account.
Photochemical Studies. As mentioned in our introduc-
tion, tropolonate ion is known to be a good chromophoric
chelating agent for transition metals12,13 or a sensing probe for
biological systems by enhancing their fluorescent properties.14
Thus, we also wanted to explore if this photoactivity can also
be exploited in the promotion of acyl-transfer reactions. We
started the investigation by looking at the UV−vis absorption
and photoluminescent spectra of tetrabutylammonium tropol-
onate (3-TBA) in acetonitrile. This tropolonate salt was used
in preference over potassium tropolonate (3-K) because of its
superior solubility in acetonitrile and some other organic
solvents. 3-TBA displayed two absorption maxima in
acetonitrile at ∼350 nm and ∼410 nm (Figure 6, top-left).
Excitations at 400 or 420 nm both gave weak luminescence
with emission peaks at ∼460−470 nm.
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Figure 6. (Top-left) UV−vis absorption and photoluminescence spectra of 3-TBA; (top-right) photochemical esterification reactions with 3-TBA
catalyst; (bottom-right) UV−vis absorption and photoluminescence spectra of 9a; (bottom-left) calculated free-energy profile [kcal/mol, TD-M06-
2X/6-311+G(2d,2p)-SMD(MeCN)//TD-M06-2X/6-31+G(d,p)-SMD(MeCN)] and optimized structures of transition states for the tropolonate-
catalyzed blue light-mediated acylation reaction.
We envisioned that such photochemical properties would
enable tetrabutylammonium tropolonate to act as a photo-
sensitizer to promote the acyl-transfer reaction under
investigation in this work. Indeed, under otherwise identical
conditions, 3-TBA could efficiently catalyze the esterification
of a range of alcohols and phenols under blue light irradiation
[400−420 nm blue light-emitting diode (LED), Figure 6, top-
right] at ambient temperature instead of 70 °C. Without light,
the reaction outcomes were rather unsatisfactory, indicating
that blue light indeed promoted the reaction (Figure 6, top-
right, 4e−5e and 4k−5k). Without 3-TBA catalyst, poor
efficiency was also observed (Figure 6, top-right, 4e−5e and
4k−5k) with starting materials degrading into unidentified
mixture of compounds. Therefore, it can be concluded that 3-
TBA can act as a photosensitizer or -catalyst for the acyl-
transfer reaction between alcohols or phenols and acid
anhydrides. The photochemical procedure proved to be useful
in the esterification reactions of biologically relevant systems
such as estrogen and cholesterol (4q and 8d, Figure 6), where
better product yields were achieved at a more physiologically
suitable temperature range.
We again resorted to time-dependent (TD)-DFT calcu-
lations to elucidate the reaction mechanism for the photo-
chemical tropolonate-catalyzed acylation reaction, with 1a and
2a as model substrates (Figure 6, bottom). Interestingly, the
NICS(1)zz values of the tropolonate ion and the methyl
tropolate ester are 23.9 and 26.9 ppm, respectively, which
indicate that these species change from aromatic at ground
state to antiaromatic at S1 excited state. Our calculations also
suggests that at S1 excited state, the formation of the product
and tropolonate ion from the methyl tropolate ester could take
place easily because of large antiaromaticity relief.24 We found
that this reaction occurs in both ground and excited states. The
first step of this reaction is the nucleophilic attack of
tropolonate ion 3 to acetic anhydride via TS-5 giving
tropolone acetate 9a in the ground state. It should be noted
here that in the absence of the potassium counterion, the
activation pathways seen in Figure 4 might not occur at all.
The activation barrier of TS-5 is calculated to be 23.2 kcal/mol
relative to 3. To proceed, tropolone acetate 9a is excited by
blue LED to form 9a*. 9a was confirmed to absorb weakly at
∼404 nm (Figure 6, bottom). The nucleophilic attack of 2,4,6-
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trichlorophenol 1a to 9a* can then take place via TS-6,
providing product and regenerating tropolonate ion 3 via the
relaxation of 3*. The second step of this reaction occurs in the
S1 excited state with the calculated activation barrier of 17.4
kcal/mol relative to 9a*. It is important to also note that we
could not locate any feasible mechanism for this reaction if it
only occurs in the ground or excited state. Because of the weak
luminescence of the 3-TBA catalyst and its tropolonate esters
9, we did not pursue its catalytic capability further at this stage
but instead focused on developing novel tropolone-based
photosensitizers for better catalytic efficiency. This work is
underway in our laboratories and will be reported in due
course.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acscatal.0c03702
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was funded by the Australian Research Council
(grant FT180100260 to T.V.N. and DP200100063 to T.V.N.
and R.M.K.). R.M.K. is grateful for the generous support from
the German Science Foundation. D.J.M.L., D.P.P., and A.H.D.
thank the Australian Government RTP program and UNSW
for sponsoring their Ph.D. scholarships. C.E. gratefully
acknowledges RWTH Aachen University for a doctoral
scholarship. T.V.N. and R.M.K. also thank UA/DAAD Joint
Research Cooperation Scheme seeding grant #RG172186/
57388073 for travel support. Preliminary experiments carried
out by Dr Katharina Hock (RWTH Aachen) and Dr Uyen
Tran (UNSW) and sample screening analysis by Jasnoor Mann
(UNSW) are acknowledged.
CONCLUSION
■
We have established a new acyl-transfer catalytic motif based
on the interesting structural properties of the nonbenzenoid
aromatic tropolone. We showed that tropolonate salts, due to
their strong nucleophilicity and photochemical activity, can
promote the coupling reaction between alcohols and carboxylic
acid anhydrides or chlorides to give products with excellent
efficiency under thermal or blue light-irradiated conditions. A
combination of experimental studies and computational
calculations suggests interesting mechanistic insights and
warrants further works to fully explore the potential of this
new acyl-transfer catalytic system.
REFERENCES
■
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acscatal.0c03702.
■
sı
Experimental details and spectroscopic data for all
products (PDF)
Full Gaussian reference, Cartesian coordinates, and
electronic and free energies (PDF)
(2) (a) Larionov, E.; Zipse, H. Organocatalysis: acylation catalysts.
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AUTHOR INFORMATION
Corresponding Authors
■
Binh Khanh Mai − Department of Chemistry, University of
Pittsburgh, Pittsburgh, Pennsylvania 15260, United States;
orcid.org/0000-0001-8487-1417; Email: binh.mai@
pitt.edu
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Rene M. Koenigs − School of Chemistry, University of New
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Institute of Organic Chemistry, RWTH Aachen, Aachen
D52074, Germany; orcid.org/0000-0003-0247-4384;
Email: rene.koenigs@rwth-aachen.de
Thanh Vinh Nguyen − School of Chemistry, University of New
South Wales, Sydney, New South Wales 2052, Australia;
orcid.org/0000-0002-0757-9970; Email: t.v.nguyen@
unsw.edu.au
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Authors
Demelza J. M. Lyons − School of Chemistry, University of New
South Wales, Sydney, New South Wales 2052, Australia
Claire Empel − School of Chemistry, University of New South
Wales, Sydney, New South Wales 2052, Australia; Institute of
Organic Chemistry, RWTH Aachen, Aachen D52074, Germany
Domenic P. Pace − School of Chemistry, University of New
South Wales, Sydney, New South Wales 2052, Australia
An H. Dinh − School of Chemistry, University of New South
Wales, Sydney, New South Wales 2052, Australia
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