Realizing metal-free carbene-catalyzed carbonylation reactions with CO

Article abstract of DOI:10.1021/jacs.0c09938

Many organic and main-group compounds, usually acids or bases, can accelerate chemical reactions when used in substoichiometric quantities, a process known as organocatalysis. In marked contrast, very few of these compounds are able to activate carbon monoxide, and until now, none of them could catalyze its chemical transformation, a classical task for transition metals. Herein we report that a stable singlet ambiphilic carbene activates CO and catalytically promotes the carbonylation of an oquinone into a cyclic carbonate. These findings pave the way for the discovery of metal-free catalyzed carbonylation reactions.

Full text of DOI:10.1021/jacs.0c09938

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Realizing Metal-Free Carbene-Catalyzed Carbonylation Reactions
with CO
́
Jesse L. Peltier, Eder Tomas-Mendivil, Daniel R. Tolentino, Max M. Hansmann, Rodolphe Jazzar,
and Guy Bertrand*
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ABSTRACT: Many organic and main-group compounds, usually acids or bases, can accelerate chemical reactions when used in
substoichiometric quantities, a process known as organocatalysis. In marked contrast, very few of these compounds are able to
activate carbon monoxide, and until now, none of them could catalyze its chemical transformation, a classical task for transition
metals. Herein we report that a stable singlet ambiphilic carbene activates CO and catalytically promotes the carbonylation of an o-
quinone into a cyclic carbonate. These findings pave the way for the discovery of metal-free catalyzed carbonylation reactions.
t the end of the 19th century, Sabatier discovered that
thin slivers of reduced nickel could promote the addition
concluded that only carbenes featuring a sufficiently small
singlet−triplet (S/T) gap could yield stable ketenes.13 This
result prompted us to consider the bicyclic (alkyl)(amino)-
carbene BiCAAC14 and the cyclic (alkyl)(amino)carbene
CAAC,15,16 which feature S/T gaps of 45.7 and 49.2 kcal/
mol, respectively. Indeed, these two carbenes instantaneously
react with carbon monoxide in benzene or THF at room
temperature to afford the corresponding ketenes Ket-1 and the
previously reported Ket-2,12a which were isolated in 85 and
90% yield, respectively (Scheme 1A).
The second step involves the reaction between amino-
ketenes and an o-quinone to give the corresponding lactones,
akin to the reactivity of alkyl- and aryl-substituted ketenes
(RR′CCO).17 Indeed, the reactions of Ket-1 and Ket-2
with 9,10-phenanthrenequinone (Quin) at room temperature
quickly and quantitatively led to the corresponding [4 + 2]
cycloadducts Lact-1 and Lact-2 as single diastereomers
(Scheme 1A), despite the introduction of a new chiral center
(vide infra).
To check the feasibility of the reductive elimination, we first
treated Lact-1 and Lact-2 with elemental sulfur, a classical
trapping agent for singlet carbenes.10a In the case of Lact-1,
heating at 80 °C for 16 h in THF was necessary to obtain
Thio-1 along with the release of the cyclic carbonate Carbo-1
(Scheme 1B). With Lact-2 derived from CAAC, we were
pleased to observe the clean formation of the thiolactam Thio-
2 along with a quantitative amount of carbonate Carbo-1 after
16 h at room temperature. Since Lact-1 and Lact-2 can be
regarded as products resulting from the insertion of carbenes
into the C−O single bond of the corresponding cyclic
A
of H2 to ethylene. Afterward, it was widely accepted that a
transition metal was required to activate small molecules in all
chemical and biological systems1 and to promote their reaction
with substrates. However, in the past decade, a handful of
main-group compounds have been shown to achieve the same
task,2 as exemplified by the hydrogenation of multiple bonds
using frustrated Lewis pairs.3 In 2007, we wrote that in
possessing a lone pair of electrons and an accessible vacant
orbital, singlet carbenes resemble transition metal centers and
thus could potentially mimic their chemical behavior.4,5
Indeed, some stable electrophilic carbenes can split enthalpi-
cally strong bonds like B−H, Si−H, P−H, N−H,6 and even
C−H7 and C−F bonds8 as well as challenging molecules like
H2 and NH3.4,9 Additionally, it has recently been shown that
reductive elimination at carbon can also occur.10 However, the
third essential step of a classical transition metal catalytic cycle,
namely, the transformation of the reactants, is hampered by the
lack of reactivity of C−H and C−F bonds. This analysis
prompted us to turn our attention to catalytic carbonylation
reactions with carbon monoxide, a task that to date has been
restricted to transition metals.11 Although CO is one of the
most inert molecules (with a bond dissociation energy of 1072
kJ/mol vs 432 kJ/mol for H2), it is known to react with
ambiphilic carbenes to give the corresponding ketenes.12 The
latter are very reactive, and thus, the working hypothesis relied
on the possibility that a ketene would react with a substrate
and that the resulting adduct would undergo a reductive
elimination, releasing the product and regenerating the
carbene. As a proof of principle, herein we report that select
singlet carbenes can promote the stoichiometric and even
catalytic carbonylation of an o-quinone to give a cyclic
carbonate.
Received: September 16, 2020
We started our investigation by performing stoichiometric
reactions representing the different elementary steps of the
putative catalytic cycle. First, the carbene must react with CO
to form a ketene. From a computational study, Frenking et al.
https://dx.doi.org/10.1021/jacs.0c09938
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
© XXXX American Chemical Society
A

Journal of the American Chemical Society
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Communication
quinone is due to the better leaving group ability of the
phenoxy anion, which facilitates the formation of the opened
betaines and thus hampers the observation of the kinetic
diastereomers.
Scheme 1. Stoichiometric Reactions Mimicking the
Elementary Steps of the Postulated Catalytic Cycle
To close the catalytic cycle, we reacted the spirolactones
with carbon monoxide (Scheme 1E). With Lact-1, no reaction
was observed at room temperature, even under 4 atm CO.
However, heating a C6D6 solution of Lact-1 under 4 atm CO
at 80 °C for 16 h led to Carbo-1 and a black residue from
BiCAAC decomposition that was not characterized. With
Lact-2 under 4 atm CO at room temperature, the solution
turned slightly blue, indicating the formation of Ket-2, which
was confirmed by the observation of the characteristic 13C
NMR signal of CCO at 278.8 ppm.12a Notably, 1 atm CO was
insufficient to observe any reaction, which demonstrates that
this process is dependent on the CO pressure.
Encouraged by these results, we investigated the hypo-
thetical catalytic process computationally (B3LYP-D3/TZVP),
as shown in Scheme 2, employing BiCAAC, sterically
a
Scheme 2. Proposed Mechanism and Energy Diagram
carbonates, we also investigated the reaction of BiCAAC and
CAAC with Carbo-1 and observed the clean formation of the
corresponding spirolactones Lact-1 and Lact-2, again as single
diastereomers (Scheme 1C). These results demonstrate the
reversibility of the reductive elimination step.
To understand the diastereoselectivity of the process, we
reacted BiCAAC with ethylene carbonate (Carbo-2) in C6D6
at room temperature. The 1H NMR analysis showed the
formation of a 74:26 mixture of the diastereomers Lact-3′ and
Lact-3 (Scheme 1D). When the mixture was heated at 80 °C,
the ratio was reversed toward the thermodynamic product
(Lact-3). This mixture also reacted at 80 °C with elemental
sulfur, leading to Thio-1 and Carbo-2. Importantly, these
observations strongly suggest that unlike in organometallic
catalysis,18 the reductive elimination involves a polar stepwise
process via opened (o-Bet) and closed (c-Bet) betaines.19 We
believe that the diastereoselectivity observed with the o-
a
DFT calculations were performed at the B3LYP-D3/TZVP level.
Gibbs energies are shown in kcal/mol.
encumbered CAAC, and small CAACMe for comparison. The
formation of ketenes is predicted to be exergonic in every case.
Similarly, the reactions between the ketenes and Quin leading
to spirolactones (Lact) are exergonic; the latter are the most
stable intermediates and thus can be regarded as the resting
state of the catalytic cycle. It should be noted that the relative
energies between o-Bet, c-Bet and free carbonate plus CAAC
are negligible and that the overall transformation is exergonic.
Importantly, calculations indicate that the reductive elimi-
nation step should be possible with all of the carbenes,
although it is significantly easier with the bulky CAAC than
with BiCAAC and CAACMe.
B
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Communication
Together, the experimental and computational results
discussed above suggested that a catalytic reaction should be
achievable. We were delighted to find that indeed, with only 10
mol % CAAC in benzene after 40 h at 60 °C under 4 atm CO,
the carbonylation of Quin was achieved and gave Carbo-1 in
80% isolated yield (Scheme 3).
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.0c09938.
■
sı
General methods and materials, experimental procedures
and characterization data, X-ray structure determination,
computational details, and NMR spectra (PDF)
Crystallographic data in CIF format for Lact-1, Lact-2,
Carbo-1, I, and II (ZIP)
Scheme 3. Organocatalytic Carbonylation of Quin
AUTHOR INFORMATION
Corresponding Author
■
Guy Bertrand − UCSD−CNRS Joint Research Chemistry
Laboratory (UMI 3555), Department of Chemistry and
Biochemistry, University of California San Diego, La Jolla,
California 92093-0358, United States; orcid.org/0000-
0003-2623-2363; Email: guybertrand@ucsd.edu
On the other hand, all attempts to use BiCAAC and
CAACMe as catalysts failed, even when more forcing
experimental conditions were used. We discovered that
BiCAAC and the small CAACMe readily react with Quin,
leading irreversibly to adducts I20 and II,21 respectively, thus
quenching the catalytic cycle (Scheme 4). As a control
experiment, we confirmed that the bulky CAAC did not react
with the o-quinone even at 80 °C and thereby leads to a viable
catalyst.
Authors
Jesse L. Peltier − UCSD−CNRS Joint Research Chemistry
Laboratory (UMI 3555), Department of Chemistry and
Biochemistry, University of California San Diego, La Jolla,
California 92093-0358, United States; orcid.org/0000-
0002-3493-2127
́
Eder Tomas-Mendivil − UCSD−CNRS Joint Research
Chemistry Laboratory (UMI 3555), Department of Chemistry
and Biochemistry, University of California San Diego, La Jolla,
California 92093-0358, United States; Department of Applied
Chemistry, Faculty of Chemistry, University of the Basque
Scheme 4. Deactivation of BiCAAC and CAACMe
́
Country (UPV-EHU), Donostia-San Sebastian 20018,
Gipuzkoa, Spain
Daniel R. Tolentino − UCSD−CNRS Joint Research
Chemistry Laboratory (UMI 3555), Department of Chemistry
and Biochemistry, University of California San Diego, La Jolla,
California 92093-0358, United States
Max M. Hansmann − UCSD−CNRS Joint Research Chemistry
Laboratory (UMI 3555), Department of Chemistry and
Biochemistry, University of California San Diego, La Jolla,
California 92093-0358, United States; orcid.org/0000-
0003-3107-1445
Rodolphe Jazzar − UCSD−CNRS Joint Research Chemistry
Laboratory (UMI 3555), Department of Chemistry and
Biochemistry, University of California San Diego, La Jolla,
California 92093-0358, United States; orcid.org/0000-
0002-4156-7826
Until now, carbene organocatalysis has been limited to
weakly basic carbenes featuring large HOMO/LUMO gaps,
allowing them to behave as good leaving groups (e.g., thiazol-
2-ylidenes,22 1,2,4-triazol-5-ylidenes,23 and 1,2,3-triazol-4-
ylidenes24). However, this work demonstrates that strongly
ambiphilic carbenes such as CAACs are highly desirable in
small-molecule catalysis. Their electronic properties allow
them to mimic all of the elementary steps involved in transition
metal catalytic cycles, but their mechanism can be different, as
shown by the reductive elimination process described herein,
thus opening up new possibilities in catalytic transformations.
Transition-metal-catalyzed carbonylation reactions are widely
applied in industry on a large scale, employing CO gas as the
C1 source.11 We are aware that the carbonylation of quinones
is not of synthetic utility, but since not only carbenes but also
other low-valent main-group compounds such as silylenes,
phosphinidenes, and borylenes are able to bind CO,25 these
findings pave the way for the discovery of other metal-free
catalyzed carbonylation reactions.
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.0c09938
Notes
The authors declare no competing financial interest.
Metrical data for the solid-state structures of Lact-1, Lact-2,
Carbo-1, I, and II have been deposited with the Cambridge
Crystallographic Data Centre under reference numbers
CCDC-1999858, CCDC-1999855, CCDC-1999854, CCDC-
1999857, and CCDC-1999856, respectively.
ACKNOWLEDGMENTS
■
We are grateful to the U.S. Department of Energy, Office of
Science, Office of Basic Energy Sciences, Catalysis Science
Program, under Award DE-SC0009376 for financial support of
this work and to the Tribal Membership Initiative fellowship
administered by the UC San Diego Graduate Division for
C
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Communication
11559−11563. (c) Eichhorn, A. F.; Fuchs, S.; Flock, M.; Marder, T.
B.; Radius, U. Reversible oxidative addition at carbon. Angew. Chem.,
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(11) (a) Beller, M.; Wu, X.-F. Transition Metal Catalyzed
Carbonylation Reactions; Springer: Berlin, 2013. (b) Peng, J. B.;
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financial support of J.L.P. E.T.-M. is thankful to the Basque
Government for a Doctoral Research Staff Improvement
Program scholarship and to Dr. Zoraida Freixa for her support.
M.M.H. is thankful for a Feodor-Lynen scholarship from the
Alexander von Humboldt foundation.
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