Strategic Utilization of Multifunctional Carbene for Direct Synthesis of Carboxylic–Phosphinic Mixed Anhydride from CO2

Article abstract of DOI:10.1002/anie.201609710

Direct synthesis of carboxylic–phosphinic mixed anhydrides has been achieved by treating carbon dioxide with N-phosphine oxide-substituted imidazolylidenes (PoxIms) that contain both nucleophilic carbene and electrophilic phosphorus moieties. This novel mixed anhydride was efficiently derivatized into an ester, an amide, and an unsymmetrical ketone via transformation into its corresponding imidazolium salt followed by a dual substitution reaction. The presented work used well-designed multifunctional carbene reagents to establish a novel utility for carbon dioxide in organic synthesis.

Full text of DOI:10.1002/anie.201609710

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201609710
German Edition: DOI: 10.1002/ange.201609710
Carbon Dioxide Fixation
Strategic Utilization of Multifunctional Carbene for Direct Synthesis of
Carboxylic–Phosphinic Mixed Anhydride from CO2
Yoichi Hoshimoto,* Takahiro Asada, Sunit Hazra, Takuya Kinoshita, Panukorn Sombut,
Ravindra Kumar, Masato Ohashi, and Sensuke Ogoshi*
Abstract: Direct synthesis of carboxylic–phosphinic mixed
anhydrides has been achieved by treating carbon dioxide with
N-phosphine oxide-substituted imidazolylidenes (PoxIms) that
contain both nucleophilic carbene and electrophilic phospho-
rus moieties. This novel mixed anhydride was efficiently
derivatized into an ester, an amide, and an unsymmetrical
ketone via transformation into its corresponding imidazolium
salt followed by a dual substitution reaction. The presented
work used well-designed multifunctional carbene reagents to
establish a novel utility for carbon dioxide in organic synthesis.
Recently, we reported the synthesis of N-phosphine oxide-
substituted imidazolylidenes (PoxIms) as a novel class of
isolable N-heterocyclic carbenes (NHCs) (Scheme 1).[4,5]
M
ixed anhydrides comprising carboxylic and phosphinic
acids are frequently employed for the preparation of carbonyl
compounds such as carbamates and peptides.[1] In particular,
mixed anhydrides could be of great utility in organic synthesis
when three conditions exist: when the compounds are stable
towards thermal disproportionation, when they can be
reacted with external nucleophiles specifically at the carbon
center of the carbonyl moiety, and when they are isolable as
shelf-stable solids.[1d] These anhydrides are prepared in the
presence of a suitable base by either the reaction of
a carboxylic acid with a phosphinic chloride or the reaction
of an activated carboxylic acid derivative with a phosphinic
acid; however, the concomitant production of a significant
amount of salt is inevitable. Furthermore, carboxylic acid
derivatives can be prepared from carbon dioxide.[2] Thus, the
direct synthesis of carboxylic–phosphinic mixed anhydrides
from carbon dioxide is attractive from the aspects of step- and
atom-economy.[3]
Scheme 1. PoxIm plays a role of a) two distinct Lewis bases (previous
work) and b) both a nucleophile and an electrophile (this work).
PoxIms are equipped with two distinct Lewis bases, phosphine
oxide and carbene, that led to the formation of either classical
phosphine oxide–borane at À908C or carbene–borane
adducts at room temperature (Scheme 1a). The later adduct
was used as a precursor for frustrated Lewis pairs[6] compris-
ing carbene and borane.[4] On the other hand, the phosphine
oxide moiety can also function as an electrophile (Sche-
me 1b). Given this intriguing multifunctionality, we envi-
sioned that the reaction of PoxIms with carbon dioxide could
give an imidazolium-2-carboxylate followed by the nucleo-
philic addition of the carboxylate to the phosphine oxide to
directly yield a novel type of carboxylic–phosphinic mixed
anhydride. The formation of imidazolium-2-carboxylates by
the reaction of NHCs and carbon dioxide was studied
extensively.[7] Furthermore, the imidazole moiety in the
expected mixed anhydride can be easily transformed into an
imidazolium moiety, which can be applied to the preparation
of a variety of carbonyl compounds via dual substitution
processes. Herein, we describe the use of strategically
designed multifunctional carbenes as powerful tools for
carbon dioxide transformation.
Treatment of PoxIm 1a with carbon dioxide (5 atm) in
toluene resulted in the formation of mixed anhydride 2a,
which was isolated in > 99% yield (Scheme 2). This reaction
also proceeded quantitatively under 1 atm of carbon dioxide.
The molecular structure of 2a was identified by NMR
spectroscopy and X-ray crystallography (Figure S6 in the
Supporting Information), clearly showing the fixation of
carbon dioxide and a migration of the phosphine oxide moiety
to the carboxylic portion.[8] In the 13C NMR spectrum, the
[*] Prof. Dr. Y. Hoshimoto, T. Asada, Dr. S. Hazra, T. Kinoshita,
P. Sombut, Dr. R. Kumar, Prof. Dr. M. Ohashi, Prof. Dr. S. Ogoshi
Department of Applied Chemistry, Faculty of Engineering
Osaka University
Suita, Osaka 565-0871 (Japan)
E-mail: hoshimoto@chem.eng.osaka-u.ac.jp
ogoshi@chem.eng.osaka-u.ac.jp
Prof. Dr. Y. Hoshimoto
Frontier Research Base for Global Young Researchers, Graduate
School of Engineering, Osaka University
Suita, Osaka 565-0871 (Japan)
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201609710.
ꢀ 2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co.
KGaA. This is an open access article under the terms of the Creative
Commons Attribution Non-Commercial NoDerivs License, which
permits use and distribution in any medium, provided the original
work is properly cited, the use is non-commercial, and no
modifications or adaptations are made.
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Scheme 3. Reaction of 1g with CO2, giving 2g and 2g’. Yields were
determined by 31P NMR analysis at 228C. Molecular structure of
2g·B(C6F5)3 is shown with ellipsoids set at 30% probability. H, F (on
C6F5), CH3 (on tBu), and solvated C6H6 molecules are omitted for
clarity.
Scheme 2. Reactions of PoxIms 1a–e and SPoxIm 1 f with CO2.
Isolated yields are given in parenthesis. [a] Reaction at 808C for 1 h.
resonances of C1 and C2 are observed at dC 137.8 (2JC,P
=
4.0 Hz) and 153.0 ppm (3JC,P = 10.0 Hz), respectively, both of
to CD3CN again afforded 2g as the sole product in > 99 yield,
which indicates that the intermolecular addition giving 2g’ is
reversible. Treatment of a mixture of 2g and 2g’ in C6D6 with
B(C6F5)3 (1.0 equiv) resulted in the quantitative formation of
2g·B(C6F5)3 as a sole product (Scheme 3), which also shows
that the imidazole nitrogen in 2g would be key for the
generation of 2g’.
The formation of an imidazolium-2-carboxylate inter-
mediate (A) was confirmed in CD2Cl2 at À908C when the
reaction of 1b with carbon dioxide (1 atm) was monitored by
NMR spectroscopy (Scheme 4). A 16% yield of 2b was also
which appeared with the simultaneous disappearance of the
2
resonance of the carbene carbon (dC 221.6 ppm, JC,P
=
26.0 Hz). In the 31P NMR spectrum, the resonance of the
phosphinate moiety is observed at dP 69.2 ppm, following
a downfield shift via the migration of the phosphorus center
(1a: dP 61.4 ppm). PoxIms 1b–e also reacted with carbon
dioxide to give the corresponding mixed anhydride 2b–e, all
of which were isolated as solids in excellent yields. The
isolated solids of 2b–e can be stored in the presence of air and
moisture (20–308C, 30–50% humidity) for at least 2 days
without significant decomposition. Furthermore, no dispro-
portionation was observed under ambient conditions in C6D6
for at least 21 days, which was confirmed using 2b. Thus, these
mixed anhydrides are ideal reagents for transfer of the
carbonyl group. In fact, 2b was converted into N-isopropyl
imidazole derivatives efficiently (see the Supporting Infor-
mation for details). N-Phosphine oxide-substituted imidazo-
linylidene (SPoxIm) 1 f gradually reacted with carbon dioxide
at room temperature to afford 2 f in 76% yield after 11 h.
Heating promoted the reaction, and afforded 2 f in > 99%
yield after 1 h at 808C.
Scheme 4. The formation of A and its conversion into 2b. Yields are
determined by NMR analyses. [a] 2b was also formed in 16% yield.
detected when the reaction mixture was cooled to À908C in
the NMR apparatus. An anti-orientation is proposed for the
PoxIm 1g containing bis-N-phosphine oxide groups did
not react with carbon dioxide under the ambient conditions,
although 1g was totally consumed in toluene at 808C after 1 h
(Scheme 3). The quantitative formation of 2g was confirmed
when the product was dissolved in CD3CN at 228C; however,
a mixture of 2g and 2g’ was given in yields of 58 and 42%,
respectively, after changing the solvent from CD3CN to C6D6
(Figure S8–S10). The formation of 2g’, which was generated
via the intermolecular addition of imidazole nitrogen to N-
phosphine oxide in another 2g, was expected according to
NMR analyses in C6D6 (showing the resonances attributed to
the imidazolium moiety) as well as to ESI-MS analysis (2g’ of
n = 2 was detected).[8] Replacement of the solvent from C6D6
À
phosphine oxide moiety against the C1 C2 bond based on the
results of theoretical studies (see below). The conversion of A
into 2b was observed even at 08C, and the quantitative
formation of 2b was confirmed when the reaction mixture was
allowed to warm to room temperature, suggesting the rate-
limiting step includes an irreversible transformation from A
to 2b.
Plausible reaction mechanisms are further evaluated by
DFT calculation at the B3LYP/6-31G + (d) level of theory (in
toluene, at 258C, Figure 1a), and 2c was employed as a model
substrate.[8] Two conformers, aPxm and sPxm, are found
depending on the orientation of the phosphine oxide moiety
2
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Figure 1. a) Plausible mechanisms for the production of the mixed anhydride (mxd ahd). The anti and syn conformers for PoxIm are denoted as
aPxm and sPxm, respectively. Relative Gibbs free energies (kcalmolÀ1) in toluene (SMD implicit solvation model) with respect to [CO2 +aPxm]
(= +0.0) are shown, calculated by DFT at the B3LYP/6-31G+(d) level of theory (298.15 K). Optimized structure of b) TS2b, c) aPxm-CO2, d) TS4b
and e) sPxm-CO2 are also given. Hydrogen atoms are omitted for clarity. Selected distances [ꢁ] and angles [8]: b) C1···C2 2.210, C2–O1(O2) 1.193,
N2–P 1.795; O1-C2-O2 152.5, C1-N2-P-O3 174.0, c) C1–C2 1.550, C2–O1(O2) 1.245–1.249, N2–P 1.845, O3–P 1.505; O1-C2-O2 132.3, C1-N2-P-
O3 177.5, d) N2···P 2.144, O2···P 2.045, C1–C2 1.491, C2–O1 1.227, C2–O2 1.302, O3–P 1.518; O2-P-O3 151.4, O1-C2-O2 126.6, e) C1–C2 1.557,
C2–O1(O2) 1.243–1.245, N2–P 1.822; O1-C2-O2 133.6, C1-N2-P-O3 10.0.
À
with respect to the lone pair of the carbene. The relative
energy of sPxm is higher than that of aPxm by + 5.5 kcal
molÀ1. The theoretical activation energy of the rotation of the
phosphine oxide is + 12.5 kcalmolÀ1, indicating its facile
occurrence at 258C.[4] While it was difficult to form the
classical carbene–borane adduct using B(C6F5)3 and the anti-
conformer PoxIm with N-aryl groups due to the steric
repulsion between them, the formation of aPxm-CO2
(+ 5.2 kcalmolÀ1) by the reaction of aPxm with CO2 is
expected to proceed more easily than the formation of
sPxm-CO2 (+ 10.8 kcalmolÀ1). The latter takes place via the
transformation of aPxm to sPxm followed by the reaction
with CO2. TS2b (+ 15.7 kcalmolÀ1) is a more stable activation
complex than TS2a (+ 19.8 kcalmolÀ1). The optimized struc-
ture of TS2b is given in Figure 1b, where the carbene is
approaching CO2 (C1···C2 = 2.210 ꢀ) and the angle between
the planes of the imidazolylidene ring and CO2 is out of
perpendicular (67.58). A similar plane angle of 71.48 is
observed in the optimized structure of aPxm-CO2 (Fig-
ure 1c), which is significantly smaller than the corresponding
plane angle of 89.08 found in the IPr–CO2 adduct,[7c] but
larger than the angles in the imidazolium-2-carboxylate with
the carboxylate–phosphine oxide bond (O2 P) occur in
a concerted manner.[9] The plane angle between the imida-
zolium and the carboxylate moieties in TS4b becomes 5.08 with
interatomic distances of N2···P and P···O2 of 2.144 and
2.045 ꢀ, respectively (Figure 1d). The P atom adopts a dis-
torted trigonal bipyramidal geometry with O2 and O3 atoms
at the axial position. These calculation results are consistent
with experimental observation, which means that the path-
ways affording mxd ahd via TS4b, in which overall activation
energy is + 21.0 kcalmolÀ1, are thus likely.
Treatment of 2b (3.40 g, 10.82 mmol) with an equimolar
amount of MeOTf in CH2Cl2 (5 mL) gave imidazolium salt 3b
(5.17 g, 10.81 mmol), which was isolated in 99% yield as
a shelf-stable solid under a dried nitrogen atmosphere
(Scheme 5a). The two potential leaving groups in 3b are
the imidazolium and the phosphinate moieties, which makes
it a useful synthetic precursor for a variety of carbonyl
compounds. In fact, dual substitution reactions of 3b with two
different nucleophiles in a one-pot manner gave ester 5 (1st;
ZnAr2: 2nd; MeOH) and amide 6 (1st; ZnAr2: 2nd; CyNH2) in
excellent yields, where Ar is p-OMeC6H4 (Scheme 5b). In situ
formation of 4b and the zinc phosphinate salt by the reaction
of 3b and ZnAr2 (1.0 equiv) was directly observed by NMR
analyses (Figures S17, S18). Furthermore, 3b was utilized as
a novel precursor for the preparation of unsymmetrical
ketones (Scheme 5c). Efficient synthetic method for unsym-
metrical ketones is particularly of high utility when it can be
carried out in a one-pot and straightforward manner.[10] The
reaction of 4b generated in situ with NaCH2NO2 and
NaCH(CO2Et)2 in DMF afforded unsymmetrical ketones 7
and 8 in 90 and 93% isolated yield, respectively.[11] Synthesis
of unsymmetrical diaryl ketone was also examined to give 9 in
36% isolated yield by utilizing two equivalents of ZnAr2 at
the second substitution step. It should be noted that the
present synthetic method for unsymmetrical ketones was
conducted at mild temperature without tedious experimental
procedures such as strict regulation of the reaction temper-
ature and the concentration of nucleophiles. Thus, prelimi-
bis-N-methyl groups (29.18).[7b] The bond length of C1 C2 in
À
aPxm-CO2 is 1.550 ꢀ, which is comparable with those found
in other reported NHC–CO2 adducts showing experimental
results of 1.52–1.54 ꢀ.[7] A slight elongation of the C1 C2
À
bond and an increment of the plane angle are found in sPxm-
CO2 (1.557 ꢀ and 86.68, Figure 1e) compared with those in
aPxm-CO2. Although the formation of sPxm-CO2 might also
be possible under the reaction conditions, sPxm-CO2 is
expected to undergo rapid decarboxylation and eventually
give aPxm-CO2. A direct transformation of aPxm-CO2 to
sPxm-CO2 via TS3 (+ 25.2 kcalmolÀ1) would hardly occur
since the theoretical activation barrier for this step is much
larger than that for the migration of the phosphine oxide to
give mxd ahd via TS4b (+ 21.0 kcalmolÀ1). Theoretical pre-
diction for the formation of TS4b suggests that cleavage of the
À
imidazolium–phosphine oxide bond (N2 P) and formation of
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transformation of CO2 into an unsymmetrical ketone is also
demonstrated. Remarkably, PoxIm played dual key roles. It
was nucleophile in the activation of carbon dioxide, and it also
acted as an electrophile to transfer the phosphine oxide
moiety to the carboxylic portion. The present work shows that
strategically designed multifunctional carbene reagents offer
a novel utility for carbon dioxide in organic synthesis.
Acknowledgements
This work was supported by Grants-in Aid for Young
Scientists (A) (JSPS KAKENHI Grant Number
JP25708018), Encouragement for Young Scientists (B)
(JSPS KAKENHI Grant Number JP15K17824), and Scien-
tific Research on Innovative Areas “Stimuli-responsive
Chemical Species (No. 2408)” (JSPS KAKENHI Grant
Number JP15H00943) and “Precisely Designed Catalysts
with Customized Scaffolding (JSPS KAKENHI Grant
Number JP15H05803)”. Y.H. acknowledges support from
the Frontier Research Base for Global Young Researchers,
Osaka University, on the program of MEXT.
Keywords: carbon dioxide fixation · mixed anhydrides ·
N-heterocyclic carbenes · one-pot reactions ·
unsymmetrical ketones
Scheme 5. Synthetic applications of mixed anhydride 2b: a) synthesis
of imidazolium salt 3b and b,c) its one-pot derivatization. 4b was not
isolated in the dual substitution reactions. Isolated yields are given.
[a] NMR yield. [b] A mixture of keto/enol (8:1) isomers. [c] A mixture of
keto/enol (5.9:1) isomers. [d] 1,2-Dichloroethane was used through
dual substitutions.
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nary results shown herein ensure the promising reactivity of
3b as a precursor for unsymmetrical ketones. The key for
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first substitution step in order to avoid an overreaction
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A one-pot transformation of CO2 proceeding via key
intermediate 3b was conducted to afford 8 in 75% isolated
yield (Scheme 6), which also manifests this straightforward
transformation processes of carbon dioxide toward unsym-
metrical ketones.
[3] In situ preparation of potassium t-butylcarbonate by the reaction
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of a mixture of keto/enol (8:1) isomers is given.
In summary, a synthetic route affording a novel carbox-
ylic–phosphinic mixed anhydride directly from carbon diox-
ide has been achieved by using N-phosphine oxide substituted
imidazolylidenes (PoxIms). The obtained carboxylic–phos-
phinic mixed anhydrides were utilized in the preparation of
a variety of carbonyl compounds. A thorough one-pot
4
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Manuscript received: October 4, 2016
Final Article published: && &&, &&&&
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Communications
Carbon Dioxide Fixation
Y. Hoshimoto,* T. Asada, S. Hazra,
T. Kinoshita, P. Sombut, R. Kumar,
M. Ohashi, S. Ogoshi*
&&&& — &&&&
Strategic Utilization of Multifunctional
Carbene for Direct Synthesis of
Carboxylic–Phosphinic Mixed Anhydride
from CO2
CO2 transformation on strategy: Strategic
utilization of N-phosphine oxide-substi-
tuted imidazolylidenes enabled a direct
preparation of a novel carboxylic–phos-
phinic mixed anhydride from CO2, which
was followed by efficient derivatization
into a variety of carbonyl compounds.
6
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ꢀ 2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!