Site-selective C-H bond carbonylation with CO2 and cobalt-catalysis

Article abstract of DOI:10.1039/c8cy02060d

Utilization of anthropogenic greenhouse gas CO₂ for catalytic C-C bond formation via conversion to essentially valuable C1 synthons like CO is very challenging. The requirement of an efficient catalyst that has the ability to convert CO₂ into CO and activate inert C-H bonds is the bottleneck. We herein demonstrate a tandem approach accomplished in a two-chamber system for efficient fluoride-mediated generation of CO from CO₂ using disilane as a deoxygenating reagent and utilization of the in situ-produced CO gas for C-H bond carbonylation using earth-abundant cobalt catalysts. The ease of handling CO₂ gas at atmospheric pressure allows us to prepare ¹³C labelled compounds which are otherwise difficult to achieve. The procedure developed makes it possible to utilize CO₂ as a CO source, which can be widely applied as a C1 synthon that can be incorporated between C-H and N-H bonds of aromatic, hetero-aromatic and aliphatic carboxamides for the synthesis of various cyclic imides including spirocycles in a site-selective fashion. The late-stage derivatization of a well-known angiotensin receptor blocker (ARB), Telmisartan, and a well-known drug for very low-density lipoproteins (VLDLs), Gemfibrozil, is demonstrated. Further, to showcase the generality of the reaction, various pharmacologically important and privileged scaffolds like xanthone, coumarin and isatin have been synthesized with CO₂ under atmospheric pressure.

Full text of DOI:10.1039/c8cy02060d

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Site-selective C–H bond carbonylation with CO2
and cobalt-catalysis†
Cite this: Catal. Sci. Technol., 2018,
8, 5963
*
Nagaraju Barsu, Deepti Kalsi and Basker Sundararaju
Utilization of anthropogenic greenhouse gas CO2 for catalytic C–C bond formation via conversion to es-
sentially valuable C1 synthons like CO is very challenging. The requirement of an efficient catalyst that has
the ability to convert CO2 into CO and activate inert C–H bonds is the bottleneck. We herein demonstrate
a tandem approach accomplished in a two-chamber system for efficient fluoride-mediated generation of
CO from CO2 using disilane as a deoxygenating reagent and utilization of the in situ-produced CO gas for
C–H bond carbonylation using earth-abundant cobalt catalysts. The ease of handling CO2 gas at atmo-
spheric pressure allows us to prepare 13C labelled compounds which are otherwise difficult to achieve.
The procedure developed makes it possible to utilize CO2 as a CO source, which can be widely applied as
a C1 synthon that can be incorporated between C–H and N–H bonds of aromatic, hetero-aromatic and ali-
phatic carboxamides for the synthesis of various cyclic imides including spirocycles in a site-selective fash-
ion. The late-stage derivatization of a well-known angiotensin receptor blocker (ARB), Telmisartan, and a
well-known drug for very low-density lipoproteins (VLDLs), Gemfibrozil, is demonstrated. Further, to show-
case the generality of the reaction, various pharmacologically important and privileged scaffolds like xan-
thone, coumarin and isatin have been synthesized with CO2 under atmospheric pressure.
Received 4th October 2018,
Accepted 22nd October 2018
DOI: 10.1039/c8cy02060d
rsc.li/catalysis
photo- as well as electrochemical methods for the conversion
of CO2 to CO.17–19 It should be noted however that the cata-
Introduction
Production of value-added building blocks such as alcohols,
aldehydes and carboxylic acids by utilizing most abundant
lytic systems in these reactions are not combined with classi-
cal catalytic carbonylations due to operational complexities.
In fact, the optimization is far more challenging.
sources is
a
challenging task in synthetic organic
chemistry.1–4 In contrast to the traditional synthesis, atom-
efficient carbonylation using diatomic CO gas as a C1 source
is a widely practiced process for the production of oxidized
hydrocarbons in both academia and industry.5–8 However,
fundamental drawbacks associated with CO such as flamma-
bility, toxicity and the requirement of high-pressure necessi-
tate precautionary steps in its handling.9 To circumvent this
problem, CO surrogates such as formates, aldehydes, metal
carbonyls, etc. have been employed in lieu of the poisonous
diatomic gas.10,11 Incidentally, the scope of these reactions is
too narrow due to the release of additional non-innocent mol-
ecules along with the in situ generated CO. Alternatively, the
abundant and non-flammable greenhouse gas such as CO2
can be used instead of toxic CO, provided that effective cleav-
age of the carbon–oxygen (CO) bond can be achieved.12–16
In this regard, notable advances have been achieved through
To achieve an effective carbonylation by integrating in situ
generated CO from CO2 or its surrogates, Skrydstrup and co-
workers elegantly demonstrated carbonylative cross coupling
reactions in a two-chamber system using COware, where in
one chamber CO is released and in the other chamber CO is
consumed (Fig. 1A).20 To achieve such a process, the carbon-
ylation has to occur with atmospheric CO gas. In contrast, di-
rect carbonylation of the C–H bond can be much more ele-
gant provided that the catalyst has the ability to activate inert
C–H bonds and bind to π-acidic CO.21 In this regard,
Murahashi reported perhaps the first catalytic C–H bond car-
bonylation of benzaldimine using low-valent cobalt under
high CO pressure and high temperature.22 Recently, Daugulis
reported the first C–H bond carbonylation of arenes catalyzed
by high-valent cobaltIJIII) using CO at atmospheric pressure and
room temperature (Fig. 1B).23 Very recently, Sundararaju,24
Gaunt25 and Lei26 independently developed cobalt-catalyzed
C(sp3)–H bond carbonylation using atmospheric CO to access
various bio-active succinimide derivatives (Fig. 1B). Based
on these results and the literature precedent,27–31 we sought
to couple reduction of CO2 with C–H bond activation for
direct carbonylation under cobalt catalysis.23–26 To realize
Department of Chemistry, Indian Institute of Technology Kanpur, Uttar Pradesh,
208016, India. E-mail: basker@iitk.ac.in
† Electronic supplementary information (ESI) available: Experimental procedures
and spectroscopic data for all new compounds (1H NMR, 13C NMR, and HRMS),
including images of NMR spectra. See DOI: 10.1039/c8cy02060d
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Table 1 Optimization studies and control experimentsa
Entry
Change in conditions
Yieldc (%)
1
2
3
4
5
6
7
8
None
78
60
32
30
n.d.
n.d.
64
Chlorobenzene instead of TFT as a solvent
1 equiv. of PhCO2Na used
MnIJOAc)3·2H2O instead of Ag2CO3
No cobalt
No CO2 in chamber A
10 mol% of [Co] used
CoIJacac)3 was used without PhCO2Na
70
a
All reactions were performed in the 0.3 mmol scale using 1a as a
b
limiting reagent. 1.5 equivalents of disilane were used with respect
to 1a.
c
Isolated yield of 2a; Q = 8-quinoline; TFT = α,α,α-
trifluorotoluene.
Fig. 1 Overview of catalytic C–H carbonylation.
amount of base proved to be deleterious to the product
formation (entry 3). Changing the oxidant from AgIJI) to
MnIJIII) dramatically reduced the yield of phthalimide 2a
(entry 4). Based on control experiments, we found that the co-
balt and CO2 are mandatory for the carbonylation to proceed
(entries 5 and 6). Reduction in the catalyst loading or usage
of CoIJIII) as a catalyst without any additional carboxylate
ligand provided results (entries 7–8) lower than those of the
initial experiment (entry 1). Overall, our optimized result is
gratifyingly comparable to that reported by Daugulis and co-
workers, who employed CO directly for carbonylation of
benzamide derivatives.23,32
this goal, we began our investigations in a two chamber
system, wherein CO is released in one chamber with an
appropriate reductant in the presence of fluoride and the
released CO is consumed in the second chamber for C–H
bond carbonylation, which is catalyzed by CoIJIII) (Fig. 1C).
Indeed, we have achieved site-selective C(sp2)–H and C(sp3)–
H bond carbonylation efficiently using the stated protocol,
yielding phthalimide and succinimide derivatives in good-
to-excellent yields.
Results and discussion
With the optimized conditions, the efficiency of the reac-
tion was further scrutinized for various arylamides
substituted at the para-position with diverse electron-
releasing (–Me, –tBu, –OMe and –SMe) and withdrawing
groups (F, Cl, Br, CF3 and CO2Me) (Scheme 1). Notably, only
a marginal difference was observed in terms of the reactivity
with electron-rich amides yielding rather better yields (2c–2f)
than the e-poor amides (2g–2k). A similar trend was observed
for meta- as well as ortho-substituted amides, indicating a
marginal influence of the inductive effect (2l–2s). Benzo-
annulation as well as heteroarylannulation yielded the prod-
ucts with comparable efficacy (2t–2u). Under the same reac-
tion conditions, the carbonylation of sulfonamides led to
saccharins (2v–2z), albeit in moderate yields. Next, we under-
took the challenge of introducing heterocycles, which could
potentially poison the catalyst by strongly coordinating to
cobalt as depicted in Scheme 1 (bottom). The versatile
groups and their presence facilitate their functioning as phar-
macological agents. Heterocycles such as pyrimidine and
pyridine afforded carbonylated products in excellent yields,
but the reaction of 7-azaindole led to the product in a lower
yield (3a–3c). Substitution of the amide with five membered
To begin with, we wanted to consider a known reaction that
delivers CO from CO2 for adaptation to a two-chamber proto-
col for C–H bond carbonylation (Table 1). The reduction of
CO2 by 1,1,2,2-tetramethyl-1,2-diphenylsilane at 1–2 atmo-
spheric pressure is one such reaction that is well known to
produce CO,31 cf. Table 1. We, therefore, utilized this process
for generation of CO in one chamber. The CO released in one
chamber, i.e., A, was carried forward to the second chamber,
i.e., B, in which C–H bond carbonylation of benzamides is
accomplished with a Co-catalyst, cf. Table 1. To start with,
8-aminoquinoline substituted benzamide was employed as a
representative substrate and carbonylation was investigated
using CO2 as a CO source in the two-chamber setup. The car-
bonylation was conducted in α,α,α-trifluorotoluene as the sol-
vent at 100 °C for 24 h using the catalytic system that com-
prised of CoIJacac)2 (20 mol%), PhCO2Na (20 mol%) and
Ag2CO3 (2.5 equiv.). Remarkably, the formation of expected
phthalimide 2a occurred in a facile manner leading to an iso-
lation of the product in 78% (Table 1, entry 1). When chloro-
benzene was used as the solvent in place of TFT, a rather
lower yield of 2a was obtained (entry 2). Increasing the
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Scheme 1 Cobalt-catalyzed C(sp2)–H bond carbonylation – scope of carboxamides and sulfonamides.
heterocyclic rings, namely, pyrazole, 2-thiophene and
3-thiophene, did not affect the reactions, leading to products
in good yields (3d–3f). In the same vein, substitution of
benzamides at the para-position and at the hindered
ortho-position with other biologically active scaffolds like oxa-
zoline gave moderate yields of the products (3g–3h).
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To further expand the scope of the two-chamber protocol,
able yield. Furthermore, challenging unsubstituted propa-
namides could also be activated due to higher relative
strength, yielding the product (5o). The reaction of amides
substituted with C5-functionalized quinoline proceeded
smoothly leading to the products in decent yields (5p–q).
To underscore the synthetic utility of this two-chamber
methodology, the carbonylation reaction was applied for late-
stage functionalization of drug molecules such as Gemfibro-
zil and Telmisartan (Scheme 3A). The former serves to lower
the lipid level and the latter functions as an angiotensin II re-
ceptor blocker. The carbonylation of these drugs yielded the
corresponding products in 52 and 30% isolated yields, re-
spectively (Scheme 3A). Further diversity of the two-chamber
reaction was demonstrated by applying the methodology for
the synthesis of various biologically active scaffolds such as
succinimide (10),33 isatin (11),34 coumarin (12)35 and xan-
thone (13)36 in moderate-to-good yields (Scheme 3B). Addi-
tionally, the method was proved to be more sustainable by
successfully performing Hiyama–Denmark carbonylative
cross-coupling between liberated disiloxane from the first
chamber and aryl iodide under the reported conditions of
the reaction (Scheme 3C).37
carbonylation was tested for amide containing C(sp3)–H
bonds with a slight modification of the conditions. Accord-
ingly, the reactions were conducted at atmospheric pressure
of CO2 in the presence of 10 mol% CoIJacac)2, 1.5 equiv.
PhCO2Na and 3.0 equiv. of Ag2CO3 in chlorobenzene for 30 h
at 160 °C. The products were isolated in respectable yields
for substrates containing quaternary alpha carbon atoms
(Scheme 2, 5a–i & p–q). Carbonylation of α,α-disubstituted
propanamide with varied substituents that range from di-
methyl to alkyl to cycloalkyl groups led to the products in
moderate-to-excellent yields (5a–d). 5/6-Membered spirocyclic
succinimides were accessed in good yields (5e–f). Next the re-
activity comparison of the internal C(sp3)–H bond versus the
terminal C(sp3)–H bond was investigated. The results suggest
that the activation of the terminal β-C–H bond is facile com-
pare to that of the internal benzylic β-C–H bond for direct
carbonylation (5g–h). Subsequently, we examined the sub-
strates containing the aryl C–H (sp2-H) bond and aliphatic
C–H (sp3-H) bond. The results suggest that the activation of
the C(sp3)–H bond is favoured over C(sp2)–H bonds, possibly
due to favored 5-membered metallacycles, leading to anticon-
vulsant 3-methyl-3-phenyl-1-(quinolin-8-yl)pyrrolidine-2,5-dione
as the major product (5i).
To gain insights into the reaction mechanism, control ex-
periments and preliminary mechanistic investigations were
conducted (Scheme 4).
Further extension of the scope towards amide-containing
α-monosubstituted propanamides yielded the desired
succinimides (5j–n) in moderate yields. Moreover, privileged
scaffolds like phthalimide protected amino acid (5n) could
be activated to yield the corresponding product in a respect-
Scheme 3 Diversification. Reaction conditions for the synthesis of
10–13: for succinimide (10): (i) 0.2 mmol of amide, PdIJOAc)2 (10 mol%),
AgOAc (2 equiv.), TEMPO (2 equiv.), KH2PO4 (2 equiv.), hexane (0.1 M)
for 24 h at 130 °C; for isatin (11): (ii) 0.3 mmol of amine, PdCl2IJPPh3)2
(5 mol%), CuIJOPiv)2 (2 equiv.), DMSO/toluene (1 : 1) for 30 h at 100 °C;
for coumarin (12): (iii) 0.3 mmol of phenol, Cp·CoIJCO)I2 (10 mol%),
Ag2CO3 (1 equiv.), CuIJOAc)2·H2O (1 equiv.), o-xylene (0.2 M) for 24 h at
30 °C; for xanthone (13): (iv) 0.3 mmol of ether, PdIJOAc)2 (5 mol%),
K2S2O8 (2 equiv.), 2,2,2-trifluoroacetic acid (0.2 M) for 6 h at 50 °C.
Scheme
2
Cobalt-catalyzed C(sp3)–H bond carbonylation. The
number in parenthesis is the ratio of diastereomers.
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the radical in the reaction pathways for C(sp3)–H bond car-
bonylation of pivalamide and not with C(sp2)–H carbonyla-
tion of benzamide (Scheme 4B). Third, stoichiometric experi-
ments were conducted with an isolated cobaltacycle for
carbonylation using in situ-generated CO in α,α,α-
trifluorotoluene for 2 h. The expected phthalimide 2a was
obtained in 43% yield along with the recovery of the starting
benzamide 1a in 31% yield. This result suggests that the iso-
lated cobaltacycle is possibly involved in the catalytic cycle
(Scheme 4C). Fourth, KIE experiments were conducted in par-
allel for carbonylation of benzamide (1b), aliphatic amide
(4e), and their deuterium analogues under standard reaction
conditions. These experiments provided kH/kD values of 1.53
0.47 and 1.69 0.47, respectively. Although the KIE values
are low, they are significant enough and hence it is very diffi-
cult to conclude at this stage that the C–H activation is not
the rate-determining step (Scheme 4D).
Conclusions
In summary, direct conversion of CO2 to CO, and its incorpo-
ration between C–H and N–H bonds of amides to yield useful
succinimides, phthalimides, saccharins and their derivatives
is compellingly demonstrated by a two-chamber protocol.
This unconventional deoxygenation of CO2 and the insertion
of CO between C–H bonds in a simple laboratory setup are
heretofore unprecedented. With the developed protocol, di-
rect handling of toxic carbon monoxide is avoided. The site-
selective C–H functionalization could be applied to other car-
bonylation reactions to access diverse valuable products like
isatins, coumarins and xanthones. Furthermore, the reaction
could be applied for late-stage functionalization of block-
buster drugs such as Telmisartan and Gemfibrozil.
Conflicts of interest
The authors declare no conflict of interest.
Acknowledgements
Scheme 4 Mechanistic investigations.
Financial support provided by SERB (EMR2016/000136) to
carry out this work is gratefully acknowledged. N. B. and D. K.
thank CSIR and IITK for their fellowship. The PK Kelkar
Young Faculty Award given to B. S. is gratefully acknowledged.
B. S. acknowledges Johnson Matthey for generously providing
the precious metals.
First, validation of the fact that a stoichiometric amount
of CO is produced from CO2 in the two-chamber protocol was
established by employing 13C-labelled CO2 in both sp2 and
sp3 C–H carbonylations (Scheme 4A). The incorporation of
13C-labelled CO in the respective products was confirmed by
13C NMR and mass spectrometric analysis. This clearly
endorses the role of CO2 as a C-1 source. Second, to examine
the involvement of a radical in the reaction pathways, carbon-
ylation reactions were conducted in the presence of 1 equiv.
of TEMPO with benzamide and pivalamide under standard
reaction conditions, where the former provided the
phthalimide product without any reduction in the yield,
while the latter led to succinimide in a significantly lower
yield. These results suggest the possibility of involvement of
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