Rhodium(i)-catalyzed Pauson-Khand-type reaction using formic acid as a CO surrogate: An alternative approach for indirect CO2 utilization

Article abstract of DOI:10.1039/c8gc03933j

Formic acid is found to be an ideal CO surrogate for the rhodium(i)-catalyzed Pauson-Khand-type (PK-type) reaction of various substituted 1,6-enynes to afford bicyclic cyclopentenones in moderate to good yields. High TON value of up to 263 and good results in the gram-scale experiment were also obtained, demonstrating the efficacy of this methodology. In addition, heterocyclic molecules of pharmaceutical importance were also furnished via inter- or intra-molecular hetero-PK-type reactions, further broadening the application of current strategy. In this protocol, formic acid was utilized as a bridging molecule for the conversion of CO₂ to CO, since formic acid is manufactured via catalytic hydrogenation of CO₂ and releases CO in the presence of acetic anhydride readily. Therefore, this methodology represents a green and indirect approach for chemical valorization of CO₂ in the preparation of value-added compounds.

Full text of DOI:10.1039/c8gc03933j

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DOI: 10.1039/C8GC03933J
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Rhodium(I)-catalyzed Pauson-Khand-type reaction using formic
acid as a CO surrogate: an alternative approach for indirect CO2
utilization
32231345677Received 00th
January 20xx,
Accepted 00th January 20xx
Xian-Dong Lang,a Fei You,a Xing He,a Yi-Chen Yua and Liang-Nian He*a,b
DOI: 10.1039/x0xx00000x
www.rsc.org/
Formic acid is found to be an ideal CO surrogate for the rhodium- established to be efficient catalysts for inter and intramolecular
catalyzed Pauson-Khand-type (PK-type) reaction of various reactions; and the addition of chiral ligands helps to generate
substituted 1,6-enynes to afford bicyclic cyclopentenones in bicyclic cyclopentenones with two chiral centers; (b) the
moderate to good yields. High TON value up to 263 and nice substrates of Pauson-Khand type annulation can be extended to
results in gram scale experiment were also obtained, allene-alkyne,13 allene-alkene,14 alkynyl-carbodiimide,15 1-
demonstrating the efficaciousness of this methodology. In yne/ene-vinylcyclopropanes,16
alkynylboronic
esters-
addition, heterocyclic molecules of pharmaceutical importance norbornadiene (NBD)17 and diene-ynes18 to access structurally
were also furnished via inter- or intramolecular hetero-PK-type interesting and biologically active molecules; (c) the toxicity
reactions, further broadening the application of current strategy. and the gaseous nature of CO represent major operational
In this protocol, formic acid can be regarded as a bridging drawbacks in the activity of both academia and industrial
molecule from CO2 to CO, since formic acid is manufactured via sectors. To circumvent such problems, aldehydes,19 alcohols20
catalytic hydrogenation of CO2 and releases CO in the presence of and benzyl formate21 have been exploited as non-gas CO
acetic anhydride readily. Therefore, this methodology represents a surrogates in an alternative way to perform the reaction with
green and indirect approach for chemical valorization of CO2 in the stoichiometric CO extruded in situ. The tandem
preparation of value-added compounds.
decarbonylation and subsequent carbonylative annulation can
be promoted by a single rhodium compound. Furthermore,
supercritical CO2 can also be used as a green substitute for the
conventional organic solvent.22
Introduction
During the past decades, transition metal-mediated
Formic acid is an appealing hydrogen source and can be
23
carbonylation reactions have attracted considerable interest and produced by catalytic hydrogenation of CO2 and oxidation of
constituted as a highly convergent route to access various biomass.24 Meanwhile, formic acid can also be viewed as a
carbonyl molecules.1 In this vein, Pauson-Khand reaction, the liquid replacement for CO gas with manipulation simplicity and
[2+2+1] carbonylative cycloaddition of an alkene, alkyne, and has been exploited in various organic synthesis. For example,
carbon monoxide has assumed its magnified importance as an Zhou,25 Fu26 and Shi27 have independently reported that the
elegant protocol for the construction of cyclopentenone hydrocarboxylation of unsaturated hydrocarbons proceeded
scaffolds, which exist pervasively in synthetically useful and efficiently with formic acid as the CO precursor, giving the
pharmaceutically attractive molecules.2 Since its discovery by corresponding carboxylic acids in high yields. Additionally,
Pauson, Khand, and co-workers in 1973,3 the prosperity of various palladium-catalyzed carbonylative Sonogashira and
Pauson-Khand reaction has been witnessed in several Suzuki reaction, reductive carbonylation of aryl halides have
categories, such as: (a) in addition to the dicobalt octacarbonyl,4 been explored by Wu and co-workers,28 delivering a range of
other transition metal complexes including Ti,5 Ru,6 Rh,7 Ir,8 functionalized carbonylative compounds efficiently without
Ni,9 Mo,10 Fe,11 and heterobimetallic complexes12 have been using external toxic CO gas. Our group has also reported that
selective reductive methylation of amines proceed smoothly
with the catalysis of Cu(OAc)2 in the presence of phenylsilane
a. State Key Laboratory and Institute of Elemento-organic Chemistry, Nankai
University, College of Chemistry, Tianjin 300071, (P. R. China)
b. Collaborative Innovation Center of Chemical Science and Engineering (Tianjin),
Tianjin 300071, P. R. China. Fax: +86-022-23503878; E-mail address:
heln@nankai.edu.cn; Tel: +86-022-23503878
as a reductant, using formic acid as a valuable C1 source.29
Inspired by the attractive features of formic acid as a CO gas
substitute in the presence of organic anhydrides as additives
and intriguing applications of cyclopentenone frameworks,
which is scientifically useful and biologically interesting
†
Electronic
Supplementary
Information
(ESI)
available.
See
DOI: 10.1039/x0xx00000x
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carbocycles, we herein would like to report that the necessary for the reaction to proceed smoothly, anVdiefwoArrmticilecOancliinde
DOI: 10.1039/C8GC03933J
[RhCl(cod)]2 catalyzed Pauson-Khand-type reaction proceeded acts as the actual carbonyl source in the present reaction (for
smoothly with formic acid as a CO source from various detailed optimization results, see Table S1 in the Supporting
functional groups substituted diester-linked and oxygen- Information).
tethered 1,6-enynes, affording the cyclocarbonylated products
Subsequently, screening of various commercially available
in moderate to good yields (Scheme 1). In addition, rhodium and iridium complexes suggests that the nature of the
heterocyclic molecules of pharmaceutical importance were also ligand markedly affects the reaction outcome, with the neutral
furnished via inter- or intramolecular hetero-PK-type reactions. complex [RhCl(cod)]2 delivering 78% yield of 2a. The iridium
This protocol features operational simplicity, external CO gas- counterpart
i.e.
[IrCl(cod)]2
also
promoted
the
free, low catalyst loading and excellent functional group cyclocarbonylation slightly less efficiently, nevertheless
tolerance. Because formic acid can be easily produced via the reasonable yield of 2a was still furnished. Generally, the
hydrogenation of CO2 and release CO easily under certain monophosphine rhodium complex i.e. Rh(PPh3)3Cl gave poor
conditions in the presence of acetic anhydride, formic acid can activity in the CO gas participated annulation, and silver salt
be recognized as a bridging molecule from CO2 to CO, and this e.g. AgOTf is necessary to initiate the reaction.7a However,
protocol represents a green and indirect approach for chemical Rh(PPh3)3Cl and RhCl3 were also found to be catalytically
valorization of CO2 in the preparation of value-added active, affording 2a in 33% and 35% yields, respectively.
compounds, which may hold much industrial potential.
Therefore, we deduced that formate anion may act as a proper
ligand to accelerate the reaction. No reaction occurred in the
absence of [RhCl(cod)]2, showing that the catalyst is
indispensable to the carbonylative annulation. The
abovementioned results implied that a suitable catalyst is
required for both the decarbonylation reaction and subsequent
carbonylative annulation.
Afterward, modulation of acetic anhydride amount
disclosed that the molar ratio of formic acid to acetic anhydride
has prominent influence on the reaction, with the ratio of 2:1
furnishing the desired cyclic enone in 86% yield, being in
accordance with the reported results by Shi and coworkers, 27
but different from others,25,26,28 suggesting that the molar ratio
varies with specific reaction scenarios. Variation of the ratio
CO2
R
H2
X
O
HCOOH
Ac2O
R
X = O or X = C(COOEt)2
Carbo Pauson-Khand reaction
Ph
[Rh]
O
O
CO
+
n-Pr
n-Bu2O
Ph
Ph
R
N
N
N
C Y
N
O
Y = O or N
Hetero-Pauson-Khand-type reaction
(inter- or intramolecular)
Scheme 1 Utilization of formic acid as a bridging molecule from CO2 to CO in
Pauson-Khand annulation.
from 2:1 led to lower yield of 2a.
Ph
Ph
Results and discussion
E
E
[RhCl(CO)2]2
O
CO
H OH
+
E
E
Ac2O, solvent
130 oC, 24 h
It is generally accepted that formic acetic anhydride is
thermolabile,25-27 which can be conveniently prepared in situ
through the reaction between formic acid/salt and acetic
anhydride and likely undergoes decomposition to give acetic
acid with simultaneous release of CO. We envisioned that the
bicyclic cyclopentenones can be afforded via [2+2+1]
intramolecular carbonylative cycloaddition of 1,6-enyne
utilizing the stoichiometric CO released from formic acid in the
presence of acetic anhydride without using external toxic CO
gas. To validate our hypothesis, the cyclocarbonylation of
diethyl 2-allyl-2-(3-phenylprop-2-yn-1-yl)malonate (1a) using
formic acid or its salt as CO source and acetic anhydride as the
additive was established as the model reaction, which is
operated in a glass pressure tube, and the results are depicted in
Scheme 2. To our delight, in the presence of 2 mol%
[RhCl(CO)2]2, the desired carbonylated product diethyl-7-oxo-
8-phenyl-bicyclo[3.3.0]oct-1(8)-ene-3,3-dicarboxylate (2a) was
E = COOEt
E = COOEt
1a
2a
effect of formic acid and its alkali salts in the presence of 1.5 mmol Ac2O
HCOOLi H2O
13%
HCOONa
4%
HCOOK
3%
HCOOCs
0
HCOONH4
3%
HCOOH
56%
effect of rhodium and Iridium catalysts instead of 2 mol% [RhCl(CO)2]2
RhCl3 3H2O
35%
Rh(PPh3)3Cl
33%
Rh(acac)3
5%
[RhCl(coe)]2
[RhCl(CO)2]2
56%
63%
[Cp*RhCl2]2
[Rh(CO)2acac
[IrCl(cod)]2
[RhCl(cod)]2
78%
10%
60%
46%
effect of acetic anhydride amounts
(0.75 mmol)
86%
(0.25 mmol)
13%
(1 mmol)
85%
(2 mmol)
45%
(0.5 mmol)
14%
(1.5 mmol)
78%
o
effect of solvents
delivered in 56% yield at 130 C for 24 h with the 1:1 molar
ratio of formic acid and acetic anhydride. Replacement of
formic acid with the corresponding alkali salts (HCOOX, X =
Li, Na, K, Cs and NH4) has detrimental effect on the titled
reaction, with sharp decrease of 2a yield witnessed. A trace
amount of 2a was obtained in the absence of formic acid and
acetic anhydride, indicating that formic acetic anhydride is
Bu2O
86%
DMF
9%
DMAc
11%
DMSO
3%
NMP
0
THF
21%
Toluene
68%
Scheme 2 Optimization of key reaction parameters.
2 | J. Name., 2012, 00, 1-3
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R
In this respect, it is reported that the concentration of CO in the
catalytic system can be regulated by the amount of acid
anhydride added,26 and high concentration of CO may lead to
deceleration of the Pauson- Khand-type reaction, even under
atmospheric pressure.30 Therefore, the ratio of 2:1 may be most
suitable for both the decarbonylation and carbonylative
annulation in this study. Replacement of acetic anhydride with
trifluoroacetic anhydride (TFAA) or benzoic anhydride (Bz2O)
led to lower yield of 2a.
Solvent appears to be another crucial factor, and the
reaction performed best in n-Bu2O (butyl ether). Subsequently,
variation of reaction parameters e.g. catalyst loading,
temperature and reaction time was investigated as shown in
Table S5 in the Supporting Information. The yield of 2a kept
almost levelled off with catalyst loading increasing from 2
mol% to 3 mol%, whereas sharp decrease in yield was
witnessed when lower catalyst loading was used. Similar to the
study of catalyst loading, we found that the reaction was very
temperature-sensitive and 2a yield kept nearly unchanged when
increasing the temperature, but decreased when performing at a
lower temperature. The investigation thereafter disclosed that
24 h was the suitable reaction time.
View Article Online
DOI: 10.1039/C8GC03933J
H3CO
2c
2d
2e
, R = CH3, 83%
, R = F, 77%
, R = Cl, 79%
, R =Br, 76%
Ph
EtOOC
EtOOC
EtOOC
EtOOC
EtOOC
EtOOC
O
O
O
2f
2a
2b
, 81%
, 86%
Cl
Cl
Cl
H3CO
O
O
O
O
O
O
O
O
2g
, 82%
2h
2i
, 53%
, 49%
2j
, 72%
OCH3
Cl
F
Br
O
O
O
O
O
O
O
O
, 80%
2l
, 70%
2k
, 69%
2n
2m
, 74%
a All the reactions were carried out with the 1,6-enyne (0.5 mmol), [RhCl(cod)]2
(4.9 mg, 2 mol%), HCOOH (0.0684 g, 1.5 mmol), Ac2O (0.0766 g, 0.75 mmol), n-
Bu2O (3 mL), 130 oC, 24 h. b Isolated yield.
Scheme 3 Pauson-Khand-type reactions of various substituted 1,6-enyne
with formic acid.
Ph
[RhCl(cod)]2, Ac2O
Ph
CO
O
+
H
OH
O
O
n-Bu2O, 130 oC, 24 h
Having identified this green and alternative catalytic
system for Pauson-Khand-type reaction, our attention then
focused on the substrate portfolio. As shown in Scheme 3,
various functional groups such as OCH3, CH3, F, Cl, and Br
substituted diester-linked 1,6-enynes were found to be
amenable to the Pauson-Khand type reaction to furnish the
corresponding products 2b-2f in moderate to good yield.
Subsequently, steric and electronic effects of substituents at the
phenyl ring were evaluated using various oxygen-tethered 1,6-
enynes. To our delight, 2g was obtained in 82% yield without
any substitute at the phenyl ring. Slightly lower yields of 2h
and 2i were delivered because of the steric hindrance of ortho-
substitutes, which prevents the yne moiety from coordinating
with the Rh center. In addition, comparable yields were also
obtained using meta-dichloro and para-halogen substituted 1,6-
en-ynes as the substrates (2j-2m), indicating that electronic
effect also has little influence on the reactivity.
To demonstrate the efficaciousness of this protocol, a
gram-scale synthesis was performed using 1g as the
prototypical substrate under the optimized conditions (Scheme
4). To our delight, 64% yield of its corresponding product was
obtained. We also carried out the cocyclization of 1a with two
catalyst/substrate ratios, and moderate value of TON was
obtained, as shown in Table 1. In the presence of 1.2 mg
catalyst, 59% yield of the target product was afforded (TON:
121, TOF: 2.5 h-1, respectively). Further decrease in the catalyst
amount led to higher value of TON and TOF (entry 2 vs. 1).
For the sake of comparison, the substitution of formic acid with
CO gas balloon led to slightly higher efficiency (entry 3 vs.
entry 2). All the above mentioned results substantiated the
efficiency of current protocol.
1g
10 mmol, 1.72 g
2g
Yield: 64%
Scheme 4 Gram-scale synthesis of 2g.
Table 1 Evaluation of efficiency of the current protocol.a
2a
1a
(mmol)
[RhCl(cod)]2
(mg)
Entry
Yieldb
(%)
TONc
TOFd
1
2
0.5
0.5
0.5
1.2
0.3
0.3
59
32
38
121.2
263.2
312.3
2.5
5.5
6.5
3e
a
All the reactions were carried out in 25 mL pressure tube, 1a (0.1714 g, 0.5 mmol),
HCOOH (0.0684 g, 1.5 mmol), Ac2O (0.0766 g, 0.75 mmol), n-Bu2O (3 mL), 130 oC, 48 h.
b Determined by GC using biphenyl as internal standard.
c TON (turnover number): mole of 2a/mole of [RhCl(cod)]2.
d TOF (turnover frequency): m(2a)/m([RhCl(cod)]2)·t (hour).
e In the presence of 1 atm free CO gas.
of structurally divergent heterocyclic compounds of
pharmaceutical importance. We are pleased to find that the
current methodology is applicable to various intra- and
intermolecular hetero-Pauson-Khand-type reaction using formic
acid as an efficient CO-releasing molecule, in which little
difference of catalytic efficiency was observed in comparison
with the reactions involving gaseous CO.31 For example,
intramolecular
aza-Pauson-Khand-type
reaction
of
alkynecarbodiimide 1o proceeded efficiently in the presence of
dppp (1,3-bis(diphenylphosphino)propane), giving pyrrolo-
[2,3-b]indol-2-one 2o in up to 69% yield (Scheme 5a).
Furthermore, intermolecular cocyclization of propyl isocyanate
1p, diphenylacetylene and extruded CO from decarbonylation
of formic acid proceeded rapidly, affording the corresponding
maleimide 2p in 84% yield under the modified reaction
conditions (Scheme 5b). Consequently, all the above-mentioned
Distinct from conventional carbo Pauson-Khand reaction,
heterocumulenic Pauson-Khand-type reaction represents a
powerful and synthetically useful strategy for the construction o
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results revealed the outstanding efficiency in valuable corresponding cyclopentenones in 49-86% yields. V6i4ew%ArYticileeOldnlionef
heterocyclic molecules preparation, further extending the 2g in gram-scale experiment and high TDOONI: 1v0a.1l0u3e9/(Cu8pGCto03296333J)
application of current methodology.
were obtained, demonstrating the excellent efficaciousness of
this methodology. It is worthy to note that heterocyclic
molecules of pharmaceutical importance were also furnished
via inter- or intramolecular hetero-PK-type reactions, further
extending the application of current strategy. Further in-depth
study of this reaction awaits further study.
Ph
O
[Rh(cod)Cl]2 ( 5 mol%)
dppp (11 mol%)
a
(
)
N
HCOOH
+
Ac2O, n-Bu2O
130 oC, 24 h
N
N
C
N
1o
2o
, 69%
[Rh(cod)Cl]2 ( 5 mol%)
dppp (11 mol%)
O
N
C O
n-Pr
Ph
HCOOH
+
1p
N
b
(
)
Ac2O, n-Bu2O
130 oC, 24 h
O
Ph
, 84%
Conflicts of interest
Ph
Ph
2p
There are no conflicts to declare.
Scheme 5 Formic acid participated heterocumulenic Pauson-Khand-type
reactions.
Acknowledgements
Another point of particular note is that after the reaction,
free CO gas was detected,32 which is the evidence of
thermolability of formic acetic anhydride and may suggest that
free CO gas is involved in the reaction pathway. Therefore, on
the basis of the reported literature and experimental results, a
possible reaction mechanism is depicted in Scheme 6. Firstly,
formic acid reacts with the acetic anhydride to generate the
corresponding mixed anhydride,25-27 which decomposes to
gaseous CO in situ under prolonging heating. Then the CO gas
coordinates to the rhodium complex A, leading to a rhodium-
carbonyl species B. The intermediate B then activates the 1,6-
enynes via coordinating to the double and triple bonds, forming
the Rh-enyne complex C. Subsequently, oxidative addition
occurs to give the rhodacycle complex D. Finally, CO
migratory insertion and reductive elimination occur to form the
bicyclic cyclopentenone as a target product.
This work was financially supported by National Key Research
and Development Program (2016YFA0602900), National
Natural Science Foundation of China (21421062, 21672119),
the Natural Science Foundation of Tianjin Municipality
(16JCZDJC39900).
Notes and references
1
For a general review, see: H. M. Colquhoun, D. J. Thompson
and M. V. Twigg, Carbonylation; Plenum Press: New York,
1991.
2
3
4
L. L. Shi and Z. Yang, Eur. J. Org. Chem., 2016, 2016, 2356-
2368.
I. U. Khand, G. R. Knox, P. L. Pauson, W. E. Watts and M. I.
Foreman, J. Chem. Soc., Perkin Trans. 1, 1973, 975-977.
For selected examples for cobalt-catalyzed Pauson-Khand
reaction, see: (a) N. Jeong, S. H. Hwang and Y. Lee, J. Am.
Chem. Soc., 1994, 116, 3159-3160; (b) M. Hayashi, Y.
Hashimoto, Y. Yamamoto, J. Usuki and K. Saigo, Angew.
Chem. Int. Ed., 2000, 39, 631-633; (c) M. E. Krafft, L. V. R.
Boñaga, J. A. Wright and C. Hirosawa, J. Org. Chem., 2002,
67, 1233-1246; (d) Y. F. Wang, L. M. Xu, R. C. Yu, J. H.
Chen and Z. Yang, Chem. Commun., 2012, 48, 8183-8185;
(e) S. Orgue, T. Leon, A. Riera and X. Verdaguer, Org. Lett.,
2015, 17, 250-253.
Ph
Ph
O
C
O
C
O
O
Rh
Cl
E
Ph
Cl
Rh
CO
HCOOH
+
Ac2O
Rh Cl
O
A
D
O
O
CO
H
O
CH3
For selected examples for Pauson-Khand reaction promoted
by Ti, Ru, Rh and Ir, see Ref. 5-9, respectively:
5
6
(a) F. A. Hicks, N. M. Kablaoui and S. L. Buchwald, J. Am.
Chem. Soc., 1999, 121, 5881-5898; (b) F. A. Hicks and S. L.
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B. Zhao, Y. Ding and G. Zhao, J. Org. Chem., 1998, 63,
9285-9291; (d) F. A. Hicks, N. M. Kablaoui and S. L.
Buchwald, J. Am. Chem. Soc., 1996, 118, 9450-9451.
(a) T. Kondo, N. Suzuki, T. Okada and T. Mitsudo, J. Am.
Chem. Soc., 1997, 19, 6187-6188; (b) T. Morimoto, N.
Chatani, Y. Fukumoto and S. Murai, J. Org. Chem., 1997, 62,
3762-3765; (c) N. Chatani, T. Morimoto, Y. Fukumoto and S.
Murai, J. Am. Chem. Soc., 1998, 120, 5335-5336; (d) H.
Miura, K. Takeuchi and T. Shishido, Angew. Chem. Int. Ed.,
2016, 55, 278-282; (e) T. Kondo, M. Nomura, Y. Ura, K.
Wada and T. Mitsudo, J. Am. Chem. Soc., 2006, 128, 14816-
14817.
CO
Rh Cl
Ph
Rh Cl
CO
B
C
O
Ph
O
Scheme 6 Postulated reaction mechanism.
Conclusions
In summary, we have shown the feasibility of the rhodium
catalyzed PK-type reaction using formic acid as a bridge
molecule from CO2 to CO to obviate the arduous management
of gaseous reagents. The reactions were conducted in a glass
pressure tube with operational simplicity. In addition, a variety
of 1,6-enyne substrates bearing electron-withdrawing or
electron-donating substituents were smoothly converted to the
7
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Green Chemistry
Page 6 of 6
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DOI: 10.1039/C8GC03933J
Rhodium(I)-catalyzed Pauson-Khand-type reaction using
formic acid as a CO surrogate: an alternative approach for
indirect CO2 utilization
As a bridging molecule from CO2 to CO, formic acid is successfully applied to
rhodium-catalyzed carbo and hetero-Pauson-Khand-type reaction, affording moderate to high
yields of bicyclic cyclopentenones. TON and gram-scale experiments were also conducted to
demonstrate the efficaciousness of this methodology.
R
X
O
Carbo Pauson-Khand reaction
[Rh]
CO2
Ph
HCOOH
O
O
n-Pr
Ac2O
Ph
Ph
N
N
N
O
Hydrogenation
Hetero-Pauson-Khand-type reaction
(inter- or intramolecular)
Avoidance of handling carbon monoxide
Operationally convenient
Wide functional group tolerance
Carbo and hetero-PK-type reaction
High TON value was obtained
Amenable to gram-scale synthesis