Reductive CO2Fixation via the Selective Formation of C-C Bonds: Bridging Enaminones and Synthesis of 1,4-Dihydropyridines

Article abstract of DOI:10.1021/acs.orglett.0c02963

Herein, a selective tandem C-C bond-forming reaction with CO2 was developed to realize the bridging of enaminones and synthesis of 1,4-dihydropyridines, respectively. n-Butylamine significantly promoted this CO2 deoxymethylenation procedure catalyzed by 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) and ZnCl2. The mechanism involving the formation of bis(silyl)acetal, nucleophilic addition, and amine elimination was also interpreted to clarify the bridging of two molecules of enaminones with CO2 and the generation of dihydropyridine derivatives.

Full text of DOI:10.1021/acs.orglett.0c02963

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Letter
Reductive CO2 Fixation via the Selective Formation of C−C Bonds:
Bridging Enaminones and Synthesis of 1,4-Dihydropyridines
Yulei Zhao,* Xuqiang Guo, Xin Ding, Zheng Zhou, Man Li, Nan Feng, Bowen Gao, Xu Lu, Yunlin Liu,
and Jinmao You
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ABSTRACT: Herein, a selective tandem C−C bond-forming reaction with CO2 was developed to realize the bridging of
enaminones and synthesis of 1,4-dihydropyridines, respectively. n-Butylamine significantly promoted this CO2 deoxymethylenation
procedure catalyzed by 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) and ZnCl2. The mechanism involving the formation of
bis(silyl)acetal, nucleophilic addition, and amine elimination was also interpreted to clarify the bridging of two molecules of
enaminones with CO2 and the generation of dihydropyridine derivatives.
bridging of two molecules of amines with CO2 through the
organocatalyzed reduction of CO2 to methylene.3b
s one of the most basic one-carbon (C1) structural units,
Athe methylene structure is widely identified in bioactive
molecules and higher value-added chemical structures.1 Over
the past years, carbon dioxide (CO2) has been regarded as a
ubiquitous, green, and recyclable C1 source.2 The precise
introduction of methylene blocks can be achieved by the
deoxymethylenenation of CO2, which has attracted interest
from research on the utilization of CO2. Previously, the
oxidation state of carbon (C4+) in CO2 was reduced to 2+, 0,
or 2− valence states (Scheme 1a) through a 2-, 4-, or 6-
electron reduction process3 to achieve the methylation,4
hydroxylmethylation,5 deoxymethylenenation,6 or formylation
reaction.7 Among them, limited attention was paid to
deoxymethylenenation of CO2. Specifically, CO2 can first be
reduced by 2-electron reduction to obtain C2+ species, but the
subsequent reduction of C0 to the C2− species is often faster
than the reduction of C2+ to C0 species (Scheme 1a), making it
difficult to utilize the C0 species.6a,3
The introduction of suitable nucleophilic substrates and the
development of mild catalytic systems are feasible solutions to
selective deoxymethylenenation. In 2015, Bontemps and Sabo-
Etienne et al. developed a (dihydrido)iron complex-catalyzed
selective reduction of CO2 into the methylene group with the
hydroborane as a reductant.6b In addition, the symmetric
Csp2−Csp3 bonds were constructed with 2,4-ditert-butylphenol
as the C-nucleophile (Scheme 1b, total yield for two steps:
39%). In the same year, Cantat and co-workers realized the
They found that malonate could efficiently replace the amine
reagents to promote the challenging formation of Csp3−Csp3
bonds from CO2 (Scheme 1c, 58%). In addition, the
deoxymethylenation of CO2 was also conducted for the
synthesis of aminals (betaine-catalyzed, reported by He et al.6a
and Han et al.3a), dimethoxymethane (Co-catalyzed, reported
by Klankermayer et al.6c), dithioacetals (under distinctive
NaBH4/I2 system, reported by Xi et al.6d), and spiro-
indolepyrrolidines (TBD-catalyzed, reported by Xia et al.6e),
etc.
In all of the above studies, the methylene group was
introduced from CO2 mainly in a controllable manner.
However, these deoxymethylenation reactions primarily
emphasized the construction of C−X bonds (where X = N,
O, or S). Limited attention was paid to the efficient
construction of C−C bonds. Because of our ongoing interest
in developing efficient tandem reaction methods,8 the present
research aimed to develop such an alkylamine-promoted
deoxymethylenation reaction of CO2 for the formation of
Received: September 4, 2020
https://dx.doi.org/10.1021/acs.orglett.0c02963
© XXXX American Chemical Society
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A

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Scheme 1. CO2 Deoxymethylenation for the Formation of
Scheme 2. Optimization of the Reaction Conditions
C−C Bonds
C−C bonds. Specifically, the bridging of two molecules of
enaminones with CO2 and the synthesis of 1,4-dihydropyr-
idines were realized.
In terms of high O-,9 N-,10 or C-nucleophilicity,11
enaminones were regarded as useful precursors for synthesis
of value-added chemicals. Initially, the reaction of acyclic
enaminone 1a, CO2, and phenylsilane (PhSiH3) was
investigated under different conditions (Scheme 2). To our
delight, when the reaction was performed in dry MeCN with
1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) and ZnCl2 as the
catalyst under 1 atm of CO2, the bis-enaminone 2a was
selectively obtained, achieving 88% yield upon isolation
(Scheme 2, entry 1). Control experiments then were
performed to determine the usefulness of each component
(Scheme 2, entries 2−5). Without PhSiH3, no reaction
occurred (Scheme 2, entry 2). In contrast, when the reaction
was conducted in the absence of n-BuNH2, the yield of 2a only
reached 47% (Scheme 2, entry 3), implying the significant role
of n-BuNH2 in promoting the reaction. As a possible reason, a
symbiotic relationship may exist between the CO2 and amine.
CO2 might react with the primary amine to form carbamic
acid, thus essentially increasing the effective concentration of
CO2 in the solution12 and aiding the subsequent reduction
steps. Without TBD, 2a was not detected (Scheme 2, entry 4),
revealing that the TBD was an indispensable catalyst. In the
absence of ZnCl2, the yield of 2a sharply decreased to 39%
(Scheme 2, entry 5), identifying ZnCl2 to be a favorable
synergistic catalyst. As the amount of PhSiH3, n-BuNH2, TBD,
and ZnCl2 was decreased, 2a was also isolated smoothly in a
90% yield (entry 6). When the amount of TBD and ZnCl2 was
further reduced to 0.15 equiv, the yield of 2a was slightly
reduced (86%; see Scheme 2, entry 7 vs entry 6). If the dosage
of butylamine was reduced from 4 equiv to 2 or 1 equiv, the
yield decreased to 83% or 78% (Scheme 2, entries 8 and 9). A
a
Isolated yield, n.r. = no reaction, n.d. = no detected.
further reduction of the amount of silane to 2 equiv led to a
sharp decrease in the yield of 2a (Scheme 2, entry 10).
Based on the significant role of butylamine in promoting the
reaction, other amines were also screened (Scheme 2, entries
11−15). When other monobasic primary alkylamines (e.g.,
benzylamine and tert-butylamine) were used to replace n-
butylamine, good yields of 2a could still be obtained (Scheme
2, entries 11 and 12). Nonetheless, ethylenediamine showed
little effect in promoting this reaction (Scheme 2, entry 13).
Similarly, diethylamine and triethylamine had no prominent
promoting effect, only furnishing 2a in a 50% and 34% yield,
respectively (Scheme 2, entries 14 and 15). Combined with the
above results, we used n-butylamine in subsequent inves-
tigations. When glycine betaine (GB) rather than TBD was
adopted, the yield of 2a only reached 61% (Scheme 2, entry
16). Common basic catalysts (e.g., DBU, Cs2CO3, and t-
BuONa) and Lewis acids (e.g., AlCl3, FeCl3, and AgNO3) also
failed to result in a better yield of 2a. When trifluorometha-
nesulfonic acid (HOTf) was taken as the Brønsted acid
catalyst, only a 32% yield of 2a was generated. Finally, different
silanes were screened. Other hydrosilanes, including Ph3SiH,
B
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Et3SiH, EtO3SiH, Ph2SiH2, and polymethylhydrosiloxane were
almost ineffective in the formation of 2a, which may be
attributed to the reduction ability of hydrosilane.13 In addition,
when tetrahydrofuran (THF), N,N-dimethylformamide
(DMF), 1,4-dioxane, toluene, and 1,2-dichloroethane (DCE)
were used as the solvent, the yield of 2a was no better than that
obtained using acetonitrile. Notably, when the reaction
temperature in the second step (Temp.2) was increased to
120 °C, 2a had a tendency to be further converted to 1,4-
dihydropyridine.
Under the best reaction conditions (Scheme 2, entry 6), the
substrate scope was examined, as shown in Scheme 3. It was
found that the reaction of CO2 with acyclic or cyclic
enaminones contributed to a good yield of bis-enaminone
formation. First, the R1 substituents on the acyclic enaminones
were studied. Generally, cyclic enaminones bearing electron-
donating substituents proceeded well (2b−2f, 82%−94%), e.g.,
in substrates with groups such as 4-methoxyphenyl, 4-
(dimethylamino)phenyl, and 4-alkylphenyl. Meanwhile, elec-
tron-withdrawing substituents (e.g., 4-chlorophenyl) seemed to
further accelerate the cyclization reaction to form 1,4-
dihydropyridine derivatives. The alkyl group such as tert-
butyl was suitable for this reaction to produce 2h. Second, the
R2 substituents were examined (2i−2n). When R2 was n-butyl,
the desired bis-enaminone 2i was generated in a moderate
yield. Furanyl was applicable as well to the production of 2j in
a 66% yield under optimal reaction conditions. The substrates
with electron-donating (e.g., 2k, R2 = 3,4,5-trimethoxyphenyl,
90%; 2l, R2 = p-methoxyphenyl, 83%) groups were tolerable to
furnish the corresponding products in good yields. When the
R2 substituent was p-chlorophenyl, the bis-enaminone 2n was
obtained in a moderate yield. Finally, the cyclic enaminones
were investigated (2o−2t). Good yields of the corresponding
bis-enaminones were produced, regardless of electron-donating
(2p, R1 = p-methoxyphenyl, 83%) or electron-withdrawing
(2q, R1 = p-chlorophenyl, 85%; 2r, R1 = p-fluorophenyl, 80%)
groups on the benzene ring. Enaminones that were prepared
from 1,3-cyclohexanedione and 1,3-cyclopentanedione also
performed well, resulting in the 62% and 71% yields of the
desired 2s and 2t, respectively. In addition, when the reaction
time (t2) was shortened, substrates containing electron-
withdrawing trifluoromethyl and ester groups were also
tolerated (2u, −CF3, 65%; 2v, −CO2Et, 43%). When different
enaminones were used simultaneously (e.g., 1b:1o = 1:1), 2b,
2o and the corresponding unsymmetric bis-enaminone were
obtained as a mixture. The structure of the products was
characterized by nuclear magnetic resonance (NMR) analysis
and further confirmed by X-ray analysis of 2o.
a
Scheme 3. Scope of the Reaction
In the above research process, some bis-enaminone, such as
2g and 2n, had a tendency to be converted to 1,4-
dihydropyrimidine derivatives, which have been widely
exploited as reducing agents14 and are very attractive
heterocycles in medicinal chemistry and pharmacology,
because of their wide range of biological activities.15 Afterward,
the conditions were optimized. To be more specific, when the
reaction temperature was increased to 120 °C, this tandem
cyclization reaction was promoted to furnish 1,4-dihydropyr-
imidine derivatives. To evaluate the scope of this reaction with
respect to the formation of 1,4-dihydropyrimidines, we
preliminary studied reactions of CO2 with acyclic or cyclic
enaminones (see Scheme 4). The substrates with electron-
withdrawing (e.g., 3b, R1 = p-chlorophenyl, 81%; 3d, R2 = p-
chlorophenyl, 89%) or electron-donating (e.g., 3c, R1 = p-
methoxyphenyl, 89%; 3e, R2 = p-methoxyphenyl, 71%) groups
were all found to help generate the target products in good
yields. In addition, fused-dihydropyridine derivatives could also
be generated from the reaction between cyclic enaminones and
CO2 (3f−3i, 77%−95%).
Next, the reaction mechanism was explored and studied.
When formaldehyde rather than CO2 and PhSiH3 was directly
adopted, 2a and 3a were successfully produced (Scheme 5a).
In particular, more 3a could be obtained at 120 °C, revealing
the higher temperature as a favorable condition for the
production of 3a. These results also suggest that formaldehyde
or its equivalents (e.g., bis(silyl)acetal) may be intermediates in
the tandem reaction. Moreover, when 2a was subjected to the
standard conditions, 3a was isolated in good yield (Scheme
5b), revealing that dihydropyridine was transformed from bis-
enaminone 2.
a
b
Yields are of isolated products. Part of the product irrepressibly
c
Drawing upon our experimental results and the related
knowledge in the literatures, we propose the following catalytic
converted to 1,4-dihydropyridine. Poor solubility in acetonitrile.
Using MeCN:THF (2:1, 3 mL) as the solvent. t2 = 3 h. t2 = 9 h.
d
e
C
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enaminone 1a. Note that the formation of imine (TBD-
catalyzed reduction−condensation of CO2 with primary
amine) in the same system has been supported in our previous
work8c and Xia’s work.6e Subsequently, an elimination reaction
(IV or IV′ to V) occurs to generate terminal olefin V, as
supported by density functional theory (DFT) calculations in
our previous work.8c The bridging of enaminones (to form bis-
enaminone 2a) is realized through the nucleophilic addition
between V and 1a. Intermediate VI then is yielded by further
intramolecular nucleophilic addition of nucleophilic N atom to
the β-position of the carbonyl group on the other side. Finally,
after eliminating aniline, 1,4-dihydropyrimidine 3a is gener-
ated. In this procedure, ZnCl2, as a beneficial catalyst, may
coordinate with the carbonyl group to enhance its electro-
philicity,17 thereby promoting the corresponding nucleophilic
addition or elimination process. In addition, the role of n-
butylamine may also be reflected in the reaction with CO2 to
form carbamic acid, which essentially increases the concen-
tration of CO2 in the solution.12
Scheme 4. Scope of the Reaction to Form 1,4-
Dihydropyridines
a
In summary, we have developed a new reductive tandem
Csp2−Csp3 bond-forming reaction with CO2. The bridging of
enaminones and the synthesis of 1,4-dihydropyridines were
achieved separately. Control experiments revealed that n-
butylamine significantly promoted this TBD- and zinc-
chloride-catalyzed CO2 deoxymethylenation procedure. A
possible mechanism involving the formation of bis(silyl)acetal,
nucleophilic addition, and amine elimination was postulated to
clarify the generation of bis-enaminone and dihydropyridine
derivatives. Further investigations of the reaction mechanism
and the synthetic utility are ongoing in our laboratory.
a
b
Yields are of isolated products. Using MeCN:THF (2:1, 3 mL) as
the solvent.
Scheme 5. Verification of Intermediates
ASSOCIATED CONTENT
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* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c02963.
1
Experimental details, synthetic procedures, H and 13C
NMR spectra for new compounds (PDF)
cycle for this tandem CO2 deoxymethylenenation reaction (see
Scheme 6). The reaction is initiated by TBD-catalyzed
reduction of CO2, producing bis(silyl)acetal II.16 Then,
nucleophilic substitution between 1a and II affords inter-
mediate IV (Scheme 6, path a). Alternatively, bis(silyl)acetal II
undergoes a condensation reaction with n-butylamine (or
aniline formed in the last step) to form imine III,6b which is
further transformed to IV′ via the nucleophilic addition with
Accession Codes
CCDC 1896690 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
Scheme 6. Proposed Mechanism
D
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AUTHOR INFORMATION
Corresponding Author
■
Yulei Zhao − School of Chemistry and Chemical Engineering,
Qufu Normal University, Qufu 273165, China; orcid.org/
0000-0002-8628-2680; Email: zhaoyulei86@126.com
(3) (a) Xie, C.; Song, J.; Wu, H.; Zhou, B.; Wu, C.; Han, B. ACS
Sustainable Chem. Eng. 2017, 5, 7086−7092. (b) Frogneux, X.;
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́
Blondiaux, E.; Thuery, P.; Cantat, T. ACS Catal. 2015, 5, 3983−3987.
(4) For methylation, see: (a) Wang, M.-Y.; Wang, N.; Liu, X.-F.;
Qiao, C.; He, L.-N. Green Chem. 2018, 20, 1564−1570. (b) Liu, X.-F.;
Qiao, C.; Li, X.-Y.; He, L.-N. Green Chem. 2017, 19, 1726−1731.
(c) Fang, C.; Lu, C.; Liu, M.; Zhu, Y.; Fu, Y.; Lin, B.-L. ACS Catal.
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Chem., Int. Ed. 2014, 53, 10476−10480. (e) Li, Y.; Cui, X.; Dong, K.;
Junge, K.; Beller, M. ACS Catal. 2017, 7, 1077−1086.
Xuqiang Guo − School of Chemistry and Chemical Engineering,
Qufu Normal University, Qufu 273165, China
Xin Ding − School of Chemistry and Chemical Engineering,
Qufu Normal University, Qufu 273165, China
Zheng Zhou − School of Chemistry and Chemical Engineering,
Qufu Normal University, Qufu 273165, China
Man Li − School of Chemistry and Chemical Engineering, Qufu
Normal University, Qufu 273165, China
Nan Feng − School of Chemistry and Chemical Engineering,
Qufu Normal University, Qufu 273165, China
Bowen Gao − School of Chemistry and Chemical Engineering,
Qufu Normal University, Qufu 273165, China
Xu Lu − School of Chemistry and Chemical Engineering, Qufu
Normal University, Qufu 273165, China
Yunlin Liu − School of Chemistry and Chemical Engineering,
Guangzhou University, Guangzhou 510006, China;
orcid.org/0000-0001-5902-9885
Jinmao You − School of Chemistry and Chemical Engineering,
Qufu Normal University, Qufu 273165, China; Northwest
Institute of Plateau Biology, Chinese Academy of Science, Xining
810001, China
(5) For hydroxylmethylation, see: (a) Chen, X.-W.; Zhu, L.; Gui, Y.-
Y.; Jing, K.; Jiang, Y.-X.; Bo, Z.-Y.; Lan, Y.; Li, J.; Yu, D.-G. J. Am.
Chem. Soc. 2019, 141, 18825−18835. (b) Qiu, J.; Gao, S.; Li, C.;
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C.; Fu, H.-C.; He, L.-N. Angew. Chem., Int. Ed. 2017, 56, 7425−7429.
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(b) Jin, G.; Werncke, C. G.; Escudie, Y.; Sabo-Etienne, S.; Bontemps,
S. J. Am. Chem. Soc. 2015, 137, 9563−9566. (c) Schieweck, B. G.;
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Bernskoetter, W. H. ACS Catal. 2018, 8, 1338−1345. (b) Ren, X.;
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9209−9212.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c02963
Notes
The authors declare no competing financial interest.
(8) (a) Zhao, Y.; Xu, M.; Zheng, Z.; Yuan, Y.; Li, Y. Chem. Commun.
2017, 53, 3721−3724. (b) Zhao, Y.-L.; Cao, Z.-Y.; Zeng, X.-P.; Shi, J.-
M.; Yu, Y.-H.; Zhou, J. Chem. Commun. 2016, 52, 3943−3946.
(c) Zhao, Y.; Liu, X.; Zheng, L.; Du, Y.; Shi, X.; Liu, Y.; Yan, Z.; You,
J.; Jiang, Y. J. Org. Chem. 2020, 85, 912−923. (d) Zhao, Y.; Zhang, Z.;
Liu, X.; Wang, Z.; Cao, Z.; Tian, L.; Yue, M.; You, J. J. Org. Chem.
2019, 84, 1379−1386. (e) Zhao, Y.; Guo, X.; Wang, Z.; Shen, D.;
Chen, T.; Wu, N.; Yan, S.; You, J. Eur. J. Org. Chem. 2020, 2020,
978−984. (f) Chen, G.-S.; Chen, S.-J.; Luo, J.; Mao, X.-Y.; Chan, A.
S.-C.; Sun, R. W.-Y.; Liu, Y.-L. Angew. Chem., Int. Ed. 2020, 59, 614−
621. (g) Luo, J.; Chen, G.-S.; Chen, S.-J.; Li, Z.-D.; Zhao, Y.-L.; Liu,
Y.-L. Adv. Synth. Catal. 2020, 362, 3635−3643. (h) Zhao, Y.; Guo, X.;
Du, Y.; Shi, X.; Yan, S.; Liu, Y.; You, J. Org. Biomol. Chem. 2020, 18,
6881−6888.
ACKNOWLEDGMENTS
■
We thank the Natural Science Foundation of Shandong
Province (No. ZR2018BB026) and the Doctoral Start-Up
Scientific Research Foundation of Qufu Normal University.
DEDICATION
This paper is dedicated to Zhao’s upcoming child.
■
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