Computer-Assisted Design of Ionic Liquids for Efficient Synthesis of 3(2H)-Furanones: A Domino Reaction Triggered by CO2

Article abstract of DOI:10.1021/jacs.6b08895

A strategy for the highly efficient synthesis of 3(2H)-furanones by hydration of diyne alcohols catalyzed by base-functionalized ionic liquids under atmospheric-pressure CO₂ that was developed through computer-assisted design is reported. The best range of basic ionic liquids as catalysts was predicted at first, and [HDBU][BenIm] exhibited the highest catalytic activity. Through a combination of NMR spectroscopic investigations and quantum-chemical calculations, the results indicated the importance of the basicity of the anion and the species of cation in the ionic liquid.

Full text of DOI:10.1021/jacs.6b08895

Communication
pubs.acs.org/JACS
Computer-Assisted Design of Ionic Liquids for Efficient Synthesis of
3(2H)‑Furanones: A Domino Reaction Triggered by CO2
Kaihong Chen,† Guiling Shi,† Weidong Zhang,† Haoran Li,† and Congmin Wang*,†,‡
†Department of Chemistry, ZJU-NHU United R&D Center, Zhejiang University, Hangzhou 310027, China
‡Key Laboratory of Biomass Chemical Engineering of Ministry of Education, Zhejiang University, Hangzhou 310027, China
S
* Supporting Information
pharmaceuticals.8 Obviously, the basicity of the catalyst plays a
ABSTRACT: A strategy for the highly efficient synthesis
of 3(2H)-furanones by hydration of diyne alcohols
catalyzed by base-functionalized ionic liquids under
atmospheric-pressure CO2 that was developed through
computer-assisted design is reported. The best range of
basic ionic liquids as catalysts was predicted at first, and
[HDBU][BenIm] exhibited the highest catalytic activity.
Through a combination of NMR spectroscopic inves-
tigations and quantum-chemical calculations, the results
indicated the importance of the basicity of the anion and
the species of cation in the ionic liquid.
significant role in these reactions: weak basicity results in low
catalytic activity, while strong basicity leads to side reactions.9
However, the choice of basic catalyst is often based on
experience and experimental trial and error. Therefore, a
strategy that can predict the best range of basicities before
experiment will have a profound influence in this field.
Herein we demonstrate a new method for the highly efficient
synthesis of 3(2H)-furanones from diyne alcohols through a
one-pot domino hydration reaction triggered by atmospheric-
pressure CO2. Through quantum-chemical calculations, a
proper range of basicities was designed and predicted at first,
and [HDBU][BenIm] was found to be very effective in this
reaction during experiments, in good agreement with the
prediction. The roles of the anion and the cation were also
investigated through a combination of density functional theory
(DFT) and NMR methods.
In a base-catalyzed reaction, can we select one of the most
effective ILs before the experiment by simply using DFT
calculations? One of criteria that represent basicity is the pKa
value. We wonder whether the catalytic activity can be
evaluated just by comparing the pKa values of the raw material
and ILs.
The hydration of 2-methyl-6-phenylhexa-3,5-diyn-2-ol (1a)
was selected as the model reaction to investigate the effect of
different bases. At first, the pKa value of 1a was calculated,10
and the pKa values (Figure 1) of various common anions
(Figure 2) were obtained from previous works.11 However, 1a
exhibited stronger basicity than Im, which was considered as a
strong base, where the pKa values were 20.5 and 14.2,
onsiderable attention has been drawn to CO2 capture and
C
utilization (CCU), as CO2 is the main reason leading to
climate change. On one hand, CO2, an abundant and nontoxic
C1 resource, has been converted into fuels and various useful
chemicals, like urea, cyclic carbonates, and oxazolidinones.1 On
the other hand, CO2 can be utilized to tune the physical
properties of liquids or be a cocatalyst that is effective and
accessible.2 A CCU process under mild conditions is still
challenging because CO2 is at the most oxidized state of carbon
and is thermodynamically stabilized. Ionic liquids (ILs) exhibit
outstanding performance in CO2 capture because of their
unique properties.3 Recently, it was also discovered that many
CCU processes (CO2 acting as reactant or cocatalyst) can
perform well under atmospheric-pressure CO2 by tuning the
properties of ILs. The ILs act both as absorbents and as
activators in these reactions.4
3(2H)-Furanones are fundamental structural elements in
many natural products, some of which exhibit antitumor,
antibiotic, and antiulcer activities.5 Thus, effective strategies for
their synthesis need to be developed. Acid-catalyzed cycliza-
tion−dehydration of 1-hydroxy-2,4-diketones is one of the
traditional methods.6 What’s more, transition metal catalysts,
like Au, Ag, Cu, Pd, Pt, and Hg, and base-catalyzed cyclization
were developed to afford 3(2H)-furanones.2h,i,7 However, these
approaches usually require an excess of strong acid or base,
volatile organic solvents, tedious processes for purification, and
metal catalysts that are of poor reusability. Therefore, a
challenging approach is to develop a novel method for the
efficient synthesis of 3(2H)-furanones that is metal- and
solvent-free.
Figure 1. pKa values of the complex of 1a and CO2, 1a, and various
azoles.
Base-catalyzed reactions, such as the aldol reaction, Morita−
Baylis−Hillman reaction, and Rauhut−Currier reaction, are
important strategies to afford various natural products and
Received: August 24, 2016
Published: October 18, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.6b08895
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
yield, respectively. With moderate basicity, to our delight,
[HDBU][BenIm] improved the yield of 2a to 84%. The
control experiments without CO2 (Table 1, entry 9) or without
base (entry 10) revealed that CO2 and the base are essential to
this reaction. This reaction did not proceed when Et3N was
used as the catalyst (entry 11), indicating the importance of the
basicity of the base catalyst. Furthermore, the traditional
superbase DBU could only give a moderate yield (entry 12).
The use of traditional inorganic bases such as NaOH, NaTriz,
and Na2CO3 did not promote the reaction (entries 13−15).
This poor catalytic activity should be ascribed to their poor
CO2 activation. After screening of the reaction conditions
(Table S1), the yield of 2a improved to 92% when the reaction
was performed at 80 °C for 2 h (entry 16).
Clearly, [BenIm] is a suitable anion in this reaction on the
basis of either the DFT prediction or the experimental results.
However, why does the cation of the IL have such a significant
effect on this reaction, with superbase-derived protic ILs
normally exhibiting higher catalytic activity than aprotic ILs?
To begin with, the basicities of these ILs should be taken into
account, since this reaction is sensitive to the basicity of the
catalyst. Therefore, NMR spectroscopic experiments were
utilized to compare their basicities (Figure 3). When the H
Figure 2. Structures of cations and anions used in this work.
respectively. It was reported that CO2 can facilitate hydrogen
abstraction.2b,4e Therefore, CO2 was taken into consideration in
calculating the pKa value of 1a. Interestingly, this value declined
to 11.2, which is between those of BenIm and Triz. Did this
mean that BenIm or Triz anions would show higher catalytic
reactivity than other anions?
To verify this hypothesis, the hydration of 1a was carried out
at 90 °C under CO2 at atmospheric pressure (balloon). As
shown in Table 1, when [N4444][Im] was used as the base, 1a
Table 1. Effect of Different Basic Ionic Liquids on the
a
Synthesis of 3(2H)-Furanones
b
entry
base
T (°C)
t (h)
yield (%)
1
2
3
4
5
6
7
8
[N4444][Im]
[N4444][BenIm]
[N4444][Triz]
[N4444][Bentriz]
[N4444][Tetz]
[HMTBD][BenIm]
[HDBU][BenIm]
[HTMG][BenIm]
[HDBU][BenIm]
−
90
90
90
90
90
90
90
90
90
90
90
90
90
90
90
80
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
2
61
72
70
60
12
72
84
47
<1
<1
<1
65
<1
<1
<1
92
c
9
10
11
12
13
14
Et3N
DBU
NaOH
Figure 3. 1H spectra of (a) BenIm, (b) [N4444][BenIm], (c)
[HTMG][BenIm], (d) [HDBU][BenIm], and (e) [HMTBD]-
[BenIm].
NaTriz
d
15
Na2CO3
16
[HDBU][BenIm]
a
Reaction conditions: 1a (0.6 mmol), H2O (6 mmol), base (0.6
b
mmol), and CO2 (∼1 bar, balloon). Determined by GC using an
internal standard. Without CO2. 0.3 mmol of Na2CO3 was added.
c
d
in BenIm was abstracted, the H signal of C2−H moved from
8.22 to 7.67 ppm (Figure 3a,b). Surprisingly, that signal just
moved less than 0.1 ppm in these three superbase-derived
protic ILs. For example, that signal was at 8.21 and 8.18 ppm in
[HTMG][BenIm] and [HDBU][BenIm], respectively (Figure
3c,d). As for [HMTBD][BenIm], where the cation had the
strongest basicity in these protic ILs, the H signal of C2−H
moved only 0.06 ppm, to 8.16 ppm (Figure 3e). Clearly, these
PILs mainly existed in the state of molecular pairs rather than
ion pairs, as mainly appeared in [N4444][BenIm]. Moreover, the
natural bond orbital (NBO) charges of these ILs were also
calculated (Table S2). It was found that the NBO charge on N
varied from −0.583 to −0.580 in these PILs, which is lower
than that in [N4444][BenIm] (−0.644). Accordingly, the
basicities of these PILs are lower than that of [N4444][BenIm].
was converted completely and a 61% yield of 2a was obtained.
On the contrary, only a little bit of 1a was converted when a
weaker base IL, [N4444][Tetz], was used, while higher yields of
2a were obtained when the moderately basic ILs [N4444]-
[BenIm] and [N4444][Triz] were used (72 and 70%,
respectively). Clearly, it worked out to be consistent with our
prediction.
With [BenIm] as the anion, we also investigated the catalytic
activity of many superbase-derived protic ILs (PILs). The
basicity of these superbases could be ordered as MTBD > DBU
> TMG. When [HMTBD][BenIm] and [HTMG][BenIm]
were applied in this reaction, 2a was obtained in 72 and 47%
B
DOI: 10.1021/jacs.6b08895
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
Figure 4. Computational studies of the reaction mechanism.
Therefore, the appropriate basicity of [HDBU][BenIm] should
be one of the reasons for its high catalytic activity.
the larger difference in NBO charges, the easier this reaction
would proceed.
With the optimized conditions in hand, the utility and
generality of this reaction was also investigated (Table 2). First,
Subsequently, the mechanism of this domino reaction was
investigated by DFT calculations (Figures 4 and S1).10 At first,
1a reacts with CO2 in the presence of [BenIm], overcoming an
energy barrier of 14.3 kcal/mol. Then, the intramolecular
cyclization of 3 requires an activation free energy of 23.3 kcal/
mol, making it the rate-determining step in this reaction. The
cyclic carbonate 4 is hydrolyzed and releases CO2 to afford 6
under the base [BenIm]. Finally, 8 is formed through
isomerization of 6 with the base catalyst, and intramolecular
cyclization of 8 affords the product 2a.
Table 2. Substrate Scope of the [HDBU][BenIm]-Catalyzed
a
Domino Reaction
b
entry
substrate
R1
R2
R3
t (h)
yield (%)
c
1
1a
1b
1c
1d
1e
1a
H
Me
F
H
−
−
H
−
−
−
6
82 (77)
89
2
3
4
5
2.5
2.5
2
87
−
−
H
Et
86
Therefore, the intramolecular cyclization of 3 was picked up
to investigate the difference between protic ILs and aprotic ILs
in this reaction (Figure 5).12 The energy barrier would be
−(CH2)5−
H
2.5
2
84
d
6
H
84
a
Reaction conditions: substrate (0.6 mmol), H2O (6 mmol),
[HDBU][BenIm] (0.6 mmol), and CO2 (1 bar, balloon) at 80 °C.
b
Determined by GC using an internal standard; the yield in
c
parenthesis is an isolated yield. Reaction conditions: 1a (6 mmol,
1.1 g), H2O (60 mmol), [HDBU][BenIm] (6 mmol), and CO2 (1 bar,
d
balloon) at 80 °C. [HDBU][BenIm] was used for the fifth time.
this reaction proceeded on a 10 mmol scale of 1a, and an 82%
yield of 2a was obtained after 6 h (entry 1). Diyne alcohols
bearing either electron-donating (entry 2) or electron-with-
drawing groups (entry 3) reacted smoothly with water under
the optimized conditions, and the corresponding products 2b
and 2c were obtained in 89% and 87% yield, respectively. When
there was a large substituent group at R2 or R3 (e.g., ethyl or
cyclohexyl), the reaction also gave the desired product in high
yield (entries 4 and 5). Moreover, [HDBU][BenIm] could be
recycled easily without significant loss of catalytic activity after
the fifth time (entry 6).
In conclusion, a strategy for the highly efficient synthesis of
3(2H)-furanones by hydration of diyne alcohols catalyzed by
base-functionalized ionic liquids under atmospheric-pressure
CO2 has been developed. Notably, the best range of basicities
of catalysts was predicted prior to the experiment. Furthermore,
the catalytic reactivity of protic ILs (e.g., [HDBU][BenIm])
was better than that of aprotic ILs (e.g., [N4444][BenIm]).
Through a combination of NMR spectroscopic investigations
and quantum-chemical calculations, we found the moderate
basicity to be the main reason for such high catalytic activity.
Finally, this method could be extended to other diyne alcohols,
Figure 5. Structures of TS2 with (a) [HDBU][BenIm] (ΔG = 24.6
kcal/mol) or (b) [N1111][BenIm](ΔG = 25.6 kcal/mol) as the base.
higher while considering cations; a similar phenomenon was
also reported in SO2 absorption.13 The energy demand for the
cyclization process with [HDBU][BenIm] was 24.6 kcal/mol,
while it was 25.6 kcal/mol with [N1111][BenIm] as the base.
Interestingly, the basicity of [N1111][BenIm] should be stronger
than that of [HDBU][BenIm] (Figure 3). At this point, the
NBO charges of 3 with different catalysts were also calculated.
As shown in Figure S2, the NBO charges of C and O were
0.126 and −0.799, respectively, with [BenIm] as the base.
When the cations were taken into consideration, these
differences were much smaller. When [HDBU][BenIm] was
used, the difference in the NBO charges of C and O was 0.861,
while it was 0.855 with [N1111][BenIm] as the catalyst. Clearly,
C
DOI: 10.1021/jacs.6b08895
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
F.; Yang, Z. Z.; Yu, B.; Zhang, H. Y.; Xu, H. J.; Hao, L. D.; Han, B. X.;
Liu, Z. M. Chem. Sci. 2015, 6, 2297. (c) Hu, J.; Ma, J.; Zhu, Q.; Zhang,
Z.; Wu, C.; Han, B. X. Angew. Chem., Int. Ed. 2015, 54, 5399. (d) Luo,
X. Y.; Chen, K. H.; Li, H. R.; Wang, C. M. Int. J. Hydrogen Energy
2016, 41, 9175. (e) Chen, K. H.; Shi, G. L.; Dao, R. N.; Mei, K.; Zhou,
and [HDBU][BenIm] is reusable. The method developed in
this work paves the way for other CCU processes and base-
catalyzed reactions.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.6b08895.
■
X. Y.; Li, H. R.; Wang, C. M. Chem. Commun. 2016, 52, 7830. (f) Calo,
́
S
V.; Nacci, A.; Monopoli, A.; Fanizzi, A. Org. Lett. 2002, 4, 2561.
(5) (a) Stierle, A. A.; Stierle, D. B.; Patacini, B. J. Nat. Prod. 2008, 71,
856. (b) Jerris, P. J.; Smith, A. B., III. J. Org. Chem. 1981, 46, 577.
(c) Smith, A. B., III; Guaciaro, M. A.; Schow, S. R.; Wovkulich, P. M.;
Toder, B. H.; Hall, T. W. J. Am. Chem. Soc. 1981, 103, 219.
(d) Felman, S. W.; Jirkovsky, I.; Memoli, K. A.; Borella, L.; Wells, C.;
Russell, J.; Ward, J. J. Med. Chem. 1992, 35, 1183.
Experimental procedures and DFT calculation details
(PDF)
(6) (a) Inoue, Y.; Ohuchi, K.; Imaizumi, S. Tetrahedron Lett. 1988,
29, 5941. (b) Smith, A. B., III; Levenberg, P. A.; Jerris, P. J.;
Scarborough, R. M., Jr.; Wovkulich, P. M. J. Am. Chem. Soc. 1981, 103,
1501. (c) Curran, D. P.; Singleton, D. H. Tetrahedron Lett. 1983, 24,
2079.
AUTHOR INFORMATION
Corresponding Author
*chewcm@zju.edu.cn
Notes
■
(7) (a) Crone, B.; Kirsch, S. F. J. Org. Chem. 2007, 72, 5435. (b) Egi,
M.; Azechi, K.; Saneto, M.; Shimizu, K.; Akai, S. J. Org. Chem. 2010,
75, 2123. (c) He, H. T.; Qi, C. R.; Hu, X. H.; Ouyang, L.; Xiong, W.
F.; Jiang, H. F. J. Org. Chem. 2015, 80, 4957. (d) Kusakabe, T.;
Takahashi, T.; Shen, R.; Ikeda, A.; Dhage, Y. D.; Kanno, Y.; Inouye, Y.;
Sasai, H.; Mochida, T.; Kato, K. Angew. Chem., Int. Ed. 2013, 52, 7845.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We acknowledge the support of the National Key Basic
Research Program of China (2015CB251401), the National
Natural Science Foundation of China (21176205 and
21322602), and the Fundamental Research Funds of the
Central Universities.
́
(e) Kirsch, S. F.; Binder, J. T.; Liebert, C.; Menz, H. Angew. Chem., Int.
Ed. 2006, 45, 5878. (f) Marson, C. M.; Edaan, E.; Morrell, J. M.; Coles,
S. J.; Hursthouse, M. B.; Davies, D. T. Chem. Commun. 2007, 2494.
(g) Poonoth, M.; Krause, N. J. Org. Chem. 2011, 76, 1934.
(8) (a) Denmark, S. E.; Stavenger, R. A. Acc. Chem. Res. 2000, 33,
432. (b) Johnson, J. S.; Evans, D. A. Acc. Chem. Res. 2000, 33, 325.
(c) Xie, P. Z.; Huang, Y. Org. Biomol. Chem. 2015, 13, 8578. (d) Wei,
Y.; Shi, M. Chem. Rev. 2013, 113, 6659. (e) Xie, P. Z.; Huang, Y. Eur. J.
Org. Chem. 2013, 2013, 6213.
REFERENCES
■
(1) (a) Sakakura, T.; Choi, J. C.; Yasuda, H. Chem. Rev. 2007, 107,
2365. (b) Porosoff, M. D.; Yan, B. H.; Chen, J. G. Energy Environ. Sci.
2016, 9, 62. (c) Seki, T.; Baiker, A. Chem. Rev. 2009, 109, 2409−2454.
(d) Lu, X. B.; Zhang, Y. J.; Jin, K.; Luo, L. M.; Wang, H. J. Catal. 2004,
227, 537. (e) Shi, F.; Zhang, Q. H.; Ma, Y. B.; He, Y. D.; Deng, Y. Q. J.
Am. Chem. Soc. 2005, 127, 4182. (f) Wang, Y. B.; Wang, Y. M.; Zhang,
W. Z.; Lu, X. B. J. Am. Chem. Soc. 2013, 135, 11996.
(9) (a) Gao, F.; Huang, Y. Adv. Synth. Catal. 2014, 356, 2422.
(b) Sun, X.; Sengupta, S.; Petersen, J. L.; Wang, H.; Lewis, J. P.; Shi, X.
Org. Lett. 2007, 9, 4495. (c) Chen, J. B.; Natte, K.; Spannenberg, A.;
Neumann, H.; Langer, P.; Beller, M.; Wu, X. F. Angew. Chem., Int. Ed.
2014, 53, 7579.
(10) All of the calculations were performed with the Gaussian 09
package. Details of the calculations and the Gaussian 09 reference are
shown in the Supporting Information.
(11) (a) Yagil, G. Tetrahedron 1967, 23, 2855. (b) Bruice, T. C.;
Schmir, G. L. J. Am. Chem. Soc. 1958, 80, 148. (c) Catalan, J.; Abboud,
J. L. M.; Elguero, J. Adv. Heterocycl. Chem. 1987, 41, 187.
(12) To save time but still obtain convincing results, [N1111] was used
as the cation in the DFT calculation instead of [N4444].
(13) Chen, K. H.; Lin, W. J.; Yu, X. N.; Luo, X. Y.; Ding, F.; He, X.;
Li, H. R.; Wang, C. M. AIChE J. 2015, 61, 2028.
(2) (a) Jessop, P. G.; Heldebrant, D. J.; Li, X.; Eckert, C. A.; Liotta,
C. L. Nature 2005, 436, 1102. (b) Liu, Y.; Jessop, P. G.; Cunningham,
M.; Eckert, C. A.; Liotta, C. L. Science 2006, 313, 958. (c) Sugawara,
Y.; Yamada, W.; Yoshida, S.; Ikeno, T.; Yamada, T. J. Am. Chem. Soc.
2007, 129, 12902. (d) Xiong, D. Z.; Cui, G. K.; Wang, J. J.; Wang, H.
Y.; Li, Z. Y.; Yao, K. S.; Zhang, S. J. Angew. Chem., Int. Ed. 2015, 54,
7265. (e) Jessop, P. G.; Mercer, S. M.; Heldebrant, D. J. Energy
Environ. Sci. 2012, 5, 7240. (f) Sugawara, Y.; Yamada, W.; Yoshida, S.;
Ikeno, T.; Yamada, T. J. Am. Chem. Soc. 2007, 129, 12902. (g) He, H.
T.; Qi, C. R.; Hu, X. H.; Guan, Y. Q.; Jiang, H. F. Green Chem. 2014,
16, 3729. (h) Qi, C.; Jiang, H.; Huang, L.; Yuan, G.; Ren, Y. Org. Lett.
2011, 13, 5520. (i) Yuan, G.; He, Z.; Zheng, J.; Chen, Z.; Huang, H.;
Shi, D.; Qi, C.; Jiang, H. Tetrahedron Lett. 2011, 52, 5956.
(3) (a) Bates, E. D.; Mayton, R. D.; Ntai, I.; Davis, J. H. J. Am. Chem.
Soc. 2002, 124, 926. (b) Cadena, C.; Anthony, J. L.; Shah, J. K.;
Morrow, T. I.; Brennecke, J. F.; Maginn, E. J. J. Am. Chem. Soc. 2004,
126, 5300. (c) Gurkan, B. E.; de la Fuente, J. C.; Mindrup, E. M.;
Ficke, L. E.; Goodrich, B. F.; Price, E. A.; Schneider, W. F.; Brennecke,
J. F. J. Am. Chem. Soc. 2010, 132, 2116. (d) Wang, C. M.; Luo, X. Y.;
Luo, H. M.; Jiang, D. E.; Li, H. R.; Dai, S. Angew. Chem., Int. Ed. 2011,
50, 4918. (e) Soutullo, M. D.; Odom, C. I.; Wicker, B. F.; Henderson,
C. N.; Stenson, A. C.; Davis, J. H. Chem. Mater. 2007, 19, 3581.
(f) Liu, Z. M.; Wu, W. Z.; Han, B. X.; Dong, Z. X.; Zhao, G. Y.; Wang,
J. Q.; Jiang, T.; Yang, G. Y. Chem. - Eur. J. 2003, 9, 3897. (g) Luo, X.
Y.; Guo, Y.; Ding, F.; Zhao, H. Q.; Cui, G. K.; Li, H. R.; Wang, C. M.
Angew. Chem., Int. Ed. 2014, 53, 7053. (h) Chen, F. F.; Huang, K.;
Zhou, Y.; Tian, Z. Q.; Zhu, X.; Tao, D. J.; Jiang, D. E.; Dai, S. Angew.
Chem., Int. Ed. 2016, 55, 7166. (i) Yang, Z. Z.; Zhao, Y. N.; He, L. N.
RSC Adv. 2011, 1, 545. (j) Yang, Z. Z.; He, L. N.; Gao, J.; Liu, A. H.;
Yu, B. Energy Environ. Sci. 2012, 5, 6602. (k) Cui, G. K.; Wang, J. J.;
Zhang, S. J. Chem. Soc. Rev. 2016, 45, 4307.
(4) (a) Zhao, Y. F.; Yu, B.; Yang, Z. Z.; Zhang, H. Y.; Hao, L. D.;
Gao, X.; Liu, Z. M. Angew. Chem., Int. Ed. 2014, 53, 5922. (b) Zhao, Y.
D
DOI: 10.1021/jacs.6b08895
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX