Aromatic-Amide-Derived Nonbiaryl Atropisomer as Highly Efficient Ligand for Asymmetric Silver-Catalyzed [3 + 2] Cycloaddition

Article abstract of DOI:10.1021/acscatal.5b01685

In this work, we have successfully determined that the aromatic amide-derived nonbiaryl atropisomer/silver complex (silver-Xing-Phos) is an effective catalyst system for the solvent-dependent exo-selective cycloaddition of glycine aldimino esters with cha

Full text of DOI:10.1021/acscatal.5b01685

Letter
pubs.acs.org/acscatalysis
Aromatic-Amide-Derived Nonbiaryl Atropisomer as Highly Efficient
Ligand for Asymmetric Silver-Catalyzed [3 + 2] Cycloaddition
†
,§
§
,†
§
,†,§
Xing-Feng Bai, Zheng Xu, Chun-Gu Xia,* Zhan-Jiang Zheng, and Li-Wen Xu*
†State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics (CAS) and University of the
Chinese Academy of Sciences, Lanzhou 730000, P. R. China
§
Key Laboratory of Organosilicon Chemistry and Material Technology of Ministry of Education, Hangzhou Normal University,
Hangzhou 311121 , P. R. China
*
S Supporting Information
ABSTRACT: In this work, we have successfully determined that the
aromatic amide-derived nonbiaryl atropisomer/silver complex
(
silver-Xing-Phos) is an effective catalyst system for the solvent-
dependent exo-selective cycloaddition of glycine aldimino esters with
chalcones or less-reactive methyl cinnamates to give the correspond-
ing chalcone- or cinnamate-derived pyrrolidines with multiple
stereogenic centers in good yields and high diastereoselectivities as
well as excellent enantioselectivities. Remarkably, it is the first
example of highly enantioselective silver-catalyzed [3 + 2] cyclo-
addition of methyl cinnamates with glycine aldimino esters.
KEYWORDS: asymmetric catalysis, pyrrolidines, silver, homogeneous catalysis, atropisomers, cycloaddition
he functionalized pyrrolidine ring is an important core
structure of numerous five-membered azaheterocyclic-
involving enones or alkyl cinnamates as the dipolarophiles so
7
−11
T
far.
Furthermore, the sole example of catalytic asymmetric
unit-containing natural products and also considered to be a
privileged scaffold in medicinal chemistry because its derivatives
are valuable backbones for the discovery of new biologically
[3 + 2] cycloaddition of alkyl cinnamate with aldimino ester
was reported by Kobayashi et al. in 2008, in which the
moderate ee value of the desired product was achieved in the
11
1
active molecules or therapeutic agents. As a result, the
presence of calcium catalyst system (from Ca(OiPr) and chiral
2
enantioselective construction of pyrrolidine rings has drawn2
considerable attention from synthetic chemists in recent years.
Among these, the asymmetric metal-catalyzed [3 + 2]
cycloaddition was proved to be one of the most convergent
and atom-economical protocols for the stereoselective synthesis
bisoxazoline ligand). Interestingly, there is no report on the
enantioselective synthesis of exo-adducts through silver-
catalyzed cycloaddition of chalcones with aldimino esters so
far. Thus, accordingly, the development of new methods for the
enantioselective construction of chalcone- or cinnamate-derived
pyrrolidines with high level of diastereo- and enantioselectivity
is still in great demand.
3
of pyrrolidine derivatives. Since the pioneering work of
employing stoichiometric amounts of chiral metal complex
4
reported by Grigg and Allway, as well as the first catalytic
Our initial investigation began with the catalytic asymmetric
cycloaddition of chalcone 1a and glycine aldimino ester 2a in
asymmetric cycloaddition with aldimino esters reported by
5
12
Zhang and co-workers, much attention have been paid to the
the presence of Xing-Phos and AgF (Scheme 1 and Table S1
of Supporting Information (SI)). Although good yield (>95%)
was obtained under the reaction conditions, disappointingly,
the pyrrolidine adduct was obtained in poor diastereoselectivity
development of enantioselective [3 + 2] cycloaddition over the
past decade, in which various activated olefins, such as
malonates, maleimides, acrylates, nitroalkenes, and complicated
alkenes bearing electron-withdrawing groups, have been utilized
as dipolarophiles in such asymmetric [3 + 2] cycloaddition
(
see Scheme 1, exo/endo/endo’/exo’ = 52/39/9/0) and varied
enantioselectivity for the three diastereoisomers (94% ee/64%
6
ee/49% ee/-). This result supported the binuclear silver-Xing-
reactions. However, both chalcones and alkyl cinnamates,
12
Phos (CCDC 1042429) exhibited poor activity and selectivity
which have been employed widely for the catalytic synthesis of
chiral carbonyl compounds via conjugate addition reactions,
have not yet been successfully applied as dipolarophiles in the
catalytic asymmetric [3 + 2] cycloadditions with glycine imino
esters. Only limited examples of cycloaddition reaction have
been reported for the synthesis of functionalized pyrrolidines
under the reaction conditions. The solvent effect on the
Received: August 3, 2015
Revised: September 2, 2015
©
XXXX American Chemical Society
6016
DOI: 10.1021/acscatal.5b01685
ACS Catal. 2015, 5, 6016−6020

ACS Catalysis
Letter
Scheme 1. Initial Result in the Asymmetric Silver-Catalyzed
Cycloaddition of Chalcone 1a with Glycine Aldimino Ester
Scheme 2. Ligand Survey for Silver-Catalyzed Cycloaddition
of 1a and 2a
2
a in the Presence of Aromatic Amide-Derived Non-Biaryl
Atropisomer (Xing-Phos)
catalytic activity and the enantioselectivity of the silver catalyst
system in this reaction are summarized in Table S1 (see SI).
The enantio- and exo/endo-selectivity proved obviously
dependent on the reaction medium. Most of solvents evaluated
in this work, such as Et O, DCM, tert-butylmethyl ether, and
2
toluene, gave promising enantioselectivities, but the diastereo-
meric ratios was quite poor. Notably, despite the relatively
similar polarity of 1,4-dioxane in comparison to that of other
ethers, the 1,4-dioxane significantly does impact the exo/endo
ratio. This remarkably solvent-controlled diastereoselectivity
stands in marked contrast to the tetrahydrofuran (endo-selective
in dioxane versus exo-selective in THF). Gratifyingly, we found
that alcoholic solvent gave high diastereoselectivity (up to 99:1
exo/endo) and good enantioselectivity (95.5:4.5 e.r. in EtOH) in
this reaction. We suggested that the polar protic solvent with
strong hydrogen-bonding-accepting properties might interrupt
the formation of the weak intermolecular interaction between
two AAA ligands, whereas nonprotic solvents have a stabilizing
effect for dimeric silver-Xing-Phos complex. We have also
found that the binuclear silver-Xing-Phos could be transferred
into mononuclear silver complex with two Xing-Phos molecules
with the aid of protic alcohol (see ESI-MS analysis of Figure
S1), and thus, the noncovalent interaction came from solvent
and Xing-Phos should not be neglected in the asymmetric
cycloaddition reaction.
Having established the optimal reaction conditions with
present silver-Xing-Phos, we explored various chalcones to
examine the catalytic activation of Xing-Phos in the silver-
catalyzed cycloaddition reactions. All the experimental results in
this catalytic asymmetric [3 + 2] cycloaddition are summarized
in Scheme 3. It can be seen that a wide range of aldimino esters
bearing electron-rich, electron-neutral, or electron-deficient
groups on the phenyl ring, reacted with chalcones to afford
the corresponding exo-3 in high yields (80−97%) and excellent
enantioselectivities (88−96% ee) as well as good diastereose-
lectivities (up to >98/2 exo/endo). Noticeably, almost the same
level of enantioselectivities and yields was observed regardless
of the different substituent patterns on the aromatic ring, as
well as the electron-withdrawing or electron-donating nature of
the substituents. Thus, as shown in Scheme 3, all the desired
products (23 examples) in this reaction were obtained in good
results. Notably, the relative and absolute configuration of the
cycloadduct 3a was determined to be (2R,3R,4S,5S) by X-ray
diffraction analysis (CCDC 1401264).
Encouraged by these results, we next investigated the
combinational use of ethanol and THF for this reaction. To
our delight, employing the ethanol/THF (1:1) as solvent, the
cycloadduct 3a was exclusively obtained in good yield (>95%)
with excellent enantioselectivity (97.5:2.5 er) albeit sacrificed
diastereoselectivity (96:4 exo/endo) in comparison to that in
ethanol. The excellent result with high exo/endo ratio and
enantioselectivity led us to test the possibly privileged role of
Xing-Phos in this reaction through the controlled experiment
with other phosphine ligands under the similar reaction
conditions (−20 °C, 2.5 mol % AgF, 5.5 mol % ligand (Ag:L
=
1:2.2), in EtOH/THF (1:1)). As expected, aromatic amide-
derived ligand (L1), several representative ligands (L2-L6),
13
such as Ming-Phos (L2) and other commercial available P-
ligands (L3-L6), shown in Scheme 2 provided varied diastereo-
and enantioselectivities (2−88% ee for exo-3a), which indicated
the stereoselectivity in this reaction is significantly dependent
on the nature of the chiral ligand. These results clearly
supported the powerful potential of Xing-Phos as both ligand
and organic promoter in this enantioselective cycloaddition
reaction.
Remarkably, to highlight the usability of aromatic amide-
derived nonbiaryl atropisomer/silver complex (silver-Xing-
Phos) in the catalytic asymmetric cycloaddition with challeng-
ing and less reactive alkenes, we explored the [3 + 2]
cycloaddition of alkyl cinnamates and glycine aldimino esters
for synthesis of 1,3-diester-containing pyrrolidines. In 2008,
6
017
DOI: 10.1021/acscatal.5b01685
ACS Catal. 2015, 5, 6016−6020

ACS Catalysis
Letter
Scheme 3. Asymmetric Cycloaddition of Chalcones with
Glycine Aldimino Esters Catalyzed by Xing-Phos-AgF
system
Guided by the above results and the application of silver-
Xing-Phos, we began our optimization of reaction conditions by
testing the effect of solvents and additives (see Table S2 of
Supporting Information). As expected, the [3 + 2] cyclo-
addition of methyl cinnamate with glycine aldimino ester was
quite sluggish under the optimization reaction conditions for
chalcones, which further confirmed the less-reactive activity of
alkyl cinnamates in this reaction. On the basis of the screening
studies of the catalytic asymmetric cycloaddition of methyl
cinnamate 4a with glycine aldimino ester 2a (Table S2), the
major findings including: 1) The addition of inorganic base to
promote the addition and cyclization reaction is necessary. It
was found that potassium carbonate was a suitable base in this
reaction; 2) Similarly to the catalytic asymmetric cycloaddition
of chalcones with glycine aldimino esters, the silver-catalyzed [3
+
2] cycloaddition of methyl cinnamate 4a with Xing-Phos as
the ligand was sensitive to the solvent. Gratifyingly, under the
optimized reaction conditions, none of the other diaster-
eoisomers (exo′ and endo′-5a) was detected, and the reaction
reached completion in THF/MeOH (1:1) at −20 °C and
delivered a high level of diastereoselectivity (exo/endo = 96/4)
with excellent enantioselectivity (>90% ee).
Having observed that highly efficient silver-catalyzed syn-
thesis of cinnamate-derived pyrrolidine can be achieved with
highly stereoselective control in the presence of Xing-Phos, we
decided to investigate the substrate scope of this process
(
Scheme 4). We were pleased to find that a wide array of
methyl cinnamates containing methyl and chloride groups on
aromatic rings as well as glycine aldimino esters derived from
aromatic aldehydes bearing electron-deficient and electron-
neutral substituents on the aryl rings reacted smoothly with
glycine aldimino esters. Moreover, the enantioselectivities of
the desired products were extremely high (90−98% ee). This
result also supports the crucial role of Xing-Phos in this
reaction because of the high level of enantio- and diastereo-
selectivity in this reaction. The absolute configuration of the
product 5l was determined to be (2R,3R,4S,5S) by using of X-
ray crystallographic analysis of exo-5l (CCDC 1401265). In
addition, superior effects on activity and stereoselectivity were
confirmed in the mixed solvent (THF/MeOH = 1:1)
illustrating the unique features of Xing-Phos, in particular the
hemilability and nonrigidity of Xing-Phos as well as the
14
conformational preferences-directed flexibility of Xing-Phos.
Although the possibility of the concerted mechanism cannot
be excluded in the base-free conditions, we suggested that the
catalytic asymmetric [3 + 2] cycloaddition was initiated
probably through the Michael addition on the basis of the
experimental results (Schemes 1−5 and Tables S1−5 of SI). As
shown in Figure 1, we proposed a plausible stepwise
mechanism to account for this silver-catalyzed stereospecific
formal [3 + 2] cycloaddition (Figure 1). Initially, coordination
of glycine aldimino ester 2 to the chiral silver-Xing-Phos
complex led to the formation of the key silver-bound
azomethine ylide dipole (IV) that would attack the chalcone
or methyl cinnamate via 1,4-conjugate addition. Notably, the
reactive intermediate in the first step of conjugate addition
Kobayashi and co-workers reported the first enantioselective
and endo-selective cycloaddition of challenging ethyl cinnamate
with glycine Schiff base; however, the enantioselectivity did not
11
yet reach a sufficient level (91% yield and 61% ee). This fact
could be at least partly explained by the challenge or difficulty
in controlling the stereoselectivity of cycloaddition due to the
presence of two coordinately similar ester groups on two
substrates, respectively. Herein, we describe the first silver-
catalyzed synthesis of cinnamate-derived pyrrolidines with exo-
selectivity through the catalytic asymmetric cycloaddition of
less-reactive alkyl cinnamates with glycine aldimino esters.
1
reaction was monitored by H NMR, in which we can find the
acyclic adduct in the initiated stage of silver-catalyzed conjugate
addition of glycine aldimino ester 2a to methyl cinnamate 4a in
pure MeOH (Figure S2 and Table S5 of SI). It was also found
that the Michael adduct 6 could be easily transferred into 7 and
3a at room temperature in several minutes (see Scheme 5 and
Table S5 of Supporting Information). However, the role of
6
018
DOI: 10.1021/acscatal.5b01685
ACS Catal. 2015, 5, 6016−6020

ACS Catalysis
Letter
Scheme 4. Asymmetric Cycloaddition of Methyl Cinnamate
with Glycine Aldimino Esters Catalyzed by Silver-Xing-Phos
Scheme 5. Determination of Possible Silver-Catalyzed
Michael-Initiated Cycloaddition of 1a and 2a
excess amount of Xing-Phos in the cycloaddition is unclear at
present. It is possible that the Xing-Phos could activate the
diastereoselective cycloaddition by solvent-controlled non-
covalent interaction between ligand and substrate and in
favor of regeneration of AgL (L = Xing-Phos) complex in the
2
last step. The major difference between chalcone and methyl
cinnamate is only the degree of difficulty for the formation of
silver-bound enolate intermediate in the initial step of conjugate
addition as well as the reactivity of the enolate intermediate in
the next cyclization step.
Notably, there are two possibilities for the addition of silver-
bound azomethine ylide dipole (IV) to chalcone or methyl
cinnamate. One scenario is that the intermediate IV
nucleophilically attacks electrophilically activated chalcone or
methyl cinnamate by favorable syn-fashion (path 1, Figure 2).
The other possibility is that nucleophilic addition of the
intermediate IV with different chirality to activated alkene
occurred through the path 2, which led to the formation of anti-
side product (6). However, it is a disfavored model because of
the sandwich-type steric repulsion. Thus, it is possible that an
exo approach of chalcone or methyl cinnamate controlled by
the silver catalyst arose predominately from the steric repulsion
and noncovalent interaction between the multifunctional Xing-
Phos ligand and activated olefin substrate during the intra-
molecular cyclization of the silver-Xing-Phos-bounded enolate
intermediate (Figure 1).
Figure 1. Proposed catalytic cycle of silver-catalyzed cycloaddition in
the presence of Xing-Phos: a plausible stepwise Mechanism.
Figure 2. Models for possible states to rationalize the stereoselectivity
of Michael addition of 2 to 1/4.
6
019
DOI: 10.1021/acscatal.5b01685
ACS Catal. 2015, 5, 6016−6020

ACS Catalysis
Letter
In summary, the novel silver-Xing-Phos is an effective catalyst
system for the catalytic asymmetric cycloaddition of glycine
aldimino esters with chalcones or less-reactive methyl
cinnamates to give the corresponding chalcone- or cinna-
mate-derived pyrrolidines with multiple stereogenic centers in
good yields and high diastereoselectivities (up to >98:2 dr) as
well as excellent enantioselectivities (up to 99:1 er).
Remarkably, we reported the first example of highly
enantioselective and exo-selective silver-catalyzed [3 + 2]
cycloaddition of methyl cinnamates or chalcones with glycine
aldimino esters.
(5) Longmire, J. M.; Wang, B.; Zhang, X. M. J. Am. Chem. Soc. 2002,
24, 13400−13401.
1
(
(
(
6) Recent reviews of catalytic asymmetric [3 + 2] cycloaddition
a) Adrio, J.; Carretero, J. C. Chem. Commun. 2011, 47, 6784−6794.
b) Moyano, A.; Rios, R. Chem. Rev. 2011, 111, 4703−4832. (c) Xing,
Y.; Wang, N. X. Coord. Chem. Rev. 2012, 256, 938−952. (d) Maroto, E.
E.; Izquierdo, M.; Reboredo, S.; Marco-Martínez, J.; Filippone, S.;
Martín, N. Acc. Chem. Res. 2014, 47, 2660−2670.
(7) (a) Liu, T. L.; He, Z. L.; Li, Q. H.; Tao, H. Y.; Wang, C. J. Adv.
Synth. Catal. 2011, 353, 1713−1719. (b) Liu, T. L.; He, Z. L.; Wang,
C. J. Chem. Commun. 2011, 47, 9600−9602. (c) Ma, T.; Fu, X.; Kee,
C. W.; Zong, L.; Pan, Y.; Huang, K. W.; Tan, C. H. J. Am. Chem. Soc.
2
011, 133, 2828−2831. (d) Nie, J.; Hua, M. Q.; Xiong, H. Y.; Zheng,
Y.; Ma, J. A. J. Org. Chem. 2012, 77, 4209−4216. (e) Sheshenev, A. E.;
Boltukhina, E. V.; White, A. J. P.; Hii, K. K. Angew. Chem., Int. Ed.
ASSOCIATED CONTENT
■
*
S
Supporting Information
2
013, 52, 6988−6991. (f) Vidadala, S. R.; Golz, C.; Strohmann, C.;
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.5b01685.
Daniliuc, C. G.; Waldmann, H. Angew. Chem., Int. Ed. 2015, 54, 651−
655. (g) Konno, T.; Watanabe, S.; Takahashi, T.; Tokoro, Y.;
Fukuzawa, S. -i. Org. Lett. 2013, 15, 4418−4421. (h) Oura, I.; Shimizu,
K.; Ogata, K.; Fukuzawa, S. -i. Org. Lett. 2010, 12, 1752−1755.
Experimental details, additional reaction data, and NMR
and HPLC spectra of products (PDF)
Crystallographic data file 1 (CIF)
(
8) Hernan
Carretero, J. C. Org. Lett. 2009, 11, 393−396.
9) Najera, C.; Retamosa, M.; de Gracia Retamosa, M.; Martín-
Rodríguez, M.; Sansano, J. M.; de Cozar, A.; Cossío, F. P. Eur. J. Org.
Chem. 2009, 2009, 5622−5634.
10) (a) Zhang, C.; Yu, S. B.; Hu, X. P.; Wang, D. Y.; Zheng, Z. Org.
́ ́
dez-Toribio, J.; Arrayas, R. G.; Martín-Matute, B.;
(
́
Crystallographic data file 2 (CIF)
́
AUTHOR INFORMATION
(
■
Lett. 2010, 12, 5542−5545. (b) Han, M. L.; Wang, D. Y.; Zeng, P. W.;
Corresponding Authors
E-mail: liwenxu@hznu.edu.cn.
E-mail: cgxia@licp.cas.cn.
Zheng, Z.; Hu, X. P. Tetrahedron: Asymmetry 2012, 23, 306−312.
*
(11) Tsubogo, T.; Saito, S.; Seki, K.; Yamashita, Y.; Kobayashi, S. J.
*
Am. Chem. Soc. 2008, 130, 13321−13332.
12) Bai, X. F.; Song, T.; Xu, Z.; Xia, C. G.; Huang, W. S.; Xu, L. W.
Angew. Chem., Int. Ed. 2015, 54, 5255−5259.
13) Zhang, Z. M.; Chen, P.; Li, W.; Niu, Y.; Zhao, X. L.; Zhang, J.
Angew. Chem., Int. Ed. 2014, 53, 4350−4354.
14) (a) Clayden, J. Chem. Commun. 2004, 127−135. (b) Clayden, J.;
Lund, A.; Vallverdu, L. S.; Helliwell, M. Nature 2004, 431, 966−971.
c) Dai, W. M.; Yeung, K. K. Y.; Liu, J. T.; Zhang, Y.; Williams, I. D.
Notes
(
The authors declare no competing financial interest.
(
ACKNOWLEDGMENTS
■
(
This project was supported by the National Natural Science
Founder of China (No. 21173064, 21133011, and 21472031),
and Zhejiang Provincial Natural Science Foundation of China
(
Org. Lett. 2002, 4, 1615−1618. (d) Dai, W. M.; Yeung, K. K. Y.; Wang,
Y. Tetrahedron 2004, 60, 4425−4430. (e) Clayden, J.; Kubinski, P. M.;
Sammiceli, F.; Helliwell, M.; Diorazio, L. Tetrahedron 2004, 60, 4387−
(LR14B030001) is appreciated.
4
397. (f) Philipova, I.; Stavrakov, G.; Dimitrov, V. Tetrahedron:
REFERENCES
■
Asymmetry 2012, 23, 927−930.
(
1) For representative reviews, see (a) Marti, C.; Carreira, E. M. Eur.
J. Org. Chem. 2003, 2003, 2209−2219. (b) Moonen, K.; Laureyn, I.;
Stevens, C. V. Chem. Rev. 2004, 104, 6177−6216. (c) Bellina, F.; Rossi,
R. Tetrahedron 2006, 62, 7213−7256. (d) Pandey, G.; Banerjee, P.;
́
Gadre, S. R. Chem. Rev. 2006, 106, 4484−4517. (e) Alvarez-Corral,
M.; Munoz-Dorado, M.; Rodriguez-García, I. Chem. Rev. 2008, 108,
174−3198. (f) Narayan, R.; Potowski, M.; Jia, Z. J.; Antonchick, A.
P.; Waldmann, H. Acc. Chem. Res. 2014, 47, 1296−1310.
g) Hashimoto, T.; Maruoka, K. Chem. Rev. 2015, 115, 5366−5412
and references cited therein..
2) (a) Albrecht, Ł.; Jiang, H.; Jørgensen, K. A. Angew. Chem., Int. Ed.
011, 50, 8492−8509. (b) Gonzalez, A. Z.; Benitez, D.; Tkatchouk, E.;
Goddard, W. A.; Toste, F. D. J. Am. Chem. Soc. 2011, 133, 5500−5507.
c) Jui, N. T.; Garber, J. A.; Finelli, F. G.; MacMillan, D. W. C. J. Am.
̃
3
(
(
2
́
(
Chem. Soc. 2012, 134, 11400−11403. (d) Trost, B. M.; Lam, T. M.;
Herbage, M. A. J. Am. Chem. Soc. 2013, 135, 2459−2461. (e) Sunsdahl,
B.; Smith, A. R.; Livinghouse, T. Angew. Chem., Int. Ed. 2014, 53,
1
(
4352−14356.
3) (a) Maroto, E. E.; Filippone, S.; Suar
de Cozar, A.; Cossío, F. P.; Martin, N. J. Am. Chem. Soc. 2014, 136,
05−712. (b) Wang, Z.; Luo, S.; Zhang, S.; Yang, W. L.; Liu, Y. Z.; Li,
H.; Luo, X.; Deng, W. P. Chem. - Eur. J. 2013, 19, 6739−6745.
c) Hut’ka, M.; Tsubogo, T.; Kobayashi, S. Adv. Synth. Catal. 2013,
55, 1561−1569. (d) Jing, X.; He, C.; Dong, D.; Yang, L.; Duan, C.
Angew. Chem., Int. Ed. 2012, 51, 10127−10131. (e) Hernandez-
Toribio, J.; Padilla, S.; Adrio, J.; Carretero, J. C. Angew. Chem., Int. Ed.
012, 51, 8854−8858.
4) Allway, P.; Grigg, R. Tetrahedron Lett. 1991, 32, 5817−5820.
́
́
ez, M.; Martinez-Alvarez, R.;
́
7
(
3
́
2
(
6
020
DOI: 10.1021/acscatal.5b01685
ACS Catal. 2015, 5, 6016−6020