Hafnium-catalyzed direct amide formation at room temperature

Article abstract of DOI:10.1021/acscatal.5b00385

Herein, the first example of a metal-catalyzed protocol for direct amidation of nonactivated carboxylic acids at ambient temperature (26 °C) is presented. The mild reaction conditions give rise to high yields of a range of amides in reaction times as short as 90 min, employing a commercial hafnium complex, [Hf(Cp)₂Cl₂], as catalyst. Amino acids are transformed into their corresponding amides without racemization, and the catalyst displays full selectivity for the amidation of carboxylic acids over esters. Electronic properties of the carboxylic acids were found to have a strong influence on the rate of the amidation reaction, and the need for a balanced amount of molecular sieves was observed to be highly important for optimal reaction outcome.

Full text of DOI:10.1021/acscatal.5b00385

Research Article
pubs.acs.org/acscatalysis
Hafnium-Catalyzed Direct Amide Formation at Room Temperature
Helena Lundberg and Hans Adolfsson*
Department of Organic Chemistry, The Arrhenius Laboratory, Stockholm University, SE-10691 Stockholm, Sweden
S
* Supporting Information
ABSTRACT: Herein, the first example of a metal-catalyzed protocol for direct
amidation of nonactivated carboxylic acids at ambient temperature (26 °C) is
presented. The mild reaction conditions give rise to high yields of a range of
amides in reaction times as short as 90 min, employing a commercial hafnium
complex, [Hf(Cp)2Cl2], as catalyst. Amino acids are transformed into their
corresponding amides without racemization, and the catalyst displays full
selectivity for the amidation of carboxylic acids over esters. Electronic properties
of the carboxylic acids were found to have a strong influence on the rate of the amidation reaction, and the need for a balanced
amount of molecular sieves was observed to be highly important for optimal reaction outcome.
KEYWORDS: hafnium, amidation, carboxylic acid, Lewis acids, catalysis
temperatures typically required are not suitable for highly
functionalized or sensitive substrates. Hence, catalysis is an
attractive alternative to enable atom-economical formation of
amides from carboxylic acids and amines under milder
conditions. The two main classes of synthetic catalysts for
direct amidation are boronic acids and esters along with Lewis
acidic metal complexes. In the first class, boric acid12,13 as well
as substituted arylboronic acids14−16 have been used as
catalysts. These protocols all require elevated reaction temper-
atures (generally 85−110 °C), with the exception of o-
iodophenylboronic acid17,18 and 2-furanylboronic acid,19
which are the only synthetic amidation catalysts active at 25
°C. In addition, water formed in the reaction needs to be
removed in order to obtain good yields of the amide products,
most commonly by azeotropic distillation or the use of
molecular sieves. In the second class of catalysts, complexes
based on e.g. titanium,20−22 zirconium,9,23,24 zinc,25,26 and
iron27 have been reported. Similar to the case for the boron-
based catalysts, the metal-catalyzed protocols all require
elevated temperatures and an efficient removal of water.
INTRODUCTION
■
The amide bond is found in a wide variety of chemical products
such as pharmaceuticals, agrochemicals, and polymers, and
products containing the functionality are synthesized on a
multiton scale every year. Nature uses enzymes to form amides
and peptides from carboxylic acids and amines in a catalytic
fashion, generating water as the only byproduct. However, this
green reaction has proved complicated to mimic for the
synthetic chemist and only a limited number of catalytic
protocols for the transformation is known.1,2 To date, the most
common way to form amides synthetically from carboxylic
acids is via activation of the carboxylic acid, either by a coupling
reagent3 or by the formation of an acid chloride (the Schotten−
Baumann reaction), followed by aminolysis of the activated
carboxylic derivative. This fact was clearly illustrated in a survey
performed by three major pharmaceutical companies, which
showed that 93% of all N-acylations in the synthesis of 128
drug candidates were carried out by stoichiometric activation of
the carboxylic acid.4 Despite the efficiency of these methods,
the stoichiometric activation is waste intensive and introduces
extra steps to the synthesis, along with the need for additional
purifications. These issues are circumvented by the use of
catalysis. Amide formation avoiding poor atom economy
reagents was highlighted as a key research area by the ACS
Green Chemistry Institute Pharmaceutical Roundtable in
2005,5 and the need for further development in the field is
great.
RESULTS AND DISCUSSION
■
We have previously reported on the use of titanium and
zirconium complexes in catalytic amounts for the formation of
Scheme 1. Hafnium-Catalyzed Direct Amidation of
Phenylacetic Acid and Benzylamine
Amides can be formed from several classes of starting
materials, including esters, aldehydes, alcohols, and nitriles.6
The use of carboxylic acids as starting materials in direct
amidations is generally considered to be the most challenging,
due to the possibility of salt formation between the acid and the
amine upon mixing. This problem can be overcome by
performing reactions at elevated temperature (generally around
160 °C), at which moderate to good yields of amides are
obtained. The efficiency of direct thermal amidation has been
shown to be highly substrate dependent,7−11 and the high
Received: February 20, 2015
Revised: April 10, 2015
© XXXX American Chemical Society
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ranging between 70 and 120 °C. The latter is a significant
drawback for the amidation of sensitive substrates, and we were
therefore interested to find a milder catalytic protocol which
would enable the formation of amides even at room
temperature. We set out to evaluate different Lewis acidic
metal complexes in catalytic amounts for the direct amidation
of phenylacetic acid (1a) (see the Supporting Information for a
complete list of complexes and solvents screened). The
sandwich complex bis(dicyclopentadienyl)hafnium dichloride
was identified as the most active catalyst, giving rise to 71%
isolated yield of the amide formed from phenylacetic acid (1a)
and benzylamine (2a) after 48 h in diethyl ether (Scheme 1).
Molecular sieves were added to scavenge water formed in the
reaction and to prevent the hydrolytically unstable catalyst from
decomposing.28 The amount of molecular sieves is important
for the reaction outcome, and 0.75 g of activated powdered 4 Å
sieves was found to be optimal for the transformation of 0.5
mmol of carboxylic acid into amide (vide infra).
Table 1. Evaluation of Reaction Mixture Concentration
amt of
amine
(equiv)
isolated
yield
(%)
[acid]
(M)
reaction
time (h)
entry amide carboxylic acid
1
2
3
4
5
6
3a
3a
3a
3a
3a
3a
phenylacetic
acid
0.05
0.05
0.05
0.1
1
24
24
24
24
24
24
38
56
81
86
81
84
phenylacetic
acid
1.5
2
phenylacetic
acid
phenylacetic
acid
2
phenylacetic
acid
0.2
2
phenylacetic
acid
0.4
2
The amount of amine 2a was found to be important for the
reaction outcome. Increasing the amount of amine resulted in a
significant improvement of the yield of amide product 3a after
24 h reaction time (Table 1, entries 1−3). The standard
conditions were hence set to a 2:1 molar ratio of amine to
carboxylic acid. Optimizing the reaction conditions with regard
to the concentration of the reaction mixture, we found
significant differences between structurally different acids. In
the amidation of phenylacetic acid (1a) with benzylamine, the
acid concentration had little effect on the outcome of the
7
3b
3b
3b
3b
3b
3b
valeric acid
valeric acid
valeric acid
valeric acid
valeric acid
valeric acid
0.05
0.1
2
2
2
2
2
4
48
48
24
24
96
48
17
76
80
85
63
75
8
9
0.2
10
11
12
0.4
0.05
0.05
primary, secondary, and tertiary amides.22−24 A range of
differently substituted amides were formed in good yields, using
a catalyst loading of 2−20 mol % and a reaction temperature
Table 2. Substrate Scope
entry
amide
acid
amine
amt of Hf (mol %)
[acid] (M)
isolated yield (%)
a
1
3a
3b
3c
3d
3e
3f
phenylacetic acid
valeric acid
benzylamine
benzylamine
benzylamine
benzylamine
benzylamine
benzylamine
benzylamine
benzylamine
benzylamine
benzylamine
benzylamine
benzylamine
benzylamine
benzylamine
benzylamine
pentylamine
5
5
0.1
87
2
0.4
76
83
88
3
Boc-Gly-OH
5
0.1
4
Z-Gly-OH
5
0.2
b
5
Boc-Ala-OH
5
0.2
87
b
6
Boc-Met-OH
5
0.1
96
b
7
3g
3h
3i
Boc-Leu-OH
5
0.1
81
b
8
Boc-Pro-OH
5
0.1
88
b
9
Boc-Trp-OH
5
0.4
73
c
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
3j
cyclohexanecarboxylic acid
benzoic acid
10
10
10
10
10
10
5
0.4
76/87
c
3k
3l
0.4
52/80
c
cinnamic acid
0.4
60
3m
3n
3o
3p
3q
3r
trichloroacetic acid
dichloroacetic acid
chloroacetic acid
phenylacetic acid
phenylacetic acid
phenylacetic acid
phenylacetic acid
phenylacetic acid
phenylacetic acid
phenylacetic acid
phenylacetic acid
phenylacetic acid
0.05
0.05
0.05
0.1
76
d
92
79
90
90
79
83
dodecylamine
5
0.1
cyclohexylamine
10
10
10
10
10
10
10
0.2
3s
3t
2-thiophenemethylamine
(R)-phenylethylamine
1,4-dioxa-8-azaspiro[4.5]decane
4-methoxybenzylamine
4-chlorobenzylamine
4-fluorobenzylamine
0.05
0.4
b
78
3u
3v
3w
3x
0.1
81
82
86
95
0.05
0.05
0.05
a
b
c
d
24 h reaction time. >99% ee determined by HPLC (AD column 90/10 i-hex/iPrOH). 4 equiv of amine. 90 min reaction time.
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Scheme 2. Selective Monoacylation of a Diamine: Aromatic Amine Remains Unchanged
Figure 1. Steric hindrance in the amine affecting the reaction outcome. Reaction conditions: phenylacetic acid (0.5 mmol), amine (1.0 mmol),
Hf(Cp)2Cl2 (10 mol %), Et2O (10 mL), 4 Å MS (powder, 0.75 g), 26 °C, 48 h.
Figure 2. Steric hindrance in the β position in the carboxylic side chain of the amino acid heavily affecting the reaction outcome. Reaction
conditions: carboxylic acid (0.5 mmol), benzylamine (1.0 mmol), Hf(Cp)2Cl2 (10 mol %), Et2O (10 mL), 4 Å MS (powder, 0.75 g), 26 °C, 48 h.
Scheme 3. Ester Formation Catalyzed by Hf(IV) at High Reaction Temperatures (A) but Not at Room Temperature (B)
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flask and was isolated in 77% yield (1.2 g) after extractive
workup and column chromatography.
Scheme 4. Competition Experiment between an Ester and a
Carboxylic Acid Showing That the Catalyst Is Fully Selective
toward Amidation of the Acid
Anilines are poor nucleophiles in comparison to aliphatic
amines and need high reaction temperatures in order to work
successfully in direct amidations.15,20,29 All attempts to use even
the electron-rich p-anisidine in the amidation of phenylacetic
acid 1a failed using hafnium catalysis. However, the poor
reactivity of anilines under optimized reaction conditions
enabled the selective acylation of the nonaromatic amine of
2-aminobenzylamine, as illustrated in Scheme 2. In contrast,
aliphatic diamines (e.g. p-xylylenediamine and 1,3-diaminopro-
pane) are incompatible with the reaction conditions and result
in no product formation when they are mixed with phenylacetic
acid. In addition, ammonia (NH3 in THF, 0.4 M) failed to form
the corresponding phenylacetamide.
Although the cyclic amine 1,4-dioxa-8-azaspiro[4.5]decane
works well in the amidation reaction (Table 2, entry 21),
secondary acyclic amines fail as coupling partners, which is in
line with what has previously been reported for boronic acid
catalysis at room temperature.17,18 The reason for this is not
clear, but the increased steric hindrance might play a role. The
results from the condensation of a series of benzylamines with
phenylacetic acid under identical reaction conditions are shown
in Figure 1. As can be seen, increasing the size of the
substituents on the amine lead to a dramatic drop in isolated
amide yield. Whereas optimizations of the reaction conditions
could circumvent this steric effect and enable a reasonable yield
for amide 3t (Table 2, entry 20), other amines such as the
secondary N-methylbenzylamine remained unreactive and
failed to form the desired amide 3ab.
Increasing the steric hindrance in a series of carboxylic acids
did not affect the reaction outcome in the same way (Figure 2).
Interestingly, a significant change in reactivity was only seen
when the β position of the amino acid was doubly substituted,
as in the case of Boc-protected valine (3ad) and isoleucine
(3ae), and in these cases the reaction did not proceed at all.
Despite various attempts to optimize the reaction conditions
for the formation of amide 3ad, no product was formed. The
failure of this substrate to react can possibly be explained by
steric interactions, which hinder the acid to adopt an
appropriate binding mode to hafnium in order to enable
amidation.
In addition to direct amidation of carboxylic acids, group IV
metal complexes are known to catalyze other transformations of
carboxylic derivatives. For example, Yamamoto and co-workers
have shown that HfCl4·2THF and Hf(OtBu)4 catalyze the
esterification of carboxylic acids with alcohols in refluxing
toluene with excellent yields (Scheme 3, reaction A).30 In
addition, Collins et al. recently demonstrated that both intra-
and intermolecular esterifications can be performed using
Hf(OTf)4 as a catalyst in refluxing toluene.31 However, when
we used the hafnocene complex under the reaction conditions
optimized for amidation, no esterification between phenylacetic
acid and ethanol was observed. Instead, the catalyst was
reaction (Table 1, entries 3−6). However, in the corresponding
amidation of valeric acid, the acid concentration effectively
influenced the isolated yield of amide 3b (Table 1, entries 7−
10). It was also found that a high yield of amide 3b could be
obtained at an acid concentration of 0.05 M when the reaction
time was prolonged (Table 1, entry 11), as well as when a larger
excess of the amine was used (Table 1, entry 12).
With this knowledge at hand, we investigated the scope of
the hafnium-catalyzed direct amidation protocol at room
temperature. The reaction conditions were optimized for each
substrate with respect to catalyst loading and molar
concentration. A reaction time of 48 h was chosen as standard
to allow even slow-reacting substrates to reach a high
conversion. It should be noted, however, that several substrates
are fully converted long before this time, some already after 90
min (Table 2, entry 14). Moreover, it was found that the
catalyst loading could be decreased from 10 to 5 mol % for a
number of substrates. A further decrease in catalyst loading
resulted in lower isolated yields, and a control experiment
without the hafnium complex present did not give rise to any
product. Vigorous stirring and freshly activated molecular sieves
were in all cases crucial for an efficient reaction outcome. The
substrate evaluation presented in Table 2 shows that several
substrates work well under the catalytic conditions. No
racemization was detected in the amino acid amide products
(Table 2, entries 5−9) after the reaction. Protection groups
such as Boc (tert-butylcarbonyloxy) (Table 2, entries 3 and 5−
9) and Cbz (benzylcarbonyloxy) (Table 2, entry 4) are stable
under the reaction conditions. Benzoic acid and cinnamic acid
were converted into their corresponding benzylamides in
moderate to good yields (Table 2, entries 11 and 12). These
substrates are particularly interesting, since conjugated acids are
known to react poorly in catalytic direct amidation, unless
elevated reaction temperatures are used.15,18,22 In addition,
sterically demanding carboxylic acids and amines were
successfully transformed into the corresponding amides
(Table 2, entries 10 and 18). Good yields were also obtained
using both electron-rich and electron-poor benzylamines
(Table 2, entries 22−24) as well as electron-poor carboxylic
acids (Table 2, entries 13−15). Of the latter, amide 3o is of
special interest, since the α-halo substituent can easily serve as a
handle for further synthetic manipulations. In addition, amide
3e was synthesized on a 5 mmol scale in a round-bottomed
Scheme 5. Amidation of Phenylacetic Acid with the Amino Ester Methyl (4-Methylamino)benzoate
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Figure 3. Reaction profiles for four different carboxylic acids with different pKas. Reaction conditions: carboxylic acid (0.5 mmol), benzylamine (1.0
1
mmol), Hf(Cp)2Cl2 (10 mol %), Et2O (10 mL), 4 Å MS (powder, 0.75 g), 26 °C, 48 h. Yields were determined by H NMR.
Figure 4. Reaction profiles of phenylacetic acid reacted with differently substituted benzylamines. Reaction conditions: phenylacetic acid (0.5 mmol),
1
amine (1.0 mmol), Hf(Cp)2Cl2 (10 mol %), Et2O (10 mL), 4 Å MS (powder, 0.75 g), 26 °C, 48 h. Yields determined by H NMR.
inhibited by the presence of alcohol, which was further
demonstrated in a competition experiment between benzyl-
amine and benzyl alcohol, resulting in only 7% product with full
chemoselectivity for the amide 3a (Scheme 3, reaction B).
Interestingly, this reactivity is the opposite of that described for
Hf(OTf)4, which was found to be fully selective for
esterification over amidation.31 The low tolerance of Hf-
(Cp)2Cl2 toward free hydroxyl groups under the reaction
conditions explain why Boc-Ser-OH, Boc-Tyr-OH, and
mandelic acid failed to form their correspondning benzyla-
mides.
Furthermore, group IV metal complexes were reported by
Porco and co-workers to catalyze the aminolysis of esters to
form amides.32 The authors present results with up to 95%
yield of the amide product at room temperature in THF, using
10 mol % of Zr(OtBu)4 and 10 mol % of 1-hydroxy-1-
azabenzotriazole (HOAt) as catalysts. However, the ester
functionality was found to be inert under the hafnium-catalyzed
conditions and only starting material was recovered when ethyl
phenylacetate was mixed with 2 equiv of benzylamine under the
standard conditions. This chemoselectivity was further
confirmed in a competitive experiment between ethyl phenyl-
acetate, p-tolylacetic acid, and benzylamine, which resulted in
full selectivity for the amidation of the carboxylic acid and 78%
isolated yield of amide 3af (Scheme 4).
The inertness of esters under the hafnium-catalyzed
amidation conditions suggested that this functionality can act
as a protecting group for additional carboxylic acid groups
present in the substrates. Gratifyingly, the concept proved
successful, as demonstrated in the reaction between methyl 4-
(aminomethyl)benzoate and phenylacetic acid, which resulted
in 84% isolated yield of amide 3ag with the ester functionality
untouched (Scheme 5).
It has previously been demonstrated that the pKa of the
carboxylic acid is of importance for the reaction outcome for
both uncatalyzed and catalyzed direct amidations.8 Comparing
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Figure 5. Dependence of the reaction outcome on the amount of molecular sieves present in the reaction mixture. Reaction conditions: phenylacetic
acid (0.5 mmol), benzylamine (1.0 mmol), Hf(Cp)2Cl2 (10 mol %), Et2O (10 mL), 4 Å MS (powder, 0.75 g), 26 °C, 48 h.
three carboxylic acids with different pKas, Whiting and co-
workers showed that bromoacetic acid (pKa 2.69 in water) did
not react to form amide products under either thermal or boric
or o-iodophenylboronic acid catalysis during 48 h of reaction
time in toluene (120 or 50 °C), whereas phenylbutyric acid
(pKa 4.76) reacted to form amides with a series of amines.
Benzoic acid (pKa 4.19) demonstrated a reactivity between the
other two. These results were in contrast to those reported by
Loupy and co-workers, who did not find any strong correlations
between reactivity and pKa for the thermal direct amidation at
150 °C under microwave irradiation.33 Similarly, no correlation
between pKa and product yield was seen in the hafnium-
catalyzed amidation reaction. As demonstrated in Table 2
(entries 1 and 13−15), high yields of the corresponding benzyl
amides were obtained after 90 min to 48 h from four different
carboxylic acids with pKas (H2O) ranging between 0.7 and 4.3,
indicating that the pKa does not have a significant effect on the
thermodynamics of the reaction. However, the pKa of the
carboxylic acid is of importance for the reaction kinetics. As can
be seen in Figure 3, the three chloride-containing acids form
amide products at a higher rate in comparison to phenylacetic
acid during the first 90 min. Interestingly, dichloroacetic acid is
the fastest to react and forms the corresponding benzyl amide
in an NMR yield of around 90% after only 90 min, a yield
confirmed by workup and isolation of the product (Table 2,
entry 14). This observation suggests that there are two
counteracting effects at play, which need to be balanced. On
the one hand, a high electron-withdrawing capacity of the
substituents on the carboxylic acid makes the carbonyl carbon
more electrophilic and hence more susceptible for attack from
the amine. On the other hand, the high acidity of strong acids is
likely to protonate the amine to a greater extent, thereby
effectively decreasing the concentration of the nucleophile.
The electronic nature of the amine also affects the reaction
outcome. As previously mentioned, anilines are not suitable as
coupling partners, likely due to the low electron density of the
aromatic nitrogen. On comparison of a series of differently
substituted benzylamines, both electron-rich and electron-poor
analogues as reaction partners with phenylacetic acid resulted in
high amide yields (Table 2, entries 1 and 22−24). Analogous to
the results for the carboxylic acids, the thermodynamics of the
reaction is not strongly affected by the pKa of the amines.
However, comparing the reaction profiles (Figure 4), one can
see that the initial rates of the halo-substituted amines are
somewhat lower and display a more pronounced sigmoidal
shape in comparison to the two more electron rich amines.
Interestingly, the most electron rich species, p-methoxybenzyl-
amine, is not significantly faster in comparison to the
unsubstituted benzylamine, which might be explained by the
relatively small difference in basicities among the examined
amines (Figure 4).
Furthermore, water was found to play an important role in
the hafnium-catalyzed direct amidation of carboxylic acids.
Figure 5 shows a chart of the isolated yield of amide 3a, where
the reactions were run with different amounts of powdered
molecular sieves, all other parameters being equal. As can be
seen, a maximum in isolated yield after 48 h is reached at 0.75 g
of molecular sieves for a reaction of 0.5 mmol of carboxylic
acid. A decrease as well as an increase in the amount of
molecular sieves resulted in a lower product yield. This
observation points toward a dual role of water in the reaction.
On the one hand, group IV metal complexes are known to be
hydrolytically unstable28 and molecular sieves are likely
required in the reaction vessel to prevent water formed in the
condensation to decompose the catalyst and/or reactive
intermediates in the catalytic cycle. On the other hand, the
presence of some water seems to be needed for an efficient
reaction, possibly by partially hydrolyzing the metal complex to
form the active catalyst in a fashion similar to what was
proposed by Shteinberg and co-workers for a titanium-based
amidation catalyst.34,35 Hall and co-workers have recently
suggested that molecular sieves might act as a reversible
reservoir of water, which they speculate enhances the catalytic
activity of o-iodophenylboronic acid in direct amidations at
room temperature,18 and it is not unlikely that the molecular
sieves play an analogous role in the hafnium-catalyzed
amidation reaction.
The nature of direct amidation has previously been described
to be governed by several different factors such as stability of
the ammonium carboxylate salts, the pKas of the carboxylic
acids, and the balance between amine nucleophilicity and
basicity.8 These factors, together with the concentration
dependence observed for several substrates (vide supra),
point toward a complex sequence of coupled equilibria
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amount of water appears to play an important role in the
outcome of the amidation reaction. Mechanistic studies of
group IV metal catalyzed direct amidation are ongoing, and the
results from these studies will be communicated in due time.
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CONCLUSIONS
■
The development of mild and environmentally friendly
processes for the effective formation of amides is of great
interest to chemists in both industry and academia. The mild
method presented herein is the first example of a hafnium-
catalyzed protocol for the direct amidation of nonactivated
carboxylic acids. The method gives rise to high yields of a range
of differently substituted amides in reaction times as short as 90
min. Additionally, the protocol is the first example of a metal-
catalyzed amidation process performed at ambient temperature,
and the full conservation of enantiomeric purity of the
asymmetric starting materials is a testimony to the mildness
of the reaction conditions. The catalyst is highly selective
toward amidation of carboxylic acids over esters, enabling the
latter to function as protecting groups for additional carboxylic
acid moieties present in the substrates. The reaction outcome
was shown to be affected by the amount of activated molecular
sieves present and the concentration of the reaction mixture, as
well as the steric and electronic nature of the substrates.
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ASSOCIATED CONTENT
* Supporting Information
The following file is available free of charge on the ACS
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S
Publications website at DOI: 10.1021/acscatal.5b00385.
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AUTHOR INFORMATION
Corresponding Author
*E-mail for H.A.: hansa@organ.su.se.
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́ ́
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Notes
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The authors declare no competing financial interest.
(33) Perreux, L.; Loupy, A.; Volatron, F. Tetrahedron 2002, 58,
2155−2162.
ACKNOWLEDGMENTS
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The K & A Wallenberg Foundation and the Swedish Research
Council are gratefully acknowledged for financial support. We
thank Drs. Fredrik Tinnis and Tobias Ankner for fruitful
discussions.
(34) Shteinberg, L. Y.; Shein, S. M.; Mishchenko, S. E. Zh. Org. Khim.
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DOI: 10.1021/acscatal.5b00385
ACS Catal. 2015, 5, 3271−3277