1,3-Dipolar Cycloadditions of Stabilized Azomethine Ylides and Electrophilic Alkenes Mediated by a Recyclable TSIL·AgOAc Catalyst

Article abstract of DOI:10.1002/ejoc.201900724

The synthesis of a task-specific ionic liquid (TSIL) based on a BMIM ionic liquid phosphane is achieved by modification of the original procedure. It is used as a ligand with AgOAc, and the resulting complex is fully characterized. This catalytic system promotes the 1,3-dipolar cycloaddition of stabilized azomethine ylides and electrophilic alkenes using a green methodology. The green character was analyzed using an overall set of parameters finding the optimal conditions and individualized separation/purification methods for each cycloadduct. The determination of the most important green metrics is reported in this survey, demonstrating the value of this methodology on a laboratory scale or even at an industrial level.

Full text of DOI:10.1002/ejoc.201900724

A Journal of
Accepted Article
Title: 1,3-Dipolar cycloadditions of stabilized azomethine ylides and
electrophilic alkenes mediated by a recyclable TSIL·AgOAc
catalyst
Authors: Jose Miguel Sansano, David Lledó, and Guillermo Grindlay
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201900724
Link to VoR: http://dx.doi.org/10.1002/ejoc.201900724
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10.1002/ejoc.201900724
European Journal of Organic Chemistry
COMMUNICATION
1,3-Dipolar cycloadditions of stabilized azomethine ylides and
electrophilic alkenes mediated by a recyclable TSIL·AgOAc
catalyst
David Lledó,[a] Guillermo Grindlay,[a] and José Miguel Sansano.*[b]
12 13 ]
Abstract: The synthesis of a task-specific ionic liquid (TSIL) based on
a BMIM-ionic liquid phosphane is achieved by modification of the
original procedure. It is used as ligand with AgOAc, and the resulting
complex is fully characterized. This catalytic system promotes the 1,3-
dipolar cycloaddition of stabilized azomethine ylides and electrophilic
alkenes using a green methodology. The green charater is analized
in an overall set of parameters finding the optimal conditions and
individualized separation/purification methods for each cycloadduct.
The determination of the most important green metrics is reported in
this survey, demonstrating the value of this methodology in laboratory
scale or even at an industrial level.
efficient in a multicomponent fashion.[ 11
Even reactions
where products were isolated in good yield by simple filtration[14]
can be considered a good green approach. 2,2,2-Trifluoroethanol
as a modern green solvent,[15] PEG-400,[16] ethyl lactate as a bio
based green solvent,[17] and ethanol,[18] were also considered new
green alternatives.
Concerning solvents, easily separable/extractable ionic liquids
were also involved in 1,3-DC of azomethine ylides generated by
decarboxylative route of iminium salts. The first [3+2]
cycloaddition of azomethine ylides employing an ionic liquid
medium allowed to prepare functionalized spiropyrrolidines.[19]
Other
examples
using
1-butyl-3-methylimidazolium
tetrafluoroborate, [BMIM]BF4,[20,21] or [BMIM]Br[22] as ionic liquids,
or ionic liquid-multicomponent-MW combinations,[ 23 , 24 ] were
described. In other contributions, the catalytic effect of the ionic
liquid was demonstrated, due to the interaction of the heterocyclic
hydrogen bonded at the 2-position of [BMIM]Br with the carbonyl
group of the conjugated ketone.[ 25 , 26 ] In the same line,
spiro[pyrrolidine-benzofuran-3-one] were isolated in high yields
(73-99 %) from the corresponding imino esters and cyclic
unsaturated ketones using ionic liquids as catalysts.[27]
Introduction
The 1,3-dipolar cycloadditions (1,3-DCs) involving azomethine
ylides and electron-poor alkenes are characterized by their total
atom economy.[1,2] In addition, high stereospecificity, regio-, and
diastereoselectivity are very frequently achieved due the high
control of the geometry of the dipole, as well as the preferential
dipolarophile approach. These features convert this cycloaddition
in a very attractive subject for implementing the green chemistry
concepts.[3]
The 1,3-DCs performed with azomethine ylides generated via
decarboxylative route of iminium salts have been very frequently
associated to green processes. For example, heterogeneous
graphene oxide,[ 4 ] zinc oxide nanoparticles,[ 5 ] heterogeneous
silver nanoparticle,[6] bentonite,[7] were used as recyclable catalyst.
The ability to run these reactions using multicomponent versions[8]
and the employment of more efficient microwave-assisted
heating,[9,10] were also adapted. Sequential thermal (MW) [3+2]
cycloaddition involving imino esters generated in situ was also
The employment of
a
chiral framework, containing an
imidazolium-group-tagged planar chiral ferrocenyl oxazoline
phosphane (FimiOAXP) ligand was designed. Here, the absence
of an acidic hydrogen at the 2-position of the imidazolium ring
avoided the formation of a copper(I) carbene, generating a
conventional chiral ligand·copper(I) complex instead. In the
presence of a weak base, the products were obtained, from
iminoesters and several dipolarophiles, in satisfactory yields and
with excellent enantioselectivity (up to 99% ee).[28] These are
examples of the so called task-specific ionic liquid (TSIL), which
are designed and prepared for special reactions.
In this work, the synthesis and use of a (TSIL) BMIM-ionic liquid-
phosphane·AgI, as recoverable catalyst, in the 1,3-DC of
stabilized azomethine ylides and electrophilic alkenes is
performed. We design an ambitious environmental friendly
cycloaddition where, multicomponent process, room temperature
(energetic efficiency), green solvent, inherent atom economy with
no waste chemicals, high to excellent both diastereoselectivity
and chemical yield, separation/isolation and purification of the
cycloadduct in a simple and safer way, are the parameters to
control.
[a]
Mr. D. Lledó and Prof. G. Grindlay.
University of Alicante, Department of Analytical Chemistry, Nutrition
and Food Sciences, PO Box 99, 03080 Alicante, Spain.
https://cvnet.cpd.ua.es/curriculum-breve/es/lledo-garcia-david/32643
https://cvnet.cpd.ua.es/curriculum-breve/es/grindlay-lledo-
guillermo/9734
[b]
Prof. J. M. Sansano.
University of Alicante, Department of Organic Chemistry, Centro de
Innovación en Química Avanzada (ORFEO-CINQA) and Instituto de
Síntesis Orgánica (ISO), PO Box 99, 03080 Alicante, Spain.
https://cvnet.cpd.ua.es/curriculum-breve/es/sansano-gil-jose-
miguel/10547
Results and Discussion
Corresponding author: jmsansano@ua.es
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
In the enantioselective 1,3-DC of imino esters and maleimides,
our group reported the application of a very stable (even to light
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10.1002/ejoc.201900724
European Journal of Organic Chemistry
COMMUNICATION
exposure) chiral Binap·AgClO4, which was reused up to five
catalytic cycles without apparent decrement of efficiency.[29] Due
to the interaction of the phosphino group with the silver cation and
continuing with the development of new catalysts for this 1,3-DC,
we envisaged ligand 1[30] as a good candidate for this purpose.[31]
1-Butyl-3-methylimidazolium hexafluorophosphate [BMIM]PF6
was dissolved in extra-dry dichloromethane and treated with n-
completely analogous and reproducible (Table 1, entries 12 and
13). The third experiment with the same catalyst gave a lower,
although important, yield (85%) in very good dr, whilst the 4th cycle
was not so productive (Table 1, entries 14 and 15, respectively).
Table 1. Optimization of the 1,3-DC between 2a and NMM promoted by
AgOAc·1 complex.
Buli
at
-78
°C
followed
by
the
addition
of
chlorodiphenylphosphane giving final product 1 in 82% yield
(Scheme 1).
AgOAc·1
Solvent
(mL)
Base
t (h)
Yield
(%)[a]
88
endo/exo[b]
Scheme 1. Synthesis of ionic liquid ligand 1.
(mol%)
1
4
PhMe (4)
CH2Cl2 (4)
Et2O
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
----
17
17
17
17
17
17
17
17
4
88:12
85:15
88:12
85:15
85:15
88:12
88:12
83:17
95:5
This ligand 1 was immediately combined with a silver salt
and tested in the 1,3-DC between imino ester 2a and N-
methylmaleimide (NMM) as benchmark reaction (Scheme 2, and
Table 1). Too many parameters and reagents must be tested but
we initially evaluated a reaction with 0.15 mmol of both 2a and
NMM in toluene (3 mL) and triethylamine (1 equiv), together with
a solution of 20 mg BMIM, which contained AgOAc (1 mg, 0.006
mmoL) and 1 (3.12mg, 0.007 mmoL). The mixture of BMIM,
AgOAc and 1, was stirred for 1 hour previously, in the absence of
light, before the addition of the other two components (Table 1).
The employment of ethanol afforded higher yields than the
reactions performed in dichloromethane, diethyl ether, THF,
acetonitrile and isopropyl alcohol although all of them afforded
similar diastereoselectivities (Table 1, entries 2-7). Finally,
ethanol was selected because the reaction afforded a very clean
1H NMR spectra and it is considered a valid green solvent. The
presence of an added base as triethylamine was not necessary
(Table 1, entry 8). The acetate anion could efficiently act as base
and at this point, other basic silver salts as carbonate, benzoate
or oxide were tested obtaining both lower chemical yields and
diastereoselectivities of the final cycloadduct endo-3a (data not
shown in Table 1).[32] The most surprising result was obtained
during the optimization of the amounts of catalyst, solvent, and
time using the Plackett-Burman experimental design (see SI).[33]
When the reduction of the catalyst loading (1 mol%) was done the
reaction was completed in four hours almost quantitatively and
with a 95:5 endo:exo ratio (Table 1, entry 9). The optimal amount
of ethanol was 4 mL for the reaction of 0.2 mmol of substrate 2a
(Table 1, entries 9 and 10). Using the best conditions described
before, the reaction was run in the absence of AgOAc, that means,
using both ionic liquids the process did not take place
demonstrating the scarce catalytic activity of these species in this
1,3-DC between imino ester 2a and NMM (Table 1, entry 11).
For the purification of compound endo-3a, the mixture was
refluxed at 78 °C for 30 min and the cycloadduct recrystallized at
4 °C taking advantage of the high solubility of these ionic liquids
in ethanol after refluxing for 30 minutes. After this treatment, no
epimerization of product endo-3a occurred. Next, this solution,
containing BMIM, AgOAc and 1, was submitted to several
identical processes, finding that the second process was
2
4
80
3
4
81
4
4
THF
86
5
4
MeCN
88
6
4
PriOH (4)
EtOH (4)
EtOH (4)
EtOH (4)
EtOH (0.5)
EtOH (4)
78
7
4
90
8
4
86
9
1
----
99
10
11
1
---c
----
4
82
88:5
----
4
----
----
12
13
14
15
1 (1st)
EtOH (4)
EtOH (4)
EtOH (4)
EtOH (4)
----
----
----
----
4
4
4
4
99
99
85
65
95:5
95:5
95:5
90:10
1 (2nd
)
1 (3rd)
1 (4th)
[a] Isolated yield after the purification step. [b] Determined by 1H NMR of the
isolated compound. [c] Only 20 mg BMIM and 3.12 mg of 1 were added.
The catalytic complex AgOAc·1 was also characterized (Figure 1)
obtaining the most representative data from the 31P NMR
experiments performed in deuterated chloroform. Meanwhile free
phosphane 1 gave a signal at -26.5 ppm the silver complex
spectra showed a peak at 22.6 ppm. The clear singlets observed
in solution revealed that there is not the existence of different
aggregates of the silver complex. In addition, the MS-ESI spectra
also supported this fact because m/z signals at 430 and 432
(cationic forms) corresponded to the existence of monomeric
species (see SI).
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COMMUNICATION
AgOAc·1
T
Yield
R1
R2
endo-3
endo/exo[b]
(%)[a]
(mol%)
(°C)
25
25
78
78
1
2
3
4
1
1
4
4
H
Me
Ph
Me
Me
3a[c]
3b[d]
3c[e]
3d[e]
96
79
66
92
94:6
H
93:7
Me
Bn
93:7
>99:<1
[a] Isolated yield after the purification step. [b] Determined by 1H NMR of
the isolated compound. [c] The reaction was completed in 4 h. For the
purification, the mixture was refluxed at 78 °C for 30 min and the
cycloadduct recrystallized at 4 °C. [d] The reaction was completed in 17 h.
For the purification, the mixture was refluxed at 78ºC for 30 min and the
cycloadduct recrystallized at 4 °C. [e] The reaction was completed in 17 h.
For the purification, EtOH was evaporated and MeOH was added allowing
the recrystallization at -25 °C.
Figure 1. 31P NMR shifts of ionic liquid 1 and complex 1·AgOAc.
Other different representative dipolarophiles were assessed.
For example, methyl acrylate reacted at room temperature (rt) in
isopropyl alcohol (also considered as an appropriate green
solvent) affording endo-3e in high yield and excellent
diastereomeric ratio (>99:<1, Table 3, entry 1). The bulkier tert-
The multicomponent version of this transformation was
carried out using the same reaction conditions but with
benzaldehyde and glycine methyl ester hydrochloride instead of
the preformed imino ester 2a. The generation of the free
aminoester was attempted using trimethylamine in the presence
of the catalysts and the dipolarophile, but the yield obtained of
the cycloadduct endo-3a was very low (<34%), even increasing
the amount of complex 1·AgOAc up to a 4 mol%. Besides, the
reaction of imine 2a and NMM did not occurred neither in the
absence of solvents nor in the presence of water.
Continuing with the reaction between several dipole
precursors 2 and maleimides we found very interesting results. N-
Phenymaleimide reacted as NMM affording a lower yield (79%)
and similar diastereoselectivity (Table 2, compare entries 1 and
2). In both reactions, the purification of cycloadducts endo-3a and
endo-3b was achieved by refluxing the mixture at 78 °C for 30 min,
followed by recrystallization at 4 °C. α- Substituted imino esters,
derived from alanine (2b) and phenylalanine (2c), needed to react
in a higher temperature (refluxing EtOH) and using higher amount
of catalyst to yield compounds endo-3c and endo-3d in 66 and
92% yield, respectively (Table 2, entries 3 and 4). So, the reaction
demonstrated to be sensitive to this substitution due to steric
reasons. The diastereoselectivities were very high, especially in
the case of the reaction with the phenylalanine derivative (>99:<1).
The purification of these two last heterocycles was accomplished
by evaporating the EtOH followed by the addition of the minimum
amount of MeOH, which dissolved perfectly the ionic liquid and
allowed the selective recrystallization of the corresponding
cycloadducts at -25 °C (see, Table 2).
butyl acrylate afforded
a lower yield (76%) and good
diastereoselectivity (92:8) using the same reaction conditions
(Table 3, entry 2). The reaction with dimethyl fumatate required a
higher temperature (70 °C) to proceed (68% isolated yield). This
temperature was the cause of
a
decrement of the
diastereoselection (Table 3, entry 3). Chalcone afforded major
endo-3h cycloadduct in moderate isolated yield and with a notable
diastereoselectivity (Table 3, entry 4). However, an opposite
approach of the 1,2-bis(phenylsulfonyl)ethylene (BPSE) to the W-
shape dipole took place, giving exclusively the exo-3j cycloadduct
(see Figure 2) in both high yield and diastereoselectivity (Table 3,
entry 5).
Another key detail of this full process involving 1,3-DC was
the purification protocol (see Table 3). Each cycloadduct could be
efficiently separated and purified using environmentally benign
solvents and recrystallization tasks. In the most cases, the
precipitation/crystallization of prolinates preferentially occurred
and the washing step solubilized the remaining ionic liquid (Table
3, entries 1, 2 and 4). However, in entry 3 of the Table 3, the
complete BMIM precipitation took place at -25 °C obtaining pure
compound endo-3g. The purification of the disulfone exo-3j was
very difficult, but finally the solubility of the ionic liquid mixture in
a 5:5:1 AcOEt:water:MeCN solution allowed to obtain pure
cycloadduct (Table 3, entry 5).
Table 3. Scope of the 1,3-DC between imino ester 2a and other dipolarophiles
promoted by AgOAc·1 complex.
Table 2. Scope of the 1,3-DC between imino esters 2 and maleimides promoted
by AgOAc·1 complex.
Z
R1
Solvent
3
Yield
(%)[a]
82
endo/exo[b]
1
2
CO2Me
CO2But
H
H
PriOH
PriOH
3e[c]
3f[d]
>99:<1
92:8
76
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COMMUNICATION
EtOH
3
CO2Me
CO2Me
3g[e]
68
75:25
70 °C
EtOH
EtOH
4
5
COPh
Ph
3h[f]
3j[g]
45
89
89:11
9:91
SO2Ph
SO2Ph
[a] Isolated yield after the purification step. [b] Determined by 1H NMR of the
isolated compound. [c] For the purification, the solvent was evaporated and
the crude product was washed with n-PrOH. [d] For the purification, the
solvent was evaporated the reaction crude washed with tert-butyl methyl
ether. [e] For the purification the crude mixture was stored at -25°C overnight
(BMIM precipitation occurred). [f] For the purification, the crude mixture was
refluxed for 30 min and then recrystallization at 4 °C overnight. [g] For the
purification, the solvent was evaporated, then AcOEt (5 mL) was added and
the organic phase washed with water (5mL) + CH3CN (1 mL).
The relative stereochemistry of all compounds was
1
assigned by comparison of the H NMR and 13C NMR signals of
the know compound described in the literature.[29] This major
endo-cycloadduct, obtained with a 2,5-cis-arrangement, is the
result of the formation of the most stable W-shape dipole whose
geometry was controlled by this new ionic liquid phosphane
1·silver(I) complex. Then, the dipolarophile approached via an
endo-fashion, that means, the electron-withdrawing group
interacted with the metal center (Figure 2). However,
stereoelectronic
reasons
can
justify
the
opposite
diastereoselection in the reaction with BPSE. This dipolarophile
could interact with one of the phenyl groups bonded to the
phosphorous atom, which facilitated the exo-approach (Figure 2).
The hexafluorophosphate anion was not crucial in the explanation
of this reverse diastereoselectivity because is placed in a long
distance of the metal center.
Figure 2. Endo- and exo-approaches onto the azomethine ylide with W-
geometry. Basic model of minimal energy justifying the exo-preference of BPSE.
For a comprehensive evaluation of the greenness and the
eco-scale level of this 1,3-DC the reactions of the formation of
endo-3a (A) and exo-3j (B) cycloadducts, following the Schemes
of Tables 2 and 3, will be selected for the full analysis. Also, the
whole process (C) to prepare endo-3a (supposing a 96% yield)
from glycine methyl ester hydrochloride, benzaldehyde and
triethylamine will be assessed (Scheme 2). A comparison
between (A) and the analogous processes run in the presence of
toluene (Tol) and dichloromethane (Dcm) will be also show in
Table 4. Thus, green metrics calculations such as atom economy
(AE), atom efficiency (AEf), carbon efficiency (CE), reaction mass
efficiency (RME), optimum efficiency (OE), process mass
intensity (PMI), E-factor (E), solvent intensity (SI) water
intensity[34] and eco-scale tool (E-S) (Table 4)[35] were performed.
The AE and CE were 100% except for method C due to the
elimination of triethylammonium chloride as secondary product.
RME and OE were identical to each other. RME, which is
considered as useful metric to determine the greenness of the
reaction considering reactant mass, atom economy and yield,
was very high (96 and 89%) for both A and B reactions,
respectively. Clearly, Tol and Dcm methods are not as efficient
and green reactions as A and B. A notable 62% of RME was
determined for the whole process C. Together with values of OE,
the higher the value of these five measurements is, the greener
the process is (Table 4, columns 2-6). The last parameter (E-S)
takes in consideration the quality (from the ecological and
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10.1002/ejoc.201900724
European Journal of Organic Chemistry
COMMUNICATION
economical points of view) of the organic synthesis based on yield, B had a very high metrics (85 and 151, respectively) (Table 5). WI
safety, cost, general conditions, work-up and purification. The E-
S value of process A is 85 (0-100 points range) which is
considered an excellent economical/ecological procedure (>75).
In addition, procedure B has the penalty of using several solvents
to perform the final liquid-liquid extraction and purification,
meanwhile the whole reaction C has the penalty of toxicity of the
commercially available basic components. Despite of these
penalties, B and C got 71 and 73 marks, respectively, which are
in the range of acceptable processes (>50) but very close to the
excellence limit (Table 4). E-S values of Tol and Dcm are notably
less advantageous economical/ecological methods although both
acceptable even (>50).
index was 0 except for the example of exo-3j (B).
Table 5. Green metrics (PMI, E, SI and WI) for processes A and B.
Process
A
PMI (g/g)
52
E (g/g)
51
SI (g/g)
51
WI (g/g)
0
A*[a]
1.04
56
0.04
57
0
0
Tol
56
0
Dcm
B
87
86
85
0
66
65
151
58
[a] The solvent and the catalyst were recovered
and reused, so both of them were not included
in the equation.
Scheme 2. Synthesis of endo-3a from commercially available precursors
(process C).
Table 4. Green metrics (AE, AEF, CE, RME, OE, MP and E-S) for processes A,
B and C.
AE
AEf
(%)
96
CE
(%)
100
100
100
100
71
RME
(%)
96
OE
(%)
96
E-S
(0-100)
85
Process
(%)
100
100
100
100
65
A
Tol
Dcm
B
88
79
88
58
80
78
80
52
89
89
89
71
C
62
62
96
73
Conclusions
In this work, the preparation and use of an ionic liquid
phosphane·silver acetate complex was efficiently focussed on the
search of a green 1,3-DC between azomethine ylides and
electrophilic alkenes. The green process was achieved using the
minimal amount of an environmental benign solvent as ethanol
(no solvent and water were not productive). The reaction was fast
and completely reproducible. The separation of the catalytic
complex plus solvent was demonstrated. The stability of the
complex is good enough to recover it and reuse up to four batches.
The small amount of the catalyst required and its easy synthesis
contributed to increase the green character of this 1,3-DC.
Another crucial point in a green process is the purification, and
using this methodology it was possible to design an individual
purification protocol for each compound. The metrics are
excellent in the reaction involving NMM with methyl
benzylideneaminoglycinate especially AE, CE, RME, PMI, E and
the eco-scale parameter.
In the next PMI, E, SI and WI metrics, the lower values
guarantee the better processes (Table 5). PMI and E values are
in the range of 52 and 66 in the preparation of endo-3a (A) and
exo-3j (B) cycloadducts. However, a much more interesting 1.04
PMI and 0.04 E values could be obtained taking in account that
the catalyst and the solvent were recycled in the preparation of
endo-3a (A*).34 Solvents constitute an important contribution in
the solvent intensity (SI) parameter, thus the processes Dcm and
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10.1002/ejoc.201900724
European Journal of Organic Chemistry
COMMUNICATION
We gratefully acknowledge financial support from the Spanish
Ministerio de Ciencia, Innovación y Universidades (projects
CTQ2013-43446-P, CTQ2016-81893REDT and CTQ2014-
51912-REDC) the Spanish Ministerio de Economía, Industria y
Competitividad, Agencia Estatal de Investigación (AEI) and
Fondo Europeo de Desarrollo Regional (FEDER, EU) (projects
CTQ2016-76782-P, CTQ2016-80375-P and CTQ2016-81797-
REDC), the Generalitat Valenciana (PROMETEOII/2014/017),
and the University of Alicante. D. Lledó thanks the Spanish
Ministerio de Educación for the given fellowship FPU15/00097.
Experimental Section
General 1,3-dipolar cycloaddition: To a solution of BMIM (20 mg)
containing AgOAc (1 mg, 0.006 mmol) and the TSIL (3.12mg, 0.007 mmol)
was allowed to stir for 1 hour in darkness to form the metallic complex.
Then, the dipole and the dipolarophile were added together with the
solvent and the mixture was stirred in darkness for the following 17 hours
at a determined temperature. The purification steps are explained for each
cycloadduct along the text.
See supporting information for additional details.
Acknowledgments
Keywords: Green chemistry • dipolar cycloaddition •
azomethine ylides • TSIL • silver complex
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10.1002/ejoc.201900724
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Entry for the Table of Contents (Please choose one layout)
Layout 1:
COMMUNICATION
One of the best environmentally
friendly 1,3-dipolar cycloadditions
between imino esters and electrophilic
alkenes was run using a versatile,
efficient, stable and fully recoverable
TSIL·AgOAc catalyst. Green metrics
are excellent.
Green 1,3-dipolar cycloaddition
David Lledó, Guillermo Grindlay, José
Miguel Sansano *
Page No. – Page No.
1,3-Dipolar cycloadditions of
stabilized azomethine ylides and
electrophilic alkenes mediated by a
recyclable TSIL·AgOAc catalyst
Notes and references
[1]
[2]
[3]
[4]
[5]
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10.1002/ejoc.201900724
European Journal of Organic Chemistry
COMMUNICATION
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