Mechanochemically Activated Liebeskind-Srogl (L-S) Cross-Coupling Reaction: Green Synthesis of meso-Substituted BODIPYs

Article abstract of DOI:10.1021/acs.organomet.0c00037

In the past decade mechanochemistry has become a highly versatile solvent-free methodology that fulfills several principles of so-called green chemistry. In this regard, we describe a mechanochemical activation protocol for the synthesis of several releva

Full text of DOI:10.1021/acs.organomet.0c00037

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Mechanochemically Activated Liebeskind−Srogl (L-S) Cross-
Coupling Reaction: Green Synthesis of meso-Substituted BODIPYs
́
́
̃
Mario Perez-Venegas, Miriam N. Villanueva-Hernandez, Eduardo Pena-Cabrera,* and Eusebio Juaristi*
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ABSTRACT: In the past decade mechanochemistry has become a highly
versatile solvent-free methodology that fulfills several principles of so-called
green chemistry. In this regard, we describe a mechanochemical activation
protocol for the synthesis of several relevant meso-aryl-substituted BODIPYs
via the Liebeskind−Srogl cross-coupling reaction with excellent yield and
very short reaction times relative to alternative procedures in solution,
thereby avoiding the use of bulk solvent. In contrast with previous assertions
that the L-S reaction requires anaerobic conditions to avoid the oxidation of
Cu(I) to Cu(II), our results seem to indicate that this paradigm may not be
altogether accurate, since the L-S reactions studied here took place under aerobic conditions, possibly suggesting an alternative
reaction mechanism.
meso-Substituted boron-dipyrromethenes (BODIPYs) are
highly fluorescent compounds that constitute a valuable class
of photodynamic therapy (PDT) agents,1 with additional
extensive applications in imaging,2 in ion sensing,3 or as
components of organic solar cells.4 Previously, meso-
substituted BODIPYs were synthesized following the strategies
described by Kreuzer5a and Biellmann5b (Figure 1). More
theless, experimentally the L-S coupling reaction presently
requires the use of an inert atmosphere and extremely dry
conditions, restricting its synthetic application in coupling
reactions in academic organic synthesis. Furthermore, those
experimental conditions also result in increased cost in
industrial settings. Finally, high-boiling-point and toxic solvents
such as toluene are usually employed; thus, it is clear that a
more environmentally benign protocol for the L-S coupling
reaction is badly needed.9
In this context, in the last two decades synthetic organic
chemistry has taken advantage of energetically more efficient
strategies that employ nonconventional sources of energy.10
Particularly noteworthy is the use of mechanical energy to
synthesize organometallic complexes and catalysts.11,12 Fur-
thermore, advances in the still very young area of organo-
metallic mechanosynthesis are enabling the development of
mechanochemical metal-catalyzed processes in which the
catalyst is efficiently generated by mechanochemical procedur-
es.12b
Figure 1. BODIPY serendipitously synthesized by Krauzer (1),5a
Biellmann’s BODIPY (2),5b and the iconic para-substituted phenyl
BODIPY (3a).
Bearing in mind the current need to develop more efficient
and less polluting processes using nonconventional sources of
energy13 and the relevant application of the L-S protocol in
organic synthesis,14 we deemed it essential to explore the L-S
cross-coupling reaction mediated by mechanical activation,
eliminating the use of special reaction conditions and
recently, a significant number of substituted BODIPYs have
been prepared through efficient strategies that employ the
cross-coupling reaction known as the Liebeskind−Srogl (L-S)
protocol.6
The L-S cross-coupling reaction, i.e., the C−C bond
formation between thioorganics and boronic acids or organo-
stannanes,7 is a process that takes place in the presence of a
catalytic amount of Pd(0) and a stoichiometric amount of a
Cu(I) carboxylate, under neutral conditions.7,8 The remarkable
results obtained when the L-S protocol is applied motivated
the examination of alternative strategies such as the Suzuki−
Miyaura, Stille, and Sonogashira coupling reactions in the
synthesis of analogous meso-substituted BODIPYs.9 Never-
Received: January 20, 2020
https://dx.doi.org/10.1021/acs.organomet.0c00037
© XXXX American Chemical Society
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conducting a greener coupling strategy in line with the
principles of green chemistry.
the Supporting Information). Therefore, a 25 Hz frequency
was used for subsequent reactions.
Accordingly, in this communication, we report the effective
application of the L-S cross-coupling reaction under mechan-
ical conditions. Considering the rather good results obtained
when this coupling reaction was applied in the synthesis of
meso-substituted BODIPYs under traditional solution con-
ditions,6 we proceeded to evaluate the mechanochemically
activated process by adequate modification of the previously
reported protocol (Scheme 1).
As has been demonstrated, the nature of the milling jar and
balls plays a crucial role in the efficiency of the
mechanochemical process.17 In the present work, stainless
steel (SS), agate, and Teflon (PTFE) components were
compared in the L-S coupling reaction. It was found that the
mechanochemical process proceeds in a cleaner way when an
SS milling jar and balls are used. In contrast, with Teflon
components a lower yield was obtained (see the Supporting
Information). This effect seems to correlate with the hardness
of the vial and ball materials, reaching the best results when a
denser and harder material, in this case SS (ca. 7.67 g cm−3 and
Rockwell C60, respectively)17a was used. Additional advan-
tages encountered in the use of SS components are convenient
scale-up processes and easier cleaning up of the reaction vessel,
in addition to greater compatibility with the reagents needed
for the coupling process. Therefore, we decided to use a
metallic milling jar and balls during the optimization process.
As was already mentioned, when the L-S cross-coupling
reaction is performed under mechanochemical conditions, it is
not essential to use a dry and deoxygenated solvent. This
practical observation facilitated the evaluation of a great variety
of LAG additives as assistants in the mechanochemical
protocol, providing mobility inside the milling jar in moderate
quantities.15 Surprisingly, polar solvents such as 1,4-dioxane
and tert-butyl methyl ether significantly decrease the reaction
yield (67% and 43%, respectively; see also the Supporting
Information). Lower yields of the desired BODIPYs were also
observed when a nonpolar solvent such as toluene was used
(see the Supporting Information). The observed reduction in
reaction yield in polar media could be associated with excessive
stabilization of the metallic complexes by the LAG
additive.18,19 Notwithstanding, given the minimal amount of
additive used, a direct association of the LAG effect with the
observed yield is not clear at this moment. In this context, the
optimum loads of boronic acid, palladium catalyst, phosphine
adjuvant, and copper carboxylate were also determined in the
mechanically activated L-S reaction. As it turned out, the
original amounts of reagents employed in the solution
reactions were also the most convenient for the mechano-
chemical procedure (see the Supporting Information).
Optimal L-S cross-coupling reaction conditions (1 equiv of
Biellmann’s BODIPY 2, 3 equiv of boronic acid 4, 2.5% mol of
Pd2(dba)3, 7.5% mol of TFP, 3 equiv of CuTC, 0.2 mL of
THF, SS milling jar and balls, and 25 Hz frequency for 30 min
in an MM200 Mixer Mill (Retsch)) were then used to
synthesize a variety of meso-substituted aryl BODIPYs by using
different arylboronic acids, as shown in Scheme 2 (see also the
Supporting Information).
High reaction yields were obtained when unsubstituted 4b
or electron-deficient boronic acids 4c,e were used, providing
BODIPYs 3b,c,e in 95%, 82% and 86% yield, respectively. In
contrast, the electron-rich substituted arylboronic acid 4d
afforded BODIPY 3d in a low 50% yield. Saliently, reaction
yields of compounds 3b,e,f were higher under mechanical
activation relative to the same process in solution (see the
Supporting Information). In this regard, the reaction time
required for the preparation of compounds 3b,f was
significantly shorter (by 25% and more than 200%,
respectively) relative to procedures under the traditional
solution conditions (see the Supporting Information).
Scheme 1. L-S Cross-Coupling Reaction under Traditional
Reaction Conditions (Upper Reaction) and under
a
Mechanochemical Activation (Lower Reaction)
a
The three balls on the arrow represent the mechanochemical
method.16
Initially, the L-S cross-coupling reaction was carried out
under the reported conditions using a dry and deoxygenated
solvent (tetrahydrofuran (THF), 2.5 mL) and under an argon
atmosphere (Scheme 1; see also the Supporting Information).
Gratifyingly, after 1 h of continuous stirring at 55 °C BODIPY
3a was obtained in very high yield (96% yield of crude
product), although it was contaminated with traces of several
byproducts. Surprisingly, when the mechanochemical L-S
coupling reaction was conducted without special precautions
(i.e., reagent-grade solvent that had been exposed to air before
use and not under hermetically closed conditions) analytically
clean BODIPY 3a was synthesized in 93% yield. All reagents
were placed in an agate milling jar charged with one agate ball,
in the presence of 0.2 mL of THF as adjuvant to improve the
solubility of the reagents (liquid-assisted grinding, LAG15).
The resulting mixture was milled for 90 min at 25 Hz of
frequency (see also the Supporting Information) (Scheme 1).
As can be appreciated from Scheme 1, the rather similar
results obtained when the L-S coupling reaction is activated
mechanically clearly demonstrates the applicability of the
mechanochemical protocol, with the additional advantage that
a minimal amount of solvent is required in this case.
With the aim to optimize the mechanochemically activated
L-S reaction, this reaction was subjected to different milling
times and vibration frequencies. Additionally, the constituting
material of the milling jar and balls was evaluated, and the
nature of LAG additives was examined in the mechanochem-
ical procedure. In this regard, the reaction progress was
followed by thin-layer chromatography, confirming that the
reaction was complete after 30 min of milling. At this point, the
desired BODIPYs could be isolated in high yield (ca. 90%).
Nevertheless, additional mechanical activation (i.e., more than
30 min of milling) promotes the production of undesirable
byproducts. Mechanical energy directly associated with the
milling frequency is a determining parameter for the
appropriate conduction of the coupling process.17 Hence,
lower frequencies considerably reduce the reaction yield (see
B
https://dx.doi.org/10.1021/acs.organomet.0c00037
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Communication
preparation of several novel BODIPYs. This strategy avoids
the use of special reaction conditions, such as inert atmosphere
and dry solvents, and employs a minimal amount of liquid
additive (LAG) in the coupling reaction. The optimal
conditions found for the mechanochemical procedure were
used to synthesize a family of meso-aryl-substituted BODIPYs
(3a−f) with good results (50−95% yields) in a short time (45
min). Furthermore, sustainable techniques, such as the
employment of recyclable copper, were successfully applied,
confirming the adaptability of mechanochemical procedures in
organometallic processes. In the same way, a greener strategy
using ethyl acetate as an LAG additive in a larger-scale reaction
allows for the isolation of the BODIPYs by simple
crystallization. In contrast with previous assertions that the
L-S requires anaerobic conditions to avoid the oxidation of
Cu(I) to Cu(II), our results seem to indicate that this
paradigm may not be altogether accurate, since the L-S
reactions studied here took place under aerobic conditions,
possibly suggesting an alternative reaction mechanism.
Scheme 2. Arylboronic Acid Scope for the
Mechanochemical L-S Cross-Coupling Reaction
a
a
An SS milling jar and balls were used.
As indicated in previous reports,19 the nature of the
copper(I) reagent is determining for the adequate outcome
of the L-S reaction. Interestingly, mechanochemical cross-
coupling reactions have been successfully carried out using a
copper milling jar and balls.12 Furthermore, several examples of
mechanochemical transformations taking place by activation
with copper reagent substitutes such as brass balls have been
reported.20 These precedents suggested the potential applica-
tion of copper components in the ball mill as a convenient
option in the application of mechanochemical synthetic
strategies. Thus, aiming to eliminate the use of copper(I)
thiophene-2-carboxylate (CuTC) in the mechanochemically
activated L-S coupling reaction, we used copper balls.
Unfortunately, the reaction proceeded in a very low yield
(2%; see the Supporting Information for additional details).
Finally, aiming to increase the amount of BODIPY
synthesized in the mechanochemical reaction, we carried out
the reaction on a larger scale according to a slightly modified
protocol. In particular, ethyl acetate, a green solvent, was
employed successfully as an LAG additive in the cross-coupling
reaction. At the end of the mechanochemical process, the
product was extracted with fresh ethyl acetate and washed with
basic aqueous solution. This ethyl acetate solvent was
efficiently recovered by distillation and BODIPY 3 was readily
crystallized to provide the pure product in 73% yield (Scheme
3).
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.0c00037.
■
sı
Full experimental details, tables with all relevant results,
and NMR spectra (PDF)
AUTHOR INFORMATION
■
Corresponding Authors
́
Eduardo Pena-Cabrera − Departamento de Quimica,
̃
Universidad de Guanajuato, Guanajuato 36050, Mexico;
orcid.org/0000-0002-2069-6178; Email: eduardop@
ugto.mx
́
Eusebio Juaristi − Departamento de Quimica, Centro de
́
Investigacion y de Estudios Avanzados, 07360 Ciudad de
́
Mexico, Mexico; El Colegio Nacional, 06020 Ciudad de
́
Mexico, Mexico; orcid.org/0000-0003-0936-7020;
Email: ejuarist@cinvestav.mx, juaristi@relaq.mx
Authors
Mario Perez-Venegas − Departamento de Quimica, Centro de
́
́
́
Investigacion y de Estudios Avanzados, 07360 Ciudad de
́
Mexico, Mexico
Miriam N. Villanueva-Hernandez − Departamento de
Quimica, Universidad de Guanajuato, Guanajuato 36050,
́
́
Mexico
In conclusion, a convenient L-S cross-coupling protocol
activated by mechanical energy was developed for the
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.0c00037
Scheme 3. Larger Scale Process for the Mechanochemical
LS Cross-Coupling Reaction Using Ethyl Acetate (EA) for
LAG additive
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are indebted to the Consejo Nacional de Ciencia y
́
Tecnologia (CONACyT, project numbers A1-S-44097,
253623, and 123732) and SEP-CINVESTAV project No.
126. M.P.-V. thanks the CONACyT for Ph.D. scholarship No.
70766.
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