Electro-Olefination—A Catalyst Free Stereoconvergent Strategy for the Functionalization of Alkenes

Article abstract of DOI:10.1002/chem.202001394

Conventional methods carrying out C(sp²)?C(sp²) bond formations are typically mediated by transition-metal-based catalysts. Herein, we conceptualize a complementary avenue to access such bonds by exploiting the potential of electrochemistry in combination with organoboron chemistry. We demonstrate a transition metal catalyst-free electrocoupling between (hetero)aryls and alkenes through readily available alkenyl-tri(hetero)aryl borate salts (ATBs) in a stereoconvergent fashion. This unprecedented transformation was investigated theoretically and experimentally and led to a library of functionalized alkenes. The concept was then carried further and applied to the synthesis of the natural product pinosylvin and the derivatization of the steroidal dehydroepiandrosterone (DHEA) scaffold.

Full text of DOI:10.1002/chem.202001394

A Journal of
Accepted Article
Title: Electro-olefination - a Catalyst Free Stereoconvergent Strategy
for the Functionalization of Alkenes
Authors: Andreas N. Baumann, Arif Music, Jonas Dechent, Nicolas
Müller, Thomas C. Jagau, and Dorian Didier
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To be cited as: Chem. Eur. J. 10.1002/chem.202001394
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Electro-Olefination - a Catalyst Free Stereoconvergent Strategy for
the Functionalization of Alkenes
Andreas N. Baumann,[+] Arif Music,[+] Jonas Dechent, Nicolas Müller, Thomas C. Jagau and Dorian
Didier*
Dedicated to Prof. Rolf Huisgen on the occasion of his 100th birthday
[*]
A. N. Baumann, A. Music, J. Dechent, N. Müller, Dr. T.-C. Jagau, Dr. D. Didier
Department of Chemistry and Pharmacy
Ludwig-Maximilians-University Munich
Butenandtstraße 5-13, Haus F, 81377 Munich (Germany)
Dorian.Didier@cup.uni-muenchen.de
[+]
These authors contributed equally to this work
Abstract: Conventional methods carrying out C(sp2)-C(sp2) bond
formations are typically mediated by transition-metal based catalysts.
Herein, we conceptualize a complementary avenue to access such
bonds by exploiting the potential of electrochemistry in combination
with organoboron chemistry. We demonstrate a transition metal
catalyst-free electrocoupling between (hetero)aryls and alkenes via
readily available alkenyl-tri(hetero)aryl borate salts (ATBs) in a
stereoconvergent fashion. This unprecedented transformation was
investigated theoretically and experimentally and led to a library of
functionalized alkenes. The concept was then carried further and
applied to the synthesis of the natural product pinosylvin and the
derivatization of the steroidal dehydroepiandrosterone (DHEA)
scaffold.
electrochemical oxidation led to the selective formation of
heterocoupled biaryls (Scheme 1B).[7]
As an alternative route to conventional cross coupling reactions,
the catalyst free Zweifel olefination cannot be neglected.[8] This
powerful methodology enables the stereospecific formation of
alkenes from the corresponding alkenyl-organoborinates, as
exemplified recently by the groups of Aggarwal and Morken
(Scheme 1C).[9] In addition, we demonstrated that the logical
combination of different organometallic reagents[10] with boron
alkoxides could lead to the formation of the required bis-
organoborinates in an efficient one-pot process.[11] Based on
these findings, we decided to examine the reactivity of alkenyl-
triaryl borate salts (ATBs) in order to develop an electro-
olefination reaction (Scheme 1D).
Despite its young history of only a few decades, the Suzuki-
Miyaura reaction is one of the most utilized reaction in modern
organic chemistry.[1,2] The palladium-catalyzed coupling of
boronic acids with organohalides was not only awarded with the
Nobel prize in 2010, in fact, a recent study ranks the Suzuki-
Miyaura coupling as one of the most frequently used reactions (5th
place) in medicinal chemistry.[1] Besides, many other transition-
metal-mediated cross-couplings, namely Stille, Heck, Negishi,
Sonogashira, Hiyama and Kumada are likewise powerful tools to
forge new C-C bonds.[3] Such indispensable strategies
undoubtedly display many advantages and have inspired us to
challenge the formation of C-C bonds without the need of the
commonly used transition-metal catalysts, thus breaking new
grounds in the field of cross-coupling reactions. We first started
our ambitious concept by replacing the catalyst with an
electrochemical setup. Innate advantages, including the use of
inexpensive and reusable electrodes, reaction tuneability and
scalability do not only rely on the modern and cutting-edge work
from Baran, but trace back to many other advances in
electrochemical synthesis since the pioneering works of Volta and
Faraday in the 19th century.[4]
We already employed electrochemistry to initiate aryl-aryl bond
formation - inspired by the work of Geske[5] and Waldvogel[6]
(Scheme 1A) - introducing new hetero-substituted tetraarylborate
salts (TABs). We demonstrated that the formation of
“unsymmetrical“ TAB salts is enabled by a triple ligand exchange
reaction on commercially available organotrifluroborate species
employing aryl-Grignard reagents. Submitting those TABs to mild
Scheme 1. Electrochemical / Transition-metal catalyst free C-C-couplings.
1
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ATBs (2) are underexplored salts, the only representative
compound being triphenylvinyl borate which can be synthesized
by treatment of tetravinyltin with triphenylborane.[12] In order to
investigate the electro-olefination and expand the structural
variety of ATBs, we aimed to simplify their access. Therefore, we
built on our previously described strategy for the synthesis of
hetero-substituted tetraarylborate salts (TABs), and decided to
make ATBs accessible by a triple ligand exchange reaction onto
atom present on each of the aryl groups, an Eox value of +0.81 V
vs. SCE was measured for 2a, similar to the one of 2c (+0.82 V
vs. SCE). However, in the absence of electron-withdrawing
substituents, the oxidation potential of 2b was decreased to
+0.67 V vs. SCE. As expected, it can be concluded that alkenyl
groups are easier to oxidize and that the oxidation potential varies
with the electronic nature of substituents on the moieties
surrounding the boron atom. From a chemoselectivity perspective,
the favorable oxidation of the olefin leaves no other path for the
reaction but to transfer one of the remaining aryl moieties, thereby
avoiding the undesirable formation of biaryl homocoupling
compounds.
the corresponding potassium alkenyl-trifluoro borates
1
(Molander salts),[13] employing ex situ generated Grignard or
organozinc reagents.[14]
We anticipated that the removal of an electron through an
oxidation process should occur preferentially on the alkenyl
moiety, avoiding the energetically disfavored dearomatization of
one of the aryl groups. As a proof of concept, we first synthesized
the model systems 2a[15] and 2b, possessing respectively para-
fluorophenyl and phenyl moieties in addition to the -styryl
substituent (Figure 1). To describe the change in the electronic
structure upon oxidation of 2a and 2b, spin and charge densities
were computed based on Mulliken population analysis of the DFT
results. Charge densities were additionally computed using the
CHarges from ELectrostatic Potentials using a Grid-based
(ChElPG) method.[14] Blue areas (Figure 1A) represent positive
spin densities after oxidation. Only the alkenyl substituent is
selectively oxidized in both cases while the charge and spin
densities of the other aromatic substituents only change
insignificantly, confirming our assumptions.
80
70
60
50
40
30
20
10
0
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
2.2
2.4
2.6
Electron equiv. [F]
0.002
0
Scheme 2. 1H NMR studies of the transformation of 2a into 3a and 3ab under
electrochemical oxidation and conversion-rate (galvanostatic) experiment with
n-undecane as internal standard.
-0.002
-0.004
-0.006
-0.008
-0.01
-0.012
2a was chosen to test and optimize the reaction conditions.[14]
Inexpensive and reusable glassy carbon electrodes (GCE)
proved to deliver the desired stilbene derivative 3a with optimal
conversions in acetonitrile at 25 °C. Following the transformation
by 1H NMR (Scheme 2) showed that the borate salt 2a is
selectively oxidized into product 3a. Full conversion can be
observed after 2.2 F in 1H NMR studies and conversion-rate
experiments of the electro-olefination using GC revealed that an
optimal yield was obtained after 2 F. Remarkably, further
oxidation resulted in consumption of the reaction product.
-0.2
-0.1
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
Eox vs. SCE [V]
Figure 1. A) Spin density after oxidation of 2a and 2b. B) ATB salts 2a, 2b and
tetraphenyl borate with experimental oxidation potentials and cyclic voltammetry
calibrated to the reversible ferrocene oxidation (Fc/Fc+).
The oxidation potentials of ATB salts 2a and 2b were determined
by cyclic voltammetry and compared to the value measured for
commercial sodium tetraphenyl borate (Figure 1B). With a fluoride
1
Although no biaryl byproduct was detected in H NMR, traces
2
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were found in GC. Interestingly, a third minor compound 3ab can
be observed, which was identified as the epoxy-stilbene
derivative of 3a. This side reaction will be discussed later with the
mechanistic considerations (Scheme 7).
or - in cases of sensitive functional groups - organozinc
species.[14]
Therefore, we started investigating the scope of the
transformation using borate salts without prior purification. The
reaction was first evaluated engaging (E)-alkenyltrifluoroborates
1a-g as starting materials in this two-pot sequence. Upon
generation of the desired borate intermediates, those were
treated with an aqueous solution to remove remaining inorganic
salts and were subjected to electrochemical oxidation conditions
after switching the solvent to acetonitrile. The results are depicted
in Scheme 3. With electron-withdrawing substituents present on
the aryl moieties, (E)-alkenes 3a-b were obtained in up to 69%
yield over two steps. In the case of p-CN-substituted phenyl
groups, the corresponding organozinc species had to be
employed, lowering the overall yield of the 2-pot procedure (3c,
29%). This consequent decrease in yield can be attributed to the
lower reactivity of organozinc derivatives in ligand exchange
reactions. Electron-donating and neutral aryl substituents
furnished the desired (E)-alkenes 3d-e in moderate to good yields
(42 and 74%). Varying the substitution pattern on the alkenyl
moiety did not influence the course of the reaction, and 3f-g were
isolated in 55 to 71%. Heteroaryl groups were also tolerated in the
electro-olefination process, furnishing structures 3h-j in up to 68%
yield. Interestingly, trisubstituted double bonds also led to the
corresponding olefinated aryl derivative 3k in good yield (70%).
The formation of the borate salt proved however difficult when an
acrylate derivative was used. The introduction of 3-pyridylzinc
onto a trifluoroboryl acrylate and subsequent electro-olefination
only gave 25% of product 3l. Noteworthy, all derivatives were
obtained with excellent (E/Z) ratios, up to 99:1
Z-alkenyl trifluoroborates were employed next. Following the
same two-pot protocol, the freshly generated Z-alkenyl-triaryl
borates were engaged crude in the electro-olefination under
oxidative conditions. Diversely substituted aryl moieties were able
to perform the coupling reaction, furnishing compounds 3m-s in
reasonable yields (43 to 64%). It is however interesting to notice
that all derivatives were isolated as trans-isomers. Given that
either of the starting material (E or Z) gives the same
thermodynamic E-isomers after electro-coupling, the strategy is
stereoconvergent (Scheme 3). As it will be discussed in the
mechanistic part, we assume that the oxidation of the double bond
into a radical cationic species allows for the resulting bonding
system to freely rotate and adopt the thermodynamically more
stable configuration before abstraction of the boron-containing
moiety (Scheme 7).
Our study of the electro-olefination was pursued with the use of
-substituted alkenyl borates (Scheme 4). The simple acyclic
isopropenyl borate salt delivered product 4a in 41% yield. Cyclic
alkenyl groups were then investigated in the presence of electron-
rich, -neutral and -poor aromatic systems, and gave compounds
4b-f with up to 75% yield. Borate salts containing heteroatoms in
the cycloalkenyl scaffolds such as 3,6-dihydro-2H-pyranyl, -
thiopyranyl and 1,2,3,6-tetrahydropyridinylunderwent successful
electro-olefinations, delivering trisubstituted olefins 4g-o in
moderate to good yields. We lastly demonstrated the reaction to
be compatible in the presence of ketal functionalities (4p-r, up to
86%).
Scheme 3. Two-pot borate salt formation / electro-olefination sequence -
Synthesis of acyclic alkenes. aYields are stated as isolated yields over two-steps.
bGC-ratios determined from crude mixtures. cElectrochemical oxidation in EtOH
as solvent instead of MeCN at 25°C and open to air.
The synthesis of alkenyl-borate salts can be followed by 11B NMR
and proved quantitative when employing either Grignard reagents
3
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inversion of the double bond configuration, as the reaction
proceeds through two consecutive stereospecific steps (1,2-
metallate rearrangement and antiperiplanar -elimination). The
(Z)-isomer can therefore be synthesized using Zweifel conditions
(Scheme 5C) and (Z)-3a was isolated in 86% yield (E/Z
ratio < 1:99). Noteworthy, stereodivergent Zweifel protocols have
been developed. Even though the presented method might be
less versatile than these contributions, our strategy avoids the use
of highly toxic chemicals such as BrCN and PhSeCl.[16]
Scheme 4. Two-pot borate salt formation / electro-olefination sequence -
Synthesis of cyclic alkenes. aYields are stated as isolated yields over two-steps.
Next, we applied the method to the derivatization of more
challenging structures to demonstrate the synthetic potential of
our ATB salts. Dehydroepiandrosterone (DHEA) was derivatized
into a TBS-protected ether and the carbonyl function transformed
into the corresponding alkenyltrifluoroborate 1o. The addition of
arylmagnesium bromide reagents to 1o, followed by electro-
olefination under the optimized oxidative conditions described
above furnished functionalized molecules 5a and 5b in up to 70%
yield (Scheme 5A). In addition, -Styryltrifluoroborate 1a was
employed as substrate for the synthesis of the natural product
pinosylvin (Scheme 5B). 3,5-Dimethoxyphenylmagnesium
bromide was introduced to perform the triple ligand exchange
reaction and gave the intermediate alkenyltriaryl borate species.
Subsequent electro-olefination and demethylation with BBr3
furnished 5c in 35% yield over three steps with perfect control of
Scheme 5: aYields are stated as isolated yields over two-steps. bYield over three
steps. cGC-ratios determined from crude mixtures. dStarting from 2a. eStarting
from (Z)-2a.
Lastly, we set out to ascertain the mechanism of this intriguing
reaction, building on conversion experiments, cyclovoltammetry
and theoretical considerations (Figure 1 and Scheme 2).
Crossover experiments were conducted by mixing different borate
salts under electrochemical conditions, confirming the absence of
products resulting from intermolecular reactions and ruling out the
possibility of intermolecular processes.[14]
After selective oxidation of the alkenyl moiety, a rearrangement
takes place. To study the nature of this rearrangement, we
synthesized borate salts containing more than a single styryl
group (6a-b, Scheme 6), employing styryl-Grignard reagents as
(E/Z)-mixtures, and submitted them to our electrocoupling
conditions. As a reference, the desired compound 3a was
obtained as the sole compound from 2a. With a salt bearing two
styryl groups (6a), a product ratio of 73:27 of 3a and the diene 7
the diastereoselectivity (E/Z
=
99:1). Furthermore, the
chemoselectivity was investigated on our benchmark salt 2a
under distinct oxidative conditions (Scheme 5C). As already
mentioned before, the electro-olefination occurs in
a
stereoconvergent manner. We selectively obtain the stilbene
derivative 3a using (E)-2a or (Z)-2a in moderate to good yields. In
contrast, typical Zweifel conditions led to a stereospecific
4
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was obtained (E/Z = 85:15). This result points out that the transfer
of a vinyl group is not preferred over the transfer of an aryl group,
and therefore indicates that the rearrangement is more likely to
go through a -bond breaking process rather than a -addition,
as for the latter a unfavorable dearomatization has to occur.
Example 6b (possessing three styryl moieties) further supports
this hypothesis, as 7 was obtained in 54% and 3a in 46% GC-
ratio. The non-statistical distribution of products 3a and 7 in both
experiments also indicates that the aryl moiety is - in such cases -
a better transferable ligand than the styryl group.
Scheme 7: Proposed mechanism for the electro-olefination of ATB 2a.
In conclusion, we have developed a new conceptual approach to
alkene derivatives through electro-olefination. A simple strategy
was assembled for the synthesis of alkenylborate salts (ATBs) via
ligand exchanges on potassium trifluoroborates. No purification of
these salts was required for the sequence to be pursued and
deliver the expected coupling compounds in moderate to good
yields under electrochemical oxidation. Such method represents
an original and stereoconvergent alternative to the formation of
functionalized olefins, opening new ways of thinking about C-C
bond disconnections.
Scheme 6: Electrocoupling of different mixed potassium tetraorganoborate
salts. [a]In situ generated following general procedure D[14] as follows for 6a: 0.5
mmol 1p and 1.0 mmol styrylmagnesium bromide. For 6b: 0.5 mmol potassium
trifluoro(4-fluorophenyl)borate and 1.5 mmol styrylmagnesium bromide.
[b]Product distribution ratios are determined by GC analysis on crude mixtures
without isolation. Homocoupled biaryls are omitted and not included in the GC-
ratios for more clarity.
In summary, the alkenyl moiety is more prone to oxidation than
the aryl groups (as concluded from quantum-chemical
calculations and selectivity experiments, see Figure 1 and
Scheme 6) and leads to an intermediate alkyl radical cationic
species [A] (Scheme 7). We then propose that further
intramolecular -addition of one of the aryl moieties undergoes a
rearrangement[17] towards intermediate [B] in which the C-C alkyl
radical bond can freely rotate and lead to the thermodynamically
favored trans product (E)-3a. Oxygen probably interacts with the
reaction intermediates under formation of structure [C], as 3ab
was observed in traces under air and isolated in 37% yield when
the reaction was carried out under oxygen atmosphere. It is
however important to note that product 3ab does not come from
the oxidation of product 3a under electrochemical conditions, as
confirmed by control experiments, indicating a radical pathway.[14]
Based on cyclovoltametry (Figure 1), galvanostatic experiments
(Scheme 2) and our findings in the previous work on biaryl electro-
coupling,[7] we assume that no second oxidation has to occur
during the formation of the desired product 3a.
Acknowledgements
Robert J. Mayer (LMU Munich) is kindly acknowledged for his help
with cyclovoltammetry, as well as Hendrik Eijsberg (UCLdV,
Poitiers) for helpful comments in the preparation of this
manuscript. We thank the Chemical Industry Fund (FCI), the
Deutsche Forschungsgemeinschaft (DFG grant DI 2227/2-1 + JA
2794/1-1, SFB749), and the Ludwig-Maximilians University,
Munich (LMUExcellent) for financial support.
Keywords: Olefination • Electrochemistry • Stereoconvergent •
Organoborates • Catalyst-free
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6
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Entry for the Table of Contents
Electro-Olefination: The electrocoupling between (hetero)aryls and alkenes via alkenyl-triaryl borates (ATBs) is demonstrated. ATBs
are synthesized by a triple ligand exchange reaction on potassium alkenyltrifluroborates with organometallic species and directly
oxidized in an electrochemical two-pot coupling towards functionalized alkenes in a stereoconvergent fashion.
Institute and/or researcher Twitter usernames: @didier_group
7
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