Electrochemical Generation of Hypervalent Bromine(III) Compounds

Article abstract of DOI:10.1002/anie.202104677

In sharp contrast to hypervalent iodine(III) compounds, the isoelectronic bromine(III) counterparts have been little studied to date. This knowledge gap is mainly attributed to the difficult-to-control reactivity of λ³-bromanes as well as to their challenging preparation from the highly toxic and corrosive BrF₃ precursor. In this context, we present a straightforward and scalable approach to chelation-stabilized λ³-bromanes by anodic oxidation of parent aryl bromides possessing two coordinating hexafluoro-2-hydroxypropanyl substituents. A series of para-substituted λ³-bromanes with remarkably high redox potentials spanning a range from 1.86 V to 2.60 V vs. Ag/AgNO₃ was synthesized by the electrochemical method. We demonstrate that the intrinsic reactivity of the bench-stable bromine(III) species can be unlocked by addition of a Lewis or a Br?nsted acid. The synthetic utility of the λ³-bromane activation is exemplified by oxidative C?C, C?N, and C?O bond forming reactions.

Full text of DOI:10.1002/anie.202104677

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Accepted Article
Title: Electrochemical Generation of Hypervalent Bromine(III)
Compounds
Authors: Igors Sokolovs, Nayereh Mohebbati, Robert Francke, and
Edgars Suna
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Electrochemical Generation of Hypervalent Bromine(III)
Compounds
Igors Sokolovs,[a,c] Nayereh Mohebbati,[b,c] Robert Francke,*[b,c] and Edgars Suna*[a,d]
[a]
Dr. I. Sokolovs, Prof. Dr. E. Suna
Latvian Institute of Organic Synthesis
Aizkraukles 21, LV-1006 Riga, Latvia
E-mail: edgars@osi.lv
[b]
N. Mohebbati, Dr. R. Francke
Leibniz Institute for Catalysis
Albert-Einstein-Str. 29a, 18059 Rostock, Germany
E-Mail: robert.francke@catalysis.de
Dr. I. Sokolovs, N. Mohebbati, Dr. R. Francke
Institute of Chemistry, Rostock University
Albert-Einstein-Str. 3a, 18059 Rostock, Germany
Prof. Dr. E. Suna
[c]
[d]
Faculty of Chemistry, University of Latvia
Jelgavas 1, LV-1004 Riga, Latvia
Supporting information (SI) for this article is given via a link at the end of the document.
Abstract: In sharp contrast to hypervalent iodine(III) compounds, the
isoelectronic bromine(III) counterparts have been little studied to date.
This knowledge gap is mainly attributed to the difficult-to-control
reactivity of λ3-bromanes as well as to their challenging preparation
from the highly toxic and corrosive BrF3 precursor. In this context, we
present a straightforward and scalable approach to chelation-
stabilized λ3-bromanes by anodic oxidation of parent aryl bromides
transition metal-free regioselective C-H amination of non-
activated alkanes.[12] Although these notable accomplishments
highlight the remarkable synthetic potential of bromine(III)
species,[13a] the hypervalent bromine chemistry appears to be
significantly less developed as compared to that of iodine(III)
compounds. This striking bias apparently is to be attributed to the
relatively poor stability and the high oxidizing power of bromine(III)
reagents,[13b] properties that have created a common perception of
difficult-to-control reactivity and poor functional group compatibility.
Furthermore, there is an apparent lack of convenient methods for
the synthesis of bromine(III) species.
The synthesis of Br(III) reagents is typically accomplished by a
ligand exchange reaction between bromine trifluoride (BrF3) and
nucleophilic arene derivatives such as arylsilanes and
arylstannanes.[13] For example, BrF3 reacts with phenyl
trifluorosilane to form a relatively unstable phenyl-λ3-bromane 1[14]
that is suitable both as reagent and as entry point for the synthesis
of other derivatives via ligand exchange (Figure 1, top).[15,16]
Bromine trifluoride has been also used by Martin et al. for the
preparation of chelation-stabilized aryl-λ3-bromane 3d.[17]
However, an important limitation of these approaches is the use
of BrF3, a highly toxic and extremely reactive liquid that requires
dedicated equipment (such as PTFE vessels etc.) and specific
experimental techniques for its safe handling. Notably, high
oxidation potentials of aryl bromides (e.g. 2.0 V vs. Ag/0.01M
AgNO3 for PhBr in CH3CN)[18] and poor stability of the resulting
bromine(III) species complicates the direct two-electron oxidation
by chemical oxidants, a method that is routinely employed for the
synthesis of hypervalent iodine(III) counterparts from
iodoarenes.[19] Clearly, the development of a convenient synthetic
approach to λ3-bromanes would open the door for the rapid
development of hypervalent Br(III) chemistry. Herein we report a
straightforward and high yielding synthesis of Martin’s
hypervalent bromine(III) species 3 by anodic oxidation of the
parent aryl bromide 2 (Figure 1, bottom). The extremely high
reactivity of arylbromonium species[13a] is tamed by the two
coordinating ortho substituents, allowing 3 to be stored as a
possessing
two
A
coordinating
hexafluoro‐2‐hydroxypropanyl
substituents.
series of para-substituted λ3-bromanes with
remarkably high redox potentials spanning a range from 1.86 V to
2.60 V vs. Ag/AgNO3 was synthesized by the electrochemical method.
We demonstrate that the intrinsic reactivity of the bench-stable
bromine(III) species can be unlocked by addition of a Lewis or a
Brønsted acid. The synthetic utility of the λ3-bromane activation is
exemplified by oxidative C-C, C-N, and C-O bond forming reactions.
The chemistry of hypervalent halogen species has experienced
tremendous development in the last decades, and hypervalent
iodine(III) compounds have become mainstream reagents in
contemporary organic synthesis.[1] Recent selected applications
of hypervalent iodine(III) compounds involve oxidative
heterocycle formation,[2] atom transfer reactions[3] such as
alkynylation[4]
and
trifluoromethylation,[5]
oxidative
rearrangements[6] and C-H functionalization.[7] Additionally,
electrochemically generated iodine(III) derivatives have been
frequently utilized as ex-cell mediators (electro-generated
reagents) in organic electrosynthesis.[8] The corresponding
isoelectronic hypervalent bromine(III) species feature superior
reactivity to that of iodine(III) counterparts owing to the higher
oxidizing ability, stronger electrophilicity and better nucleofugality
of the bromanyl unit.[9] Not surprisingly, the unique properties of
bromine(III) reagents have allowed for the development of
unprecedented synthetic transformations such as Hofmann
rearrangement of sulfonamides to the corresponding N-
arylsulfamoyl fluorides,[10] unusual Bayer-Villiger-type oxidation of
open-chain aliphatic aldehydes,[9a] oxidative coupling of alkynes
and primary alcohols to form conjugated enones[11] as well as
1
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10.1002/anie.202104677
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Table 1. Optimization of the electrochemical synthesis of 3a.
Deviations
from standard
conditions
Mass
balance
[%]a
Entry
2a [%]a
3a [%]a
1
2
none
43
40
93
54
60
50
44
51
38
37
30
17
18
55
52
98
92
99
100
100
94
99
98
94
97
97
92
89
TFE
3
CH3CN
6
4
BDD(+)
46
5
RVC(+)
40
6
0.1 TBA–ClO4
0.1 TBA–PF6
j = 5 mA/cm2
j = 15 mA/cm2
j = 20 mA/cm2
q/mol = 3.0 F
q/mol = 4.0 F
q/mol = 5.0 F
44
7
55
8
47
Figure 1. New approach to aryl-3-bromanes.
9
56
10
11
12
13
60
bench-stable reagent for extended periods of time. However, the
reactivity of the electrochemically generated Br(III) species can be
unlocked by a Lewis acid (such as TMSOTf and BF3OEt2) or a
Brønsted acid (TfOH) additive as described below.
67
75 (63)b
71
Recently we reported on electrosynthesis of hypervalent
iodine(III) species by anodic oxidation of aryl iodides in an
undivided cell using HFIP as solvent.[20] We hypothesized that the
electrochemical oxidation would be suitable also for the synthesis
of aryl-λ3-bromanes provided that they are compatible with anodic
oxidation conditions. Hence, the air- and moisture-stable[17b] aryl-
λ3-bromane 3a was chosen as the target substrate to verify the
feasibility of its electrochemical synthesis. The published
conditions for anodic oxidation of iodoarenes into λ3-iodanes were
used as the starting point for the preparation of λ3-bromane 3a
(Table 1).
Gratifyingly, electrochemical oxidation using glassy carbon (GC)
as the working electrode and platinum foil as the counter
electrode in a TBA–BF4/HFIP electrolyte at 10 mA cm-2 afforded
the desired 3a in 55% yield (entry 1) after passing 2 F per mole of
starting material. The molecular structure of 3a was confirmed by
single crystal X-ray analysis (Table 1, graphics). Although the use
of 2,2,2-trifluoroethanol (TFE) as a solvent gave similar results
(entry 2), we refrained from using it in further experiments
because of its high toxicity. In contrast, acetonitrile turned out to
be unsuitable due to anodic degradation (see p. S47), resulting in
poor conversion to 3a (entry 3). The replacement of working
electrode material to BDD or RVC (entry 4 and 5) gave no
increase in product yield. TBA–PF6 can be used as an alternative
to TBA–BF4 (entry 7 vs. 1), whereas TBA–ClO4 turned out to be
inferior in terms of yield (entry 6). Lower current density (5 mA cm-
2) resulted in two-fold increase of the reaction time and slightly
reduced product yield (entry 8 vs entry 1). Higher current densities
(15 and 20 mA cm-2, entries 9 and 10, respectively) led to
increased conversion at the expense of selectivity. Thus, a current
density of 10 mA cm-2 was used in further optimization
experiments. The increase of passed charge equivalents from 2.0
F up to 3.0 F resulted in the increase of λ3-bromane 3a yield from
a Yields and mass balances determined by 1H NMR spectroscopy of the post-
electrolysis solution using 1,2,3,4-tetrafluorobenzene as the internal standard
(0.3 mmol scale). b Isolated yield.
55% to 67% (entry 11 vs 1). Higher yields of 3a and improved
conversion was observed after passing 4.0 F. Nevertheless,
further increase of the passed charge amount did not result in
significant improvements, and
a concomitant formation of
degradation products was observed (entry 13 vs 12).
With the optimized conditions in hand (entry 10), the scalability of
the developed electrochemical synthesis of λ3-bromane 3a was
examined. We were pleased to find that the galvanostatic
electrolysis in an undivided electrochemical cell could be readily
performed on a 10 mmol scale to afford 3.50 g of 3a in a single-
batch (72% yield). Notably, λ3-bromane 3a was isolated from the
electrolysis mixture as >95% pure (1H-NMR assay) pale-yellow
solid by simple filtration through a short silica plug followed by
extractive workup (see Figure 2, bottom right).
After elaboration of the electrochemical protocol for synthesis of
λ3-bromane 3a, we intended to vary the substitution in para
position to create a reagent platform with tunable reactivity and
electrochemical properties. To establish access to compounds
3a–f, the synthetic routes depicted in Figure 4 were developed.
Introduction of 2-hydroxy-1,1,1,3,3,3-hexafluoropropanyl group in
positions 2 and 5 of an aryl bromide was readily accomplished in
two steps from 2-bromoisophthalic acids 5a–f. The first step
involved elaboration of acids 5a–f into the corresponding acid
chlorides, followed by the reaction with 2,2,2-trifluoroethanol to
afford bis-trifluoroethyl esters 6a–f. Subsequent treatment of
esters 6a,d,e with TMSCF3 (7 equiv) in the presence of catalytic
TBAF (10 mol%)[21] afforded the desired alcohols 2a,d,e in 57-
2
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Figure 2. Synthesis of bromides 2a-g. a Average yield of two runs on 0.3 mmol scale. b Yield on 10.0 mmol scale.
72% yields. The synthesis of 2b,c,f required the addition of
excess (4 equiv) of TBAF[22] and TMSCF3 (6 equiv)[23] to furnish
products in 62-69% yields. The nitration of 2a afforded bromide
2f. The starting 2-bromoisophthalic acids 5a–e were obtained by
the oxidation of the corresponding commercially available 2-
bromo-m-xylenes 4a–e with KMnO4.[24] Acid 5f was synthesized
by chemoselective cross-coupling of the two iodo-moieties in 7
(prepared by iodination of 4-bromobenzotrifluoride)[25] with
ethynyltrimethylsilane under Sonogashira cross-coupling
conditions,[26] followed by oxidative cleavage of the acetylene
subunit in 8 with KMnO4 (Figure 2). Finally, application of the
optimized electrolysis conditions (Table 1, entry 10) to the
oxidation of 2b-g rendered λ3-bromanes 3b-g in 24 - 66% yields.
It should also be noted that attempts toward conversion of related
bromoarenes carrying one or two chelating moieties remained
unsuccessful (for details see p. S21).
For characterization of the electrochemical properties of the
bromoarene/λ3-bromane redox couples, all synthesized bromides
2a–g were studied by cyclic voltammetry, using a 0.1 M
NBu4BF4/HFIP electrolyte, a glassy carbon working electrode and
a Ag/0.01 M AgNO3 reference (E0 = −87 mV vs Fc/Fc+ couple;[27]
for more details, see p. S43). In the range between 0 and 2.7 V,
each of the bromoarenes exhibits a single irreversible feature
associated with the oxidation of Br(I) to Br(III). The corresponding
half-peak potentials (EP/2) are situated in the range between 1.86
Figure 3. Top: Background and iR drop corrected linear sweep voltammograms
(LSV) of aryl bromides 2a-g (c = 5 mM) recorded at 10 mV s-1. Bottom: Plot of
the half-peak potentials EP/2 (extracted from the LSVs) vs. the σp substituent
V (2e, R = OMe) and 2.60 V (2g, R = NO2; see table in Figure 3).
It follows that i) the anodically generated bromanes can be
considered as strong oxidants due to the high EP/2 values and ii)
+
constants.
3
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that with the compounds synthesized thus far, the potential of 2 is
flexibly tunable within a range of 0.66 V. A Hammett treatment
(Figure 3, bottom right) using 휎푃+ substituent coefficients[28] shows
that EP/2 is dependent on the electron donating or withdrawing
ability of the substituent R and follows a linear trend according to
equation 1,
Lewis acid.[32] Indeed, a nearly quantitative formation of biaryl 10a
(98% yield) was observed after 3h at -30 oC (Figure 4) when 3a
was used together with TMSOTf[33] (1:2 ratio, respectively).
Alternatively, BF3∙OEt2 could be employed for the activation of
bromane 2a in HFIP as the solvent (35% yield of 10a). Finally,
TfOH as additive (1.05 equiv) also effected the 3a-mediated biaryl
formation in DCM (93% yield of 10a). Hence, both Lewis and
Brønsted acids are suitable for the enhancement of λ3-bromane
reactivity possibly by increase of cationic character of 3a upon
interactions with the Lewis basic oxygen atom.[32,34] Noteworthy,
CV studies showed that an in-cell mediated biaryl coupling is not
possible, since the substrates are easier to oxidize than the
bromoarenes (for details see p. S46).
The scope of substrates for the λ3-bromane-mediated biaryl
formation was briefly explored using TMSOTf as the activator
(Figure 4A). The reaction features remarkable functional group
compatibility. Not only halides are tolerated (biaryls 10a–d) but
also allylic and propargylic subunits as well as primary alcohols
(10e,f,g, respectively) are compatible with the 3a-mediated biaryl
formation. Biaryls 10h and 10i as well as bi-isoflavone derivative
10j can be also obtained using TMS-OTf activation of λ3-bromane
3a (Figure 4A).
(1) 퐸푃/2(ퟐ) = 2.25 푉 + 0.47 푉 ∙ 휎푃+
wherein the slope provides a measure of the influence of the
substituents upon the observed potential, while the intercept
refers to the EP/2 of the unsubstituted compound of the series.[29]
The stabilization of the characteristic pseudo-trigonal bipyramidal
geometry of λ3-bromanes 3 by 2‐benzobromaoxole rings (for X-
ray structure of 3a, see Table 1 graphics)[30] endows Martin’s
bromine(III) species with remarkable stability. Given the high
oxidizing power and strong electrophilicity of λ3-bromanes,[13a] the
observed stability is striking. Not only it allowed for the
electrosynthesis, isolation and purification of λ3-bromanes 3a-g,
but also made possible their convenient handling and storage for
extended periods of time. In the meantime, the synthetic
application of λ3-bromanes 3a–g would require activation to
unlock their intrinsic reactivity. To this end, oxidative biaryl
coupling[31] was chosen as a model reaction to develop the in-situ
bromane activation (Figure 4). As anticipated, the non-activated
3a did not effect the homocoupling of benzodioxole 9a in DCM at
room temperature after 18 h. We envisioned that the reactivity of
λ3-bromane 3a could be enhanced by weakening the stabilizing
effect of the two chelating ortho-substituents using a suitable
We were pleased to find that λ3-bromane 3a also effected the
oxidative amidation of N,N-dimethylanilines 11a–h with imides
(phthalimide and succinimide) as well as with various amides
such as pyrrolidin-2-one, oxindole and cinnamamide (Figure
4B).[35] Relatively electron-rich anilines 11a,b,d are suitable as
Figure 4. Synthetic applications of electrogenerated bromane reagent 3a.
4
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substrates. Notably, the oxidative amination with λ3-bromane 3a
is compatible with the presence of vinylic, propargylic and primary
alcohol moieties in anilines (11f–h) as well as with bromine and
aldehyde substituents (11c,e). Finally, λ3-bromane 3a was also
found to be suitable for the oxidative cyclization of Schiff bases
17a–c to benzoxazoles (18a–c; Figure 4C). Further studies to
expand the application scope of λ3-bromane 3a are ongoing in our
laboratories.
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In summary, an efficient, reliable and inexpensive approach to
Martin’s hypervalent bromine (III) species is reported. The key
step of the synthesis is anodic oxidation of the parent aryl
bromides in an undivided cell using a glassy carbon as the
working electrode, platinum as the counter electrode, TBA–BF4
as the supporting electrolyte and HFIP as the solvent. The use of
an undivided cell under constant current conditions allowed for
easy scale-up of the electrolysis as demonstrated by the
preparation of the bench-stable λ3-bromane in multi-gram
quantities. A series of aryl bromides with oxidation half-peak
potentials spanning the range from 1.86 V to 2.60 V were
converted into the corresponding λ3-bromanes. Cyclic
voltammetry studies showed good correlation between the half-
peak potentials and Hammett substituent coefficients for aryl
bromides. The aryl bromides could be readily obtained from 2-
bromoisophthalic acids in three-steps. The reactivity of Martin’s
λ3-bromane is sufficient for oxidative amidation and benzoxazole
formation and could be further enhanced by Lewis or Brønsted
acid additives as demonstrated by the successful application of
Martin’s hypervalent bromine (III) species in the biaryl coupling.
The developed electrochemical approach to λ3-bromanes offers
considerable advantages as compared to the existing methods
that rely on highly toxic, hazardous and difficult-to-handle BrF3 as
the key reagent. Therefore, we believe that our approach may
open the door to the development of unprecedented synthetic
transformations that would benefit from the unique properties of
hypervalent bromine(III) species.
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This work was funded by ERDF (Post-Doc Latvia) project No.
1.1.1.2/VIAA/2/18/377 for I. Sokolovs and the German Research
Foundation (DFG, Grant No. FR 3848/1-2). RF is grateful for
financial support by the DFG (Heisenberg Program, Grant No. FR
3848/4-1). We thank Dr. Alexander Villinger (Institute of
Chemistry, Rostock University) for X-ray structural analysis.
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3288.
Keywords: electrochemistry • hypervalent bromine • cyclic
voltammetry • oxidative coupling • anodic oxidation
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Entry for the Table of Contents
A straightforward electrochemical synthesis of chelation-stabilized hypervalent bromine(III) compounds is presented for the first time.
The electrolysis proceeds at room temperature in an undivided cell under galvanostatic conditions and renders the λ3-bromanes in
good yields on the gram scale from bromoarene precursors. The reactivity of λ3-bromanes can be enhanced by Lewis or Brønsted
acid additives as demonstrated in λ3-bromane-mediated oxidative biaryl formation.
Institute and/or researcher Twitter usernames: @FranckeLab, @likat_rostock, @osi_lv, @edgars_suna
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