Benchtop-Stabssle Hypervalent Bromine(III) Compounds: Versatile Strategy and Platform for Air- And Moisture-Stable λ3-Bromanes

Article abstract of DOI:10.1021/jacs.1c04536

We present the first synthesis of air/moisture-stable λ³-bromanes (9and10) by using a cyclic 1,2-benzbromoxol-3-one (BBX) strategy. X-ray crystallography and NMR and IR spectroscopy ofN-triflylimino-λ³-bromane (12) revealed that the bromine(III) center is effectively stabilized by intramolecular R-Br-O hypervalent bonding. This strategy enables the synthesis of a variety of air-, moisture-, and benchtop-stable Br-hydroxy, -acetoxy, -alkynyl, -aryl, and bis[(trifluoromethyl)sulfonyl]methylide λ³-bromane derivatives.

Full text of DOI:10.1021/jacs.1c04536

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Benchtop-Stable Hypervalent Bromine(III) Compounds: Versatile
Strategy and Platform for Air- and Moisture-Stable λ3‑Bromanes
Kazunori Miyamoto,*,∥ Motomichi Saito,∥ Shunsuke Tsuji, Taisei Takagi, Motoo Shiro,
Masanobu Uchiyama,* and Masahito Ochiai*
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ABSTRACT: We present the first synthesis of air/moisture-stable λ3-bromanes (9 and 10) by using a cyclic 1,2-benzbromoxol-3-
one (BBX) strategy. X-ray crystallography and NMR and IR spectroscopy of N-triflylimino-λ3-bromane (12) revealed that the
bromine(III) center is effectively stabilized by intramolecular R−Br−O hypervalent bonding. This strategy enables the synthesis of a
variety of air-, moisture-, and benchtop-stable Br-hydroxy, -acetoxy, -alkynyl, -aryl, and bis[(trifluoromethyl)sulfonyl]methylide λ3-
bromane derivatives.
he invention of new chemical reagents is a fertile area of
research in synthetic chemistry, providing novel strategies
T
for efficient synthetic access to compounds of commercial and
academic importance. For example, hypervalent iodine(III)
reagents (denoted here as λ3-iodanes) are an important class of
organic reagents, serving as unique electrophiles/oxidants that
are employed extensively in academia and industry.1−3 In
contrast, the chemistry of λ3-bromanes has been little
investigated, mainly due to the lack of efficient, user-friendly
preparation methods.4,5 Much of the challenge involved in
preparing λ3-bromanes resides in the innate thermodynamic
barrier to oxidation of Br(I) arising from its great electro-
negativity (Br: 2.96 > I: 2.66),6 as well as its large ionization
(IP) potential [e.g., PhBr (IP: 8.98 eV) > PhI (IP: 8.69 eV).7,8
The first synthesis of the Br(III) analogue, λ3-bromane, by
Sandin and Hay dates back to 1952.9 They elegantly overcame
the energetic impasse by leveraging intramolecular λ3-bromane
formation with the pyrolysis reaction of the diazonium salt of
2-amino-2′-bromobiphenyl (1), obtaining cyclic o,o′-dipheny-
lene-λ3-bromane (2; Figure 1A). The use of the “Martin
ligand” is an excellent alternative strategy for the preparation of
stable λ3-bromane (4; Figure 1B).10,11 These approaches,
however, are less than ideal in terms of versatility. In 1984,
Frohn and co-workers revolutionized this field by the synthesis
of difluoro-λ3-bromane 6 (Frohn’s reagent):12 the use of the
substitution reaction of BrF3 with arylsilanes 5 circumvents the
unfavorable oxidation process. Importantly, Frohn’s reagent 6
serves as a pivotal platform for various λ3-bromanes (7; Figure
1C).13−18 Furthermore, difluoro-λ3-bromane 6 promotes
unprecedented reactions owing to the hypernucleofugality of
the ArBr(III)+ group:19 examples include Hofmann rearrange-
ment of sulfonamide20 and Baeyer−Villiger oxidation of
primary aliphatic aldehydes.21 However, excessive reactivity
and the need for strict exclusion of moisture (6 immediately
decomposes into ArBr, HF, and O2 via oxidation of water)
have limited the utility of Frohn’s protocol.12
Figure 1. Background of Ar−λ3-bromane chemistry. (A) The first
synthesis by Sandin and Hay. (B) Martin’s bicyclic λ3-bromane 4. (C)
Frohn’s protocol. (D) This work. (E) Expected stabilization effect.
electronic and steric stabilization of the three-center four-
electron (3c−4e) hypervalent bond on the Br(III) center by
using the 1,2-benzbromoxol-3-(1H)-one (BBX) ligand (Figure
1D,E).22 The similar 1,2-benziodoxol-3-(1H)-one (BIX)
Received: May 1, 2021
Published: June 14, 2021
We herein present the first versatile synthetic methodology
for benchtop-stable λ3-bromanes (9 and 10) through
https://doi.org/10.1021/jacs.1c04536
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ligand has been historically used for the preparation of various
λ3-iodanes.23−26 Another key to the present achievement is the
use of the N-triflylimino group as an exocyclic changeable
ligand, which can be transformed into various functionalities
through electrophilic substitution reactions on the Br(III)
center. This strategy allows us to synthesize a series of air/
moisture/heat/light-stable λ3-bromanes coordinated with
nitrogen, oxygen, and carbon moieties at the apical (exocyclic)
position.
Tf-BIX-λ3-iodane 15 was totally unproductive (Scheme 1C).
C−H amination transforms stable, abundant, and inert
hydrocarbons into viable building blocks for the robust
construction of C−N bonds.29 BBX-imino-λ3-bromanes 12a
thus prepared facilitated C−H amination of cyclohexane (16)
without any catalyst, while in marked contrast, no reaction
occurred with the corresponding BIX-λ3-iodane 15.
We next focused on the transformation of the imino-λ3-
bromanes 12 to various BBX-λ3-bromanes (Scheme 2A).
In order to synthesize BBX-λ3-bromane 9, we commenced
our study by the direct oxidation of 2-bromobenzoate
derivatives (8) with various oxidants (Martin’s approach).
However, these trials resulted in the oxidation (fluorination) of
the aromatic ring or decarboxylation in preference to the
desired oxidation of bromine (Scheme S1A). All attempts at
ligand exchange (Frohn’s approach) of arylstannanes/boranes/
silanes with BrF3 were unsuccessful (Scheme S1B). Then, we
focused on the ylide transfer reaction of imino-λ3-bromane 11a
(Ar = p-CF3C6H4), developed by our group in 2007,18,27 to o-
bromobenzoates 8. Gratifyingly, the treatment of 11a with an
excess amount (50 equiv) of ethyl o-bromobenzoate (8a) at
room temperature gave the desired N-triflylimino-λ3-bromane
12a in 79% yield (Scheme 1A). Use of trimethylsilyl o-
Scheme 2. Ligand Exchange Reactions of 12
Scheme 1. Synthesis, Structure, and Reactivities of N-
Triflylimino-BBX-λ3-bromanes (12)
Lewis/Brønsted acid-catalyzed ligand exchange of the imino
group to Tf2C− or phenyl ligand produced the corresponding
bromonium ylide 18 and phenyl-λ3-bromane 19, respectively,
in high yields. Strong coordination of the CO2Et ligand was
1
revealed by H/13C NMR spectra and X-ray crystal structure
analysis (Figure S3). This may be the reason why these BBX-
λ3-bromanes 18 and 19, which can potentially serve as an
electrophilic bis(triflyl)methylating and arylating agents,
respectively, in solution,4,5,30−32 both turned out to be stable
in the isolated form on the benchtop for at least 6 months. On
the other hand, all attempts at ligand exchange to alkynyl and
vinyl groups were unsuccessful; instead, preferential aziridina-
tions of these π bonds occurred.
Gratifyingly, saponification of 12a followed by careful
neutralization of the resulting sodium o-bromosylbenzoate
(20) afforded 1-hydroxy-1,2-benzbromoxol-3(1H)-one (hy-
droxy-BBX-λ3-bromane, 21) in 66% yield (over 2 steps;
Scheme 2). Treatment of 12b with NaOH (2 equiv) also
resulted in the clean formation of 20 in 97% yield. Since λ3-
bromanes with a hydroxyl ligand have never been synthesized,
we will discuss in detail the physicochemical properties of 21.
Evidence supporting the presence of the Br−OH bond was
bromobenzoate 8b resulted in smooth NTf transfer accom-
panied by desilylation to afford the corresponding N-
triflylimino-λ3-bromane 12b in 80% yield. In sharp contrast
with acyclic λ3-bromanes, these BBX-imino-λ3-bromanes 12
exhibited extremely high air/moisture/thermal stability. The
isolated powders 12a and 12b can be stored under ambient
conditions of temperature, air, moisture, and lighting without
decomposition for at least 4 months. Indeed, no sign of
decomposition was detected in CH2Cl2 solution after one
month.
1
obtained by means of FT-IR and H NMR analyses, which
showed a characteristic absorption at ν 2650−2400 cm−1
(solid state) and a broad signal of a proton exchangeable
with D2O at δ 7.05 ppm (in CD3CN, 22 °C), respectively,
similar to those observed with the I(III) analogue, hydroxy-
BIX-λ3-iodane 22.33−35 Compound 21 showed higher
solubility than 22 under the same conditions: 21 is sparingly
soluble in CH3CN, CH3CN−H2O, H2O, and 1,1,1,3,3,3-
hexafluoro-2-propanol (HFIP), but not in less polar solvents
(dichloromethane, chloroform, THF, diethyl ether, ethyl
acetate, benzene, and hexanes). In solution, 21 displays a
strong absorption band at λ = 268−275 nm (in CH3CN) or at
λmax = 278 nm (in H2O), but is transparent in the visible to
NIR (λ ≤ 1000 nm) region. The isolated colorless fluffy
The structures of imino-λ3-bromanes 12a and 12b were
unambiguously determined by single-crystal X-ray diffraction
analyses (Scheme 1B and Figure S2); the strong intramolecular
coordination of CO to Br (12a: 2.641(2) Å; 12b: 2.504(1)
Å) is presumably responsible for the stability of the BBX-λ3-
bromanes (Figures S1 and S2). Notably, BBX-λ3-bromanes 12
retain strong nucleofugality of the BBX unit; a unique
aziridination reaction of an olefin (13) was observed under
mild conditions,18,28 whereas the λ3-iodine(III) analogue, N-
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needles of 21 are air/moisture-stable, and no sign of
decomposition was observed on standing under air and normal
lighting for at least 4 months. This compound can be kept in a
refrigerator (4 °C) indefinitely, while it gradually decomposes
with a half-life time (t1/2) of 7.5 days in 1:1 CD3CN−D2O
solution (0.016 M, 22 °C), with evolution of O2 gas.
Scheme 3. Reactions of 21 with Various Nucleophiles*
Recrystallization of 21 from 1:1 CH3CN-H2O at 4 °C
afforded colorless prisms with a melting point of 116−117 °C
(decomp.) that were suitable for X-ray crystallography
(Scheme 2C and Figures S5 and S6A). Compound 21 has
distinct T-shaped trigonal bipyramidal geometry around the
Br(III) atom, with a near-linear O1−Br1−O3 triad
(174.43(14)°), and forms a five-membered ring through
intramolecular Br1−O1 interaction (2.226(3) Å). All atoms
(Br1, C1, C6, C7, and O1) in the five-membered ring are
essentially in the same plane and the RMS atomic deviation
from the least-squares plane is 0.049(5) Å. The exocyclic Br1−
O3 distance of 1.824(3) Å is comparable to the predicted value
for covalent radius (1.80 Å) but considerably shorter than the
reported value of 2.01−2.06 Å for the Br(III)−O(Ac) bond in
(diacetoxybromo)arene 7 (X = Y = OAc), owing to the trans
influence on the Br(III) center.36−38 The presence of a
hydrogen atom H1 attached to O3 could not be determined
owing to low electron density, but the intermolecular O2···O3
(symmetry operation 3: −X + 1/2 + 1, Y + 1/2, Z + 1/2)
distance of 2.626(5) Å strongly suggests the presence of a
hydrogen atom between these two oxygen atoms with efficient
hydrogen bonding. The O−X(III)···O−X(III) (X = Br or I)
intermolecular interactions observed in the crystal structure of
22 was not seen in 21, probably due to the lower halogen-bond
donor ability of Br(III) than I(III).39 These structural
differences are likely to be related to the solubility difference
between 21 and 22 (vide supra).
*
a
b
Yields were determined by 1H NMR. Isolated yields. Yields of
autoxidation products 27−29 are calculated based on 21. The 19F
NMR yield.
c
Hydroxy-BBX-λ3-bromane 21 thus prepared turned out to
serve as a potent oxidizing agent through Br(III)−OH bond
activation (Scheme 3). For instance, 21 undergoes homolytic
Br−OH cleavage even at room temperature. Thermolysis of 21
in benzene at 80 °C produced 2-bromobiphenyl (23; 58%
yield), along with a small amount of phenol (24) (3% yield;
Scheme 3A). The formation of these products strongly
suggests that homolytic cleavage of the Br−OH bond and
successive decarboxylation of radical I take place, resulting in
the formation of 2-bromophenyl radical II. Under the same
conditions, iodine(III) analogue 22 was recovered intact.40
Further, 21 also served as an excellent radical initiator in the
autoxidation of aliphatic hydrocarbons (Scheme 3B). Exposure
of indane to 21 in CH2Cl2 at 40 °C under oxygen afforded 1-
indanone (25) in moderate yield; the initial hydrogen
abstraction from indane is mediated by the Br radical I, and
hence, 2-bromobenzoic acid (26) is exclusively produced.
Encouraged by this, we next investigated autoxidation of
cyclohexane, which has a much higher bond dissociation
energy (benzylic C−H in indane: 85.9 kcal/mol vs Cy−H:
99.5 kcal/mol).41 This process is extremely important in
industry (as the first step of 6-nylon/6,6-nylon synthesis) and
generally requires a transition metal catalyst such as Co(III) at
elevated temperature and/or high pressure of O2.42 Pleasingly,
21 could effectively initiate autoxidation of cyclohexane at 80
°C in air (1 atm), and after 24 h, a 57:43 mixture of
cyclohexanol (27) and cyclohexanone (28)(so-called KA oil)
was obtained in 82% yield along with cyclohexyl hydroperoxide
(29; 67%). In addition, 21 functions as a nonradical oxygen
transfer agent (Scheme 3C). For instance, 21 oxidizes diphenyl
sulfide at room temperature under argon to give the
corresponding sulfoxide 30 in 92% yield. The absence of
overoxidized diphenyl sulfone as well as the dependence of the
rate on the concentration of sulfide suggest that 21 serves as an
electrophilic oxygen donor.43 In the presence of base, oxidation
of α,β-unsaturated ketone to afford epoxide 31 proceeded at
room temperature in high yield. This reaction probably
involves Michael addition of 21, followed by reductive
elimination, as an electron-withdrawing group accelerated the
rate of oxidation.44,45 Oxidation of weakly nucleophilic HFIP
with 21 proceeded at room temperature via a ligand exchange-
1
reductive elimination sequence, based on the H NMR/ESI-
MS data (Figure S8). None of these transformations
proceeded to a discernible extent with iodine(III) analogue
22 under the same reaction conditions.
Next, the potential to further transform the hydroxy-BBX-λ3-
bromane (21) into various λ3-bromanes was examined
(Scheme 4). Quantitative acetylation of 21 with acetic
anhydride proceeded at room temperature to give acetoxy-
BBX-λ3-bromane 33. More interestingly, treatment of 21 with
in situ-generated alkynyl(difluoro)borane 34 followed by
alkaline hydrolysis selectively afforded β-(triisopropylsilyl)-
ethynyl-BBX-λ3-bromane 35 in 30% yield. In marked contrast
to acyclic analogues,13,38 BBX-λ3-bromanes 33 and 35 can be
handled under ambient conditions without decomposition.46
In conclusion, our use of the BBX group opens up a new
strategy for the synthesis of λ3-bromanes that overcomes the
requirement for strict exclusion of moisture under dark and
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Scheme 4. Synthesis of Acetoxy- (33) and Alkynyl-λ3-
bromane (35)
Taisei Takagi − Graduate School of Pharmaceutical Sciences,
The University of Tokyo, Tokyo 113-0033, Japan
Motoo Shiro − Rigaku Corporation, Akishima, Tokyo 196-
8666, Japan
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.1c04536
Author Contributions
∥These authors contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
cryogenic conditions normally demanded by the use of λ3-
bromanes. The BBX strategy has enabled the isolation of
various λ3-bromanes with heteroatom/carbon ligands, which is
a long-cherished dream in the field of hypervalent bromine-
(III) chemistry. Some synthetic applications of this approach
have been demonstrated. We believe our BBX strategy
represents a fundamental advance in the preparation of λ3-
bromanes, and will have broad synthetic utility. Indeed, its
practical convenience and efficiency should make it widely
applicable both in the laboratory and in industry.
■
This work was supported by grants from JSPS KAKENHI (B)
(No. 17H03017; to K.M.) and KAKENHI (S) (No.
17H06173; to M.U.), as well as grants from JST CREST
(No. JPMJCR19R2; to M.U.), Nagase Science and Technol-
ogy Development Foundation, and Sumitomo Foundation (to
M.U.).
REFERENCES
■
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.1c04536.
■
(2) Zhdankin, V. V. Hypervalent Iodine Chemistry: Preparation,
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sı
Experimental procedures and characterization data
(PDF)
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Accession Codes
CCDC 2072450, 2076045, 959764, 959765, and 959767−
959769 contain the supplementary crystallographic data for
this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/data_request/cif, by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Authors
■
(8) The oxidation potentials of 2-halobenzoate esters were also
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Kazunori Miyamoto − Graduate School of Pharmaceutical
Sciences, The University of Tokyo, Tokyo 113-0033, Japan;
Graduate School of Pharmaceutical Sciences, University of
Tokushima, Tokushima 770-8505, Japan; orcid.org/
0000-0003-1423-6287; Email: kmiya@mol.f.u-tokyo.ac.jp
Masanobu Uchiyama − Graduate School of Pharmaceutical
Sciences, The University of Tokyo, Tokyo 113-0033, Japan;
Research Initiative for Supra-Materials (RISM), Shinshu
University, Ueda 386-8567, Japan; orcid.org/0000-0001-
6385-5944; Email: uchiyama@mol.f.u-tokyo.ac.jp
Masahito Ochiai − Graduate School of Pharmaceutical
Sciences, University of Tokushima, Tokushima 770-8505,
Japan; Email: bmasahito@gmail.com
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Shunsuke Tsuji − Graduate School of Pharmaceutical
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NOTE ADDED AFTER ASAP PUBLICATION
■
This paper originally published ASAP on June 14, 2021. An
update to the Supporting Information was made regarding the
large-scale synthesis of 12b and a new version was posted on
June 16, 2021.
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9331
https://doi.org/10.1021/jacs.1c04536
J. Am. Chem. Soc. 2021, 143, 9327−9331