Additive- and Metal-Free, Predictably 1,2- and 1,3-Regioselective, Photoinduced Dual C-H/C-X Borylation of Haloarenes

Article abstract of DOI:10.1021/jacs.6b05436

We report herein a simple, additive- and metal-free, photoinduced, dual C-H/C-X borylation of chloro-, bromo-, and iodoarenes. The reaction produces 1,2- and 1,3-diborylarenes on gram scales under batch and continuous flow conditions. The regioselectivity of the dual C-H/C-X borylation is determined by the solvent and the substituents in the parent haloarenes.

Full text of DOI:10.1021/jacs.6b05436

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Communication
Additive- and Metal-Free, Predictably 1,2- and 1,3-Regioselective,
Photoinduced Dual C–H/C–X-Borylation of Haloarenes
Adelphe M. Mfuh, Vu T. Nguyen, Bhuwan Chhetri, Jessica E. Burch, John
D. Doyle, Vladimir N. Nesterov, Hadi D. Arman, and Oleg V. Larionov
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b05436 • Publication Date (Web): 27 Jun 2016
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Additive- and Metal-Free, Predictably 1,2- and 1,3-
Regioselective, Photoinduced Dual C–H/C–X-Borylation of
Haloarenes
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Adelphe M. Mfuh,a Vu T. Nguyen,a Bhuwan Chhetri,a Jessica E. Burch,a John D. Doyle,a Vladimir N.
Nesterov,b Hadi D. Arman,a and Oleg V. Larionov*a
a Department of Chemistry, University of Texas at San Antonio, San Antonio, Texas 78249, United States
b Department of Chemistry, University of North Texas, Denton, Texas 76201, United States
Supporting Information Placeholder
corresponding boronic acids and esters by a photoinduced boryla-
tion in the absence of additives and catalysts.19,20 Homolytic and
heterolytic cleavage of the Ar–X bond followed by the homolytic
substitution at boron21 in B2(OR)4 were proposed as key steps in
this process.
ABSTRACT: We report herein a simple, additive- and metal-
free, photoinduced, dual C–H/C–X-borylation of chloro-, bromo-,
and iodoarenes. The reaction produces 1,2- and 1,3-diborylarenes
on gram scales under batch and continuous flow conditions. The
regioselectivity of the dual C–H/C–X-borylation is determined by
the solvent and the substituents in the parent haloarenes.
Scheme 1. Photoinduced 1,2- and 1,3-Selective Dual C–
H/C–X Borylation Reaction of Haloarenes
Aromatic di- and polyboronic acids (PBAs) are valuable organo-
boron materials with important roles in synthetic organic chemis-
try,1 molecular self-assembly,2 anion recognition,3 molecular
optoelectronics,4 and materials science.5 Regioselective installa-
tion of two boryl groups in the aromatic ring is challenging and is
usually accomplished by borylation of the preinstalled C–X bonds
using transition metal catalysis6,7 or by generating dimagnesiated
or dilithiated intermediates.8 Alternatively, Rh- and Ir-catalyzed
C–H-borylation9 has also been used to prepare diborylarenes and
related PBAs.10 Other methods include the Zn-catalyzed 1,2-
selective dual C–H/C–X-borylation of iodo- and bromoarenes
described by Marder,11 and the Pt- and Cu-catalyzed 1,2-
diborylation of in situ-generated arynes developed by Yoshida.12
However, the use of highly reactive, air- and moisture- sensitive
reagents, expensive ligands and catalysts based on heavy metals,
in addition to low regioselectivity and low yields limit the syn-
thetic scope, practicality, and scalability of these methods.
A
photoinduced diamine-mediated borylation of (pseu-
do)haloarenes was also reported by Li.22 Computational studies
by Li and Lin support involvement of aryl radicals and triplet aryl
cations in the homolytic substitution step.22b Further investigation
of the additive-free borylation reaction showed that small
amounts of 1,2- and 1,3-C–H/C–X diborylation products 1 and 2
(Scheme 2) were formed from iodobenzene in acetonitrile. Addi-
tional studies identified isopropanol as the optimal solvent for the
1,2-selective dual C–H/C–X borylation. The ratio of 1,2- and 1,3-
isomers 1 and 2 was 5.3:1. The 1,4-isomer was not observed in
this case. Monoborylation and unselective diborylation were ob-
served in a number of other solvents (see Table S1 in SI).
Photoinduced dissociation of Ar–X bonds is an efficient method
of generation of reactive intermediates that are typically inacces-
sible from the ground state.13 In addition to homolytic cleavage of
the Ar–X bond, photoheterolysis of haloarenes produces π5σ1
triplet aryl cations. Due to the combined radical and cationic
character of these species, their reactivity is distinctively different
from the reactivity of the ground state aryl cations and aryl radi-
cals.14 The multitude of reactivity patterns available after photoac-
tivation of haloarenes has enabled a number of important carbon–
heteroatom and carbon–carbon bond-forming reactions, including
photoinduced nucleophilic substitution,15 alkylation,16 arylation,17
and photocyclization18 reactions.
Remarkably, the regioselectivity of the reaction switched in favor
of the 1,3-isomer 2, when the borylation was carried out in hex-
afluoroisopropanol (HFIP). The diborylation products were iso-
lated in 85% yield. The 1,3- 1,2-, and 1,4-isomers were formed in
a ratio of 29:4:1. The reaction proceeded with a good photochem-
ical quantum yield ( = 0.18,  = 254 nm).
The experimental and computational data available for the pho-
toinduced Ar–X monoborylation,19,22 point to a solvent-cage-
assisted radical pathway (Scheme 2) for the dual 1,2-C–H/C–X
borylation reaction in isopropanol. Photoinduced homolysis is
very efficient for bromo- and iodoarenes in low- and medium
polarity solvents.13,14 Following the initial homolytic substitution
at B2pin2 with the photogenerated aryl radical,19,22 addition of the
Bpin radical to PhBpin may be enabled by the cage effect of the
more confining medium (e.g. compare viscosities of acetonitrile
We report herein that haloarenes undergo a photoinduced regiose-
lective 1,2- and 1,3-C–H/C–X-diborylation that is enabled by a
remarkable solvent effect. The diborylation reaction proceeds on
preparative scales without removal of air or moisture in batch and
flow and does not require catalysts or additives (Scheme 1).
We recently reported that haloarenes, including fluoroarenes, as
well as quaternary arylammonium salts can be converted to the
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0.377 cP,23 and isopropanol 2.859 cP,24 at 15 C). The 1,2-
regioselectivity of the diborylation process is determined by the
stabilization of the radical intermediate 3 by the conjugation with
the boryl group.25 Use of solvents with higher viscosity to sup-
press out-of-cage crossover reactions has previously been report-
ed for other photoinduced radical processes.26
(e.g. as in 11). On the other hand, if a stronger electron-
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withdrawing group is present in the para position of the haloarene,
the diborylation pattern in HFIP is expected to switch to the 1,2-
diborylation. Similarly, if a strongly electron-releasing substituent
is present in the meta position of the haloarene, 1,2-diborylation is
also expected to become predominant. Since the boryl group is a
relatively weak electron-acceptor (p = 0.12 for B(OH)230), the
substitution pattern can change from 1,3 to 1,2, depending on the
position of other substituents in the haloarene, and it can be pre-
dicted from their electronic properties.
Hexafluoroisopropanol has found use in organic chemistry and
electrochemistry, especially as a solvent for processes that involve
arene cation radical intermediates, due to a combination of high
polarity (EN = 1.068), high ionizing power (YOTs = 3.79), and
T
very low nucleophilicity (NOTs = –4.23).27 Additionally, as a weak
hydrogen-bond acceptor and a strong hydrogen-bond donor, hex-
afluoroisopropanol has a strong propensity to solvate anions, fur-
ther increasing the kinetic stability of cationic and cation radical
species.27a,b
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Table 1. Scope of the Photoinduced 1,3-Selective Dual C–
H/C–X-Borylation Reaction of Haloarenes.a
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Scheme 2. Plausible Reaction Pathways for the Photoin-
duced Dual C–H/C–X-Borylation
Several experiments were performed to test the hypothesis that
the reaction in HFIP proceeds via an electrophilic borylation of
the intermediate ArBpin with pinB–X. First, the dual C–H/C–X
borylation of iodobenzene was carried out in the presence of 2-
tolylBpin (4 equiv.). A crossover reaction of pinB–I was expected
to produce 1,3-diborylarene 4 (Table 1 and SI) in the case of the
electrophilic borylation. However, the crossover product 4 was
not formed. Further, since chloroarenes also undergo the dual C–
H/C–X borylation, in the case of electrophilic borylation of Ar-
Bpin, addition of pinB–Cl was expected to increase the yield of
the corresponding 1,3-diborylation product. However, addition of
pinB–Cl (2 equiv.) to the diborylation reaction of 1-chloro-2-
fluorobenzene did not lead to an increase in the yield of 1,3-
diborylation product 5 (Table 1). Additionally, no accumulation
of the monoborylation product (PhBpin) was observed at the early
stages of the diborylation of iodobenzene (See Figure S1 in the
SI). Taken together, these results indicate that electrophilic
borylation of ArBpin with pinB–X may not be involved in the
photoinduced dual C–H/C–X borylation process. In addition to
homolysis, photoinduced heterolytic cleavage of C–X bonds to
give the 51 triplet aryl cations can take place in polar solvents,
both directly, and by a homolysis followed by a charge transfer.14
Formation of allylbenzene in the photolysis of ArX in the pres-
ence of allyltrimethylsilane has been used to detect involvement
of photoheterolytic Ar–X bond dissociation pathways.14b In pre-
liminary experiments, photolysis of iodo- and bromobenzene in
HFIP in the presence of allyltrimethylsilane led to formation of
allylbenzene that was not observed in isopropanol, pointing to the
potential involvement of the heterolysis of haloarenes in HFIP. It
is possible that heterolytic cleavage of the C–X bond in the very
polar HFIP leads to the generation of the triplet aryl cation 6 that
can react with the sp2-sp3 diboron anion 7.19,22,28 Subsequent re-
combination of the intermediates (8 and 9) will favor the 1,3-
addition (10), since the 1,2- and 1,4-addition will produce cationic
intermediates that are destabilized by the -acceptor boryl group29
a Iodoarenes were used unless specified otherwise; a and b denote
the position of the C–X bond in the parent haloarene. Reaction
conditions for small scale experiments: haloarene (0.6 mmol),
o
B2Pin2 (1.2 mmol), HFIP (6 mL), 15 C, UV lamp (254 nm, 4.5
b
c
mW/cm2).
Ratio of 1,3/1,2/1,4-isomers: 42:5:1.
1,3/1,2/1,4-isomers: 29:4:1.
Ratio of
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The scope and the influence of substituents on the regioselectivity
The structures of diborylarenes 21, 27, 29, and 37 were confirmed
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were investigated next (Table 1). 2- and 4-alkyliodobenzenes
afforded the 1,3-products in excellent yields (4, 12-14) and as
single diastereomers, indicating that the 2- and 4-alkyl groups
reinforce the 1,3-regioselectivity determined by the Bpin group.
Similarly, high yields were observed for haloarenes with the 2,3-
dialkyl substitution pattern (15-17). Benzylic positions in these
substrates were not affected. 2- and 4-Fluoro and chloro groups
were also tolerated (5, 18, and 19). 1,3-Diborylation products
were formed in these cases, indicating that halogens serve as -
donor groups in the diborylation reaction. Methyl group in the
ortho position proved to have a stronger effect than a chloro group
in the meta position of the haloarene (product 20). 2-
Trifluoromethoxy group also showed good directing ability, rein-
forcing the 1,3-diborylation pattern (product 21). The photoin-
duced diborylation also afforded the 1,3-diborylnaphthalene 22
and the sterically encumbered 1,3-diboryl derivative of mesity-
lene 23. In addition, a cyano group in the benzylic position is also
well-tolerated (24 and 25). An electron acceptor group in the meta
position in haloarenes was expected to reinforce the 1,3-
diborylation pattern. Indeed, several iodoarenes bearing electron-
acceptor groups were tested, including the medicinally relevant
pentafluorosulfanyl31 and trifluoromethyl groups, as well as Bpin,
and cyano groups. In all cases, the expected 1,3,5-trisubstituted
products 26-30 were formed as single regioisomers. The reaction
produced the less sterically hindered of the two possible electron-
ically-favored 1,3-regioisomers. For example, for 2-substituted
iodoarenes, only the hydrogen that is para to the substituent was
displaced by the Bpin group (e.g. as in 1,3-diborylarene 4). Bro-
mo- and chloroarenes can also participate in the diborylation reac-
tion, giving rise to 1,3-diborylarene products 2, 4, and 5 in good
yields. The diborylation products were purified by evaporating the
more volatile components of the reaction mixture (typically,
B2pin2, ArBpin, and pinacolone) on a rotary evaporator equipped
with an electrothermal vacuum oven.
by X-ray crystallography. The diborylation reactions were suc-
cessfully carried out on gram scales in quartz test-tubes and FEP
plasticware without prior deoxygenation of reaction mixtures.
Diborylation products 5, 12, 15, 21, 26 (Table 1), as well as 38
and 48 (Table 2) were prepared in 1.1–5.0 g quantities. Further
experiments showed that the dual C–H/C–X borylation reaction
can also be translated to a continuous flow process, and several
1,3- and 1,2-diborylarenes were synthesized in flow on prepara-
tive scales (4, 29, 30, 37, 38, and 48, 0.9–2.9 g). Importantly,
hexafluoroisopropanol can be efficiently recovered by distillation
(95% recovery) from the reaction mixture and reused in another
diborylation reaction.
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Table 2. Scope of the Photoinduced 1,2-Selective Dual C–
H/C–X Borylation Reaction of Haloarenes.a
The scope of the 1,2-diborylation was investigated next (Table 2).
Diborylation in isopropyl alcohol proceeded with high 1,2-
selectivity, and the corresponding products 31 and 32 were ob-
tained in good yields. Two 1,2-diboronates 33 and 34 were
formed in a ratio of 4:1 with 4-iodo-o-xylene, indicating that the
second Bpin addition occurs preferably away from the present
substituents. Interestingly, when both meta positions in the io-
doarene were substituted with a methyl and a fluoro substituent,
the diborylation in isopropyl alcohol produced 1,2-isomers 35 and
36 in a 3.4:1 ratio. In this case, substitution of the hydrogen para
to the fluoro group was the more favored pathway.
Following the general mechanistic blueprint, the diborylation in
HFIP was expected to produce 1,2-diborylarenes from haloarenes
bearing electron-withdrawing groups (EWG) in the para position,
and from haloarenes with electron-releasing substituents in the
meta position. When para-EWG-substituted iodoarenes were
subjected to the dual C–H/C–X borylation reaction, 1,2-
diborylarenes 37-40 were obtained in excellent yields as single
regioisomers. Similarly, 3-alkyl-substituted iodoarenes produced
the corresponding 1,2-diborylarenes 41-44 as single regioisomers
and in good yields. Haloarenes bearing heteroatom substituents
(fluoro, chloro, trifluoromethoxy groups) also produced 1,2-
diborylarenes (45-47). In addition, difluoro- and trifluoro-
substituted 1,2-diborylarenes 48 and 49 were prepared in good
yields. Interestingly, 1,2-diborylation was favored, when the fluo-
ro group was in the meta position with respect to the iodide in the
haloarene, while the ortho position was occupied by a methyl
group. In this case regioisomers 50 and 51 were obtained in a
ratio of 6.2:1. Chloro- and bromoarenes can also be successfully
converted to the corresponding 1,2-diboryl derivatives (e.g. 38,
41, 42, 44-46).
a
Reaction conditions: see footnote for Table 1, IPA = isopropyl
alcohol. The reactions were carried out in HFIP unless specified
otherwise. b Ratio of 1,2/1,3-isomers: 5.3:1.
In conclusion, this paper describes a simple, scalable, additive-
and heavy metal-free method of photoinduced dual C–H/C–X
borylation of haloarenes. The reaction produces 1,2- and 1,3-
diborylarenes regioselectively and in good to excellent yields
under mild conditions from chloro-, bromo-, and iodoarenes. The
regioselectivity of the reaction is determined by the solvent (hex-
afluoroisopropanol or isopropanol) and the substituents in the
aromatic ring, and it can be readily predicted from the electronic
properties of the substituents.
ASSOCIATED CONTENT
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Supporting Information
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Experimental and spectral details for all new compounds and all
reactions reported. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
oleg.larionov@utsa.edu
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
Financial support by the Welch Foundation (AX-1788), the NSF
(CHE-1455061), NIGMS (SC3GM105579), and UTSA is grate-
fully acknowledged. Mass spectroscopic analysis was supported
by a grant from the NIMHD (G12MD007591).
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