Pyridine-catalyzed radical borylation of aryl halides

Article abstract of DOI:10.1021/jacs.6b11813

A pyridine-catalyzed transition-metal-free borylation reaction of haloarenes has been developed based on the selective cross-coupling of an aryl radical and a pyridine-stabilized boryl radical. Arylboronates were produced from haloarenes under mild conditions. This borylation reaction features a broad substrate scope, operational simplicity, and gram-scale synthetic ability.

Full text of DOI:10.1021/jacs.6b11813

Communication
pubs.acs.org/JACS
Pyridine-Catalyzed Radical Borylation of Aryl Halides
Li Zhang and Lei Jiao*
Center of Basic Molecular Science (CBMS), Department of Chemistry, Tsinghua University, Beijing 10084, China
S
* Supporting Information
achievements represent a new dimension in arylboronate
synthesis; however, they often suffer from defects such as high
cost for the borylation reagent, low reactivity, or operational
inconvenience.
ABSTRACT: A pyridine-catalyzed transition-metal-free
borylation reaction of haloarenes has been developed
based on the selective cross-coupling of an aryl radical and
a pyridine-stabilized boryl radical. Arylboronates were
produced from haloarenes under mild conditions. This
borylation reaction features a broad substrate scope,
operational simplicity, and gram-scale synthetic ability.
Our idea for realizing a new transition-metal-free approach to
arylboronates was inspired by two recent advances in radical
chemistry. In recent years it has been disclosed that, in the
presence of an organic additive and a base, aryl halides can be
transformed to aryl radicals via a single electron transfer (SET)/
carbon-halogen bond cleavage sequence,12 representing a novel
mode of haloarene activation.13 Meanwhile, very recently Li and
Zhu reported the homolytic cleavage of the B−B bond by a Lewis
base, 4-cyanopyridine (1a),14 to produce a pyridine-stabilized
boryl radical.15 Weenvisioned that, if the transient aryl radical and
the stabilized boryl radical were generated in one reaction system,
selective cross-coupling of these two species might occur to
produce arylboronate (Scheme 1b), subjecting it to the persistent
radicaleffect.16 Thisstrategywouldprovideanewradicalpathway
for the borylation of haloarenes. Although in some previous
reports possible radical mechanisms were proposed,5b,d,9,10a,b,d
the cross-coupling of two radical species for arylboronate
formation has not been achieved to date.
rylboronates have been widelyused in molecular science,1 in
A
particular as reagents and building blocks in synthetic
chemistry.2 As a result, methods that can efficiently synthesize
arylboronates, especially functionalized ones, are highly de-
manded. Haloarenes are the most commonly employed
precursors to arylboronates (Scheme 1a). A classical method
Scheme 1. Previous Borylation Methods and the Proposed
Radical Borylation Reaction
We started to test this concept on a model borylation reaction
of iodoarene 2a employing bis(pinacolato)diboron (B2pin2)
(Table 1). First, when 4-cyanopyridine (1a) was used to activate
B2pin2, potassium tert-butoxide (t-BuOK) was employed as the
base, and the reaction was carried out in tetrahydrofuran (THF);
no borylation product was observed, and most of the iodoarene
remained intact (entry 1). When sodium methoxide was used in
place of t-BuOK, the desired borylation product 5a was observed
in a low yield, together with a significant amount of
dehalogenation product, anisole (entry 2). Given that hydrogen
atom transfer (HAT) between the aryl radical and THF might be
the reason for dehalogenation, methyl tert-butyl ether (MTBE)
was employed as the solvent to suppress the formation of anisole.
Indeed, the yield improved remarkably (entry 3), and a more
concentrated reaction afforded an even better yield (entry 4).
Replacing 1a by another electron-deficient pyridine, 4-trifluoro-
methylpyridine (1b), diminished the reactivity remarkably (entry
5), while simple pyridine (1c) was as efficient as 1a (entry 6).
Control experiments showed that the reaction could not proceed
without either the pyridine catalyst or the base, supporting the
proposed reaction pathway.
for the preparation of arylboronates is stoichiometric metalation
of haloarenes (by Li or Mg) and subsequent nucleophilic
substitution utilizing trialkyl borates as the boron source.3 The
transition-metal catalyzed methods developed over the past
decades enabled an efficient approach to arylboronates employ-
ing diboron or hydroborate species as the boron source, which
features a broad substrate scope and good functional group
compatibility.4−6 Recently, new pathways have been discovered
to prepare arylboronates from haloarenes7−9 without the need for
stoichiometric metalation or a transition metal catalyst.10,11 Ito
reported the borylation of aryl iodides and bromides under basic
conditions using a silylborane reagent;7 Zhang reported a base-
promoted borylation of aryl iodides using a diboron reagent;8 Li,
Larionov, and Fu independently reported the photoinduced
borylation of haloarenes with a diboron reagent.9 These
Bromoarenes are more easily available than iodoarenes and are
widely utilized in industry and the laboratory, and we hoped to
apply the present borylation method to bromoarenes. However,
the optimized conditions were found to be inefficient (entry 7).
Received: November 15, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.6b11813
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
a
a
Table 1. Optimization of the Borylation Reaction
Table 2. Scope of Haloarenes
a
Reaction conditions: 2a or 3a (0.5 mmol, 1 equiv), base (2 equiv),
B2pin2 (2 equiv), additive (20 mol %), solvent (2 mL), sealed tube, 85
°C for 12 h. The conversions and yields were determined by GC using
b
n-dodecane as an internal standard. Anisole was obtained in 57%
c
d
yield as the byproduct. 0.4 mL of solvent was used. Reaction time
e
was 3 h. 1 mL of solvent was used.
To solve this problem, pyridine catalysts 1c−e with different
electronic natures were attempted, but none of them produced
improved results (entries 8−10). Considering that a pyridine
molecule that can further stabilize the boryl radical might be
helpful, we employed 4-phenylpyridine (1f) as the catalyst and
indeedobservedasignificantincreaseintheyieldof5a(entry11).
When MeOK was used instead of MeONa, the yield was further
improved (entry 12). Application of these conditions to
iodoarene 2a afforded similarly good results (entries 13 and
14). Therefore, the optimal conditions for pyridine-catalyzed
borylation of iodo- and bromoarenes (entry 12) were utilized in
the following studies.
a
Reaction conditions: 2, 3, or 4 (0.5 mmol, 1 equiv), 1f (20 mol %),
MeOK (2 equiv), diboron reagent (2 equiv) in 0.4 mL of MTBE, 85
°C for 12 h. Yields of isolated products are reported, and GC yields are
b
shown in parentheses. Together with 4.3% diborylation product 5ad′.
c
d
1.3 equiv of MeOK and 1.3 equiv of B2pin2 were used. Contained
e
f
g
9.7% transesterification product 5n. NMR yield. E/Z = 89:11. 40
mol % of 1f was used.
This borylation reaction exhibits a broad substrate scope with
respect to haloarenes (Table 2). In general, aryl halides with
varioussubstituents, aswellasheteroarylandalkenylhalides, were
compatible substrates. Haloarenes with electron-donating
substituents were transformed to the corresponding arylboro-
natesingoodyields,regardlessofthesubstitutionpattern(5a−g).
A sterically hindered substrate 2h participated in the reaction well
to afford the borylation product 5h in 53% yield. The reactions of
electron-deficient haloarenes, such as chloro-, fluoro-, and
trifluoromethyl-substituted ones, produced borylation products
in good to moderate yields (5i−p). Notably, activated aryl
chlorides 4n and 4p could also undergo the borylation reaction.
Biphenyl, fluorenyl, and naphthyl halides, together with some
heteroaromatic halides, took part in the borylation reaction
smoothly to produce arylboronates 5q−v. However, 3-
bromofuran (3w) and 3-iodopyridine (2x) proved to be poor
substrates. In many cases aryl bromides were also good substrates
compared with the corresponding aryl iodides, albeit lower yields
were obtained. Interestingly, olefin-containing haloarenes 2y and
2z were also compatible, despite the fact that diboronation of a
CC bond could occur in the presence of B2pin2 and a base.17 In
addition to aryl halides, alkenyl iodides 2aa−ac afforded the
corresponding alkenylboronates 5aa−ac in good yields, further
broadening the scope of this borylation method. Finally, this
reaction also worked well with bis(neopentyl glycolato)diboron
and bis(1,1,3-trimethylethylene glycolato)diboron to produce
arylboronates 6a and 7a in good yields.
Interestingly, dihaloarene 2ad could undergo selective mono-
or diborylation reaction (Scheme 2). Monoborylation product
5ad was obtained as the major product by applying a lower
loading of base and B2pin2, while diborylation product 5ad′ was
tuned to the sole product by increasing the loading of base and
B2pin2. This feature is different from that observed in the base-
promoted homolytic aromatic substitution reaction, in which
selective monosubstitution of a dihaloarene was difficult.13a,g
B
DOI: 10.1021/jacs.6b11813
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Journal of the American Chemical Society
Communication
In the reaction of 2a and a preformed diboron ate complex
(B2pin2·MeOK)18 in the presence of 10 mol % pyridine 1f,
addition of THF (0.1−1.0 mL) resulted in the formation of a
significant amount of HAT product, anisole, and the product
distributionArH/ArBpincorrelatedwellwiththeamountofTHF
added (left plot). This observation was in agreement with a
competition scenario, in which the hydrogen atom donor THF
competed with the boryl species to react with the aryl radical.
Meanwhile, it was observed that, with a fixed amount of added
THF (0.2 mL), the loading of pyridine catalyst 1f (0−20 mol %)
had a remarkable impact on the product ratio, ArBpin/ArH: the
preference for borylation significantly increased with increasing
amount of 1f (right plot). In the competition kinetics, the
promoting effect of pyridine clearly indicated the involvement of
pyridine-related boryl species in the formation of the C−B bond
and excluded the simple SRN1-type reaction between the aryl
radical and boryl species unrelated to pyridine (e.g., the ate
complex).
Third, EPR experiments confirmed the formation of radical
species under the reaction conditions (Scheme 3c). Radical
species could be produced by the reaction of pyridine 1f with the
preformed diboron ate complex, but not with diboron itself
(entries 1 and 2). A similar EPR signal was observed for the
borylation reaction mixture (entry 3). This result confirmed the
radical nature of the reaction and suggested that the homolysis of
diboron to form the pyridine-stabilized boryl radical occurred at
the stage of the ate complex.
Scheme 2. Selective Borylation Reaction of Dihaloarene 2ad
The borylation reactions of several aryl iodides were conducted
on gram scale employing inexpensive pyridine (1c) as the catalyst
(Scheme 1). In these reactions the yields were comparable to
those in small scale reactions, affording grams of arylboronates in
one batch. Since the diboron reagent is much less expensive than
the silylborane reagent,7 this method provides a simple and cost-
effective approach to arylboronates in a transition-metal-free
manner.
Preliminary mechanistic studies were conducted to disclose
some mechanistic information in this radical borylation process.
First, the generation of aryl radical from the haloarene under the
borylation conditions was confirmed by inter- and intramolecular
trapping reactions (Scheme 3a). When conducted inbenzene, the
Scheme 3. Preliminary Mechanistic Studies
Given the experimental evidence and the reported stabilization
effect of pyridine derivatives on boryl radicals,14,19 a plausible
reaction mechanism was proposed (Scheme 4). A methoxide
Scheme 4. Plausible Reaction Mechanism
anion reacts with B2pin2 to produce the ate complex A, which can
further react with pyridine to afford intermediate B. Homolytic
cleavage of the adduct B generates a pair of radicals, pyridine-
stabilized boryl radical C and methoxyboronate radical anion D.
The latter acts as a strong electron donor20 to undergo SET to an
aryl halide, producing an aryl radical after carbon-halogen bond
cleavage. Finally, capture of the aryl radical by boryl radical C
produces the borylation product.
In conclusion, a pyridine-catalyzed radical borylation reaction
of aryl and alkenyl halides has been realized, which is efficient,
easy-to-use, and scalable. This reaction utilizes the cross-coupling
of a transient radical and a persistent radical, representing a new
application of aryl radical produced in a transition-metal-free
reaction system.
reaction of 2a afforded the borylation product 5a together with a
phenylation product 8a, which was attributed to trapping of the
aryl radical by benzene. On the other hand, when aryl iodide 2ae
with an ortho-allyloxy side chain was subjected to the standard
conditions, cyclic alkylboronate 5ae was obtained, indicating an
intramolecular trapping of the aryl radical by the olefin moiety,
followed by coupling with the boryl species.10e
Second, the involvement of both aryl radical and pyridine-
related boryl species in carbon−boron bond formation was
confirmed by a series of competition experiments (Scheme 3b).
C
DOI: 10.1021/jacs.6b11813
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Journal of the American Chemical Society
Communication
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ASSOCIATED CONTENT
* Supporting Information
TheSupportingInformationisavailablefreeofchargeontheACS
Publications website at DOI: 10.1021/jacs.6b11813.
■
S
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Experimental procedures, additional information, and
characterization data (PDF)
AUTHOR INFORMATION
Corresponding Author
*Leijiao@mail.tsinghua.edu.cn
ORCID
Lei Jiao: 0000-0002-8465-1358
Notes
The authors declare no competing financial interest.
■
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ACKNOWLEDGMENTS
■
The National Natural Science Foundation of China (Grant No.
21390403) and the Thousand Talents Plan for Young
Professionals are acknowledged for financial support. The
technology platform of CBMS is acknowledged for providing
instrumentation.
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DOI: 10.1021/jacs.6b11813
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