Electrophilic borylation of terminal alkenes with BBr3/2,6-disubstituted pyridines

Article abstract of DOI:10.1021/acs.orglett.8b00335

A variety of terminal alkenes, as well as heteroaromatic compounds, are borylated by the combined use of BBr₃/2,6-dichloropyridine (B3) or BBr₃/2,6-lutidine (B5). α,α-Diarylalkenes prefer the former reagent combination, while other alkenes prefer the latter. Mechanistic considerations strongly suggest that the former and latter reactions proceed through electrophilic substitution reactions with BBr₃ and [BBr₂·B5]⁺BBr₄^-, respectively.

Full text of DOI:10.1021/acs.orglett.8b00335

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Electrophilic Borylation of Terminal Alkenes with BBr3/2,6-
Disubstituted Pyridines
Shinya Tanaka,* Yuki Saito, Takaya Yamamoto, and Tetsutaro Hattori*
Department of Biomolecular Engineering, Graduate School of Engineering, Tohoku University, 6-6-11 Aramaki-Aoba, Aoba-ku,
Sendai 980-8579, Japan
S
* Supporting Information
ABSTRACT: A variety of terminal alkenes, as well as
heteroaromatic compounds, are borylated by the combined
use of BBr3/2,6-dichloropyridine (B3) or BBr3/2,6-lutidine
(B5). α,α-Diarylalkenes prefer the former reagent combination,
while other alkenes prefer the latter. Mechanistic considerations
strongly suggest that the former and latter reactions proceed
through electrophilic substitution reactions with BBr3 and [BBr2·B5]+BBr4 , respectively.
−
lkenylboronic esters are important reagents for the
synthesis of substituted alkenes through Suzuki−Miyaura
base, would be applicable to the borylation of alkenes. Herein,
we report the borylation of terminal alkenes with BBr3 in the
presence of 2,6-disubstituted pyridines.
A
or other coupling reactions.1 A conventional method for the
preparation of alkenylboronic esters involves the hydroboration
of alkynes, and various catalysts have been developed to effect
this transformation.2 Transition-metal-catalyzed borylation3
and dehydrogenative borylation4 with hydroboronates or
diboron compounds have an advantage over the hydroboration,
in that they use inexpensive and readily available alkenes as
starting materials. In addition, these reactions provide facile
access to β,β-disubstituted alkenylboronates. However, they
often suffer from low yields because of competing side
reactions, such as alkene hydroboration and hydrogenation.
In this respect, the palladium-catalyzed Heck-type reaction of
alkenes with B-chlorocatecholborane5 is a fascinating alternative
that avoids these side reactions. The ruthenium-catalyzed cross
metathesis between an alkene and a vinylboronic ester6 is
another potent method for the preparation of an alkenylboronic
ester.
First, α,α-diphenylethylene (1a) was borylated using a
dichloromethane solution of BBr3 in toluene, which was the
best solvent for the carboxylation of alkenes,12a in the presence
of various 2,6-disubstituted pyridines (Table 1). To avoid
decomposition in an aqueous medium, the resulting
dibromovinylborane was converted into pinacol ester 2a by
the treatment of the reaction mixture with pinacol under basic
conditions prior to aqueous workup. Among the dihalopyr-
idines tested, 2,5-dichloropyridine (B1) was most effective,
affording ester 2a in 20% yield together with recovered 1a
(70%) (entries 1−3). The use of dibromopyridine (B2), which
was the best-performing amine in combination with EtAlCl2 for
the carboxylation of 1a,12 resulted in a considerably reduced
yield of recovered 1a (20%), while the yield of 2a was
maintained. Increasing the basicity of B1 by replacing the
chlorine atom(s) with alkyl group(s) did not improve the yield
of 2a, irrespective of the bulkiness of the alkyl substituents
(entries 4−6). Consequently, reaction conditions were further
examined using dichloride B1. The removal of toluene from the
mixed solvent slightly improved the yield of 2a (entry 7), while
lowering the reaction temperature for borylation to 0 °C
further improved the yield, as well as the mass balance of the
starting and product alkenes (entry 8). The best yield (81%)
was finally obtained by optimizing the molar equivalences of
BBr3 and B1 (1a/BBr3/B1 = 1:3:3) (entry 9).13 It should be
noted that BCl3 did not borylate 1a in combination with any of
the amines except B4 (Table S2 in the Supporting Information
(SI)).13
It is known that arenes and alkynes are electrophilically
borylated with boron species. Ingleson and co-workers reported
the borylation of aromatic and heteroaromatic compounds with
a borenium species generated from an amine/BCl3/AlCl3
mixture.7−9 Blum and co-workers reported the borylative
cyclization of alkynes with B-chlorocatecholborane.10,11 How-
ever, the electrophilic borylation of alkenes has yet to be
achieved, presumably due to the marked tendency for alkenes
to undergo cationic polymerization under Lewis acidic
conditions. Recently, we found that aryl- and alkyl-substituted
alkenes are carboxylated with CO2 by the combined use of
EtAlCl2 and 2,6-dibromopyridine.12 This reaction proceeds
through the base-promoted electrophilic substitution of an
alkene with EtAlCl2 to form a vinylaluminum ate complex,
followed by the carbonation of this complex with CO2. It is
noteworthy that the bulky weak base enabled the alumination
of alkenes while suppressing their decomposition and/or
polymerization without Lewis acid deactivation. We envisaged
that this concept, with a suitably chosen boron reagent and
α-Methylstyrene (1b) was borylated under the optimized
conditions using dichloride B1 as the base to give the desired
boronic ester 2b in a poor yield (entry 1 in Table 2).
Received: January 30, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b00335
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
the less-basic tetramethylpyrazine (B9), with the same steric
environment around the nitrogen atom as that of B5, were
ineffective (entries 5−8). On the other hand, 2-methylquino-
line (B10), with similar steric bulk and basicity to B5, exhibited
comparable performance to B5 (entry 9 compared with entry
4). These observations indicate that appropriate steric bulk and
basicity are necessary for the base to promote the borylation of
1b. Optimization of the molar equivalences of BBr3 and B5
(1b/BBr3/B5 = 1:4:4) improved the yield of the ester (87%)
along with its E/Z selectivity (78/22) (entry 11).13 The
reaction time could be shortened to 1 h by increasing the
temperature to room temperature without any reduction in
yield and stereoselectivity (entry 12); the reaction conditions
employed in entry 12 were adopted as standard hereafter.
Various alkenes were borylated under the standard
conditions using 2,6-lutidine (B5) as the base (Scheme 1).
Table 1. Borylation of α,α-Diphenylethylene (1a) with BBr3
in the Presence of Various Bases
a
b
c
c
entry
base
pKa
solvent
2a (%)
20
1a (%)
f
f
f
f
f
f
1
2
3
4
5
6
7
B1
B2
B3
B4
B5
B6
B1
B1
B1
−2.8
−2.2
−1.9
1.9
toluene−CH2Cl2
toluene−CH2Cl2
toluene−CH2Cl2
toluene−CH2Cl2
toluene−CH2Cl2
toluene−CH2Cl2
CH2Cl2
70
20
47
58
86
>99
49
45
15
14
n.d.
22
6.5
n.d.
n.d.
28
5.5
−2.8
−2.8
−2.8
d
8
de
,
CH2Cl2
55
Scheme 1. Borylation of Various Alkenes and
a b c
, ,
gh
,
9
CH2Cl2
81 (69)
Heteroaromatic Compounds
a
Reaction conditions: 1a (0.50 mmol), BBr3 (1.2 molar equiv as a 1.0
M solution in CH2Cl2), base (1.0 molar equiv), solvent (1.0 mL), rt,
24 h; then pinacol (3.0 molar equiv), Et3N (15 molar equiv), rt, 1 h.
b
c
Calculated value for the conjugate acid. See ref 14 for details. 1H
d
NMR yield determined using CH2Br2 as the internal standard. 0 °C
for borylation. BBr3 (3.0 molar equiv) and B1 (3.0 molar equiv) were
e
f
g
used. CH2Cl2 was incorporated from the BBr3 solution. Isolated
h
yield. iPr2NEt was used instead of Et3N.
Table 2. Borylation of α-Methylstyrene (1b) with BBr3 in the
a
Presence of Various Bases
b
c
c
entry
base
pKa
2b (%)
E/Z
1
2
3
4
5
6
7
8
9
10
B1
B2
B4
B5
B6
B7
B8
B9
B10
Et3N
B5
B5
−2.8
−2.2
1.9
4
100/0
−
−
n.d.
n.d.
64
a
6.5
64/36
Reaction conditions: 1 (0.50 mmol), BBr3 (4.0 molar equiv as a 1.0
M solution in CH2Cl2), B5 (4.0 molar equiv), CH2Cl2 (1.0 mL), rt, 1
5.5
n.d.
n.d.
n.d.
n.d.
66
−
−
−
−
i
h; then pinacol (4.0 molar equiv), Pr2NEt (15 molar equiv), rt, 1 h.
5.8
b
c
Isolated yield. 1H NMR yield is shown in parentheses. E/Z ratio was
5.1
d
1
determined by H NMR analysis. The reaction was conducted using
BBr3 (3.0 molar equiv), B1 (3.0 molar equiv), and pinacol (3.0 molar
1.9
5.1
56/44
e
f
equiv). Borylated at −40 °C for 24 h. Borylated at 0 °C for 24 h.
10.8
6.5
n.d.
87
80 (77) ,
−
78/22
80/20
g
The reaction was conducted using BBr3 (3.0 molar equiv), B5 (3.0
d
11
molar equiv), and pinacol (3.0 molar equiv).
de
,
fg
12
6.5
a
Reaction conditions: 1b (0.50 mmol), BBr3 (3.0 molar equiv as a 1.0
As for α,α-diarylethylenes, better yields were obtained under
the reaction conditions optimized for α,α-diphenylethylene
(1a); consequently, results obtained under the latter conditions
are listed in Scheme 1. α,α-Diphenylethylenes bearing
substituents at the para-positions of the benzene rings were
borylated in good yields irrespective of the electron-donating or
-withdrawing nature of the substituents, to give 2c and 2d;
however α,α-diphenylpropylene did not afford any 2e. Various
straight-chain terminal alkenes gave the corresponding boronic
esters 2f−k with complete E-stereoselectivity in moderate-to-
high yields. Notably, allylbenzene gave the (E)-vinylboronic
ester 2g as a single product, unlike the corresponding boryl-
M solution in CH2Cl2), base (3.0 molar equiv), CH2Cl2 (1.0 mL), 0
°C, 24 h; then pinacol (3.0 molar equiv), Et3N (15 molar equiv), rt, 1
b
c
1
h. Calculated value. See ref 14 for details. Determined by H NMR
d
analysis. BBr3 (4.0 molar equiv), base (4.0 molar equiv), and pinacol
e
f
(4.0 molar equiv) were used. Borylated at rt for 1 h. Isolated yield is
g
shown in parentheses. iPr2NEt was used instead of Et3N.
Rescreening of the bases revealed that 2,6-lutidine (B5) was
more promising, affording 2b in a moderate yield (64%) with
an E/Z ratio of 64/36 (entries 2−4). The more-bulky di-tert-
butylpyridine (B6) and less bulky monomethylpyridine (B7)
and pyridine (B8), all having similar basicity to B5, as well as
B
DOI: 10.1021/acs.orglett.8b00335
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Heck reaction that reportedly gives double-bond and E/Z-
isomerization products.6 Bromo and siloxy groups were
tolerated under the reaction conditions. An α,α-dialkylethylene
also gave the desired boronic ester 2l, but a trialkyl-substituted
internal alkene did not give 2m. Taking advantage of this
behavior, (−)-limonene, with both internal and terminal double
bonds, was borylated to afford 2n in which the terminal double
bond of limonene was selectively borylated. Furthermore, this
reaction could also be applied to heteroaromatic compounds;
1-methylindole and 2-methylthiophene were borylated in high
yields to give 2o and 2p, respectively.
observation as indicating that an analogous borenium species,
namely [BBr2·B5]+BBr4 , was generated from two BBr3 and
−
one B5 molecule; that is, BBr3 serves as both the donor and
acceptor of Br−. The borylation of 1b was then attempted using
BBr3/B5 (2.0:1.0), as well as BCl3/AlCl3/B5 (1.0:1.1:1.05).
However, both reactions failed (entries 3 and 1 in Table 3). On
Table 3. Borylation of 1b with Various Combinations of
a
Lewis Acid(s) and B5
We carried out NMR experiments to gain insight into the
reaction mechanism. A 1:1 mixture of BBr3 and dichloropyr-
idine (B1) in CD2Cl2 was analyzed by 11B NMR spectroscopy
to investigate the acid−base interaction between them; the
spectrum exhibited a singlet at a chemical shift (δ 38.1 ppm)
close to that of BBr3 (39.0 ppm) (Figure 1a). The
b
entry
BX3 (x)
AlY3 (y)
z
yield (E/Z)
1
2
3
4
5
BCl3 (1.0)
BCl3 (1.0)
BBr3 (2.0)
BBr3 (2.0)
BCl3 (3.0)
AlCl3 (1.1)
1.05
2.0
1.0
2.0
3.0
n.d.
AlCl3 (1.1)
8 (56/44)
n.d.
−
−
−
44 (61/39)
3 (67/33)
a
Reaction conditions: 1b (0.50 mmol), BBr3 (3.0 molar equiv as a 1.0
M solution in CH2Cl2), base (3.0 molar equiv), CH2Cl2 (1.0 mL), 0
°C, 24 h; then pinacol (3.0 molar equiv), Et3N (15 molar equiv), rt, 1
b
c
1
h. Calculated value. See ref 14 for details. Determined by H NMR
d
analysis. BBr3 (4.0 molar equiv), base (4.0 molar equiv), and pinacol
e
f
(4.0 molar equiv) were used. Borylated at rt for 1 h. Isolated yield is
g
shown in parentheses. iPr2NEt was used instead of Et3N.
the other hand, we found that the reaction proceeded for each
reagent combination when the amount of base equaled the total
amount of the Lewis acid(s) employed (entries 2 and 4).
Previously we reported that B2 is necessary in the EtAlCl2/B2-
mediated carboxylation of alkenes with CO2, as it is required to
abstract a proton from the initially formed alkene−EtAlCl2
adduct.12a Therefore, we conclude that, in addition to the base
used for the formation of the borenium species, an additional
amount of base is required to abstract a proton from the
alkene−borenium adduct. Replacement of BBr3 with BCl3
greatly reduced the yield of boronic ester 2b, even when
excess reagents were used in a 1:1 (BCl3/B5) ratio (entry 5),
indicating that BCl3 hardly forms a borenium species in its self-
bimolecular B5-mediated reaction. On the basis of these results,
a feasible reaction mechanism for the borylation with BBr3/B5
is shown in Scheme 3. Borenium species A, generated from
Figure 1. 11B NMR spectra of 1:1 mixtures of (a) BBr3 and B1 and (b)
BBr3 and B5 in CD2Cl2.
corresponding species exhibited the same splitting pattern to
1
that of B1 in its H NMR spectrum, but the signals were
slightly shifted to lower fields (Figure S1). These observations
indicate that the acid−base equilibrium between BBr3 and B1
lies far to the dissociation side. Therefore, we conclude that the
B1-mediated borylation proceeds via a zwitterionic species
formed through the addition of BBr3 to an alkene, followed by
the abstraction of a proton by B1, and the subsequent
elimination of bromide from the resulting borate complex
(Scheme 2). It seems that proper basicity and bulkiness (i.e., as
those of B1) are necessary for the base to abstract a proton
from the zwitterionic species without deactivating BBr3.
Scheme 3. Feasible Mechanism for the B5-Mediated
Borylation with BBr3
Scheme 2. Feasible Mechanism for the B1-Mediated
Borylation with BBr3
On the other hand, the 11B NMR spectrum of a 1:1 mixture
of BBr3 and lutidine (B5) exhibited several singlets assignable
to tetracoordinate borate complexes in the negative chemical
shift region (Figure 1b). Furthermore, a broad singlet was
observed at lower field (47.2 ppm) than that of BBr3, consistent
with the formation of a borenium species.7b,e,f,15 On the basis of
BBr3 and B5, electrophilically attacks an alkene to form adduct
B, from which a free base (B5) abstracts a proton at the α-
position to the boron, affording the desired vinylborane. In this
mechanism, a rather strong base is necessary to push out a
bromide from BBr3, and it has to have enough steric bulk so as
the fact that borenium species [BCl2·B5]+AlCl4 is generated
−
from BCl3, AlCl3, and B5 (1.0:1.1:1.05),7e we interpret this
C
DOI: 10.1021/acs.orglett.8b00335
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
1191. (g) Iwadate, N.; Suginome, M. Chem. Lett. 2010, 39, 558.
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coordination of another base molecule.
In conclusion, we have developed a method for the
electrophilic borylation of alkenes by the combined use of
BBr3/2,6-dichloropyridine (B1) or BBr3/2,6-lutidine (B5). The
active electrophiles for the former and latter reactions are BBr3
(5) Reid, W. B.; Spillane, J. J.; Krause, S. B.; Watson, D. A. J. Am.
Chem. Soc. 2016, 138, 5539.
−
and [BBr2·B5]+BBr4 , respectively. A base is necessary in each
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M. J. Chem. Commun. 2015, 51, 2878.
reaction to facilitate the abstraction of a proton from the
alkene−electrophile adduct, in addition to the formation of a
borenium species in the latter reaction.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b00335.
Supplemental data, experimental procedures, character-
ization data, and NMR spectral charts (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
(8) (a) Ishida, N.; Moriya, T.; Goya, T.; Murakami, M. J. Org. Chem.
2010, 75, 8709. (b) Prokofjevs, A.; Kampf, J. W.; Vedejs, E. Angew.
Chem., Int. Ed. 2011, 50, 2098.
*E-mail: shinya.tanaka.d8@tohoku.ac.jp.
*E-mail: tetsutaro.hattori@tohoku.ac.jp.
(9) (a) Kitani, F.; Takita, R.; Imahori, T.; Uchiyama, M. Heterocycles
2017, 95, 158. (b) Yin, Q.; Klare, H. F. T.; Oestreich, M. Angew.
Chem., Int. Ed. 2017, 56, 3712.
(10) (a) Faizi, D. J.; Issaian, A.; Davis, A. J.; Blum, S. A. J. Am. Chem.
Soc. 2016, 138, 2126. (b) Faizi, D. J.; Davis, A. J.; Meany, F. B.; Blum,
S. A. Angew. Chem., Int. Ed. 2016, 55, 14286. (c) Issaian, A.; Faizi, D. J.;
Bailey, J. O.; Mayer, P.; Berionni, G.; Singleton, D. A.; Blum, S. A. J.
Org. Chem. 2017, 82, 8165.
(11) (a) Warner, A. J.; Lawson, J. R.; Fasano, V.; Ingleson, M. J.
Angew. Chem., Int. Ed. 2015, 54, 11245. (b) Warner, A. J.; Churn, A.;
McGough, J. S.; Ingleson, M. J. Angew. Chem., Int. Ed. 2017, 56, 354.
(12) (a) Tanaka, S.; Watanabe, K.; Tanaka, Y.; Hattori, T. Org. Lett.
2016, 18, 2576. (b) Tanaka, S.; Chiba, M.; Saito, Y.; Yamamoto, T.;
Hattori, T. Bull. Chem. Soc. Jpn. 2017, 90, 419.
ORCID
Shinya Tanaka: 0000-0001-9741-6253
Tetsutaro Hattori: 0000-0001-8422-4674
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by JSPS KAKENHI Grant Number
K17K144430.
■
REFERENCES
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DOI: 10.1021/acs.orglett.8b00335
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