Direct Introduction of a Dimesitylboryl Group Using Base-Mediated Substitution of Aryl Halides with Silyldimesitylborane

Article abstract of DOI:10.1002/chem.201604021

The first dimesitylboryl substitution of aryl halides with a silylborane bearing a dimesitylboryl group in the presence of alkali-metal alkoxides is described. The reactions of aryl bromides or iodides with Ph₂MeSi?BMes₂and Na(OtBu) afforded the desired aryl dimesitylboranes in good to high yields and with high borylation/silylation ratios. Selective reaction of the sterically less-hindered C?Br bond of dibromoarenes provided monoborylated products. This reaction was used to rapidly construct a D-π-A aryl dimesityl borane with a non-symmetrical biphenyl spacer.

Full text of DOI:10.1002/chem.201604021

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Accepted Article
Title: Direct Introduction of a Dimesitylboryl Group Using Base-Mediated Substituti-
on of Aryl Halides with Silyldimesitylborane
Authors: Eiji Yamamoto; Kiyotaka Izumi; Ryosuke Shishido; Tomohiro Seki; Noriaki
Tokodai; Hajime Ito
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COMMUNICATION
Direct Introduction of a Dimesitylboryl Group Using Base-
Mediated Substitution of Aryl Halides with Silyldimesitylborane
Eiji Yamamoto, Kiyotaka Izumi, Ryosuke Shishido, Tomohiro Seki, Noriaki Tokodai, Hajime Ito*
Abstract: The first Dimesitylboryl substitution of aryl halides with a
silylborane bearing a dimesitylboryl group in the presence of alkali-
metal alkoxides is described. The reactions of aryl bromides or
iodides with Ph2MeSi–BMes2 and Na(OtBu) afforded the desired
aryldimesitylboranes in good to high yields and with high
borylation:silylation ratios. Selective reaction of the sterically less-
hindered C−Br bond of dibromoarenes provided monoborylated
products. This reaction was used to rapidly construct a D-π-A
dimesitylarylborane with a non-symmetrical biphenyl spacer.
of the less sterically hindered bromo group of dibromoarene
substrates was highly selective and gave monoborylated
products. We demonstrate the utility of this method by rapidly
constructing a D-π-A dimesitylarylborane containing a non-
symmetrical biphenyl spacer.
Boron-containing π-conjugated compounds have received
considerable attention as an important class of organic
materials[1] with uses such as emissive materials,[2] two-photon
absorption materials,[3] ion sensors,[4] electron-transporting
materials,[5] and nonlinear optical materials.[6] The unique optical
properties of these compounds arise from the pπ-π* conjugation
between the vacant p-orbital of the boron atom and the π* orbital
of the attached carbon π-conjugated moieties. A dimesitylboryl
(BMes2) group has been employed in most of these compounds
because of its strong π-electron accepting ability and air stability.
The routes to access aryldimesitylborane are highly limited
despite considerable efforts to develop methods to synthesis
organoboron compounds.[7-10] The most popular synthesis of
aryldimesitylboranes involves nucleophilic substitution of
dimesitylboryl electrophiles (Mes2BX, X = halogen or OR) with
Scheme 1. a) Current methods for the synthesis of aryldimesitylboranes. b)
Direct dimesitylboryl substitution of aryl halide electrophiles with Ph2MeSi–
BMes2/base.
organometallic
compounds
(ArM,
M
=
Li,
Mg).
We initially reacted 1-bromo-4-ethylbenzene 2a with
Ph2MeSi–BMes2 and an alkoxy base (Table 1) using conditions
previously optimized for (pinacolato)borylation.[12a] The reaction
of 2a with Ph2MeSi–BMes2 (1.5 equiv) and KOMe (1.2 equiv) in
DME at 30 °C provided the corresponding borylated and
silylated products 3a and 4a, respectively, in 28% total yield with
a 4a:3a ratio of 39:61 (entry 1). The yields and B:Si selectivity
were largely dependent on the solvents and bases used for the
reaction. The use of 1,4-dioxane:hexane (1:1) and Na(OtBu)
provided the substituted products in high yield with high B:Si
selectivity (entry 2, total 80% yield, B:Si = 90:10, isolated yield of
3a: 57%). It is worthy of note that a high stirring speed (800 rpm)
is important to assure the product yield. When neat 1,4-dioxane
was used, the yield and selectivity were slightly lower (entry 3,
78%, B:Si = 87:13). The use of hexane resulted in no reaction
(Table 1, entry 4). Performing the reaction in DME resulted in
exclusive silyl substitution (entry 5, 24%, B:Si = 0:100).
Performing the reaction at 30 oC reduced the yield and
selectivity (entry 6, 33%, B:Si = 86:14). The use of the
alternative bases Li(OtBu) and K(OtBu) afforded very little
substituted products (Table 1, entries 7 and 8). Using a less
bulky base (NaOMe) promoted the reaction, albeit with lower
yield and B:Si selectivity compared with those achieved using
Na(OtBu) (entry 9, 76%, B:Si = 77:23). Attempting the reaction
with 1-chloro-4-ethylbenzene gave no expected products, while
1-ethyl-4-iodobenzene afforded the products in comparable yield
and B:Si ratio to those achieved with 2a (entry 10 and 11,
respectively).
Aryldimesitylboranes are also synthesized by reaction of aryl
boronic acid esters and MesLi.[11] These reactions are laborious
and must be individualized for each aryldimesitylborane
prepared. The use of organometallic reagents limits reaction
scope and these reagents require multi-step syntheses, with low
total yields (Scheme 1a). There are no direct methods to
synthesize ArBMes2 from ArX. We recently reported a transition-
metal-free boryl substitution of organohalides
using
(phenyldimethylsilyl)pinacolborane [PhMe2Si–B(pin)] and an
alkoxy base. This reaction allows the direct boryl substitution of
aryl, alkyl, alkenyl and heteroaryl halides without using
organometallic reagents.[12] We expected that the use of a silyl
borane[13] with a diarylboryl group instead of B(pin) could provide
a
potent complementary method for the synthesis of
triarylboranes. Herein, we report the first direct dimesitylboryl
substitution of aryl halides using
(diphenylmethylsilyl)dimesitylborane (Ph2MeSi–BMes2)[14] and
Na(OtBu), which provides the corresponding
aryldimesitylboranes in good to high yield with high
borylation:silylation (B:Si) ratios (Scheme 1b). Boryl substitution
[*]
Dr. E. Yamamoto, K. Izumi, R. Shishido, Dr. Prof. T. Seki, N.
Tokodai, Dr. Prof. H. Ito.
Division of Applied Chemistry & Frontier Chemistry Center,
Graduate School of Engineering
Hokkaido University, Sapporo 060-8628 (Japan)
E-mail: hajito@eng.hokudai.ac.jp
[**]
Supporting information for this article is given via a link at the end of
the document.
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Table 1. Optimization of the reaction conditions.[a]
terminal epoxide 2p also underwent the reaction to provide the
desired product in 69(55)% yield with a B:Si ratio of 80:20.
Table 2. Substrate scope: dimesitylborylation of aryl bromides with Ph2MeSi–
BMes2 and Na(OtBu).[a]
Entry
Solvent
Base
Total Yield
(%)[b]
B:Si[c]
1
DME
KOMe
28
39:61
90:10
87:13
2
1,4-dioxane/hexane[e]
1,4-dioxane
Na(OtBu)
Na(OtBu)
Na(OtBu)
Na(OtBu)
Na(OtBu)
Li(OtBu)
K(OtBu)
NaOMe
80 (57)[f]
3
78
0
4
hexane
5
DME
24
33
0
0:100
86:14
6[d]
7
1,4-dioxane/hexane[e]
1,4-dioxane/hexane[e]
1,4-dioxane/hexane[e]
1,4-dioxane/hexane[e]
1,4-dioxane/hexane[e]
1,4-dioxane/hexane[e]
8
2
30:70
77:23
9
76
0
10[g]
11[h]
Na(OtBu)
Na(OtBu)
76
91:9
[a] Conditions: Ph2MeSi–BMes2 (0.30 mmol), aryl halide (0.20 mmol), base
(0.24 mmol) in solvent (2 mL) at 50 °C. [b] GC yield (3a+4a). [c] B:Si ratio
(3a:4a). [d] Reaction temperature: 30 °C. [e] 1:1 (v/v) Mixture of 1,4-dioxane
and hexane solution was used. [f] Isolated yield of 3a is shown in
parentheses. [g] p-Chloroethylbenzene was used as
a substrate. [h] p-
[a] Conditions: Ph2MeSi–BMes2 (0.30 mmol), aryl halide (0.20 mmol),
Na(OtBu) (0.24 mmol) in 1:1 (v/v) mixture of 1,4-dioxane and hexane (total 2
mL) at 50 °C for 24 h. Yield (3) and borylation:silylation ratios (3:4) are based
on 1H NMR analysis with isolated yields of 3 shown in parentheses. [b]
Reaction time: 36 h. [c] Isolated yield of 3.
Iodoethylbenzene was used as a substrate.
With the optimized conditions in hand, we proceeded to
examine the substrate scope of this borylation (Table 2).
Phenylbromide (2b) and 3-tolylbromide (2c) reacted to give the
corresponding borylated products 3b and 3c, respectively, in
good yields with good B:Si selectivities [3b: 74(72)%, B:Si = 91:9,
3c: 83(66)%, B:Si = 92:8]. 2-Tolylbromide (2d) reacted with
lower yield and selectivity [3d: 31(20)%, B:Si = 67:33]. 1-
Naphthylbromide provided the borylated product in good yield
with good selectivity [3e: 70(66)%, B:Si = 90:10]. Substrates
containing an electron-donating methoxy or dimethylamino
group afforded the expected products in high yields with high
selectivity [3f: 87(73)%, B:Si = 89:11, 3g: 74(50)%, B:Si = 96:4].
Reaction of electron-deficient substrates, such as those
containing a chloro or trifluoromethyl group, also proceeded to
give the products in good yield [3h: 74(69)%, B:Si = 91:9, 3i:
58(49)%, B:Si = 84:16]. The use of substrates bearing an
aromatic alkene or alkyne moiety afforded the corresponding
products in moderate to high yields [3j: 80(42)%, B:Si = 90:10,
3k: 48(40)%, B:Si = 93:7]. Aryldimesitylboranes containing a
terphenyl, 9,9-dimethylfluorenyl or heteroaryl group also
proceeded in good yield with high selectivity [3l: 54%, B:Si =
89:11, 3m: 79(61)%, B:Si = 89:11, 3n: 79(62)%, B:Si = 89:11,
3o: 49(42)%, B:Si = 89:11]. Furthermore, substrate having a
We proceeded to examine the selective dimesitylborylation
of dihaloarenes. A directing group with a lone pair is necessary
for selective mono-metallation of multi-halogenated arenes when
using organometallic reagents. Mono-metallation is useful for
short-step syntheses of various functionalized arenes.[15]
However, the necessity of the directing group strongly limits the
reaction scope. When 1,4-dibromo-2-methylbenzene 2q was
subjected to our standard borylation conditions C4 was selective
borylated in high yield with high B:Si selectivity [Scheme 2,
78(57)%, B:Si
= 92:8, C4-substitution only]. Conventional
lithiation-borylation procedures provided the borylated products
with low site selectivity (90%, C1:C4 = 66:34, inseparable).
Borylation of 2,4-dibromo-1-methylbenzene (2s) afforded the
C4-substituted product 3s selectively, albeit in low yield. The
sterically hindered iodo
group
of
4-bromo-1-iodo-2-
methylbenzene (2r) underwent exclusive boryl substitution to
give product 3r in good yield with a high B:Si ratio [67(51)%,
B:Si = 95:5].
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Figure 1. Emission spectra of 5q and 5t (ex = 365 nm). Inset shows
photographs of 5q and 5t taken under UV light.
Crystallographic analyses indicate that introduction of
methyl group into a biphenyl bridge unit alter conformation of the
resulting D-π-A compounds. X-ray diffraction analyses indicate
that 5q and 5t crystallize into I2/a (monoclinic) and P-1 (triclinic)
respectively (Figure 2, and see the SI for details), indicating that
the methyl-substituted 5q is in less symmetric crystalline lattice.
5q exhibits rotational disorder at the methyl-substituted benzene
ring (Figure 2). Conformations of molecules in two crystals 5q
and 5t are different: dihedral angle of two benzene rings of
biphenyl moiety of 5q, which does not have methyl group, is
27.29°, while the corresponding dihedral angle of methyl-
substituted 5t is 70.49°. This would be the origin of the
aforementioned lower em of 5q (Figure 1) as well as lower
melting temperature of 5q (236 °C for 5q; 262 °C for 5t). This
synthetic procedure can contribute the modified conformation of
the “π” segment of D-π-A fluorophores and this may further
result in the tuning of their solid structure and the photophysical
properties.
Scheme 2. Diarylborylation of dihaloarenes with Ph2MeSi–BMes2 and
Na(OtBu). Reactions were carried out at 50 oC for 24 h. Yield is based on 1H
NMR analysis of the crude products. Isolated yield of the borylated product is
shown in parentheses. B:Si ratio and ratios of regioisomers were determined
based on 1H NMR analysis.
The utility of this selective dimesitylborylation was
demonstrated by the rapid construction of a donor-(π-spacer)-
acceptor (D-π-A) system[2d] with a non-symmetrical biphenyl
spacer (Scheme 3). Aryl bromide 3q was prepared in one step
from dihaloarene 2q and successfully underwent Suzuki–
Miyaura coupling using XPhos ligand to afford the corresponding
new compound 5q in 90% yield. This good yield can be
achieved only with the present synthetic procedure. The Mes2B
group is known to serve as an effective coupling partner under
typical
Suzuki–Miyaura
coupling
conditions,[16]
and
unsatisfactory yields of desired coupling products using
Pd(PPh3)4 have been reported in the literature,[17] probably
because of a competing undesired coupling process. We
confirmed that the compound 5q shows strong luminescence in
CH2Cl2 (emission quantum yield em is 36%, Figure S2) and in
the solid state (em,max = 431 nm, em = 65%; Figure 1). It is
noted that 5t also displays strong luminescence with higher em
in the solid state (m,max = 421 nm, em = 95%; Figure 1).
Figure 2. Single crystal structures of 5q and 5t. In 5q, two methyl groups were
observed because of the rotational disorder.
A possible mechanism for our dimesitylboryl substitution is
depicted in Scheme 4.[12,18] Ph2MeSi–BMes2 initially reacts with
Na(OtBu) to give the corresponding ate complex A. The
nucleophilic Ph2MeSi group of A attacks the bromine atom in the
arylbromide to give the aryl anion intermediate B. Subsequent
nucleophilic attack of the carbanion species to the boron atom in
the Mes2B(OtBu) affords triarylborane 3. This proposed
mechanism follows the same process as the mechanism for
boryl substitution with PhMe2Si–B(pin) and an alkoxy base.[12]
However, the optimized conditions for dimesityl borylation are
slightly different from those for the previously reported borylation
[PhMe2Si–B(pin) (1.5 equiv) and KOMe (1.2 equiv) in DME at
30 °C].[12a] An important difference is that our reaction requires
the sterically hindered tert-butoxide base to ensure high B:Si
ratio, while the use of a small methoxide base is important for
high B:Si ratio in the previously reported borylation. We
speculated that the coordination of the solvent or additional
alkoxide to the boron centre may affect the reactivity and
selectivity because the dimesitylboryl group has stronger Lewis
acidity than a pinacol boryl group.
Scheme 3. Rapid construction of a D-π-A dimesitylarylborane with a non-
symmetrical biphenyl spacer.
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In conclusion, we developed
a method to synthesize
aryldimesityl boranes using Ph2MeSi–BMes2 and Na(OtBu) for
the first time. This reaction afforded aryl dimesitylboranes in
good to high yields with high B:Si ratios. In addition, the site-
selective dimesitylborylation of dibromoarenes was achieved,
which can provide rapid access to various aryl dimesitylboranes.
The utility of this method was demonstrated by the synthesis of
a D-π-A dimesitylarylborane with a non-symmetrical biphenyl
spacer.
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Acknowledgements
We thank Professor. Todd B. Marder for a fruitful discussion.
This work was financially supported by the MEXT (Japan)
program “Strategic Molecular and Materials Chemistry through
Innovative Coupling Reactions” of Hokkaido University. This
work was also supported by JSPS KAKENHI (grant numbers
15H03804, 15K13633 and 262447). E. Y. was supported by a
Grant-in-Aid for JSPS Fellows.
Keywords: silylborane • triarylborane • boryl substitution •
dimesitylborylation • base-mediated borylation
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Dimesitylboryl substitution of aryl
halides using Ph2MeSi–BMes2 and
Na(OtBu) was developed as a novel
direct method to synthesize
Eiji Yamamoto, Kiyotaka Izumi, Ryosuke
Shishido, Tomohiro Seki, Noriaki
Tokodai, Hajime Ito*
Page No. – Page No.
aryldimesityl boranes. This reaction
afforded aryl dimesityl boranes in
good to high yields with high B:Si
ratios. In addition, the site-selective
dimesityl borylation of dibromoarenes
was achieved, which can provide
rapid access to various aryl
Title: Direct Introduction of a
Dimesitylboryl Group Using Base-
Mediated Substitution of Aryl Halides
with Silyldimesitylborane
dimesitylboranes.
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