Metal-Free Selective Borylation of Arenes by a Diazadiborinine via C-H/C-F Bond Activation and Dearomatization

Article abstract of DOI:10.1021/jacs.9b06022

A newly developed annulated 5-chlorinated 1,3,2,5-diazadiborinine derivative (4) selectively activates a C-H bond of benzene (C₆H₆) and 1,3-di(trifluoromethyl)benzene, as well as a C-F bond in partially fluorinated arenes, to furnish borylation products under catalyst-, metal-, and irradiation-free conditions. Moreover, 4 readily undergoes a reversible dearomative coupling reaction with polycyclic aromatic hydrocarbons to afford diboration products. The latter represents the first reversible intermolecular dearomative diboration of arenes.

Full text of DOI:10.1021/jacs.9b06022

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Communication
Metal-free Selective Borylation of Arenes by a Diazadiborinine
via C#H/C#F Bond Activation and Dearomatization
Yuanting Su, Dinh Cao Huan Do, Yongxin Li, and Rei Kinjo
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b06022 • Publication Date (Web): 16 Aug 2019
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Journal of the American Chemical Society
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Metal-free Selective Borylation of Arenes by a Diazadiborinine via
C‒H/C‒F Bond Activation and Dearomatization
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Yuanting Su,† Dinh Cao Huan Do,† Yongxin Li,† Rei Kinjo*,†
†Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang
Technological University, 637371, Singapore
Supporting Information Placeholder
diazadiborinine (Figure 1e). We also present its reactivity
towards a series of aromatic hydrocarbons and partially
fluorinated arenes.
ABSTRACT: A newly developed annulated 5-chlorinated
1,3,2,5-diazadiborinine derivative (4) selectively activates a
C‒H
bond
of
benzene
(C6H6)
and
1,3-
di(trifluoromethyl)benzene, as well as, a C‒F bond in
partially fluorinated arenes, to furnish borylation products
under the catalyst-, metal-, and irradiation-free condition.
Moreover, 4 readily undergoes a reversible dearomative
coupling reaction with polycyclic aromatic hydrocarbons to
afford diboration products. The latter represents the first
reversible intermolecular dearomative diboration of arenes.
C–H borylation
(a)
R
R
B
B
R'3 or 2R'4
H
B
R'2
 or Lewis acids
or hv
I
C–F borylation
(b)
(c)
R
R
B
2R'4
F
B
R'2
hv
II
Organoboranes are ubiquitous in various organic
transformations being found as synthetic intermediates, and
hence, its simple and convenient synthesis has received
significant attention.1 Among them, direct borylation of non-
activated aromatic hydrocarbons has been the subject of a
number of landmark studies over the past several decades, as
it represents straightforward conversion of inert raw
materials into value-added chemical building blocks. To
achieve this task, various transition metal catalysts, as well as,
other metal-containing molecules, have been dominantly
employed for borylation of arenes.2,3
Dearomative borylation
Ar
N
Ar
Ar
Ar
Ph
Ph
Ph Ph
B
hv
N
hv
B
B
B
Ph

Mes
Mes
R
Ph
R
III
IV
N
N
H
N
B
H
N
B
V
On the other hand, metal-free borylation of aromatic
hydrocarbons has been described in the literature,4 that
involve (i) C‒H bond borylation of arenes I under heating
and Lewis acidic conditions or irradiation of light (Figure
1a),5 (ii) photo-induced C‒F bond borylation of fluoroarenes
II (Figure 1b),6 (iii) intramolecular dearomative borylation of
tetracoordinate boron species III and IV by photochemical
reaction or intermolecular trapping of borylene V (Figure
1c).7,8,9 Despite recent progress in this area, facile metal-free
borylation of C‒F bonds of aromatic hydrocarbons, especially
without photolysis, still remains extremely challenging, and
to our knowledge, intermolecular and thermally reversible
dearomative borylation is hitherto unprecedented. Recently,
we have reported that annulated 1,3,2,5-diazadiborinine VI
featuring a relatively small HOMO‒LUMO gap (2.84 eV)
cleavages the H‒H bond of dihydrogen and an N‒H bond of
ammonia under mild conditions (Figure 1d).10 With this
approach in hand, we envisaged that reducing the steric
congestion around the boron center of 1,3,2,5-
diazadiborinine would allow access to the more reactive
derivative, which may be applied for metal-free borylation of
arenes, even without photo irradiation. Herein, we report the
synthesis, single crystal X-ray diffraction analysis and
(d)
(e)
Previous work
This work
Ph
Ph
Ph
Ph
Ph
Ph
B
B
C–H borylation
Ph
C–F borylation
B
N
N
N
N
Ph
Mes
Ph
Mes
Ph
Mes
N
N
N
N
B
Dearomative borylation
Mes
Ph
VI
Cl
Figure 1. Reported examples of metal-free borylation of
arenes via (a) C‒H bond activation, (b) C‒F bond activation,
and (c) photo-induced dearomatization. (d) Our previous
work, and (e) present work.
We began our investigation with the preparation of
1,3,2,5-diazadiborinine featuring a small Cl substituent on
the boron atom between two carbon atoms (Scheme 1).
Treatment of compound
1 with one equivalent of
dichlorophenylborane gave compound 2 in 68% yield, which
was further reacted with two equivalents of hydrogen
chloride, affording a 2,5,5-trichloro-1,3,2,5-diazadiborinine
derivative 3 as a white powder in 65% yield. Reduction of 3
computational studies of
a
5-chlorinated 1,3,2,5-
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with four equivalents of potassium graphite in toluene under
six-membered ring, indicating that the C‒H activation
proceeds regio-and stereoselectively. Note that formation of
5 and 6 represents the first examples of C‒H activation of
arenes by diazadiborinine.11 DFT calculations at the M06-
2X/Def2-SVP level shows that C‒H activation of benzene by
4 may proceed in a concerted manner with the activation
barrier of 24.4 kcal·mol–1 (Figure 3). NBO analysis indicates
that the Mulliken charge of the boron between two nitrogen
atoms is more positive than that of the boron between two
carbon atoms (Figure S99 and Table S3), supporting the
regioselectivity of the reaction.
ambient condition afforded, after work up, 5-chlorinated
1,3,2,5-diazadiborinine derivative 4 as an orange powder in
62% yield. Spectroscopically, the 11B NMR spectrum of 4
displays a singlet for the boron atom between two carbon
atoms at δ = 10.5 ppm and a broad peak for the boron
between two nitrogen atoms at δ = 25.8 ppm, both of which
are shifted downfield relative to those of the precursor 3 (δ =
‒6.2 and 4.0 ppm), and comparable to those of VI (δ = 9.8
and 25.4 ppm).10 Structurally, 4 features a geometry (Figure
2), which is nearly identical to that of VI. Nevertheless, the
space-filling model of 4 clearly shows the steric congestion
around the boron atom between two carbon atoms is
significantly less than that of VI, owing to the replacement of
the phenyl group in VI by the smaller chloride in 4.
Theoretically, the energy gap between the HOMO and the
LUMO (Figure S98) of 4 is estimated to be 2.85 eV, which is
comparable to that (2.84 eV) of VI. 10
1
2
3
4
5
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4
CF3
CF3
90 oC, 30 h
120 oC, 18 h
H
H
F3C
CF3
Ph
Ph
N
Ph
N
Ph
Ph
N
Ph
Ph
Cl
B
B
5
B
B
6
Ph
Ph
Ph
Ph
N
Mes
Ph
Mes
Ph
Ph
Mes
Ph
B
B
2
B
Ph Cl2
N
N
N
N
N
N
N
N
Ph
Mes
Ph
Ph
Ph
CH2Cl2
Mes
Mes
N
N
N
N
B
Cl
H
Cl
H
Mes
Mes
Ph
(72%)
NiPr2
1
NiPr2
(68%)
(76%)
Cl
Scheme 2. Reactions of
4
with benzene and 1,3-
toluene
[iPr2NH2]Cl
HCl Et2O
(2 eq.)
di(trifluoromethyl)benzene.
Cl Ph
Ph
rel (kcal·mol1
)
G
Ph
Ph
Ph
B
B
N
N
KC8 (4 eq.)
toluene
 KCl
N
N
H
Ph
Ph
Ph
Ph
Mes
Ph
Ph
B
N
N
N
N
N
B
B
Ph
B
N
N
Mes
Mes
TS: 24.4
Mes
Cl
Cl
Cl
Cl
Mes
Ph
Ph
N
4
(62%)
3
(65%)
0.0
Mes
Scheme
1. Synthesis of
5-chlorinated
1,3,2,5-
diazadiborinine 4 (Mes = 2,4,6-trimethylphenyl).
27.1
5

4
+
Figure 3. Plausible reaction pathway for the formation of 5.
Interestingly, when partially fluorinated arene derivatives
were employed as the reactants, chemoselective C-F bond
activation was observed (Scheme 3).3, 12 Thus, treatment of 4
with pentafluorobenzene at 80 oC for 12 h afforded 7 in 70%
yield. In the 11B NMR spectrum of 7, two broad resonances
were detected at ‒11.0 and 4.8 ppm. The 19F NMR spectrum
of 7 displays four multiplets at ‒129.3 ppm, ‒143.2 ppm,
‒160.4 ppm and ‒161.6 ppm for the fluorine atoms
substituted on the arene ring and one broad singlet at ‒153.8
ppm for the boron-bound fluorine atom. The CH in the
fluorinated aryl group is signaled at 115.1 ppm in the 13C
Figure 2. Solid-state structure of 4 (Hydrogen atoms are
omitted for clarity), overlaid with the Space-filling model.
1
(DEPT135) NMR spectrum and 6.14 ppm in the H NMR
spectrum, respectively. An X-ray diffraction analysis
revealed that 7 is formed via the cleavage of a C‒F bond of
pentafluorobenzene (Figure S74). The fluorinated aryl group
is bound to the boron atom between two carbon atoms and
the fluorine atom is attached on the boron atom between two
nitrogen atoms with the B‒F bond distance of 1.406(4) Å,
which are on the same side with respect to the B2C2N2 six-
membered ring. Previous experimental and computational
studies have indicated that activation of the C‒F bond in
pentafluorobenzene typically becomes more favorable in the
Next, we examined the reactivity of 4 towards benzene and
its derivative (Scheme 2). Heating a benzene solution of 4 at
o
90 C for 30 h led to a colorless solution. After workup,
compound 5 was isolated as a white solid in 76% yield.
Analogously,
treatment
of
4
with
1,3-
o
di(trifluoromethyl)benzene at 120 C afforded compound 6
as a white powder in 72% yield. Compounds 5 and 6 have
been characterized by standard spectroscopic, analytical and
crystallographic methods, confirming that the aryl group is
attached on the boron between two nitrogen atoms and the
H atom forms a bond with the boron atom between two
carbon atoms. Both the aryl group and the H atom on the
boron atoms are on the same side with respect to the B2C2N2
order para > meta > ortho fluorine atom with respect to C‒H
In marked contrast, the C‒F bond cleavage of
pentafluorobenzene by 4 regioselectively occurred at ortho
12
bond.3,
position with respect to the C‒H bond, the least plausible
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position. Similarly, the C‒F bond activation over the C-H
with polycyclic aromatic hydrocarbons (Scheme 4). Thus,
treatment of with excess amount of naphthalene
4
1
2
3
4
5
6
7
8
bond cleavage was observed in the reaction of 4 with 1,2,4,5-
tetrafluorobenzene, and 8 was obtained in 76% yield.
Treatment of 4 with less fluorinated arenes, such as 1,2,3,5-
tetrafluorobenzene and 1,2,4-trifluorobenzene, afforded 9
(74% yield) and 10 (82% yield), respectively, the
regioselectivity of which are also in contrast to the previous
reports (Scheme 3). Moreover, when 1-chloro-4-
fluorobenzene was employed, 4 cleaved the C‒F bond, rather
than the weaker C‒Cl bond, to furnish 11 in 86% yield.
F
exclusively afforded 12 in 89% yield, which was fully
characterized by standard spectroscopic and analytical
techniques, and its structure in the solid state was
determined by single-crystal X-ray diffraction (Figure S79).
The carbon atoms at the 1- and 2-position of naphthalene
regioselectively bind to the boron between two carbon atoms
and the boron between two nitrogen atoms in 4, respectively.
Remarkably, upon heating a C6D6 solution at 80 oC for 3.5 h,
12 undergoes a retro[4+2] cycloaddition to reproduce 4 and
naphthalene in a equilibrium. Van’t Hoff Analysis revealed
the free energy of 12 (ΔG = -11.6 kcal·mol–1, ΔH = -26.5
kcal·mol–1, ΔS = -50.1 cal·mol–1·K–1) is lower than that of 4
and naphthalene (Figures S67 and S68). Analogously,
reactions of 4 with biphenylene, anthracene, and tetracene
readily afforded the dearomatic product 13 (93% yield), 14
(85% yield), and 15 (92% yield), respectively, and all
products 13–15 were thoroughly characterized (Figure S80-
82). Two boron atoms in 4 regioselectively bind to the
carbons at the 2- and 3-position of biphenylene to afford 13,
and at the 1- and 2-position of anthracene and tetracene to
form 14 and 15, respectively, all of which are formed by
binding at the thermodynamically favored positions of the
substrates.13 Analogous to 12, compounds 13–15 undergo
retro [4+2] cycloaddition reactions at 80–140 oC to
regenerate 4 and the corresponding arene in an equilibrium
process. Note that the formation of 12–15 represents the first
examples of intermolecular and thermally reversible
dearomative diboration of arenes.8,14,15
9
F
Ph
Ph
Ph
7
8
(70%)
(76%)
(74%)
(82%)
(86%)
10
11
12
13
14
15
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B
B
R
N
N
Ph
Ph
4
9
N
N
10
11
Mes
Mes
Cl
R
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
Cl
: 80 oC
12 h
: 60 oC
12 h
: 80 oC
12 h
: 80 oC
4 h
: 80 oC
4 h
7
8
9
10
11
Scheme 3. Reactions of 4 with partially fluorinated arenes.
R2
rel (kcal·mol1
)
R1
TS-o: 31.5
TS-m: 27.6
TS-p: 22.1
G
R3
F
F
Ph
N
F
N
B
Ph
B
Ph
Mes
N
Ph
Ph
Cl
0.0
N
H
H
Ph
H
Mes
H
F
H
Ph
F
F
F
F
B
B
B
B
F
Ph
Ph
Ph
Ph
Ph
N
N
N
N
4
+
Ph
Ph
Ph
Ph
B
N
N
N
N
Ph
Mes
Ph
Mes
Cl
Cl
N
N
Ph
Ph
Mes
Mes
Mes
N
N
F
B
12
(80%)
13
(93%)
Mes
Cl
R1
F
R3
o: R1 = H, R2 = R3 = F
m: R2 = H, R1 = R3 = F
p: R3 = H, R1 = R2 = F
R2
C6H6, 60 oC, 2 d
C6H6, 60 oC, 12 h
80 oC, C6H6
80 oC, C6H6
80 oC, C6H6
140 oC, THF
7-o (7): 97.4
7-p: 98.0
7-m: 99.2
4
product
Figure 4. Concerted C‒F bond activation of
pentafluorobenzene by 4.
C6H6, 60 oC, 12 h
H
C6H6, 50 oC, 12 h
H
Ph
H
H
Ph
B
To gain insight into the reaction mechanism, the pathways
for the reaction of 4 with pentafluorobenzene were explored
theoretically using M06-2X/Def2-SVP functional (Figure 4).
The activation barriers for the direct C-F bond cleavage are
found to be 31.5 kcal·mol–1 (TS-o), 27.6 kcal·mol–1 (TS-m)
and 22.1 kcal·mol–1 (TS-p), respectively, which is
inconsistent with the experimentally observed ortho-C–F
bond activation, and thus unlikely to occur. In addition, 7-o
is also predicted to be the thermodynamically least stable
isomer, thus, 7-o may be formed through a kinetically
controlled pathway (Figure S102). While the detail of the
selectivity of the reaction is still unclear, the symmetry of the
frontier molecular orbitals, charge distribution in the
substrates, as well as, the steric factor, would affect the
activation barrier of the rate-determining step (for Mulliken
charges and MOs of 1,2,4,5-C6F4H2, 1,2,3,5-C6F4H2, 1,2,4-
C6F3H3, p-C6ClFH4, see Table S3).
B
N
B
Ph
Ph
N
N
N
N
Ph
B
Ph
Ph
N
Ph
Mes
Ph
N
N
Ph
Mes
Cl
Cl
15
Mes
Mes
14
(85%)
(92%)
Scheme 4. Reactions of 4 with polycyclic aromatic
hydrocarbons.
In summary, we have demonstrated that an annulated, 5-
chlorinated 1,3,2,5-diazadiborinine 4 selectively cleaves the
C‒H bond in arenes and C‒F bond in partially fluorinated
arenes, as well as, undergoes reversible dearomative coupling
reaction with polycyclic aromatic hydrocarbons. The former
represents a rare example of metal-free borylation of arenic
C‒F bonds whereas the latter can be viewed as dearomative
diboration.
While no intermediates were experimentally detected
during the formation of compounds 5‒11, dearomative
diboration products were obtained from the reactions of 4
ASSOCIATED CONTENT
Supporting Information
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Borylation of Haloarenes and Quaternary Arylammonium Salts. J.
The Supporting Information is available free of charge on the
ACS Publications website. Complete experimental data and
computational details including Cartesian coordinates for
stationary points (PDF) Crystallographic data (CIF) of 1-15
(ZIP) [CCDC: 1916645-1916659].
Am. Chem. Soc. 2016, 138, 2985-2988. (b) Mfuh, A. M.; Schneider,
B. D.; Cruces, W.; Larionov, O. V. Metal- and Additive-free
Photoinduced Borylation of Haloarenes. Nat. Protoc. 2017, 12, 604.
(c) Chen, K.; Cheung, M. S.; Lin, Z.; Li, P. Metal-free Borylation of
Electron-rich Aryl (Pseudo)halides Under Continuous-flow
Photolytic Conditions. Org. Chem. Front. 2016, 3, 875-879.
1
2
3
4
5
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7
8
(7) (a) Fox, W. B.; Wartik, T. Reaction of Diboron Tetrach Reaction
of Diboron Tetrachloride with Aromatic Substances. J. Am. Chem.
Soc. 1961, 83, 498-499. (b) Wilkey, J. D.; Schuster, G. B. 2,5,7,7-
AUTHOR INFORMATION
Corresponding Author
*rkinjo@ntu.edu.sg
Tetraphenyl-7-boratabicyclo[4.1.0]hepta-2,4-diene:
The
First
Isolation and Characterization of a Boratanorcaradiene. J. Am. Chem.
Soc. 1988, 110, 7569-7571. (c) Wilkey, J. D.; Schuster, G. B.
Photochemistry of Tetraarylborate Salts (Ar4B-): Formation of
9
Notes
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The authors declare no competing financial interests.
2,5,7,7-Tetraphenyl-7-boratabicyclo[4.1,0]hepta-2,4-diene
(a
Boratanorcaradiene) by Irradiation of (p-Biphenylyl)triphenyl
Borate. J. Am. Chem. Soc. 1991, 113, 2149-2155.
ACKNOWLEDGMENT
(8) (a) Rao, Y.-L.; Amarne, H.; Zhao, S.-B.; McCormick, T. M.;
Martić, S.; Sun, Y.; Wang, R.-Y.; Wang, S. Reversible Intramolecular
C-C Bond Formation/Breaking and Color Switching Mediated by a
N,C-Chelate in (2-ph-py)BMes2 and (5-BMes2-2-ph-py)BMes2. J.
Am. Chem. Soc. 2008, 130, 12898-12900. (b) Rao, Y.-L.; Amarne,
H.; Wang, S. Photochromic Four-coordinate N,C-chelate Boron
Compounds. Coord. Chem. Rev. 2012, 256, 759-770. (c) Mellerup,
S. K.; Wang, S. Photoresponsive Organoboron Systems. In Main
Group Strategies towards Functional Materials; Baumgartner, T.,
Jakle, F., Eds.; John Wiley & Sons Ltd.: Hoboken, NJ, 2018; p 47−78.
(d) Mellerup, S. K.; Wang, S. Isomerization and Rearrangement of
Boriranes: from Chemical Rarities to Functional Materials. Sci.
China Mater. 2018, 61, 1249−1256.
This work was financially supported by Nanyang
Technological University (NTU), Singapore, and the
Singapore Ministry of Education (MOE2015-T2-032) (R.K.).
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a
(15) For the influence of the Cl-substituent on the B atom in 4, see
the Supporting Information.
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SYNOPSIS TOC
1
2
3
4
5
6
Ar(or F)
H
Ar
E
B
B
B
B
B
N
N
(E = H, F)
N
N
N
N

B
H
regioselective
H(or Ar)
reversible
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
5
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