Synthesis of 1,2-borazaronaphthalenes from imines by base-promoted borylation of C-H bond

Article abstract of DOI:10.1021/jo5029437

A new route from benzylic imines permits the synthesis of 1,2-borazaronaphthalenes in good yields. The reaction involves formation of the enamidyl dibromoborane, which undergoes base-promoted borylation of the nearby aromatic C-H bond. Electrophilic attack of the boron species onto the benzylic arene is supported by the slow borylation of arenes substituted with electron-withdrawing groups. The resultant boron bromides can be easily substituted with lithium reagents to provide a range of products. The electronic properties of these 1,2-borazaronaphthalene derivatives have been investigated by UV-vis and fluorescence spectroscopy.

Full text of DOI:10.1021/jo5029437

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pubs.acs.org/joc
Synthesis of 1,2-Borazaronaphthalenes from Imines by Base-
Promoted Borylation of C−H bond
Xinwang Liu, Pengbo Wu, Jianfeng Li, and Chunming Cui*
State Key Laboratory of Elemento-Organic Chemistry, Nankai University, Cooperative Innovation Center of Chemical Science and
Engineering (Tianjin), Tianjin 300071, People’s Republic of China
S
* Supporting Information
ABSTRACT: A new route from benzylic imines permits the synthesis of 1,2-borazaronaphthalenes in good yields. The reaction
involves formation of the enamidyl dibromoborane, which undergoes base-promoted borylation of the nearby aromatic C−H
bond. Electrophilic attack of the boron species onto the benzylic arene is supported by the slow borylation of arenes substituted
with electron-withdrawing groups. The resultant boron bromides can be easily substituted with lithium reagents to provide a
range of products. The electronic properties of these 1,2-borazaronaphthalene derivatives have been investigated by UV−vis and
fluorescence spectroscopy.
isomers A, B, and their derivatives have been extensively
studied, while isomer C has not been reported so far.5e,i,j,7e,9
Although a few effective approaches for the synthesis of 1,2-
azaborines have been developed, they generally required either
metal catalysts or multiple steps.6d,10 In some cases, the starting
materials are not easily available and the 1,2-azaborine products
are difficult to functionalize.4a,5a Herein, we report the synthesis
of 1,2-borazaronaphthalene derivatives starting with C-benzyl-
substituted imines through two steps. The reaction involved a
base-promoted C−H borylation of the benzyl rings with a B−
Br bond, thus establishing a convenient synthetic route to
various substituted BN naphthalene derivatives.
INTRODUCTION
■
Polycyclic aromatic hydrocarbons (PAHs) are an important
class of organic compounds that have received significant
attention in organic electronics, dyes, sensors, and liquid crystal
displays.1 Replacement of a CC bond in PAHs with a B−N
bond leads to analogous BN aromatic systems because of the
isoelectronic relationship of the B−N with a CC bond.
Despite their similarities, they also showed dramatically
different reactions and optical and electronic properties due
to the polarized B−N bond.2 As early as 1958, Dewar reported
the first synthesis of the singly BN-substituted aromatic
compound.3 Since then, a number of BN aromatic compounds
and conjugated systems have been reported. With the
increasing demands of new molecular materials in recent
years, Piers, Ashe, Liu, and others have carried out extensive
efforts in this area.4−7
Borazaronaphthalene is isosteric with naphthalene, which
represents one of the smallest PAHs with many distinct features
from benzene. Because of the BN unit in the molecule, there
exist a number of isomers depending on the positions of the B
and N atoms. Three of the isomers are shown in Figure 1.
Theoretical calculations and experimental studies revealed that
these isomers may display different stability, reactivity, and
optical properties.4b,6p,8 The synthesis and properties of
RESULTS AND DISCUSSION
■
Deprotonation of imine 1a with n-BuLi and subsequent
treatment with BBr3 resulted in the formation of 1b. No
reaction was observed upon heating of 1b at 80 °C for 1 h in
C6D6 as disclosed by NMR analysis (see Figure S1 in the
Supporting Information). However, addition of 1 equiv of NEt3
in the reaction media and subsequent heating in toluene at 80
°C for 1 h led to the formation of a new species in 67% yield,
whose 11B NMR exhibited a singlet at δ 37.1 ppm,
corresponding to a three-coordinate boron species. The
NMR data indicated the formation of the 1,2-borazaronaph-
thalene 1 (Scheme 1).
This finding prompted us to extend this methodology to the
investigation of the scope of the reaction and the factors that
have pronounced effects on the reaction rate. The key step in
this reaction is the C−H borylation of the benzyl ring of the
Received: December 29, 2014
Published: February 24, 2015
Figure 1. Structures of three borazaronaphthalene isomers.
© 2015 American Chemical Society
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Scheme 1. Synthesis of 1,2-Borazaronaphthalene 1
a,b
Table 1. Synthesis of Various Substituted BN Naphthalene Derivatives by Base-Mediated Borylation from Imines
a
Reaction conditions: imines (3 mmol), n-BuLi (3.0 mmol) in toluene for step 1; NEt3 (3.0 mmol), XBBr2, or BCl3 (3.0 mmol), reaction
b
temperatures and times listed for step 2. Isolated yields.
imine. The closely related borylation has been reported to be
promoted by Lewis acids such as AlCl3 at high temperatures in
a Friedel−Crafts cyclization pattern.3,7b The present base-
promoted reaction did not require any metal additive under
relatively mild conditions; thus, it might be useful for the
synthesis of boron-containing conjugated rings. The same
reactions with PhBBr2 or BCl3, instead of BBr3, also led to the
formation of the corresponding BN naphthalenes 2 and 10.
However, the formation of 2 and 10 required more harsh
conditions and a longer reaction time, indicating the Lewis
acidity of boron halides and the strength of B−X bonds have
pronounced effects on the reaction rates (Table 1).
A series of imines with different substituents have been
synthesized for the generation of the corresponding BN
naphthalene derivatives (Table 1). The imines shown in
Table 1 were synthesized by the reactions of N-phenyl-
pivalimidoyl chlorides with benzylmagnesium chlorides in
THF. The N-aryl and C-phenyl/t-Bu-substituted imines worked
well to yield the expected BN naphthalenes in modest yields
while N-n-Bu substituted imine 8a led to a sluggish borylation
reaction, and many attempts to isolate the desired product were
unsuccessful to date. The bulky substituents on the NC
moiety of the imines apparently accelerate the reaction (2 vs 4).
The electronic factors on the benzyl ring have significant effects
on the reaction rate. Introduction of a fluorine atom in the
benzyl ring apparently slowed down the reaction (8 vs 1). The
imine with a 3-methyl-substituted benzyl group led to the
isolation of only one isomer 7 possibly due to the steric effect
of the methyl group. To further simplify the synthetic process,
one-pot reactions of the imines with BBr3NEt3 in the presence
of various bases at high temperatures have been investigated.
However, the reactions proved to be sluggish, and no expected
cyclic products can be observed by NMR analysis.
Compounds 1−10 have been fully characterized by multiple
nuclear NMR spectroscopy and HRMS. The 11B NMR spectra
of 1−10 display similar resonances in the very narrow range of
δ 36.2−40.0 ppm. Compounds containing a B−X (X = Cl or
Br) bond are sensitive to moisture, but the B−Ph-substituted
compounds proved to be stable at least for 5 h in dilute acidic
(1.0 M HCl) and basic aqueous (1.0 M NaOH) solutions.
To confirm the molecular structures of these species, X-ray
single-crystal analysis has been conducted for compound 3 and
4 (for the crystal structure of 4, see Figure S3 in the Support
Information). The colorless crystals of 3 suitable for X-ray
analysis were obtained from n-hexane.
The structure shown in Figure 2 indicates the planar fused
bicyclic six-membered ring contains a BN unit. The B−N bond
length of 1.430(4) Å is comparable to analogous distances
Figure 2. ORTEP illustration of 3 with ellipsoids drawn at 50%
probability. Hydrogen atoms have been omitted for clarity.
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reported for 2,1-borazaronaphthalenes 1.424(5) Å.7e The N1−
B1−C1−C2 skeleton is planar within experimental error.
The successful synthesis of compounds 1, 3, and 5−8
containing a B−Br bond allowed us to investigate the
substitution reactions with various organolithium compounds
for the synthesis of different B-substituted derivatives.6a,9a The
results are summarized in Table 2. The bromide ligand can be
indicated by NMR analysis, and the resulting products have not
been isolated so far.
1
These new compounds have been characterized by H, 13C,
and 11B NMR spectroscopy and HRMS. The crystal structure
of 18 has been established by X-ray single-crystal analysis
(Figure 3). Compound 18 was obtained by reaction of 7 with
Table 2. Synthesis of B-Substituted Derivatives by
a,b
Substituent Reaction
Figure 3. ORTEP illustration of 18 with ellipsoids drawn at 50%
probability. Hydrogen atoms have been omitted for clarity.
the alkynyllithium LiCC−C6H4−OMe. The X-ray analysis
confirmed the methyl group in the benzyl ring lies in the para
position of the boron atom. The N-substituted phenyl ring is
approximately perpendicular to the central BN borazaronaph-
thalene ring with a dihedral angle of 86.65(13)°. The B−C
C−C unit is almost linear. The small dihedral angle of
22.04(12)° between the two aromatic rings attached to the C
C triple bond indicates some degree of π−π interactions
between the two rings through the triple bond.6l
BN aromatic compounds have been studied as luminescent
materials. Having these novel BN naphthalene derivatives in
hand, we are interested in the investigation of their electronic
structures by UV−vis and fluorescence spectroscopy (Figures 4
and 5 and Table 3).
The UV−vis spectra of the B-alkynyl-substituted derivatives
in CH2Cl2 exhibited the absorption maxima in the range of
349−352 nm, somewhat red-shifted compared with the other
1,2-borazaronaphthalenes with an aromatic ring on the boron
atom. These B-phenyl 1,2-borazaronaphthalenes exhibited a
similar absorption maxima with known 2,1-borazaronaphtha-
a
Reaction conditions: 1,2-borazaronaphthalenes bromide (3 mmol),
b
R3Li (3 mmol) in Et2O from −78 °C to rt. Isolated yields.
easily substituted by phenyl, naphthalenyl, thienyl, and alkynyl
groups to give compounds 2 and 11−19 in good yields under
mild conditions. However, reactions of 1 with MeLi or 1-
propenylmagnesium bromide under the same conditions led to
the formation of a mixture that cannot be separated and
identified definitely.
Compounds 2 and 11−19 were obtained as white crystalline
materials. Crystals of 12 are stable in air for several days while
the other crystalline compounds are stable in air for several
months. These materials were found to be stable enough for
the purification by chromatography on silica gel. It is noted that
the substitution reaction can be performed in situ without the
isolation of the BN naphthalene boron bromides.
Reaction of 3 with amine PhCH2NH2 in toluene at room
temperature led to the formation of the B−NHCH2Ph
substituted BN cycle 20 as an air-stable white solid in 67%
yield (Scheme 2), indicating that the bromide can be easily
substituted by a nucleophile. However, reactions of 3 with
MeOH and water under similar conditions were sluggish as
Scheme 2. Aminolysis of the B−Br Bond
Figure 4. UV−vis spectra of selected 1,2-borazaronaphthalenes. All
tests were performed in CH2Cl2 solution at 10−5 M.
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Compounds 13−19 with an alkynyl group on the boron
atom only show one emission maximum ranging from 416 to
443 nm, which can be regarded as the emissions due to the BN
naphthalene core. These emission maxima are red-shifted in
comparison to those (the high energy emissions) observed in 2,
9, 11, and 12, indicating the more pronounced interaction of
the alkynyl π system with the BN naphthalene unit than the
aromatic rings on the boron atom in 2 and 9, 11, and 12. The
absolute quantum yields for the selected compounds 4, 12, 15,
and 16 are given in Table 3.
CONCLUSION
■
In conclusion, we have established a convenient and reliable
base-promoted borylation route to novel 1,2-borazaronaph-
thalenes starting from easily available imines under mild
conditions. This method allowed the synthesis of various
substituted BN naphthalene derivatives. The ease of the
generation of the ring system with a B−Br bond is especially
important for the modification of the electronic structures of
this class of the BN aromatic system as observed in the
conjugation enlargement by a CC triple bond. This
luminescent BN system encouraged us to employ this route
to synthesize large π systems, which will be reported in due
course.
Figure 5. Normalized fluorescence emission spectra of selected 1,2-
borazaronaphthalenes. All tests were performed in n-hexane solution at
10−5 M and λex = 340 nm.
a
Table 3. Fluorescence Data for Selected Compounds
b
b
compd
λem (nm)
ΦF
compd
λem (nm)
ΦF
2
388, 582
405
14
15
16
17
18
19
432
424
435
420
416
443
4
0.14
0.15
0.11
0.11
9
385, 570
384, 590
397, 580
433
11
12
13
EXPERIMENTAL SECTION
General Considerations. All operations involving air- and
moisture-sensitive compounds were carried out under an atmosphere
of dry argon by using a modified Schlenk line and glovebox techniques.
■
a
All experiments were performed in n-hexane solution at 10−5 M, λex
b
=340 nm. Absolute quantum yields were determined by a calibrated
integrating sphere system.
1
All solvents were freshly distilled from Na. The H, 13C, 11B, and 19F
NMR spectra were recorded on a 400 MHz NMR spectrometer.
Chemical shifts are referenced against external Me4Si (1H, 13C), BF3·
Et2O (11B), and CFCl3 (19F). High-resolution mass spectra (HRMS)
were obtained on a Q-Tof spectrometer. Imines were synthesized by a
modified method (Scheme 3).11
lene derivatives (ca. 320 nm, type A in Figure 1) in the UV−vis
spectra.9a,b
Compounds 2, 9, 11, and 12 with two neighboring aromatic
rings on the BN rings have similar fluorescence emission
patters, in which two emission bands were observed. The
emission maxima at 388, 384, and 397 nm for 2, 11, and 12 are
more intense than those in the low energy region. In contrast,
the emission maximum at 385 nm for 9 is only slightly weaker
than the one at 570 nm. Compound 4 with the bulky 2,6-i-
Pr2C6H3 group on the nitrogen atom only shows one strong
emission maximum at 405 nm. This suggests that the high
wavelength emission bands (582, 570, 590, 580 nm) for 2, 9,
11, and 12 are relevant to the π−π interaction between the
neighboring aromatic rings on the BN ring. The steric
hindrance in 4 due to the bulky Ar group led to unfavorable
π−π overlap between the Ar and phenyl rings and, thus, the
disappearance of the emission band. Compound 9 displays the
relatively strong emission arising from the phenyl−phenyl
interactions, probably due to the more efficient π−π overlap as
a result of the less hindered environment.
General Procedure for the Synthesis of Imines. N-(3,3-
Dimethyl-1-phenylbutan-2-ylidene)aniline (1a). To N-phenylpivali-
midoyl chloride (9.8 g, 50 mmol) in 50 mL of THF was slowly added
benzylmagnesium chloride in THF (2.0 M, 25 mL, 50 mmol) at 0 °C.
After the reaction was stirred at 0 °C for 12 h, 30 mL of water was
added to quench the reaction. The mixture was extracted with Et2O
(30 mL × 3). The extract was dried over MgSO4. Removal of the
solvent and reduced pressure distillation gave 1a as a pale yellow oil
(9.5 g, 76%). Bp: 82 °C (1.0 mmHg). 1H NMR (400 MHz, CDCl3): δ
7.32−7.13 (m, 5H), 7.05 (d, J = 7.3 Hz, 2H), 6.98 (t, J = 7.4 Hz, 1H),
6.71 (d, J = 7.4 Hz, 2H), 3.78 (s, 2H), 1.28 (s, 9H). 13C NMR (101
MHz, CDCl3): δ 176.8, 150.3, 136.2, 127.6, 127.4, 127.2, 124.8, 121.4,
117.7, 39.9, 34.2, 27.8. HRMS (ESI): m/z calcd for C18H22N [M +
H]+ 252.1747, found 252.1743
N-(3,3-Dimethyl-1-phenylbutan-2-ylidene)-4-fluoroaniline (2a).
Obtained as a colorless oil (10.9 g, yield 81%). Bp: 85 °C (1.0
1
mmHg). H NMR (400 MHz, CDCl3): δ 7.25−7.11 (m, 3H), 7.00−
6.94 (m, 2H), 6.92−6.84 (m, 2H), 6.62−6.54 (m, 2H), 3.75 (s, 2H),
1.25 (s, 9H). 13C NMR (101 MHz, CDCl3): δ 179.2, 158.9 (d, J =
Scheme 3. Synthesis of Benzylic Imines 1a−8a
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241.4 Hz), 147.4, 137.1, 128.5, 128.3, 126.0, 120.0 (d, J = 8.1 Hz),
115.3 (d, J = 22.2 Hz), 41.0, 35.1, 28.7. 19F NMR (376 MHz, CDCl3):
δ −122.2. HRMS (ESI): m/z calcd for C18H21FN [M + H]+ 270.1653,
found 270.1652
cooled to −78 °C, and PhBBr2 or BBr3 (3 mmol) was added. The
reaction was slowly warmed to room temperature and stirred
overnight. After filtration, NEt3 (3 mmol, 0.42 mL) was added to
the filtrate, and the reaction was heated at 80 or 120 °C for several
hours depending on the substrates. After filtration to remove NEt3·
HBr, the solvent was removed under reduced pressure to give crude
product. The crude product was purified by crystallization from n-
hexane or toluene at −40 °C.
N-(3,3-Dimethyl-1-phenylbutan-2-ylidene)-3,4-dimethoxyaniline
(3a). Obtained as a sticky pale yellow oil (9.6 g 62%). Bp: 165 °C (1.0
mmHg). 1H NMR (400 MHz, CDCl3): δ 7.23−7.06 (m, 3H), 6.98 (d,
J = 7.4 Hz, 2H), 6.71 (d, J = 8.3 Hz, 1H), 6.19 (d, J = 8.4 Hz, 1H),
6.14 (s, 1H), 3.80 (s, 3H), 3.74 (s, 2H), 3.67 (s, 3H), 1.24 (s, 9H). 13C
NMR (101 MHz, CDCl3): δ 178.7, 149.0, 145.2, 144.6, 137.6, 128.5,
128.2, 125.9, 111.6, 109.6, 103.7, 56.1, 55.6, 40.9, 35.3, 28.8. HRMS
(ESI): m/z calcd for C20H26NO2 [M + H]+ 312.1958, found 312.1953
N-(3,3-Dimethyl-1-phenylbutan-2-ylidene)-2,6-diisopropylaniline
(4a). Obtained as a sticky pale yellow oil (13.2 g, 79%). Bp: 171 °C
1-Bromo-2-phenyl-3-tert-butyl-1,2-dihydrobenzo[c][1,2]-
azaborinine (1). The reaction was heated at 80 °C for 1 h. Compound
1
1 was obtained as a white solid (0.68 g, 67%). Mp: 98 °C. H NMR
(400 MHz, C6D6): δ 8.62 (d, J = 7.7 Hz, 1H), 7.51−7.41 (m, 2H),
7.38−7.25 (m, 1H), 7.06−6.99 (m, 3H), 6.99−6.93 (m, 2H), 6.90 (s,
1H), 1.06 (s, 9H). 13C NMR (101 MHz, C6D6): δ 151.9, 146.0, 141.6,
136.0, 132.0, 131.8, 127.6, 126.9, 125.7, 110.4, 37.7, 32.3, the
resonance for the carbon atom adjacent to the boron could not be
observed. 11B NMR (128 MHz, C6D6): δ 37.1. HRMS (EI): m/z calcd
for C18H19BBrN [M]+ 339.0789, found 339.0795.
3-tert-Butyl-1,2-diphenyl-1,2-dihydrobenzo[c][1,2]azaborinine
(2). PhBBr2 was used, and the reaction was performed at 120 °C for 6
h. Compound 2 was obtained as a white solid by crystallization from
toluene (0.53 g, 52%). Mp: 128−129 °C. 1H NMR (400 MHz, C6D6):
δ 7.98 (d, J = 7.7 Hz, 1H), 7.65 (d, J = 7.9 Hz, 1H), 7.53 (t, J = 7.0 Hz,
1H), 7.25 (t, J = 7.3 Hz, 1H), 7.19 (s, 2H), 7.13−7.01 (m, 4H), 7.01−
6.92 (m, 2H), 6.85−6.76 (m, 3H), 1.22 (s, 9H). 13C NMR (101 MHz,
C6D6): δ 151.1, 145.8, 142.1, 136.7, 132.6, 131.8, 131.1, 127.1, 126.8,
126.7, 126.6, 126.3, 124.8, 110.4, 37.8, 32.6, the resonances for the
carbon atoms adjacent to the boron could not be observed. 11B NMR
(128 MHz, C6D6): δ 39.3. HRMS (ESI): m/z calcd for C24H25BN [M
+ H]+ 338.2075, found 338.2069
1
(1.0 mmHg). H NMR (400 MHz, CDCl3): δ 7.16−7.06 (m, 3H),
7.05−6.94 (m, 3H), 6.87−6.79 (m, 2H), 3.55 (s, 2H), 2.63 (hept, J =
6.8 Hz, 2H), 1.33 (s, 9H), 1.16 (d, J = 6.9 Hz, 6H), 1.11 (d, J = 6.7 Hz,
6H). 13C NMR (101 MHz, CDCl3): δ 174.9, 146.0, 136.8, 135.1,
129.4, 128.0, 125.8, 122.6, 122.3, 41.5, 36.5, 29.0, 28.2, 23.5, 21.2.
HRMS (ESI): m/z calcd for C24H34N [M + H]+ 336.2686, found
336.2683.
N-(3,3-Dimethyl-1-(m-tolyl)butan-2-ylidene)aniline (5a). The re-
action of Grignard reagent with imidoyl chloride was performed in
Et2O. Compound 5a was obtained as a pale yellow oil (9.5 g, 72%).
1
Bp: 91 °C (1.0 mmHg). H NMR (400 MHz, CDCl3): δ 7.14−7.07
(m, 2H), 7.00 (t, J = 7.6 Hz, 1H), 6.88−6.81 (m, 2H), 6.76−6.66 (m,
2H), 6.57 (dd, J = 8.4, 1.1 Hz, 2H), 3.61 (s, 2H), 2.18 (s, 3H), 1.14 (s,
9H). 13C NMR (101 MHz, CDCl3): δ 177.9, 151.4, 137.7, 137.1,
129.5, 128.7, 128.0, 126.6, 125.4, 122.4, 118.8, 40.9, 35.2, 28.9, 21.3.
HRMS (ESI): m/z calcd for C19H24N [M + H]+ 266.1903, found
266.1901
1-Bromo-3-tert-butyl-2-(2,6-diisopropylphenyl)-1,2-
dihydrobenzo[c][1,2]azaborinine (3). Obtained as a white solid (0.90
g, 71%). Single crystals of 3 suitable for X-ray diffraction were obtained
N-(1-(2-Fluorophenyl)-3,3-dimethylbutan-2-ylidene)aniline (6a).
The reaction of Grignard reagent with imidoyl chloride was performed
in Et2O. Compound 6a was obtained as a pale yellow oil (8.8 g, 65%).
1
by cooling a solution of 3 in n-hexane. Mp: 115−116 °C. H NMR
1
Bp: 93 °C (1.0 mmHg). H NMR (400 MHz, CDCl3): δ 7.20 (t, J =
(400 MHz, C6D6): δ 8.63 (d, J = 7.7 Hz, 1H), 7.47−7.38 (m, 2H),
7.29−7.19 (m, 2H), 7.09−7.03 (m, 3H), 2.64 (hept, J = 6.8 Hz, 2H),
1.17 (d, J = 6.7 Hz, 6H), 1.15−1.11 (m, 15H). 13C NMR (101 MHz,
C6D6): δ 152.8, 145.7, 142.9, 141.9, 136.4, 132.2, 128.8, 126.6, 125.5,
124.3, 113.6, 39.2, 34.1, 28.6, 25.2, 23.9, the resonance for the carbon
atom adjacent to the boron could not be observed. 11B NMR (128
MHz, C6D6): δ 36.5. HRMS (EI): m/z calcd for C24H31BBrN [M]+
423.1728, found 423.1731
7.8 Hz, 2H), 7.17−7.10 (m, 1H), 7.07−6.98 (m, 2H), 6.98−6.87 (m,
2H), 6.67 (dd, J = 8.3, 0.9 Hz, 2H), 3.75 (s, 2H), 1.26 (s, 9H). 13C
NMR (101 MHz, CDCl3): δ 177.0, 160.3 (d, J = 245.5 Hz), 151.1,
130.2 (d, J = 4.0 Hz), 128.7, 127.8 (d, J = 8.1 Hz), 124.3 (d, J = 16.2
Hz), 123.7 (d, J = 4.0 Hz), 122.6, 118.6, 115.1 (d, J = 22.2 Hz), 41.0,
28.6, 27.8 (d, J = 3.0 Hz). 19F NMR (376 MHz, CDCl3): δ −116.5.
HRMS (ESI): m/z calcd for C18H21FN [M + H]+ 270.1653, found
270.1649
3-tert-Butyl-2-(2,6-diisopropylphenyl)-1-phenyl-1,2-
dihydrobenzo[c][1,2]azaborinine (4). Obtained as a white solid by
recrystallization from toluene (0.95 g, 75%). Single crystals of 4
suitable for X-ray diffraction were obtained by cooling a solution of 4
in toluene. Mp: 188−189 °C. 1H NMR (400 MHz, C6D6): δ 8.11 (d, J
= 7.8 Hz, 1H), 7.59 (d, J = 7.9 Hz, 1H), 7.51−7.44 (m, 1H), 7.23−
7.17 (m, 3H), 7.07 (dd, J = 5.2, 1.8 Hz, 3H), 7.01 (dd, J = 6.9, 2.7 Hz,
2H), 6.96 (d, J = 7.7 Hz, 2H), 2.88−2.70 (m, 2H), 1.22 (s, 9H), 1.13
(d, J = 6.8 Hz, 6H), 0.49 (d, J = 6.7 Hz, 6H). 13C NMR (101 MHz,
CDCl3): δ 152.2, 145.6, 142.1, 141.5, 136.9, 135.3, 130.6, 127.8, 127.0,
126.5, 125.8, 123.8, 123.6, 112.4, 39.2, 33.9, 28.0, 24.2, 23.9, the
resonances for the carbon atoms adjacent to the boron could not be
observed. 11B NMR (128 MHz, C6D6): δ 40.0. HRMS (ESI): m/z
calcd for C30H37BN [M + H]+ 422.3014, found 422.3016
1-Bromo-3-tert-butyl-2-(3,4-dimethoxyphenyl)-1,2-
dihydrobenzo[c][1,2]azaborinine (5). Obtained as a white solid by
recrystallization from Et2O (0.60 g, 55%). Mp: 112−114 °C. 1H NMR
(400 MHz, C6D6): δ 8.66 (d, J = 7.7 Hz, 1H), 7.47 (q, J = 7.7 Hz, 2H),
7.30 (t, J = 7.2 Hz, 1H), 6.95 (s, 1H), 6.61 (d, J = 8.4 Hz, 1H), 6.56 (s,
1H), 6.42 (d, J = 8.4 Hz, 1H), 3.33 (s, 3H), 3.28 (s, 3H), 1.16 (s, 9H).
13C NMR (101 MHz, C6D6): δ 152.2, 149.1, 148.9, 141.4, 138.4,
N-(1,2-Diphenylethylidene)aniline (7a).12 Obtained as a yellow
solid (8.5 g, 63%) by recrystallization from Et2O. 1H spectral data were
the same with the reported data.
N-(3,3-Dimethyl-1-phenylbutan-2-ylidene)butan-1-amine (8a).
Obtained as a colorless liquid (8.8 g, yield 76%). Bp: 66 °C (1.0
mmHg). 1H NMR (400 MHz, CDCl3): δ 7.34−7.26 (m, 2H), 7.22 (q,
J = 7.1 Hz, 1H), 7.09 (d, J = 7.3 Hz, 2H), 3.76 (s, 2H), 3.28 (t, J = 7.0
Hz, 2H), 1.66−1.49 (m, 2H), 1.40−1.26 (m, 2H), 1.16 (s, 9H), 0.91
(t, J = 7.4 Hz, 3H). 13C NMR (101 MHz, CDCl3): δ 128.5, 128.0,
126.0, 51.7, 40.8, 33.4, 33.2, 28.3, 20.6, 14.1. HRMS (ESI): m/z calcd
for C16H26N [M + H]+ 232.2060, found 232.2063.
Synthesis of 1b. To imine 1a (3.0 mmol) in 30 mL of toluene was
added n-BuLi (2.4 M, 1.3 mL, 3.0 mmol) at room temperature, and
the mixture was stirred at room temperature for 30 min. The resulting
suspension was cooled to −78 °C, and neat BBr3 (3.0 mmol) was
added. The reaction was slowly warmed to room temperature and
stirred overnight. After filtration, the solvents were removed under
reduced pressure to give 1b as a colorless liquid (1.1 g, 88%). This
compound cannot be further purified by distillation but can be directly
1
used for the further reaction. H NMR (400 MHz, C6D6): δ 7.56 (m,
135.8, 131.7, 126.7, 125.5, 123.5, 115.9, 110.2, 110.1, 55.2, 55.0, 37.6,
32.3, the resonance for the carbon atom adjacent to the boron could
not be observed. 11B NMR (128 MHz, C6D6): δ 37.2. HRMS (EI): m/
z calcd for C20H23BBrNO2 [M]+ 399.1000, found 399.0994
2H), 7.35 (m, 2H), 6.88−7.13 (m, 6H), 6.50 (s, 1H, t-BuC=CH), 1.03
(s, 9H, t-Bu). 11B NMR (128 MHz, C6D6): δ 29.2. HRMS (EI): m/z
calcd for C18H20BBr2N [M]+ 419.0050, found 419.0048
General Procedure for the Synthesis of 1,2-Borazaronaph-
thalene. n-BuLi (2.4 M, 1.3 mL, 3 mmol) was added to imine (3.0
mmol) in 30 mL of toluene, and the reaction was stirred at room
temperature for 30 min. The resulted lithium salt suspension was
1-Bromo-3-tert-butyl-2-(4-fluorophenyl)-1,2-dihydrobenzo[c]-
1
[1,2]azaborinine (6). White solid (0.48 g, 45%). Mp: 94−96 °C. H
NMR (400 MHz, C6D6): δ 8.59 (d, J = 7.7 Hz, 1H), 7.49−7.40 (m,
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Featured Article
2H), 7.34−7.24 (m, 1H), 6.86 (s, 1H), 6.72−6.58 (m, 4H), 1.00 (s,
9H). 13C NMR (101 MHz, C6D6): δ 162.0 (d, J = 247.4 Mz), 151.7,
141.9, 141.5, 136.0, 133.2 (d, J = 8.1 Hz), 132.1, 126.9, 125.8, 114.8
(d, J = 22.2 Hz), 110.5, 37.5, 32.2, the resonance for the carbon atom
adjacent to the boron could not be observed. 11B NMR (128 MHz,
C6D6): δ 36.6. 19F NMR (376 MHz, C6D6): δ −113.9. HRMS (EI):
m/z calcd for C18H18BBrFN [M]+ 357.0695, found 357.0697
126.9, 126.5, 126.4, 126.3, 126.3, 124.7, 124.6, 124.5, 124.5, 110.1,
37.9, 32.7. 11B NMR (128 MHz, CDCl3): δ 41.1. HRMS (ESI): m/z
calcd for C28H27BN [M + H]+ 388.2231, found 388.2226
3-tert-Butyl-2-phenyl-1-(thiophene-2-yl)-1,2-dihydrobenzo[c]-
[1,2]azaborinine (12). White solid (0.57 g, 55%). Mp: 134−136 °C.
1H NMR (400 MHz, CDCl3): δ 7.93 (d, J = 7.8 Hz, 1H), 7.66 (q, J =
8.1 Hz, 2H), 7.40 (d, J = 4.7 Hz, 1H), 7.35 (t, J = 7.2 Hz, 1H), 7.27−
7.19 (m, 5H), 7.04−6.99 (m, 2H), 6.86 (d, J = 3.3 Hz, 1H), 1.27 (s,
9H). 13C NMR (101 MHz, C6D6): δ 151.2, 145.6, 141.9, 136.4, 132.9,
131.8, 131.0, 128.5, 127.2, 126.8, 126.6, 126.4, 124.7, 110.5, 37.5, 32.4,
the resonances for the carbon atoms adjacent to the boron could not
be observed. 11B NMR (128 MHz, CDCl3): δ 37.8. HRMS (ESI): m/z
calcd for C22H23BNS [M + H]+ 344.1639, found 344.1636
3-tert-Butyl-2-phenyl-1-(phenylethynyl)-1,2-dihydrobenzo[c]-
[1,2]azaborinine (13). Purified by chromatography on silica gel with
EtOAc/n-hexane (1/10, v/v) as the eluent. White solid (0.82 g, 76%).
Mp: 157 °C. 1H NMR (400 MHz, CDCl3): δ 8.41 (d, J = 7.4 Hz, 1H),
7.75−7.63 (m, 2H), 7.54−7.37 (m, 6H), 7.32−7.25 (m, 3H), 7.24−
7.16 (m, 2H), 6.95 (s, 1H), 1.31 (s, 9H). 13C NMR (101 MHz,
CDCl3): δ 151.9, 147.4, 141.2, 135.1, 132.2, 131.3, 131.2, 128.3, 128.0,
127.5, 127.0, 126.4, 124.6, 123.7, 110.4, 109.4, 37.7, 32.4, the
resonances for the carbon atoms adjacent to the boron could not be
observed. 11B NMR (128 MHz, C6D6): δ 31.1. HRMS (ESI): m/z
calcd for C26H25BN [M + H]+ 362.2075, found 362.2081
3-tert-Butyl-1-((4-fluorophenyl)ethynyl)-2-phenyl-1,2-
dihydrobenzo[c][1,2]azaborinine (14). White solid (0.83 g, 73%).
Mp: 137−138 °C. 1H NMR (400 MHz, CDCl3): δ 8.37 (d, J = 7.6 Hz,
1H), 7.72−7.63 (m, 2H), 7.52−7.36 (m, 6H), 7.20−7.12 (m, 2H),
6.99−6.90 (m, 3H), 1.30 (s, 9H). 13C NMR (101 MHz, CDCl3): δ
162.6 (d, J = 251.5 Hz), 151.9, 147.4, 141.2, 135.0, 134.2 (d, J = 8.1
Hz), 131.4, 131.2, 127.6, 126.9, 126.4, 124.7, 119.8 (d, J = 3.0 Hz),
115.4 (d, J = 22.2 Hz), 109.5, 109.2, 37.7, 32.4, the resonances for the
carbon atoms adjacent to the boron could not be observed. 11B NMR
(128 MHz, CDCl3): δ 30.7. 19F NMR (376 MHz, CDCl3): δ −110.6.
HRMS (ESI): m/z calcd for C26H24BFN [M + H]+ 380.1980, found
380.1982
3-tert-Butyl-1-((4-methoxyphenyl)ethynyl)-2-phenyl-1,2-
dihydrobenzo[c][1,2]azaborinine (15). White solid (0.88 g, 75%).
Mp: 167−168 °C. 1H NMR (400 MHz, CDCl3): δ 8.39 (d, J = 7.5 Hz,
1H), 7.71−7.61 (m, 2H), 7.51−7.37 (m, 6H), 7.13 (d, J = 8.6 Hz,
2H), 6.91 (s, 1H), 6.79 (d, J = 8.6 Hz, 2H), 3.82 (s, 3H), 1.29 (s, 9H).
13C NMR (101 MHz, CDCl3): δ 159.7, 151.8, 147.5, 141.1, 135.1,
1-Bromo-3-tert-butyl-6-methyl-2-phenyl-1,2-dihydrobenzo[c]-
1
[1,2]azaborinine (7). White solid (0.69 g, 65%). Mp: 109 °C. H
NMR (400 MHz, C6D6): δ 8.55 (d, J = 7.9 Hz, 1H), 7.25 (s, 1H), 7.13
(s, 1H), 7.06−6.94 (m, 5H), 6.88 (s, 1H), 2.22 (s, 3H), 1.09 (s, 9H).
13C NMR (101 MHz, C6D6): δ 151.7, 145.9, 141.8, 141.7, 135.8,
131.6, 127.3, 127.2, 126.6, 110.0, 37.5, 32.1, 21.5, the resonance for the
carbon atom adjacent to the boron could not be observed. 11B NMR
(128 MHz, C6D6): δ 36.6. HRMS (EI): m/z calcd for C19H21BBrN
[M]+ 353.0945, found 353.0949
1-Bromo-3-tert-butyl-5-fluoro-2-phenyl-1,2-dihydrobenzo[c]-
[1,2]azaborinine (8). The reaction was performed at 100 °C for 6 h.
Compound 8 was obtained as a white solid (0.46 g, 43%). Mp: 93−94
1
°C. H NMR (400 MHz, C6D6): δ 8.29 (d, J = 7.6 Hz, 1H), 7.39 (s,
1H), 7.13−6.97 (m, 5H), 6.95−6.86 (m, 2H), 1.01 (s, 9H). 13C NMR
(101 MHz, C6D6): δ 158.5 (d, J = 250.5 Hz), 152.8, 145.5, 131.5,
131.4 (d, J = 4.0 Hz), 129.9 (d, J = 7.1 Hz), 127.7, 127.5, 125.9 (d, J =
7.1 Hz), 116.2 (d, J = 19.2 Hz), 101.6 (d, J = 6.1 Hz), 37.9, 32.0, the
resonance for the carbon atom adjacent to the boron could not be
observed. 11B NMR (128 MHz, C6D6): δ 36.6. 19F NMR (376 MHz,
C6D6): δ −124.5. HRMS (EI): m/z calcd for C18H18BBrFN [M]+
357.0695, found 357.0701
1,2,3-Triphenyl-1,2-dihydrobenzo[c][1,2]azaborinine (9). PhBBr2
was used, and the reaction was performed at 120 °C for 6 h.
Compound 9 was obtained as a white solid (0.51 g, 52%) by
chromatography on silica gel with n-hexane as elute. Mp: 123−124 °C.
1H NMR (400 MHz, CDCl3): δ 7.93 (d, J = 7.7 Hz, 1H), 7.68 (dt, J =
14.7, 7.2 Hz, 2H), 7.37 (dd, J = 10.6, 3.9 Hz, 1H), 7.25−7.11 (m,
10H), 7.00−6.79 (m, 6H). 13C NMR (101 MHz, CDCl3): δ 144.9,
144.7, 141.2, 138.9, 136.6, 133.2, 131.1, 129.8, 129.7, 127.5, 127.4,
127.0, 126.8, 126.7, 126.2, 125.6, 124.7, 113.7, the resonances for the
carbon atoms adjacent to the boron could not be observed. 11B NMR
(128 MHz, CDCl3): δ 39.0. HRMS (ESI): m/z calcd for C26H21BN
[M + H]+ 358.1762, found 358.1758
3-tert-Butyl-1-chloro-2-phenyl-1,2-dihydrobenzo[c][1,2]-
azaborinine (10). BCl3 was used, and the reaction was performed at
120 °C for 24 h. Compound 10 was obtained as a white solid by
133.8, 131.4, 131.1, 127.5, 126.8, 126.3, 124.5, 115.8, 113.7, 110.6,
109.2, 55.2 37.7, 32.4, the resonances for the carbon atoms adjacent to
the boron could not be observed. 11B NMR (128 MHz, CDCl3): δ
30.7. HRMS (ESI): m/z calcd for C27H27BNO [M + H]+ 392.2180,
found 392.2175
1
recrystallization from n-hexane (0.65 g, 73%). Mp: 95 °C. H NMR
(400 MHz, C6D6): δ 8.52 (d, J = 7.7 Hz, 1H), 7.45 (tt, J = 7.9, 4.2 Hz,
2H), 7.28 (ddd, J = 8.0, 6.4, 1.9 Hz, 1H), 7.08−6.92 (m, 5H), 6.85 (s,
1H), 1.06 (s, 9H). 13C NMR (101 MHz, C6D6): δ 151.3, 144.5, 141.9,
134.0, 131.7, 131.3, 127.7, 127.2, 126.6, 125.2, 109.4, 37.4, 32.0. 11B
NMR (128 MHz, C6D6): δ 36.2. HRMS (EI): m/z calcd for
C18H19BClN [M]+ 295.1294, found 295.1296
General Procedure for the Substitution of the Bromide by
Lithium Salts. Synthesis of 3-tert-Butyl-1,2-diphenyl-1,2-
dihydrobenzo[c][1,2]azaborinine (2). To 1 (1.02 g, 3 mmol) in 15
mL of Et2O was added 3 mmol of PhLi (generated in situ) at −78 °C.
The mixture was slowly warmed to room temperature and stirred
overnight. To this solution was added 20 mL of water to quench the
reaction. The mixture was extracted by Et2O (30 mL × 3). The extract
was dried over MgSO4, and the solvent was removed under vacuum.
Compound 2 was obtained as a white solid by recrystallization from
Et2O (0.69 g, 68%).
3-tert-Butyl-2-(4-fluorophenyl)-1-(phenylethynyl)-1,2-
dihydrobenzo[c][1,2]azaborinine (16). White solid (0.84 g, 74%).
Mp: 167−169 °C. 1H NMR (400 MHz, CDCl3): δ 8.36 (d, J = 7.6 Hz,
1H), 7.65 (q, J = 8.1 Hz, 2H), 7.44 (t, J = 7.0 Hz, 1H), 7.35 (dd, J =
8.5, 5.0 Hz, 2H), 7.31−7.26 (m, 3H), 7.25−7.18 (m, 2H), 7.13 (t, J =
8.5 Hz, 2H), 6.91 (s, 1H), 1.28 (s, 9H). 13C NMR (101 MHz,
CDCl3): δ 161.6 (d, J = 247.5 Hz), 151.7, 143.4, 141.1, 135.1, 132.7
(d, J = 8.0 Hz), 132.1, 131.3, 128.5, 128.2, 126.5, 124.8, 123.5, 114.3
(d, J = 23.2 Hz), 110.7, 109.6, 37.6, 32.4, the resonances for the carbon
atoms adjacent to the boron could not be observed. 11B NMR (128
MHz, CDCl3): δ 32.0. 19F NMR (376 MHz, CDCl3): δ −115.5.
HRMS (ESI): m/z calcd for C26H24BFN [M + H]+ 380.1980, found
380.1978
3-tert-Butyl-2-(3,4-dimethoxyphenyl)-1-(phenylethynyl)-1,2-
dihydrobenzo[c][1,2]azaborinine (17). White solid (0.97 g, 77%).
Mp: 132−133 °C. 1H NMR (400 MHz, CDCl3): δ 8.38 (d, J = 7.6 Hz,
1H), 7.73−7.59 (m, 2H), 7.51−7.40 (m, 1H), 7.31−7.21 (m, 5H),
7.04−6.83 (m, 4H), 3.99 (s, 3H), 3.87 (s, 3H), 1.32 (s, 9H). 13C NMR
(101 MHz, CDCl3): δ 152.1, 147.9, 147.8, 141.2, 140.3, 135.0, 132.1,
131.2, 128.3, 128.1, 126.3, 124.6, 123.7, 123.2, 115.3, 110.0, 109.6,
109.5, 56.1, 37.8, 32.5, the resonances for the carbon atoms adjacent to
the boron could not be observed. 11B NMR (128 MHz, CDCl3): δ
Compounds 11−19 were prepared similarly as described for 2.
Only spectroscopic data are given below.
3-tert-Butyl-1-(naphthalen-1-yl)-2-phenyl-1,2-dihydrobenzo[c]-
[1,2]azaborinine (11). White solid (0.73 g, 63%). Mp: 168−169 °C.
1H NMR (400 MHz, CDCl3): δ 7.82−7.72 (m, 2H), 7.69−7.62 (m,
2H), 7.58 (d, J = 8.3 Hz, 1H), 7.42−7.36 (m, 2H), 7.32−7.24 (m,
3H), 7.19 (dd, J = 12.8, 6.4 Hz, 2H), 7.10 (td, J = 7.8, 1.3 Hz, 2H),
7.01 (d, J = 7.9 Hz, 1H), 6.94 (t, J = 7.4 Hz, 1H), 6.75 (td, J = 7.8, 1.2
Hz, 1H), 1.34 (s, 9H). 13C NMR (101 MHz, CDCl3): δ 151.8, 145.7,
141.2, 136.2, 135.7, 132.5, 131.5, 131.1, 130.2, 130.1, 129.8, 128.1,
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Featured Article
32.6. HRMS (ESI): m/z calcd for C28H29BNO2 [M + H]+ 422.2286,
found 422.2284
ACKNOWLEDGMENTS
■
We are grateful to the 973 program (Grant No.
2012CB821600) and the National Natural Science Foundation
of China (Grant Nos. 2139040, 21421062, and 21472098) for
support of the work.
3-tert-Butyl-1-((4-methoxyphenyl)ethynyl)-6-methyl-2-phenyl-
1,2-dihydrobenzo[c][1,2]azaborinine (18). White solid (0.87 g, 72%).
Mp: 128−129 °C. Crystals of 18 suitable for X-ray diffraction were
1
obtained by cooling a solution of 18 in Et2O. H NMR (400 MHz,
CDCl3): δ 8.15 (d, J = 7.7 Hz, 1H), 7.37−7.26 (m, 6H), 7.17 (s, 1H),
7.00 (d, J = 8.7 Hz, 2H), 6.73 (s, 1H), 6.67 (d, J = 8.7 Hz, 2H), 3.70
(s, 3H), 2.42 (s, 3H), 1.16 (s, 9H). 13C NMR (101 MHz, CDCl3): δ
159.6, 151.9, 147.6, 141.4, 141.2, 135.1, 133.8, 131.4, 127.4, 126.8,
126.3, 126.1, 115.9, 113.6, 110.3, 108.9, 55.2, 37.7, 32.4, 22.0, the
resonances for the carbon atoms adjacent to the boron could not be
observed. 11B NMR (128 MHz, CDCl3): δ 32.4. HRMS (ESI): m/z
calcd for C28H29BNO [M + H]+ 406.2337, found 406.2330
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(m, 3H), 7.42−7.31 (m, 4H), 7.31−7.21 (m, 3H), 7.21−7.13 (m, 3H),
1.29 (s, 9H). 13C NMR (101 MHz, CDCl3): δ 158.5 (d, J = 252.5 Hz),
152.8, 147.1, 132.2, 131.2, 130.7, 129.7 (d, J = 8.0 Hz), 128.5, 128.1,
127.6, 127.2, 125.0, 123.4, 115.6 (d, J = 20.2 Hz), 110.8, 101.0, 100.9,
38.1, 32.4, the resonances for the carbon atoms adjacent to the boron
could not be observed. 11B NMR (128 MHz, CDCl3): δ 30.6. 19F
NMR (376 MHz, CDCl3): δ −124.9. HRMS (ESI): m/z calcd for
C26H24BFN [M + H]+ 380.1980, found 380.1983
Synthesis of N-Benzyl-3-tert-butyl-2-(2,6-diisopropylphenyl)-
benzo[c][1,2]azaborinin-1(2H)-amine (20). To a mixture of 3 (1.0
mmol, 0.42 g) and NEt3 (0.14 mL, 1.0 mmol) in 15 mL of toluene was
added PhCH2NH2 (0.11 mL, 1 mmol). The mixture was stirred at
room temperature for 6 h. After removal of solvents, the residual was
extracted with 15 mL of Et2O. The solution was concentrated and
stored at −40 °C for 8 h to give 20 as a white solid (0.30 g, 67%). Mp:
121 °C. 1H NMR (400 MHz, CDCl3): δ 8.06 (d, J = 7.5 Hz, 1H), 7.58
(dt, J = 23.4, 7.3 Hz, 2H), 7.48−7.20 (m, 9H), 6.64 (s, 1H), 3.90 (s,
2H), 2.85 (m, 2H), 1.28 (d, J = 6.8 Hz, 6H), 1.19 (s, 9H), 1.10 (d, J =
6.7 Hz, 6H). 13C NMR (101 MHz, CDCl3): δ 150.1, 146.6, 141.9,
136.9, 130.7, 129.8, 127.7, 127.5, 126.4, 126.1, 125.8, 124.8, 123.4,
122.9, 105.7, 45.5, 37.1, 31.8, 27.5, 24.8, 21.8. 11B NMR (128 MHz,
CDCl3): δ 28.7. HRMS (ESI): m/z calcd for C31H40BN2 [M + H]+
451.3279, found 451.3284.
Reactions of 1 with MeLi or 1-Propenylmagnesium Bromide.
Reactions of 1 with MeLi or 1-propenylmagnesium bromide in Et2O
from −78 °C to room temperature led to the formation of a
complicated mixture from which the desired substitution products
cannot be isolated in pure form.
Reaction of 3 with MeOH. The reaction of 3 with MeOH was
performed in Et2O. The corresponding product can be observed by
NMR but cannot be obtained in pure form.
Stability of 4 in Aqueous Solution. At room temperature, 0.10 g
of 4 was added to 20 mL of 1.0 M HCl and NaOH aqueous solutions,
respectively. The mixtures were stirred at room temperature for 5 h.
Extraction of the aqueous solutions with Et2O (20 mL × 2) resulted in
the recovery of 4 in 90% and 92% yields, respectively.
ASSOCIATED CONTENT
* Supporting Information
■
S
NMR experiments for the effects of NEt3 on the borylation
reaction, crystallographic data, and selected bond parameters
for compounds 3, 4, and 18 and 1H, 11B, and 13C spectra of the
1,2-borazaronaphthalene derivatives reported in this paper.
This material is available free of charge via the Internet at
http://pubs.acs.org.
̀
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L.; Maziere, A.; Lovinger, G. J.; Li, B.; Zakharov, L. N.; Dargelos, A.;
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AUTHOR INFORMATION
Corresponding Author
*E-mail: cmcui@nankai.edu.cn.
Notes
■
The authors declare no competing financial interest.
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DOI: 10.1021/jo5029437
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