Additive- and Photocatalyst-Free Borylation of Arylazo Sulfones under Visible Light

Article abstract of DOI:10.1021/acs.joc.8b01662

We developed a photocatalyst-free and additive-free, visible light induced borylation reaction using arylazo sulfones as starting material. This protocol shows some advantages such as mild conditions, simple equipment, and wide substrate scope, which gives a complementary protocol for the preparation of arylboronates.

Full text of DOI:10.1021/acs.joc.8b01662

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Note
Additive- and Photocatalyst-free Borylation
of Arylazo Sulfones under Visible Light
Yuliang Xu, Xinying Yang, and Hao Fang
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.8b01662 • Publication Date (Web): 26 Sep 2018
Downloaded from http://pubs.acs.org on September 27, 2018
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Additive- and Photocatalyst-free Borylation of Arylazo Sulfones under
Visible Light
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Yuliang Xu, Xinying Yang and Hao Fang*
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Department of Medicinal Chemistry, Key Laboratory of Chemical Biology (Ministry of Education),
School of Pharmacy, Shandong University, 44 West Wenhua Road, Jinan, Shandong 250012, PR
China.
Email: haofangcn@sdu.edu.cn.
Abstract: We developed a photocatalyst-free and additive-free, visible light induced borylation
reaction using arylazo sulfones as starting material. This protocol shows some advantage such as
mild conditions, simple equipment, and wide substrate scope, which gives a complementary
protocol for the preparation of arylboronates.
Aryl boronic acids and arylboronates are important building blocks in chemical, medicinal and
material science areas.1 Their preparation fall among one of the most active areas in the realm of
organic synthesis. The classical method involves reactions of aryl Grignard or aryl lithium with
trialkoxyboranes, followed by transesterification or hydrolysis. This methodology suffers from
limited functional group tolerance and the requirement of anhydrous conditions.2 Later on, a variety
of transition metals such as palladium,3 copper,4 nickel,5 and zinc6 have been applied in transition
metal-catalyzed borylation reactions. In order to reduce the costs and metal residue in the final
medicinal products, more attention has been focused on the transition metal-free methods. Although
Sandmeyer-type borylation of aryl amine and base-mediated borylation of aryl halides have been
reported,7 harsh conditions and longer reaction time are usually required.
Recently, several groups reported metal-free UV photoinduced borylation,8 in which specialized
equipment was required. Evidently, it is more convenient and practical to use visible-light and
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regular glassware. Very recently, photocatalysis has opened up new avenues for the generation of
radical species under visible-light-induced conditions.9 This new paradigm has found numerous
applications in synthetic chemistry including borylation reactions. For example, Yan reported
visible light induced borylation of aryldiazonium salts (Figure 1, Path A).10 Fu reported visible light
photoredox borylation of aryl halides;11 These reactions are convenient and efficient, however, they
inevitably required expensive photocatalysts, ligands, additives or bases. Recently, Glorius
disclosed the first example of transition metal-free, visible light induced decarboxylative borylation
of aryl N-hydroxyphthalimide esters.12 The starting material of the reaction is aryl carboxylic acids.
Unlike carboxyl group, the NH2 group is ortho-para directing group. We herein report a
complementary visible-light-induced borylation method for arylamine derivatives and this method
does not require expensive photocatalyst or ligand under mild condition. Our preliminary
mechanistic studies support the free radical mechanism for the photoinduced borylation process.
Based on Yan’s research, we conjectured that the colored arylazo compounds prepared from
colorless aryldiazonium salts could produce the aryl radicals under visible light. Arylazo sulfones
are stable and easy to obtain from aryldiazonium salts. Inspired by Protti and Fagnoni’s recent work
of transition metal-free arylation of arylazo sulfones,13 we present a catalyst and additive-free,
visible-light-induced borylation reaction involving arylazo sulfones as aryl radical precursors
(Figure 1).
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Figure 1. Outline of This Work
We started our investigation by using 4-methoxyphenylazo mesylate (1a) with
bis(pinacolato)diboron (B2pin2) under visible-light induced condition (Table 1). A solution of 1a
and 1.5 equiv of B2pin2 in acetonitrile was placed in a Schlenk tube under N2 atmosphere, and
irradiated with a 30 W blue LED light without photocatalyst. To our delight, the desired arylboronate
3a was observed in 81% yield as detected by 1H NMR analysis of the crude reaction mixture (entry
1). Next, other solvents (methanol, DMF, THF, ethyl acetate and toluene) were screened, but no
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improvement was observed (entry 2 to entry 6) compared with acetonitrile as solvent. Efforts to
improve the yield then shifted to examine the additive effect. The use of NaBF4 (entry 7), pyridine
(entry 9) and N,N,N’,N’-tetramethyldiaminomethane (TMDAM, entry 8) as additive provides much
lower yield. Surprisingly, 10% water as cosolvent (entry 11) increased the yields to 86%, and the
byproduct 4a was not detected. When 2.0 equiv of B2pin2 was used, 97% yield was observed (entry
12). The yield of 3a decreased when reaction was irradiated with CFL lamp (entry 13) or white LED
light (entry 14). Control experiment was performed in the absence of light, the yield was much
lower (17%, entry 15).
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Table 1. Reaction Optimizationa
2
addtive
(eq.)
convn
(%)
100
100
100
84
yield (%)b
entry
(eq.)
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
solvent
CH3CN
CH3OH
DMF
3a
4a
8
1
2
3
4
5
6
7
8
none
81
79
19
25
72
35
68
55
none
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19
24
10
22
6
none
THF
none
EtOAc
toluene
CH3CN
CH3CN
none
100
100
100
100
none
NaBF4 (1)
TMDAM
(1)
16
9
1.5
1.5
1.5
2
CH3CN
Pyridine (1)
none
100
100
100
100
80
72
82
86
97
35
45
17
10
n.d.
n.d.
n.d.
10
10
CH3CN/H2O (19/1)
CH3CN/H2O (9/1)
CH3CN/H2O (9/1)
CH3CN/H2O (9/1)
CH3CN/H2O (9/1)
CH3CN/H2O (9/1)
11
none
12
none
13 c
14 d
15e
2
none
2
none
87
5
2
none
30
n.d.
aReaction conditions: N2 atmosphere and 30 W blue LED light irradiation, 1a (0.1 mmol),
B2pin2 (0.15-0.2 mmol), additive (0.5 mmol), solvent (1 mL), rt., 17 h in a Schlenk tube.
bDetermined by 1H NMR with 1,3,5-trimethoxybenzene as an internal standard. cThe reaction
was carried out under 30 W CFL lamps. dThe reaction was carried out under 30 W white LED
light. eNo light.
With the optimized condition in hand, we next examined substrate scope of this transformation
(Scheme 1). A variety of arylazo sulfones can be used regardless of their electronic nature. Notably,
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both electron-rich and electron-deficient arylazo sulfones could be tolerated. para-Substituted
arylazo sulfones gave moderate to excellent yields. These new borylation conditions possessed
excellent functional group compatibility, such as ether (3a), cyano (3b), ketone (3c), nitro (3e),
carboxyl (3m), alkyl (3h and 3j) and aryl (3d) groups. Chloro- (3o), bromo-(3p) and fluoro-(3n)
groups were tolerated in the reaction, where those products were difficult to be prepared by the
conventional transition metal-catalyzed borylation of arylhalides. meta-Substituted substrates
produced borylation products in moderate yields (3q, 3r and 3s). However, ortho-substituted
arylazo sulfones afforded lower yield (3t and 3u) due to steric hindrance. Noteworthy, the arylazo
sulfones can generate reactive intermediates under solar light,14 we also investigated the borylation
of arylazo sulfones under solar light. Sun light was convenient to induce the borylation of 1a to
obtain the 3a, but the isolated yield was only 50%.
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Scheme 1. Substrate Scope for Visible-Light Photoinduced Borylation of 1 with 2a
aReaction conditions: N2 atmosphere and 30 W blue LED light irradiation, 1 (0.5 mmol), 2
(1 mmol), solvent (5 mL), rt., 17 h in a Schlenk tube. Isolated yields and NMR yields are
shown in parentheses. bThe reaction was carried out under natural sunlight (17 h).
We further attempted reaction of arylazo sulfones with a more atom economical borylating
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reagent bisboronic acid. By condition optimization, the change from the previous conditions was
using MeOH as the solvent. Because of the inconvenience to isolate the arylboronic acid, aqueous
KHF2 was added to obtain the aryltrifluoroborate (Scheme 2). In this study, the yields were about
30% and the reaction was inferior to the borylation using B2pin2 as the borylating reagent. When
bisboronic acid was as the borylating reagent, the conversion rate of arylazo sulfones was high, the
unsatisfied yields may be due to the low reactivity between the bisboronic acid and aryl radical.
Since ArBpin is the most widely used reagents for cross-coupling reactions, we did not further
optimize the reaction for other borylating reagents such as bis(neopentanediolato)diboron and
bis(catecholato)diboron.
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Scheme 2. Visible-Light Photoinduced Borylation with Bisboronic Acida.
aReaction conditions: N2 atmosphere and 30 W blue LED light irradiation, Arylazo Sulfone
(1 mmol), bisboronic acid (2 mmol), MeOH (10 mL), rt, 17 h in a Schlenk tube, then KHF2.
Isolated yields.
We hypothesize that borylation of arylazo sulfones proceed though a radical mechanism. To gain
insight into the reaction mechanism, radical trapping experiments were undertaken (Scheme 3). The
addition of 2,2,6,6-tetramethyl-1-piperidinoxyl (TEMPO) to the reaction was observed to suppress
the reaction (equation 1). The borylation product was obtained in only 16% yield together with the
TEMPO adduct in 29% yield. On the other hand, TEMPO was added to the reaction mixture without
B2pin2 and the aryl-TEMPO adduct was isolated in 50% yield (equation 2), which suggests the
reaction involves a radical mechanism. Although the mechanism is not entirely clear, aryl radicals
can be obtained via homolysis of N-S bond occurring from the 1nπ* state and loss of a N2
molecule,13 it is possible that the radical reacts with sp2-sp3 Lewis base-diboron adduct to produce
the arylboronates.
Scheme 3. Preliminary Mechanistic Studies.
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In conclusion, we developed a catalyst-free, visible-light photoinduced borylation reaction using
arylazo sulfones at room temperature. As a substrate, arylazo sulfones with various functional
groups can be prepared easily. Significantly, the reaction system is operationally simple without any
photocatalyst or additive. This protocol shows advantages for the synthesis of arylboronates and
represents an important approach to the reported C-B bond formation methods. We believe that this
work will find wide applications in some fields. Further studies on the mechanism are under way in
our laboratory.
EXPERIMENTAL SECTION
General experimental information:
Only if otherwise mentioned, all solvents and reagents were commercially available and utilized
without any purification. Reactions were monitored by thin layer chromatography (TLC), and
organic solutions were concentrated under reduced pressure on Eyela rotary evaporator. The
products were obtained by column chromatography on silica gel (200-300 mesh).
1H-NMR and 13C-NMR spectra were recorded on a Bruker Avance 400 spectrometer at ambient
temperature. Data for 1H-NMR are reported as follows: chemical shift (ppm, scale), multiplicity (s
= singlet, d = doublet, t = triplet, q = quartet, m = multiplet), coupling constant (Hz), and integration.
Data for 13C-NMR are reported in terms of chemical shift (ppm, scale), multiplicity, and coupling
constant (Hz). High-resolution mass spectra were recorded by TOF using the electrospray ionization
(ESI) or electron impact ionization (EI) method.
General procedure for the synthesis of arylazo sulfones (1a~1u)13
Arylamine (10 mmol) as dissolved in a mixture of 5 mL of distilled water and 4 mL of 50%
hydrofluoroboric acid. After cooling the reaction mixture to 0 °C using ice bath and the sodium
nitrite (760 mg in 4 mL distilled water ) was added dropwise in 5 min interval of time. The resulting
mixture was stirred for 1h and the precipitate was collected by filtration and redissolved in minimum
amount of acetone. Diethyl ether was added until precipitation of aryl diazonium tetrafluoroborate,
which is filtered, washed several times with diethyl ether. To a cooled (0 °C) suspension of the
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appropriate diazonium salt in CH2Cl2 was added sodium methanesulfinate (1.2 equiv) in one portion.
The temperature was allowed to rise to room temperature and the solution stirred overnight. The
resulting mixture was then filtered and the obtained solution evaporated. The crude product was was
purified by dissolution in cold CH2Cl2 and precipitation by adding n-hexane.
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1-(4-methoxyphenyl)-2-(methylsulfonyl)diazene (1a).13 Yield: 53% (1.1 g). Yellow solid. Mp: 81-
83 °C. 1H NMR (400 MHz, CDCl3) δ 7.95 (d, J = 9.0 Hz, 2H), 7.04 (d, J = 9.0 Hz, 2H), 3.94 (s,
3H), 3.19 (s, 3H). 13C{1H} NMR (101 MHz, CDCl3) δ 165.7, 143.2, 127.4, 115.0, 56.0, 34.9.
4-((methylsulfonyl)diazenyl)benzonitrile (1b).13 Yield: 85% (1.8 g). Yellow solid. Mp: 113-115 °C.
1H NMR (400 MHz, CDCl3) δ 8.06 (d, J = 8.5 Hz, 2H), 7.91 (d, J = 8.5 Hz, 2H), 3.28 (s, 3H).
13C{1H} NMR (101 MHz, CDCl3) δ 150.8, 133.8, 124.8, 118.1, 117.4, 35.0.
1-(4-((methylsulfonyl)diazenyl)phenyl)ethan-1-one (1c).13 Yield: 44% (1.0 g). Yellow solid. Mp:
120-122 °C. 1H NMR (400 MHz, CDCl3) δ 8.15 (d, J = 8.7 Hz, 2H), 8.03 (d, J = 8.7 Hz, 2H), 3.26
(s, 3H), 2.69 (s, 3H). 13C{1H} NMR (101 MHz, CDCl3) δ 196.8, 151.2, 141.5, 129.7, 124.6, 34.9,
27.0.
1-([1,1'-biphenyl]-4-yl)-2-(methylsulfonyl)diazene (1d). Yield: 54% (1.0 g). Brown solid. Mp: 118-
120 °C. 1H NMR (400 MHz, CDCl3) δ 8.03 (d, J = 8.5 Hz, 2H), 7.80 (d, J = 8.5 Hz, 2H), 7.67 (d, J
= 7.1 Hz, 2H), 7.48 (m, 3H), 3.24 (s, 3H). 13C{1H} NMR (101 MHz, CDCl3) δ 148.2, 148.1, 139.1,
129.2, 129.0, 128.3, 127.4, 125.3, 34.9. HRMS (EI) m/z: Calcd for C13H12N2O2S [M+]: 260.0619,
Found: 260.0612.
1-(methylsulfonyl)-2-(4-nitrophenyl)diazene (1e).15 Yield: 48% (1.2 g). Orange solid. Mp: 113-
115 °C. 1H NMR (400 MHz, CDCl3) δ 8.46 (d, J = 9.0 Hz, 2H), 8.12 (d, J = 9.0 Hz, 2H), 3.29 (s,
3H). 13C{1H} NMR (101 MHz, CDCl3) δ 151.9, 151.1, 125.2, 35.0.
1-(methylsulfonyl)-2-(4-(trifluoromethoxy)phenyl)diazene (1f). Yield: 18.6% (500 mg). Yellow
solid. Mp: 74-76 °C. 1H NMR (400 MHz, CDCl3) δ 8.03 (d, J = 9.0 Hz, 2H), 7.42 (d, J = 8.4 Hz,
2H), 3.24 (s, 3H). 13C{1H} NMR (101 MHz, CDCl3) δ 154.0, 146.8, 126.5, 121.3, 120.2 (q, J =
260.0 Hz), 34.9. HRMS (EI) m/z: Calcd for C8H7F3N2O3S [M+]: 268.0129, Found: 268.0132.
Ethyl 4-((methylsulfonyl)diazenyl)benzoate (1g). Yield: 55% (1.4 g). Yellow solid. Mp: 74-76 °C.
1H NMR (400 MHz, CDCl3) δ 8.25 (d, J = 8.5 Hz, 2H), 8.00 (d, J = 8.5 Hz, 2H), 4.44 (q, J = 7.1
Hz, 2H), 3.25 (s, 3H), 1.44 (t, J = 7.1 Hz, 3H). 13C{1H} NMR (101 MHz, CDCl3) δ 165.2, 151.2,
135.9, 131.0, 124.3, 61.9, 34.9, 14.3. HRMS (ESI) m/z: Calcd for C10H13N2O4S [M+H]+: 257.0591,
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Found: 257.0595.
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1-(4-(tert-butyl)phenyl)-2-(methylsulfonyl)diazene (1h).13 Yield: 56% (1.3 g). Yellow solid. Mp:
70-72 °C. 1H NMR (400 MHz, CDCl3) δ 7.89 (d, J = 8.7 Hz, 2H), 7.59 (d, J = 8.7 Hz, 2H), 3.21 (s,
3H), 1.38 (s, 9H). 13C{1H} NMR (101 MHz, CDCl3) δ 159.9, 147.0, 126.7, 124.6, 35.6, 34.8, 31.0.
1-(methylsulfonyl)-2-(3-(trifluoromethyl)phenyl)diazene (1i). Yield: 56% (1.4 g). Yellow solid.
Mp: 68-70 °C. 1H NMR (400 MHz, CDCl3) δ 8.23 (s, 1H), 8.15 (d, J = 8.0 Hz, 1H), 7.94 (d, J = 7.8
Hz, 1H), 7.76 (t, J = 7.9 Hz, 1H), 3.27 (s, 3H). 13C{1H} NMR (101 MHz, CDCl3) δ 149.0, 132.6 (q,
J = 33.7 Hz), 131.3 (q, J = 3.5 Hz), 130.6, 127.2, 123.2 (q, J = 272.7 Hz), 121.7 (q, J = 3.8 Hz),
35.0. HRMS (EI) m/z: Calcd for C8H7F3N2O2S [M+]: 252.0180, Found: 252.0175.
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1-(methylsulfonyl)-2-(p-tolyl)diazene (1j).15 Yield: 36% (715 mg). Orange solid. Mp: 104-106 °C.
1H NMR (400 MHz, CDCl3) δ 7.85 (d, J = 8.3 Hz, 2H), 7.37 (d, J = 8.3 Hz, 2H), 3.21 (s, 3H), 2.48
(s, 3H). 13C{1H} NMR (101 MHz, CDCl3) δ 147.2, 147.0, 130.4, 124.8, 34.8, 22.0.
1-(methylsulfonyl)-2-(4-(trifluoromethyl)phenyl)diazene (1k). Yield: 44% (1.2 g). Yellow solid.
Mp: 111-113 °C. 1H NMR (400 MHz, CDCl3) δ 8.07 (d, J = 8.2 Hz, 2H), 7.87 (d, J = 8.4 Hz, 2H),
3.27 (s, 3H). 13C{1H} NMR (101 MHz, CDCl3) δ 150.7, 136.0 (q, J = 33.1 Hz), 127.0 (q, J = 3.7
Hz), 124.7, 123.2 (q, J = 273.0 Hz), 34.9. HRMS (EI) m/z: Calcd for C8H7F3N2O2S [M+]: 252.0180,
Found: 252.0178.
1
1-(methylsulfonyl)-2-phenyldiazene (1l).13 Yield: 65% (1.2 g). Orange solid. Mp: 61-63 °C. H
NMR (400 MHz, CDCl3) δ 7.96 (d, J = 7.6 Hz, 2H), 7.68 (t, J = 7.4 Hz, 1H), 7.59 (t, J = 7.6 Hz,
2H), 3.23 (s, 3H). 13C{1H} NMR (101 MHz, CDCl3) δ 149.0, 135.3, 129.8, 124.6, 34.8.
4-((methylsulfonyl)diazenyl)benzoic acid (1m). Yield: 66% (1.5 g). Yellow solid. Mp: 133-134 °C.
1H NMR (400 MHz, DMSO-d6) δ 13.58 (s, 1H), 8.21 (d, J = 8.5 Hz, 2H), 8.06 (d, J = 8.5 Hz, 2H),
3.49 (s, 3H). 13C{1H} NMR (101 MHz, DMSO-d6) δ 166.7, 151.3, 131.5, 124.6, 35.3. HRMS (ESI)
m/z: Calcd for C8H7N2O4S [M-H]-: 227.0132, Found: 227.0131.
4-((methylsulfonyl)diazenyl)benzoic acid (1n). Yield: 69% (1.4 g). Yellow solid. Mp: 64-66 °C. 1H
NMR (400 MHz, CDCl3) δ 8.05 – 7.97 (m, 2H), 7.28 (dd, J = 11.7, 5.2 Hz, 2H), 3.23 (s, 3H). 13C{1H}
NMR (101 MHz, CDCl3) δ 166.9 (d, J = 259.4 Hz), 145.6 (d, J = 2.9 Hz), 127.3 (d, J = 9.9 Hz),
117.1 (d, J = 23.3 Hz), 34.9. HRMS (EI) m/z: Calcd for C6H4N2F [M-CH3SO2]+: 123.0359, Found:
123.0361.
1-(4-chlorophenyl)-2-(methylsulfonyl)diazene (1o).13 Yield: 50% (1.3 g). Yellow solid. Mp: 61-
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63 °C. 1H NMR (400 MHz, CDCl3) δ 7.91 (d, J = 8.8 Hz, 2H), 7.57 (d, J = 8.8 Hz, 2H), 3.23 (s,
3H). 13C{1H} NMR (101 MHz, CDCl3) δ 147.4, 141.8, 130.2, 125.8, 34.9.
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1-(4-bromophenyl)-2-(methylsulfonyl)diazene (1p).15 Yield: 56% (1.7 g). Yellow solid. Mp: 132-
133 °C. 1H NMR (400 MHz, CDCl3) δ 7.83 (d, J = 8.8 Hz, 2H), 7.74 (d, J = 8.8 Hz, 2H), 3.23 (s,
3H). 13C{1H} NMR (101 MHz, CDCl3) δ 147.7, 133.2, 130.6, 125.9, 34.9.
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3-((methylsulfonyl)diazenyl)benzoic acid (1q). Yield: 22% (500 mg). Yellow solid. Mp: 121-
123 °C. 1H NMR (400 MHz, CDCl3) δ 8.70 (s, 1H), 8.41 (d, J = 7.7 Hz, 1H), 8.20 (d, J = 8.0 Hz,
1H), 7.74 (t, J = 7.9 Hz, 1H), 3.27 (s, 3H). 13C{1H} NMR (101 MHz, CDCl3) δ 166.5, 149.1, 135.9,
133.2, 131.3, 128.4, 124.8, 35.3. HRMS (ESI) m/z: Calcd for C8H7N2O4S [M-H]-: 227.0132, Found:
227.0133.
1-(methylsulfonyl)-2-(3-nitrophenyl)diazene (1r). 15 Yield: 57% (1.3 g). Yellow solid. Mp: 125-
126 °C. 1H NMR (400 MHz, CDCl3) δ 8.80 (t, J = 1.9 Hz, 1H), 8.54 (dd, J = 8.2, 1.2 Hz, 1H), 8.32
(t, J = 12.5 Hz, 1H), 7.84 (t, J = 8.1 Hz, 1H), 3.30 (s, 3H). 13C{1H} NMR (101 MHz, CDCl3) δ
149.3, 149.1, 130.9, 129.5, 128.8, 119.5, 35.1.
1-(3-chlorophenyl)-2-(methylsulfonyl)diazene (1s). Yield: 64% (1.4 g). Yellow solid. Mp: 80-
82 °C. 1H NMR (400 MHz, CDCl3) δ 7.95 – 7.91 (m, 1H), 7.87 (d, J = 7.9 Hz, 1H), 7.64 (dd, J =
8.0, 0.7 Hz, 1H), 7.55 (t, J = 8.0 Hz, 1H), 3.24 (s, 3H). 13C{1H} NMR (101 MHz, CDCl3) δ 149.7,
135.9, 134.9, 130.8, 123.7, 123.6, 34.9. HRMS (EI) m/z: Calcd for C6H4N2Cl [M-CH3SO2]+:
139.0063, Found: 139.0068.
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1-(methylsulfonyl)-2-(o-tolyl)diazene (1t). Yield: 71% (1.4 g). Yellow solid. Mp: 94-96 °C. H
NMR (400 MHz, CDCl3) δ 7.71 (d, J = 8.1 Hz, 1H), 7.55 (t, J = 7.4 Hz, 1H), 7.41 (d, J = 7.7 Hz,
1H), 7.31 (t, J = 7.7 Hz, 1H), 3.20 (s, 3H), 2.67 (s, 3H). 13C{1H} NMR (101 MHz, CDCl3) δ 147.1,
141.7, 135.5, 132.1, 127.0, 116.5, 34.8, 17.6. HRMS (EI) m/z: Calcd for C8H10N2O2S [M+]:
198.0463, Found: 198.0461.
1-(2-chlorophenyl)-2-(methylsulfonyl)diazene (1u).16 Yield: 64% (1.4 g). Yellow solid. Mp: 118-
120 °C. 1H NMR (400 MHz, CDCl3) δ 7.78 (dd, J = 8.2, 1.2 Hz, 1H), 7.62 (m, 2H), 7.45 – 7.38 (m,
1H), 3.24 (s, 12H). 13C{1H} NMR (101 MHz, CDCl3) δ 145.0, 138.5, 136.1, 131.5, 127.8, 118.1,
34.7.
General procedure for the preparation of arylboronic esters (3a~3u)
B2pin2 (1 mmol), Arylazo Sulfones (0.5 mmol) were added to a Schlenk tube which was purged
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thoroughly with N2. MeCN (4.5 mL) and water (0.5 ml) ware added via syringe and and the reaction
mixture was stirred at room temperature with the irradiation of a 30 W blue LED or natural sunlight
for about 17 h. The solution was then concentrated under reduced pressure, diluted with EtOAc and
washed with water. The aqueous layer was washed with EtOAc, the combined organic layers were
dried over MgSO4, and concentrated under reduced pressure. The crude product was purified by
column chromatography.
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General Procedure for the Preparation of trifluoroborates (6a~6d)
B2(OH)4 (2 mmol), Arylazo Sulfones (1 mmol) were added to a Schlenk tube which was purged
thoroughly with N2. MeOH (10 mL) was added via syringe and the reaction mixture was stirred at
room temperature with the irradiation of a 30 W blue LED for about 17 h. The solution was then
concentrated under reduced pressure. The concentrated crude reaction was taken up in MeOH (15
mL) and cooled to 0 ºC, to this cooled mixture was added 3.5 equivalents of a 4.5 M aqueous KHF2
solution, and the reaction was stirred for 10 min at 0 ºC before removing the bath and allowing the
mixture to stir at rt for 20 min. he mixture was concentrated under reduced pressure. Acetone (50
mL) was added into the flask and the solution was passed through Celite pad. The collected solvent
was concentrated and then dissolved in a minimal volume of acetone (3 mL). The addition of Et2O
(25 mL) led to the precipitation of the desired product. The collected solid was washed with Et2O.
2-(4-methoxyphenyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane
(3a).8b
Rf=0.5
(petroleum
ether/ethyl acetate=20/1). Eluent: petroleum ether/ethyl acetate=200/1. Yield: 80% (94 mg). A 50%
(58 mg) yield was obtained under natural sunlight. Pale yellow oil. 1H NMR (400 MHz, CDCl3) δ
7.75 (d, J = 8.6 Hz, 2H), 6.90 (d, J = 8.6 Hz, 2H), 3.83 (s, 3H), 1.33 (s, 12H). 13C{1H} NMR (101
MHz, CDCl3) δ 162.2, 136.5, 113.3, 83.6, 55.1, 24.9.
4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzonitrile (3b).8b Rf=0.5 (petroleum ether/ethyl
acetate=5/1). Eluent: petroleum ether/ethyl acetate=100/1. Yield: 58% (66 mg). Pale yellow solid.
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Mp: 93-95 °C. H NMR (400 MHz, CDCl3) δ 7.88 (d, J = 8.1 Hz, 2H), 7.64 (d, J = 8.2 Hz, 2H),
1.35 (s, 12H). 13C{1H} NMR (101 MHz, CDCl3) δ 135.1, 131.2, 118.9, 114.5, 84.5, 24.9.
1-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)ethan-1-one (3c).8b Rf=0.3 (petroleum
ether/ethyl acetate=20/1). Eluent: petroleum ether/ethyl acetate=100/1. Yield: 65% (80 mg). White
solid. Mp: 65-67 °C. 1H NMR (400 MHz, CDCl3) δ 7.93 (d, J = 7.9 Hz, 2H), 7.89 (d, J = 8.1 Hz,
2H), 2.62 (s, 3H), 1.36 (s, 12H). 13C{1H} NMR (101 MHz, CDCl3) δ 198.5, 139.0, 134.9, 127.3,
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84.2, 26.8, 24.9.
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2-([1,1'-biphenyl]-4-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (3d).8b Rf=0.5 (petroleum
ether/ethyl acetate=20/1). Eluent: petroleum ether/ethyl acetate=200/1. Yield: 70% (100 mg).
Mahogany solid. Mp: 108-110 °C. 1H NMR (400 MHz, CDCl3) δ 7.89 (d, J = 7.4 Hz, 2H), 7.65 –
7.58 (m, 4H), 7.44 (t, J = 7.5 Hz, 2H), 7.35 (t, J = 7.3 Hz, 1H), 1.36 (s, 12H). 13C{1H} NMR (101
MHz, CDCl3) δ 143.9, 141.0, 135.3, 128.8, 127.6 127.3, 126.5, 83.9, 24.9.
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4,4,5,5-tetramethyl-2-(4-nitrophenyl)-1,3,2-dioxaborolane (3e).7a Rf=0.4 (petroleum ether/ethyl
acetate=5/1). Eluent: petroleum ether/ethyl acetate=200/1. Yield: 70% (87 mg). Mahogany solid.
Mp: 104-105 °C. 1H NMR (400 MHz, CDCl3) δ (d, J = 8.6 Hz, 2H), 7.96 (d, J = 8.5 Hz, 2H), 1.37
(s, 12H). 13C{1H} NMR (101 MHz, CDCl3) δ 149.8, 135.7, 122.4, 84.6, 24.9.
4,4,5,5-tetramethyl-2-(4-(trifluoromethoxy)phenyl)-1,3,2-dioxaborolane (3f).7h Rf=0.6 (petroleum
ether/ethyl acetate=20/1). Eluent: petroleum ether/ethyl acetate=200/1. Yield: 54% (78 mg). Yellow
solid. Mp: 58-60 °C. 1H NMR (400 MHz, CDCl3) δ 7.84 (d, J = 8.4 Hz, 2H), 7.20 (d, J = 7.9 Hz,
2H), 1.34 (s, 12H). 13C{1H} NMR (101 MHz, CDCl3) δ 151.7, 136.5, 120.4 (q, J = 257.4 Hz), 119.9,
84.1, 24.9.
Ethyl 4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzoate (3g).8b Rf=0.4 (petroleum
ether/ethyl acetate=20/1). Eluent: petroleum ether/ethyl acetate=200/1. Yield: 75% (103 mg). Red
oil. 1H NMR (400 MHz, CDCl3) δ 8.02 (d, J = 7.9 Hz, 2H), 7.86 (d, J = 8.0 Hz, 2H), 4.38 (q, J =
7.1 Hz, 2H), 1.40 (t, J = 7.1 Hz, 3H), 1.36 (s, 12H). 13C{1H} NMR (101 MHz, CDCl3) δ 166.7,
134.6, 132.7, 128.6, 84.2, 61.1, 24.9, 14.3.
2-(4-(tert-butyl)phenyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (3h).7h Rf=0.6 (petroleum
ether/ethyl acetate=20/1). Eluent: petroleum ether/ethyl acetate=200/1. Yield: 55% (72 mg). Pale
yellow solid. Mp: 58-60 °C. 1H NMR (400 MHz, CDCl3) δ 7.76 (d, J = 8.2 Hz, 2H), 7.41 (d, J =
8.3 Hz, 2H), 1.33 (d, 21H). 13C{1H} NMR (101 MHz, CDCl3) δ 154.5, 134.7, 124.7, 83.6, 34.9,
31.2, 24.9.
4,4,5,5-tetramethyl-2-(3-(trifluoromethyl)phenyl)-1,3,2-dioxaborolane (3i).7a Rf=0.7 (petroleum
ether/ethyl acetate=20/1). Eluent: petroleum ether/ethyl acetate=200/1. Yield: 44% (60 mg). Pale
yellow oil. 1H NMR (400 MHz, CDCl3) δ 8.06 (s, 1H), 7.97 (d, J = 7.4 Hz, 1H), 7.70 (d, J = 7.8 Hz,
1H), 7.49 (t, J = 7.6 Hz, 1H), 1.36 (s, 12H). 13C{1H} NMR (101 MHz, CDCl3) δ 138.0 , 131.4 (q, J
= 3.7 Hz), 130.0 (q, J = 32.0 Hz), 128.0, 127.8 (q, J = 3.7 Hz), 124.3 (q, J = 272.4 Hz), 84.3, 24.9.
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4,4,5,5-tetramethyl-2-(p-tolyl)-1,3,2-dioxaborolane (3j).8b Rf=0.6 (petroleum ether/ethyl
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acetate=20/1). Eluent: petroleum ether /ethyl acetate=200/1. Yield: 32% (35 mg). Yellow oil. H
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NMR (400 MHz, CDCl3) δ 7.70 (d, J = 7.7 Hz, 2H), 7.19 (d, J = 7.6 Hz, 2H), 2.36 (s, 3H), 1.34 (s,
12H). 13C{1H} NMR (101 MHz, CDCl3) δ 141.4, 134.8, 128.5, 83.6, 24.9, 21.8.
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4,4,5,5-tetramethyl-2-(4-(trifluoromethyl)phenyl)-1,3,2-dioxaborolane (3k).8b Rf=0.7 (petroleum
ether/ethyl acetate=20/1). Eluent: petroleum ether/ethyl acetate=200/1. Yield: 44% (60 mg). Yellow
solid. Mp: 69-71 °C. 1H NMR (400 MHz, CDCl3) δ 7.91 (d, J = 7.8 Hz, 2H), 7.61 (d, J = 7.9 Hz,
2H), 1.36 (s, 12H).13C{1H} NMR (101 MHz, CDCl3) δ 134.0, 131.8 (q, J = 32.0 Hz), 123.3 (q, J =
3.8 Hz), 123.1 (q, J = 272.5 Hz), 83.2, 23.8.
4,4,5,5-tetramethyl-2-(4-(trifluoromethyl)phenyl)-1,3,2-dioxaborolane (3l).8b Rf=0.5 (petroleum
ether/ethyl acetate=20/1). Eluent: petroleum ether/ethyl acetate=200/1. Yield: 49% (50 mg). Yellow
oil. 1H NMR (400 MHz, CDCl3) δ 7.81 (d, J = 7.6 Hz, 2H), 7.46 (t, J = 7.4 Hz, 1H), 7.37 (t, J = 7.3
Hz, 2H), 1.35 (s, 12H). 13C{1H} NMR (101 MHz, CDCl3) δ 134.8, 131.3, 127.7, 83.8, 24.9.
4,4,5,5-tetramethyl-2-(4-(trifluoromethyl)phenyl)-1,3,2-dioxaborolane (3m).7c Rf=0.7 (ethyl
acetat). Eluent: petroleum ether/ethyl acetate=20/1). Yield: 60% (75 mg). White solid. Mp: 230-
232 °C. 1H NMR (400 MHz, CDCl3) δ 8.10 (d, J = 8.0 Hz, 2H), 7.91 (d, J = 8.0 Hz, 2H), 1.37 (s,
12H).13C{1H} NMR (101 MHz, CDCl3) δ 171.9, 134.8, 131.4, 129.2, 84.3, 24.9.
2-(4-fluorophenyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (3n).8b Rf=0.6 (petroleum ether/ethyl
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acetate=20/1). Eluent: petroleum ether/ethyl acetate=200/1. Yield: 65% (72 mg). Yellow oil. H
NMR (400 MHz, CDCl3) δ 7.79 (dd, J = 8.4, 6.3 Hz, 2H), 7.05 (t, J = 8.9 Hz, 2H), 1.34 (s, 12H).
13C{1H} NMR (101 MHz, CDCl3) δ 164.1 (d, J = 250.4 Hz), 136.0 (d, J = 8.2 Hz), 113.8 (d, J =
20.2 Hz), 82.9, 23.8.
2-(4-chlorophenyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (3o).8b Rf=0.6 (petroleum ether/ethyl
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acetate=20/1). Eluent: petroleum ether/ethyl acetate=200/1. Yield: 71% (85 mg). Yellow oil. H
NMR (400 MHz, CDCl3) δ 7.73 (d, J = 8.3 Hz, 2H), 7.34 (d, J = 8.3 Hz,2H), 1.34 (s, 12H). 13C{1H}
NMR (101 MHz, CDCl3) δ 137.5, 136.1, 128.0, 84.0, 24.9.
2-(4-bromophenyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (3p).7a Rf=0.6 (petroleum ether/ethyl
acetate=20/1). Eluent: petroleum ether/ethyl acetate=200/1. Yield: 69% (98 mg). White solid. Mp:
64-66 °C. 1H NMR (400 MHz, CDCl3) δ 7.66 (d, J = 8.2 Hz, 2H), 7.50 (d, J = 8.2 Hz, 2H), 1.34 (s,
12H). 13C{1H} NMR (101 MHz, CDCl3) δ 136.3, 131.0, 126.2, 84.1, 24.9.
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2-(4-bromophenyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (3q).8b Rf=0.5 (petroleum ether/ethyl
acetate=2/1). Eluent: petroleum ether/ethyl acetate=30/1. Yield: 40% (49 mg). White solid. Mp:
209-211 °C. 1H NMR (400 MHz, CDCl3) δ 8.58 (s, 1H), 8.21 (d, J = 7.8 Hz, 1H), 8.05 (d, J = 7.3
Hz, 1H), 7.49 (t, J = 7.6 Hz, 1H), 1.37 (s, 12H). 13C{1H} NMR (101 MHz, CDCl3) δ 172.1, 140.0,
136.6, 132.9, 128.8, 127.9, 84.2, 24.9.
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2-(4-bromophenyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (3r).7a Rf=0.7 (petroleum ether/ethyl
acetate=5/1). Eluent: petroleum ether/ethyl acetate=120/1. Yield: 56% (70 mg). Yellow solid. Mp:
40-41 °C. 1H NMR (400 MHz, CDCl3) δ 8.65 (d, J = 1.3 Hz, 1H), 8.30 (dd, J = 8.2, 1.2 Hz, 1H),
8.10 (d, J = 7.3 Hz, 1H), 7.54 (t, J = 7.8 Hz, 1H), 1.37 (s, 12H). 13C{1H} NMR (101 MHz, CDCl3)
δ 147.9, 140.7, 129.4, 128.8, 125.9, 84.6, 24.9.
2-(3-chlorophenyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (3s).7a Rf=0.8 (petroleum ether/ethyl
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acetate=20/1). Eluent: petroleum ether/ethyl acetate=200/1. Yield: 40% (48 mg). Yellow oil. H
NMR (400 MHz, CDCl3) δ 8.65 (d, J = 1.3 Hz, 1H), 8.30 (dd, J = 8.2, 1.2 Hz, 1H), 8.10 (d, J = 7.3
Hz, 1H), 7.54 (t, J = 7.8 Hz, 1H), 1.37 (s, 12H). 13C{1H} NMR (101 MHz, CDCl3) δ 134.6, 134.1,
132.7, 131.3, 129.2, 84.2, 24.9.
4,4,5,5-tetramethyl-2-(o-tolyl)-1,3,2-dioxaborolane (3t).8b Rf=0.7 (petroleum ether/ethyl
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acetate=20/1). Eluent: petroleum ether/ethyl acetate=200/1. Yield: 27% (3o mg). Yellow oil. H
NMR (400 MHz, CDCl3) δ 7.77 (d, J = 7.0 Hz, 1H), 7.40 – 7.29 (m, 1H), 7.20 – 7.14 (m, 2H), 2.55
(s, 3H), 1.36 (s, 12H). 13C{1H} NMR (101 MHz, CDCl3) δ 144.9, 135.9, 130.8, 129.8, 124.7, 83.4,
24.9, 22.3.
4,4,5,5-tetramethyl-2-(o-tolyl)-1,3,2-dioxaborolane (3u).11 Rf=0.7 (petroleum ether/ethyl
1
acetate=20/1). Eluent: petroleum ether/ethyl acetate=200/1. Yield: 27% (30 mg). Yellow oil. H
NMR (400 MHz, CDCl3) δ 7.68 (d, J = 7.1 Hz, 1H), 7.38 – 7.30 (m, 2H), 7.25 – 7.20 (m, 1H), 1.37
(s, 12H). 13C{1H} NMR (101 MHz, CDCl3) δ 139.6, 136.4, 131.9, 129.4, 125.8, 84.2, 24.8.
Potassium (4-methoxyphenyl)trifluoroborate (6a).8b Yield: 37% (80 mg). White solid. Mp: 240-
242 °C. 1H NMR (400 MHz, DMSO-d6) δ 7.21 (d, J = 8.1 Hz, 2H), 6.65 (d, J = 8.0 Hz, 2H), 3.66
(s, 3H). 13C{1H} NMR (101 MHz, DMSO-d6) δ 157.6, 132.7, 112.3, 55.0.
Potassium (4-biphenyl)trifluoroborate (6b).17 Yield: 31% (80 mg). White solid. Mp: > 250 °C. 1H
NMR (400 MHz, DMSO-d6) δ 7.60 (d, J = 7.4 Hz, 2H), 7.48 – 7.35 (m, 6H), 7.29 (t, J = 7.3 Hz,
1H). 13C{1H} NMR (101 MHz, DMSO-d6) δ 141.9, 137.3, 132.4, 129.2, 127.0, 126.8, 125.1.
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Potassium (4-fluorophenyl)trifluoroborate (6c).8b Yield: 20% (40 mg). White solid. Mp: > 250 °C.
1H NMR (400 MHz, DMSO-d6) δ 7.31 (t, J = 7.4 Hz, 2H), 6.87 (t, J = 9.0 Hz, 2H). 13C{1H} NMR
(101 MHz, DMSO-d6) δ 161.4 (d, J = 238.4 Hz), 133.3 (d, J = 6.6 Hz), 113.1 (d, J = 18.4 Hz).
Potassium (4-fluorophenyl)trifluoroborate (6d).8b Yield: 31% (80 mg). White solid. Mp: 184-
186 °C. 1H NMR (400 MHz, DMSO-d6) δ 7.72 (d, J = 7.7 Hz, 2H), 7.45 (d, J = 7.9 Hz, 2H), 4.27
(q, J = 7.1 Hz, 2H), 1.31 (t, J = 7.1 Hz, 3H). 13C{1H} NMR (101 MHz, DMSO-d6) δ 167.1, 131.9,
127.6, 127.1, 60.5, 14.7.
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Radical capturing experiments
(1) B2pin2 (1 mmol), 4-nitrophenylazo mesylate (0.5 mmol) and 2,2,6,6-tetramethylpiperidine-1-
oxy radical (TEMPO) (1 mmol) were added to a Schlenk tube which was purged thoroughly with
N2. MeCN (4.5 mL) and water (0.5 ml) ware added via syringe and and the reaction mixture was
stirred at room temperature with the irradiation of a 30 W blue LED for about 17 h. The solution
was then concentrated under reduced pressure, diluted with EtOAc and washed with water. The
aqueous layer was washed with EtOAc, the combined organic layers were dried over MgSO4, and
concentrated under reduced pressure. The crude product was purified by column chromatography,
yielding compound 7 (40 mg, 29%), compound 3e (20 mg 16%).
(2) 4-nitrophenylazo mesylate (0.5 mmol) and 2,2,6,6-tetramethylpiperidine-1-oxy radical
(TEMPO) (1 mmol) were added to a Schlenk tube which was purged thoroughly with N2. MeCN
(4.5 mL) and water (0.5 ml) ware added via syringe and and the reaction mixture was stirred at room
temperature with the irradiation of a 30 W blue LED for about 17 h. The solution was then
concentrated under reduced pressure, diluted with EtOAc and washed with water. The aqueous layer
was washed with EtOAc, the combined organic layers were dried over MgSO4, and concentrated
under reduced pressure. The crude product was purified by column chromatography, yielding
compound 7 (70 mg, 50%).
2,2,6,6-tetramethyl-1-(4-nitrophenoxy)piperidine (7).18 White solid. Mp: 84-86 °C. 1H NMR (400
MHz, CDCl3) δ 8.14 (d, J = 9.4 Hz, 2H), 7.26 (br, 2H), 1.66 – 1.58 (m, 5H), 1.49 – 1.40 (m, 1H),
1.25 (s, 6H), 0.99 (s, 6H).13C{1H} NMR (101 MHz, CDCl3) δ 168.7, 141.1, 125.6, 114.2, 60.9, 39.7,
32.3, 20.5, 16.9.
Supporting Information
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The Supporting Information is available free of charge on the ACS Publications website.
Photograph of the reaction setup, study of key reaction parameters and NMR spectra (PDF)
Notes
9
The authors declare no competing financial interest.
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ACKNOWLEDGMENT
We are very grateful to Professor Xiaoxun Li at School of Pharmacy, Shandong University, for
providing valuable suggestions. This work was supported by National Natural Science Foundation
of China (Grant No. 21672127), Key Research and Development Project of Shandong Province
(Grant No. 2017CXGC1401) and the Fundamental Research Funds of Shandong University (Grant
No. 2016JC018).
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