One-pot Suzuki coupling of aromatic amines via visible light photocatalyzed metal free borylation using t-BuONO at room temperature

Article abstract of DOI:10.1016/j.tetlet.2016.02.097

A convenient and efficient metal free borylation of aromatic amines has been achieved using tertiary butyl nitrite and B₂Pin₂ (bis(pinacolato)diborane) under irradiation with blue LED light at room temperature. This protocol has been successfully extended to subsequent Suzuki coupling in the same pot. Thus a series of functionalized aryl boronates and biaryls are obtained in high yields in a shorter reaction period starting from relatively cheap aryl amines in one-pot avoiding isolation of potentially unstable and hazardous intermediates.

Full text of DOI:10.1016/j.tetlet.2016.02.097

Accepted Manuscript
One-pot Suzuki coupling of aromatic amines via visible light photocatalyzed
metal free borylation using t-BuONO at room temperature
Sabir Ahammed, Shiny Nandi, Debasish Kundu, Brindaban C. Ranu
PII:
S0040-4039(16)30206-4
DOI:
http://dx.doi.org/10.1016/j.tetlet.2016.02.097
TETL 47372
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
27 January 2016
24 February 2016
25 February 2016
Please cite this article as: Ahammed, S., Nandi, S., Kundu, D., Ranu, B.C., One-pot Suzuki coupling of aromatic
amines via visible light photocatalyzed metal free borylation using t-BuONO at room temperature, Tetrahedron
Letters (2016), doi: http://dx.doi.org/10.1016/j.tetlet.2016.02.097
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1
Tetrahedron Letters
One-pot Suzuki coupling of aromatic amines via visible light photocatalyzed
metal free borylation using t-BuONO at room temperature
Sabir Ahammed a, Shiny Nandia, Debasish Kundua and Brindaban C. Ranu a,*
a
Department of Organic Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India. Tel: (+91) 33 2473 4971, Fax: (+91)
33-24732805, E-mail: ocbcr@iacs.res.in
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
A convenient and efficient metal free borylation of aromatic amines has been achieved using
tertiary butyl nitrite and B2Pin2 (bis(pinacolato) diborane ) under irradiation with blue LED
light at room temperature. This protocol has been successfully extended to subsequent Suzuki
coupling in the same pot. Thus a series of functionalized aryl boronates and biaryls are obtained
in high yields in a shorter reaction period starting from relatively cheap aryl amines in one-pot
avoiding isolation of potentially unstable and hazardous intermediates.
Available online
Keywords:
Visible light
Borylation
Suzuki coupling
Amines
2009 Elsevier Ltd. All rights reserved.
Biaryls
Introduction
single electron transfer process is the key concept in this
protocol.13 Very recently, Yan and co-workers reported a metal
Boronates and boronic acids are of much importance in
organic synthesis, pharmaceutical research and material science.1
Suzuki-Miyaura reaction is a landmark in involving aryl
boronates and boronic acids in cross coupling reaction.2 Thus the
synthesis of functionalized aryl boron compounds has received
much attention in recent years. Traditionally, aryl boronic
compounds were prepared by the reaction of aryl Grignard or
aryl lithium reagents with trialkyl boronates.3 However, these
reactions are associated with the limitation of substrate scope and
tedious reaction procedure. Several transition metal catalyzed
borylation have been developed in last decades. In 1995, Miyaura
et al. reported palladium catalyzed borylation of aryl halide with
HBPin or B2Pin2.4 An alternative Cu catalyzed borylation of aryl
halide was also developed.5 Hartwig and co-workers reported
borylation via C-H activation by using iridium catalyst,6 whereas
Pucheault and co-workers demonstrated borylation of aryl
diazonium salt using iron and Gr-IV metallocene under mild
conditions.7 However, use of expensive ligand, catalyst and the
chance of metal contamination in the final product has prompted
the necessity of transition metal free less expensive protocols.
Wang and co-workers developed a milder metal free one pot
borylation with aryl amine and B2Pin2 using t-BuONO as a
diazotizing agent and benzoyl peroxide as an additive at room
temperature.8a They have also observed that heating may be used
to perform the reaction in absence of BPO.8b
free borylation using aryl diazonium tetrafluoroborates, prepared
from aryl amine separately, under visible light.14 Being
encouraged by our recent results on one-pot C-H heteroarylation
and heteroaryl selenylation from aromatic amine using t-BuONO
via an aryl/heteroaryl radical,15 we report here a direct one pot
borylation of aryl amines with B2Pin2 under visible light (blue
LED) using eosin Y as a photocatalyst at room temperature
(Scheme 1A) and subsequent same-pot Suzuki coupling leading
to the synthesis of biaryls from aryl amines directly (Scheme
1B).
Scheme 1A. Visible light photocatalyzed borylation of aryl amines.
Scheme 1B. One pot visible light photocatalyzed borylation and subsequent
Suzuki coupling.
In recent times, visible light photocatalysis has emerged as a
powerful green tool in organic transformations. Various C-C,9 C-
N,10 C-P,11 C-S12 bond formations have been accomplished by
this protocol. The generation of aryl radical from aryl diazonium
salt using a photocatalyst in presence of visible light through a
Anilines are attractive starting materials as many of them are
commercially available and cheap. They can be also prepared

2
Tetrahedron Letters
Table 2
easily from nitroarene precursors. Thus access to biaryls in one
Visible light photocatalyzed borylation of aryl aminea
pot from anilines under mild conditions bypassing the
intermediate steps is of much importance in contemporary
organic synthesis.
Results and discussion
To standardize the reaction conditions, several experiments were
performed for a representative reaction of an aromatic amine, 4-
methoxyaniline and bis(pinacolato) diborane (B2Pin2) with
variation of the amount of photocatalyst, solvent and time. The
results are shown in Table 1. It was found that the best yield was
obtained in acetonitrile at room temperature (Table 1, entry 11)
using 0.01 equiv. of eosin Y and tert-butyl nitrite (1.1 equiv.)
under blue LED light for 2 h. Although the yield of product using
DMSO is comparable, those in relatively nonpolar solvents such
as THF, dioxane, and toluene are not satisfactory. The reaction
did not progress in absence of a photocatalyst (Table 1, entry 13)
as well as in absence of blue-LED (Table 1, entry 12). Although
[Ru(bpy)3]Cl2 works efficiently for this reaction (Table 1, entry
9), eosin Y, being relatively cheap, is selected as the catalyst of
choice. The reactions with the other photocatalysts such as Rose
Bengal and fluorescein (Table 1, entries 9 and 10) were not
satisfactory. In absence of t-BuONO, no product was formed
(Table 1, entry 14).
Table 1
Optimization of reaction conditionsa
Entry
1
Photocatalyst
eosin Y
eosin Y
eosin Y
eosin Y
eosin Y
eosin Y
eosin Y
Ru(bpy)2Cl2
rose bengal
fluorescein
eosin Y
eosin Y
-
Solvent
toluene
THF
Time (h)
Yield (%)
aYields refer to those of purified products characterized by 1H and 13C
NMR spectroscopic data.
6
6
6
6
6
3
3
3
3
3
2
2
6
2
-
18
trace
64
81
85
86
82
trace
-
2
3
1,4-dioxane
DMF
Both electron donating (-OMe, -Me, -Et) and electron
withdrawing (-NO2, -CN, -CF3, -F, -COOH) groups attached with
aromatic ring are compatible with the reaction conditions.
However, the electron-withdrawing groups at the para and meta
positions exhibit good reactivity compared to the electron-
donating one.
4
5
DMSO
DMSO
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
6
7
8
The present photocatalyzed procedure offers distinctive
advantages over other procedures reported by Wang et al
earlier8a,8b in terms yield as illustrated by a comparison of results
in Table 3. In addition the reactions are performed at room
temperature in our procedure whereas these were carried out at
60 -80 oC in earlier ones.8a,8b
9
10
11
12b
13
14c
88
-
-
Table 3
eosin Y
-
Comparison of results of borylation under thermal and photocatalysis
(our procedure) conditions
aReaction conditions: amine (0.5 mmol), bis(pinacolato) diborane (0.5
mmol), t-BuONO (1.1 mmol, added drop wise for 2 min.), photocatalyst
(0.01 mmol), blue LED. bReaction was carried out in the absence of blue
LED. cIn absence of t-BuONO.
Substrate
Ref. 8a
(yield)
30%
Ref. 8b
(yield)
52%
Our work
(yield)
88%
Thus, in a typical experimental procedure, the tert-butyl nitrite
was added to a stirred mixture of aromatic amine and
bis(pinacolato) diborane in acetonitrile followed by a catalytic
amount of eosin Y at room temperature and the reaction mixture
was stirred under blue LED for 2 h (TLC). The reaction proceeds
efficiently with meta- and para-substituted aryl amines, while the
reactions with ortho substituted aryl amines furnished relatively
low yields of product (Table 2, 3c, 3e, 3g, 3h). This result might
be might be attributed to steric hindrance of the ortho substituent.
Aniline, p-toluidine and p-anisidine provided high yields without
any difficulty. The reaction showed excellent functional group
tolerance. The halogen groups (-Cl, -Br) remained intact after
borylation and they can be used for further functionalization.
54%
41%
54%
67%
83%
92%
A possible reaction pathway is proposed as outlined in Scheme 2.
The reaction is completely arrested in presence of 2,2,6,6-

3
tetyramethylpiperidine-1-oxyl TEMPO) and thus the reaction is
likely to proceed through a radical pathway.
same-pot Suzuki coupling leading to the synthesis of biaryls in
a shorter reaction period from inexpensive aryl amines without
isolation of potentially hazardous intermediates. Significantly,
the reaction tolerates a broad range of functionalities. We are not
aware of any procedure for direct borylation of anilines followed
by subsequent Suzuki coupling in one-pot at room temperature
under visible light. This procedure allows a straightforward
access to biaryls and we believe, this will be of much application
in organic synthesis.
Acknowledgments
We gratefully acknowledge the financial support from DST, New
Delhi, for the J.C. Bose Fellowship to B.C.R. (Grant No.
SR/S2/JCB-11/2008). S.A. and D.K. thank CSIR, New Delhi, for
their fellowships. S.N. thanks Indian Academy of Sciences for
offering her a summer project fellowship during 2014.
Supplementary Material
Scheme 2. Plausible mechanism.
1H NMR, 13C NMR data and H NMR, 13C NMR spectra of all
compounds listed in Table 2 and Table 3 can be found in the
online version, at http://
1
The aryl diazonium ion formed by the reaction of amine and t-
BuONO generates an Ar radical by a single electron transfer
from activated eosin Y under visible light.14 In another cycle the
tert-butoxide moiety from t-BuONO combines with B2Pin2 to
form the intermediate 2a which interacts with the aryl radical to
produce the aryl boronate ester 3 together with the species 2b
which transfers one electron to continue the cycle and oxidizes
itself to 2c.
References and notes
1.
(a) Lam, P. Y. S.; Clark, C. G.; Saubern, S.; Adams, J.; Winters, M.
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(a) Miyaura, N.; Yamada, K.; Suzuki, A. Tetrahedron Lett. 1979,
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Commun., 1979, 866; (c) Miyaura, N.; Suzuki, A. Chem.
Rev., 1995, 95, 2457.
To find a useful application of this borylation reaction we then
performed Suzuki coupling of the crude boronate with aryl iodide
in the same pot in presence of PdCl2 to produce the
corresponding biaryl. Several diversely substituted aryl amines
combine with the substituted aryl iodides to provide a series of
functionalized biaryls in high yields by this one-pot procedure
(Table 4).
2.
3.
4.
(a) Brown, H. C.; Srebnik, M.; Cole, T. E. Organometallics 1986,
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7508; (b) Ishiyama, T.; Miyaura, N. J. Synth. Org. Chem. Jpn.
1999, 57, 503; (c) Ishiyama, T.; Miyaura, N. J. Organomet. Chem.
In general, the reactions are clean and high yielding. The
compounds are obtained in high purity after purification by
simple standard column chromatography.19 All reactions are
performed at room temperature under mild conditions and thus it
has tolerance to a wide spectrum of functionalities.
2000,
611,
392;
(d)
Miyaura
N.
In
Catalytic
Heterofunctionalization (Eds.: A. Togni, H. Gr_tzmacher), Wiley-
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Table 4
One pot visible light photocatalyzed borylation of aryl amines and
subsequent Suzuki couplinga
7.
8.
(a) Marciasini, L. D.; Richy, N.; Vaultier, M.; Pucheaulta, M. Adv.
Synth. Catal. 2013, 355, 1083; (b) Marciasini, L. D.; Vaultier, M.;
Pucheault, M. Tetrahedron Lett. 2014, 55, 1702.
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Int. Ed. 2010, 49, 1846; (b) Qiu, D.; Jin, L.; Zheng, Z.; Meng, H.;
Mo, F.; Wang, X.; Zhang, Y.; Wang, J. J. Org. Chem. 2013, 78,
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422.
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(a) Nicewicz, D. A.; MacMillan, D. W. C. Science 2008, 322, 77;
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Du, J.; Espelt, L. R.; Guzei, I. A.; Yoon, T. P. Chem. Sci. 2011, 2,
2115; (d) Ischay, M. A.; Anzovino, M. E.; Du, J.; Yoon, T. P. J.
Am. Chem. Soc. 2008, 130, 12886; (e) Ischay, M. A.; Lu, Z.; Yoon,
T. P. J. Am. Chem. Soc. 2010, 132, 8572; (f) Pham, P. V.; Nagib, D.
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M. R.; Stephenson, C. R. J. Chem. Soc. Rev. 2011, 40, 102; (i) Lu,
aYields refer to those of purified products characterized by 1H and 13C
NMR spectroscopic data.
In conclusion, an efficient and general method for the direct
conversion of aniline to aryl boronates at room temperature under
visible light photocatalysis is developed. This procedure avoids
isolation and handling of hazardous diazonium salt and use of
any metal. This protocol is successfully extended to sequential

4
Tetrahedron Letters
Z.; Shen, M.; Yoon, T. P. J. Am. Chem. Soc. 2011, 133, 1162; (j)
anisidine (61 mg, 0.5 mmol) and eosin Y (0.01 mmol) in
acetonitrile (3 mL). The resulting mixture was stirred at room
temperature under irradiation with blue LED for 2 h (TLC). This
mixture after being diluted with ethyl acetate (5 mL) was filtered
through celite and the filtrate was extracted with ethyl acetate
(3X10 mL). The extract was washed with brine, dried over
anhydrous Na2SO4, and evaporated to leave the crude product
which was purified by column chromatography over silica gel
with hexane-ethyl acetate (98:2) as eluent to furnish pure 2-(4-
methoxyphenyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane as a light
yellow viscous liquid (3d, 208 mg, 88%); IR (neat) 2978, 2933,
Yoon, T. P.; Ischay, M. A.; Du, J. Nat. Chem. 2010, 2, 527; (k)
Rueping, M.; Leonori, D.; Poisson, T. Chem. Commun. 2011, 47,
9615; (l) Hari, D. P.; Koenig, B. Org. Lett. 2011, 13, 3852; (m)
Neumann, M.; Fueldner, S.; Koenig, B.; Zeitler, K. Angew. Chem.
Int. Ed. 2011, 50, 951; (n) Nguyen, J. D.; Tucker, J. W.;
Konieczynska, M. D.; Stephenson, C. R. J. J. Am. Chem. Soc. 2011,
133, 4160; (o) Zou, Y.-Q.; Lu, L.-Q.; Fu, L.; Chang, N.-J.; Rong, J.;
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Lacôte, E.; Malacria, M.; Ollivier, C. Angew. Chem. Int. Ed. 2011,
50, 4463; (q) McNally, A.; Prier, C. K.; MacMillan, D. W. C.
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2839, 2526, 2050, 1950, 1911, 1724, 1605, 1570 cm-1; H NMR
1
(500 MHz, CDCl3) δ 1.33 (s, 12H), 7.82 (s, 3H), 6.89 (d, J = 8.0
Hz, 2H), 7.75 (d, J = 8.0 Hz, 2H); 13C NMR (125 MHz, CDCl3) δ
24.9 (4C), 55.2, 83.6 (2C), 113.4 (2C), 136.6 (2C), 162.3. The
spectroscopic data is in full agreement with those reported for an
authentic sample.14 This procedure was followed for all the
reactions listed in Table 2. All of these products (3a,14 3b,14 3c,16a
10. Zhu, M.V.; Zheng, N. Synthesis 2011, 2223.
11. Rueping, M.; Zhu, S.; Koenigs, R. M. Chem. Commun. 2011, 47,
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52, 7860.
3d,14 3e,14 3f,8a 3g,14 3h,14 3i,14 3j,8a 3k,8a 3l,8a 3m,14 3n,8c 3o,16b
)
13. (a) Hari, D. P.; Schroll, P.; Konig, B. J. Am. Chem. Soc. 2012, 134,
2958; (b) Hari, D. P.; Hering, T.; König, B. Org. Lett. 2012, 14,
5334; (c) Hering, T.; Hari, D. P.; König, B. J. Org. Chem. 2012, 77,
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are known compounds, and their spectroscopic data are in
agreement with those previously reported.
Experimental Procedure for the Synthesis of 4-methoxy-2-methyl-
1,1’-biphenyl (4c): Tert-butyl nitrite (155 mg, 1.1 mmol) was
added drop wise to a mixture of bis(pinacolato) diborane (127 mg,
0.5 mmol), 4-anisidine (61 mg, 0.5 mmol) and eosin Y (0.01
mmol) in acetonitrile (3 mL). The resulting mixture was stirred at
room temperature under irradiation with blue LED for 2 h (TLC).
Acetonitrile was then evaporated and DMF-water (1:1, 4 mL) was
added followed by PdCl2 (3 mg, 5 mol %), K3PO4 (138 mg, 0.65
mmol) and 1-iodo-2-methylbenzene (131 mg, 0.6 mmol)
respectively. The reaction mixture was stirred for 4 h at room
temperature (TLC) and was extracted with ethyl acetate (3x10
mL). The extract was washed with water (5 mL) and brine (5 mL)
and dried over Na2SO4. The crude product was purified by column
chromatography over silica gel (hexane) to afford pure 4-
methoxy-2-methyl-1,1’-biphenyl as a yellow viscous liquid (172
mg, 87%). 1H NMR (500 MHz, CDCl3) δ 2.39 (s, 3H), 3.94 (s,
3H), 7.05-7.08 (m, 2H), 7.33-7.38 (m, 6H); 13C NMR (125 MHz,
CDCl3)
δ
20.6, 55.3,
113.6 (2C), 125.8, 127.1, 129.9,
130.3(2C),130.4, 134.5, 135.5, 141.7, 158.6. These data are in
perfect match with those reported for an authentic sample.17c This
procedure was followed for all the reactions listed in Table 3. All
of these products (4a,17a 4b,17b 4c,17c 4d,17d 4e,17a 4f,17e 4g,17f 4h,17g
)
19.
General Experimental Procedure for the Synthesis of Aryl and
Hetero-aryl boronates. Representative Procedure for the Synthesis
of 2-(4-Methoxyphenyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane:
Tert-butyl nitrite (155 mg, 1.1 mmol) was added drop wise to a
mixture of bis(pinacolato) diborane (127 mg, 0.5 mmol), 4-
are known compounds, and their spectroscopic data are in
agreement with those previously reported. Although these
experiments were performed with 0.5 mmol scale similar results
were obtained in higher (10-15 mmol) scale.
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One-pot Suzuki coupling of aromatic amines via visible light photocatalyzed
metal free borylation using t-BuONO at room temperature
Sabir Ahammed a, Shiny Nandi a, Debasish Kundua and Brindaban C. Ranu a,*

5
Highlights
 Metal free borylation of anilines is achieved at room temperature under visible light.
 One-pot Suzuki coupling of anilines to biaryls via borylation is demonstrated.

A series of functionalized biaryls are obtained in high yields starting from anilines.