Visible-light-mediated Gomberg-Bachmann reaction: An efficient photocatalytic approach to 2-aminobiphenyls

Article abstract of DOI:10.1016/j.tetlet.2019.02.022

An efficient, metal-free and highly selective protocol for the synthesis of 2-aminobiphenyls via visible-light-mediated arylation of anilines using aryl diazonium salts (Gomberg-Bachmann reaction) has been reported. The reaction is an example of photoredo

Full text of DOI:10.1016/j.tetlet.2019.02.022

Accepted Manuscript
Visible-light-mediated Gomberg-Bachmann reaction: an efficient photocata-
lytic approach to 2-aminobiphenyls
Ritu Kapoor, Ruchi Chawla, Lal Dhar S. Yadav
PII:
S0040-4039(19)30143-1
https://doi.org/10.1016/j.tetlet.2019.02.022
TETL 50616
DOI:
Reference:
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
14 December 2018
6 February 2019
9 February 2019
Please cite this article as: Kapoor, R., Chawla, R., Yadav, L.D.S., Visible-light-mediated Gomberg-Bachmann
reaction: an efficient photocatalytic approach to 2-aminobiphenyls, Tetrahedron Letters (2019), doi: https://doi.org/
1
0.1016/j.tetlet.2019.02.022
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Graphical Abstract
Visible-light-mediated Gomberg-Bachmann reaction: Leave this area blank for abstract info.
an efficient photocatalytic approach to
2
-aminobiphenyls
Ritu Kapoor, Ruchi Chawla, Lal Dhar S. Yadav
R
R
R
R
N
N
eosin Y (1 mol %)
ArN BF
Ar
2
4
MeCN/H O (10: 1)
2
green LEDs, rt
R'
R'
2
0 examples
yield up to 96%

1
Tetrahedron Letters
journal homepage: www.elsevier.com
Visible-light-mediated Gomberg-Bachmann reaction: an efficient photocatalytic
approach to 2-aminobiphenyls
Ritu Kapoor, Ruchi Chawla, Lal Dhar S. Yadav*
Green Synthesis Lab, Department of Chemistry, University of Allahabad, Allahabad 211 002, India
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
An efficient, metal-free and highly selective protocol for the synthesis of 2-aminobiphenyls via
visible-light-mediated arylation of anilines using aryl diazonium salts (Gomberg-Bachmann
reaction) has been reported. The reaction is an example of photoredox catalysed direct C(sp )-H
arylation of anilines using eosin Y as an organophotoredox catalyst. The presented visible-light-
mediated environmentally sustainable approach has the potential to earn a decent position
among the classical and contemporary methods for the synthesis of aminobiaryls.
2
Available online
2
009 Elsevier Ltd. All rights reserved.
Keywords:
Anilines
Aryl diazonium salts
Visible light
Aminobiphenyls
Aryl-aryl radical coupling reactions seem to offer a very
using the same salts via visible-light photoredox catalysis.
However, to the best of our knowledge, there is no report in the
literature on the visible-light-mediated arylation of anilines using
arenediazonium salts as aryl radical precursors (visible-light-
mediated Gomberg-Bachmann reaction). Notably, Heinrich et al.
have also previously reported the radical Gomberg-Bachmann
reaction using anilium hydrochlorides, diazonium salts and
convenient route for the synthesis of biaryls but are known to be
associated with a number of challenges. The unavoidable side
reactions accompanying the desired intermolecular radical aryl-
1
aryl couplings are, in fact, the major challenge. Circumvention
of this problem is generally done by using substrate as the
2
solvent or by carrying out the intramolecular version of the
3
specific reaction. An impeccable solution to this issue can be the
3
substoichiometric amount of TiCl in acidic aqueous conditions
use of visible-light for such reactions keeping in mind the unique
selectivity displayed by visible-light-mediated radical reactions.4
These reactions also offer additional advantages of pronounced
sustainability in terms of cost and environmental concerns.
However, only a few visible-light-mediated radical aryl-aryl
where large excess (20 equiv) of anilium hydrochlorides was
used to avoid homocoupling. An excess use of the substrate
11a
and reductant makes the method less economical and it also
suffers from limited substrate scope and selectivity issues
(formation of meta substituted product along with ortho
substituted product). However, the same group has also
previously reported a number of other radical arylations of
anilines using arylating agents other than diazonium salts which
5
couplings have been reported till date and the scope of new
developments in this area is enormous.
The aromatic nucleus of anilines is ubiquitous in an
array of natural products and pharmaceuticals. It also finds
important applications in numerous other industries, viz. dye,
1
1b-d
address the regioselectivity issue.
6
NR2
R2N
rubber, explosives etc. Typically, arylation of anilines can be
DIPEA
7
8
9
a)
efficiently accomplished by the Suzuki, Stille, or Ullmann
Het
X
Het
CH3CN, N2, 25 o
455 nm
C
EWG
EWG
R
cross-coupling reactions which require transition metals and
prefunctionalization of the aniline. The direct C-H arylation of
anilines via visible-light-mediation can prove to be a state-of-the-
art alternative to the aforementioned typical reactions.
X = Br, Cl
R
R1
R
R1
Ru(bpy) Cl .6H O (1 mol %), [Ni]
3
2
2
R
b)
c)
N
N
HO
Ar
TsCl, visible light, rt
Ar
Aryl diazonium salts are well known aryl radical
Present work
R
R
precursors which can be easily photoreduced to give aryl
R
R
N
N
eosin Y (1 mol %)
1
0
radicals. In 2011, the research group of Sanford has reported
ligand directed C−H arylation reactions using aryl diazonium
salts using a combination of visible-light photoredox and
ArN BF4
Ar
2
MeCN/H2O (10: 1)
green LEDs, rt
1
2
3
R'
R'
palladium catalysis.5a Also, König et al. and Heinrich et al.
5b
5c
Scheme 1. Visible-light-mediated arylation of anilines.
have independently reported arylation of arenes/heteroarenes
*Corresponding author. Tel.: +91 532 2500652; fax: +91 532 2460533; E-mail: ldsyadav@hotmail.com (L.D.S. Yadav).

2
Recently, König and co-workers have described the visible-light-
in the presence of eosin Y as an organophotoredox catalyst
under visible-light irradiation (Scheme 1c).
2
mediated radical C(sp )-H arylation of anilines with acceptor-
substituted (hetero)aryl halides (Scheme 1a).5d Yu et al. have
reported the arylation of aniline with phenols via visible-light
photoredox/nickel dual catalysis but under their reaction
To test the feasibility of our proposed strategy, initial
trials were carried out using N,N-dimethyl-p-toluidine 1a and
phenyl diazonium tetrafluoroborate 2a as model substrate/reagent
and eosin Y as an organophotoredox catalyst (1 mol %) in
acetonitrile under irradiation with green light emitting diodes
(LEDs; 2.50 W, λ = 535 nm). Pleasingly, the desired product 3a
was formed in 66% yield in 12 hours (Table 1, entry 1). Before
3
conditions, arylation of the C(sp )-H bonds of anilines has taken
place (Scheme 1b).12 All the above factors and our previous work
on visible-light photoredox catalysis,13 prompted us to develop a
more efficient photocatalytic protocol for the Gomberg-
Bachmann reaction. We report herein the results obtained when
anilines 1 were treated with arenediazonium tetrafluoroborates 2
proceeding
further,
we
conducted
some
control
Table 1. Optimization of experimental conditionsa
N
N BF
2
N
Eosin Y
4
COOH
Br
Reaction
Br
Conditions
HO
O
O
1a
2a
3a
Br
Br
Entry
Photocatalyst (mol %)
Eosin Y (1 mol %)
-
Solvent
Time (h)
Yield (%)b
66
traces
traces
60
65
78
70
87
79
81
64
88
87
87
58
87
1
2
3
MeCN
MeCN
MeCN
H O
2
DMF
DMSO
MeCN/H
MeCN/H
DMF/H
DMSO/H
MeCN/H
MeCN/H
MeCN/H
MeCN/H
MeCN/H
MeCN/H
MeCN/H
12
18
18
18
18
18
12
12
12
12
12
12
12
14
12
12
12
c
Eosin Y (1 mol %)
Eosin Y (1 mol %)
Eosin Y (1 mol %)
Eosin Y (1 mol %)
Eosin Y (1 mol %)
Eosin Y (1 mol %)
Eosin Y (1 mol %)
Eosin Y (1 mol %)
Rose Bengal (1 mol %)
4
5
6
7
8
9
2
2
O (5 : 1)
O (10 : 1)
2
O (10 : 1)
O (10 : 1)
1
1
1
1
1
1
1
0
1
2
3
4
5
6
7
2
2
O (10 : 1)
O (10 : 1)
O (10 : 1)
O (10 : 1)
O (10 : 1)
O (10 : 1)
O (10 : 1)
d
Ru(bpy)
3
Cl
2
(1 mol %)
2
2
2
2
2
2
e
Eosin Y (1 mol %)
Eosin Y (1 mol %)
Eosin Y (0.5 mol %)
Eosin Y (2 mol %)
Eosin Y (1 mol %)
f
1
a
78
Reaction conditions: 1a (1 mmol), catalyst (1 mol %), 2a (3 equiv.), solvent 5 mL, green LEDs, 12-18 h, under a nitrogen atmosphere.
b
c
d
e
f
Isolated yield.
No light.
Irradiation with blue LEDs.
5
equiv. 2a.
Solvent 7 mL.
experiments to test the necessity of photocatalyst and visible-
light in our reaction. The absence of either of the two reaction
parameters led to the formation of product in traces (entries 2
and 3). Thus, we were encouraged to employ a wide set of
reaction conditions to decide the best solvent system and
catalyst for our proposed scheme. The results of some selected
optimization conditions have been reported in Table 1. As
regards the solvent, the reaction was tried in MeCN, water,
DMF and DMSO (entries 1, 4-6). In fact, the solvent
combination of acetonitrile and water in 10:1 ratio gave the best
yield of product under the initial reaction conditions (entries 4-
reaction time from 12 to 14 h, there was no increase in the yield
of the product (entry 14). The catalyst loading of 1 mol % was
found to be the optimum loading for the reaction (entry 8 versus
15 and 16). It was also observed that on diluting the reaction
mixture, the yield of the product considerably decreased (entry
17).
Once the optimum reaction conditions were decided,
we moved on to investigate the scope of the reaction with
respect to the aniline and arenediazonium salt (Table 2). Both
alkyl and methoxy substituents on the aromatic ring were well
tolerated in the case of anilines, affording the corresponding
biaryls 3a-c in high yields. Also, Cl, F and CN groups were
compatible with the established photoredox conditions (3d-g).
When no substituent at position 4 of the aromatic ring of N,N-
dimethylaniline was present, both o- and p-substituted products
were obtained in the ratio 2:1, respectively (3h). Not only N, N-
dimethylaniline but anilines with different substituents at the
nitrogen atom also performed well in the reaction (3i, j). It was
observed that arylation occurs predictably at the ortho- and
para-positions with respect to the amino substituent.
Specifically, N,N-dialkylated anilines have been used as
substrates by us because mono-alkylated and non-alkylated
anilines quickly form triazenes under the present reaction
1
0). In view of our prime concern to develop a metal-free
protocol, eosin Y (entries 8, 11 and 12) was identified as the
best catalyst under irradiation with visible-light (green LEDs).
The role of eosin Y as an organophotoredox catalyst is well-
documented in the literature14 and previously our research
group has also reported a number of synthetic transformations
using this organic dye as a photoredox catalyst.13a-g
The diazonium salt was used in excess (3 equiv) with
respect to aniline because dimerisation of a small amount of
aryl radicals was also observed as a side reaction. Furthermore,
the yield of biaryl did not increase on using a modest excess of
the diazonium salt (entry 8 versus 13). On increasing the

3
conditions. As far as arenediazonium salts are concerned, a
broad range of substituents were found to be tolerated. Both
electron-withdrawing and electron-donating substituents on the
aromatic ring of diazonium salts provided the corresponding
biaryls in good to moderate yields (3k-s). The diazonium salts
bearing strongly electron-withdrawing substituents such as p-
(3r and 3s). However, anilines 1 bearing an electron-donating
group on the aromatic ring appear to react faster and afford
slightly higher yields in comparison to those having an electron-
withdrawing group (Table 2, products 3a-c versus 3d-g). In the
case of diazonium salts 2, electron- withdrawing group on the
aromatic ring gave better yield of the products in shorter
NO
2
or p-CN were also compatible with our reaction conditions
Table 2. Scope of aniline and arenediazonium salta
R
R
R
R
N BF
2
4
N
N
eosin Y (1 mol %)
R''
MeCN/H O (10: 1 )
2
R''
green LEDs, rt
R'
R'
1
2
3
b,c
(
28-96%)
N
N
N
N
4
2
2'
OCH3
3c, 12 h, 89%
Cl
3b, 12 h, 79%, 4:1:1.5d
3a, 12 h, 87%
3d, 18 h, 44%
N
N
N
2
N
2
'
Cl
4
F
CN
3g, 18 h, 28%
3h,12 h, 71%, 2:1e
3
e, 18 h, 36%, 4:1:1.5d
3f, 18 h, 39%
Et
Et
N
Bn
Bn
N
N
N
3j, 12 h,38%, 2:1e
3l, 18 h, 70%
3
i, 12 h, 54%, 1:1e
3k, 18 h, 72%
OMe
Cl
F
t-Bu
N
N
N
N
3
m, 18 h, 74%
3n, 18 h, 69%
3o, 12 h, 92%
CN
3p, 12 h, 95%
CF3
NO2
N
N
N
N
3q,12 h, 96%
3r,12 h, 68%
3s,12 h, 65%
3t, 12 h, 76%
a
b
c
For experimental procedure, see Ref. 17.
Isolated yield of the purified products 3.
All compounds gave satisfactory spectral ( H NMR, 13C NMR and HRMS) data.
Ratio of regiomers arylated at labeled carbon 2, 2’ and 4, respectively.
Ratio of ortho- and para-phenyl regiomers, respectively.
1
d
e

4
reaction time in comparison to those having an electron-
donating group (Table 2, products 3k-n versus 3o-s). The high
yields for products 3a, 3b, 3o, 3p and 3q are remarkable
because compounds bearing a benzylic position are usually
prone to hydrogen abstraction by the aryl radical as reported in
many Gomberg-Bachmann reactions.15 However, the high
yields in our case can be attributed to the simultaneous
been proposed in Scheme 2. A SET from the excited state of
eosin Y to the arenediazonium salt 2 leads to the formation of
aryl radical I. This aryl radical I is trapped by the aryl ring of
aniline 1 to form radical intermediate II. The oxidation of
radical II by the eosin Y radical cation generates the
carbocation intermediate III and completes the visible-light-
mediated catalytic cycle. Finally, rearomatization of
carbocation III by proton loss provides the 2-aminobiphenyl
product 3.
In summary, we have developed a metal-free visible-
light-mediated radical approach for the introduction of aryl
group into the aniline ring. It offers an efficient and highly
selective method for the synthesis of 2-aminobiphenyls from
anilines and aryl diazonium salts employing eosin Y as an
activation of the aryl ring of aniline via a SET from R
2
N- group
under visible light photoredox catalysis which leads to arylation
1
6
of the aromatic ring exclusively. Naphthyldiazonium salt also
gave a decent yield of the product (3t) under our reaction
conditions.
No biaryl formation could be detected when two
equivalents of TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxyl)
were added to the reaction mixture under standard conditions
suggesting a radical pathway. The formation of phenyl-TEMPO
organophotoredox
Environmental
catalyst
sustainability,
under
cost-effectiveness,
mild
conditions.
broad
adduct was confirmed by its MS (HRMS (ESI): calcd for
substrate scope and simple methodology are some of the
adorning features of the present protocol which makes its
addition to the arsenal of synthetic strategies creditable.
+
C
15
H
23NO [M+H] 234.1858, found 234.1855). Based on this
5
b,10
result and previous literature,
a plausible mechanism has
N BF
2
4
+
N + BF4
2
R
R
I
2
N
eosin Y*
eosin Y
1
N
green
light
R
eosin Y
II
N
N
H
R
R
-HBF4
3
BF4
III
Scheme 2. Plausible mechanism for the formation of 2-aminobiphenyls.
Acknowledgements
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1
5312;
We sincerely thank SAIF, Punjab University, Chandigarh, for
providing microanalyses and spectra. R.K. is thankful to the
DST-SERB, New Delhi for the award of Start-Up Grant project
(d) Marzo L, Wang S, König B. Org Lett. 2017; 19: 5976.
Manufacture and Uses of the Anilines: A Vast Array of Processes and
6
7
.
Products. In Patai’s Chemistry of Functional Groups, ed. Rappoport Z,
John Wiley & Sons, Chichester, UK, 2009.
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(
2
(
References and notes
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(
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3
4
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49: 1566.
(
(
(
(
b) Narayanam JM, Stephenson CR. Chem Soc Rev. 2011; 40: 102;
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5
. (a) Kalyani D, McMurtrey KB, Neufeldt SR, Sanford MS. J Am Chem Soc.
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2
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5
9
647.
16. Johnson TC, BL Elbert, Farley AJM, Gorman TW, Genicot C,
1
1
2. Gui Y-Y, Wang Z-X, Zhou W-J, Liao L-L, Song L, Yin Z-B, Li J, Yu
D-G. Asian J Org Chem. 2018; 7: 537.
3. (a) Tripathi S, Kapoor R, Yadav LDS. Adv Synth Catal. 2018; 360: 467;
Lallemand B, Pasau P, Flasz J, Castro JL, MacCoss M, Dixon DJ,
Paton RS, Schofield CJ, Smith MD, Willis MC Chem. Sci. 2018; 9:
6
29.
(
2
(
2
b) Singh M, Yadav AK, Yadav LDS, Singh RKP. Tetrahedron Lett.
018; 59: 450;
c) Yadav VK, Srivastava VP, Yadav LDS. Tetrahedron Lett.
016; 57: 2502;
1
7. General procedure for the synthesis of compounds 3: A solution of
aniline 1 (1 mmol), arenediazonium salt (3 equiv.) and eosin Y (1 mol
3 2
%) in CH CN/H O 10:1 (5 mL) was irradiated with visible light (green
light emitting diodes (LEDs), λmax = 535 nm, 2.5 W) under a nitrogen
atmosphere with stirring at rt for 12/18 h (Table 2). After completion of
the reaction (monitored by TLC), water (5 mL) was added and the
mixture was extracted with ethyl acetate (3 × 5 mL). The combined
(d) Yadav AK, Yadav LDS. Tetrahedron Lett. 2015; 56: 686;
(e) Keshari T, Yadav VK, Srivastava VP, Yadav LDS. Green Chem.
2
(
2
014; 16: 3986;
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014; 55: 1788;
2 4
organic phase was dried over anhydrous Na SO , filtered, and
concentrated under reduced pressure. The resulting crude product was
purified by silica gel chromatography using a mixture of hexane/ethyl
acetate (49:1) as eluent to afford an analytically pure sample of product
(g) Yadav AK, Yadav LDS. Tetrahedron Lett. 2014; 56: 6696.
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1
76;
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86;
j) Yadav AK, Yadav LDS. Green Chem. 2016; 18: 4240;
3
.
(
Characterization data of representative compounds 3:
6
1
8
Compound 3a: H NMR (400 MHz, CDCl
3
) δ: 7.58 (d, J = 7.6 Hz,
(
2
2
H), 7.34 (t, J = 7.4 Hz, 2H), 7.28-7.24 (m, 1H), 7.05 (d, J = 10.8 Hz,
H), 6.97 (d, J = 8.0 Hz, 1H), 2.50 (s, 6H), 2.31 (s, 3H). C NMR (100
(k) Yadav VK, Srivastava VP, Yadav LDS. Tetrahedron Lett. 2016;
1
3
5
7: 155;
MHz, CDCl
3
) δ: 148.8, 141.8, 134.3, 132.3, 130.8, 128.6, 128.4, 128.2,
(
(
6
(
l) Yadav AK, Yadav LDS. Green Chem. 2015; 17: 3515;
m) Singh AK, Chawla R, Yadav LDS. Tetrahedron Lett. 2015; 56:
53;
n) Kapoorr R, Singh SN, Tripathi S, Yadav LDS. Synlett.
015; 26: 2101;
+
1
2
26.4, 117.6, 43.5, 20.5. HRMS (ESI): calcd for C15
12.1434, found 212.1436. Compound 3g:
H17N [M+H]
1
8 1
H NMR (400 MHz,
3
CDCl ) δ: 7.52-7.38 (m, 6H), 7.33-7.30 (m, 1 H), 6.95 (d, J = 7.5 Hz,
1
3
1
1
H), 2.65 (s, 6H). C NMR (100 MHz, CDCl
3
) δ: 154.1, 140.3, 135.5,
2
32.2, 131.8, 128.6, 127.9, 127.2, 119.8, 116.7, 101.9, 42.5. HRMS
(o) Srivastava VP, Yadav AK, Yadav LDS. Synlett. 2014; 25: 665;
(p) Yadav AK, Yadav LDS. Tetrahedron Lett. 2014; 55: 2065;
(q) Yadav AK, Srivastava VP, Yadav LDS. RSC Adv. 2014; 4: 24498;
+
(ESI): calcd for
C
15
H
14
N
2
[M+H] 223.1230, found 223.1227.
Compound 3n: H NMR (400 MHz, CDCl ) δ: 7.54-7.50 (m, 2H), 7.07
d, J = 10.8 Hz, 2H), 7.03-6.98 (m, 2H), 6.95 (d, J = 10.8 Hz, 1H), 3.85
1
3
(
(
1
1
4. (a) Hari DP, König B. Chem Commun. 2014; 50: 6688;
b) Srivastavaa V, Singh PP. RSC Adv. 2017; 7: 31377.
1
3
s, 3H), 2.54 (s, 6H), 2.30 (s, 3H). C NMR (100 MHz, CDCl
3
) δ:
(
1
5
58.2, 141.8, 134.3, 133.8, 130.8, 129.6, 128.6, 126.4, 117.5, 113.6,
5.2, 43.30, 20.50. HRMS (ESI): calcd for C16H19NO [M] 241.1467,
5. Beadle JR, Korzeniowski SH, Rosenberg DE, Garcia-Slanga BJ, Gokel
GW. J Org Chem. 1984; 49: 1594.
+
found 241.1468.
8. Shi R, Niu H, Lu L, Lei A. Chem Commun. 2017; 53: 1908.
1
Highlights
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Metal-free one-pot approach to 2-
aminobiphenyls.
2
The direct C(sp )-H arylation of
anilines with diazonium salts.
Eosin Y as an organophotoredox
catalyst.
Visible-light-mediated Gomberg-
Bachmann reaction.

6
Tetrahedron
Graphical Abstract
Visible-light-mediated Gomberg-Bachmann reaction: Leave this area blank for abstract info.
an efficient photocatalytic approach to
2
-aminobiphenyls
Ritu Kapoor, Ruchi Chawla, Lal Dhar S. Yadav
R
R
R
R
N
N
eosin Y (1 mol %)
ArN BF
Ar
2
4
MeCN/H O (10: 1)
2
green LEDs, rt
R'
R'
2
0 examples
yield up to 96%