Catalyst free, base free microwave irradiated synthesis of aryl nitrites from potassium aryltrifluoroborates and bismuth nitrate

Article abstract of DOI:10.1016/j.tetlet.2014.01.102

A mixture of bismuth nitrate pentahydrate and potassium aryltrifluoroborate in toluene under microwave heating at 120 C for 20 min provides an interesting and mild reaction protocol for the synthesis of aryl nitrite. The conversion to aryl nitrites from aryltrifluoroborates without transition metal catalyst and base in high yields is remarkable.

Full text of DOI:10.1016/j.tetlet.2014.01.102

Tetrahedron Letters 55 (2014) 1726–1728
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Catalyst free, base free microwave irradiated synthesis of aryl nitrites
from potassium aryltrifluoroborates and bismuth nitrate
⇑
Mohammad Al-Masum , Rebecca L. Welch
Department of Chemistry, Tennessee State University, 3500 John A Merritt Blvd, Nashville, TN 37209, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
A mixture of bismuth nitrate pentahydrate and potassium aryltrifluoroborate in toluene under micro-
wave heating at 120 °C for 20 min provides an interesting and mild reaction protocol for the synthesis
of aryl nitrite. The conversion to aryl nitrites from aryltrifluoroborates without transition metal catalyst
and base in high yields is remarkable.
Received 14 December 2013
Revised 20 January 2014
Accepted 22 January 2014
Available online 6 February 2014
Ó 2014 Elsevier Ltd. All rights reserved.
Keywords:
Bismuth nitrate as nitrite source
Catalyst free reaction
Potassium aryltrifluoroborates
Microwave
Nitration of aromatic compounds is an extensively studied field
of organic synthesis.1 Nitro group addition provides important
physical and chemical properties to target compounds. The metab-
olism of aromatic amines and aromatic nitro compounds is effec-
the bismuth nitrate, we swapped the hydrated base with an
anhydrous variant: NaH2PO4, and found an interesting change
(Scheme 2). 1:6 ratio of styryltrifluoroborate and bismuth nitrate
pentahydrate showed the predicted styryl nitrite without homo-
coupling product, but compared to the organonitrite reaction pro-
cess run with sodium nitrite5, this process is sluggish and
reproducibility is also low. Concurrently, when we applied bismuth
nitrate pentahydrate with potassium p-tolylltrifluoroborate, clean
transformation to p-tolylnitrite was observed (Scheme 3). Many
control experiments were run to optimize reaction conditions,
and the best conditions for complete conversion to p-tolylnitrite
were found to be one equivalent of p-tolyltrifluoroborate and one
equivalent of bismuth nitrate in toluene microwaved at 120 °C
for 20 min (Table 1). p-Tolylnitrite 3a from p-tolyltrifluoroborate
tively interconnected. So, it is important to see
a new
development in aryl nitrite synthesis. Compared to classical strong
conditions for the nitration process, a new milder technique of
nitration of aromatic compounds by cross-coupling of aryl halides
with sodium nitrite and potassium nitrite has been developed
recently.2 Potassium organotrifluoroborates have already proven
to be unique boron reagents for organic transformations. Molander
has pioneered several new developments using organotrifluorobo-
rate salts.3 We also use organotrifluoroborates in various organic
transformations.4 Very recently, we reported the palladium cata-
lyzed cross-coupling of potassium styryl- and aryltrifluoroborates
with sodium nitrite to give the corresponding styryl-, and arylni-
trites.5 In this work, we found bismuth nitrate is a good nitrite
source that formed aryl nitrites when microwaved with potassium
aryltrifluoroborates without adding transition metal catalyst and
base (Scheme 1).
1a and bismuth nitrate pentahydrate
2 is a representative
procedure (Table 1, entry 1). To an oven dried reaction tube with
magnetic stirrer bar were added p-tolyltrifluoroborate (49.4 mg,
0.25 mmol), bismuth nitrate (121.3 mg 0.25 mmol), and anhydrous
toluene (2.0 mL) under an inert atmosphere of Argon, followed by
insertion of the reaction tube into the microwave and heating at
120 °C for 20 min. The resulting reaction product was filtered
Our report of PdCl2(dtbpf)-catalyzed cross-coupling reaction of
styryl- and aryltrifluoroborates with sodium nitrite in the presence
of NaH2PO4ÁH2O encouraged us to seek the effect of bismuth
nitrate for a similar study. Under those conditions, however, reac-
tion failed. Bismuth nitrate arrives from the vendor in hydrated
form: Bi(NO3)3Á5H2O. To counterbalance the hydrated nature of
Bi(NO3)3.5H2O
Z
Z
NO2
High yield
BF3K
Toluene
MW, 120 °C, 20 min
⇑
Scheme 1.
Corresponding author. Tel.: +1 615 963 5339; fax: +1 615 963 5326.
E-mail address: malmasum@tnstate.edu (M. Al-Masum).
http://dx.doi.org/10.1016/j.tetlet.2014.01.102
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.

M. Al-Masum, R. L. Welch / Tetrahedron Letters 55 (2014) 1726–1728
1727
PdCl2(dtbpf)
Z
Z
+
Bi(NO3)3.5H2O
BF3K
Z = H, Me, Cl, F3C
NO2
NaH2PO4
mw, 120 °C, 30 min
Styryltrifluoroborate
Bi(NO3)3.5H2O
Styryl Nitrite
1
1
1
4
None
Nonea
Nitrite product
1
6
observed in GC-MS
aReported condition with base NaH2PO4 (Tetrahedr on Lett.
2013
, 54, 1141.)
Scheme 2. Styrylnitrites from cross-coupling of styryltrifluoroborates and Bi(NO3)3Á5H2O in toluene.
(Table 1, entry 2). No nitration product was observed when
4-pyridinyltrifluoroborate was employed as arylating agent (Ta-
ble 1, entry 9). Most likely, the basic nature of the pyridine moiety
is competing with the trifluoroborate moiety of aryltrifluoroborate.
A proposed mechanism that accounts for this nitration is portrayed
in Scheme 4.
The microwave heated minute reaction between potassium
aryltrifluoroborates and bismuth nitrate is a new process and a
clear improvement on existing nitration techniques. In summary,
we find a new application of potassium aryltrifluoroborates with
bismuth nitrate and present an efficient process for the synthesis
of aryl nitrites without catalyst and base. Application of this nitra-
tion process in natural product synthesis and in highly substituted
aryltrifluoroborates is currently under investigation.
H3C
H3C
+
Bi(NO3)3.5H2O
BF3K
NO2
1a
2
3a
Ratio
1a : 2
Toluenea
Run
Catalyst
Base
1
2
1 : 4
1 : 6
PdCl2(dtbpf)
PdCl2(dtbpf)
NaH2PO4
NaH2PO4
4 mL
4 mL
3
4
1 : 1
1 : 1
PdCl2(dtbpf)
PdCl2(dtbpf)
NaH2PO4
NaH2PO4
4 mL
2 mL
Acknowledgments
5
1 : 1
None
NaH2PO4
2 mL
Financial support from US department of education Title III
Grant, Tennessee State University is thankfully acknowledged. Re-
becca Welch also gratefully acknowledges the receipt of a MARC
6
1 : 1
None
None
2 mL
7
1 : 1
None
None
8 mLb
Table 1
Reaction of aryltrifluoroborates 1 and Bi(NO3)3Á5H2O 2 for arylnitrites 3a
a0.25 mmol scale. b1.0 mmol scale. Best condition was found in Run 6.
1
Me
NO2
99% yields
Me
BF3K
1a
Scheme 3.
3a
2
3
MeO
NO2
3b
49 %
80 %
MeO
BF3K
1b
through a sintered funnel with a Celite bed using 50 mL ethyl ace-
tate. The product solution was analyzed by GC–MS, evaporated,
and purified by preparative TLC. Mass spectra, 1H NMR, and 13C
NMR confirmed the product. LRMS: Calculated for C7H7NO2 M+
137. Found: 137. 1H NMR (Acetone-d6, 300 MHz) d 8.14–7.47 (m,
4H), 2.47 (s, 3H); 13C NMR (Acetone-d6, 75.5 MHz) d 147.2, 131.1,
130.5, 129.8, 124.2, 21.5. Reactions were carried out via standard
dry system under an inert atmosphere of Argon or in open air. Both
systems were equally effective but the Argon atmosphere condi-
tion provided cleaner looking result. All potassium aryltrifluorobo-
rates shown in Table 1 were prepared from the corresponding
arylboronic acids by applying the known method.6
The Prokash group used a mixture of silver nitrate salt and chlo-
romethylsilane as nitrating agent and produced nitroarenes when
treated with arylboronic acid at room temperature for 30–72 h in
poor to moderate yields (20–98%).7 In this Letter the nitration pro-
cess is mild and yields are high. Bismuth nitrate pentahydrate is a
successful Lewis acid and provides a nitro group when reacted
with potassium aryltrifluoroborate as its Lewis base. The high elec-
tron affinity of bismuth toward fluorine enhances the nitration
process.8 In case of 4-methoxyaryltrifluoroborate, yield was low
NO2
BF3K
1c
3c
MeS
NO2
4
5
99 %
90 %
MeS
BF3K
1d
3d
F3CO
NO2
F3CO
BF3K
3e
1e
F3C
NO2
6
98 %
F3C
BF3K
3f
1f
7
8
9
F
NO2
60 %
96 %
F
BF3K
1g
3g
Cl
NO2
Cl
BF3K
1h
3h
No reaction
N
BF3K
1i
a
0.25 mmol of arylfluoroborate, 0.25 mmol of Bi(NO3)3Á5H2O, and 2.0 mL of
toluene microwaved at 120 °C for 20 min. Preparative TLC technique was used for
purification.

1728
M. Al-Masum, R. L. Welch / Tetrahedron Letters 55 (2014) 1726–1728
Olah-Prakash
x(CH3)3SiCl
+
M(NO3)x
+
MClx
x(CH3)3Si O NO2
OH
Si(CH3)3
HO
HO
OH
B
NO2
B
O
Si(CH3)3
NO2
O
NO2
Lewis Acid
Our Approach
F
B
F
B
NO2
F
F
F
O
O
NO2
Bi
NO2
O
Me
Me
O
NO2
Lewis Acid
Lewis Base
NO2
Me
Arylnitrite
Scheme 4.
4. (a) Al-Masum, M.; Liu, K.-Y. Tetrahedron Lett. 2011, 52, 5090–5093; (b) Al-
Masum, M.; Ng, E.; Wai, M. C. Tetrahedron Lett. 2011, 52, 1008–1010; (c) Kabalka,
G. W.; Al-Masum, M. Org. Lett. 2006, 8, 11–15; (d) Al-Masum, M.; Alam, S.
Tetrahedron Lett. 2009, 50, 5201–5204; (e) Kabalka, G. W.; Dadush, E.; Al-
Masum, M. Tetrahedron Lett. 2006, 47, 7459–7461; (f) Kabalka, G. W.; Al-Masum,
M.; Mereddy, A. R.; Dadush, E. Tetrahedron Lett. 2006, 47, 1133–1136; (g)
Kabalka, G. W.; Al-Masum, M. Tetrahedron Lett. 2005, 46, 6329–6331.
5. Al-Masum, M.; Saleh, N.; Islam, T. Tetrahedron Lett. 2013, 54, 1141–1144.
6. Vedejs, E.; Fields, S. C.; Hayashi, R.; Hitchcock, S. R.; Powell, D. R.; Schrimpf, M. R.
J. Am. Chem. Soc. 1999, 121, 2460–2470.
7. (a) Prakash, G. K. S.; Panja, C.; Mathew, T.; Surampudi, V.; Petasis, N. A.; Olah, G.
A. Org. Lett. 2004, 6, 2205–2207; (b) Sun, H.-B.; Hua, R.; Yin, Y. J. Org. Chem. 2005,
70, 9071–9073.
8. Electron affinity of Bi, Si, Sn, and F are 91.2 KJ/mol, 133.6 KJ/mol, 107.3 KJ/mole,
and 329 KJ/mol, respectively; electronegativity Bi, Si, Sn, and F are 2.02, 1.9,
1.96, and 3.98 respectively; valence of Bi, Si, Sn, and F are 5, 4, 4, and 1,
respectively. Norman, N. C. Chemistry of Arsenic, Antimony and Bismuth. Springer
publisher, 1998.
fellowship from NIH’s MARC program for her undergraduate re-
search study (National Institutes of Health 2T34GM007663-32).
References and notes
1. (a) Schofield, K. Aromatic Nitration; Cambridge University Press: Cambridge,
1980; (b) Olah, G. S.; Malhotra, R.; Narang, S. C. Nitration: Methods and
Mechanism; VCH Publishers: New York, 1989; (c) Bak, R. R.; Smallridge, A. J.
Tetrahedron Lett. 2001, 42, 6767–6769; (d) Ridd, J. H. Acta Chem. Scand. 1998, 52,
11–22; (e) Mori, T.; Suzuki, H. Synlett 1995, 383–392; (f) Tani, K.; Lukin, K.;
Eaton, P. E. J. Am. Chem. Soc. 1997, 119, 1476–1479; (g) Eberson, L.; Radner, F.
Acc. Chem. Res. 1987, 20, 53–59; (h) Suzuki, H.; Murashima, T.; Shimizu, K.;
Tsukamoto, K. Chem. Lett. 1991, 817–818.
2. (a) Fors, B. P.; Buchwald, S. L. J. Am. Chem. Soc. 2009, 131, 12898–12899; (b)
Saito, S.; Koizumi, Y. Tetrahedron Lett. 2005, 46, 4715–4717.
3. (a) Molander, G. A.; Figueroa, R. Aldrichchimica Acta 2005, 38, 49–56; (b)
Molander, G. A.; Ellis, N. Acc. Chem. Res. 2007, 40, 275–286; (c) Darses, S.; Genet,
J.-P. Chem. Rev. 2008, 108, 288–325.