Tetrahedron Letters
A novel route to organonitrites by Pd-catalyzed cross-coupling of sodium nitrite
and potassium organotrifluoroborates
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Mohammad Al-Masum , Nabil Saleh, Tasfia Islam
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:
Microwave irradiated palladium-catalyzed cross-coupling reaction of potassium styryltrifluoroborates
and sodium nitrite gives the corresponding styryl nitrites in high yields. Potassium aryltrifluoroborates
also furnish aryl nitrites under same reaction condition. This unprecedented cross-coupling is an interest-
ing development and has the potential to lead to new nitration protocols.
Received 8 December 2012
Revised 15 December 2012
Accepted 17 December 2012
Available online 22 December 2012
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Sodium nitrite in cross-coupling
Organotrifluoroborates
Organonitrites
Microwave
Potassium organotrifluoroborates are air- and water-stable un-
ique solid compounds. Their use in organic transformations is
increasing dramatically.1 Organonitro compounds, especially, aro-
matic nitro compounds are important moieties in various pharma-
ceuticals, dyes, and materials. Instead of using strong acid
conditions (HNO3/H2SO4) for nitration which is often poor regiose-
lective, our efforts in developing organonitrite by cross-coupling
reaction are exciting and creative. Buchwald’s recent report on Pd-
catalyzed conversion of aryl chlorides to nitroaromatics2a and Sai-
to’s report on Cu-catalyzed coupling of aryl iodides to nitroaromat-
ics2b are pioneering works of cross-coupling chemistry involving
potassium and sodium nitrite as coupling partners. Our recent suc-
cess in constructing direct aroylation reaction by the Pd-catalyzed
cross-coupling reactions of potassium organotrifluoroborates with
aroyl halides has motivated us to involve in another new form of
cross-coupling reaction of metal nitrite with potassium organotri-
fluoroborates.3 In this work, we report microwave irradiated
palladium catalyzed new reaction for the cross-coupling of organo-
trifluoroborates with sodium nitrite.
This is an interesting development that has potential to lead a
new nitration protocol inspiring to the conventional chemistry. Re-
cently, we have applied several Pd-complexes as catalyst for cross-
coupling chemistry involving potassium organotrifluoroborates.
One of them is PdCl2(dtbpf)2 complex and is very effective. To ex-
tend further application of this Pd-complex, we scanned it along
with many bases and solvents for the cross-coupling of styryltriflu-
oroborates and sodium nitrite. Key activities compared the effects
of bases and solvents on the formation of aromatic nitro com-
pounds to optimize the reaction condition. The short lists are
shown in Table 1. The reaction mixture of styryltrifluoroborate, so-
dium nitrite, NaH2PO4ÁH2O, and Pd-complex in toluene is largely in
an insoluble state. Since the reaction product was detected rela-
tively clear in toluene, we focused in increasing the solubility of
the heterogeneous mixture by adding more toluene and found bet-
ter results. The optimized condition is shown in Scheme 1. A good
number of styryltrifluoroborates were synthesized in the labora-
tory and used with sodium nitrite for cross-coupling reaction and
listed in Figure 1. Styrylnitrite 3a from the cross-coupling of
potassium styryltrifluoroborate 1a and sodium nitrite 2 is shown
as a representative procedure. 0.25 mmol of styryltrifluoroborate
1a, 69.0 mg (1.0 mmol) of NaNO2 2, and 138.0 mg (1.0 mmol) of
NaH2PO4ÁH2O were taken into a microwave reaction vial followed
by placing the septum cap. The argon was flushed through the
reaction vial by piercing a needle into the septum. The cap was un-
bolted and 3.0 mg (0.0075 mmol) of PdCl2(dtbpf)2 catalyst was
transferred into the reaction vial very quickly and the cap was
placed back. The ready to use reaction vial was flushed with argon
again for a few seconds. While the toluene bottle was still under
argon pressure 5.0 mL of the toluene was added into the reaction
mixture via oven dry syringe. The reaction vial was positioned in
a microwave device (maximum power setting 250 W) and heated
at 120 °C for 30 min. After the reaction is complete, the brown or-
ganic layer was pipetted. The remaining solid in the reaction vial
was washed with ethyl acetate three times, pipetted, and collected
with brown organic layer in a round bottom flask. A small amount
of silica gel was added to the round bottom flask. Then the
solvent was evaporated. The silica adsorbed reaction product was
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Corresponding author.
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.