Poly(4-vinylpyridine)-nitrating mixture complex (PVP-NM): Solid nitrating mixture equivalent for safe and efficient aromatic nitration

Article abstract of DOI:10.1039/c5gc00458f

Friedel-Crafts type aromatic nitration has served as an indispensable reaction within both industrial and academic applications. However, growing concern over the use of copious amounts of strong acids has prompted the search for more environmentally friendly alternatives. Polymer-bound Bronsted acids, on the other hand, have been shown useful as convenient alternatives to liquid acids. Nitric acid and sulfuric acids have, therefore, been combined, both individually and as a mixture, with poly(4-vinylpyridine). The new solid acid systems have been used to nitrate both activated and deactivated arenes under mild conditions and proved to be effective nitrating agent.

Full text of DOI:10.1039/c5gc00458f

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Poly(4-vinylpyridine)-Nitrating Mixture Complex (PVP-NM): Solid
Nitrating Mixture Equivalent for Safe and Efficient Aromatic
Nitration
†
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
G. K. Surya Prakash,* Laxman Gurung, Kevin E. Glinton, Kavita Belligund, Thomas Mathew* and
George A. Olah
www.rsc.org/
Friedel-Crafts type aromatic nitration has served as an indispensable reaction within both industrial and academic
applications. However, growing concern over the use of copious amounts of strong acids has prompted the search for
more environmentally friendly alternatives. Polymer-bound Brønsted acids, on the other hand, have been shown useful as
convenient alternatives to liquid acids. Nitric acid and sulfuric acids have, therefore, been combined, both individually and
as a mixture, with poly(4-vinylpyridine). The new solid acid systems have been used to nitrate both activated and
deactivated arenes under mild conditions and proved to be effective nitrating agent.
Introduction
generation of nitronium cation. The classic two-acid system
1
, 2
Since the initial reports by Mitscherlich in 1834, the nitration called nitrating mixture has, for decades, been a mixture of
of organic compounds has maintained its position as one of nitric acid (nitrating agent) and concentrated sulfuric acid (acid
the most useful and widely studied reactions in organic catalyst), although several other acids have been used in the
chemistry. This addition/substitution of a nitro group at a place of sulfuric acid. These have included mixtures of nitric
carbon, oxygen or nitrogen center yields products of immense acid with aluminum chloride, polyphosphoric acid, perchloric
importance to almost all facets of the field. Industrially, the acid, methanesulfonic acid, hydrogen fluoride and even
reaction is used to produce nitrobenzene and aniline, superacids such as boron trifluoride, triflic acid, fluorosulfonic
1
, 3
explosives, precursors for dyes, pharmaceuticals, polymers and acid and many others.
many other applications. With such an array of uses, it is no
wonder that nitration continues to be the subject of countless
publications and studies and among various methods,
electrophilic aromatic nitration has been tremendously helpful
as well. For instance, both C. K. Ingold and Robert Robinson
used nitration to help elucidate mechanistic aspects of
3
-5
aromatic reactivity. Furthermore; the reaction has proven
useful in explaining substituent activating-deactivating effects,
directing effects and the flow of electrons in electrophilic
6
-8
substitution reaction.
The generally accepted Ingold-Hughes mechanism for
+
2
nitration involves the generation of nitronium (NO ) cation
Scheme 1 Hughes-Ingold mechanism of electrophilic aromatic
nitration
followed by its subsequent electrophilic substitution to an
9
aromatic center (Scheme 1). The mechanism has been
1
0
revisited using high level theoretical studies. The nitronium
cation is usually generated in situ by the use of neat nitric acid
or by adding a second, stronger acid to help accelerate the
However, though liquid based systems are highly effective
in nitrating aromatic substrates, when carried out on a large
scale, separation of the products is often tedious and usually
leads to large waste streams that can be both dangerous and
difficult to dispose of. Therefore, there has emerged a growing
need for the development of better and more ecologically
sustainable nitrating agents. A growing area of research has
involved the development of nitrogen-containing salts as a
cheaper, more accessible source of nitronium ions.
Loker Hydrocarbon Research Institute and Department of Chemistry,
University of Southern California, Los Angeles, CA, 90089-1661, USA.
Fax: +1 213-7405087; E-mail: gprakash@usc.edu; tmathew@usc.edu
† Electronic Supplementary Information (ESI) available: Experimental
details and characterization data. See DOI: 10.1039/x0xx00000x
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Nitrates of metals such as sodium, potassium and silver, for
Green Chemistry
instance, can be activated in the presence of an acid to yield
free nitronium ions or polarized complexes capable of
electrophilic aromatic nitration. Along with these, nitration
reactions have also been described using alkyl nitrates, nitryl
1
, 2
halides, silyl nitrates and the various oxides of nitrogen. Also
commonly employed, are the nitronium salts. Though known
1
1
for decades, nitronium salts were not particularly practical as
nitrating agents until Olah and coworkers were able to
12, 13
significantly improve and simplify their synthesis and use.
Fig. 1 Acid-functionalized polymeric reagents
In addition to these, some of the most attractive nitrating
mixtures are those employing solid acid catalysts to activate
nitrating reagents. Such methods generally allow for safer and
more controlled reactions with far less acid waste. Many of the
solid acids can also be easily separated from reaction mixtures
and regenerated for repeated use. Over the years, many
It has long been known that nitrogen bases form salts or
hydrogen bonded complexes upon neutralization with strong
2
8
acids and such salts have been exploited in a variety of ways.
Kawabata,
for example,
described
the
use of
poly(vinylpyridine) hydrochloride salt (Figure 2,
a
) as an acid
1
4, 15
catalyst for the acetalization of carbonyls and the esterification
diverse solid acids such as montmorillonite clays,
19
zeolites,
Nafion -H, sulfated zirconia and even silica supported
2
9
1
6, 17
®
18
of carboxylic acids. The group found the polymer complex
less hygroscopic than the monomer and more tolerant of
sensitive functionalities than other acidic resins. Later, a
similar form of the hydrochloride reported by Krieger and
associates was found to be a highly efficient catalyst in the
2
0
21
sulfuric acid and nitric acid have been reported for
nitration.
On the other hand, the use of polymer-bound or polymer-
supported reagents in synthetic organic chemistry has seen an
exponential growth since Merrifield’s pioneering work on solid
3
0
tetrahydropyranylation of alcohols and phenols. Menger and
2
2
Chu had performed the same reaction with the p-
phase peptide synthesis and the significant advances in the
area of protein synthesis. Polymeric reagents are usually solid
and reasonably stable, thus their primary advantage over
other reagents types are the ease with which they may be
toluenesulfonate salts of poly(vinylpyridine) earlier (Figure 2,
b
3
and c). The same complexes were also later found useful in
1
3
2
the reverse hydrolysis of the ethers by Li and Ganesan.
2
3
removed and recycled after reactions. Polymeric support can
also help modulate the reactivity of reagents making it
possible to use them in excess, scale up and even automate
reactions without significant deleterious effects. The benefits
associated with polymer supported reagents are often
countered by several drawbacks such as the difficulty
sometimes associated with their characterization, the thermal
and chemical sensitivity of some polymers and sometimes
longer reactions that give poor yields. In general however, the
development of polymeric reagents is now seen as an
important and influential contribution to both industrial and
academic research and the field has continued to grow.
Fig. 2 Acid-functionalized poly(vinyl pyridines)
Polymeric amine-acid complexes have been extensively
studied by Olah and coworkers during the last four decades.
Amine-anhydrous hydrogen fluoride (HF) complexes have been
used in nucleophilic fluorination, bromination and alkylation
Polymeric synthetic reagents exist in many forms.
However, of particular interest are those containing acidic
functionalities. Reagents of this kind rely upon several
different methods for incorporating acidic species into
polymers. Some of them (Figure 1), such as the familiar
3
3, 34
reactions.
In particular, pyridine and PVP (poly(4-vinyl
pyridine)) form stable polyhydrogen fluoride complexes
(
Olah’s Reagents) with varying amounts of hydrogen fluoride
3
5-37
®
24
25
(Figure 2,
d
).
Amines can act a reservoir for HF and the
Nafion -H or poly(vinylphosphonic acid) (
a), consist of
volatility of HF is significantly reduced by amine complexation.
Therefore, the complexes can be considered as “green”
reagents and it is generally much easier to modulate their
polymerized acidic monomers while many others consist of
2
b), and a growing class of
6
acid-functionalized polystyrenes (
2
7
polymer-coordinated Lewis acids (c, d).
2
acidity and catalytic activity than the free acid itself. PVP-SO is
another polymeric amine-acid complex that has been prepared
and subsequently used as a solid mild acid catalyst for Strecker
3
8
synthesis of α-aminonitriles. Recently, we have also prepared
PVP-TfOH complex for the Friedel-Crafts hydroxyalkylation and
3
9
acylation of arenes.
However, the use of polymer supported reagents in
4
0
aromatic nitration has very rarely been explored . In
continuation of our previous efforts towards various polymeric
2
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amine-acid complexes for a variety of organic transformations, refrigerator. The complexes were further examined by
we anticipated poly(4-vinylpyridine) to be an effective polymer Scanning Electron Microscopy and were found to remain
support for acids such as nitric acid and sulfuric acid and the consistent in both particle size and shape when compared to
resultant mixed acid-solid polymer complexes to be strong acid the parent polymer (Figure 4,
b and c).
catalyst/nitrating agent system capable of effecting aromatic Finally, we prepared poly(4-vinylpyridine)-nitrating mixture
nitration via Ingold-Hughes mechanism for nitration. (PVP-NM) complex from 2% cross linked poly(4-vinylpyridine),
Accordingly, we prepared poly(4-vinylpyridine) complexes of sulfuric acid and nitric acid. The two acids were first mixed
both concentrated nitric acid (HNO
acid (H SO ) as well as poly(4-vinylpyridine)-nitrating mixture amounts to form PVP-NM complex with one molar equivalent
PVP-NM) complex consisting of equal parts concentrated of monomer complexed with 7.5 molar equivalents of each
3
) and concentrated sulfuric together at -78 ºC before adding poly(4-vinylpyridine) in small
2
4
(
4
1
3 2 4
nitric and sulfuric acid (HNO :H SO ). We found that these acid. The complex contains 36% nitric acid, 56% sulfuric acid
solid complexes are safe and efficient solid nitrating mixture and 8% poly(4-vinylpyridine) by weight and is a free-flowing,
equivalents for aromatic nitration and our results are reported white solid that fumes slightly in air but remains
herein.
microscopically uniform as observed in SEM studies (Figure 4,
).
d
Results and Discussion
We began by preparing poly(4-vinylpyridinium) poly(nitric
3
acid) or PVP-HNO (Figure 3, 1) from poly(4-vinylpyridine) and
fuming nitric acid (≥ 99%). Commercially available poly(4-vinyl
pyridine) 2% cross-linked with divinylbenzene was carefully
added to nitric acid that had been cooled to -78 ºC and mixed
thoroughly. A 1:7.5 molar ratio loading of polymer to acid led
to the formation of a fluffy, free-flowing, white solid although
molar ratios from 1:4 to 1:8 can also be obtained. The resulting
complex is found to contain 82% nitric acid by weight and can
be stored in a closed Nalgene bottle for several months in the
refrigerator. It is worth mentioning that for complex formation
and preparation of solid complexes ideal for organic reactions,
the use of cross-linked polymers is required. Non-cross-linked
poly(4-vinylpyridinium) polymer was found to always lead to a
sticky, semi-solid mass.
a
b
c
d
Fig. 4 Surface morphology of (a) PVP and (b) PVP-HNO
PVP-H SO and (d) PVP-NM
3
(c)
2
4
To assess their capabilities as acid catalysts, the complexes
were subsequently used as nitrating reagents for the nitration
of various aromatics. To begin with, we conducted nitration
reactions of aromatics with a combination of PVP-HNO
complex and PVP-H SO complex. Equal portions of each
3
2
4
complexes were added one after the other to a solution of the
substrate and then stirred for a particular time and
temperature (Table 1). We found that reactions generally took
place at room temperature within two hours for activated
aromatic systems while even deactivated arenes could be
nitrated at moderate temperatures within the same
timeframe. Yields in all cases were generally good though
nitration of nitrobenzene was not found to occur even with
prolonged heating. It is interesting to note that the exclusive
mononitration of aromatic substrates was observed in all
cases. Also, all systems showed a general preference for para-
substituted products, with some deactivated systems reaching
up to ~ 7:1 ratio of para to ortho products.
Fig. 3 Possible structures of solid PVP complexes of acids
Poly(4-vinylpyridinium) poly(sulfuric acid) or PVP-H
2 4
SO
(
Figure 3, 2) was also prepared from 2% cross-linked poly(4-
vinylpyridine) and concentrated sulfuric acid in a similar
manner. The acid in this case formed a free-flowing peach
colored solid with a loading molar ratio of 1 equivalent of
polymer to 7.5 equivalents of acid and ratios from 1:4 to 1:8
are also possible. The complex containing 87% sulfuric acid by
weight was stored in a closed Nalgene bottle in the
The nitrating capacity of PVP-NM, obtained from PVP and
nitrating mixture prepared from nitric and sulfuric acids, was
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found to be superior to that of the two individual complexes 30 min for completion. The exception was nitrobenzene,
PVP-HNO and PVP-H SO added separately to the arenes one which, despite prolonged heating, remained inert to nitration.
after the other (Table 2). This is possibly due to presence of As in the case of PVP-HNO /PVP-H SO persistent
3
2
4
3
2
4
, a
both the acids in the same polymer network, which helps more preference for the para-substituted isomer, which attracts a
intimate interaction between the acids. While product yields higher value than the other isomers, was again observed with
from the separate addition of the complexes remained modest mononitration at high selectivity. In terms of para-selectivity,
in most cases, our new direct PVP-NM complex was able to there is no difference between PVP-NM and the combined acid
nitrate activated arenes in very high yields. Even the
deactivated arene bromobenzene was nitrated in 94% yield
Table 2 Nitration Reactions with PVP-NM
(Table 2, entry i) while other deactivated arenes were also
3 2 4
Table 1 Nitration Reactions with PVP-HNO and PVP-H SO
complexes. However, in terms of activity, neat PVP-NM
complex seems to be fairly more active than the combined
acid complexes as is evident in higher yields of the products
when PVP-NM was used (Table 2) than when combined acid
complexes were used (Table 1). Moreover, with PVP-NM small
amounts of dinitrated products were observed in some cases.
But exclusively mononitrated products can be obtained either
by decreasing the reaction time (Table 2, entry c) or by
decreasing the amount of PVP-NM (Table 2, entry f). For
further comparison, the reactions were conducted under
similar conditions using pure nitrating mixture made from
conc. sulfuric acid and fuming nitric acid. The reactions were
not as clean as observed in reactions using PVP-NM (Table 3).
For example, nitration of benzene showed only 51% selectivity
for mononitration (49% of di and trinitro). In most cases,
nitrated in high yields (Table 2, Entries g, h). It is worthwhile to
mention that previously reported poly(4-vinylpyridine)
polymer supported solid nitrating reagents were limited to
nitration of only activated arenes and were unable to nitrate
4
0a-b
even a mildly activated arene toluene.
Also, the reactions
were performed under reflux conditions and for longer times.
4
On the other hand, we were able to nitrate toluene in 99%
0b
yield with PVP-NM, our new (4-vinylpyridine) polymer
supported solid nitrating reagent. Moreover, except for the
nitration of benzene, all other nitrations were carried out at
very mild conditions (at room temperature) and were
complete within 2 hours, with some reactions taking as little as
4
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especially reactions with electron rich aromatics (toluene,
Therefore, attempt was made to recover PVP after the first
ethylbenzene, n-propylbenzene etc.) gave complex mixtures of reaction and reuse it to make new PV-PNM. After the first
isomers from mono and dinitration (major) with significant reaction with PVP-NM (11.16g), the reaction mixture was
drop in selectivity. This clearly indicates that PVP-NM is milder, filtered and the solid residue was made basic with saturated
much safer and provides high selectivity for mononitration NaHCO
3
solution, filtered again, washed with water followed
by acetone, and dried in vacuum. The recycled PVP was mixed
with the nitrating mixture (1:1 mixture of HNO and H SO ) to
with reaction control over dinitration (Table 3).
3
2
4
Table 3 Nitration Reactions with Nitrating Mixture
make PVP-NM (recycled) (10.75g, 96% recovery). When the
nitration of ethylbenzene was repeated with PVP-NM made
from recycled PVP, nitrated ethylbenzene (mixture of
regioisomers) was obtained in 95% yield (Scheme 2), which is
comparable to the yield obtained with fresh PVP-NM (Table 2,
entry b).
NO2
Nitrating Mixture
CH2Cl2
R
R
Entry
a
Arene
Temp (°C) Time (h)
Mononitro (o:m:p) (i) Di/Trinitro
NO2
50
2
2
51%
49%
di:tri = 19:1)
(
CH3
CH3
b
RT
8%
di, 92%
(
32:4:64)
O2N
O2N
Conclusions
C2H5
C2H5
11%
c
d
e
RT
RT
RT
RT
RT
1.5
2
di, 89%
di, 75%
100%
In conclusion, we prepared free-flowing stable solid complexes
of poly (4-vinylpyridine) with nitric and sulfuric acids both
separately as well as together (nitrating mixture) and
investigated their application as solid nitrating mixture
(
5:4:81)
C3H7
CH3
C3H7
25 %
(33:4:66)
O2N
3 2 4
equivalents. Both the PVP-HNO /PVP-H SO and PVP-NM were
2
0.5
2
__
__
(
di:tri = 6:1)
able to successfully nitrate both activated and deactivated
aromatic compounds. In particular, the PVP-NM complex was
found to be a safe and efficient “green” reagent system for
Friedel-Crafts nitration of aromatics. Mild reaction conditions,
mononitration with high selectivity, high isolated yields, high
para-selectivity especially in the case of highly deactivated
haloarenes, short reaction times and the recyclability of the
polymer support are some of the salient features of the
methodology.
H3C
f(ii)
di, 100%
F
F
g
98%
di, 2%
di, 7%
(1:6)
O2N
Cl
Cl
Cl
h
RT
2
93%
(1:7)
O2N
O2N
Br
i
j
RT
75
2
94%
di, 6%
100%
1
:1)
NO2
__
24
Acknowledgements
(
di:tri = 6:1)
Support by the Loker Hydrocarbon Research Institute is
gratefully acknowledged.
(
i) Isomeric ratio as well as mono-di/trinitro raio were determined by NMR and
GC/MS; (ii) 2.5 equiv. of nitrating mixture was used.
Notes and references
We also investigated the recovery and recycling of the solid
PVP-NM complex. After the first reaction with ethylbenzene
1
2
3
4
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(
Scheme 2), the solid residue was allowed to settle, the liquid
was pipeted out of the reaction vessel and the solid was
washed with dichloromethane. Ethylbenzene and
dicloromethane were added to the washed solid residue and
stirred. Even after 12 h, no reaction was observed, which
shows that no more nitrating mixture was left in the reaction
vessel after the first reaction and subsequent washing of the
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Scheme 2 Recyclability test for PVP-NM
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Green Chemistry
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DOI: 10.1039/C5GC00458F
Table of Contents
Poly(4-vinylpyridine)-Nitrating Mixture Complex (PVP-
NM): Solid Nitrating Mixture Equivalent for Safe and
Efficient Aromatic Nitration
G. K. Surya Prakash,* Laxman Gurung, Kevin E. Glinton, Kavita
Belligund, Thomas Mathew,* and George A. Olah
PVP-NM prepared from poly(4-vinylpyridine) and nitrating mixture (1:1 mixture of
HNO and H SO ) is a safe and effective nitrating agent for both activated and
3 2 4
deactivated arenes under mild conditions. The polymer can be retrieved and reused
for further complexation and the complex can be used for nitration without losing
the efficacy.