SOLVENT -FREE MICROWAVE ASSISTED ORGANIC
SYNTHESIS
A BRIEF REVIEW OF LITERATURE
PartB
Chapterd
Review of Literature
S O L V E N T -F R E E
M IC R O W A V E
A S S IS T E D
O R G A N IC
S Y N T H E S IS
- A R E V IE W
In tro d u c tio n :
G re e n chem istry, a relatively new field o f research and developm ent,
prom ises to provide the science and technology b a se n ecessary for
accom plishing m a n y o f the objectives o f an eco-efficient society in the post
m odern world o f the 21st century. G re e n chem istry seeks to d evelop and
deploy chem ical processes that can reduce o r elim inate the use and
generation o f h a za rdo u s substances. G ree n chem istry is being practiced
within at least fo u r regim es: traditional basic academ ic research, industryspecific
research
and
developm ent,
governm ent-industry,
academ ic
collaborations and technology generation and transfer am ong governm ent
National laboratories.
O rg a n ic synthesis directed tow ards G re e n C h em istry h ave attained
increasing
interest
in
recent
ye a rs 1-4 which
should
lead
to
new
environm entally benign procedures to sa ve resources and energy. G reen
chem istry can be defined as environm entally benign chem ical synthesis. T h e
dim ension o f environm ental problem s has increased substantially in the past
decade and will continue to expand in the future too. Therefore, attempts
have been m ade to design synthesis for m anufacturing processes in such a
w a y that the w aste products are m inim ized, th ey h a ve no effect on the
environm ent and their disposal is convenient. G re e n chem istry prom otes an
opportunity to readdress som e o f these problem s. It differs from conventional
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Chapter 6
chemistry and has an upper hand in terms of nature of reactants, reaction
conditions and time, economic feasibility and making use of raw materials in
nature. The green chemistry philosophy provides an opportunity to present
Avoid waste
chemistry in a refreshing and positive manner and represents the pillars that
support our sustainable future.
The principles of Green Chemistry5"6, proposed by Paul Anastas and John
W arner are summarised as follow s:
The twelve principles of Green Chemistry:
1)
Prevent waste
2)
Design safer chemicals and products
3)
Design less hazardous chemical synthesis
4)
Use renewable feed stocks
5)
Use catalysts, not stoichiometric reagents
1 99
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6)
Avoid chemical derivatives
7)
Maximize atom economy
8)
Use safer solvents and reaction conditions
9)
Increase energy efficiency
10)
Design chemicals and products which degrade after use
11)
Analysis in real time to prevent pollution
12)
Minimize the potential for accidents.
Chapter 6
The principles of green chemistry can be applied broadly to areas like
synthesis, catalysis7, reaction conditions, analysis and monitoring,
extraction, separations, computational chemistry and process modeling etc.
Green chemistry is also applicable to all sectors of the chemical industry
ranging from pharmaceuticals and specially chemicals to high volume
manufacture of bulk chemicals. The focus of the industry has now shifted to
reduce or eliminate the use of organic solvents during manufacturing and
processing. This involves “closed loop system” leading to reduction and/or
recycling switching to “solvent-free” process or solvent alternatives.
A possible alternative for the use of organic solvents is the extensive
utilization of water as a solvent8,9 and chemistry in aqueous medium offers
many advantages for a clean green chemistry. Also, promoting a reaction
photochemically rather than thermally is greener, since light is the green
reagent par excellence. Photochemistry10 offers an inexhaustible source of
new chemistry via chemical reactions of the excited state, which are much
faster and dependent little on conditions. Similarly, reactions by a microwave
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Chapter 6
have a advantage of having to use less or no solvent, very little time, no side
products and better yields with better selectivities compared to those carried
out by conventional means.
Solvent Free Organic Synthesis:
Among the most promising procedures for Green Chemistry, solventfree techniques hold a strategic position as solvents are very often toxic,
expensive and problematic to use and to remove. The concept of solventfree reactions is not a new one and chemistry texts of the late 19thand 20th
century contain numerous examples of reactions carried out using neat
reactants. Many excellent examples exist of solvent free reactions in single
crystals including photochemical reactions11, which may proceed under
topotactic control12, reactions of salts, including those to produce species
that might otherwise be difficult to access14, reactions of inclusion
complexes15 or co-crystals and reactions of solid monomers to form
polymers16. In fact, solvent-free reactions and the reactions carried out in a
homogenous phase where all reactants are dissolved in a mutual solvent
can be considered as one of the big break through in modem chemistry.
Solvent-free reaction makes it possible to reduce consumption of
environmentally unfriendly solvents and utilize scaled down reaction vessels.
Moreover, the use of a catalyst for solvent-free reaction can achieve energy
savings. Several techniques for the efficient use of solvent-free reaction
have been developed.
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Chapters
Solvent Free Technique or Dry Reaction Technique:
Generally, for a solid-state or solvent free reaction, three technique
are employed.
Reagents Supported on Mineral Oxides in “Dry Media”
Aluminas, silicas, clays or zeolites can be selected as acidic or basic
supports17depending on the type of organic reaction. Alumina alone can act
as a base towards a rather acidic molecule but if a strong base is necessary,
KF on alumina18, which can ionize carbon acids upto pka *3519, is used. On
the other hand, montmoriilonites (clays) such as K10 or KSF offer acidities
very close to nitric or sulfuric acids.20
in this technique, reactants are individually impregnated onto the
solid-support via a saturated solution in an adequate solvent if they are
solids, with subsequent solvents removal and reactions are performed by
simple mixing of the supported reagents in the absence of any organic
solvent.
Solid-Liquid solvent-free phase Transfer Catalysis (PTC):
This method21 is specific for anionic reactions as it involves “anionic
activation.” A catalytic amount of a tetraalkyiammonium salt or a cation
complexing agent is added to the mixture of both pure reactants. Reactions
occur in the liquid organic phase which consists only of the electrophile R-X.
The presence of a solvent is prejudicial as it induces a dilution of reactants
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Review of Literature
and, consequently, a decrease in reactivity. The electrophile RX, acts as
both the reactant and the organic phase for the reaction (Scheme B.VI.1)
Scheme B.V1.1
Solvent-free solid-liquid phase transfer catalysis
Solid Phase M*. Nu + R4N+, X-
Liquid Phase RNu + F^N+, X*
.,T= ^
R4N+, Nu, M*X-
--------------- R4N+, Nu + R-X
Solid Phase M*. Nir or Nu — H+Base
Catalyst f^N+, X*
Liquid Phase R - X (then R - Nu)
Aliquat 336 MehTOctg, Cf
Both electrophile and
organic phase
TBAB Bu4 N*. Br“
Heterogenous Reactions without any Catalyst or Support:
In this case, the heterogenous reactions are carried out between neat
reactants in quasi equivalent amounts without any adduct22,23. The simple
heterogenous mixture of two neat products (one solid and one liquid) can
eventually lead to a reaction; either the solid is partially soluble in the liquid
with a reaction occurring in the liquid phase or the liquid is adsorbed onto the
surface of the solid with a reaction occurring at the interphase. In both
situations, the presence of a solvent is deleterious due to dilution of the
reactants.
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O ften, the products of solid state reactions turnout to b e different from
those obtained in solution phase. T h is is b ecause o f specific spatial
orientation o r packing o f the reacting m olecules in the crystalline state. T h is
is true not on ly o f the crystals o f single com pounds, but also o f cocrystallized
solids o f two o r even m ore reactant m olecules. T h e host-guest interaction
com plexes obtained b y sim ply mixing the com ponents intimately also adopt
ordered structure. T h e orientational requirem ents o f the substrate m olecules
in the crystalline state have provided excellent opportunities to achieve high
degree o f stereoselectivity in the products. T h is has m ade it possible to
synthesize chiral m olecules from prochiral ones either b y com plexation with
chiral hosts o r form ation o f interm ediates with chiral partners.
S olvent-free
organic synthesis
leads
to
a
clean,
efficient and
econom ical technology safety is largely increased, w ork-up is considerably
simplified, cost is reduced, increased am ount o f reactants can b e used in the
sam e equipm ents and reactivities and som etim es selectivities are enhanced
without dilution. D u e to all these advantages, there is an increasing interest
in the use o f environm entally benign reagent and procedures o r in other
w ords, the ab sence o f solvent coupled with high yields and short reaction
time often associated with reactions o f this type m ake these procedures ve ry
attractive fo r synthesis.
Solvent Free Synthesis using Focussed Microwaves:
T h e em erging area o f green chem istry envisa g e s m inim um hazard as
the perform ance criteria while designing n ew chem ical processes. N ew
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Review of Literature
strategies have recently been developed aim ed at w orking without solvent.
Furtherm ore, it is also possible to activate p ro ce sse s b y physical m eans
such a s ultrasound, pressure o r m icrow aves. A m o n g these n ew n o n conventional m ethods in organic synthesis, m icrow ave irradiation takes a
particular place as it induces specific interactions betw een m aterials and
w a ve s o f electrom agnetic nature assimilated to dielectric heating.
M icrowave (M W ) irradiation, as an unconventional energ y source has
been used for a variety o f applications including organic syn th esis .24'28
W h erein chem ical reactions are accelerated b ecause o f selective absorption
o f M W energ y b y polar m olecules, n on -p ola r m olecules being inert to the
MW
dielectric loss. T h e
com bination
o f solvent-free
procedures and
m icrow ave irradiation can b e used to carry out a w ide range o f reactions
within short reaction time and with high conversions and selectivity. A lso , the
yields and purity o f the products are greatly im proved com pared to those
obtained b y conventional heating. T h is approach is efficient, e a s y to perform ,
econom ic and less polluting. S o m e m ajor advan tag es o f the organic
synthesis under m icrow aves are
• V e ry rapid reactions, requiring few m inutes, brought about b y high
and h om og en ous tem peratures and com bined with pressure effects (if
conducted in closed vessels).
•
H igher degree o f purity achieved d ue to short residence time at high
tem peratures, no local overheating, m inor decom position and m inor
occurrence o f se co n da ry reactions.
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PartB
•
Review of Literature
Chapter 6
Y ie ld s are often better, obtained within shorter tim es to give purer
product.
T h e solvent-free conditions are especially adapted to m icrow ave
activation as reactions can be run safely u n der atm ospheric pressure in the
presence o f significant am ounts o f products. T h e application o f m icrow ave
irradiation u n der solvent-free conditions, ena bles organic reactions to occur
expeditiously at am bient pressure29"31, thus providing unique chem ical
processes with special attributes such a s enh an ced reaction rates, higher
yields and the associated e ase o f m anipulation. T h e s e reactions can be
perform ed in open glass containers sim ply b y mixing o f neat reactants with
the catalyst/ prom oter o r their absorption on m ineral o r “d o p e d ” supports and
exposing the reaction mixture briefly to m icrow aves.
A P P L IC A T IO N S IN S Y N T H E T IC O R G A N IC T R A N S F O R M A T IO N S U S IN G
M IC R O W A V E S U N D E R S O L V E N T -F R E E C O N D IT IO N S ;
T h e use o f solvent-free reaction conditions has been w idely reported
as an appropriate m ethodology for the reduction o f w aste and minimization
o f energ y use, in lines with the principles o f green chem istry. T h e re are a
large n um bers o f solvent-free reactions o f a m yriad o f types described in the
chem ical literature32'34. O n e o f the m ost striking characteristics o f solventfree reactions is the high degree o f conversion to a single product that is
achieved in m a n y c a ses and reduction in the n u m b e r o f by-products form ed.
S o m e striking exam ples o f synthetically useful reaction u n d er m icrow ave in
d ry m edia are -
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Review of Literature
A. Functional Group Transformations:
Cleavage reactions are expedited by microwave (MW) exposure of
protected molecules on mineral oxides or benign “doped” reagents. For
example - a solvent-free deprotection procedure uses relatively benign
reagent, ammonium persulfate on clay for regeneration of carbonyl
compounds from the corresponding semicarbazone and phenylhydrazone
derivative35 by microwave or ultrasound irradiation (Scheme B.VI.2,
Reaction
1).
Microwave
irradiation
of
phenylhydrazones
and
tosylhydrazones with phosphoric acid35 also regenerate the carbonyl
compound in excellent yield (Scheme B.VI.2, Reaction 2).
Scheme B.VI.2
R e a c tio n 1
R = -NHPh, -NHTs
Similarly, ketones can be regenerated in a MW oven by simply
admixing neat ketoximes with periodate on wet silica and then exposing
them to MW irradiation (Scheme B.VI.3, Reaction 1). The role of the surface
is very critical as the deoximation reaction that occurs on silica surface
predominantly delivers the Beckmann rearranged products on clay. Another
facile method of microwave assisted regeneration of aldoximes using
207
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Chapter 6
Review of Literature
phosphoric acid in solvent-free condition at am bient pressure has been
reported recently (Schem e B.VI.3, R eaction 2)37.
S chem e B.VI.3
Reaction 1
Reaction 2
R1 \ _
/= N O H
R7
2
^> = N O H
Periodate - moist silica
MW, 1-2.5 min
h 3p o 4
-------------------------
R\
’>=
R/
2
R> = °
MW, 35-120 secs
A no th e r im portant conversion is the thiocarbonyis to carbonyls.
Unfortunately, this transform ation by conventional m ethod have certain
lim itations such as the use o f stoichiom etric am ounts o f the oxidants tha t are
often inherently toxic or require longer reaction tim e or involve tedious
process. These difficulties have been overcom ed by developing an efficient
solvent-free dethiocarbonylation process using clayfen [Iron (III) nitrate on
clay] o r clayan [A m m onium nitrate on clay] th a t is accelerated by M W
irradiation (Schem e B.VI.4)38.
S ch e m e B .V I.4
0.
Clayfen (or Clayan)
M W , 1.5-2 min
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Review of Literature
P a r tB
C h a p te rs
B. Condensation Reaction:
Several MW-assisted condensation reactions such as Aidol and
Knoevenagel etc. have been accompanied using relatively benign reagents
such as ammonium acetate. MW-expedited dehydration reaction using
montmorillonite K10 clay39 (Scheme B.VI.5 & 6) or Envirocat reagent40,
EPZG® (Scheme B.VI.5&6) have been accomplished in a rapid synthesis of
imines and enamines via the reactions of primary and secondary amines
with aldehydes and ketones respectively.
Scheme B.VI.5
(90 - 97%)
Scheme B.VI.6
0
EPZG
o r C la y
M W
Similarly, Henry reaction, the condensation reaction of nitroalkanes
with carbonyl compounds to generate nitroalkenes, also proceeds rapidly via
this MW approach in the presence of only catalytic amounts of ammonium
209
PartB
Review of Literature
Chapter 6
acetate, thus avoiding the use of large excess of polluting nitrohydrocarbons
usually employed in these reactions (Scheme B.VI.7)41.
Scheme B.VI.7
Recently, it has been shown that a solvent-free and catalyst-free
reaction of hydrazines with carbonyl compounds is possible upon microwave
irradiation in household MW oven (Scheme B.VI.8)42.
Scheme B.VI.8
C. Oxidation Reactions;
Metal-based reagents have found extensive use as oxidants in
organic synthesis. The utility of such reagents in the oxidation process is
compromised due to several reasons such as potential danger in handling of
metal complexes, inherent toxicity, cumbersome product isolation and waste
disposal problems. Immobilization of metallic reagents on solid supports has
addressed some of these limitations and provided an attractive alternative in
210
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Chapter 6
Review of Literature
organic synthesis because of the selectivity and prevention of leaching of the
metals into environment.
Recently, several MW-assisted oxidative protocols43-45 have been
introduced which are applicable to both alcohols (Scheme B.VI.9) and
sulfides (Scheme B.VI.10) using a wide variety of supported oxidants.
Scheme B.VI.9
CuS04/AI203, 2-3.5 min
Scheme B.VI.10
O
N a l 0 4 - s ilic a
Rr 8- ^ ------------------IB D - A lu m in a , 4 0 - 9 0
II
R -S -F L
1 II
sec
o
2
Similarly, arenes are oxidized into ketones within 10-30 minutes using
KMn04 impregnated onto alumina in dry media under focussed
microwaves45 (Scheme B.VI.11) instead of several days under classical
conditions.
2 11
PartB
Chapter 6
Review of Literature
Scheme B.VI.11
D. Reduction Reactions:
The solid-state selective reduction of carbonyl compounds occurs
readily with alumina-supported sodium borohydride (NaBH. ). However, a
4
catalytic amount of clay can also be used for borohydride reduction in the
48
same pot, thus providing a simple route to secondary and tertiary amines.
Clay serves the dual purpose of a Lewis acid and also provides water from
its interlayers that enhance the reducing ability of NaBH (Scheme B.VI.12).
4
Scheme B.VI.12
R.
1
R2
OH
NaBH4- AI2Q3
MW
R2
h 2N
-
R1 clay/ MW*~
Rf \ _ N __ R
3
NaBH4- c la y RfH20, M W
R'
R,
ti
H
Another striking example is the Leuckart reductive amination of
ketones under solvent-free microwave irradiation (Scheme B.VI.13)49. With
MW irradiation, this reaction could be carried out in excellent yields with
short reaction time.
212
PartB
Chapter 6
Review of Literature
Scheme B.VI.13
Rv
1
HCONH, + HCOOH
R1N
O
*2
MW
NHCHO
r;
E. Synthesis of Heterocyclic Compounds:
Heterocyclic chemistry has been a major beneficiary of MW-expedited
solvent-free chemistry utilizing mineral supported reagents developed over
the last decade.50A wide variety of heterocyclic compounds can be rapidly
assembled employing this solvent-free approach. For example, synthesis of
pharmacologically active 2-aroylbenzo[b]furans,51 Diels-Alder cycloaddition
reaction of vinyl pyrazoles52, synthesis of aziridines53 etc. under microwave
irradiation which overcomes the most important disadvantages of the
classical conditions.
It has been reported recently that 1, 3-Dipolar cycloaddition of organic
azides to acetylenic amides undersolvent-free microwaves irradiation
produces the corresponding N-substituted 1,2,3-triazoles in excellent yields
(Scheme B.VI.14)54.
Scheme B.VI.14
N=N
CONHCH2Ph
Multi-component condensation (MCC) strategy also enables the
creation of a diversity in a single step by varying the reacting components.
213
PartB
Chapters
Review of Literature
The generation of small-molecules libraries is indeed facilitated when the
ease of reaction manipulation of a reaction process is coupled with efficient
protocols.
Very recently, 1,4-disustituted-1,2,3-triazoles has been prepared from
the corresponding alkyl-halides, sodium azides and alkynes using multicomponent technique under microwave irradiation (Scheme B.VI.16)55.
Scheme B.VI.15
R
R
Br + NaN3+
— R1
H
Cu(O), CuS04 / MW
tBuOH, H20 , 10-15 min
nN
n^
r
F. Miscellaneous Reactions;
Several other reagents can be used under these solvent-free
conditions to expedite organic reactions. The crossed Cannizzaro reaction
can be accomplished with paraformaldehyde on barium hydroxide surface.56
Even more recently, an environmentally friendly Suzuki cross-coupling
reaction has been developed that uses small amount of recyclable
polyethylene glycol (PEG) as the reaction medium and palladium chloride as
a catalyst.57
Room temperature ionic liquids (RTIL’s) consisting predominantly of
dialkylimidazolium cations and various anions, have received wide attention
due to their potential in a variety of commercial applications such as
electrochemistry, heavy metal ion extraction, phase-transfer catalysis,
214
PartB
Chapter 6
Review of Literature
polym erization a n d as substitutes for traditional volatile o rg a n ic solvents.
S o m e ionic liquids bearing tetrafluoroborate a n io n s h a v e b e e n p re pared
recently
by
e x p o sin g
N .N '-d ia lkylim id a zo liu m
chlorid e
and
a m m o n iu m
tetrafluoroborate salt to M W irradiation (S c h e m e B .V I.1 6 )58,59.
S c h e m e B .V I.1 6
/ = \
^
R -X
------------ ►
MW
^ ' ©
K
L ,
^
R
215
N H 4B F 4 „
MW
/© \
^
R
PartB
Conclusion
Chapter 6
Conclusion:
In conclusion, this eco-friendly solvent-free microwave approach
opens up numerous possibilities for conducting rapid organic synthesis and
functional group transformations more efficiently. Additionally, there are
distinct advantages of these solventiess protocols since they provide
reduction or elimination of solvents thereby preventing pollution in organic
synthesis “at source”. The chemo-, regio- or stereoeiective of high value
chemical entities and parallel synthesis to generate a library of small
molecules will add to the growth of microwave-enhanced reactions in the
near future.
216
PartB
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Chapter 6
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