Tetrahedron 68 (2012) 949e958
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Tetrahedron report number 961
Organic reactions in subcritical and supercritical water
Nermin Simsek Kus *
Department of Chemistry, Faculty of Arts and Science, Mersin University, 33343 Mersin, Turkey
a r t i c l e i n f o
Article history:
Received 4 October 2011
Available online 29 October 2011
Keywords:
Pressured hot water
Subcritical water
Supercritical water
Near-critical water
Contents
1.
2.
3.
4.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 949
Properties of water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 950
Synthesis reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 950
3.1.
Alkylation reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 950
3.2.
Condensation reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 950
3.3.
Coupling reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 951
3.4.
Cyclization reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 951
3.5.
Decomposition reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 952
3.6.
Decarboxylation reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 952
3.7.
DielseAlder reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 952
3.8.
Disproportionation reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 952
3.9.
Elimination reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 953
3.10.
Ene reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 953
3.11.
Isomerization reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 953
3.12.
HeD exchange reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 953
3.13.
Hydrogenation/dehydrogenation reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 954
3.14.
Hydrolysis reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 954
3.15.
Oxidation reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 955
3.16.
Organometallic reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 955
3.17.
Rearrangement reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 955
3.18.
Transformation reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 956
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 956
References and notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 957
Biographical sketch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 958
1. Introduction
* Corresponding author. Tel.: þ90 3243610001; fax: þ90 3243610046; e-mail
address: [email protected].
0040-4020/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2011.10.070
There are many important studies of the extraordinary properties of subcritical and supercritical water for chemical reactions in
the literature.1e5 The term ‘superheated water’ refers to liquid
950
N. Simsek Kus / Tetrahedron 68 (2012) 949e958
water under pressure between 100 C and its critical temperature,
374 C. The properties of superheated water, subcritical water, and
supercritical water change with temperature and density. Many of
the anomalous properties of water are due to very strong hydrogen
bonding. Over the superheated temperature range the extensive
hydrogen bonds break down, changing the properties more than
usually expected by increasing temperature alone. Water becomes
less polar and behaves more like an organic solvent, such as
methanol or ethanol. The ionization constant of near-critical water
is several orders of magnitude higher than that of ambient water,
thus providing a source of hydronium and hydroxide ions, which
can act as catalytically active species in chemical reactions, such as
FriedeleCrafts alkylation.6 In addition, while reactions in supercritical water have mainly been applied in chemical reactions to
break up bonds,7 for waste destruction by supercritical water oxidation, the milder temperature regime of hot liquid water allows
for bond formation, i.e., synthesis of organic compounds.8,9 Thus,
water can be used as a solvent, reagent, and catalyst in industrial
and analytical applications, including extraction, chemical reactions, and cleaning.
material synthesis, waste destruction, plastic recycling, and biomass processing as reaction media.9,16e18
3.1. Alkylation reactions
Alkylphenols are important intermediates for both industrial
and synthetic organic chemistry as antioxidants, pharmaceuticals,
and polymers. The production of alkylphenols exceeds 450,000 t/
year on a worldwide basis.19 Arai et al. showed that the regioselectivity of phenol 1 alkylation with propionaldehyde 2 could be
controlled in supercritical water without a catalyst at 673 K and
60 min reaction time at a water density of 0e0.5 g cm3. The sum of
the yields of phenolic compounds 4 and 5 increased with increasing
water density. The sum of the yield of the main liquid products was
about 30%. Gas analyses showed that the products were CO and
C2H6 in the pyrolysis of propionaldehyde20 (Scheme 1).
2. Properties of water
Subcritical water is liquid water under pressure at temperatures
between the usual boiling point (100 C) and the critical temperature (374 C). It is also known as superheated water and pressurized hot water. Many of the anomalous properties of water are due
to very strong hydrogen bonding. For example, the dielectric constant of water decreases from 3 ¼80 at STP to 3 ¼31 at 225 C,
P¼100 bar, and finally to 3 ¼6 at the critical point, Tc¼374.15 C,
Pc¼221.2 bar,10 due to the steady decrease in the effectiveness of
hydrogen bonds with increasing temperature. Above the subcritical
temperature range, the extensive hydrogen bonds break down,
changing the properties more than usually expected by increasing
the temperature alone.11 Water effectively becomes less polar and
behaves more like an organic solvent, such as methanol or ethanol.
Organic molecules often show a dramatic increase in solubility
in water as the temperature rises, partly because of the polarity
changes, and the solubility of sparingly soluble materials tends to
increase with temperature as they have a high enthalpy of solution.
The solubility of organic materials and gases increases by several
orders of magnitude and the water itself can act as a solvent, reagent, and catalyst in industrial and analytical applications, including extraction, chemical reactions, and cleaning.
At temperatures below 300 C, water is fairly incompressible,
which means that pressure has little effect on its physical properties, provided it is sufficient to maintain water in its liquid state.
This pressure is given by the saturated vapor pressure, and it can be
found in stream tables or calculated.12 As a guide, the saturated
vapor pressure at 121 C is 100 kPa, at 150 C it is 470 kPa, and at
200 C it is 1550 kPa. The critical point is 21.7 MPa at a temperature
of 374 C, above which water is supercritical rather than superheated. Above about 300 C, water starts to behave as a near-critical
liquid, and the physical properties start to change more significantly with pressure.
3. Synthesis reactions
The last two decades have seen significant growth in the development of water-based synthetic chemistry, reflected in a new
appreciation for the unique characteristics of water as a solvent.13,14
The dramatic decrease in solvent viscosity, increase in substrate
solubility, and enhanced sensitivity of solvating properties of the
subcritical fluid with respect to temperature and pressure make it
attractive as a potential solvent.15 Recently, subcritical and supercritical water have been applied intensively for chemical synthesis,
Scheme 1. Synthesis of alkylphenols 4 and 5 in supercritical water (SCW).
The FriedeleCrafts reaction can be achieved in subcritical media.21 Eckert et al. have reported the alkylation6 and acetylation of
phenol 1 and resorcinol22 (Scheme 2) without an acid catalyst in an
industrial synthesis in near-critical water. The FriedeleCrafts alkylation of phenol 1 and p-cresol with tert-butanol was found to be
reversible, with equilibrium yields of approximately 20% after 50 h
at 275 C, without any added catalyst. The FriedeleCrafts acetylation of phenol 1 with acetic acid gave use the to 20 -hydroxy acetophenone 8, 40 -hydroxy acetophenone 9 with a combined
equilibrium yield of 8%. The acetylation of resorcinol successfully
produce primaril 2,4-dihydroxy acetophenone, with an equilibrium
yield of more than 50% in 12 h. Thus, the methods were advantageous in terms of both green chemistry and good economics.
OH
OH
+ t -BuOH
SCW
t-Bu
OH
+
t -Bu
1
6
OH
+ MeCOOH
SCW
OH
7
OH
COMe
+
COMe
1
8
9
Scheme 2. Alkylation and acetylation reactions of phenol 1 in subcritical water.
3.2. Condensation reactions
Condensation reactions are very important for organic synthesis. Eckert et al. investigated the ClaiseneSchmidt condensation of
benzaldehyde 10 with 2-butanone 11 and demonstrated the ability
to conduct conventional acid- or base-catalyzed reactions using
subcritical water without the addition of a catalyst (Scheme 3).23
Condensation reaction of benzaldehyde 10 with 2-butanone 11 at
low efficiency (0e20%) in subcritical water gives 12 and 13, Although these reactions did not produce yields of industrial interest,
this condensation reaction demonstrated that near-critical water is
a medium with great potential for organic reactions without the
addition of acids or bases.
N. Simsek Kus / Tetrahedron 68 (2012) 949e958
O
H
+
10
O
O
O
SCW
11
951
I
+
12
19
13
Scheme 3. ClaiseneSchmidt condensation of benzaldehyde with 2-butanone in subcritical water.
24
In another study, Comisar and Savage synthesized benzalacetone 15 from the crossed aldol condensation of benzaldehyde 10
and acetone 14, and chalcone 17 from the crossed aldol condensation of benzaldehyde 10 and acetophenone 16, at temperatures of
250, 300, and 350 C. The maximum molar yield of the benzalacetone 15 synthesis was 24% at 250 C and 5 h, while the maximum
yield of the chalcone 17 was 21% at 250 C and 15 h. This reaction
used high-temperature water, which can make the process more
environmentally benign by not using an acid or base (Scheme 4).
SCW
+
20
21
Scheme 6. Heck reaction in subcritical water.
Recently, the author has developed a practical procedure for the
oxidation of primary aromatic amines to their corresponding diazene compounds, in subcritical water with molecular oxygen. This
simple, economical, and environmentally friendly method is useful
for the transformation of substituted primary aromatic amines 22
in to the corresponding diazene derivatives 23 (Scheme 7).27 The
yields of the oxidation reactions of arylamines with an electrondonating group are 80e95%. Oxidation of arylamines with an
electron-withdrawing substituents resulted in the formation of the
corresponding oxide products in 50e60% yields because of the
unreacted starting materials.
NH 2
R
SCW
R
22
NN
R
23
Scheme 7. Synthesis of diazene derivatives in subcritical water.
Scheme 4. Synthesis of benzalacetone and chalcone in high-temperature water.
Cellulose in the form of paper products, cellulosic fiber, and
cellulose derivatives, is greatly used in everyday life. However,
many of the conventional techniques for manufacturing cellulosic
materials are harmful to the environment. In an environmentally
friendly study by Arai et al.,25 cellobiose hydrolysis and retro-aldol
condensation reactions were studied in subcritical and supercritical water. Cellobiose 19 degradation mainly proceeds by hydrolysis and retro-aldol condensation. Hydrolysis of cellobiose
gives two glucose molecules and retro-aldol condensation can
occur at the reducing end of the cellobiose to form glucosylerythrose and glycolaldehyde. The glucosyl-erythrose hydrolyzes
to glucose via retro-aldol condensation, and then glucose can
convert into glyceraldehyde, dihydoxyacetone, and some organic
acids (Scheme 5).
A novel method for the synthesis of disulfides was developed by
Ozen and Aydin.28 In this method, molecular oxygen is used to
oxidize thiols to their corresponding disulfides in subcritical water
in >90% yields. The oxidation of thiols to disulfides in subcritical
water was carried out as isothermal experiments at 100 C using
different amounts of oxygen (PO2¼5e20 bar) in the absence of
metal catalysts (Scheme 8).
2RSH
SCW, O 2
RSSR
R = alkyl, substituted phenyl
Scheme 8. Synthesis of disulfides from thiols in subcritical water.
Parsons et al.29 performed palladium-catalyzed alkeneearene
coupling reactions in superheated and supercritical water. These
reactions are simple and proceed similarly to those of their counterparts in traditional, organic solvents. The coupling reactions
proceed relatively cleanly at 260 C (superheated) and 400 C
(supercritical) in the presence of NH4HCO3 and the coupled product
25 obtained from toluene 24, although the yields of the coupled
products are limited by deactivation of the palladium catalyst. Part
of this study is presented in Scheme 9.
R
SCW
Scheme 5. Reaction of cellobiose 18 degradation in subcritical and supercritical water.
3.3. Coupling reactions
In a study by Gron et al.,26 hydrothermal media at subcritical
conditions were shown to influence chemical pathways for the
model Heck reaction studied and may have an important future as
solvents for synthetic organic applications. In a typical reaction, 12ml 316 stainless steel reactors were loaded with 1 mmol of iodobenzene 19, 3 or 5 mmol of cyclohexene 20, 3 mmol of NaOAc,
0.06 mol of Pd(OAc)2, and 0.36e10.1 g of water (Scheme 6). The
yield of 21 (12e47%) in the coupling reactions of iodobenzene 19
and cyclohexene 20 is affected by the addition of ionic salts, the
pressure, and the density of water.
24
R
25
R= PhCH=CH2
CH2 =CMeEt
CH2 =CHCH 2OH
CH2 =CHCH 2Cl
Scheme 9. Pd-catalyzed akleneearene coupling reactions in superheated and supercritical water.
3.4. Cyclization reactions
The Nazarov reaction is an acid-catalyzed cyclization of divinyl
ketones to 2-cyclopentenones.30e32 Aaltonen et al. obtained the
first results of a potentially sustainable method to carry out the
Nazarov reaction in near-critical water or in near-critical water/
carbon dioxide system.33 The Nazarov reaction of transetransdibenzylidene acetone 26 in a near-critical water gives a 39% yield
of the abnormal product isomer, 2,3-diphenyl-2-cyclopentenone
27, instead of the conventional classical Nazarov product, 3,4-
952
N. Simsek Kus / Tetrahedron 68 (2012) 949e958
diphenyl-2-cyclopentenone 28. Hence, there is no need to use
a strongly acidic solvent/catalyst system in the reaction. This
methodology may also be applicable to other pericyclic reactions of
general and industrial interest (Scheme 10).
O
O
Ph
O
Ph
SCW
Ph
or
Ph
Ph
27
26
Ph
because of the widespread use of pentachlorophenol as a probable
human carcinogen. In a study by Jae-Duck et al.,47 the hydrothermal
decomposition of pentachlorophenol to intermediate products,
such as tetrachlorophenol and trichlorophenol was investigated in
a tubular reactor for subcritical and supercritical water with a sodium hydroxide additive at a temperature range of 300e420 C and
a fixed pressure of 25 MPa, with a residence time that ranged from
10 to 70 s.
28
3.6. Decarboxylation reactions
Scheme 10. Nazarov reaction in subcritical water.
3.5. Decomposition reactions
The reactivity and decomposition of organic compounds in
subcritical water and in supercritical water have received a great
deal of attention.34,35 In a study by Savage and Hunter36 bisphenol
A 29 was converted in to 4-isopropenylphenol 30 and 1 after
60 min of reaction with a specialty chemical product in subcritical
water. However, in another study by Savage et al., bisphenol E 32
decomposed in high-temperature liquid water at 250e350 C to
form 4-vinylphenol 33 and phenol 1 as primary products (Scheme
11).37
Unsubstituted indoles have been prepared by classical methods
requiring the removal of a 2-carboxyl group. There are two important methods, including pyrolysis, or heating with various derivatives of copper, in quinoline.48 However, in a simple method,
indole-2-carboxylic acid 37 is quantitatively decarboxylated to 38
after 20 min at 255 C in water (Scheme 13). However, 2carbethoxyindole underwent a low conversion into indole under
the same conditions.
Scheme 13. Decarboxylation reaction of 2-indole-2-carboxylic acid 37 in hightemperature water.
In another study,17 ester 39 was hydrolyzed for 6 h at 290 C and
the product was decarboxylated to give styrene 40 in 69% yield
(Scheme 14).
Scheme 11. Decomposition reactions of bisphenol A and bisphenol E in high-temperature water.
In industrial chemistry, subcritical and supercritical water have
been investigated in the field of waste treatment, especially in the
presence of a large excess of oxygen (Scheme 12). For example, PVC
was oxidized to carbon dioxide, water, and hydrogen chloride
(sodium chloride) without any catalyst (Eq. 1).38e40 The degradation or gasification of cellulose gives glucose, fructose, cellobiose,
and hydrogen gas, without any additive or catalyst in subcritical
and supercritical water (Eq. 2).41e43 In subcritical and supercritical
water, ethane-1,2-diol 35 and terephthalic acid 36 were also obtained from the hydrolysis of polyethylene terephthalate 34 (Eq.
3).44e46
(O 2, NaOH)
SCW
PVC
SCW
Cellulose
O
O
O
34
SCW
O
CO 2 + H2 O + HCl (NaCl)
eq. 1
Glucose + Fructose + Cellobiose + H 2
eq. 2
HO
OH + HO 2C
CO 2H
Scheme 14. Decarboxylation reaction of ethyl cinnamate 39 in high-temperature
water.
3.7. DielseAlder reactions
In recent years, water above the critical temperature (374 C)
has received much attention as a novel medium for DielseAlder
reactions, due to the high solubility of organic reactants. The
DielseAlder reaction is a very important synthetic method for the
production of polycyclic ring systems. A study by Kolis and Korzenski49 showed that supercritical water is an attractive medium
for the DielseAlder reaction and that the reaction times of a variety
of cycloaddition reactions are enhanced significantly in supercritical water, relative to reactions run under more conventional conditions (Scheme 15). The cycloaddition of cyclopentadiene 41 with
dimethyl maleate 42 using an ethanolewater co-solvent mixture
along with bakers’ yeast as the catalyst led to appreciable stereoselectivity.50 The addition reaction gave 43 and 44 in 78% yield.
However, 43 and 44 are obtained in yields of 86% in subcritical
water after 1 h.
eq. 3
+
n
35
36
Scheme 12. Use of subcritical and supercritical water in industrial chemistry.
41
CO2Me
SCW
CO2 Me
CO 2Me
86%
CO2Me
42
43
+
CO2Me
CO2 Me
44
Scheme 15. DielseAlder reactions in supercritical water.
Pentachlorophenol is one of the most widely used biocides in
the form of insecticides, fungicides, herbicides, molluscicides, algicides, antimicrobial agents, disinfectants, and wood preservatives, and also for adhesives, latex paints, paper coatings,
coatings in re-usable bulk food-storage containers, photographic
solutions leather tanneries, and pulp and paper mills in the United
States. Organic waste materials require disposal or destruction
3.8. Disproportionation reactions
Disproportionation reactions of diarylmethanol derivatives 45
have also been applied to various diarylmethylamine derivatives 45
to give disproportionation products (46 and 47) in supercritical
N. Simsek Kus / Tetrahedron 68 (2012) 949e958
water. Hatano et al. have developed a novel disproportionation
method for diarylmethanol derivatives and diarylmethylamine
derivatives in good yields (35e50%) (Scheme 16).51
953
3.11. Isomerization reactions
Sattar et al.55 prepared carvacrol 58 from carvone 57 in a steam
bath for several hours in high yield in subcritical water (Scheme 20).
O
OH
SCW
95%
57
58
Scheme 20. Isomerization reaction of carvone 57 in subcritical water.
Scheme 16. Disproportionation reactions of diarylmethanol derivatives in supercritical water.
3.12. HeD exchange reactions
3.9. Elimination reactions
In a study by Kuhlmann et al.,52 cyclohexanol and methylcyclohexanol derivatives underwent dehydration to cyclohexene
derivatives in deuterium oxide at 250e300 C (Scheme 17). From an
18-h treatment of cyclohexanol 48 in superheated water at 278 C,
an 85% yield of the cyclohexene 49 was obtained.
Scheme 17. Elimination reactions of cyclohexanol derivatives 48 in high-temperature
water.
In addition, cis- and trans-2-methyl-cyclohexanol were found to
undergo elimination to 1-methyl-cyclohexene with 100% selectivity, but in low yields, in 60 min at 300 C.52
Carlsson et al.53 showed the conversion of citric acid 50 in to
methacrylic acid 53 at 250 C, with compressed (34.5 MPa) liquid
water to form citraconic acid 51 and itaconic acid 52 with a combined selectivity that exceeded 90% (Scheme 18).
Exchange reactions involving the displacement of hydrogen
bonded to carbon by deuterium are of interest in a broad variety of
disciplines, such as the preparative chemistry of isotopically labeled
materials, fundamental studies of carbonehydrogen bond activation processes, and studies of the nature of catalysts.56 There are
many studies of HeD exchange in the literature, but there are few
such studies in subcritical or supercritical water.
Kuhlmann et al.52 have suggested reactivity in superheated
water according to acidities within a molecule. Acids with roomtemperature pKa values as low as 50 can be deuterated within
reasonable heating times with sufficiently concentrated 600 C
media.
D2O has demonstrated potential for HeD exchange in organic
compounds at high temperatures and pressures. Hydrogen-atom
exchange of aromatic compounds in neutral near-critical D2O
could be improved by using a polymer-supported sulfuric acid
catalyst at 325 C with high yields (>90%). Boix and Poliakoff performed an HeD exchange reaction of phenol, aniline (Scheme 21),
quinoline, and substituted aromatic hydrocarbons at 325 C for 2 h
in D2O/Deloxan and HeD exchange products 59 were obtained in
high yields.57
Scheme 18. Conversion of citric acid in to methacrylic acid.
3.10. Ene reactions
The observations described above show that water can be an
efficient solvent in both the acceleration of ene reactions and the
inhibition of side reactions. Laitinen et al.54 studied the ene reaction
of allylbenzene 54 and N-methylmaleimide 55 in subcritical water,
the high vapor pressure of water favored a pericyclic association
between the polar and the apolar starting compounds in 5e6%
yields at 310 C (Scheme 19). Subcritical water is inappropriate for
this reaction, because it hydrolyzes N-methylmaleimide. When
subcritical ethanol was used, the yield of the ene product 56
reached 40% in 480 min, and the highest trans-selectivity was 92%.
H
O
+
N Me
SCW
H
N Me
O
54
55
O
O
56
Scheme 19. Ene reaction of allylbenzene 54 and N-methylmaleimide 55 in subcritical
water.
Scheme 21. HeD exchange reaction of phenol, aniline at 325 C in D2O/Deloxan.
Yao and Evilia58 used subcritical and supercritical water for the
hydrogenedeuterium exchange of various compounds having pKa
values of up to 50 in D2O at 400 C. The HeD exchange reactions
were obtained with nearly quantitative yields.
In another study, Kuhlmann et al.52 reported the first-order
global rate constant (1.47104 s1) for hydrogenedeuterium exchange of the a-methyl protons in pinacolone at 225 C and
25.4 bar.
In a study by Yonker et al.,59 the kinetic data for the hydrogenedeuterium exchange of resorcinol in pure D2O were studied
for the first time using a flow-through capillary tubular reactor
954
N. Simsek Kus / Tetrahedron 68 (2012) 949e958
with on-line, proton, and deuterium NMR detection at high temperatures and pressure.
Hydrogen exchange was achieved rapidly and nearly quantitatively in the a and a0 positions of ketone carbonyl groups by
Kuhlmann et al. as seen in Table 1.52
were obtained at the end of the reaction. Higher conversions of DBT
and yield of BP and CHB were obtained in COeSCW than in
H2eSCW. Thus, it was shown that hydrodesulfurization reactions
can be achieved in subcritical media (Scheme 22).
Table 1
Hydrogenedeuterium exchange in ketones
Compound
% D (position)
Reaction conditions
Pinacolone
Acetone
Cyclopentanone
1,4-Cyclohexadione
Acetphenone
Deoxybenzoin
100 (a Me
97 (a,a0 Me)
100 (a,a0 Me)
100 (a,a0 Me)
>88 (a Me)
99 (a Me)
277
200
225
225
250
250
C
min
60
60
30
30
60
30
3.13. Hydrogenation/dehydrogenation reactions
The demand for lighter liquid oils is increasing. Therefore,
heavy-oil emulsions are converted in to lighter liquid oils. Arai
et al.60 reported on a new hydrogenating process for the hydrotreatment of heavy oils in supercritical water. The experiments,
involving hydrogenation of dibenzothiophene, carbazole, and
naphthalene, comprised three parts: (1) hydrogenation through
a wateregas shift reaction in supercritical water; (2) selective formation of carbon monoxide by partial oxidation in supercritical
water and through a combination of (1) and (2); and (3) hydrogenation of hydrocarbons through partial oxidation in supercritical
water. A wide range of hydrogenation applications are able to make
use of the findings presented in this work.
Dehydrogenation can be studied in subcritical water. A study by
Crittendon and Parsons61 showed that the functional-group
transformations and the extent of dehydrogenation can be controlled in experiments by judicious selection of catalysts and pH in
subcritical media. Cyclohexanol, cyclohexene, and cyclohexane
were converted in to related compounds at the end of the experiment performed in subcritical water by Crittendon and Parsons61
(Table 2).
Scheme 22. Hydrodesulfurization reactions in subcritical media.
3.14. Hydrolysis reactions
Supercritical fluids are now used in industry and science as
solvent and reaction media for many chemical applications. One of
these is hydrolysis reaction.
The author’s group has recently converted four monocyclic and
four bicyclic olefins 63 into the corresponding alcohols 64 in excellent yields (85e95%) (Scheme 23).63 The reaction times for the
bicyclic olefins were shorter than those for the monocyclic olefines
and the yields of alcohols from the bicyclic olefins were slightly
higher than those from the monocyclic olefins. It is assumed that
these conversions proceeded via a radical mechanism, because the
behavior of subcritical water as a radical oxidant in the presence of
a metal catalyst has been known for a long time.64
SCW
HO
H
63
64
Scheme 23. Synthesis of alcohols 64 from olefins 63 in subcritical water.
Mineral and Lewis acids are used as the largest set of industrial
catalysts today. The large volume of toxic waste generated from the
neutralization of these acids represents a significant challenge to
the implementation of the principles of green chemistry for acid
technologies. Liotta and Eckert et al.65,66 have shown the hydrolysis
of anisole derivatives 65, benzoate esters 67, 4-nitroaniline 69 and
N,N-dimethyl-nitroaniline 71 to their corresponding compounds
(66, 68, 70 and 72) in near-critical water (Scheme 24).
Table 2
Dehydrogenation reactions in subcritical water
Reactant
Product
HO
HO
O
Catalyst
% of Product
SnCl2
65
PtO2
54
PtO2
27
PtO2
44
PtO2þNH4OH
56
In another study, Adschiri et al.62 used a conventional NiMo/
Al2O3 hydrotreating catalyst to hydrogenate and remove sulfur
from dibenzothiophene (DBT; 60) in various atmospheres
(H2eSCW, COeSCW, CO2eH2eSCW, and HCOOHeSCW) at 673 K
and 30 MPa. Biphenyl (BP; 61) and cyclohexylbenzene (CHB; 62)
Scheme 24. Hydrolysis of aromatic compounds in near-critical water.
In addition, 4-tert-butylcyclopent-1-enoic acid 74 was obtained
from its morpholide derivative 73 in high yield after only 10 min at
N. Simsek Kus / Tetrahedron 68 (2012) 949e958
955
200 C in 2 M HCl by Strauss et al.,17 although tertiary amides can be
difficult to hydrolyze because of steric factors and low solubility
(Scheme 25).
O
N
O
O
SCW
Scheme 28. Synthesis of benzaldehyde 87 derivatives from primary alcohols 86 and
toluene derivatives 88.
OH
70%
HO
74
73
OH
SCW, O2
63%
Scheme 25. Hydrolysis of 4-[4(4-tert-butyl-1-cyclopenten-1-yl)-carbonyl]morpholine
73 in high-temperature water.
In another study, Strauss et al.17 achieved the hydrolysis of
benzonitrile 75 and n-octanonitrile 78 to afford benzoic acid 76 and
octanoic acid 79, respectively, as the major products, in water
(Scheme 26). Benzamide 77 were obtained as the minor product in
the hydrolysis of benzonitrile 75. The esters, hexyl acetate, pentyl
benzoate, and ethyl cinnamate, were also hydrolyzed to significant
extents in high yields (80e95%) within 1e3 h at 250e300 C.
89
O
SCW, O 2
O
O
75%
90
91
Scheme 29. Synthesis of lactone and ester in subcritical water.
Synthesis of cyclic imides is certainly important for both the
chemical industry and medicine.70 A cyclic imide moiety can be found
in some natural products and in man-made compounds. The oxidation method in subcritical water is general, as a variety of lactams 92
were successfully transformed into cyclic imides 93 (Scheme 30).
O
n(H2 C)
NH
SCW, O 2
55-63%
O
n(H2 C)
NH
O
92
93
Scheme 30. Synthesis of cyclic imides in subcritical water.
Scheme 26. Hydrolysis reactions of nitriles in subcritical water.
Vogel and Krammer67,68 studied the kinetics of the hydrolysis
reactions of substituted acetate 80, 1,4-butanediol diacetate 82,
poly(1,4-butanediol diacetate) 84, poly(tetrahydrofuran diacetate),
poly(tetrahydrofuran), acetamide and benzamide at 23e30 MPa
and 350 C in subcritical and supercritical water (Scheme 27). The
hydrolysis products (81, 83, 85) were obtained in high yields in the
end of reactions.
MeCOOR
80
OAc
AcO
82
SCW
SCW
100%
SCW
n OAc 100%
AcO
84
Holliday et al. showed that subcritical water can be used as
a synthetic medium for reactions involving highly non-polar substrates.72 This research also demonstrated that selective oxidation
can be performed and controlled in subcritical water on a number of
different aromatic substrates, leading to the formation of oxidized
products. A number of alkyl aromatic compounds 94 were oxidized to
aldehydes 87, ketones, and acids 95 by molecular oxygen in subcritical
water mediated by various transition-metal catalysts (Scheme 31).
MeCOOH + ROH
81
HO
OH
83
R = -Me 88%
-Et 98%
-n-Bu 60%
-Bn 44%
n OH
HO
85
Scheme 31. Oxidation of alkyl aromatic compounds in subcritical water.
A number of other alkyl aromatics 96 can be selectively oxidized
at the a-position. Moreover, increasing the oxygen pressure or using longer reaction times leads to the formation of benzoic acids
957,23 (Scheme 32).
Scheme 27. Hydrolysis reactions of esters in subcritical and supercritical water.
3.15. Oxidation reactions
Recently, the author’s group has described a simple, clean, and
efficient catalytic oxidation procedure using molecular oxygen as
an oxidant achieved in oxidation reactions of different molecules.69,70 The preparation of aromatic aldehydes 87 from the corresponding primary alcohols 86 and toluene derivatives 88 is not
easily achieved using molecular oxygen in subcritical water, because aldehydes are readily converted in to carboxylic acids under
oxidizing conditions71 (Scheme 28). 1,4-Diols, such as 89 are easily
converted into lactones 90 (e.g.) and are important flavor and
aroma constituents in many natural products; the procedures work
well for the preparation of butyrolactones in subcritical water70
(Scheme 25). Furthermore, 1,3 and 1,5-diols were converted into
dialdehydes, but attempts to obtain lactones failed. Cyclic ethers,
such as 91 were also converted into lactones 90 (e.g.) with oxygen
in subcritical water70 (Scheme 29).
Scheme 32. a-Oxidation of alkyl aromatics 96 in subcritical water.
3.16. Organometallic reactions
Organosilicon molecules have been extensively utilized in modern organic synthesis.73 Itami et al. reported an extremely facile CeSi
bond cleavage of organosilicon compounds in supercritical water
(Scheme 33), which serves as a starting point toward the development of silicon-based organic syntheses in supercritical water.74
3.17. Rearrangement reactions
In a study by Comisar and Savage,75 the benzil-benzilic acid
rearrangement, a base-catalyzed reaction under conventional conditions, proceeds in neutral subcritical water with no catalyst added
956
N. Simsek Kus / Tetrahedron 68 (2012) 949e958
and obtained the ortho Claisen rearrangement product, 2allylphenol 107, in high yields (Scheme 36).
O
OH
SCW
Scheme 33. CeSi bond cleavage of organosilicon compounds in supercritical water.
(Scheme 34). The benzilic acid 98 obtained from benzil 97 in subcritical water gives benzhydrol 99, benzophenone 100, and diphenylmethane 101. Benzhydrol 99 forms nearly to the equal yields of
benzophenone 100 and diphenylmethane 101. The rearrangement
proceeds in neutral subcritical water without the addition of a base,
but the yield of the rearrangement products is nearly insensitive to pH
at near-neutral conditions. When larger amounts of base are added,
rearrangement products occur in much higher yields.
O
O
SCW
97
O
106
107
Scheme 36. Rearrangement of allyl phenyl ether 108 to 2-allylphenol 109 in hightemperature water.
In another study, Strauss et al.,17 achieved a Rupe rearrangement
of 1-ethynyl-cyclohexanol 108 to 1-acetylcyclohex-1-ene 109
without using acid at 290 C for 1 h in 45% yield (Scheme 37).
O
SCW
OH
OH
OH
OH
45%
109
108
98
Scheme 37. Rupe rearrangement of 1-ethynyl-cyclohexanol 108 to 1-acetylcyclohex1-ene 109 in high-temperature water.
99
O
+
100
101
3-Phenyl-2-enaldehydes17 111 and 112 occurred with the
MeyereSchuster rearrangement of 2-phenyl-3-butyn-2-ol 110 in
water at 200 C in a trans- to cis ratio of 2:3 (Scheme 38). Apparu and
Glenat81 obtained a mixture of aldehydes at subambient temperatures under acidic conditions.
Scheme 34. Rearrangement reaction in subcritical water.
In a study by Kuhlman et al.,52 a 1,10 -dihydroxy-1,10 -dicyclopentyl and 1,10 -dihydroxy-1,10 -dicyclohexyl rearrangement to the
corresponding ketones, with negligible alkene formation, occurred
in deuterium oxide in 60 min at 275 C. A classical method76 to
obtain pinacolone from pinacol requires boiling in 25% H2SO4 for 3 h.
Experiments by Boero et al.77 have shown that supercritical
water has the ability to accelerate and make selective synthetic
organic reactions, such as Beckmann rearrangements. Cyclohexane
oxime 102 changed into 3 -caprolactam 103 (Eq. 4) in supercritical
water with rearrangement (Scheme 35). In addition, Ikushima
et al.76 have demonstrated that pinacol (Eq. 5) and Beckmann (Eq.
4) rearrangements can be achieved using supercritical water in the
absence of acid catalysts and showed that the processes are well
suited to ecofriendly industrial applications. 2,3-dimethylbutane2,3-diol 104 transformed into 3,3-dimethylbutan-2-one 105 in
subcritical water (Eq. 5, Scheme 35).
OH
110
SCW
34%
CHO
111
CHO
+
3:2
112
Scheme 38. MeyereSchuster rearrangement of 2-phenyl-3-butyn-2-ol (110) to 3phenyl-2-enaldehydes in high-temperature water.
3.18. Transformation reactions
Koltunov and Abornev82 showed the possibility, in principle,
and prospects of using subcritical and supercritical water for the
selective transformation of tetralones (113, 115) and tetralin into
naphthols (114, 116) and naphthalene without additional reagents
and/or catalysts (Scheme 39).
Scheme 39. Selective transformation of tetralones in subcritical and supercritical
water.
4. Conclusions
Scheme 35. Pinacol and Beckmann rearrangements in supercritical water.
Although White and Wolfarth78 found that the reaction rate of
the Claisen rearrangement was enhanced in polar solvents, Grieco
et al.79,80 first demonstrated the benefits of water for the rearrangement of allyl vinyl ethers at moderate temperatures. Strauss
et al.17 heated allyl phenyl ether 106 in water at 240 C for 10 min
This paper describes applications to several important organic
reactions in subcritical and supercritical water. Important differences in the physicochemical properties of subcritical and supercritical water relative to ambient water are density, dielectric
constant, and autodissociation constant. These changes are due to
the reduction of the extensive hydrogen bonding; specifically,
water loses approximately 55e60% of the hydrogen-bonding network as the temperature is increased from 25 to 300 C.83 These
N. Simsek Kus / Tetrahedron 68 (2012) 949e958
resulting properties correspond to a moderately polar solvent and
are also tunable with changes in pressure. The reduction in dielectric constant enables a greatly enhanced solubility of relatively
non-polar organic species in subcritical and supercritical water,
although at a cost of some reduction in inorganic salt solubility.84,85
Subcritical and supercritical water can be used as a solvent, reactant
and catalyst in organic synthesis because of their unique properties.
The methods outlined here have the potential for abundant application, and it is hoped that this paper will motivate such efforts.
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958
N. Simsek Kus / Tetrahedron 68 (2012) 949e958
Biographical sketch
Nermin Simsek Kus was born on May 19, 1971 in Erzurum, Turkey. She is an Associate
Professor at the Department of Chemistry of Mersin University. She received her M.Sc.
and Ph.D. degrees in synthetic organic chemistry in 1996 and 2001, respectively, from
€ rk University under the supervision of Professor Dr. Metin Balci. Her current reAtatu
search interests are focused on organic reactions in subcritical water, photooxygenation reactions, high-temperature bromination reactions, photobromination reactions
of unsaturated bicyclic systems, and [4þ2] DielseAlder cycloaddition reactions.