Novel synthetic approach in microwave-assisted solid-supported oxidations using 'in situ' generated molecular oxygen

Article abstract of DOI:10.1016/j.tet.2007.02.061

Three different types of oxidation reactions were carried out under microwave (MW) conditions in dry media, with nearly quantitative yield, using 'in situ', yet separately generated molecular oxygen as the reactive gas. The latter is formed by a controlled decomposition of potassium chlorate (220-306 °C) adsorbed on zeolite support, and is used as a reactive oxidizing agent for the solid-supported oxidations. The MW-assisted oxidations include an oxidative decomplexation of (η⁶-arene)Cr(CO)₃ complexes to the corresponding arenes using silica as solid support (100 °C), an oxidation of fluorene to fluorenone induced by KF-alumina support (150 °C), and oxidation of benzyl alcohol to benzaldehyde using a supported ruthenium catalyst (150 °C). This synthetic approach allows to carry out in synchronized manner two different solid-supported reactions (oxygen generation and oxidation) at different temperatures and on different solid supports together in the same sealed system. It was made possible by tuning the absorption efficiency of MWs through accurate selection of the solid supports employed in the reactions. The high feasibility of this novel synthetic approach resulted from a preliminary study of the interaction between MWs and mineral oxides such as alumina, silica, clay, and zeolite particularly when mixed with additives such as water, ionic liquids or graphite (5% w/w). The use of these MW absorber additives allows the MW transparent or poorly absorbing mineral oxides to be efficiently heated to very high temperatures in few minutes.

Full text of DOI:10.1016/j.tet.2007.02.061

Tetrahedron 63 (2007) 3762–3767
Novel synthetic approach in microwave-assisted solid-supported
oxidations using ‘in situ’ generated molecular oxygen
*
Eytan Gershonov, Esther Katz, Yishai Karton and Yossi Zafrani
The Department of Organic Chemistry, Israel Institute for Biological Research, PO Box 19, Ness-Ziona 74100, Israel
Received 22 November 2006; revised 28 January 2007; accepted 15 February 2007
Available online 20 February 2007
Abstract—Three different types of oxidation reactions were carried out under microwave (MW) conditions in dry media, with nearly quan-
titative yield, using ‘in situ’, yet separately generated molecular oxygen as the reactive gas. The latter is formed by a controlled decomposition
of potassium chlorate (220–306 ꢀC) adsorbed on zeolite support, and is used as a reactive oxidizing agent for the solid-supported oxidations.
The MW-assisted oxidations include an oxidative decomplexation of (h6-arene)Cr(CO)3 complexes to the corresponding arenes using silica as
solid support (100 ꢀC), an oxidation of fluorene to fluorenone induced by KF-alumina support (150 ꢀC), and oxidation of benzyl alcohol to
benzaldehyde using a supported ruthenium catalyst (150 ꢀC). This synthetic approach allows to carry out in synchronized manner two dif-
ferent solid-supported reactions (oxygen generation and oxidation) at different temperatures and on different solid supports together in the
same sealed system. It was made possible by tuning the absorption efficiency of MWs through accurate selection of the solid supports
employed in the reactions. The high feasibility of this novel synthetic approach resulted from a preliminary study of the interaction between
MWs and mineral oxides such as alumina, silica, clay, and zeolite particularly when mixed with additives such as water, ionic liquids or graph-
ite (5% w/w). The use of these MWabsorber additives allows the MW transparent or poorly absorbing mineral oxides to be efficiently heated
to very high temperatures in few minutes.
Ó 2007 Elsevier Ltd. All rights reserved.
1. Introduction
media, which involved in situ generation of gaseous carbon
monoxide from solid molybdenum hexacarbonyl (i.e., di-
rectly in the reaction mixture, without the need for external
source).3
The use of microwave (MW) irradiation technique as an
energysourcefororganicsynthesis,1 combinedwithinorganic
solids as catalysts or as reaction media,2 is rapidly growing.
These MW-assisted, solvent-free (dry media) reactions often
involve minimal waste, efficient catalyst recycling, faster re-
action rates, increased yield, and easier set-up and work-up
procedures as compared to the classical synthetic methods.
Alumina, silica, clays, and zeolites are most popular as solid
supports, exhibiting large and distinctive surface area and
catalytic properties.2 However, due to technical limitations
regarding MW heating in conjunction to gaseous reagents
in sealed reaction vessels or under pressure, this approach
seems to be impractical for conventional MW ovens cur-
rently used for synthetic applications. The development of
a new and efficient process, which involves reactive gases
such as molecular oxygen (oxidation), hydrogen (reduction),
carbon dioxide (carboxylation), carbon monoxide (carbon-
ylation), sulfur dioxide (sulfonylation), and so forth, is there-
fore imperative for the progress of MW-assisted organic
synthesis. To the best of our knowledge, there are only
a few examples of reactions assisted by MW, in solution
The aim of this study was to explore the possibility of
a highly controlled MW-assisted synthetic process whereby
two different solid-supported reactions are carried out in one
sealed vessel, so that a reactive gas produced in the first
reaction (quantitatively) is used as a reagent by the second
reaction (an organic transformation). We hypothesized that
the specific interaction between each reaction mixture and
MWs may lead to different temperatures even though the
two reaction tubes are situated together in the same sealed
vessel and exposed to the same irradiation conditions.
2. Results and discussion
In order to prove the feasibility of this approach we chose
three different oxidation reactions as models, in which mo-
lecular oxygen was used as oxidant (Fig. 1). These were
{1} oxidative decomplexation of (h6-arene)Cr(CO)3 com-
plex to the corresponding arene.4 {2} Oxidation of fluorene
to fluorenone using KF-alumina as solid support.5 {3} Oxida-
tion of benzyl alcohol to benzaldehyde using supported
ruthenium catalyst.6 As a source for molecular oxygen,
thermal decomposition of potassium chlorate was chosen,
Keywords: Microwaves; Solid supports; Supported catalysts; Oxidation;
Oxygen.
* Corresponding author. Tel.: +972 893 817 41; fax: +972 893 815 48;
e-mail: zafrani@iibr.gov.il
0040–4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2007.02.061

E. Gershonov et al. / Tetrahedron 63 (2007) 3762–3767
3763
pressure
Models of Oxidation
O2
silica, O2
1) Ar-Cr(CO)3
or
Ar-H
MW, 100 °C
O2
O2
KF-Al2O3, O2
MW, 150 °C
Ru-Al2O3, O2
MW, 150 °C
2) Fluorene
or
Fluorenone
high
low
Temp.
Temp.
3) Benzyl alcohol
Benzaldehyde
solid support
MW, 400 °C
2KClO3
2KCl + 3O2
Figure 1. Schematic presentation for three different oxidation reactions performed by ‘in situ’, yet separately generated, molecular oxygen.
which required high temperatures and delivered fixed
amounts of reactive gas.
possess an appreciable interaction with MWs may pro-
foundly improve MW coupling of the solid supports. These
environmentally benign additives are characterized by high
boiling points, high affinity to the solid support and low re-
activity. Recently, Leadbeter et al. have shown that nonpolar
solvents, such as hexane and toluene, which are transparent
to MWs, can be heated considerably above their boiling
points by MW irradiation using a small quantity of an ionic
liquid as additive.7c One should note that for the same pur-
pose Kappe et al. have recently reported on alternative, non-
invasive approach that utilized cylinders of the MWabsorber
silicon carbide (SiC) as passive heating element in nonpolar
solvents.9
In order to attain the desired temperature and to avoid de-
sorption or decomposition of the adsorbed reactants, the pro-
cess should be controlled by tuning the coupling efficiency
of MWs by the chosen solid support. Hence, we initially in-
vestigated the coupling efficiency of various inorganic solids
with MWs (Table 1).
As expected, when properly dried, the minerals presented in
Table 1 weakly responded to MW irradiation and the maxi-
mum temperatures attained (Tmax) were between 56 and
85 ꢀC. Addition of 5% water (w/w) selectively enhanced
the maximum temperature attained and it was found that
this effect is highly dependent upon the nature of the min-
eral. Namely, only minerals such as KF-alumina, clays,
and zeolites, which effectively prevent water desorption, in-
teracted efficiently with MWs (Table 1, runs 16, 19, 23–25).
Yet, for many organic reactions that involve nonpolar reac-
tants and require high temperature and anhydrous condi-
tions, the idea of using such solid supports as reaction
media seems to be impractical. We therefore hypothesized
that additives such as ionic liquids7 and graphite,8 which
Inspection of the data presented in Table 1 reveals that
ionic liquids (5% w/w) such as 1-ethyl-3-methylimidazo-
lium tetrafluoroborate ([emim]BF4) dramatically improved
the interaction between MWs and the mineral oxides such
as silica (194 ꢀC, run 3), neutral and basic alumina (183
and 200 ꢀC, runs 7 and 11, respectively), and montmorillo-
nite K-10 (238 ꢀC, run 20). Interestingly, when graphite
(5% w/w) and inorganic solids such as alumina or silica
were mixed and exposed to MW irradiation, the thermal
behavior was utterly dependent upon the nature of the solid.
Table 1. The influence of water, ionic liquids, and graphite as additives, on the interaction between microwaves and mineral solids, when subjected to a rigorous
method consisted of 500 W limit and 220 ꢀC preselected maximum temperature for 4 min
b
b
Run
Solid used
Additivea
Tmax [ꢀC]
Run
Solid used
Additivea
Tmax [ꢀC]
1
2
3
4
5
6
7
8
SiO2
SiO2
SiO2
SiO2
Neutral-Al2O3
Neutral-Al2O3
Neutral-Al2O3
Neutral-Al2O3
Basic-Al2O3
Basic-Al2O3
Basic-Al2O3
Acidic-Al2O3
Acidic-Al2O3
None
H2O
[emim]BF4
Graphite
None
85
146
194
108
59
14
15
16
17
18
19
20
21
22
23
24
25
Acidic-Al2O3
KF-Al2O3
KF-Al2O3
KF-Al2O3
mont. K-10
mont. K-10
mont. K-10
mont. K-10
mont. KSF
mont. KSF
NH4-Y
Graphite
None
H2O
Graphite
None
H2O
[emim]BF4
Graphite
None
230e
56
218
238e
60
195
238e
105
60
190
248d,e
230d,e
H2O
88
[emim]BF4
Graphite
None
183
232e
64
107
200
60
9
10
11
12
13
H2O
H2O
[emim]BF4
None
H2O
‘Dry’c
‘Dry’c
Na-Y
186
a
Additive (5% w/w) was mixed with the appropriate inorganic solid.
Although the temperature in the irradiated solids is not uniform by definition, and its measurement is an average, the reproducibility of the measurements
b
c
(n¼3) was acceptable with ca. ꢁ5 ꢀC.
This zeolite was located in internal vessel and periodically irradiated by MW to 300 ꢀC for 2 min until no water drops were observed in the external sealed
vessel.
Temperature attained within 2.5 min.
In these cases, the material showed an excellent response to microwave irradiation and the Tmax attained was higher then the preselected 220 ꢀC due to
microwave instrument’s limitation.
d
e

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E. Gershonov et al. / Tetrahedron 63 (2007) 3762–3767
2.0 min / 500 W
Namely, with the alumina-type solids neutral-, acidic-, and
KF-alumina, a considerable increase in Tmax was observed
(232, 230, and 238 ꢀC, runs 8, 14, and 17, respectively),
while mixing graphite with silica or even with montmorillo-
nite K-10 revealed only a modest effect on Tmax (108 and
105 ꢀC, runs 4 and 21, respectively).
KClO / Zeolite
3
320
300
280
260
240
220
200
180
160
140
120
100
80
ArCr(CO) / Silica
3
7
6
5
4
3
2
1
0
O
Pressure
2
The obtained thermal data, with data on physical properties
(such as acidity/basicity, surface area, and structure) of the
above-mentioned mineral oxides, can provide an improved
tool for designing highly controlled synthetic processes
under solvent-free (dry media) MW conditions.
60
40
20
0
0
2
4
Time (min)
6
Based on the significant differences in the ability of mineral
oxides to interact with MWs as shown in Table 1, we pro-
ceeded to test the above-mentioned oxidation model reac-
tions using generation of molecular oxygen by thermal
decomposition of KClO3. Thus, an externally sealed vessel
was equipped with two separate, open, insert vials (each
coated with Teflon shawl), and the vessel was saturated
with argon gas. The first vial contained a mixture of potas-
sium chlorate (25% w/w) and ammonium-Y zeolite (dried
as mentioned above). The second vial contained tetrahydro-
naphthalene chromium tricarbonyl complex (1) adsorbed on
silica (10% w/w) as shown in Figure 1. The system, which
related to oxidation model {1}, was exposed to MW irradi-
ation at 500 W for 2 min, producing a pressure of 4 atm, in-
dicating molecular oxygen formation. After cooling to room
temperature, the pressure was carefully released and the
external vessel was opened. Simple extraction of the arene
product from the green silica mixture (indicating the pres-
ence of Cr2O3) by chloroform led to 97% yield of the product
9,10-tetrahydronaphthalene (2) (Scheme 1). The instanta-
neous liberation of O2 has occurred at a temperature between
220 and 306 ꢀC, which is far below the known decomposi-
tion temperature of potassium chlorate (ca. 400 ꢀC in the
absence of MnO2 catalyst), as shown in Figure 2. This effect
is assumed to be due to localized super-heating of the solid.
The interaction between neat KClO3 and MWs was also
tested and found to be very weak, so that under the same
MW program only a temperature of 31 ꢀC was attained.
Figure 2. Pressure and temperature profile for oxidative decomplexation of
arene chromium tricarbonyl.
the brown KF-Al2O3 mixture by chloroform led to a mixture
containing 98% fluorenone (4) and 2% bifluorenylidene (5),
as determined by 1H NMR (Table 2, run 1).
In the absence of KClO3, i.e., without oxygen release and
under air atmosphere (1 atm), the same procedure led to
the formation of a mixture of 36% fluorenone (4), 2% bi-
fluorenylidene (5) and 25% terfluorenyl (6) together with un-
reacted fluorene (3) (Table 2, run 2). The selectivity obtained
by our system strongly emphasizes the necessity of KClO3
as an oxygen source in this method.
Addition of emim[BF4] (5 wt %) to the reaction mixture de-
creased the reaction time to 4 min but yielded a mixture of
the products fluorenone (4), bifluorenylidene (5), and ter-
fluorenyl (6) in 89, 4 and 7% yield, respectively (Table 2,
run 3). However, the use of graphite as an additive not only
decreased the reaction time to 5 min but also exclusively
led to 99% fluorenone (4) (Table 2, run 4). When the reaction
mixture was irradiated for equivalent length of time (4 min)
in the absence of additives, the Tmax was only 90 ꢀC and the
yield of fluorenone product (4) was 72% (Table 2, run 7).
Due to a technical limitation regarding stirring of the reac-
tion mixture, this method was found to be restricted to a
quantity of less than 2 g in the model reaction {2}. At this
quantity, the thermal behavior of the solid supports does
not follow exactly the thermal profiles exhibited in Table 1.
Thus, increasing the loading quantity of fluorene (3) caused
a rise of the maximum temperature attained (220 ꢀC), but
decreased the yield of the reaction even after long periods
of irradiation (Table 2, runs 5 and 6). We assume that despite
the relatively high pressure of the oxygen it cannot easily
penetrate through the bulk of the KF-Al2O3 support (ca.
2 cm height in 8 mL vial) without stirring, as also visually
observed by the color gradient formed by the yellow fluore-
none product (4) along the column of the reaction mixture.
However, we believe that enlarging the external vessel would
allow situation of the solid support in such a way that will
allow efficient stirring of the reaction mixture and will
facilitate the penetration of oxygen. As a consequence, this
method may be adaptable to larger quantities.
zeolite
KClO3
MW, 2 min
220-306 °C
silica, O2
(OC)3Cr
MW, 2 min, 100 °C
2 (97 %)
1
Scheme 1. Microwave-assisted oxidative decomplexation of (h6-arene)-
Cr(CO)3 complex 1 to the corresponding arene 2.
Encouraged by these results, we proceeded to test the second
and more complicated model {2}. Namely, the oxidation of
fluorene (3) to fluorenone (4) catalyzed by the basic solid
KF-Al2O3. Here again, the first vial contained a mixture of
potassium chlorate (25% w/w) and ammonium-Y zeolite
(dried as mentioned above) and the second vial contained
fluorene (3) adsorbed on KF-Al2O3 (10% w/w). The system
was exposed to MW irradiation at 500 W and the tempera-
ture was limited to 150 ꢀC. After 12 min of irradiation, this
temperature was reached and a pressure of 4 atm (after ca.
2 min) has evolved. Simple extraction of the product from
A plausible mechanism for the formation of bifluorenylidene
(5) and terfluorenyl (6) is shown in Scheme 2. The reaction is

E. Gershonov et al. / Tetrahedron 63 (2007) 3762–3767
3765
Table 2. Results of the oxidation of fluorene to fluorenone by KF-Al2O3 and ‘in situ’ generated molecular oxygen
zeolite
KClO3
MW, 2 min
O
H
H
KF-Al2O3, O2
+
+
MW, 150 °C
3
4
5
6
Run
Additive
Tmax (ꢀC)
Timec (min)
Product distribution (%)
4
5
6
1
2
3
4
5
6
7
None
150
150
150
150
220
220
90
12
12
4
5
4
98 (93)d
36
89
99
38
2
2
4
—
7
14
—
—
25
7
—
7
Nonea
emim[BF4)
Graphite
Graphiteb
Graphiteb
None
12
4
46
72
12
—
a
Under atmospheric pressure (without KClO3).
Fluorene (0.2 g) adsorbed on 1.7 g KF-Al2O3 and 0.1 g graphite.
Total irradiation time.
Isolated yield.
b
c
d
initiated by an oxidation step to yield fluorenone (4) via a flu-
orenyl anion intermediate 7.5 The generated fluorenone (4)
undergoes a nucleophilic addition by fluorenyl anion 7 at
the carbonyl carbon followed by protonation to form fluor-
enol 8, which then leads to the observed bifluorenylidene
(5) by elimination of a water molecule catalyzed by the basic
KF-Al2O3 support (Scheme 2). The latter undergoes a further
nucleophilic addition by 7 followed by protonation that leads
to the formation of terfluorenyl (6).10 Thus, it is not sur-
prising that under air atmosphere (i.e., deficiency and low
pressure of molecular oxygen), as in the control reaction
mentioned above, the terfluorenyl product (6) was one of
the two major products with 25% yield (Table 2, run 2).
temperature was limited to 150 ꢀC (Table 3). Again, a pres-
sure of 4 atm after ca. 2 min has evolved. Simple extraction
of the product from the Ru/Al2O3 mixture by chloroform
Table 3. Results of the oxidation of benzyl alcohol to benzaldehyde cata-
lyzed by Ru/Al2O3 with ‘in situ’ generated molecular oxygen
zeolite
KClO3
MW, 2 min
O
Ru-Al2O3, O2
OH
MW, 150 °C
9
10
Run
Additive
Timea (min)
Benzaldehyde (10) (%)
Next, the approach was successfully demonstrated on the ox-
idation of benzyl alcohol to benzaldehyde, catalyzed by Ru/
Al2O3 according to model {3}.6 The first vial contained the
mixture of potassium chlorate and ammonium-Y zeolite as
described above and the second vial contained benzyl alco-
hol (9) adsorbed on Ru/Al2O3 (10% w/w). The system was
exposed to MW irradiation at 500 W for 20 min and the
1
2
3
4
None
None
10
20
20
20
60
88
8
Noneb
Graphite
95
a
Total irradiation time.
Under atmosphere pressure (without KClO3).
b
H
H
H2O
KF-Al2O3, O2
MW, 150 °C
3
6
H
KF-Al2O3
O
H
HO
KF-Al2O3
H
H2O
4
7
8
5
Scheme 2. A plausible mechanism for the formation of bifluorenylidene (5) and terfluorenyl (6) in MW-assisted oxidation of fluorene to fluorenone.

3766
E. Gershonov et al. / Tetrahedron 63 (2007) 3762–3767
gave the benzaldehyde product (10) in 88% yield, as deter-
mined by 1H NMR spectroscopy (Table 3, run 2). Irradiation
of the reaction mixture for 10 min yielded only 60% of benz-
aldehyde (10) (Table 3, run 1). Under air atmosphere (with-
out KClO3), the same procedure led to the formation of only
8% benzaldehyde (10) together with unreacted benzyl alco-
hol (9) (Table 3, run 3), emphasizing the necessity of KClO3
as an oxygen source in this system. Addition of graphite
(5 wt %) to the reaction mixture improved the yield of benz-
aldehyde product (10) (95%, Table 3, run 4).
method consisting of 500 W limit and 220 ꢀC preselected
maximum temperature for 4 min, in which the temperature,
the pressure, and the power source could be efficiently mon-
itored and controlled. All the experiments were performed in
sealed systems (PRO-24 Rotor), equipped with Pyrex insert
vessel, containing 8.0 g of the tested inorganic solid.
In cases, where an additive was used, we simply mixed
and crushed 5% (w/w) of the appropriate additive with the
mineral support.
4.2.2. Oxidations. An externally sealed vessel (HPR-1000/
10s Rotor) was equipped with two separate, open insert vials
(each coated with Teflon shawl). The first vial contained
a mixture of potassium chlorate (1.0 g, 25% w/w) and
ammonium-Y zeolite (dried as mentioned above). The sec-
ond vial contained the organic substance to be oxidized
(200 mg of 1 or 100 mg of 3 or 100 mg of 9) adsorbed on
the appropriate solid support (silica or KF-Al2O3 or Ru-
Al2O3, respectively, 10% w/w). The system was exposed
to MW irradiation at 500 W for the required time, producing
a pressure of 4–5 atm, due to molecular oxygen generation,
as illustrated in Figure 1. After cooling to room temperature,
the pressure was carefully released and the external vessel
was opened. Oxidation products (2 or 4 or 10, respectively)
were successfully extracted from the appropriate solid
support by CDCl3 (2ꢂ1 mL).
Based on 1H NMR, 13C NMR, and GC–MS data, the prod-
ucts were obtained in a relatively high degree of purity
without any additional treatment. Bifluorenylidene (5) and
terfluorenyl (6) were isolated by flash chromatography uti-
lized silica gel 60 (hexane/ethyl acetate gradient elution:
100–80% of hexane in 20 min) and identified by comparing
1H and 13C NMR and GC–MS spectra with the reported
data.11
3. Conclusion
In summary, the process described herein was made possible
by the pronounced differences between the thermal behavior
of the mineral oxides such as alumina, silica, clays, and
zeolites induced by MW irradiation (t¼4 min, 500 W,
Tmax¼56–248 ꢀC). We have shown that the temperature of
the above-mentioned inorganic solids could be carefully
tuned under MW irradiation, a feature that enabled us
to design and develop a novel synthetic approach to the
MW-assisted solid-supported oxidations with ‘in situ’, yet
separately generated, molecular oxygen. These oxidations
included an oxidative decomplexation of (h6-arene)Cr(CO)3
complexes to the corresponding arenes using silica as solid
support (100 ꢀC), an oxidation of fluorene to fluorenone
induced by KF-alumina support (150 ꢀC), and oxidation of
benzyl alcohol to benzaldehyde using supported ruthenium
catalyst (150 ꢀC). This approach consists of a unique process
in which two different reactions in different conditions are
performed in separated vessels positioned in the same sealed
system, using a single cooperative energy source. This pro-
cess that also benefits from high yield, easy set-up and work-
up procedures, and fast reaction rate may be applied to
various organic reactions involving gas or volatile reactants,
which can be ‘in situ’ generated in the sealed system under
MW conditions.
References and notes
4. Experimental
1. For general references on microwave synthesis see: (a) Kappe,
C. O.; Stadler, A. Microwave in Organic and Medicinal
Chemistry, 1st ed.; Wiley-VCH: Weinheim, 2005; (b) Loupy,
A. Microwave in Organic Synthesis, 1st ed.; Wiley-VCH:
Weinheim, 2002; (c) Kappe, C. O. Angew. Chem., Int. Ed.
2004, 43, 6250; (d) Caddick, S. Tetrahedron 1995, 51, 10403;
(e) Lidstrom, P.; Tierney, J.; Wathey, B.; Westman, J.
Tetrahedron 2001, 57, 9225.
2. (a) Loupy, A.; Petit, A.; Hamelin, J.; Texier-Boullet, F.;
Jacquault, P.; Mathe, D. Synthesis 1998, 1213; (b) Varma,
R. S. Green Chem. 1999, 43; (c) Kidwai, M. Pure Appl.
Chem. 2001, 73, 147; (d) Varma, R. S. Tetrahedron 2002, 58,
1235; (e) Pillai, U. R.; Sahle-Demessie, E.; Varma, R. S.
J. Mater. Chem. 2002, 12, 3199.
3. (a) Kaiser, N. F. K.; Hallberg, A.; Larhed, M. J. Comb. Chem.
2002, 4, 109; (b) Wannberg, J.; Larhed, M. J. Org. Chem. 2003,
68, 5750; (c) Xiongyu, W.; Larhed, M. Org. Lett. 2005, 7, 3327.
4. (a) Pape, A. R.; Kaliappan, K. P.; Kundig, E. P. Chem. Rev.
2000, 100, 2917; (b) Fomin, V. M.; Lunin, A. V. Zh. Obshch.
Khim. 1996, 66, 832.
4.1. General remarks
1H NMR (300 MHz) and 13C NMR (75.5 MHz) spectra of
CDCl3 solutions were recorded on Bruker Avance spectro-
meter. Mass Spectra were recorded on Varian Saturn 2200
instrument at 70 eV. Reactions were performed in a labora-
tory microwave oven (Ethos SYNTH MW, Milestone Inc.,
Italy). Mineral oxides were purchased from Aldrich and
dried prior to use at 150 ꢀC for 20 h. Zeolites (NH4-Y and
Na-Y) were dried under MW heating to 300 ꢀC for 2 min
(2–4 times) until no water drops were observed in the exter-
nal vessel. All other solvents and reagents were obtained
from Aldrich or Fluka and used without further purification.
Column chromatography was performed with Merck silica
gel 60 (230–400 mesh) and TLC was run on precoated
Merck silica gel plates 60 F 254 (2.0 mm).
4.2. General procedures
5. (a) Fraile, J. M.; Garcia, J. I.; Mayoral, J. A. Catal. Today 2000,
57, 3; (b) Villemin, D.; Ricard, M. React. Kinet. Catal. Lett.
1994, 52, 255.
4.2.1. Determination of microwave absorbance of min-
eral oxides. Mineral oxides were subjected to MW heating

E. Gershonov et al. / Tetrahedron 63 (2007) 3762–3767
3767
6. (a) Yamaguchi, K.; Mizuno, N. Angew. Chem., Int. Ed. 2002,
41, 4538; (b) Mallat, T.; Baiker, A. Chem. Rev. 2004,
101, 3037.
C. O. J. Org. Chem. 2002, 67, 7904; (g) Garbacia, S.; Desai,
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