Ultrasonic irradiation and the homo- and hetero-Diels-Alder reaction

Article abstract of DOI:10.1081/SCC-100105131

The effects of ultrasound on the hetero- and homo-Diels-Alder reactions have been investigated varying ultrasonic source and solvent. Irradiation enhances both reactions minimally; however, while this appears to be due to cavitation in the homo Diels-Alde

Full text of DOI:10.1081/SCC-100105131

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ULTRASONIC IRRADIATION AND THE HOMO- AND
HETERO-DIELS-ALDER REACTION
Raul Alfaro a & John V. McClusky b
a Division of Earth and Physical Sciences , The University of Texas at San Antonio , 6900
N. Loop 1604 West, San Antonio, TX, 78249, U.S.A.
b Division of Earth and Physical Sciences , The University of Texas at San Antonio , 6900
N. Loop 1604 West, San Antonio, TX, 78249, U.S.A.
Published online: 09 Nov 2006.
To cite this article: Raul Alfaro & John V. McClusky (2001) ULTRASONIC IRRADIATION AND THE HOMO- AND HETERO-DIELS-
ALDER REACTION, Synthetic Communications: An International Journal for Rapid Communication of Synthetic Organic
Chemistry, 31:16, 2513-2522, DOI: 10.1081/SCC-100105131
To link to this article: http://dx.doi.org/10.1081/SCC-100105131
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SYNTHETIC COMMUNICATIONS, 31(16), 2513–2522 (2001)
ULTRASONIC IRRADIATION
AND THE HOMO- AND
HETERO-DIELS-ALDER REACTION
Raul Alfaro and John V. McClusky*
Division of Earth and Physical Sciences,
6900 N. Loop 1604 West,
The University of Texas at San Antonio,
San Antonio, TX 78249
ABSTRACT
The effects of ultrasound on the hetero- and homo-Diels-
Alder reactions have been investigated varying ultrasonic
source and solvent. Irradiation enhances both reactions mini-
mally; however, while this appears to be due to cavitation in
the homo Diels-Alder reaction, mechanical mixing appears to
be the cause in the hetero-Diels-Alder reaction.
The Diels-Alder reaction is quite valuable in organic synthesis; how-
ever, it often requires high temperatures, high pressures, and/or Lewis acid
catalysis. The extremely high temperatures and pressures which are pro-
duced during ultrasonic cavitation1 seem ideal for promoting this reaction.
Therefore there has been significant interest in using ultrasonic irradiation
to speed this reaction, since rate enhancement should be possible whether
*Corresponding author.
2513
Copyright & 2001 by Marcel Dekker, Inc.
www.dekker.com

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ALFARO AND McCLUSKY
the reaction occurs in the collapsing bubble itself or in the immediate solvent
shell surrounding the bubble.
Alternative mechanisms include single electron transfer and other radi-
cal processes (via the electric field theory of cavitation),2–4 acid catalysis
formed from sonochemical decomposition of halogenated solvents,5 and
disruption of solvation.6 Nonetheless there are only a handful of instances
where ultrasound has increased the rate of Diels-Alder reactions, and most
of those rate enhancements have been minimal.7
The energy released during ultrasonic cavitation is related to the sol-
vent in which the cavitation is occurring:
ln k ¼ ln A À EPv=ðRToPaðꢀ À 1ÞÞ
ð1Þ
where k is the reaction rate, A and E are the Arrhenius pre-exponential and
activation parameters, Pv is the solvent vapor pressure, Pa is the acoustic
pressure at initiation of bubble collapse, R is the gas constant, g is the ratio
of specific heats, and To is the ambient temperature.6 It can be seen that
increasing solvent vapor pressure increases the energy released during cavi-
tation, although there is an upper limit at which the solvent cannot be
vaporized into the bubble and cavitation is shut off.8 Alternatively, energy
is stored in the developing bubble by vaporization of solvent; thus, the heat
of vaporization of the solvent may be important. Therefore if the rate
enhancements in Diels-Alder reactions are directly due to the ultrasonic
cavitation, the solvent should play an important role. In an effort to inves-
tigate these mechanisms, the role of solvent was investigated in the homo
Diels-Alder reaction using both an ultrasonic probe and an ultrasonic clear-
ing bath. In a related study, a hetero Diels-Alder reaction has been explored
using the same sources.
EXPERIMENTAL
Materials
1,4-naphthaquinone was purchased from Aldrich Chemical Co. with
95% purity, and the container was wrapped with aluminum foil to avoid
exposure to light. 1,4-cyclohexadiene was purchased from Acros Chemicals
with 96% purity; 1,4-napthoquinone and 1,4-cyclohexadiene were used as
received. m-Anisaldehyde (97%), p-anisaldehyde (98%), and mesitylene
(97%) were purchased from Aldrich Chemical Company. All three reagents
were distilled from NaH prior to use and stored under N2. Reagent grade
ZnCl2 was purchased from Fisher Scientific, and was further purified via high
temperature dehydration and recrystallized from dry THF under N2
atmosphere.

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2515
Instrumentation
The ultrasonic probe used in these experiments was a Cole-Parmer
(500 Watt/20kHz) ultrasonic dismembrator model 501. The unit was
fitted with a 1/8-inch diameter tapered microtip horn. The ultrasonic clean-
ing bath was a Fischer Scientific (270 Watts/43 kHz) model FS28. The hold-
ing chamber had dimensions of 28.9 (1) ꢀ 23.8 (w) ꢀ 15.2 cm (h), and a
volume capacity of 10.4 liters. Qualitative and quantitative analyses
were achieved with the use of a General Electric QE-300 Nuclear Magnetic
Resonance (NMR) spectrometer. The acquisition time was 1.6 seconds
and the delay time was set at 2 seconds. All reactions were carried out in
triplicate.
Homo Diels-Alder Reactions
Endo-Tetracyclo[10.2.2.02,11.04.9]hexadeca-4,6,8,13-tetraene-3,10-dione.
Ultrasonic Probe. 1,4-Naphthoquinone (63 mg, 0.38 mmol), 1,4-cyclo-
hexadiene (150 ml, 1.51 mmol), mesitylene (10 ml, 0.07 mmol, internal stan-
dard), and n-butanol (2.45 ml) were introduced into a rossette sonoreactor
vessel, and capped with a rubber septum through which the probe was
placed into the reaction solution. The vessel was purged with N2, immersed
in a 40ꢁC bath, and sonicated with the ultrasonic probe at approximately
110 W for six hours. Samples were taken periodically, dissolved in chloro-
1
form-d, and analyzed by H NMR.
Due to solubility limits of the napthoquinone, not all reactions were
run at the same concentration. The relative amounts of diene, dienophile,
and solvent were adjusted to give a total reaction volume of between 2.4 and
2.6 ml. Amounts for each reaction are given in Table 1.
Ultrasonic Bath. Reaction samples were prepared similarly to those
used with the ultrasonic probe; reagent amounts are given in Table 2. The
reaction was placed in either a 2-ml thick-walled conical vial (Aldrich) or a
standard 3-dram flat-bottom glass sample vial (Fischer Scientific). Vials
were purged with N2 and sealed with caps containing Teflon inserts. Vials
were suspended in the ultrasonic bath by a clamp and placed in a localized
‘‘hot spots’’ as indicated by the turbulent surface. The ultrasonic bath was
run for several hours prior to the reaction to allow thermal equilibration to
40ꢁC. Reaction solutions were sonicated for six hours, and samples were
taken periodically for analysis by 1H NMR. No differences were observed in
the reaction rates between the two types of vials.
Silent reaction. Reaction samples were prepared identically to those
used with the ultrasonic bath, Table 2. The reaction was placed in a standard

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ALFARO AND McCLUSKY
Table 1. Reagent Amounts for Homo Diels-Alder Reaction and Ultrasonic Probe
(Values in g)
1,3-Cyclo- 1,4-Naphtho-
quinone
hexadiene
Mesitylene n-Butanol Glyme Diglyme Triglyme Toluene Ethanol
0.13
0.34
0.55
0.25
0.42
0.21
0.38
0.34
0.55
0.17
0.084
0.063
0.17
0.27
0.13
0.22
0.10
0.19
0.17
0.27
0.084
0.042
0.013
0.035
0.056
0.026
0.043
0.022
0.039
0.035
0.056
0.017
0.0087
1.98
1.83
1.49
2.05
1.71
2.27
2.04
1.87
1.52
2.44
1.83
Table 2. Reagent Amounts for Homo Diels-Alder Reaction for Experiments in the
Ultrasonic Bath or Silent (Values in g)
1,3-Cyclo- 1,4-Naphtho-
hexadiene
quinone
Mesitylene n-Butanol Glyme Diglyme Triglyme Toluene Ethanol
0.067
0.17
0.25
0.13
0.25
0.13
0.17
0.17
0.25
0.084
0.055
0.033
0.084
0.13
0.085
0.017
0.026
0.013
0.026
0.013
0.017
0.017
0.026
0.0087
0.0087
1.05
0.92
0.69
0.063
0.13
1.02
1.02
0.063
0.084
0.084
0.13
1.36
0.91
0.94
0.71
0.042
0.027
1.22
1.19
3-dram flat-bottom glass sample vial with a Teflon coated ‘‘flea’’ stir bar,
purged with N2 and sealed with caps containing Teflon inserts. The reac-
tions were placed in a 40ꢁC bath and stirred gently. Samples were taken
1
periodically and analyzed by H NMR.
Data Analysis. Product and starting material concentrations were
determined by integration of the alkene resonances with respect to the inter-
nal standard mesitylene. Second order reaction rates were corrected for loss
of 1,3-cyclohexadiene by evaporation during the reaction.

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2517
Hetero Diels-Alder Reactions
2-(4-Methoxyphenyl)-2,3-dihydro-4H-pyran-4-one.
Ultrasonic Probe. Danishefsky’s diene (100 ml, 0.462 mmol), p-anisal-
dehyde (10.1 ml, 0.154 mmol), 0.85 ml of a solution of 0.200 M ZnCl2 in
THF, and mesitylene (10 ml, 0.70 mmol, internal standard) were introduced
into a rossette sonoreactor vessel, capped with a rubber septum, purged with
dry N2, placed in a 40ꢁC bath, and irradiated with approximately 110 W of
power using the ultrasonic probe. Aliquots for NMR analysis were taken
prior to irradiation and at least five times during the reaction. No side
products were noted by NMR.
Other attempted hetero-Diels Alder reactions were completed simi-
larly using identical molar amounts of the diene and aldehyde. Reactions
were followed for a minimum of 24 hours.
Ultrasonic Bath. Reaction samples were prepared identically to
those used with the ultrasonic probe. The reaction was placed in either
a 2 ml thick-walled conical vial (Aldrich) or a standard 3-dram flat-
bottom glass sample vial (Fischer Scientific). Vials were purged with N2
and sealed with caps containing Teflon inserts. Vials were suspended in
the ultrasonic bath by a clamp and placed in a localized ‘‘hot spots’’ as
indicated by the turbulent surface. The ultrasonic bath was run for several
hours prior to the reaction to allow thermal equilibration to 40ꢁC. Reaction
solutions were sonicated for six hours using a 43 kHz/270 W ultrasonic
bath (U.B.). Samples were taken periodically and analyzed by
1H NMR. No differences were observed between the reaction rates in the
two vial types.
Silent Reaction. Reaction samples were prepared identically to those
used with the ultrasonic probe. The reaction was placed in a standard
3-dram flat-bottom glass sample vial with a Teflon coated ‘‘flea’’ stir bar,
purged with N2 and sealed with caps containing Teflon inserts. The reac-
tions were placed in a 40ꢁC bath and stirred gently. Samples were taken
1
periodically and analyzed by H NMR.
RESULTS AND DISCUSSION
Homo Diels-Alder Reaction
The reaction of cyclohexadiene with napthoquinone has been examined
in several solvents and with either an ultrasonic probe or in an ultrasonic
bath, Equation 2. In all cases the reaction occurred faster during ultrasonic
irradiation, and the ultrasonic probe produced higher rates than the

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2518
ALFARO AND McCLUSKY
ultrasonic bath, Table 3. This follows the expected effect of irradiation power,
although frequency effects cannot be eliminated (however higher
frequencies typically decrease cavitation efficiency and the resultant reaction
rate). Kinetic experiments examining temperature effects demonstrate that
differences in temperature are unable to explain the observed rate differences.
The mechanism of the rate enhancement is not clear. Certainly acid
catalysis can be ruled out as there were no halogenated reagents present in
the reaction. Single electron transfer (SET) and free radical processes have
also been invoked; especially in cases in which quinones are involved. Thus
Table 3. Effect of Solvent on the Homo-Diels-Alder Reaction*
Solvent Vapor
Pressure, Torr
Rate
(Mꢂ1 secꢂ1) 40ꢁC
Relative
Rate
Solvent
Medium
Ethanol
Ethanol
Ethanol
Butanol
Butanol
Butanol
Glyme
Probe
Bath
Silent
Probe
Bath
Silent
Probe
Bath
Silent
Probe
Bath
Silent
Probe
Bath
Silent
Probe
Bath
240
240
240
23
23
23
160
160
160
5.1
5.1
5.1
0.66
0.66
0.66
59
8.4þ/ꢂ0.1 ꢀ 10ꢂ5
4.5þ/ꢂ0.2 ꢀ 10ꢂ5
4.2þ/ꢂ0.1 ꢀ 10ꢂ5
8.5þ/ꢂ0.2 ꢀ 10ꢂ5
3.2þ/ꢂ0.2 ꢀ 10ꢂ5
2.1þ/ꢂ0.1 ꢀ 10ꢂ5
8.1þ/ꢂ0.1 ꢀ 10ꢂ6
4.8þ/ꢂ0.4 ꢀ 10ꢂ6
3.9þ/ꢂ0.1 ꢀ 10ꢂ6
1.0þ/ꢂ0.1 ꢀ 10ꢂ5
6.7þ/ꢂ0.1 ꢀ 10ꢂ6
5.5þ/ꢂ0.1 ꢀ 10ꢂ6
1.5þ/ꢂ0.1 ꢀ 10ꢂ5
8.4þ/ꢂ0.1 ꢀ 10ꢂ6
6.3þ/ꢂ0.1 ꢀ 10ꢂ6
1.1þ/ꢂ0.1 ꢀ 10ꢂ5
6.0þ/ꢂ0.1 ꢀ 10ꢂ6
6.0þ/ꢂ0.1 ꢀ 10ꢂ6
2.0
1.1
1.0
4.0
1.5
1.0
2.0
1.2
1.0
1.8
1.2
1.0
2.4
1.3
1.0
1.8
1.0
1.0
Glyme
Glyme
Diglyme
Diglyme
Diglyme
Triglyme
Triglyme
Triglyme
Toluene
Toluene
Toluene
59
59
Silent
*Relative rates are given with respect to the silent reaction in that solvent.

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2519
in the reaction studied, the napthoquinone could be reduced to the radical
anion, or cyclohexadiene could be oxidized to its radical cation by the
radiomimetic effect.4 Reaction then is envisioned to occur with the radical
ions; these reactions are well known to occur with much lower activation
energies. Alternatively, a sonication enhanced redox process could take
place between the two reagents. These possibilities cannot be ruled out,
although the fact that these reactions proceed even without ultrasound
and the relatively small rate enhancement suggest that a shift in reaction
mechanism is not likely.
Equation 1 predicts a linear relationship between ln(k) and solvent
vapor pressure; this relationship was not observed either with the bath
or the probe. In all cases there was little effect of rate by the solvent
choice, although n-butanol exhibited higher rate increases than other
solvents with similar heats of vaporization or vapor pressures. Lorimer,
Mason, and coworkers have also observed little correlation between rate
enhancements and solvent vapor pressures in the solvolysis of tert-butyl
chloride in ethanol-water solutions.6 They have interpreted their results in
terms of disruption of the solvent structure by the sonocation, thereby
altering the reagent solvation and thus the reaction rate. This explanation
could explain the comparatively large rate enhancement observed for the
reaction in n-butanol which can intermolecularly hydrogen bond; however,
the low rate enhancements for ethanol, toluene, and the glyme series indi-
cate even if solvent structure does play a role, other factors are equally
important.
The low correlation between reaction rate and solvent vapor pressure
suggests that cavitation is not the direct cause. Indeed, Tuulmets has
estimated that rate enhancements arising directly from cavitation should be
approximately 1.0003. This value is derived from Suslick’s estimation of the
collapsing bubble’s size and lifetime, typical activation parameters for the
Diels Alder reaction, and Kumar’s kinetic estimate of 1010 bubbles undergo
cavitation per second per liter in water. For a direct acceleration of 2.0,
he estimates that approximately 1015 collapsing bubbles per second per
liter are needed, a value he considers too high to be consistent with the
observed data.3
Equally important, reactions between 1-methoxy-1,3-cyclohexadiene
and 2-cyclohexeneone, diethyl-maleate, and ethyl crotonate; between
1,3-cyclohexadiene and 2-cyclohexeneone; and between 1,3-cyclooctadiene
and crotononitrile, maleic anhydride, and napthoquinone were all unsuc-
cessful both as silent reactions and with the ultrasonic bath or probe.
Thus any temperature, electron transfer, or radical process which may be
initiated by ultrasonic irradiation must have only a minimal effect on these
reactions.

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ALFARO AND McCLUSKY
Hetero Diels-Alder Reaction
Hetero Diels-Alder reactions are known to occur through a different
mechanism than the homo Diels-Alder reactions; the systems studied here
involve a stepwise ring closure initiated by zinc-catalyzed nucleophilic attack
on the carbonyl by the diene. Gould and coworkers have reported that the
reaction of several substituted benzaldehydes and activated dienes in the
presence of ZnCl2 catalyst occurred only in the presence of ultrasound.9
This appeared to be an excellent series for studying the role ultrasound
plays on catalyzed hetero Diels-Alder reactions; thus, these reactions have
been examined in detail under both an ultrasonic probe and ultrasonic
cleaning bath. Unfortunately the role of solvent could not be investigated
since the ZnCl2 is insoluble in most other organic solvents. The reaction
between p-anisaldehyde and Danishefsky’s diene was completed in THF
with anhydrous zinc chloride as catalyst, Table 4, Equation 3. No reaction
occurred without the zinc chloride, either with and without sonication. It is
reported in the literature that hetero-Diels Alder reactions between several
substituted benzaldehydes and 2-triethylsilyl-1,3-pentadiene occurred only
in the presence of ultrasonic irradiation.9 Thus we have investigated several
related hetero-Diels-Alder reactions to see if similar reactivity was evi-
denced. The reaction between Danishefsky’s diene and para-anisaldehyde
proceeded roughly two times faster in the presence of ultrasonic irradiation.
However, reactions between meta-anisaldehyde and Danishefsky’s diene,
4-(dimethylamino)benzaldehyde and Danishefsky’s diene, and the para
or meta-anisaldehyde and 1-trimethylsilyl-1,3-butadiene were all unreactive,
even with increased concentrations of catalyst and higher temperatures.
Table 4. Rate of Hetero-Diels-Alder Reactions
Rate
(Mꢂ1 secꢂ1
Relative Rate
(Silent ¼ 1.0)
Medium
)
Silent
Bath
Probe
7.7þ/ꢂ0.2 ꢀ 10ꢂ4
1.4þ/ꢂ0.3 ꢀ 10ꢂ3
1.3þ/ꢂ0.1 ꢀ 10ꢂ3
1.0
1.8
1.7

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ULTRASONIC IRRADIATION
2521
Surprisingly, the rate enhancement was the same for the ultrasonic
bath and probe despite the large difference in probe intensities. To verify
this unexpected result, rate constants for the silent reaction were measured
at a variety of temperatures. These results confirm that the difference in
reaction rates between the ultrasonic bath and probe is the same as the
difference in reaction rates between the silent reaction at the two respective
temperatures. This is a strong indication that in this case, unlike the homo-
Diels-Alder reaction, the rate enhancement is not directly related to the
ultrasound. If the ultrasonic cavitation were directly altering the reaction
rate, one would expect a higher rate with the increased irradiation power
and lower frequency. Thus it is likely that the rate enhancement is simply a
mechanical effect. Even though the silent reactions were stirred, the ultra-
sonic irradiation may have caused more efficient mixing of the reaction
solution, thereby increasing the reaction rate.
CONCLUSIONS
Modest rate enhancements have been observed for ultrasonically
irradiated Diels Alder reactions between cyclohexadiene and napthoquinone
and between Danishefsky’s diene and para-methoxybenzaldehyde. Reac-
tions were not observed between many similar reagents. These results sug-
gest that ultrasonic irradiation will not cause reactions to occur which do
not occur thermally; however, reactions that occur may be accelerated mod-
erately in rate. The rate enhancement in the homo-Diels-Alder reaction
appears to be directly related to the ultrasonic irradiation, as increased
power level increased the reaction rate. However, this was not observed
for the ZnCl2-catalyzed hetero-Diels-Alder reaction; in that instance the
small rate increase is proposed to be due to increased mixing efficiency.
ACKNOWLEDGMENTS
This work was supported by awards from the Robert A. Welch
Foundation, Research Corporation, and the University of Texas at San
Antonio.
REFERENCES
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108, 5641.
2. Luche, J.L. Ultrasonics Sonochem. 1996, 3, S215.

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ALFARO AND McCLUSKY
3. Tuulmets, A. Ultrason. Sonochem. 1997, 4, 189.
4. Luche, J.L. Ultrason. Sonochem. 1997, 4, 221.
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Lee., J.; Li, J.-H.; Oya, S.; Snyder, J.K. J. Org. Chem. 1992, 57, 5301.
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Received in the USA November 10, 2000

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