Synthesis and photochemistry of 1-iodocyclohexene: Influence of ultrasound on ionic vs. radical behaviour

Article abstract of DOI:10.3390/12020188

Simultaneous application of UV light and ultrasonic irradiation to a reaction mixture containing 1-iodocyclohexene is reported. The irradiation of 1-iodocyclohexene in methanol was carried out with or without addition of zinc. The effect of ultrasound or mechanical stirring on this solid-liquid system was also compared. The irradiation of 1-iodocyclohexene in methanol in the presence of zinc increases the yield of the nucleophilic trapping product, compared with the yield after irradiation in the absence of zinc. The photodegradation of 1-iodocyclohexene was slightly accelerated after addition of zinc. A rapid formation of radical product was accompanied by substantial decrease of 1-iodocyclohexene after application of ultrasound and irradiation without the zinc. The ultrasound significantly affects the photobehaviour of this reaction, predominantly its radical route. The joint application of ultrasound and zinc contributes positively to the production of radical and ionic products. The sonochemical stirring is more effective than mechanical stirring.

Full text of DOI:10.3390/12020188

Molecules 2007, 11, 188-193
molecules
ISSN 1420-3049
http://www.mdpi.org
Full Paper
Synthesis and Photochemistry of 1-Iodocyclohexene: Influence
of Ultrasound on Ionic vs. Radical Behaviour †
Marika Blaskovicova *, Anton Gaplovsky and Jaroslav Blasko
Institute of Chemistry, Faculty of Natural Sciences, Comenius University, Mlynska dolina CH-2, SK-
842 15, Bratislava, Slovakia; Tel.: (+421) 2 602 96 319, Fax.: (+421) 2 602 96 684; E-mails:
gaplovsky@fns.uniba.sk; blasko@fns.uniba.sk
* Author to whom correspondence should be addressed, e-mail: noskova@fns.uniba.sk
† Paper presented at ECSOC-10, 1-30 November 2006
Received: 14 December 2006; in revised form: 24 January 2007 / Accepted: 25 January 2007 /
Published: 16 February 2007
Abstract: Simultaneous application of UV light and ultrasonic irradiation to a reaction
mixture containing 1-iodocyclohexene is reported. The irradiation of 1-iodocyclohexene
in methanol was carried out with or without addition of zinc. The effect of ultrasound or
mechanical stirring on this solid-liquid system was also compared. The irradiation of 1-
iodocyclohexene in methanol in the presence of zinc increases the yield of the
nucleophilic trapping product, compared with the yield after irradiation in the absence of
zinc. The photodegradation of 1-iodocyclohexene was slightly accelerated after addition
of zinc. A rapid formation of radical product was accompanied by substantial decrease of
1-iodocyclohexene after application of ultrasound and irradiation without the zinc. The
ultrasound significantly affects the photobehaviour of this reaction, predominantly its
radical route. The joint application of ultrasound and zinc contributes positively to the
production of radical and ionic products. The sonochemical stirring is more effective than
mechanical stirring.
Keywords: Ultrasound, photochemistry, 1-iodocyclohexene.

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189
Introduction
Alkyl halides exhibit competing radical and ionic photobehaviours [1]. Iodides give predominantly
ionic products. The irradiation process involves the initial homolytic cleavage of the carbon-halogen
bond, followed by electron transfer within a radical pair to generate an ion pair. Irradiation of alkyl
halides is a convenient method for generation of carbocations. The corresponding vinyl cations are
generated by irradiation of vinyl iodides. The products of photochemical reactions depend on the light
intensity, as well as on effectiveness of the reaction mixture stirring [2]. The application of the
ultrasound, especially in heterogeneous reactions, causes a mechanical effect responsible for the mass
transfer, that is, a thorough stirring of the reaction mixture, as well as the activation of the surface of
the solid reagents present [3-5]. The second effect of ultrasound, the most pronounced one in
homogenous reactions, is caused by the high temperatures (up to 5000 K) and high pressures (up to
400 atm), which are involved in the collapsing bubbles (cavities) in the ultrasound field [3-5]. The
main goal of this work was to examine simultaneous sonication and UV irradiation of 1-iodo-
cyclohexene in methanol in absence/presence of zinc as iodine scavenger.
Results and Discussion
The 1-iodocyclohexene 3 was prepared from ketone 1 by using some modifications of the general
method of Pross and Sternhell [6] (Scheme 1).
Scheme 1. Synthesis of 1-iodocyclohexene.
NH2
O
N
I
N2H4.H2O
I2
Et3N
1
3
2
Figure 1. Irradiation of 1-iodocyclohexene in methanol (λIR = 254 nm; tIR = 60 min)
monitored by GC.
350000
0 min
5 min
10 min
300000
15 min
20 min
250000
30 min
200000
150000
100000
50000
0
40 min
60 min
15,70
15,75
15,80
15,85
15,90
15,95
time

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190
Irradiation of 1-iodocyclohexene (3) in methanol (λ = 254 nm, Figure 1) afforded a mixture of the
dimethyl ketal 6, enol ether 5 and cycloalkene 4 [7] (Scheme 2). Product 6 arises from an acid-
catalyzed addition of methanol to the nucleophilic trapping product 5. Product 4 is a radical-derived
reduction product (Table 1, entry A).
Scheme 2.
I
OMe
OMe
3
6
5
H+, CH3OH
OMe
hν
electron
transfer
C+ I
CH3OH
I
C
4
Table 1. Irradiation of 1-iodocyclohexene in methanol.
time
3
6
5
4
Conditions
(min) (%) (%) (%) (%)
A
40
40
40
40
33
10
24
4
23
23
20
15
18
25
35
41
26
42
22
40
B
C
D
A - irradiation
B - irradiation and ultrasound
C - irradiation in presence of zinc and mechanical agitation
D - ultrasound and irradiation in presence of zinc
A rapid formation of 4, accompanied by a great decrease in the amount of 3 formed was observed
after application of ultrasound (Table 1, entry B). The ultrasound significantly affects photobehaviour
of this reaction, predominantly the radical route. With mechanical stirring in the presence of zinc as an
acid scavenger, the enol ether 5 was mainly obtained (Table 1, entry C). Ultrasound and irradiation in
the presence of zinc caused rapid photodegradation of 3 and an increase in the ratio of the radical
products 4D and 4C. The results obtained in our study showed that ultrasound affects the
photobehaviour of 1-iodo-cyclohexene in methanol. Ultrasound markedly influenced the lifetime of
the radical pair resulting from initial homolytic cleavage of the carbon-halogen bond. An increase of
radical product 4 was observed. The ultrasound caused rapid photodegradation of starting iodide 3
(Table 1 entries A,B). The irradiation of 1-iodocyclohexene in methanol in presence of zinc increases
the yield of 5 compared with the yield after irradiation without zinc. The sonochemistry of zinc
powder was investigated a few years ago [8]. Ultrasound creates acoustic cavitation in liquids. In the
presence of fine powders, shockwaves and turbulent flow from cavitation result in interparticle

Molecules 2007, 12
191
collisions of solids. Such collisions occur with enough force to cause changes in the morphology of
powders. After application of ultrasound on zinc powder, dramatic changes in particle morphology
were observed. Whereas the mechanical aggitation keeps zinc particles smooth (Figure 2), the
ultrasound makes the particles rough and grainy (Figure 3). The particle fragmentation improves the
mass transport. Ultrasound and zinc positively contribute to the production of 4 and 5. The rapid
photodegradation of 3 was also observed. Mechanical stirring is more or less local, being most
effective near the stirrer, but not so effective in the close proximity of the reactor walls. The
sonochemical stirring is evenly distributed over the entire reaction volume, as cavities are formed and
implode throughout the reaction mixture [4].
Figure 2. Zn powder, mechanical stirring.
Figure 3. Zn powder, ultrasound.
Conclusions
From the results of this work it follows that ultrasound significantly affects photobehaviour of 1-
iodocyclohexene predominantly its the radical route. Enhancements due to ultrasound may be
attributed to its chemical or mechanical effects, or to both simultaneously. The chemical effects of
ultrasound are due to the implosion of microbubbles, generating free-radicals with a great propensity

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192
for reaction. Mechanical effects are caused by shock waves formed during symmetric cavitation, or by
microjects formed during asymmetric cavitation.
Experimental
General
Gas-chromatographic analyses were performed on an Agilent 6890N instrument using a HP-1
capillary column (50 m x 0.32 mm ID with 1.05 µm film thickness). Cyclohexene was used as
an internal hydrocarbon standard. Proton NMR spectra were determined in chloroform-d on a Varian
Mercury Plus 300 MHz spectrometer. Mass spectra were obtained by using a MSD-Agilent 5973
Network spectrometer.
Preparation of 1-iodocyclohexene (3).
The iodide 3 was prepared from the corresponding ketone 1 by using some modifications of
general method of Pross and Sternhell [6]. Hydrazine hydrate (40.8 mL, 0.84 mol) was placed into
round-bottomed flask equipped with a magnetic stirrer, condenser and an addition funnel. A solution
of cyclohexanone (6.4 mL, 61.5 mmol) in absolute methanol (35 mL) was added very slowly (ca. 3
drops per min !!!) to the hydrazine hydrate with vigorous stirring. When the addition was complete the
reaction mixture was refluxed for 1 h and then cooled to room temperature. The methanol was
evaporated and reaction mixture was washed with chloroform, aqueous sodium chloride and dried over
anhydrous sodium sulphate. After filtration and evaporation of chloroform 6.3 g (91%) of hydrazone 2
was obtained. 1H-NMR: 4.88 (s, 2H, -NH2), 2.28 (m, 2H, C1), 2.33 (m, 2H, C5), 2.23 (m, 4H, C2, C4),
1.73 (m, 2H, C3). Iodine (27.4 g, 0.11 mol) in dry ether (170.4 mL) was added dropwise at room
temperature to a stirred solution of triethylamine (41.6 mL, 0.29 mol) and hydrazone 2 (6 g, 0.05 mol)
in dry ether (18.1 mL). The colour of the reaction mixture changed from yellow to brown after
addition of the iodine solution. The reaction was instantaneous, but stirring was continued for a further
20 min. The reaction mixture was diluted by adding dry ether (150 mL) and washed successively with
saturated aqueous sodium sulphite, 3N HCl, saturated aqueous sodium bicarbonate and aqueous
sodium chloride. The organic layer was then dried over sodium sulphate and evaporated to give crude
product 3 (1.37 g, 13%) which was purified by chromatography (silica gel, 100-250 mesh, petroleum
1
ether bp 40-60°C). H-NMR: 6.33 (sept, 1H, CH), 2.5 (m, 2H, C6), 2.1 (m, 2H, C3), 1.7 (m, 4H, C2,
C6).
Ultrasonic irradiations.
A photochemical reactor allowing simultaneous ultrasound irradiation was used. A sandwich
piezoelectric transducer (50W, 20kHz, intensity≈16W.cm-2) was attached with an epoxy resin to the
bottom of a normal photochemical reactor with internal water-cooling (50 mm diameter, 40 mL
volume). Irradiations of 3 in methanol (c = 1.63x10-3 mol/L) were carried out with a 4W (254 nm) low
pressure mercury lamp. Durring irradiation, the solution was flushed with argon.

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Sample Availability: Compound samples are not available.
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