Novel nicotinoid structures for covalent modification of wood: An environmentally friendly way for its protection against insects

Article abstract of DOI:10.1039/d0ra02071k

Timber is constantly exposed to environmental influences under outdoor conditions which limits its lifetime and usability. In order to counteract the damaging processes caused by insects, we have developed a novel and more environmentally friendly method to protect wood materials via covalent modification by organic insecticides. Starting with an important class of synthetic insecticides which are derived from the natural insecticide nicotine, various new carboxylic acid derivatives of imidacloprid were made accessible. These activated neonicotinoids were utilized for the chemical modification of wood hydroxy groups. In contrast to conventional wood preservation methods in which biocides are only physically bound to the surface for a limited time, the covalent fixation of the preservative guarantees a permanent effect against wood pests, demonstrated in standardized biological tests. Additionally, the environmental interaction caused by non-bound neonicotinoids is significantly reduced, since both, a smaller application rate is required and leaching of the active ingredient is prevented. By minimizing the pest infestation, the lifetime of the material increases while preserving the natural appearance of the material.

Full text of DOI:10.1039/d0ra02071k

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Novel nicotinoid structures for covalent
modification of wood: an environmentally friendly
way for its protection against insects†‡
Cite this: RSC Adv., 2020, 10, 15726
a
a
b
a
Martin Softje,
Sophie Acker, Rudy Plarre, Jan C. Namyslo
¨
a
and Dieter E. Kaufmann
*
Timber is constantly exposed to environmental influences under outdoor conditions which limits its lifetime
and usability. In order to counteract the damaging processes caused by insects, we have developed a novel
and more environmentally friendly method to protect wood materials via covalent modification by organic
insecticides. Starting with an important class of synthetic insecticides which are derived from the natural
insecticide nicotine, various new carboxylic acid derivatives of imidacloprid were made accessible. These
activated neonicotinoids were utilized for the chemical modification of wood hydroxy groups. In
contrast to conventional wood preservation methods in which biocides are only physically bound to the
surface for a limited time, the covalent fixation of the preservative guarantees a permanent effect against
wood pests, demonstrated in standardized biological tests. Additionally, the environmental interaction
caused by non-bound neonicotinoids is significantly reduced, since both, a smaller application rate is
required and leaching of the active ingredient is prevented. By minimizing the pest infestation, the
lifetime of the material increases while preserving the natural appearance of the material.
Received 4th March 2020
Accepted 10th April 2020
DOI: 10.1039/d0ra02071k
rsc.li/rsc-advances
a plant protection product, imidacloprid and its derivatives are
also used as active ingredients in wood preservatives. The
Introduction
6,7
The renewable natural resource wood is one of the oldest effect of the neonicotinoids is based on an interaction of the
construction materials in the world and remains of great insecticide with the nicotinic acetylcholine receptor (nAChR) of
interest due to its recoverability and its good mechanical, the nerve cells of the insects. Neonicotinoids bind to the
1
acoustical, and thermal properties. However, the CO neutral, nAChR, which causes a continuous transmission of synaptic
2
unprotected biomaterial suffers from signicant environmental impulses and thereby an over-excitation of the central nervous
8
impacts which limit its period of use. Wood products and system of the affected insects. The insecticides can be ingested
5
constructions are prone to attack or even complete destruction orally or via the cuticle. This mode of action occurs selectively
by insects such as termites or beetle larvae. In the subtropical with insects. Therefore, at the current stage of knowledge,
and tropical regions termites are widespread and the most neonicotinoids with the amounts applied against insects are of
2
,3
8
important pests on wood. Whereas in moderate regions like no harm to mammals as well as to humans. However, this class
Central to Northern Europe the house longhorn beetle and the of compounds has recently been criticized to cause damage on
9–12
common wood worm are economically most relevant. The benecial insects such as wild and honey bees.
Neo-
primary damage to the wood is caused by the feeding larvae, nicotinoids are suspected to be partly responsible for the
whereas the grown insects cause no further damage to the wood massive decline of insects in recent years. This has led to
and only pursue their reproductive instinct.
The damage caused to wood by insect attack can be signi- thiamethoxam, and clothianidin in the European Union in 2018.
cantly reduced through applying suitable insecticides, such as
To protect wood-based materials against insects,
restriction for the eld use of the neonicotinoids imidacloprid,
7
4–6
neonicotinoids. In addition to its widespread disposition as neonicotinoids are still allowed to be used both for industrial
wood preservation and also for private use. The insecticides can
7,13
be applied as surface application or by impregnation.
a
Institute of Organic Chemistry, Clausthal University of Technology, Leibnizstraße 6, However, physically bound protective layers have proved to be
3
b
8678 Clausthal-Zellerfeld, Germany. E-mail: dieter.kaufmann@tu-clausthal.de
disadvantageous in terms of durability, since they can be
washed out easily. This leads to both, reapplication at regular
intervals consuming large amounts of the insecticide and its
continuous release into the environment through a leaching
process.
Bundesanstalt f u¨ r Materialforschung und -pr u¨ fung, Unter den Eichen 87, 12205
Berlin, Germany
†
‡
1
Chemical improvement of surfaces, part 7.
Electronic supplementary information (ESI) available. See DOI:
0.1039/d0ra02071k
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Our group has developed a new and, in contrast to conven- specic synthesis pathways of the linker molecules 8–12
27–31
tional impregnation, more environmentally friendly method for following different literature procedures.
a covalent modication of wood through applying activated
The linking units 7–11 are characterized by different
14–18
benzoic acids.
By esterication of wood with benzotriazolyl- distances between the carbonyl group and the insecticidal
activated carboxylic acids, the material properties can be substructure. In order to attach the linker moieties 7–11 to
specically inuenced, while simultaneously retaining its outer imidacloprid (1), nucleophilic substitution reactions were
appearance. The modication of wood pursues the goal to carried out applying sodium hydride in dry DMF (Scheme 4),
32
increase the lifetime of the natural material in order to improve following a modied procedure from Kagabu et al. The
the attractiveness and usability of the material. The modica- substitution products 7a–11a have been isolated in good yields
tion of so and fast-growing wood species is especially impor- (Table 1). The ester groups were subsequently hydrolysed using
33,34
tant since the optimization of their properties would allow to triuoroacetic acid in DCM according to Manicardi et al.
The
use such treated wood materials as a substitute for harder and almost quantitatively obtained carboxylic acids 7b–11b were
slower growing tropical wood. The covalent modication of then activated with thionyl chloride and 1H-benzotriazole
wood using neonicotinoids with an activated acid function (Scheme 4). This reaction process, in which the carboxylic acid
opens up completely new opportunities to protect wood against chloride rst forms and then reacts in situ to the activated
35
insects. Our developed process is particularly environmentally benzotriazolate, was developed by Katritzky. The resulting
friendly because organic insecticides, such as the controversial carboxylic acid amides 7c–11c are characterized by a high
imidacloprid remains fully effective and yet will not be leached stability in combination with a high reactivity towards nucleo-
into the environment due to the hydrolytic stability especially of philes, such as lignocellulosic materials like wood. The struc-
7
the benzoates, proven in previous work. Therefore, lower tures of the products and the corresponding yields are given in
amounts of the insecticide will be required in the long term, Table 1.
since reapplication is avoided.
Attempts have also been made to substitute the butadiene
neonicotinoid 4 on the secondary amine followed by activation
analogously to Scheme 4. Unfortunately, the substitution reac-
tion failed as neonicotinoid 4 decomposed during the depro-
tonation with sodium hydride. The use of different bases such
as sodium hydroxide also remained unsuccessful. However, the
butadiene skeleton of neonicotinoid 4 provides additional
potential positions for attaching a nucleophilic substrate.
Therefore, a nucleophilic substitution in g-position of the
butadiene was carried out successfully using thiol linker 12. In
the next step, the methyl ester 12a was saponied using an
aqueous solution of sodium hydroxide. Acidication with conc.
hydrochloric acid nally gave the carboxylic acid 12b which was
then activated under the previously described conditions
leading to 12c (Scheme 5).
Results and discussion
Synthesis of the neonicotinoids
The neonicotinoid imidacloprid (1) occupies a synthetic key
position on the route to protect wood against insects. As shown
in Scheme 1 imidacloprid was synthesized from nitroguanidine
(
2) in a two-step synthetic pathway via imidazolidine 3 accord-
19–22
ing to literature.
As additional neonicotinoid we used butadiene derivative 4
which had previously been synthesized by Zapol'skii et al. starting
from 2-chloro-5-(chloromethyl)pyridine 5 (Scheme 2).
For covalent attachment of neonicotinoids 1 and 4 to wood,
implementation of a carboxylic acid function into the respective
insecticides was to achieve. This was done by substitution
reactions using suitable linking units 7–12 carrying an ester-
23–26
Covalent wood modication
protected carboxylic acid group. In addition to commercially The activated neonicotinoids 7c–12c were then applied in the
available tert-butyl bromoacetate (7), Scheme 3 shows the wood modication process, developed previously in our group
Scheme 1 Synthesis of imidacloprid (1).
Scheme 2 Synthesis of neonicotinoid 4.
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Scheme 3 Synthesis of different linking units 8–12.
Scheme 4 Synthesis of the activated neonicotinoids 7c–11c.
14–18
ꢀ
for 2 h at 50 C. During this initial process, the pore structures of
in order to improve the biological resistance against insects.
At rst, veneer chips of Scots pine sapwood were used for the the wood broaden so that the reactants can diffuse more easily
modication process. To ensure a high sample throughput, the into the material and react with the enlarged cell surface.
modications were performed in the “Synthesis 1” liquid system Furthermore, a deprotonation of the wooden hydroxy groups
parallel synthesizer. In principle, however, the functionalization already takes place during the swelling process, so that the
can also be carried out in standard Schlenk tubes. During the following esterication is facilitated. Aer the addition of the
chemical modication, a reaction occurs between the substrate activated neonicotinoid 7c–12c (7.0 mmol per gram wood), the
ꢀ
ꢀ
and the hydroxy groups of the wood biopolymers. However, the sample reacted for 24 h at 70 C and 120 C, respectively. The
alcohol groups present in the crystalline regions of the cellulose reaction conditions are summarized in Scheme 6.
form strong hydrogen bonds between the polymer strands and
Aer the modication reaction, the wood samples were
1
therefore do not react. Consequently, the number of reactive extracted and dried before determining the mass gain. The
units is signicantly lower compared to the total number of WPG (weight percentage gain) and QCO (quantity of covalently
hydroxy groups being present. Replacement experiments with bonded organo-material) values summarized in Table 2 show,
tritiated water reported by Sumi revealed that an average of 6.9– that all insecticides 7c–12c were successfully bound to pine
ꢀ
8
.0 mmol hydroxy groups per gram wood are accessible to wood leading to a signicant weight gain. At 70 C, the acet-
36
chemical reactions. In addition, the number of accessible amide 7c and the benzamide 8c were bound with high amounts
alcohol groups depends on the sample preparation, especially the to the wood polymers (0.24–0.26 mmol g ), whereas the pen-
ꢁ1
1,37,38
drying process and the moisture content of the sample.
Therefore, a mean value of 7.0 mmol of accessible hydroxy groups
toxy derivative 9c only led to a lower QCO value of 0.10 mmol
ꢁ1
g
. However, the introduction of electron-withdrawing groups
per gram wood is assumed. Prior to the modication, the wood such as a chlorine substituent in 10c or a nitro group in 11c
sample was swollen under nitrogen atmosphere in a mixture of allowed to signicantly increase the covalently bound amount
ꢁ
1
triethylamine and 4-dimethylaminopyridine (DMAP) in dry DMF of substance up to 0.29 mmol g which is comparable to the
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Table 1 Structures and yields of the synthesized neonicotinoid derivatives 7a–11c
a
b
Linker
Substituted imidacloprid
Hydrolysed imidacloprid
Activated imidacloprid
7
8
9
1
1
0
1
a t
b
Bu ¼ tert butyl. Bzt ¼ benzotriazol-1-yl.
Scheme 5 Synthesis of the activated butadiene neonicotinoid 12c.
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Scheme 6 Modification reaction of wood.
ꢀ
results of 7c and 8c. At 120 C, larger amounts of the substances
ꢀ
were usually tethered than at 70 C. Only acetamide 7c led to
ꢀ
ꢁ1
a signicantly lower QCO value at 120 C (0.06 mmol g ) than
ꢀ
ꢁ1
at 70 C (0.24 mmol g ), which can be explained by the fact that
7c decomposes at higher temperatures.
In addition to the imidacloprid substrates, the butadiene
Fig. 1 ATR-IR spectra of unmodified and modified wood in compar-
derivative 12c was also applied for wood modication and led to ison (A: nC]O, B: nC]C,arom., C: nNO2,asym., D: nC–O,asym., E: dC–H,arom.).
ꢁ
1
very similar QCO values of 0.15–0.16 mmol g
at both
temperatures. Finally, imidacloprid (1) and insecticide 4 were
bound successfully to the wood material.
ꢁ1
well as the corresponding C–O stretching bands at 1250 cm
area D). Both bands unequivocally conrm the formation of
(
a covalent bond between the wood polymers and the activated
Analytic characterization of the modied wood
substrates 7c–12c. Furthermore, slightly enlarged aromatic
The actual formation of a covalent bond between the wood
biopolymers and the applied reagent has already been
conrmed by Drafz via 2-dimensional NMR spectroscopy, or by
S ¨o je and Ehrhardt via thermal analyses (Pyrolysis GC MS).
In addition, the penetration depth of the modifying reagent into
ꢁ1
C]C deformation vibrations at about 1600 cm (area B) are
visible for the modications 8d–12d due to the introduced
aromatic groups. The corresponding aromatic deformation
7,39,40
ꢁ
1
vibrations were also detected at about 760 cm (area E). The
nal proof of a successful modication is given by a specic

the wood tissue has also been examined using m-CT tech-
band of the chemically introduced neonicotinoid. In detail, the
asymmetric stretching vibration of the nitro moiety at
39,41
niques.
Furthermore, ATR-IR spectroscopy has proven as
a very reliable and meaningful method in order to characterize
modied wood, which is the reason why this non-destructive
method has been chosen to examine all treated
ꢁ
1
1
550 cm (area C) clearly conrms the attachment of the
insecticidal group. In addition to the discussed changes in the
ATR-IR spectra caused by the formed ester bonds of the bound
insecticides, further intensity changes can be recognized in the
spectra (see Fig. S1 in the ESI‡). In fact, a slightly visible
7,14–18,39,41
samples.
Fig. 1 displays the ATR-IR spectra of untreated Scots pine
sapwood in comparison to the chemically modied samples 7d–
ꢁ
1
decrease of the hydroxyl stretching vibrations at 3340 cm for
the samples 7d–12d in contrast to unmodied wood conrms
the covalent esterication of the wood biopolymers.
12d. The biopolymers of wood itself lead to characteristic
vibration bands, which can be assigned to the structural
components of the lignocellulose material according to litera-
42–47
ture.
The spectral comparison of unmodied and modied
Biological tests of the modied wood
wood shows signicant changes caused by the formation of
covalent bonds during the modication process. The formed Aer the successful modication of the veneer chips with the
ester functions are leading to an increase of the carbonyl previously synthesized neonicotinoids 7c–12c, we aimed to
ꢁ
1
stretching vibrations at approximately 1700 cm (area A) as investigate the biological effectiveness of the functionalization.
Table 2 Results of the wood modifications using activated neonicotinoids 7c–12c (WPG and QCO values are given as well as the associated
standard deviations s for experiments that were carried out several times)
ꢀ
ꢀ
7
0 C
120 C
ꢂ s [mmol gꢁ
1
]
WPG
ꢂ s [%]
QCO
Ø
ꢂ s [mmol gꢁ1
]
Sample (reagent)
WPG
Ø
ꢂ s [%]
QCO
Ø
Ø
7
8
9
1
1
1
d (7c)
d (8c)
d (9c)
0d (10c)
1d (11c)
2d (12c)
7.2 ꢂ 0.7
9.8 ꢂ 1.7
4.3
0.24 ꢂ 0.02
0.26 ꢂ 0.05
0.10
1.9 ꢂ 0.8
19.3 ꢂ 1.3
9.4
0.06 ꢂ 0.03
0.52 ꢂ 0.03
0.21
9.9 ꢂ 0.1
0.21 ꢂ 0.00
22.3 ꢂ 2.9
0.47 ꢂ 0.06
14.2
0.29
19.3
0.40
7.2
0.15
7.5
0.16
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samples and 3 completely untreated control samples were
tested analogously to the 5 modied wood blocks.
Aer 4 weeks of the test procedure, it was observed that
66.0% of the larvae had already died on the surface of the
modied wood samples, while the remaining larvae had
gnawed into the biomaterial. Aer a total of 12 weeks, the nal
examination of the wood samples was carried out by manually
cutting up all test specimens. This procedure was used to nd
the larvae, which were then subjected to a vitality check. As
shown in Fig. 2, the larvae did not penetrate very deep into the
modied wood blocks (up to 2.5 mm) and were already located
just below the surface. On the other hand, the unmodied
0
Fig. 2 Final inspection of the modified test specimens 8d after 12
weeks: it is shown a wood block with its manually split surface were
the dead larvae were found. The larvae have only penetrated the
material to just below the surface up to 2.5 mm before they died. The samples had to be completely taken apart in order to nd the
rest of the sample remains intact and was not damaged by the larvae.
deeply inltrated larvae. At the end of the test, the mortality rate
for the functionalized samples was 100.0% (Table 3), the
extracted but unmodied samples averaged 10.9%, and the
Therefore, the modication process had to be upscaled since untreated control samples had a mortality of 4.8%.
European standardized efficacy tests require wood specimens
According to the test standard DIN EN 46-1, untreated
3
measuring 15 ꢃ 25 ꢃ 50 mm . Five of these blocks were controls are allowed to have a natural mortality rate of up to
48
modied simultaneously in a glass reactor whereby 150 mL of 30%. Therefore, the test procedure was valid and gave a very
anhydrous DMF, triethylamine and DMAP were used. For the good efficacy of the covalent modication of pine wood using
biological tests, activated benzamide 8c (1.0 mmol per gram benzamide 8c against the larvae of the domestic longhorn
wood) was applied as modication reagent. This derivative was beetle.
chosen due to the stability of its aromatic ester group and the
consistently good yields of the synthetic steps from 8a to 8c in
order to obtain the activated reagent 8c in sufficient quantities
Experimental
Preparation of the organic precursors
for the following wood modication.
The small veneer chips were already completely penetrated
by the modifying solution during the procedure described
above. While the pinewood chips were functionalized in their
entirety, it was necessary to achieve a sufficient penetration
depth of the reagents for the larger pine wood blocks. Therefore,
the reaction solution was prepared in a separate ask under
nitrogen atmosphere before being transferred by means of
a negative pressure of 0.5 bar into the glass reactor. The wood
General experimental information and synthetic procedures are
described in the ESI.‡ H NMR, C NMR and C NMR spectra
are available as well.
1
13
15
General procedure for chemical modication of wood chips
17,18
following Kaldun et al.
In general, the common amount of 7 mmol of the appropriate
wood modifying reagent was applied per 1.00 g of veneer chips
ꢀ
samples were swollen at this pressure for 2 h at 50 C before
3
of Scots pine sapwood (approx. 10 ꢃ 10 ꢃ 0.7 mm , approx.
increasing the pressure by nitrogen addition to atmospheric
ꢀ
0
0.04–0.07 g). Each reaction was carried out in the Heidolph
“Synthesis 1” parallel synthesizer in 6 mL of anhydrous DMF
with 2 equivalents of distilled triethylamine and 10 mol% of
DMAP under nitrogen atmosphere. Prior to this wood modi-
cation, the wood was extracted for 24 h in a Soxhlet apparatus
with a solvent mixture comprising toluene/acetone/methanol in
a 4 : 1 : 1 ratio. Subsequently, the pretreated wood was oven-
pressure. The samples reacted for 24 h at 70 C like previously
shown in Scheme 6. The modied samples 8d showed a bound
amount of insecticide 8c (QCO) of 0.18 ꢂ 0.01 mmol per gram
wood on average (corresponds to a WPG value of 6.7 ꢂ 0.3%).
0
The modied wood samples 8d were tested according to DIN
EN 46-1 for their biological effectiveness against the larvae of
48
the domestic house longhorn beetle (Hylotrupes bajulus). To
compare the mortalities 5 extracted but unmodied wood
ꢀ
dried at 105 C for 24 h and then stored under vacuum in
a laboratory desiccator. The wood modication reaction started
with heating the wood sample with triethylamine and DMAP in
0
Table 3 Results of the biological tests of the modified wood samples 8d (modified with 8c: WPG ¼ 6.7 ꢂ 0.2%, QCO ¼ 0.18 ꢂ 0.01 mmol)
48
against the larvae of the domestic house longhorn beetle following DIN EN 46-1. The average mortality rates of the 5 and 3 wood blocks,
respectively, as well as the associated standard deviations s are given
Surface mortality rate (Ø) ꢂ s aer
Final mortality rate
(Ø) ꢂ s [%]
Sample
4 weeks [%]
0
Modied sample 8d (extracted, modied with 8c)
Reference sample I (extracted, unmodied)
Reference sample II (untreated reference)
66.0 ꢂ 28.8
2.0 ꢂ 4.5
0.0 ꢂ 0.0
100.0 ꢂ 0.0
10.9 ꢂ 10.6
4.8 ꢂ 8.2
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ꢀ
dry DMF at 50 C for 2 h. Subsequently, the modication subjected to an optical check in order to count the larvae which
reagent was added and aerwards the temperature was died on the surface and which had gnawed into each sample.
ꢀ
ꢀ
increased to 70 C or 120 C for 24 h. Aer this reaction time, Following another rest time of eight weeks (12 weeks since the
the mixture was allowed to cool down to rt. The thus modied start of the test), the wood samples were nally examined by
wood was washed successively with THF (50 mL), chloroform manually splitting the test specimens. This procedure was used
(50 mL), and diethyl ether (50 mL). Aerwards, the wood sample to nd and count all the larvae, which had gnawed into the
was extracted again for 24 h applying the Soxhlet apparatus with samples followed by subjecting the found larvae to a vitality test.
the same solvent mixture as stated above. Finally, the modied Based on the balance of the living and dead larvae, mortality
ꢀ
wood was dried at 105 C for 24 h and nally weighed.
rates were calculated both for the surface mortality (aer four
weeks) and for the whole sample (aer 12 weeks).
General procedure for chemical modication of DIN EN 46-1
4
8
wood blocks
Conclusion
An amount of 1 mmol of the wood modifying reagent was
In the course of this work, a number of novel activated
neonicotinoids 7c–12c, based on an imidacloprid moiety, were
synthesized and consequently used for covalent wood modi-
cation. Particularly noteworthy is the benzamide 8c, which was
attached to pinewood with considerable amounts of up to
applied per 1.00 g of blocks of Scots pine sapwood (approx. 15 ꢃ
3
2
5 ꢃ 50 mm , approx. 9.00–10.00 g). The reaction was carried
out in a special glas reactor which was closed with a at ange. 5
blocks were modied simultaneously in the same vessel
whereby 150 mL of anhydrous DMF, 2 equivalents of distilled
triethylamine and 10 mol% of DMAP were used under nitrogen
atmosphere. Prior to this wood modication, the wood blocks
were extracted for 3 d in a Soxhlet apparatus with a solvent
mixture comprising toluene/acetone/methanol in a 4 : 1 : 1
0.52 mmol per gram wood. All modied wood samples were
spectroscopically analyzed. The attached fragments were
detected by ATR-IR spectroscopy, which proved the formation of
a covalent bond between the insecticide and the wood cell
surface. According to the standard DIN EN 46-1, the specimen
functionalized with imidacloprid benzamide 8c showed an
extraordinary biological activity against the larvae of the
domestic house longhorn beetle (Hylotrupes bajulus). Finally,
the successfully passed DIN-EN 46-1 test proved a mortality of
the larvae of 100% and thereby underlines the effectiveness of
this covalent modication against insects. In addition, this
newly developed wood protection process is not only charac-
terized by its biological effectiveness but also by its high
persistency. The covalent xation prevents leaching of the
insecticide, which also leads to a good environmental compat-
ibility of this unique wood protection method. The covalent
ratio. Subsequently, the pretreated wood was oven-dried at
ꢀ
1
05 C for 3 d and then stored under vacuum in a laboratory
desiccator. The modication started with the evacuation of the
wood samples in the glass reactor using a rotary vane pump.
Aerwards, a vacuum of 0.5 bar was applied using a diaphragm
pump. The reaction solution consisting of N-(1-{[4-(1H-
benzotriazole-1-carbonyl)phenyl]methyl}-3-[(6-chloropyridin-3-
ꢁ1
yl)methyl]imidazolidin-2-ylidene)nitramide (8c, 1 mmol g
wood), triethylamine and DMAP in dry DMF was prepared
during this time in a separate ask under nitrogen atmosphere.
The solution was then transferred via a metal cannula through
a septum into the glass reactor with the aid of the above

xation ought to be applicable to other types of insecticides or
mentioned negative pressure. The wood samples were heated at
fungicides, too.
ꢀ
50
C for 2 h at 0.5 bar. Subsequently, the pressure was
increased by means of nitrogen supply to atmospheric pressure.
Conflicts of interest
3
0 mL dry DMF were added and the reaction mixture was stirred
ꢀ
for 24 h at 70 C. Aer this reaction time, the mixture was
allowed to cool down to rt. The thus modied wood was washed
successively with THF (250 mL), chloroform (250 mL), and
diethyl ether (250 mL). Aerwards, the wood samples were
extracted again for 3 d applying the Soxhlet apparatus with the
same solvent mixture as stated above. Finally, the modied
There are no conicts to declare.
Acknowledgements
The authors thank Dr Gerald Dr ¨a ger, Leibniz University of
Hannover (Germany), for high resolution mass spectra. We also
thank Maike Weigert and Marko Spillner for synthetic assis-
tance, as well as Sabine Busweiler for technical support during
biological testing. We gratefully acknowledge Prof. Dr Holger
Militz and Prof. Dr Carsten Mai from the department for wood
ꢀ
wood was dried at 105 C for 3 d and nally weighed.
Procedure of the biological test of the wood samples
48
according to the DIN EN 46-1 standard
In order to compare the mortality rates ve modied wood biology and wood products, Georg-August-University G ¨o ttingen
0
blocks 8d , ve extracted but unmodied wood samples (refer- (Germany) for providing wood samples.
ence sample I) and three completely untreated control samples
(
reference sample II) were tested analogously. First, a glass pane
Notes and references
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