Insight into the Mechanism of Reversible Ring-Opening of 1,3-Benzoxazine with Thiols

Article abstract of DOI:10.1021/acs.joc.6b02727

The reversible ring-opening addition and fragmentation reaction of p-cresol-based N-phenylbenzoxazine with aliphatic and aromatic thiols was investigated in solvent-mediated and solvent-free reactions. Independently of the used thiol, N-phenylbenzoxazine and the thiols reacted to equilibrium with comparable amounts of reactants and products in aprotic solvent, whereas in protic solvent almost full conversions were reached. In contrast, thiol reactivity was a crucial factor in solvent-free reactions yielding fast and complete conversions for a more acidic thiol and balanced equilibrium concentrations in case of thiols with high pK_a values. The strong influence of thiols with low pK_a values emphasizes the relevance of the protonation step in the ring-opening reactions of 1,3-benzoxazines with thiols in absence of solvents where acidity predominates nucleophilicity. The reverse reactions, namely adduct dissociation and benzoxazine recovery, were successfully conducted at elevated temperatures and reduced pressure facilitated by the removal of the formed thiols yielding up to 95% recovered 1,3-benzoxazine. These results provide deeper understanding of the reversible ring-opening reaction mechanism of 1,3-benzoxazine with thiols.

Full text of DOI:10.1021/acs.joc.6b02727

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Article
Insight into the Mechanism of Reversible
Ring-Opening of 1,3-Benzoxazine with Thiols
Tobias Urbaniak, Marc Soto, Manuel Liebeke, and Katharina Koschek
J. Org. Chem., Just Accepted Manuscript • Publication Date (Web): 27 Mar 2017
Downloaded from http://pubs.acs.org on March 27, 2017
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Insight into the Mechanism of Reversible Ring-Opening of 1,3-Benzoxazine with Thiols
Tobias Urbaniak† §, Marc Soto†, Manuel Liebeke‡, Katharina Koschek†*
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†Fraunhofer Institute for Manufacturing Technology and Advanced Materials (IFAM), Wiener Strasse 12,
Dꢀ28359 Bremen, Germany
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§University of Bremen, Institute for Organic and Analytical Chemistry, Leobener Strasse NW2C, Dꢀ28359
Bremen, Germany
‡Max Planck Institute for Marine Microbiology, Celsiusstrasse 1, Dꢀ28359 Bremen, Germany
*Correspondence to: Katharina Koschek (katharina.koschek@ifam.fraunhofer.de)
Ring-opening ∝ thiol pKa
C6F5 > PhNO2 > Ph > CH2Ph >> Alkyl
Dissociation ∝ thiol b.p.
n-Bu ~ C6F5 ~ Cy ~ Ph >>
CH2Ph ~ n-C12H15 ~ PhNO2
ABSTRACT
The reversible ringꢀopening addition and fragmentation reaction of pꢀcresol based Nꢀphenylbenzoxazine
with aliphatic and aromatic thiols was investigated in solventꢀmediated and solventꢀfree reactions.
Independently of the used thiol, Nꢀphenylbenzoxazine and the thiols reacted to equilibrium with
comparable amounts of reactants and products in aprotic solvent, whereas in protic solvent almost full
conversions were reached. In contrast, thiol reactivity was a crucial factor in solventꢀfree reactions
yielding fast and complete conversions for a more acidic thiol and balanced equilibrium concentrations in
case of thiols with high pKa values. The strong influence of thiols with low pKa values emphasize the
relevance of the protonation step in the ringꢀopening reactions of 1,3ꢀbenzoxazines with thiols in absence
of solvents where acidity predominates nucleophilicity. The reverse reactions, namely adduct dissociation
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and benzoxazine recovery, were successfully conducted at elevated temperatures and reduced pressure
facilitated by the removal of the formed thiols yielding up to 95 % recovered 1,3ꢀbenzoxazine. These
results provide deeper understanding of the reversible ringꢀopening reaction mechanism of
1,3ꢀbenzoxazine with thiols.
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INTRODUCTION
In the last decade, reversible reactions are receiving increasing interest for the preparation of smart
materials with e.g. selfꢀhealing properties.1–3 Examples of reversible reactions applied in materials include
transesterification4,5, Diels–Alder/retro Diels–Alder6–9, [2+2] photocycloaddition10, radical reactions11–14
and olefin metathesis15.
,
Polybenzoxazines as thermosetting polymers offer a number of outstanding properties such as thermal
stability and excellent mechanical performance.16 Recently, the ring opening reaction of benzoxazines in
presence of thiols was described to be a reversible reaction, which could give access to smart materials.
Endo et al. have described this reversible reaction to proceed at room temperature without catalysts and
sideꢀproduct formation. Moreover, this reaction was transferred to bifunctional benzoxazines and dithiols
to generate polymers, which could be depolymerized and polymerized by protic/aprotic solvent change.17
Gorodisher et al. developed adhesives based on thiolꢀbenzoxazine chemistry18 and Yagci et al. described
mainꢀchainꢀtype polybezoxazines partially reacted with thiols as low temperature curing precursors for
high performance thermoset polybenzoxazines.19 Furthermore, thiolꢀbenzoxazine chemistry was used as
efficient linking reaction in block copolymer preparation.20
The ringꢀopening reaction of benzoxazines in presence of thiols was described as Catalytic Opening of the
Lateral Benzoxazine Rings by Thiols (COLBERT).18,21 This proposed mechanism contains several proton
transfer steps that could be strongly affected by the acidic nature of thiols and solvents used (i.e. protic vs.
aprotic).
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The initial thiolꢀmediated protonation step probably takes place predominantly at nitrogen (A) as it was
described for the ring opening polymerization of 1,3ꢀbenzoxazines in presence of various catalysts.22,23
The protonated benzoxazine can be considered as a system under CurtinꢀHammett conditions with rapidly
interconverting intermediates (A) and (B). A SN2 type reaction on intermediate (A) would lead to CꢀN
cleavage and formation of ꢀOꢀCH2ꢀSR, while intermediate (B) undergoes a unimolecular opening to form
acyclic iminium intermediate (C) followed by a SN1 type reaction, or alternatively a SN2 type reaction
with an appropriate thiolate (Figure 1).
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"SN2"
B
C
D
A
Figure 1. Proposed mechanism for the reversible ringꢀopening of 1,3ꢀbenzoxazines with thiols.
In solution, the equilibria strongly depend on the used solvents. In DMSO and DMF the COLBERT
reaction does not proceed, but it reaches equilibrium when performed in chloroform.17,19,24 In protic
solvents such as methanol the ringꢀopening addition reaction achieves high conversions probably due to
zwitterion formation (D). Alcohols are known to stabilize ions, for that the benzoxazine thiol adduct is
partially found in its zwitterionic form (D).17,21,24
Endo’s group described the effect of protic and aprotic solvents on the addition and fragmentation
equilibrium of benzoxazine in presence of 1ꢀoctadecanethiol,17 but studies on further factors influencing
the equilibrium reaction are missing so far. Therefore, this contribution aims to gain deeper insight
studying the reaction of Nꢀphenylbenzoxazine 1 as model compound with various aromatic and aliphatic
thiols under solventꢀmediated vs. solventꢀfree conditions. In addition to Endo’s work, variations in
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reactant concentrations as well as thiol acidity are studied and related to the proposed mechanism.
Moreover, this contribution elucidates in particular the access to a reversible ring opening addition
reaction and hence 1,3ꢀbenzoxazine recovery when performed under solventꢀfree conditions.
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RESULTS AND DISCUSSION
Reversible ringꢀopening addition and fragmentation reactions were performed with the pꢀcresol based
Nꢀphenylbenzoxazine 1 as a benzoxazine representative and aliphatic and aromatic thiols 2a-g (Figure 2).
With the aim to study the impact on the addition fragmentation equilibrium of 1 and 3a-g, the reaction setꢀ
up was varied with respect to the solvent composition, reactant concentration and bulk conditions.
O H
R
O
N
N
S
HS
R
1
3a-g
2a-g
n-C12H25
Cy
n-C4H9
a
e
f
b CH2Ph
c Ph
R =
g C6F5
pNO
d Ph-
2
Figure 2. Reversible ringꢀopening addition fragmentation reaction of 1 in presence of the thiols 2a-g.
Solvent-Mediated Reversible Ring-Opening and Fragmentation Reaction
In the first step, the influence of concentration on the reaction equilibrium was studied as a model reaction
of Nꢀphenylbenzoxazine 1 and 1ꢀdodecanethiol 2a in stoichiometric ratio in deuterated chloroform.
Thereby, the reversible reaction progress was investigated in dependence of the initial reactant
concentration ranging from 0.01 to 0.24 M with respect to both starting points, namely, the addition
reaction of Nꢀphenylbenzoxazine 1 and 2a as well as the dissociation reaction from the adduct 3a back to
Nꢀphenylbenzoxazine 1 (Figure 3).
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C0 (3)
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0
0.05
0.1
0.15
0.2
0.25
Initial concentration C0 / M
Figure 3. Equilibrium concentration of 1 vs. initial concentration of 1 or 3a determined in ring opening
1
reaction of 1 and 2a, and in a dissociation reaction of 3a in CDCl3, respectively, monitored by H NMR
(mean values with a confidence interval of 90 %).
At an initial concentration of 0.01 M Nꢀphenylbenzoxazine 1 in chloroform, chemical equilibrium was
reached after 25 h with approximately 40 % conversion of 1 to the addition product 3a. With an increasing
initial concentration of 1 the equilibrium was shifted even further to 3a. However, the conversion of
Nꢀphenylbenzoxazine 1 even at the highest initial concentration of 0.24 M does not exceed 58 % yielding
equilibrium concentrations with a slight excess of 3a. Studying the reaction starting from the addition
product 3a, the reaction proceeded equivalently showing a higher conversion with increasing initial
concentration of 3a. At its highest initial concentration 42 % of 3a were converted. Thus, the conversions
starting from 1 and 3a increased with increasing initial concentrations yielding chemical equilibrium after
25 h. At the highest initial concentration of 1 and 3a, respectively, the equilibrium concentration position
was shifted to an excess of 3a (Figure 3).
The reactions proceeded typically for chemical equilibrium reactions, namely with a favoured dissociated
state at low reactant concentrations and an increase in the relative amount of the addition product with
increasing reactant concentration. Based on the model reactions of Nꢀphenylbenzoxazine 1 with
1ꢀdodecanethiol (2a) the reversible nature of the addition dissociation reaction and the impact of reactant
concentration was proven.
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Next, the reversible ringꢀopening addition and dissociation reaction of Nꢀphenylbenzoxazine 1 was
extended to further thiols varying in their reactivity with respect to nucleophilicity and acidity. Therefore,
the impact of thiols with different pKa values was investigated by studying the reaction of
Nꢀphenylbenzoxazine 1 with 1ꢀdodecanethiol (2a), benzylthiol (2b), thiophenol (2c) and 4ꢀnitrothiophenol
(2d), respectively. The reactions were performed with an initial reactant concentration of 0.2 M in CDCl3
as an aprotic solvent and a mixture of CDCl3/MeOD as a protic solvent, respectively, and followed by
1H NMR (Figure 4).
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CDCl3
CDCl3 and MeOD (1:1)
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Figure 4. Equimolar reactions of Nꢀphenylbenzoxazine 1 (0.2 M in CDCl3 and CDCl3/MeOD,
respectively) with thiols 2a-d. Yields of adducts 3a-d were determined after 25 h reaction time at room
temperature.
In neat CDCl3 all reactions of Nꢀphenylbenzoxazine 1 and the thiols 2a-d yielded between 51 and 70 % of
the appropriate adduct 3a-d within 25 h. The aromatic thiols 4ꢀnitrothiophenol (2d) and thiophenol (2c)
yielded almost an equimolar mixture of 1 in relation to 3d and c. The aliphatic ones 2a and b caused a
slight shift in equilibrium to the addition products 3a (58 %) and b (70 %). However, in case of a solvent
mixture of CDCl3 and MeOD (1:1) the equilibria were shifted strongly to the appropriate addition
products yielding up to 95 % 3a-d after 25 h (Figure 4). That process was also shown for reactions with
cyclohexanethiol (2e), 1ꢀbutanethiol (2f) and pentafluorothiophenol (2g) that formed the corresponding
adducts 3e-g in excellent yields (>91 %).
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These observations solvents having a high impact on the equilibrium are consistent with the ones reported
by Gorodisher et al. and the studies described by Kawaguchi et al.17,21 In polar aprotic solvents (DMSO,
DMF)17 reactions do not take place, whereas less polar aprotic solvents such as chloroform gave equilibria
and a polar protic environment (MeOD:CDCl3) provided almost full conversions. A protic solvent such as
methanol promotes the first protonation step of the benzoxazine by stabilizing the resulting ionic pairs,
which provide a CurtinꢀHammettꢀlike situation due to rapid interconversion (Figure 1). Furthermore, a
protic environment would promote a proton transfer from phenol to amino group yielding zwitterion (D),
and shifting equilibrium to the addition product (Figure 1, D).
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From these results, it is clear that solvent effects dominate over thiol effects. Therefore, to determine the
influence of thiol acidity on this reaction; the next step was to investigate the reactions under solventꢀfree
conditions.
Solvent-free Ring-Opening Addition Reaction
Gorodisher et al. described that the ringꢀopening reaction of benzoxazines and thiols can be performed in
bulk.21 In order to study the impact of thiol reactivity on the equilibrium, all reactions were performed at
room temperature by dissolving Nꢀphenylbenzoxazine 1 in the corresponding thiols 2a-g, respectively,
1
and followed by H NMR in DMSOꢀd6. In case of 4ꢀnitrothiophenol (2d), which is a solid, both reagents
were homogenized by grinding (Figure S1). The reaction rate k was calculated considering the final
conversions and reaction times. Almost full conversion of 1 was observed in case of the aromatic thiols 2c
and 2d after four and one hours reaction time, respectively. The reaction mixture of the highly acidic 2g
changed from a liquid state to solid state in less than five minutes, pointing to an extremely fast reaction.
However, 1H NMR gave no clear results as in this case the equilibrium reaction did not stop in DMSOꢀd6,
acetonitrileꢀd3 or acetoneꢀd6, which was not observed up to now for other thiols . In contrast, the aliphatic
thiols 2a, 2b, 2e and 2f reacted much slower reaching the final equilibrium after 48 h.
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-3
-4
-5
-6
y = -0.16pKa - 2.6
R² = 0.98
2d
2c
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2f
2e
2b
2a
5
10
15
pKa(DMSO)
20
Figure 5. Logarithmic plot of the Brønsted relation of the benzoxazineꢀthiol reaction rate vs. thiols acid
ionization constant Ka (Reaction scheme in Figure 2).
The relationship between the logarithm of the reaction rate constant log(k) and the logarithm of the acid
ionization constant Ka is known as Brønsted catalysis law or Brønsted relation and describes the relation
of acidic strength and catalytic activity or reactivity of involved acids. Plotting log(k) as a function of the
pKa values showed a linear relationship (Figure 5) implying that the different thiols act through the same
reaction mechanism. Thus, the protonation step takes place at the same reaction site – either the oxygen or
the nitrogen – for all of the thiols (Figure 1, A and B). Additionally, the preꢀlogarithmic factor α exhibits a
low value (α = 0.16) pointing to a reactantꢀlike transition state and to the protonation as rateꢀdetermining
step.
Comparing the thiol reactivities with respect to their nucleophilicity, one would expect the opposite
situation – namely, the best result for the highly nucleophilic aliphatic thiols 2a, 2e and 2f. The
dependence of the rate constant on the acidity of the thiols and its insensitivity to their nucleophilic
strength suggests the protonation step to be the rate determining step with a favored protonation on
benzoxazines nitrogen being in rapid equilibrium with the protonated oxygen intermediate. The protonated
benzoxazine and the generated thiolate, which is a stronger nucleophile than the corresponding thiol,
overcome the low nucleophilicty of the more acidic thiols. This agrees with the studies of Gorodisher et
al. changing the residues at benzoxazines nitrogen atom. Benzoxazine derivatives with more basic amines
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reacted faster in presence of thiols.21 Hence, acidic thiols and increased basicity of benzoxazine enhance
benzoxazine/thiol reactions in bulk.
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Reversibility of Solvent-free Ring-Opening Addition Reactions
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Solvent mediated reactions of benzoxazine and thiols proved to be reversible upon adjusting the
environment with appropriate solvent compositions. However, systematic studies on the reversibility of
the solvent free reaction are missing. Benzoxazine/thiol adducts 3a-g were placed in a Schlenk tube and
exposed to vacuum, respectively, with the aim to shift the equilibrium by removing recovered thiol. The
1
amount of recovered Nꢀphenylbenzoxazine 1 was determined by H NMR measurements and correlated
with the boiling points of the appropriate thiols 2a-g (Figure 6). Adducts 3a, 3b and 3d with boiling points
above 195 °C featured Nꢀphenylbenzoxazine 1 recovery around 65 % with a higher amount of side
product formation. 3c, 3e, 3f and 3g with boiling points below 170 °C featured a recovery of 1 with yields
higher than 92 % and with high purity. As illustrative example of benzoxazine recovery, 1H NMR spectra
of 3c before and after treatment are shown in Figure 7.
Even though 1 was converted to 3 in bulk, those reactions were still chemical equilibrium reactions, as
was proven by benzoxazine recovery upon vacuum exposure. The reversible nature of the ringꢀopening,
dissociation reaction in bulk and the impact of the used thiols were proven and shown successfully.
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2a-g
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4 Pa; 90 °C
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n-C12H25
Ph
PhNO2
Cy
n-C4H9
C6F5
g
a
b
c
d
e
f
R =
100
CH2Ph
2e
2c
2f
90
80
70
60
50
2g
2d
2b
2a
80
120
160
200
240
280
b.p. of 2 / °C at 1 atm
Figure 6. Dissociation reaction of adducts 3a-g upon removal of the thiol 2a-g at 4 Pa, 90 °C. Conversion
of adducts 3a-g to Nꢀphenylbenzoxazine 1 in dependency of the corresponding boiling point of thiols
2aꢀg.
1
3c
1 recovered
aromatic
H2O DMSO
Figure 7. 1H NMRꢀspectrum (200 MHz, DMSOꢀd6) of pure Nꢀphenylbenzoxazine 1, pure
benzoxazine/thiol adduct 3c and recovered Nꢀphenylbenzoxazine 1 after 6 h in vacuum at 90 °C.
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CONCLUSIONS
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Reversible ringꢀopening addition and fragmentation reactions of pꢀcresol based Nꢀphenylꢀ1,3ꢀbenzoxazine
with various aliphatic and aromatic thiols with a wide range of acidities were shown to proceed in solvents
and in bulk. When the reaction of benzoxazine 1 and thiols 2a-g was performed in solution, the effect of
the solvent dominated over the acidity of the thiol. The polar protic solvent mixture CDCl3/MeOD 1:1
favored the ringꢀopening addition reaction of the benzoxazine and yielded excellent conversions, whereas
the slightly polar aprotic solvent CDCl3 gave equilibrium concentrations for all studied thiols. In solventꢀ
free reactions, the reaction rate was strongly related to the thiol acidity. The acidic thiols thiophenol,
4ꢀnitrothiophenol and pentafluorothiophenol with low pKa values reacted faster than the aliphatic 1ꢀ
butanethiol, 1ꢀdodecanethiol and cyclohexanethiol with higher pKa values. This indicated that the initial
protonation is the rateꢀdetermining step giving a CurtinꢀHammettꢀlike situation. Studies of the backward
reaction showed that benzoxazine could be recovered by ringꢀclosing of the benzoxazine/thiol adducts 3
using vacuum to remove the simultaneously formed thiols. Applying thiols with boiling points lower than
170 °C, the reaction yielded the recovered benzoxazines in high yields and purities.
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EXPERIMENTAL
Materials
Aniline (≥ 99.5 %), pꢀcresol (99 %), 1ꢀdodecanethiol (≥ 98 %), benzylthiol (99 %), thiophenol (97 %),
nꢀhexane (99.9 %), 4ꢀnitrothiophenol (≥ 90 %), cyclohexanethiol (97 %), 1ꢀbutanethiol (99 %),
2,3,4,5,6ꢀpentafluorothiophenol (97 %), toluene (≥ 99.3 %), paraformaldehyde (pure), methanolꢀd4
(99.8 %), chloroformꢀd (99.8 %), dimethyl sulfoxideꢀd6 (99.8 %), acetondeꢀd6 (99.8 %) and acetonitrileꢀd3
(99.8 %) were used as received.
Methods
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1HꢀNMR and CꢀNMR spectra were recorded at 200 MHz and 50 MHz, respectively. In case of CDCl3
tetramethylsilane, in case of DMSOꢀd6 residual protons and in case of MeOD/CDCl3 residual protons of
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MeOD were used as internal standard. Infrared spectroscopy was performed with a Golden Gate ATR
unit. Highꢀresolution mass spectra were recorded on a MALDIꢀTOF or a MALDIꢀOrbitrap. The MALDIꢀ
TOF samples were prepared with 2,5ꢀdihydroxybenzoic acid as matrix in acetonitrile. Measurements were
conducted in reflection mode with an acceleration voltage of 2000 V and an extraction delay time of
70 ns. The MALDIꢀ Orbitrap samples were prepared by dissolving the products in the matrix solution
(2,5ꢀdihydroxybenzoic acid at 10 mg mLꢀ1 in methanol) and depositing them using the dry droplet method
on a multigrid glass slide. Exact mass determination was performed using an atmospheric pressure
scanning microprobe MALDI source with a Q ExactiveTM HF Fourier transform orbital trapping mass
spectrometer. A nitrogen laser with a wavelength of 337 nm and 60 Hz repetition rate was used for
desorption and ionization purpose. Mass spectral acquisition was done in positive mode (m/z 50ꢀ750 Da,
mass resolving power 240.000 at 200 Da), with fixed automatic gain control and 500 ms injection time
from a minimum of 20 positions per droplet. Mass calibration was performed using matrix cluster peaks.
Melting points were determined by differential scanning calorimetry (DSC) as the onset of the
endothermic melting signal heating at 10 K minꢀ1.
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Preparation of 6-methyl-3-phenyl-3,4-dihydro-2H-1,3-benzoxazine 1
The 6ꢀmethylꢀ3ꢀphenylꢀ3,4ꢀdihydroꢀ2Hꢀ1,3ꢀbenzoxazine (1) (Nꢀphenylbenzoxazine 1) was prepared
according to the literature.17,21,25 Aniline (8.8 g, 95 mmol) and paraformaldehyde (2.9 g, 97 mmol) were
dissolved in 110 mL toluene and heated under reflux in a 250 mL threeꢀnecked roundꢀbottomed flask
equipped with a DeanꢀStark device for 18 h. Cresol (10.3 g, 94.5 mmol) and paraformaldehyde (3.8 g,
130 mmol) were added to the mixture, which was heated under reflux for 60 h. The solution was allowed
to cool to ambient temperature and the crude product was washed three times with aqueous 0.5 M NaOH,
two times with aqueous saturated NaCl solution and two times with distilled water. The organic phase was
dried with anhydrous Na2SO4, the toluene was removed at low pressure and the resulting solid was
recrystallized from nꢀhexane. The product was isolated as white crystalline solid (15.9 g, 75 %).
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1
White solid. H NMR (200 MHz, DMSOꢀd6): δ 7.28ꢀ7.17 (m, 2 H), 7.15ꢀ7.07 (m, 2 H), 6.93ꢀ6.80 (m,
3 H), 6.64ꢀ6.58 (m, 1 H), 5.40 (s, 2 H) 4.60 (s, 2 H), 2.19 (s, 3 H). 13C NMR (50 MHz, DMSOꢀd6,): δ
151.7, 147.9, 129.1, 128.1, 127.3, 120.9, 120.4, 117.3, 116.0, 78.6, 48.9, 20.2; IR (ATR): ṽ (cmꢀ1) 3028,
2908, 1599, 1493, 1367, 1217, 943, 815, 800, 745, 684. Mp (DSC onset 10 K minꢀ1): 49 °C.
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Benzoxazine/thiol reactions in solution
Study of effect of reactant concentration for the ring-opening and dissociation reaction.
Nꢀphenylbenzoxazine 1 and 1ꢀdodecanethiol 2a were dissolved in deuterated chloroform with various
concentrations from 0.01 to 0.24 M, respectively. For the dissociation reaction adduct 3a was dissolved in
deuterated chloroform with various concentrations from 0.01 to 0.24 M, respectively. The equilibrium
1
concentration of 1 was determined after a reaction time of 25 h, respectively by integrating H NMR
methylene groups signals belonging to the starting material 1, and adduct 3a. (Equation 1).
ꢇꢈ
ꢆ
2
(ꢁ)
ꢀ ꢂ ⁄ꢀ ꢂ
=
ꢅ
ꢁ
ꢁ
ꢃꢄ
ꢇꢈ
+ ꢇꢈ
ꢆ
2
(ꢉꢊ)
ꢆ
2
(ꢁ)
(1)
Study of effect of thiols 2 and solvents for the ring-opening and dissociation reaction
Nꢀphenylbenzoxazine 1 (0.2 mmol) and the appropriate thiol 2a-g (0.2 mmol) were dissolved in the
studied deuterated solvent (1 mL) under inert gas conditions and stirred at room temperature for 25 h. The
conversion was determined with 1H NMR spectroscopy.
Assuming that the benzoxazine/thiol adduct 3a-g is formed during ringꢀopening reaction exclusively, the
1
Nꢀphenylbenzoxazine 1 conversion was estimated by integrating H NMR signals belonging to the
methylene groups of the starting material and of the addition reaction product 3a-g and calculating the
relative amount of methylene groups, respectively (Equation 2).
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ꢇꢈ
ꢆ
2
(ꢉ)
⁄
ꢋꢌꢍꢎꢏ ꢐꢑ ꢉ % = 100 ∙
ꢇꢈ
+ ꢇꢈ
ꢆ
2
(ꢉ)
ꢆ
2
(ꢁ)
6
7
(2)
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4ꢀmethylꢀ2ꢀ((phenyl((dodecylthio)methyl)amino)methyl)phenol 3a
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Transparentꢀyellow oil. 1H NMR (200 MHz, DMSOꢀd6): δ 9.36 (s, 1 H), 7.21ꢀ7.07 (m, 2 H), 6.91ꢀ6.59 (m,
6 H), 4.73 (s, 2 H), 4.46 (s, 2 H), 2.59ꢀ2.46 (m, 2 H), 2.09 (s, 3 H), 1.58ꢀ1.42 (m, 2 H), 1.32ꢀ1.16 (m,
13
18 H), 0.88–0.82 (m, 3 H). C NMR (50 MHz, DMSOꢀd6): δ 152.6, 147.1, 128.8, 128.0, 127.7, 127.1,
123.7, 117.1, 114.9, 113.3, 55.0, 49.0, 31.3, 30.8, 29.8, 29.1, 29.0, 28.9, 28.8, 28.6, 22.1, 20.4, 14.0. IR
(ATR): ṽ (cmꢀ1) 3392, 3026, 2920, 2849, 1597, 1497, 1245, 945, 817, 770, 743, 695. HRMS (MALDI/
Orbitrap): m/z [M + H]+ calcd 26 for C27H42NOS]+ 428.2982; found 428.2983.
[
4ꢀMethylꢀ2ꢀ((phenyl((benzylthio)methyl)amino)methyl)phenol 3b
1
Light yellow solid. H NMR (200 MHz, DMSOꢀd6): δ 9.38 (s, 1 H), 7.36ꢀ7.07 (m, 7 H), 6.89ꢀ6.00 (m,
6 H), 4.72 (s, 2 H), 4.42 (s, 2 H), 3.82 (s, 2 H), 2.09 (s, 3 H).13C NMR (50 MHz, DMSOꢀd6): δ 152.6,
147.0, 138.8, 128.9, 128.7, 128.4, 128.0, 127.7, 127.2, 126.8, 123.6, 117.2, 114.9, 113.4, 55.0, 49.2, 35.0,
20.4. IR (ATR): ṽ (cmꢀ1) 3350, 3032, 2960, 2859, 1601, 1495, 1247, 952, 820, 764, 695. Mp (DSC onset
10 K minꢀ1): 49 °C. HRMS (MALDI/TOF): m/z [M + H]+ calcd 26 for [C22H24NOS]+ 350.1573; found
350.1562.
4ꢀmethylꢀ2ꢀ((phenyl((phenylthio)methyl)amino)methyl)phenol 3c
1
White solid. H NMR (200 MHz, DMSOꢀd6): δ 9.33 (s, 1 H), 7.49ꢀ7.40 (m, 2 H), 7.35ꢀ7.07 (m, 5 H),
6.87ꢀ6.63 (m, 6 H), 5.13 (s, 2 H), 4.34 (s, 2 H), 2.04 (s, 3 H).13C NMR (50 MHz, DMSOꢀd6): δ 152.5,
146.7, 135.6, 131.8, 129.1, 128.9, 128.0, 127.1, 126.9, 123.4, 117.7, 114.9, 113.7, 59.3, 49.3, 20.3. IR
(ATR): ṽ (cmꢀ1) 3164, 3023, 2919, 2859, 1601, 1495, 1247, 825, 769, 738, 688. Mp (DSC onset
10 K minꢀ1): 87 °C; HRMS (MALDI/Orbitrap): m/z [M + H]+ calcd 26 for [C21H22NOS]+ 336.1417; found
336.1417.
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4ꢀmethylꢀ2ꢀ((phenyl((4ꢀnitrophenylthio)methyl)amino)methyl)phenol 3d
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Yellow solid. H NMR (200 MHz, DMSOꢀd6): δ 9.38 (s, 1 H), 8.16ꢀ8.04 (m, 2 H), 7.65ꢀ7.56 (m, 2 H),
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7.25ꢀ7.12 (m, 2 H, 6.91ꢀ6.66 (m, 6 H), 5.40 (s, 2 H), 4.44 (s, 2 H), 2.05 (s, 3 H). 13C NMR (50 MHz,
DMSOꢀd6): δ 152.5, 147.2, 146.4, 144.9, 129.3, 129.0, 128.2, 128.1, 127.1, 123.7, 123.2, 118.2, 114.9,
114.0, 58.1, 49.4, 20.3. IR (ATR): ṽ (cmꢀ1) 3211, 3082, 3019, 2922, 2859, 1594, 1524, 1495, 1339, 1218,
954, 849, 824, 767, 740, 691. Mp (DSC onset 10 K minꢀ1): 86 °C; HRMS (MALDIꢀTOF): m/z [M + H]+
calcd 26 for [C21H21N2O3S]+ 381.1267; found 381.1237.
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4ꢀmethylꢀ2ꢀ((phenyl((cyclohexylthio)methyl)amino)methyl)phenol 3e
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Transparentꢀyellow oil. H NMR (200 MHz, MeOD/CDCl3): δ 7.22ꢀ7.14 (m, 2 H), 6.91ꢀ6.67 (m, 6 H),
4.64 (s,1 H), 4.62 (s, 2 H), 4.51 (s, 2 H), 2.61ꢀ2.51 (m, 1 H), 2.16 (s, 3 H), 1.92ꢀ1.86 (m, 2 H), 1.71ꢀ1.59
13
(m, 2 H), 1.30–1.16 (m, 6 H). C NMR (50 MHz, MeOD/CDCl3): δ 153.4, 148.6, 129.5, 129.2, 129.0,
129.0, 123.9, 119.4, 115.8, 115.5, 54.7, 50.5, 44.3, 34.8, 26.6, 26.2, 20.8. IR (ATR): ṽ (cmꢀ1) 3361, 3027,
2925, 2850, 1598, 1495, 1247, 1227, 948, 884, 816, 748, 691. HRMS (MALDI/Orbitrap): m/z [M + H]+
calcd 26 for [C21H28NOS]+ 342.1886; found 342.1889.
4ꢀmethylꢀ2ꢀ((phenyl((butylthio)methyl)amino)methyl)phenol 3f
1
Transparentꢀyellow oil. H NMR (200 MHz, MeOD/CDCl3): δ 7.22ꢀ7.14 (m, 2 H), 6.91ꢀ6.65 (m, 6 H),
4.65(s,1 H), 4.61 (s, 2 H), 4.53 (s, 2 H), 2.47 (t, 2 H), 2.15 (s, 3 H), 1.56ꢀ1.22 (m, 4 H), 0.83 (t, 3 H).
13C NMR (50 MHz, MeOD/CDCl3): δ 153.1, 148.2, 129.3, 129.0, 128.7, 128.7, 123.6, 119.1, 115.4, 115.2,
56.2, 50.3, 32.6, 31.9, 22.2, 20.5, 13.6. IR (ATR): ṽ (cmꢀ1) 3349, 3027, 2956, 2926, 2860, 1598, 1495,
1455, 1377, 1246, 1197, 1144, 1033, 949, 873, 747, 691. HRMS (MALDI/Orbitrap): m/z [M + H]+
calcd 26 for [C19H26NOS]+ 316.1730; found 316.1731.
4ꢀmethylꢀ2ꢀ((phenyl((2,3,4,5,6 pentafluorophenythio)methyl)amino)methyl)phenol 3g
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White solid. H NMR (200 MHz, MeOD/CDCl3): δ 7.17ꢀ7.09 (m, 2 H), 6.91ꢀ6.63 (m, 6 H), 5.00 (s,2 H),
4.65 (s, 1 H), 4.47 (s, 2 H), 2.15 (s, 3 H). 13C NMR (50 MHz, MeOD/CDCl3): δ 153.4, 147.5, 129.6, 129.4,
129.3, 129.2, 123.2, 120.4, 116.2, 115.5, 60.9, 49.6, 20.6. IR (ATR): ṽ (cmꢀ1) 3343, 2913, 1643, 1513,
1455, 1289, 1113, 978, 899, 859, 764, 697. Mp (DSC onset 10 K minꢀ1): 97 °C. HRMS
(MALDI/Orbitrap): m/z [M + H]+ calcd 26 for [C21H17NOSF5]+ 426.0946; found 426.0943.
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Benzoxazine/thiol reactions in bulk
Ring-opening reaction
The yield of 3 was estimated by integrating 1H NMR signals (recorded in DMSOꢀd6) belonging to the
methylene groups of the starting material and of the addition reaction product 3a-g, respectively, and
calculating the relative amount of methylene groups, respectively (Equation 2).
4ꢀmethylꢀ2ꢀ((phenyl((dodecylthio)methyl)amino)methyl)phenol 3a
Nꢀphenylbenzoxazine 1 (0.45 g, 2.0 mmol) and 1ꢀdodecanethiol (2a) (0.41 g, 2.0 mmol) were mixed and
the suspension was heated for five minutes at 50 °C. The resulting solution was stirred for 48 h at ambient
temperature. The product 3a was obtained without further purification as a pale yellow oil (72 %).
4ꢀMethylꢀ2ꢀ((phenyl((benzylthio)methyl)amino)methyl)phenol 3b
Nꢀphenylbenzoxazine 1 (0.45 g, 2.0 mmol) and benzylthiol (2b) (0.45 g, 2.0 mmol) were mixed and
stirred at ambient temperature and after 24 h the solution turned into an oil. After further 24 h stirring at
ambient temperature the product was isolated without further purification as a pale yellow solid (96 %).
4ꢀmethylꢀ2ꢀ((phenyl((phenylthio)methyl)amino)methyl)phenol 3c
Nꢀphenylbenzoxazine 1 (0.45 g, 2.0 mmol) and thiophenol (2c) (0.45 g, 2.0 mmol) were mixed and after
2 h stirring at ambient temperature the solution became an oil. Further stirring for 2 h at ambient
temperature yielded the product as a white solid (99 %).
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Nꢀphenylbenzoxazine 1 (0.45 g, 2.0 mmol) and 4ꢀnitrothiophenol (3d) (0.31 g, 2.0 mmol) were mixed in a
mortar and grinded for a short time. After 15 min at ambient temperature the homogeneous solid melted
and turned in to oil after 30 min. The product was yielded as a yellow solid without further purification
after 45 min (97 %).
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4ꢀMethylꢀ2ꢀ((phenyl((cyclohexylthio)methyl)amino)methyl)phenol 3e
Nꢀphenylbenzoxazine 1 (0.45 g, 2.0 mmol) and cyclohexanethiol (2e) (0.24 g, 2.0 mmol) were mixed and
the resulting solution was stirred for 48 h at ambient temperature. The product 3e was obtained without
further purification as a pale yellow oil (87 %).
4ꢀmethylꢀ2ꢀ((phenyl((butylthio)methyl)amino)methyl)phenol 3f
Nꢀphenylbenzoxazine 1 (0.45 g, 2.0 mmol) and 1ꢀbutanethiol (2f) (0.18 g, 2.0 mmol) were mixed and the
resulting solution was stirred for 48 h at ambient temperature. The product 3f was obtained without further
purification as a pale yellow oil (92 %).
4ꢀmethylꢀ2ꢀ((phenyl((2,3,4,5,6 pentafluorophenythio)methyl)amino)methyl)phenol 3g
Nꢀphenylbenzoxazine 1 (0.45 g, 2.0 mmol) and 2,3,4,5,6ꢀpentafluorothiophenol (2g) (0.40 g, 2.0 mmol)
were mixed. The resulting solution was stirred for 2 min at ambient temperature. After 5 min the oil
turned in to a solid. The product 3g was obtained without further purification as white solid. (80% in
DMSOꢀd6; containing errors due to continuing reaction in DMSO)
Dissociation reaction of benzoxazine/thiol adducts 3a-g
Benzoxazine/thiol adducts 3a-g were placed in Schlenk tubes and were heated to 90 °C. Schlenk tubes
were evacuated (4 Pa) for 6 h to remove the corresponding thiol. The conversions were calculated by
comparing integrals of the appropriate methylene groups with δ = 4.57 ppm (NꢀCH2ꢀC) from
Nꢀphenylbenzoxazine 1 and the hydroxyl groups with a chemical shift of δ = 9.7ꢀ8.8 ppm (ArꢀOH).
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Integrals in that range represents the corresponding adduct and possible side products. The conversion of
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adduct to the Nꢀphenylbenzoxazine 1 was calculated with (Equation 3).
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ꢒ
ꢓ
ꢔꢕ
ꢆ
ꢓ
( )
ꢁ
⁄
ꢋꢌꢍꢎꢏ ꢐꢑ ꢁ % = 100 ∙
(3)
ꢒ
ꢓ
ꢖꢕ ꢗ
ꢔꢕ
ꢆ
ꢆ
ꢓ
( )
ꢁ
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ACKNOWLEDGEMENTS
We acknowledge the financial support from the Bundesministerium für Bildung und Forschung (BMBF)
through the NanoMatFutur award (DuroCycleFVK 03XP0001), Prof. Martens at Carl von Ossietzky
Universität Oldenburg for the helpful discussions and the Universitat Autònoma de Barcelona where the
initial idea for the work arose.
ASSOCIATED CONTENT
Supporting Information
Compound characterization spectra (1H and 13C NMR, IR). This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*Eꢀmail: katharina.koschek@ifam.fraunhofer.de
Notes
The authors declare no competing financial interest.
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