An alternative approach to differentially substituted 2-oxazoline chalcogen derivatives

Article abstract of DOI:10.1016/j.jorganchem.2015.06.022

In this study we present an alternative method to obtain several substituted mono or bis-2-oxazolines containing a chalcogen atom as a tether element. Alkylation of 2-tosyloximethylene-2-oxazoline with an appropriate sodium, lithium or potassium alkoxide yielded the corresponding ether. Introduction of sulfur or selenium was easily accomplished using the corresponding sodium salts. The ⁷⁷Se NMR for alkyl or aryl 2-methylene-2-oxazoline selenides shows good correlation with the electronegativity pattern of substituents. Most products containing oxazolinyl-chalcogen were stable under the usual experimental conditions. However, the tellurium derivatives showed unusual sensitivity to light and oxygen, decomposing through a very complex mechanistic pathway. As a result of this oxidative process, 4,4-dimethyl-2-oxazoline-2-carbaldehyde could be isolated and fully characterized for the first time, in 17% yield.

Full text of DOI:10.1016/j.jorganchem.2015.06.022

Journal of Organometallic Chemistry 794 (2015) 11e16
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
An alternative approach to differentially substituted 2-oxazoline
chalcogen derivatives
,
, 1
Murilo B.M. de Mello a 1, Giuliano C. Clososki b, Leandro Piovan a
,
Alfredo R.M. de Oliveira a
Universidade Federal do Parana, Departamento de Química, Centro Politecnico, Curitiba 81.531-980, Brazil
, *, 1
a
ꢀ
ꢀ
b
ꢀ
^
^
~
~
ꢀ
Núcleo de Pesquisas em Produtos Naturais e Sinteticos, Faculdade de Ciencias Farmaceuticas de Ribeirao Preto, Universidade de Sao Paulo, Av. do Cafe,
~
s/nº, Ribeirao Preto, SP 14040-903, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
In this study we present an alternative method to obtain several substituted mono or bis-2-oxazolines
containing a chalcogen atom as a tether element. Alkylation of 2-tosyloximethylene-2-oxazoline with
an appropriate sodium, lithium or potassium alkoxide yielded the corresponding ether. Introduction of
sulfur or selenium was easily accomplished using the corresponding sodium salts. The 77Se NMR for alkyl
or aryl 2-methylene-2-oxazoline selenides shows good correlation with the electronegativity pattern of
substituents. Most products containing oxazolinyl-chalcogen were stable under the usual experimental
conditions. However, the tellurium derivatives showed unusual sensitivity to light and oxygen,
decomposing through a very complex mechanistic pathway. As a result of this oxidative process, 4,4-
dimethyl-2-oxazoline-2-carbaldehyde could be isolated and fully characterized for the first time, in
17% yield.
Received 23 March 2015
Received in revised form
11 June 2015
Accepted 15 June 2015
Available online 23 June 2015
Keywords:
Differentially substituted 2-oxazolines
2-oxazolinyl chalcogenides
77Se NMR of seleno 2-oxazolines
Aza-allyl telluride oxidation
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
aminosulfur trifluoride (Deoxo-Fluor) [16], and PPh3/DEAD [17].
CaO has been utilized to promote dehydration from boron esters of
2-oxazolines are important heterocyclic compounds [1] that
occur in nature as iron chelators [2], cytotoxic peptides [3], and
neuroprotective agents [4], to cite a few. They are also important in
biological systems, since they are involved in the complexation of
metallic ions such as Cu2þ and Zn2þ [5]. Moreover, 2-oxazolines
have several well-known applications in organic synthesis as pro-
tective groups [6], synthetic intermediates [7], and chiral ligands
[8].
The most common approach to synthesize 2-oxazolines in-
volves the condensation of a 1,2-amino alcohol with a carboxylic
acid or its ester derivatives [9]. Several other masked carboxylic
acid functional groups can be used, including nitriles [10], imidate
hydrochlorides [11], orthoesters [12], and iminoether hydrochlo-
rides [13]. A variety of reagents have been employed to accomplish
the cyclodehydration step, such as PPh3/DDQ [14], dieth-
ylaminosulfur trifluoride (DAST) [15], bis-(2-methoxyethyl)-
N-(2-hydroxyethyl) amides [18]. Recently, the synthesis of 2-
pyridyl-2-oxazolines was reported using Zn(OAc)2 as catalyst un-
der microwave heating [19].
2-oxazoline-inspired ligands (Fig. 1) such as BOX, PYBOX (1a-d),
and INDABOX 2, to name a few, were developed and applied to
metal catalysis involving copper, palladium, rhenium, rhodium, or
ruthenium [20]. Most of the existing methods used to obtain these
BOX-type ligands are restricted to structures with a C2-symmetry
chiral axis [21e23].
Although the ortho-lithiation of 2-aryl-2-oxazolines has been
widely used for the regioselective functionalization of arenes and
heteroarenes [24], the anion derived from 2-methyl-2-oxazoline is
the most frequently explored species. It has been employed in the
synthesis of carboxylic acid homologues [25], lactones [26], and
zinc [27] or copper [28] organometallic reagents, among others
[29].
A clever way to use 2-methyl-2-oxazolines is by placing a
leaving group at the 2-position. One example is 2-
chloromethylene-2-oxazoline, which can be obtained using a
classic condensation method by reacting chloromethyl imidate
hydrochloride and an aminoalcohol [30]. Unfortunately, this
* Corresponding author.
E-mail addresses: murilo@ufpr.br (M.B.M. de Mello), lpiovan@ufpr.br (L. Piovan),
armo@quimica.ufpr.br (A.R.M. de Oliveira).
1
http://www.quimica.ufpr.br.
http://dx.doi.org/10.1016/j.jorganchem.2015.06.022
0022-328X/© 2015 Elsevier B.V. All rights reserved.

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M.B.M. de Mello et al. / Journal of Organometallic Chemistry 794 (2015) 11e16
Fig. 1. Examples of BOX type ligands found in the literature.
Scheme 2. Synthesis of compounds 8a, 8b and 8c.
Scheme 1. General route for the synthesis of 2-oxazolinyl ethers.
method is very inefficient when 2-amino-2-methyl-1-propanol is
used [31]. To overcome this limitation, an alternative chlorination
method using t-butylhypochlorite in carbon tetrachloride was
developed by Langlois [31] and well explored by Florio [32] in
several studies. However, due to environmental restrictions, the
use of carbon tetrachloride has been banned in many countries,
including Brazil.
In order to extend the studies with 2-oxazoline chemistry, we
report the development of an alternative building block that can be
used in place of 2-chloromethylene-4,4-dimethyl-2-oxazoline. It
was used for the synthesis of 2-oxazolines with more diverse
structural features, avoiding the use of environmentally aggressive
reagents. During the synthesis of tellurium-oxazoline compounds,
an unusual photo oxidation process was observed that is similar to
the decomposition of allylic or benzylic tellurides.
Scheme 3. Synthesis of sulfide 8d and chiral sulfide 8e.
The dropwise addition of an appropriate alkoxide solution in
THF at room temperature into a solution of 5 or 6, also dissolved in
THF, followed by 2 h of reflux, afforded the desired products in good
to excellent yields (Table 1). As observed, the reaction worked very
well with several alkoxides such as alkyl (7a) (80%), allylic (7b)
(72%), propargylic (7c) (75%) or aromatic (7d) (85%). Reactions were
very clean and the products could be easily purified by column
chromatography. Lower yields were observed only when isolating
the more volatile products (7b, 7c). This method also preserves the
stereochemistry of the substrate, as can be seen for products 7e and
7f (Table 1, entries 5 and 6).
During the initial studies for preparation of compound 5, a small
amount of the BOX-like, bis-(2-methyl-2-oxazolinyl) ether (8a) was
formed as a byproduct, which prompted us to conduct further in-
vestigations. By fine-tuning the reaction conditions, either com-
pound 5 or 8a could be obtained with high selectivity and excellent
yield. The symmetrical and chiral ether 8b was achieved by using
similar conditions (Scheme 2). Additionally, the differentially
substituted ether 8c was obtained in 79% isolated yield by reacting
the sodium alkoxide from 3 with chiral tosylate 6. Thus, this
2. Results and discussion
2-hydroxy-methylene-2-oxazolines 3 and 4 were prepared in
multi-gram scale [33] and easily converted into the corresponding
tosylates 5 or 6, in 94 and 95% yields, respectively, through reaction
with tosyl chloride in THF at 0 ꢀC (Scheme 1).
Table 1
Synthesis of 2-oxazolinyl ethers.
Entry
Tosylate
Alkoxide
Product
Yielda
80%
1
5
2
5
72%
3
4
5
5
75%
85%b
5
6
5
6
85%
90%
a
Isolated yield.
Obtained using K2CO3/acetone.
b

M.B.M. de Mello et al. / Journal of Organometallic Chemistry 794 (2015) 11e16
13
yield) or reacting compound 5 with sodium n-butoxide (79% yield).
A slightly modified procedure was also used to prepare chalco-
genides 15a-c from 5. The protocol involves the generation of
lithium chalcogenolates through the reaction of elemental sulfur,
selenium, or tellurium with n-BuLi (Scheme 5) [35].
Compounds 7a and 15(a, b) were stable, fruity-smelling oils and
could be easily purified by flash column chromatography. Inter-
estingly, the telluride 15c was highly sensitive to light and oxygen
and its purification could not be achieved using conventional lab-
oratory techniques. It could only be isolated using a helium gas
atmosphere during flash chromatography and in a dark chamber.
The solvent was evaporated in a stream of dry helium gas, and the
product was immediately transferred to a NMR tube under the
same atmosphere. CDCl3 was added and the decomposition of 15c
was monitored by 125Te NMR. After 45 min exposed to air, ambient
light and at room temperature, it was not possible to observe any
tellurium containing material (see Fig. 2). The consumption of
telluride 15c was followed by the precipitation of a white solid that
was insoluble in most organic solvent systems.
Scheme 4. Synthesis of the compounds 8f, 8g, 10, 12 and 14.
The 1H or 13C NMR analysis of compound 15c showed a very
complex mixture. GC-MS analysis of the resulting solution revealed
several oxidation products, including 1-butanol, 1-butanal and
2,4,4-trimethyl-2-oxazoline (16). These were identified by com-
parison with known standard compounds.
Scheme 5. Synthesis of n-butyl chalcogen 2-oxazolinyl ethers.
Some telluride compounds are prone to decompose through
photo oxidation processes. Cava [36a] and Uemura [36b], using
dibenzyl tellurides and aryl allyl tellurides, respectively, are among
the pioneers in this field of research. Ferreira [37] described this
type of decomposition for aryl benzyl tellurides with formation of a
complex mixture of products from dimerization, extrusion of
tellurium and benzylic oxidation processes. Ouchi [38] reported a
similar observation using aryl alkyl tellurides and sunlight oxida-
tion. Oba [39] and co-workers investigated the photosensitized
oxygenation of diaryl tellurides using singlet oxygen. To the best of
our knowledge, this is the first report of oxidation using more
complex tellurides.
Some of the photo degradation products from compound 15c
were expected to be volatile, subject to loss during irradiation or
extraction. To avoid this, compound 18 was synthesized using a
decyl group side chain so less volatile byproducts would be formed.
The ditelluride 17 was obtained in 97% yield following a known
procedure [40] and converted into telluride 18 in 60% yield (Scheme
6).
strategy is a straightforward one-pot method for the synthesis of
bis 2-oxazolinyl substituted ethers (Scheme 2).
To expand the scope of this method, sulfur was introduced using
Na2S.9H2O as source of sulfide anions. Thus, reaction of sodium
sulfide (Na2S) with two equivalents of tosylate 5 furnished com-
pound 8d in 95% yield (Scheme 3). Moreover, when tosylate 6 was
used as substrate, the expected chiral derivative 8e was isolated in
94% yield (Scheme 3).
The method described here is also suitable for the synthesis of
selenium derivatives. Thus, selenides 8f and 8g were synthesized in
good yields through the reaction of sodium selenide (Na2Se) [34a]
with tosylate 5 and 6. Also, the reduction of diphenyldiselenide
[34b,c] (9) or 4-chloro-diphenyldiselenide (11) using NaBH4 in
EtOH/THF with subsequent addition of 5 gave selenides 10 and 12
in excellent yields. (For a discussion on the 77Se NMR data see S.I.).
Sulfide 14 was further obtained in 75% yield from 13 using the
same strategy (Scheme 4).
When telluride 18 was submitted to the previously described
oxidation conditions, a complex mixture was observed by GC
Ether 7a could be prepared using two strategies: alkylating the
corresponding sodium alkoxide from 3 with n-butyl bromide (94%
Fig. 2. 125Te NMR (CDCl3) spectrum showing the disappearance of butyl-2-oxazolinyl telluride (15c) signal at 393 ppm after 45 min of exposure to oxygen and light. (Ref. PhTeTePh
at 422 ppm).

14
M.B.M. de Mello et al. / Journal of Organometallic Chemistry 794 (2015) 11e16
Scheme 8. Synthesis of phenyl-2-oxazolinyl telluride (22).
Scheme 6. Synthesis of n-decyloxazolinyltelluride (18).
sample was heated to 200 ꢀC in the dark, the signal for telluride 22
disappeared and those for 2,4,4-trimethyl-2-oxazoline (16),
diphenyltelluride (23) and diphenylditelluride (21) increased,
showing thermal decomposition. When telluride 22 was heated to
200 ꢀC, protected from light and in the presence of oxygen, besides
the above products, aldehyde 24 was identified.
To track the decomposition profile, after synthesizing the
phenyl-2-oxazolinyl telluride 22, a sample was immediately puri-
fied, the solvent was evaporated and a small amount was used for
the GC analysis.
Despite our efforts, assuring inert conditions during the purifi-
cation process was difficult. For this reason, the initial profile for GC
analysis was taken immediately after the purified sample was
dried. As can be seen, the initial analysis already shows some
degradation products with the telluride 22 being the major signal
at 23.5 min. In addition, whether this process occurs thermally in
the GC injector or if the substances were already in solution prior to
injection could not be determined (Fig. 3).
To understand the oxidation process, a sample of 22 was kept
at ꢁ20 ꢀC, protected from oxygen and light, for 45 min. As expected,
no significant differences were observed from the initial profile.
When this sample was exposed to light, at ꢁ20 ꢀC and protected
from oxygen, no differences were observed either. However, when
a sample was kept in the dark, at ꢁ20 ꢀC but in the presence of
oxygen, after 45 min the GC signal for compound 22 completely
disappeared (100% conversion) and compounds 16 and 21 were
detected along with a minor amount of 23. When this last sample
was irradiated with a dichroic lamp, besides the above compounds
aldehyde 24 was detected, showing telluride 22 was not stable in
the presence of oxygen at any temperature and forms an unde-
tectable intermediate, which is converted into compounds 24 and
16 (2:1 by GC) after light irradiation.
analysis. In either reaction, with 15c or 18, we were unable to detect
alcohols or carboxylic acid derivatives (Scheme 7).
Using GC analysis, it was possible to identify oxidation products
from both substituent groups, showing a complete lack of selec-
tivity of the process.
It is well known that when aryl benzyl tellurides are submitted
to similar oxidative conditions, only their benzylic portion is
oxidized and no aryl oxidation products can be detected [37,39].
This characteristic prompted us to synthesize the phenyl-2-
oxazolinyl telluride (22) (68% yield) [41a,b]. This compound was
also isolated by flash chromatography using helium gas to pres-
surize the system, under low power red light to avoid any prema-
ture oxidation reaction (Scheme 8) [41c].
Aiming to understand the influence of the light source, the
decomposition of 22 was performed using an incandescent light
bulb, a dichroic light bulb or a high-pressure mercury UV light bulb.
All samples were kept in an open flask, irradiated during 45 min at
20 ꢀC and the result was monitored by GC analysis. The results were
essentially the same: disappearance of the starting material fol-
lowed by the appearance of a similar set of products (Scheme 9). To
ensure reproducibility, from this point on all essays were carried
out using a 25-W dichroic lamp placed 40 cm from the flask. The
dichroic light bulb was chosen since dichroic filters retain the IR
irradiation, giving an almost cold visible white light [42].
When a sample of 22 was stored in a freezer at ꢁ15 ꢀC for 5 days,
protected from light and oxygen, no decomposition was observed.
Oxidation of 22 using similar conditions was analyzed by GC-MS
and 2,4,4-trimethyl-2-oxazoline (16), diphenylditelluride (21),
diphenyl telluride (23), telluride 22 and an unknown compound
with molecular ion of m/z ¼ 127 Da were identified. The GC-IR
analysis of this unknown compound showed strong absorption at
1741 cmꢁ1, indicating a carbonyl aldehyde. This hypothesis was
confirmed by 1H NMR, 13C NMR, and high-resolution mass spectral
analysis. The 1H NMR showed an unexpected chemical shift for the
aldehyde hydrogen at 7.77 ppm. Using 13C NMR and DEPT-135
experiments, it was possible to attribute the signal at 150.7 ppm
to the carbonyl group and the 2-oxazoline carbon at 154.3 ppm.
Aldehyde 24 was also observed when compounds 15c or 18 were
exposed to the same reaction conditions (Scheme 9).
When photo degradation was carried out using methanol-d4 as
solvent, only the 2,4,4-trimethyl-2-oxazoline (16) incorporated
deuterium at the 2-methyl position. Compound (16) can be formed
through homolytic scission at the TeeC bond, as proposed by Cava
[36], followed by a proton or deuterium capture from the solvent.
No other deuterated compounds were detected by GC-MS analysis.
To verify the effect of temperature, a sample of compound 22
was prepared at 25 ꢀC, protected from light and oxygen and kept at
these conditions during 45 min. The resulting solution was
analyzed by GC, showing no significant decomposition. When this
Diaryl tellurides do not react with oxygen while in the dark or
without a sensitizer [39] and a light source. Phenyl benzyl telluride
and phenyl allyl tellurides are sensitive to ambient light/oxygen
conditions but do not react with oxygen in the absence of light [36].
Unlike these aryl telluride systems, phenyl-2-oxazolinyl telluride
has strong sensitivity to oxygen, reacting with it even at low tem-
peratures. Following Cava's proposal, we were able to react tellu-
ride 22 with oxygen to give 1,3 dipolar peroxide 25 (or the
dioxatellurirane 25a [39]). Rearrangement of 25 (or 25a) formed a
light unstable oxzolinylperoxytellurophenyl intermediate 26, from
which aldehyde 24 was formed (Scheme 10).
3. Conclusions
In summary, we have reported an alternative approach to syn-
thesize differentially substituted 2-oxazoline chalcogen derivatives.
By using this method, symmetrical or unsymmetrical (chiral or not)
chalcogenide ethers were obtained, in multi-gram scale, with good
yields and in a straightforward manner.
Scheme 7. Photo decomposition of n-decyl-2-oxazolinyl telluride (18).
Scheme 9. Products from photo oxidation of 22.

M.B.M. de Mello et al. / Journal of Organometallic Chemistry 794 (2015) 11e16
15
Fig. 3. Chromatograms of the oxidation products from phenyl-2-oxazolinyl telluride 22. Chromatogram A shows the starting material 22 as a major component. Chromatogram B
shows the products after 45 min under light and oxygen.
(200 MHz, CDCl3):
d
0.96 (t, J ¼ 6.5 Hz, 3H), 1.30 (s, 3H), 1.4 (m, 2H),
1.6 (m, 2H), 3.52 (t, J ¼ 6.5 Hz, 2H), 3.9 (s, 2H), 4.1 (s, 2H); 13C NMR
(50 MHz CDCl3):
d 13.5, 18.8, 27.9, 31.1, 64.9, 66.8, 71.0, 78.9, 162.2;
IR in cmꢁ1: 1103, 1679.
Synthesis of -phenyllteluride-metilene-4,4-dimethyl-2-
a
oxazoline (22): In a 25 mL round bottom flask equipped with
reflux condenser, 0.5 mmol, 205 mg of diphenylditelluride was
dissolved in 5.0 mL of dry THF, under dry and oxygen free nitrogen.
Lithium (10 mmol, 70 mg) was added and the solution was refluxed
for 6 h and cooled to room temperature. With a scoop, lithium was
removed and all care was taken to avoid light and oxygen. A solu-
tion of 2-oxazoline 5 (1.0 mmol, 283 mg) in 2 mL of THF was added
via cannula and stirred for 30 min. The solvent was removed and
the crude mixture was purified using flash column chromatography
under red light (1:1 hexane/ethyl acetate) and helium atmosphere,
Scheme 10. Possible mechanistic pathway for the photo decomposition of telluride 22.
The somewhat surprising behavior of telluro-2-oxazoline 22 can
be attributed to its oxy-aza-allyl system, which can stabilize elec-
trical charges or even radical species. Additionally, similar to Cava's
observations, the tellurides (15c, 18 and 22) were stable to light
irradiation unless oxygen is present. When exposed to oxygen, even
in the absence of light, tellurides were converted slowly to a
product that could not be detected by GC analysis, but once it was
exposed to light, it gave rise to aldehyde 24 and 2,4,4-trimethyl-2-
oxazoline (16). This aldehyde was isolated for the first time, in 17%
yield, and was fully characterized by HRMS, 1H RMN, 13C RMN and
IR analysis. All data are available in the supplementary material.
yielding 68% of 22, as a yellow oil. 1H NMR (200 MHz CDCl3):
d 1.16
(s, 6H), 3.60 (s, 2H), 3.89 (s, 2H), 7.24 (m, 3H), 7.81 (m, 2H); 13C NMR
(50 MHz CDCl3):
d 0.1, 27.9, 67.3, 79.5, 128.3, 129.2, 129.8, 139.2,
165.0; 125Te NMR (126 MHz CDCl3):
d 613 ppm.
Acknowledgments
~
We are grateful to the Brazilian funding agencies Fundaçao
ꢀ
Araucaria, CAPES and CNPq for financial support. We are also
grateful to the NMR Center of UFPR for all NMR analyses.
4. Experimental section
Appendix A. Supplementary data
General procedure to obtain compounds 7(aec) or 7(eef)
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jorganchem.2015.06.022.
Synthesis of a-butyloxymethylene-4,4-dimethyl-2-oxazoline
(7a): n-butanol (0.13 mL, 1.47 mmol) was dissolved in 2 mL of dry
THF and n-BuLi (1 equiv., 1.47 mmol) was slowly added. In a second
flask, compound 5 (1.35 mmol, 380 mg) was dissolved in 1 mL of
dry THF and transferred dropwise to the first flask. The resulting
solution was refluxed for 2 h and then the mixture was washed
with brine and extracted with ethyl acetate (3x15 mL). The organic
layers were combined and concentrated in a rotary evaporator. The
resulting oil was purified using flash column chromatography (1:1
hexane/ethyl acetate), yielding 80% of a colorless oil. 1H NMR
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