Decomposition of the crown ether ring in the reaction of K-, K+(15-crown-5)2 with oxetane

Article abstract of DOI:10.1016/S0022-328X(00)00229-1

A cleavage of both oxacyclic rings occurs in the reaction of K^-, K⁺(15-crown-5)₂ with oxetane in tetrahydrofuran solution. Oxetane ring opening leads to the formation of organometallic compounds, which react with the crown molecule. Potassium methoxide, potassium n-propoxide as well as potassium tetra(ethylene glycoxide) vinyl ether are the main reaction products. It means that crown ether can act both as an activator and as a reagent under studied conditions.

Full text of DOI:10.1016/S0022-328X(00)00229-1

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Journal of Organometallic Chemistry 604 (2000) 283–286
Communication
Decomposition of the crown ether ring in the reaction of K−,
K+(15-crown-5)2 with oxetane
Zbigniew Grobelny a, Andrzej Stolarzewicz a,*, Adalbert Maercker b
a Centre of Polymer Chemistry, Polish Academy of Sciences, 41-819 Zabrze, Poland
b Institut fu¨r Organische Chemie, Uni6ersita¨t Siegen, D-57068 Siegen, Germany
Received 26 January 2000; received in revised form 16 April 2000
Abstract
A cleavage of both oxacyclic rings occurs in the reaction of K−, K+(15-crown-5)2 with oxetane in tetrahydrofuran solution.
Oxetane ring opening leads to the formation of organometallic compounds, which react with the crown molecule. Potassium
methoxide, potassium n-propoxide as well as potassium tetra(ethylene glycoxide) vinyl ether are the main reaction products. It
means that crown ether can act both as an activator and as a reagent under studied conditions. © 2000 Elsevier Science S.A. All
rights reserved.
Keywords: Potassium anion; Oxetane; 15-Crown-5
1. Introduction
oxide with a larger ring than that of oxirane, was used
in the study. This compound was useful for the experi-
ments because it does not polymerize anionically.
The mechanism of reaction of K−, K+(15-crown-5)2
with methyloxirane was studied in our previous work
[1]. It was a model reaction of the initiation step of
methyloxirane polymerization by the potassium
supramolecular complex. Unexpectedly, it turned out
that in this process, both the oxirane and crown ether
rings were opened. The reaction of a metal anion with
methyloxirane resulted at first in the formation of an
organometallic compound. The latter reacted with the
crown ether causing its decomposition. The main reac-
tion products included potassium isopropoxide and
potassium tetra(ethylene glycoxide) vinyl ether. There-
fore, it was shown for the first time that the crown ether
2. Results and discussion
The literature concerning the ring opening reaction
of oxetane in an anionic process is rather scanty. Ac-
cording to our knowledge only three papers were pre-
sented in this field until now. In the first [2], oxetane
was cleaved at 0°C in tetrahydrofuran (THF) by
lithium 4,4%-di-tert-butylbiphenylide, giving lithium g-
lithiopropoxide. A similar product, i.e. potassium g-
potassiopropoxide, was found as the product of
reaction of oxetane with K−, K+(18-crown-6) complex
at −20°C [3,4]. However, other results were observed
in the present work.
Benzyl methyl ether, benzyl n-propyl ether and te-
tra(ethylene glycol) benzyl vinyl ether in 22, 25 and 38%
yield, respectively, were found as the main benzylated
products of the reaction of K−, K+(15-crown-5)2 with
oxetane at 25°C. Ethylene (9 cm3) evolved during the
ring can be opened during
organometallic compound.
Further investigations on the 15-crown-5 ring open-
ing are presented in this work. Oxetane, i.e. a cyclic
a reaction with an
* Corresponding author. Tel.: +48-32-2716077; fax: +48-32-
2712969.
E-mail address: polymer@uranos.cto.us.edu.pl (A. Stolarzewicz).
0022-328X/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(00)00229-1

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Z. Grobelny et al. / Journal of Organometallic Chemistry 604 (2000) 283–286
Scheme 1.
process. The liquid phase was then analyzed by 13C-
NMR. In the spectrum the carbon signals assigned to
CH3O, CH3CH2CH2O and CH2ꢀCHO groups were ob-
served. The carbon signals from the PhCH2O group
were also present.
Scheme 2.
In order to analyze the side-products formed in the
studied system the reaction mixture was treated with
methyl iodide. Tri- and tetra(ethylene glycol) dimethyl
ethers in 15% total yield were found similarly, as in the
case of methyloxirane [1].
It was assumed that in the beginning of the examined
process a transfer of one electron from K− to the
oxetane molecule occurs resulting in the carbonꢁoxygen
bond cleavage, similarly as described earlier [3,4]. In the
case of the system with 15-crown-5, the following
scheme for the reaction is proposed.
Scheme 3.
The unstable radical anion 2 undergoes two reac-
tions. In the first one, 2 recombines with K0 as usual
giving potassium g-potassiopropoxide 3.
Scheme 4.
In the second reaction observed for the first time in
the system with 15-crown-5 2 decomposes to ethylene 4
and the radical anion of formaldehyde 5 which is
stabilized by mesomerism.
col) dimethyl ether were not observed after methylation
at the lower temperature, i.e. the destruction of potas-
sium tetra(ethylene glycol) vinyl ether under the influ-
ence of K− to the former side-products did not occur in
this case.
The differences in the reaction mode when using the
K−, K+(15-crown-5)2 system [1] or the K−, K+(18-
crown-6) system [5], respectively, were already observed
in the reaction with methyloxirane.
Recombination of 5 with K0 gives potassium potas-
siomethoxide 6.
Then, both organometallic compounds, i.e. 3 and 6,
react with the crown ether molecule.
The reactions presented in Schemes 1–6 are very fast.
No benzylated or methylated derivatives of 3 and 6
were found in the reaction mixture. Likewise, no prod-
ucts of the reaction of organometallic compounds 3 and
6 with oxetane or the solvent were present in the
system.
3. Conclusions
It is worth noting that 3 formed in the reaction of
oxetane with K−, K+(18-crown-6) at −20°C was
found to be relatively stable [3,4], e.g. it could be
protonated or methylated. No reaction with crown
ether was observed in that system. Therefore, we have
repeated the reaction of oxetane with K−, K+(15-
crown-5)2 at −20°C. Again benzyl methyl ether, benzyl
n-propyl ether and tetra(ethylene glycol) benzyl vinyl
ether were found in the reaction mixture after benzyla-
tion. It means that the same main products were
formed at −20°C as at +25°C. However, te-
tra(ethylene glycol) dimethyl ether and tri(ethylene gly-
Until now crown ethers were known, with some
exceptions concerning the reaction with metal anions
[6–9], as the stable compounds able to enhance the
reaction rate in chemical processes. The results of the
present work confirm our earlier finding [1] that cleav-
age of the 15-crown-5 ring occurs also in the presence
of organometallic compounds resulting in potassium
tetra(ethylene glycoxide) vinyl ether. It provides evi-
dence for a more general character of the phenomenon
that the crown ether unexpectedly acts both as an
activator and as a reagent.

Z. Grobelny et al. / Journal of Organometallic Chemistry 604 (2000) 283–286
285
Scheme 5.
It seems that the kind of crown ether decides on the
process. Potassium anion and the 18-crown-6 flat 1:1
complex with potassium cation gives the relatively sta-
ble organometallic product [3,4], whereas the use of
15-crown-5 forming a sandwich 2:1 complex with K+
results in unstable compounds. The latter react instan-
taneously with the crown ether and its oxacyclic ring is
opened.
atmosphere. Then, methanol (0.08 g, 2.5 mmol) was
added dropwise while stirring at 25°C. The course of
the reaction was followed by measuring of the amount
of hydrogen evolved. After 8 h, benzyl bromide (0.43 g,
2.5 mmol) was added to the mixture. The precipitated
potassium bromide was filtered off, and benzyl methyl
ether was distilled from the solution; the fraction boil-
ing at 170°C was collected in 85% yield. 1H-NMR
acetone-d6 l: 7.30–7.24 (m, 5H, Ph); 4.34 (s, 2H,
OCH2); 3.24 (s, 3H, OCH3). 13C-NMR acetone-d6 l:
139.3 (CPh ipso); 128.7 (CPh meta); 128.0 (CPh ortho);
127.8 (CPh para); 74.7 (OCH2); 57.9 (OCH3). MS (m/e):
122 (92, M+), 105 (4), 91 (100), 77 (45), 65 (33), 51 (23),
39 (20).
4. Experimental
Gas chromatography (GC) analysis of ethylene as
well as gas chromatography–mass spectrometry (GC–
MS), 1H- and 13C-NMR analyses of benzylated or
methylated products were carried out as described in
Ref. [1].
4.2. Benzyl n-propyl ether (9)
Oxetane (Aldrich) was heated over CaH2 for 3 h
followed by distillation under dry argon atmosphere,
the fraction boiling at 50°C being collected. Potassium
and tetrahydrofuran were purified as in Ref. [1].
The K−, K+(15-crown-5)2 solution in THF was pre-
pared as in Ref. [8]. Its reaction with oxetane was
conducted at 25°C under dry argon atmosphere. A 10
cm3 sample of 0.1 M K−, K+(15-crown-5)2 was added
dropwise to 10 cm3 of 1.0 M THF solution of oxetane.
The mixture was then treated with benzyl bromide or
methyl iodide. The same procedure was used for the
process conducted at −20°C. However, the benzyla-
tion or methylation of reaction products occurred this
time after several hours. The products were identified
by matching their mass spectra, retention times, and
1H- and 13C-NMR spectra with those of model
compounds.
Potassium hydride (0.10 g, 2.5 mmol) and THF (10
cm3) were introduced into the reactor under dry argon
atmosphere. Then, n-propanol (0.15 g, 2.5 mmol) was
added dropwise while stirring at 25°C. The further
4.1. Benzyl methyl ether (12)
Potassium hydride (0.10 g, 2.5 mmol) and THF (10
cm3) were introduced into the reactor under dry argon
Scheme 6.

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Z. Grobelny et al. / Journal of Organometallic Chemistry 604 (2000) 283–286
procedure was the same as in the case of the synthesis
of benzyl methyl ether. Finally, the product was dis-
tilled from the solution; the fraction boiling at 200°C
was collected in 88% yield. 1H-NMR acetone-d6 l:
7.36–7.15 (m, 5H, Ph); 4.42 (s, 2H, OCH2); 3.35 (t,
J=6.5 Hz, 2H, OCH2); 1.56 (m, J=7.4, 6.5 Hz, 2H,
CH2); 0.9 (t, J=7.4 Hz, 3H, CH3). 13C-NMR acetone-
d6 l: 139.8 (CPh ipso); 128.7 (CPh meta); 127.8 (CPh
ortho); 127.7 (CPh para); 73.0 (OCH2Ph); 72.3 (OCH2);
23.5 (CH2); 10.9 (CH3). MS (m/e): 150 (10, M+), 107
(15), 91 (100), 79 (25), 65 (30), 51 (11), 39 (15).
References
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