Self-decomposition of K-, K+(15-crown-5)2 tetrahydrofuran solution via organometallic intermediates

Article abstract of DOI:10.1016/S0022-328X(01)00760-4

Self-decomposition of K^-, K⁺(15-crown-5)₂ tetrahydrofuran solution results in dipotassium tetraethylene glycoxide and ethylene as the main reaction products. Dipotassium triethylene glycoxide, potassium tetraethylene glyco

Full text of DOI:10.1016/S0022-328X(01)00760-4

Journal of Organometallic Chemistry 628 (2001) 65–70
www.elsevier.nl/locate/jorganchem
−
+
Self-decomposition of K , K (15-crown-5) tetrahydrofuran
2
solution via organometallic intermediates
a
a,
b
Zbigniew Grobelny , Andrzej Stolarzewicz *, Adalbert Maercker
a
Centre of Polymer Chemistry, Polish Academy of Sciences, 41-819 Zabrze, Poland
Institut f u¨ r Organische Chemie, Uni6ersit a¨ t Siegen, D-57068 Siegen, Germany
b
Received 27 November 2000; accepted 20 February 2001
Abstract
−
+
Self-decomposition of K , K (15-crown-5) tetrahydrofuran solution results in dipotassium tetraethylene glycoxide and
2
ethylene as the main reaction products. Dipotassium triethylene glycoxide, potassium tetraethylene glycoxide vinyl ether,
potassium butoxide and potassium ethoxide are found as secondary products of the reaction. Organometallic intermediate
compounds formed in the studied process are very reactive and their life-times are short. The mechanism of decomposition
−
reactions is proposed. The course of the process is found to be different from that presented earlier in the literature for the K ,
+
K (18-crown-6) system. © 2001 Elsevier Science B.V. All rights reserved.
Keywords: Potassium anion; 15-Crown-5; Potassium alkalide
1
. Introduction
furan solution was presented in Ref. [17]. It was
assumed that the potassium anion attacked at first the
crown ether molecule. The crown ring opening occurred
in this case resulting in dipotassium pentaethylene gly-
coxide and ethylene or dipotassium tetraethylene gly-
coxide and tetrahydrofuran. The linear glycoxides
formed were also cleaved by potassium anions. Further
It is known that alkali metals (except lithium) are
soluble in some linear and cyclic ethers [1–4]. Their
blue solutions contain metal anions and cations as well
as a small amount of solvated electrons. Application of
glymes, crown ethers or cryptands allow us to obtain
high metal concentrations [2,4]. Such solutions are used
in organic synthesis [5–11] and as initiators of the
anionic polymerization [12–15].
−
reactions, in which K and the next destruction prod-
ucts took part, led to a mixture of dipotassium glycox-
ides with the number of CH CH O repeating units
2 2
The stability of these solutions depends on the tem-
perature and on the kind of metal, solvent and ligand
equal from n=5 to n=1. However, dipotassium penta-
and tetraethylene glycoxides (n=5 and 4, respectively),
i.e. the initial products of 18-crown-6 ring opening, as
well as dipotassium triethylene glycoxide (n=3) were
formed in small amounts. Dipotassium diethylene gly-
coxide (n=2) seemed to be the main decomposition
product of the system. Quantitative data concerning the
amount of dipotassium ethylene glycoxide (n=1) were
not reported. Potassium glycoxides are non-volatile,
therefore, the reaction mixture was treated with
methanol for gas chromatography–mass spectrometry
−
+
[
16]. For example, K , K (18-crown-6) solution in
tetrahydrofuran starts to decompose already during its
preparation at ambient temperature [17], whereas under
−
+
the same conditions K , K (15-crown-5) system is
2
still stable during the one hour after the preparation
[
18]. However, in both cases the blue solution becomes
colorless after some time. Potassium anions vanished at
this moment [16].
A scheme of destruction reactions taking place dur-
−
+
(
GC–MS) analysis. Appropriate glycols were observed
ing discoloration of K , K (18-crown-6) in tetrahydro-
in this case in the chromatogram. Butanol was also
found in the protonated mixture. It was concluded that
the tetrahydrofuran ring was opened by potassium
anions giving butanol after protonation. However, a
*
Corresponding author. Tel.: +48-32-2716077; fax: +48-32-
2
712969.
E-mail address: polymer@uranos.cto.us.edu.pl (A. Stolarzewicz).
0
022-328X/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(01)00760-4

6
6
Z. Grobelny et al. / Journal of Organometallic Chemistry 628 (2001) 65–70
mechanism of decomposition reactions taking place in
this metal solution was not proposed.
Some years later Dye [19] suggested that an impor-
tant step of decomposition may be deprotonation of
ether (2%) as well as benzyl butyl ether (10%) and
benzyl ethyl ether (4%) were present in the chro-
matogram of the benzylated sample. Tetra- and tri-
ethylene glycol dibenzyl ethers were found to be rather
unstable at elevated temperature [24]. Therefore, their
real yields could not be estimated by GC. Di- and
monoethylene glycol dibenzyl or dimethyl ethers were
not observed in the chromatograms of benzylated and
methylated samples.
CH groups in crown ethers and cryptands by electrons
2
or metal anions. It would lead to the formation of
hydrogen atoms and carbanions, a process that would
readily lead to radical chain decomposition. Cauliez et
al. [20] investigated the decomposition of crystalline
+
−
salts M C , M% free of solvent (where M and M% are
As benzyl butyl ether and benzyl ethyl ether were
identified after benzylation, the analysis of samples
subjected to methylation was repeated. It appeared that
the expected peaks of the methyl derivative of potas-
sium butoxide and potassium ethoxide, i.e. butyl methyl
ether and ethyl methyl ether, could not be observed in
the chromatogram due to their overlapping with the
broad peak of the solvent.
n
alkali metals, C is a crown ether and n is 1 or 2), called
by Dye [21] as alkalides. Their samples were decom-
posed at room temperature under inert atmosphere.
The process took place from a few minutes for the
more sensitive compounds to several days for the more
stable species. Several gaseous products were identified
in the reaction mixture, e.g. ethylene, ethane and bu-
tane. Glymes were obtained when methyl iodide was
used as a quenching agent. No other methylated prod-
ucts were observed. This ruled out the presence of
carbanions in the reaction mixture. The authors pro-
posed that the main pathway is a ‘chemical’ reduction
of the crown ether molecule by electrons, analogous to
the reduction of vicinal dihalides [22].
3
Ethylene (10 cm ) as well as traces of ethane and
hydrogen were found in gaseous and liquid phases
before and after benzylation or methylation. No higher
hydrocarbons were observed.
In separate experiments water was added to the
methylated or benzylated samples, causing the forma-
tion of two layers. One of them was the water solution
of potassium iodide or potassium bromide and the
second a tetrahydrofuran solution of the organic com-
pounds. The pH value of both the layers was found to
be equal to 7 indicating that such inorganic compounds
Various systems containing alkali metal anions were
studied till now. However, to our best knowledge for
the first time in the present work an attempt was made
−
+
to explain the decomposition mechanism of K , K
15-crown-5)2 tetrahydrofuran solution. The selected
(
as K O or KOH had not been formed in the reaction
2
system is the most stable one from the midst of known
alkalide solutions [16].
It is worth noting that alkalides prepared with the
use of the fully methylated aza-analogue of cryptand
mixture.
The results allowed us to assume that dipotassium
tetraethylene glycoxide, i.e. the initial product of 15-
crown-5 ring opening, and ethylene were the main
reaction products in the studied system. Dipotassium
triethylene glycoxide, potassium tetraethylene glycoxide
vinyl ether, potassium butoxide and potassium ethoxide
should be treated as the secondary decomposition
products.
[
2.2.2], i.e. without oxygen atoms in the molecule, were
found to be stable for weeks at ambient temperature
23]. However, those sodide and potasside were crystals
free of the solvent.
[
It means that this system differs qualitatively and
−
+
2. Results and discussion
quantitatively from K , K (18-crown-6) described in
the literature [17]. In that work, as mentioned before,
the original crown ring opening products, i.e. dipotas-
sium pentaethylene glycoxide and dipotassium te-
traethylene glycoxide, were found only in small
amounts in the decomposed reaction mixture. On the
other hand, in the present work potassium tetraethylene
glycoxide vinyl ether, potassium butoxide and potas-
sium ethoxide were found for the first time in the
discolored potassium solution. Moreover, the presence
of hydrogen in only trace amounts in the reaction
mixture showed that the earlier hypothesis about the
−
+
Colorless mixtures obtained from K , K (15-crown-
)2 blue tetrahydrofuran solutions were analyzed by the
5
GC–MS technique after treating with methyl iodide or
benzyl bromide. This allowed the transformation of
non-volatile decomposition products to their volatile
methyl or benzyl derivatives.
Tetraethylene glycol dimethyl ether (in 64% yield)
was identified as the compound giving the main signal
in the chromatogram of methylated sample. Two other
signals of a weaker intensity were found as belonging to
triethylene glycol dimethyl ether (15%) and te-
traethylene glycol methyl vinyl ether (2%). On the other
hand, signals of some analogous products, i.e. te-
traethylene glycol dibenzyl ether, triethylene glycol
dibenzyl ether and tetraethylene glycol benzyl vinyl
deprotonation of CH groups in crown ether by elec-
2
trons or potassium anions, suggested for several alkali
metal complexes [19], cannot be accepted in the case of
potassium system with 15-crown-5. Therefore, the kind
of crown ether influences not only the stability of

Z. Grobelny et al. / Journal of Organometallic Chemistry 628 (2001) 65–70
67
alkalide solutions but also the mechanism of their
decomposition.
Based on all these findings it is proposed that in the
first step of the process the cleavage of the CꢀO bond
−
+
takes place in the crown ether molecule of K , K (15-
−
crown-5) 1 by K of 1 (Scheme 1). It results in the
2
unstable potassium organometallic alkoxide 2. After the
reductive elimination of ethylene 3, dipotassium te-
traethylene glycoxide 4 is formed. The latter can partly
decompose under the influence of 1 to potassium 2-
potassioethoxide 6 and dipotassium triethylene glycox-
ide 7 (Scheme 2). Organometallic compound 6 then
reacts rapidly with the crown ether molecule of 7
causing its ring opening and the formation of potas-
sium ethoxide 8, potassium tetraethylene glycoxide vi-
nyl ether 9 and dipotassium triethylene glycoxide 7%.
The latter contains a crown ring opening product in-
stead of one crown ether molecule. The cleavage of the
1
5-crown-5 ring by various organometallic intermediate
products was reported already in Refs. [10,25,26].
Organometallic compounds formed in the reaction of 1
with methyloxirane [10], oxetane [25] or phenyl glycidyl
ether [26] were also very unstable and reacted immedi-
ately with the crown ether. That was the reason why
they were never isolated from the reaction mixture.
However, the same crown ring opening product, i.e. 9
was found in each of those works.
Scheme 1.
Potassium anions are still present in the blue solution
until its discoloration [16]. Therefore, it was assumed
that they could react not only with 15-crown-5 but also
with 9. Decomposition products of the latter were
found to be dipotassium tetra- and triethylene glycox-
ides (4 and 7), as mentioned in Ref. [10] concerning the
reaction of 1 with methyloxirane. This might explain
the low yield of 9 in the process under study. Recently,
the reaction of methyloxirane with an excess of 1 was
conducted by us. Methyloxirane tetrahydrofuran solu-
tion was slowly dropped into the freshly prepared blue
potassium solution until the latter became colorless.
The compounds 4 and 7 were the main reaction prod-
ucts in this case and the yield of 9 was low. It confirms
that 9 is unstable in the mixture containing potassium
anions. However, the explanation of its decomposition
mechanism as well as of other unsaturated alkyl ethers
needs more experiments and it will be the subject of a
separate paper.
On the other hand, the presence of benzyl butyl ether
in benzylated samples confirms the earlier statement
that potassium anions cleave the solvent molecules [17].
−
It seems that the reaction of K with tetrahydrofuran
leads to the formation of an intermediate product, i.e.
potassium 4-potassiobutoxide 13 (Scheme 3). The latter
opens the crown ether ring forming potassium butoxide
1
4 and potassium tetraethylene glycoxide vinyl ether 9.
Taking into account the hypothesis that 13 is a stable
Scheme 2.
compound, benzyl 5-phenylpentyl ether would be ex-

6
8
Z. Grobelny et al. / Journal of Organometallic Chemistry 628 (2001) 65–70
The difference between potassium alkalide containing
8-crown-6 and 15-crown-5 found in this work is most
1
likely connected not only with the kind of crown ether
but rather with the structure of its complex with the
potassium cation. 15-Crown-5 forms a 2:1 sandwich
complex with the potassium cation whereas 18-crown-6
gives a 1:1 flat one [2]. The structure of these complexes
seems to be decisive for the course of side reactions. In
the case of 15-crown-5 two sandwich complexes are
situated on both ends of dipotassium tetraethylene gly-
coxide. In some of them, the potassium tetraethylene
glycoxide vinyl ether molecule can be present instead of
one crown ether molecule. This is possible because
linear oligoethers also form stable complexes with alkali
metal cations [28,29]. It can be assumed that such a
structure causes steric hindrance diminishing the proba-
bility of the attack of 1. Therefore, potassium anions
should react rather with the next crown molecule than
with the glycoxide one. Another situation takes place in
the 18-crown-6 system. Smaller steric hindrance causes
the easy further decomposition of dipotassium penta-
and tetraethylene glycoxides, which results in a mixture
of various products.
Scheme 3.
It is worth noting that in reactions of the potassium
solution with methyloxirane [10] or oxetane [25] the
solvent was found to be inert and crown ether reacted
exclusively with organometallic intermediate com-
pounds. However, those reactions were instantaneous.
Potassium anions vanished immediately after the mix-
ing of reagents. Therefore, they could not take part in
the relatively very slow reaction with 15-crown-5 or
with tetrahydrofuran observed in the present work.
pected in benzylated samples. However, that compound
was not found in the analyzed systems. It points to a
high reactivity of 13 and indicates that other reactions
occurring with the participation of organometallic in-
termediates can also be very rapid.
In order to confirm the participation of solvent
molecules in the decomposition process, the blue potas-
sium solution was prepared using 2-methyltetrahydro-
furan instead of unsubstituted tetrahydrofuran.
However, in such a case benzyl isopropyl ether 21 was
detected after benzylation of the colorless mixture in-
stead of the expected benzyl 1-methylbutyl ether. It
could indicate that potassium potassioisopropoxide 19
is formed initially in that reaction with the elimination
of ethylene (Scheme 4). Then, the crown molecule of 19
is opened in an intramolecular reaction resulting in
potassium isopropoxide 20 and potassium tetraethylene
glycoxide vinyl ether 9. It is not know if the open chain
product 17 is really an intermediate during the frag-
mentation of 16 to the more stable branched radical
anion 18 [27]. A concerted process would also be
reasonable.
3
. Conclusions
The mechanism of self-decomposition of K , K (15-
−
+
crown-5) tetrahydrofuran solution was proposed. In
2
the first step of this process, the cleavage of the CꢀO
bond takes place in crown ether as well as in the solvent
under the influence of potassium anions. Organometal-
lic compounds with more or less ionic potassium–car-
bon bonds are formed as intermediate products. These
compounds undergo further rapid reactions. Most in-
terestingly, the organopotassium compounds do not
cleave the solvent as usual [30] but the crown ether
which is obviously more reactive due to its more exten-
The reactions presented in Schemes 1–4 are very fast.
No methylated or benzylated derivatives of
organometallic compounds 2, 6, 13 as well as 19 were
observed in the chromatograms because of their short
life-time. Similar results were found in our earlier works
+
sive complexation of K . It confirms the results of our
−
+
previous works concerning K , K (15-crown-5) reac-
2
tions with simple cyclic ethers, as methyloxirane [10]
and oxetane [25], in which the crown ether ring was
found to be unstable in the presence of organometallic
compounds.
The hypothesis assuming the participation of
organometallic species in the decomposition of alkali
metal solutions was not discussed till now in the litera-
[
10,25,26]. On the other hand, a hypothesis assuming
the existence of organometallic intermediates was till
now not confirmed in a direct experiment. However, the
structure of end products and the knowledge of the
reactivity of such compounds show that they will take
part in the studied process.

Z. Grobelny et al. / Journal of Organometallic Chemistry 628 (2001) 65–70
69
ture and it seems to be the main novelty of this work.
It is also a very important finding of the present work
that the kind of crown ether influences not only the
stability of potassium solutions but also the mechanism
of their decomposition.
on a Varian VXR-300 multinuclear spectrometer at the
H resonance frequency of 300 MHz and C resonance
1
13
frequency of 75 MHz. CDCl was used as the solvent.
3
Chemical shifts were referenced to tetramethysilane
(
TMS) serving as an internal standard.
Tetrahydrofuran (POCH) was purified by the method
presented in Ref. [16]. 2-Methyltetrahydrofuran was
purified in the same way. 15-Crown-5 (Aldrich) was
dried under vacuum at 50°C for several hours. Potas-
sium (Aldrich) was distilled under high vacuum.
4
. Experimental
GC–MS analyses of liquid compounds were per-
−
+
About 0.1 M K , K (15-crown-5) dark blue solu-
2
formed on a 30 m long fused silica capillary column
DB-1701 using a Varian 3300 gas chromatograph
equipped with a Finnigan MAT 800 AT ion trap
detector. Diethylene glycol dimethyl ether was used as
an internal standard. The gaseous components were
examined on a Chromatron GCHF chromatograph, on
a 2 m long metal column, filled with modified Al O , at
tion was prepared by dissolving of the potassium mir-
ror in 0.2
M
15-crown-5 tetrahydrofuran or
2
-methyltetrahydrofuran solution at 20°C. The contact
time was equal to 25 min. Details of the experimental
procedure were described elsewhere in Ref. [18]. After
filtering through the glass frit the solution was left for
about 24 h at ambient temperature, until its blue color
2
3
1
13
2
0°C. H- and C-NMR spectra were recorded at 20°C
Scheme 4.

7
0
Z. Grobelny et al. / Journal of Organometallic Chemistry 628 (2001) 65–70
[
[
10] Z. Grobelny, A. Stolarzewicz, M. Czaja, W. Demuth, A. Maer-
faded. Then, the colorless mixture was quenched with
methyl iodide or benzyl bromide and analyzed by GC–
MS, GC and NMR. The methylated or benzylated
decomposition products were identified by matching
cker, J. Org. Chem. 64 (1999) 8990.
11] Z. Grobelny, A. Stolarzewicz, G. Adamus, G. Buika, J.V.
Grazulevi cˇ ius, J. Organomet. Chem. 595 (2000) 66.
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1
their mass spectra, retention times as well as H- and
1
3
C-NMR spectra with those of original compounds.
Data concerning the mass spectra and NMR spectra of
the products were already presented in the literature.
They included tetraethylene glycol dimethyl ether [10],
triethylene glycol dimethyl ether [10], tetraethylene gly-
col methyl vinyl ether [10], benzyl butyl ether [31,32],
benzyl ethyl ether [33–35] and benzyl isopropyl ether
[
15] H. Janeczek, Z. Jedli n´ ski, I. Bosek, Macromolecules 32 (1999)
503.
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87 (1986) 795.
4
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[
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Acknowledgements
The authors are indebted to Dr K. Skutil and MSc.
S. B a˛ czy n´ ski for the analysis of gaseous reaction prod-
ucts and to MSc. B. Morejko-Buz; for GC–MS analy-
sis. This work was supported by the Committee of
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