Electron-transfer processes in the reaction of K-, K+(15-crown-5)2 with carbazole: Unexpected formation of metallic potassium

Article abstract of DOI:10.1016/S0022-328X(99)00569-0

Transfer of one electron from K^- to the aromatic ring occurs in the reaction of K^-, K⁺(15-crown-5)₂ with carbazole giving K° and a radical anion. The latter rapidly decomposes with the liberation of hydrogen and formation of carbazylpotassium. It can also undergo an intramolecular proton-exchange reaction followed by a transfer of the second electron from the part of K° product. That leads to a dianion which then reacts with another carbazole molecule. Finally, carbazylpotassium and 1,4-dihydrocarbazylpotassium are formed.

Full text of DOI:10.1016/S0022-328X(99)00569-0

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Journal of Organometallic Chemistry 595 (2000) 66–69
Electron-transfer processes in the reaction of K−, K+(15-crown-5)2
with carbazole: unexpected formation of metallic potassium
Zbigniew Grobelny a, Andrzej Stolarzewicz a,*, Grazyna Adamus a, Gintaras Buika b,
;
Juozas V. Grazˇulevicˇius b
a Centre of Polymer Chemistry, Polish Academy of Sciences, PL-41-800 Zabrze, Poland
b Department of Organic Technology, Kaunas Uni6ersity of Technology, 3028 Kaunas, Lithuania
Received 15 July 1999; received in revised form 17 September 1999
Abstract
Transfer of one electron from K− to the aromatic ring occurs in the reaction of K−, K+(15-crown-5)2 with carbazole giving
K° and a radical anion. The latter rapidly decomposes with the liberation of hydrogen and formation of carbazylpotassium. It can
also undergo an intramolecular proton-exchange reaction followed by a transfer of the second electron from the part of K°
product. That leads to a dianion which then reacts with another carbazole molecule. Finally, carbazylpotassium and 1,4-dihydro-
carbazylpotassium are formed. © 2000 Elsevier Science S.A. All rights reserved.
Keywords: Carbazole; Potassium anions; Electron transfer; Intramolecular proton transfer
1. Introduction
2. Results and discussion
Alkali metals are soluble in some organic solvents in
the presence of crown ethers, giving dark blue solutions
in which the negative species are metal anions [1]. In
the case of Na, K, Rb or Cs solutions, metal anions
and cations complexed by the ligand are formed. When
solubilizing bimetallic systems, e.g. Na-containing al-
loys, sodium anions and cations of the other alkali
metal are obtained [2]. These systems have been em-
ployed as novel reagents especially for the metallation,
enolization and selective ring-opening reactions of hete-
rocyclic compounds [3–8] and also as the reducing
agents of acetylene derivatives or aromatic hydrocar-
bons [9] and ethers [10,11]. However, the literature
concerning the reaction with a compound containing
acidic hydrogen atoms is scanty [12].
When a solution of carbazole in THF was slowly
dropped into the blue K−, K+(15-crown-5)2 solution,
discoloration of the reaction mixture resulting from the
K− decay was observed at about 20 mol% excess of
carbazole relative to potassium anions. After that, no
potassium anions or carbazole remained in the system
as determined by 39K-NMR and GC–MS methods,
respectively. Hydrogen was observed to evolve, and a
yellow–white product was precipitated during the reac-
tion. A piece of solid substance of metallic color was
also found in the liquid phase.
Part of the latter substance was isolated from the
mixture. It dissolved rapidly in n-butanol with libera-
tion of hydrogen. Benzylation of the resulting solution
allowed us to identify n-butyl benzyl ether in the mix-
ture using the GC–MS method. Another part of the
solid substance was introduced into the solution of
15-crown-5 in THF. The substance dissolved giving a
deep blue color for the solution. In this solution K−
and K+(15-crown-5)2 ions were detected by 39K-NMR.
The experiments clearly showed that metallic potassium
was one of the reaction products.
In the present work, carbazole (pKa=19.9 [13]) was
selected for the study in this field and its reaction with
K−, K+(15-crown-5)2 solution in tetrahydrofuran
(THF) was examined.
When the metallic potassium had been removed,
methyl iodide was added to the reaction mixture and
* Corresponding author. Fax: +48-32-2712-969.
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(99)00569-0

Z. Grobelny et al. / Journal of Organometallic Chemistry 595 (2000) 66–69
67
Scheme 1.
carbazole and 9-methylcarbazole was found to be ca.
1:2.5, being in a good agreement with the chromato-
graphic data. A value close to that was also obtained
from the gated-decoupled 13C-NMR spectrum recorded
with 300 s delay between successive pulses.
Based on the experimental results the following
mechanism for the reaction between carbazole and
potassium supramolecular complex is proposed. In the
first step a one-electron transfer occurs from K− of 1 to
the aromatic ring of carbazole, resulting in potassium
metal 2 and radical anion 3 (Scheme 1).
Then 3 decomposes with the liberation of hydrogen
and formation of carbazylpotassium 4; compound 2
remains in the system (Scheme 2).
Compound 3 can also undergo an intramolecular
proton exchange reaction giving 6. Transfer of the
second electron from a part of K° to 6 leads to the
formation of dianion 7 (Scheme 3).
In the last step of the process, dianion 7 reacts with
carbazole to give carbazylpotassium 4% and 1,4-dihydro-
carbazylpotassium 8 (Scheme 4).
The scheme proposed above is consistent with the
literature data. For example, during reduction of ben-
zene by lithium in NH3–Et2O with ethanol, 1,4-dihy-
drobenzene is formed via an intermediate radical anion
and radical [15]. The radical anion with acidic hydro-
the liquid phase was analyzed by GC–MS. The chro-
matogram showed two signals of methylated products.
The first one was identified as 9-methylcarbazole in
70% yield. The mass spectrum of this product was
identical to the spectrum of original compound with a
molecular ion at 181. The mass spectrum of the second
product (26% as calculated from the signal intensities of
both reaction products) was very similar; a molecular
ion of 183 was found in this case.
An attempt to separate the two compounds and to
obtain them without impurities was unsuccessful. How-
ever, NMR spectroscopy was found to be useful in
identification of the unknown reaction product. The
1H-NMR spectrum of the methylated mixture showed
(aside from the signals of 9-methylcarbazole) signals
which can be ascribed, respectively, to aromatic protons
(multiplets within the range of 8.05–6.97 ppm, partially
overlapped with those from 9-methylcarbazole), pro-
tons of unsaturated aliphatic groups (two identical mul-
tiplets centered at 5.86 ppm), a methyl (singlet at 3.32
ppm) and methylene protons (two multiplets centered
at 3.25 ppm). It is worthwhile noting that analogous
signals due to CHꢀCH and CH2 protons were observed
1
earlier in the H-NMR spectrum of 1,4-dihydronaph-
thalene, obtained during the reaction of naphthalene
with metallic potassium [14]. The 13C-NMR analysis of
the product formed in the examined system seems to
confirm the structure of the unknown product. Besides
the signals characteristic for 9-methylcarbazole the
spectrum exhibits ten signals within the range from 105
to 137 ppm, which can be ascribed to six aromatic and
four olefinic carbons, a signal at 28.74 ppm due to a
CH3 group and two signals arising from CH2 groups (at
23.4 and 23.3 ppm). The assignment was supported by
analyzing the 13C-DEPT-NMR spectrum of the mix-
ture. From the NMR analysis it follows that the signal
of the unknown compound observed in the chro-
matogram is due to 1,4-dihydro-9-methylcarbazole. No
other hydrogenation products were formed as indicated
by the absence of any additional signals in the GC–MS
and NMR spectra. By integrating the NMR signals
corresponding to the methyl protons of the two com-
pounds, the molar ratio between 1,4-dihydro-9-methyl-
Scheme 2.

68
Z. Grobelny et al. / Journal of Organometallic Chemistry 595 (2000) 66–69
The mechanism of the process studied in the present
work assumed the existence of paramagnetic species.
However, the electron spin resonance (ESR) analysis
did not indicate their presence in the reaction mixture.
It could be explained by their short lifetime. All the
reactions were very fast, and therefore, the methylated
derivatives of 3, 6 and 7 were not found in the mixture
after quenching by CH3I.
The experiments were repeated several times to check
their reproducibility, and the same results were ob-
tained. However, the real novelty of this work concerns
the fact that metallic potassium was one of the reaction
products. That is a new observation in the chemistry of
metal anions.
Until now, the potassium anion has been known as a
double electron donor [17]. It reacted in two steps.
Initially, a single electron transfer took place from K−
to the acceptor molecule, resulting in the radical anion
and K°. Then K° transferred the second electron to
either the radical anion, giving dianion, or to the other
acceptor molecule, converting it into the next radical
anion. Jedlin´ski [18] studied the mechanism of these
processes. It was found that K− reacted with b-lactone
in the molar ratio of 1:1 [6]. The single-electron transfer
from the metal anion to the lactone molecule resulted in
the formation of K° and the lactone radical anion. The
latter formed enolate radical after CꢁC bond cleavage.
This radical reacted with potassium, giving the corre-
sponding enolate carbanion with K+ as a counterion.
The reaction between K− and benzophenone was car-
ried out at the molar ratio of 1:2 [19]. The single-
electron transfer from the potassium anion to the ke-
tone molecule resulted in the ketyl radical anion and
K°. The latter reacted with another benzophenone
molecule giving the next radical anion and a potassium
cation. Finally, K+ was formed from K− in these
experiments as well as in others reported in the litera-
ture [2–12]. According to our knowledge, only in the
present work was K° found as the final reaction
product of K−.
Scheme 3.
gen, formed in the reaction of potassium with
triphenylmethane, can undergo decomposition with the
liberation of hydrogen [16]. A stable dinegative ion can
be formed when naphthalene is metallated by potas-
sium in THF [14]. In the case of b-ethylnaphthalene
possessing a labile hydrogen in the substituent, the
initial aromatic dianion is isomerized into a dihy-
dronaphthalene monoanion as a result of an in-
tramolecular proton exchange reaction.
It is worth noting that potassium anions also react
with N-substituted carbazole, e.g. methylcarbazole and
vinylcarbazole. In contrast to unsubstituted carbazole,
stable radical anions were formed in these systems, their
concentration being of the order of 10−5 M as revealed
by ESR. The blue color of the metal solution changed
to dark red in this case. Metallic potassium was not
observed in the reaction mixture.
Therefore, it may be concluded that the mechanism
of the metal anion reaction depends on the presence of
acidic hydrogen atom in the reagent molecule. The K−
acts as the electron transfer agent, whereas typical
nucleophilic ions, e.g. RO−, cause deprotonation of
carbazole giving the carbazole anion in a one-step
process [20].
Scheme 4.

Z. Grobelny et al. / Journal of Organometallic Chemistry 595 (2000) 66–69
69
3. Experimental
3.2.1. 9-Methylcarbazole (5)
1H-NMR CDCl3: l 8.05–7.09 (m, 8H, Ar); 3.53 (s,
3H, CH3). 13C-NMR CDCl3: l 140.8, 125.5, 122.6,
120.1, 118.7, 108.3; (CAr, six signals); 28.66 (CH3). Mass
spectrum (m/e): 181 [M+, 100]; 180 (60); 167 (6); 152
(14); 140 (6); 90 (4); 39 (1).
3.1. Materials
THF (POCH) was purified by the method described
earlier [21]. Carbazole (Aldrich) and 9-methylcarbazole
(Aldrich) were purified by recrystallization from xylene.
15-Crown-5 (Aldrich) was dried under vacuum at 50°C
for several hours. The 0.1 M K−, K+(15-crown-5)2
dark blue solution was prepared by dissolving metallic
potassium in the 0.2 M 15-crown-5 THF solution at
20°C. The contact time was 25 min. The details of the
experimental procedures are described elsewhere [22].
3.2.2. 1,4-Dihydro-9-methylcarbazole (9)
1H-NMR CDCl3: l 8.05–6.97 (m, 4H, Ar); 5.86 (dm,
2H, ꢀCH); 3.25 (dm, 4H, CH2); 3.32 (s, 3H, CH3).
13C-NMR CDCl3: l 136.7, 132.6, 126.6, 125.6, 122.1,
120.6, 118.6, 117.8, 108.4, 105.8; (CAr, six signals+
CHꢀ, four signals); 28.74 (CH3), 23.4, 23.3 (CH2, two
signals). Mass spectrum (m/e): 183 [M+, 100]; 182 (94);
167 (56); 152 (9); 140 (6); 90 (5); 39 (1).
3.2. General procedure
A 0.1 M K−, K+(15-crown-5)2 (10 cm3) solution was
slowly titrated by 12 cm3 of a 0.1 M solution of
carbazole in THF under a dry argon atmosphere. Dur-
ing this time the reaction mixture became colorless. The
reaction was carried out at 20°C with stirring and 4 cm3
of hydrogen was found to evolve during the reaction. A
titration time of 10 min was required to obtain metallic
potassium in the form of a solid piece, weighing 0.021
g after washing in THF. At considerably shorter peri-
ods of time, the metal was usually dispersed. After
isolation of the metal, the reaction mixture was
quenched by CH3I and analyzed by GC–MS. The mass
of the methylated reaction products was equal to 0.19
g.
GC–MC analyses were performed 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 for the
yield determination.
Acknowledgements
The authors are indebted to Dr J. Grobelny for
NMR measurements and to Dr H. Janeczek for ESR
analysis.
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Analysis of hydrogen was conducted by the GC
technique on a 2.4 m long stainless steel column packed
with Al2O3 (0.02–0.03 mm) and deactivated with 5%
K2CO3, using an INCO 505 gas chromatograph with
flame ionization detector.
1H-, 13C- and 39K-NMR spectra were recorded at
20°C on a Varian VXR-300 multinuclear spectrometer
1
at the H resonance frequency of 300 MHz, 13C reso-
nance frequency of 75 MHz, and 39K resonance fre-
quency of 14 MHz. CDCl3 was used as the solvent for
1H- and 13C-NMR analysis. Chemical shifts were refer-
enced to tetramethylsilane (TMS) serving as an internal
standard in this case.
ESR measurements were carried out with an ESR
300 Bruker X-band spectrometer employing 100 kHz
field modulation and a microwave frequency of ca. 9.4
GHz. In order to determine the concentration of para-
magnetic species a double rectangular cavity was used
and a solution of 2,2-di(4-tert-octylphenyl)-1-picrylhy-
drazyl (DPPH) served as a reference.