Formation of radical anions and not deprotonation starts the reaction of γ-butyrolactone with potassium anions

Full text of DOI:10.1021/jo050776o

Formation of Radical Anions and Not
Deprotonation Starts the Reaction of
γ-Butyrolactone with Potassium Anions
lactones after treatment with methyl iodide. The process
occurs by deprotonation of the lactone.
Jedli n˜ ski11 used alkalide K , K (18-crown-6) in tetra-
hydrofuran solution to generate several γ-lactone eno-
lates. That procedure constituted a novel route to R-alkyl-
or R-acyl-γ-lactones. It was suggested that this reaction
-
+
Zbigniew Grobelny* and Andrzej Stolarzewicz
Institute of Materials Science, University of Silesia,
-
proceeded by deprotonation of the lactone by K . The
4
0007 Katowice, Poland
reaction mechanism was not proposed. However, data
concerning the reagent selected in that work allowed us
to conclude that the deprotonation of γ-butyrolactone by
Adalbert Maercker
Institut f u¨ r Organische Chemie, Universit a¨ t Siegen,
-
K could not occur.
D-57068 Siegen, Germany
The potassium anion has been known as an electron-
transfer reagent. It transfers two electrons to the acceptor
molecule in two steps, giving finally the potassium cation
grobelny@box43.pl
1
2
as it was proved in the excellent work of Perrin and
Received April 16, 2005
accepted in other papers.1
3-16
Therefore, K cannot
-
behave as a nucleophile and, consequently, cannot be able
-
+
to deprotonate γ-lactone. Furthermore, alkalide K , K -
18-crown-6) already decomposes autocatalytically to a
(
large extent during its preparation at room tempera-
ture.17 It results in a mixture of various dipotassium
glycoxides, and their influence on the formation of
enolates is not known.
A mechanism is proposed for the reaction of γ-butyrolactone
with the potassium anion as a two-electron-transfer reagent.
Potassium hydride and potassium 4-potassiobutyrate are
formed in this process as intermediates. These compounds
deprotonate γ-butyrolactone. Potassium lactone enolate,
potassium butyrate, and hydrogen are the final reaction
products.
Therefore, we decided to reinvestigate the study de-
11
-
scribed in the work. γ-Butyrolactone and alkalide K ,
+
K (15-crown-5)
2
1 in tetrahydrofuran solution were
selected as suitable model reagents for that purpose. That
-
alkalide has been found to be much more stable than K ,
+
K (18-crown-6) and free from decomposition products
even 1 h after preparation.18
Lactones form enolates very easily in the presence of
1,2
strong bases. Potassium tert-butoxide, potassium naph-
Hydrogen (0.7 mmol, 70% yield) was found to evolve
during the reaction of γ-butyrolactone with 1. GC-MS
analysis of the reaction mixture treated with benzyl
bromide revealed two benzylated derivatives, i.e., R-benz-
yl-γ-butyrolactone (1.7 mmol, 85% yield) and benzyl
butyrate (0.24 mmol, 12% yield). It means that potassium
lactone enolate, potassium butyrate, and hydrogen were
the real reaction products. The last two were identified
for the first time in this work. Yields of reaction products
indicated that the amount of hydrogen evolved is con-
nected rather with the formation of enolate than with
potassium butyrate.
3
4
thalenide, and potassium hydride transform â-lactones
to appropriate enolates. The latter are, however, unstable
and rapidly rearrange to salts of unsaturated acids.4
On the other hand, enolates obtained from γ- or
δ-lactones in the reaction with potassium naphthalenide
5
are stable. It is assumed that this reagent behaves as a
base and abstracts R-proton from the lactone molecule.
It results in an appropriate enolate and a mixture of 1,2-
and 1,4-dihydronaphthalene (in a molar ratio equal to
1
:4). The reaction mechanism is similar to that proposed
6,7
previously, for example, for water.
It shows that
potassium naphthalenide does not act as an electron-
transfer reagent in the mention systems.
The reaction mechanism is proposed on the basis of
these results and known behavior of potassium anions.
-
Several R-substituted derivatives of lactones can be
obtained by the treatment of enolates with alkyl or acyl
At first, one electron is transferred from K of 1 to the
LUMO orbital of the lactone (Scheme 1). This orbital is
8
,9
halides. For example, enolates formed in the reaction
10
of γ-lactones with lithium dialkylamides, i.e., diisopro-
pylamide or isopropylcyclohexylamide, give R-methyl-γ-
(10) Posner, G. H.; Loomis, G. J. J. Chem. Soc., Chem. Commun.
972, 892.
1
(
11) Jedli n˜ ski, Z.; Kowalczuk, M.; Kurcok, P.; Grzegorzek, M.; Ermel,
J. J. Org. Chem. 1987, 52, 4601.
(
1) Dale, J.; Schwartz, J. E. Acta Chim. Scand. 1986, B40, 559.
2) Kricheldorf, H. R.; Scharnagl, N. J. Macromol. Sci. Chem. 1989,
(12) Perrin, Ch. L.; Wang, J.; Szwarc, M. J. Am. Chem. Soc. 2000,
122, 4569.
(13) Grobelny, Z.; Stolarzewicz, A.; Maercker, A.; Krompiec, S.; Bieg,
T. J. Organomet. Chem. 2002, 660, 133.
(
A26, 951.
(
3) Jedli n˜ ski, Z.; Kowalczuk, M.; G×b8 o´ wkowski, W.; Grobelny, J.;
Szwarc, M. Macromolecules 1991, 24, 349.
(14) Grobelny, Z.; Stolarzewicz, A.; Morejko-Bu z3 , B.; Maercker, A.;
Krompiec, S.; Bieg, T. J. Organomet. Chem. 2003, 672, 133.
(15) Grobelny, Z.; Stolarzewicz, A.; Morejko-Bu z˘ , B.; Maercker, A.
J. Organomet. Chem. 2002, 660, 6.
(
4) Kurcok, P.; Matuszowicz, A.; Jedli n˜ ski, Z. Macromol. Rapid
Commun. 1995, 16, 201.
(
5) Jedli n´ ski, Z. J. Polym. Sci. Part A: Polym. Chem. 2002, 40, 2158.
6) Paul, D.; Lipkin, D.; Weissman, S. J. Am. Chem. Soc. 1956, 78,
(
(16) Grobelny, Z. Eur. J. Org. Chem. 2004, 2973 and references
therein.
1
16.
(
(
(
7) Bank, S.; Bockrath, B. J. Am. Chem. Soc. 1971, 93, 430.
(17) Jedli n´ ski, Z.; Stolarzewicz, A.; Grobelny, Z. Makromol. Chem.
1986, 187, 795.
8) Grieco, P. A. Synthesis 1975, 67 and references therein.
9) Cohen, B. J.; Kraus, M. A.; Patchornik, A. J. Am. Chem. Soc.
(18) Grobelny, Z.; Stolarzewicz, A.; Sok o´ ł, M.; Grobelny, J.; Janeczek,
H. J. Phys. Chem. 1992, 96, 5193.
1
981, 103, 7620.
1
0.1021/jo050776o CCC: $30.25 © 2005 American Chemical Society
Published on Web 08/25/2005
J. Org. Chem. 2005, 70, 8201-8203
8201

SCHEME 1a
A similar reaction mediated by KH in the presence of
8-crown-6 was previously described in ref 5.
1
Radical anion 2 can also decompose by the alkyl-
oxygen bond opening (homolytic cleavage) giving radical
anion 7 (Scheme 2). Such a phenomenon was already
-
found in the reaction of K with linear aliphatic es-
ters.1
9,20
Organopotassium salt 8 was formed after the
0
second electron transfer from K to 7. This salt and not
-
K
deprotonates γ-butyrolactone. It results in lactone
enolate 5′ and potassium butyrate 9. Their benzylated
products 6 and 10, respectively, were found in the
reaction mixture treated with benzyl bromide. It is worth
noting that for this kind of experiments the stoichiometry
fits the expected ([6] - [10]) ≈ 2[H
2
].
Unexpectedly, 8 did not react with 15-crown-5. The
benzylated product of the crown ring opening, i.e.,
tetraethylene glycol benzyl vinyl ether, was not identified
in the reaction mixture, although it was observed in
several processes with the participation of organopotas-
sium compounds.1
3-16,21
In the second series of experiments, the γ-butyrolac-
tone solution was dropped into blue alkalide solution at
the same molar ratio of reagents as in the first experi-
-
ments. At that dosage, K was still in excess in relation
to γ-butyrolactone except for the end of the reaction when
discoloration of the solution occurred. Then, an enhanced
amount of hydrogen was produced (0.9 mmol, 90% yield)
as well as R-benzyl-γ-butyrolactone 6 (1.9 mmol, 95%).
Only traces of benzyl butyrate 10 were found in that
system. Thus, dianion 3 was almost exclusively formed
at those conditions, which well confirmed the proposed
reaction mechanism. The stoichiometry fitted in that case
a
Circled K+ denotes K (15-crown-5); the second crown molecule
+
was omitted.
localized mainly on the carbonyl group. The resulting0
radical anion 2 can receive the second electron from K
giving dianion 3. It decomposes with the abstraction of
the hydride anion to produce potassium hydride 4 and
potassium enolate 5. Next, 4 reacts with another γ-bu-
tyrolactone molecule causing its deprotonation and giving
hydrogen and potassium enolate without crown ether 5′.
the equation [6] ≈ 2[H ] because [10] ≈ 0.
2
In summary, γ-butyrolactone does not undergo depro-
-
tonation by K but it receives one or two electrons from
SCHEME 2
8202 J. Org. Chem., Vol. 70, No. 20, 2005

the metal anion. The deprotonation occurs really with
intermediate reaction products, i.e., potassium hydride
rated, and the benzylated products were identified without
separation. Their mass spectra and NMR spectra were similar
to those reported already in the literature for R-benzyl-γ-
4
and potassium 4-potassiobutyrate 8. It should be
1
1,22,23
24
butyrolactone
and benzyl butyrate.
emphasized that the genuine evidence for the mechanism
involving electron transfer and radical anions is the
formation of benzyl butyrate 10 after benzylation of the
reaction mixture.
In the second version of experiments, the reagents were dosed
conversely; i.e., γ-butyrolactone tetrahydrofuran solution was
introduced to the alkalide solution. The enhanced volume of
hydrogen (20 mL) was produced in that case.
Measurements. Analysis of hydrogen was conducted by GC
Experimental Section
technique on a 2.4 m long stainless steel column packed with
2 3 2 3
Al O (0.02-0.03 mm) and deactivated with 5% K CO using a
Materials. Tetrahydrofuran (POCH) was purified as in the
gas chromatograph with flame ionization detector. GC-MS
analyses of benzylated products were performed on a 30 m long
fused silica capillary column DB-1701 using a gas chromato-
graph equipped with an ion-trap detector. Diethylene glycol
dimethyl ether was used as an internal standard for the yield
determination. NMR spectra were recorded at 20 °C on a
1
7
2
earlier work. γ-Butyrolactone was distilled over CaH ; the
fraction boiling at 80 °C (11 mmHg) was collected. 15-Crown-5
-
+
was dried under vacuum at 50 °C for several hours. K , K (15-
crown-5) (0.1 M) dark blue solution was prepared by dissolution
2
of metallic potassium in 0.2 M 15-crown-5 tetrahydrofuran
solution at 20 °C. The contact time was equal to 25 min. The
1
1
8
multinuclear spectrometer at the H resonance frequency of 300
details have been described elsewhere.
1
3
3
MHz and C resonance frequency of 75 MHz. CDCl was used
General Procedure. The reaction was conducted at 20 °C
as the solvent. Chemical shifts were referenced to tetrameth-
ylsilane (TMS) serving as an internal standard.
in a reactor described in ref 21. Tetrahydrofuran solution (10
-
+
2
mL) of K , K (15-crown-5) (1 mmol) was dropped into γ-
butyrolactone (2 mmol) dissolved in tetrahydrofuran (10 mL).
The molar ratio of the reagents allowed us the complete
conversion of γ-butyrolactone. After mixing of the reagents,
discoloration of the system was observed indicating the decay
of potassium anions. Hydrogen (16 mL) was found to evolve.
Then, the reaction mixture was treated with benzyl bromide to
identify nonvolatile reaction products. The solvent was evapo-
Acknowledgment. We are indebted to Dr. Krzysztof
Skutil for GC analysis, to Barbara Morejko-Bu z3 for GC-
MS analysis, and to Dr. Janusz Grobelny for NMR
analysis.
JO050776O
(
19) Boar, R. B.; Joukhadar, L.; McGhie, J. F.; Misra, S. C.; Barrett,
A. G. M.; Barton, D. H. R.; Prokopiou, P. A. J. Chem. Soc., Chem.
Commun. 1978, 68.
(22) Oi, S.; Moro, M.; Ito, H.; Honma, Y.; Miyano,; Inoue, Y.
Tetrahedron 2002, 58, 91.
(
20) Barrett, A. G. M.; Prokopiou, P. A.; Barton, D. H. R.; Boar, R.
(23) Tanaka, Y.; Grapsas, I.; Dakoji, S.; Cho, Y. J.; Mobashery, S.
J. Am. Chem. Soc. 1994, 116, 7475.
B.; McGhie, J. F. J. Chem. Soc., Chem. Commun. 1979, 1173.
21) Grobelny, Z.; Stolarzewicz, A.; Czaja, M.; Demuth, W.;
Maercker, A. J. Org. Chem. 1999, 64, 8990.
(
(24) Sun, S.; Edwards, L.; Harrison, P. J. Chem. Soc., Perkin Trans.
1 1998, 437.
J. Org. Chem, Vol. 70, No. 20, 2005 8203