Formation of 1,2-bis(1-methyl-1,4-dihydro-4-pyridinylidene)cyclobutane by dehydrogenated intermolecular cyclization of 4-vinyl-1-methylpyridinyl radical

Article abstract of DOI:10.1246/cl.2009.896

The one-electron reduction of 4-vinyl-1-methylpyridinium bromide with sodium amalgam in degassed acetonitrile at 0 °C leads to the formation of 1,2-bis(1-methyl-1,4-dihydro-4-pyridinylidene)cyclobutane, which is characterized by means of ¹HNMR, ¹³CNMR, and UV-vis absorption spectroscopy. Copyright

Full text of DOI:10.1246/cl.2009.896

896
Chemistry Letters Vol.38, No.9 (2009)
Formation of 1,2-Bis(1-methyl-1,4-dihydro-4-pyridinylidene)cyclobutane by Dehydrogenated
Intermolecular Cyclization of 4-Vinyl-1-methylpyridinyl Radical
Takashi Muramatsu,ꢀ1 Azumao Toyota,2 and Yusuke Satou1
1Environmental Education Center, Miyagi University of Education, Sendai 980-0845
2Department of Chemistry, Faculty of Education, Art and Science, Yamagata University, Yamagata 990-8560
(Received June 22, 2009; CL-090586; E-mail: t-mura@staff.miyakyo-u.ac.jp)
The one-electron reduction of 4-vinyl-1-methylpyridinium
bromide with sodium amalgam in degassed acetonitrile at
0 ꢁC leads to the formation of 1,2-bis(1-methyl-1,4-dihydro-4-
pyridinylidene)cyclobutane, which is characterized by means
of 1H NMR, 13C NMR, and UV–vis absorption spectroscopy.
amalgam in degassed acetonitrile (Scheme 2). Reduction was
followed by UV–vis absorption spectroscopy. Figure 1 shows
the spectral change during the course of reduction, where spec-
tral line (a) corresponds to the spectrum of 6. As the reduction
proceeds, the acetonitrile solution changed gradually from color-
less to orange. After 0.5 h of reduction, the absorption maximum
at about 260 nm decreased in intensity and the spectrum changed
into spectral line (b) with the longest wavelength absorption
band at 450 nm.
It has been established that 1-methylpyridinyl radicals with
an alkyl group at the 4-position of the pyridine ring are in equi-
librium with covalently bonded dimers.1 Through photochemi-
cal bond cleavage of dimers, the physicochemical properties of
the dimers were elucidated by kinetic EPR and CIDEP studies.
In recent years, we have studied the properties of pyridinyl dir-
adicals generated by the reduction of bispyridiniums of the type
Pyþ–(CH2)n–Pyþ (n ¼ 3{5, Pyþ = 1-methylpyridinium).2 In
the n ¼ 5 bispyridinium, the resulting diradical 1 undergoes an
intramolecular cyclization reaction to yield the corresponding
cyclomer 2, which is converted photochemically into the dispiro
compound 3 (Scheme 1). In this connection, the two-electron re-
duction of 1,2-bis(1-methyl-4-pyridyl)ethylene dibromide (4)
leads to the formation of 1,2-bis(1-methyl-1,4-dihydro-4-pyridi-
nylidene)ethane (5).3 In contrast, no chemical behavior of 4-vi-
nyl-1-methylpyridinyl radical (7) and its derivatives have so far
been studied because of the presence of a highly reactive vinyl
group.4–6 As part of a study on the chemical reactivities of such
radical species, we carried out the one-electron reduction of 4-vi-
nyl-1-methylpyridinium bromide (6) with sodium amalgam under
vacuum. We will report herein that the reduction of 6 leads to the
formation of a novel diamagnetic compound, 1,2-bis(1-methyl-
1,4-dihydro-4-pyridinylidene)cyclobutane (8) in solution.
After 4 h of reduction, the spectrum changed into spectral
line (d) and, at this stage, no spectral change was observed upon
CH CH2
CH CH2
Na-Hg
2
2
N
Br
N
CH3
CH3
6
7
H3C
N
N
CH CH2
CH CH2
H3C
H3C
N
N
CH CH2
CH CH2
H3C
2H
9a
9b
H3C
CH3
N
N
8
The starting material 6 was prepared by reaction of methyl
bromide with 4-vinylpyridine.7 It should be remarked that 6 is
stable for a period of several days at 0 ꢁC but polymerizes slowly
on standing at room temperature to a whitish substance, poly(4-
vinyl-1-methylpyridinium bromide).8 To control polymeriza-
tion, reduction of 6 was carefully carried out at 0 ꢁC with sodium
Scheme 2.
0.6
0.4
0.2
0
d
H3C
N
(CH2)5
N
CH3
CH3
1
CH3
N
c
b
H3C
CH3
h
ν
N
N
N
∆
a
3
2
200
300
400
Wavelength / nm
500
600
H
C
2e
N
CH3
H3C
N
CH CH
2 Br
N
CH3
H3C
N
C
Figure 1. Absorption spectral change during the course of re-
duction of 6 (1:0 ꢂ 10ꢃ2 mmol) with 3% sodium amalgam
(2:2 ꢂ 10ꢃ2 mmol) in degassed acetonitrile (5 mL) at 0 ꢁC: (a)
before reduction, (b) after 0.5 h, (c) after 1 h, and (d) after 4 h.
H
4
5
Scheme 1.
Copyright Ó 2009 The Chemical Society of Japan

Chemistry Letters Vol.38, No.9 (2009)
897
dimer 9b formed by the Cope rearrangement,4 where the equilib-
rium is largely shifted to the latter side. At the end of the reac-
tion, 9b is converted to 8 through an elimination of two hydrogen
atoms and a successive cyclization reaction. It is added finally
that contrary to our expectation, the reduction of 6 gives no trace
amounts of poly(4-vinylpyridine), which is clearly confirmed by
1
the H NMR spectral analysis.14
In short, the reduction of 6 is shown clearly to result in the
formation of 8 quantitatively through a dehydrogenated intermo-
lecular cyclization of 7 in degassed acetonitrile at 0 ꢁC. We are
currently extending our studies to analogous pyridinium salts to
see whether the corresponding radicals generated by reduction
undergo generally intermolecular cyclization reactions to yield
diamagnetic species with a four-membered ring.15
1
Figure 2. 400 MHz H NMR spectrum of 8 in C6D6.
further reduction. Accordingly, the reduction products dissolved
in a solution of acetonitrile were extracted with benzene and fil-
trated through a glass filter under degassed conditions.
References and Notes
1
2
Y. Ikegami, Rev. Chem. Intermed. 1986, 7, 91.
T. Muramatsu, A. Toyota, M. Suzuki, J. Am. Chem. Soc. 2005,
127, 4572, and references therein.
The reduction products in the orange benzene solution are
unstable to ambient oxygen, and after replacement of the C6H6
3
T. Muramatsu, A. Toyota, Y. Ikegami, J. Org. Chem. 1995, 60,
4925.
1
solvent with C6D6, H NMR, 13C NMR (BCM,OFR), and HC-
COSY measurements for characterization were carried out in a
4
5
F. H. Quina, D. G. Whitten, J. Am. Chem. Soc. 1977, 99, 877.
T. Nakamura, K. Takagi, Y. Sawaki, Bull. Chem. Soc. Jpn. 1998,
71, 419.
1
sealed tube under vacuum. Figure 2 shows the H NMR spec-
trum, suggesting that the chief reduction product should have a
symmetric structure because of the spectral features being quite
simple.9 The spin-decoupling techniques reveal that the four
types of 1H NMR signals observed at 5.21, 5.26, 5.38, and
5.79 ppm and the respective 13C NMR signals at 129.95,
130.01, 110.99, and 112.47 ppm exhibit a clear-cut correlation
with a molecular structure possessing two 1,4-dihydropyridinyl-
idene skeletons. The 13C signal observed at 119.37 ppm is as-
signed to the C atom at the 4-position in the 1,4-dihydropyridine
skeleton. Further, the 1H NMR signals observed at 2.48 and
1.93 ppm, together with the respective 13C signals observed at
26.26 and 39.85 ppm, are readily assigned to the protons attach-
ed to the methylene and methyl groups, respectively. It should be
noted that in tetrakis(1-ethoxycarbonyl-1,4-dihydro-4-pyridinyl-
idene)cyclobutane,10 the 13C signal observed at 113.8 ppm is as-
signed to the C(sp2) atom of the four-membered ring. In 1,2-di-
methylenecyclobutane, the 13C signal of the C(sp3) atom is ob-
served at 28.1 ppm.11 Hence, the 13C NMR signals observed at
118.66 and 26.26 ppm for the reduction product are reasonably
assigned to the C(sp2) and C(sp3) atoms of the four-membered
ring in the 1,2-dimethylenecyclobutane skeleton, respectively.
6
7
B. J. Coe, J. A. Harris, B. S. Brunschwig, J. Garin, J. Orduna,
J. Am. Chem. Soc. 2005, 127, 3284, and references therein.
Bromide 6 was synthesized by reaction of methyl bromide with
4-vinylpyridine in methanol at 0 ꢁC. 6: 400 MHz 1H NMR
(DMSO-d6): ꢀ 8.69 (d, J ¼ 5:86 Hz, 2H), 8.01 (d, J ¼ 5:86 Hz,
2H), 6.96 (dd, J ¼ 17:60, 10.82 Hz, 1H), 6.46 (d, J ¼ 17:60
Hz, 1H), 5.97 (d, J ¼ 10:82 Hz, 1H), 4.31 (s, 3H).
Formation of poly(4-vinyl-1-methylpyridinium bromide) is read-
ily confirmed by the 1H NMR spectral analysis. For the NMR da-
ta, see: B. L. Rivas, M. del Carmen Aguirre, E. Pereira, J. Appl.
Polym. Sci. 2007, 106, 89.
In Figure 2, the weak proton signals observed over the ranges
from 1.2–2.4 and 5.1–5.7 ppm will probably be assigned as aris-
ing from slight amounts of the intermediate 9a and a by-product
with a five-membered ring, where the latter is considered to be
formed by an intramolecular hydrogen transfer and a successive
cyclization reaction from 9b.15 Detailed analyses of such minor
products are now in progress.
8
9
10 For the 13C NMR data, see: M. Horner, S. Hunig, H.-U. Reißig,
¨
Liebigs Ann. Chem. 1983, 658.
11 For the 13C NMR data, see: H. U. Pfeffer, M. Klessinger, Org.
Magn. Res. 1977, 9, 121.
1
By reference to the H NMR spectrum of 5,3 the structural as-
signment can be made unambiguously such that the reduction
product should possess the novel structure of 1,2-bis(1-methyl-
1,4-dihydro-4-pyridinylidene)cyclobutane (8).12,16 From the
UV–vis absorption spectroscopy, it is also added that the spectral
features of 8 resemble quite well the spectrum of 5, especially
with the longest wavelength absorption band at about 450 nm.3
This is ascribed to the fact that 5 and 8 contain two 1-methyl-
1,4-dihydro-4-pyridinylidene groups which are directly conju-
gated together, the two chromophores being in the trans and
cis configurations, respectively.
The formation of 8 from 6 can be rationalized as follows
(Scheme 2). At 0 ꢁC, the intermediate radical 7 undergoes pre-
dominantly an intermolecular radical reaction preferentially at
the 4-position to give the 4,40-dimer 9a. This is based on AM1
calculations, showing that the spin density of 7 is largest at the
4-position.13 Successively, 9a is in equilibrium with its isomeric
12 Under degassed conditions, 8 is stable for a long period of time
at room temperature. As analogous compounds to 8, see: M.
Horner, S. Hunig, Liebigs Ann. Chem. 1983, 642.
¨
13 J. J. P. Stewart, J. Comput. Chem. 1989, 10, 221. AM1 calcula-
tions of 7 show that the spin density at the 4-position (0.32) is
twice larger than those at the 2(6)-positions (0.15) of the ring
and the ꢁ-position (0.13) of the vinyl group.
14 Z. Chen, J. Cai, X. Jiang, C. Yang, J. Appl. Polym. Sci. 2002, 86,
2687.
15 In contrast to 6, our preliminary study shows that the one-elec-
tron reduction of 4-styryl-1-methylpyridinium bromide results
in the formation of a dimeric compound with a five-membered
ring.9
16 Supporting Information is available electronically on the CSJ-
Journal Web site, http://www.csj.jp/journals/chem-lett/index.
html.
Published on the web (Advance View) August 8, 2009; doi:10.1246/cl.2009.896