NMR and computational studies of the chemical reduction of [2.2]paracyclophane: Formation of dianionic p-xylenyl oligomers

Article abstract of DOI:10.1039/b311885a

Reaction of [2.2]paracyclophane with K/Na alloy in THF gives a p-xylylenyl dianion together with its dimer and trimer which are relatively stable at low temperatures; at ambient temperatures further polymerization takes place.

Full text of DOI:10.1039/b311885a

NMR and computational studies of the chemical reduction of
2.2]paracyclophane: formation of dianionic p-xylenyl oligomers†
[
a
b
Ilya D. Gridnev* and Fabio Pichierri
a
COE Laboratory, Department of Chemistry, Graduate School of Science, Tohoku University, Sendai
9
3
80-8578, Japan. E-mail: igridnev@mail.cc.tohoku.ac.jp; Fax: +81 (0)22 217 6784; Tel: +81 (0)22 217
585
b
COE Laboratory, Department of Applied Chemistry, Graduate School of Engineering, Tohoku University,
Sendai 980-8577, Japan. E-mail: fabio@tagen.tohoku.ac.jp
Received (in Cambridge, UK) 26th September 2003, Accepted 6th November 2003
First published as an Advance Article on the web 26th November 2003
Reaction of [2.2]paracyclophane with K/Na alloy in THF gives
a p-xylylenyl dianion together with its dimer and trimer which
are relatively stable at low temperatures; at ambient tem-
peratures further polymerization takes place.
closed dianionic species 7 would be the intermediate. Otherwise, a
structural rearrangement of 2 can yield an open-shell radical anion
(8), which is then reduced to 3. Our gas-phase calculations indicate
that both the processes involving ring opening are exothermic by 23
2
1
(
2?8) and 52 (7?3) kcal mol . The pathway 2?8?3 is slightly
Since the discovery of [2.2]paracyclophane (1) in 19491 this
favored with respect to 2?7?3 (by 0.26 kcal mol ). The
formation of oligomers (e.g. 4 and 5) corresponds better to the
intermediacy of 8 being in equilibrium with the dianion 3.
21
2
molecule has been intensively studied. Synthetic methods suggest
3
4
promising materials, unique polymers, and catalysts with planar
5
1
chirality can be developed, whereas the theoretical challenge of
The most characteristic signals in the H NMR spectra of 3 are
understanding the structural features of the two adjacent coplanar
benzene rings is still under discussion.
two doublets at d = 5.30 and 5.70 which are assigned to the
negatively charged phenyls (Fig. 1a). In the C NMR spectra of 3
6
13
Much less attention has been paid to charged [2.2]para-
cyclophanes. After several characterizations of the extremely
unstable blue anion-radical (2) by ESR and optical spectroscopy,7
Pearson examined a further reduction but was hampered by “the
extremely low solubility of the paracyclophanes and other
the strong alternations of the chemical shifts of the two aromatic
CH-resonances, d = 109.7 and 130.6, as well as of the two
quarternary carbons, d = 105.3 and 150.6, are remarkable. The
spectral assignments for 3 were confirmed after performing a series
of GIAO-B3LYP/6-311++G(d,p)//HF/6-311G* chemical shift
computations (see Fig. 2).
8
experimental problems”. Furthermore, on the theoretical side, we
are unaware of any computational study of negatively charged
In the 1H and 13C NMR spectra of 4 the terminal charged
[2.2]paracyclophanes.
H
aromatic rings give signals very similar to those of 3 (d = 5.33,
We report here the results of both NMR and computational
studies of the reaction of 1 with K/Na alloy in THF. Treatment of
with K/Na alloy in THF-d at 250 °C‡ yields an intense red-
1
8
colored solution of three dianions 3–5 (Scheme 1) containing an
admixture of 1,2-di-p-tolylethane (6).§ Dianions 3–5 were stable at
2
50 °C. Monomer 3 and dimer 4 were completely characterized by
1
13
their H and C NMR spectra. The possible reduction pathways
from 1 to 3–5 were explored with the aid of quantum chemical
calculations performed on the reduced [2.2]paracyclophane species
2, 3, 7 and 8 (Scheme 2).¶
The reduction of 1 in gas-phase is an unfavourable process, as
suggested by the negative value of its electron affinity (20.49 eV)
computed at the UB3LYP/6-311++G(d,p)//UHF/6-311G* level of
theory. Nevertheless, radical anion 2 has been characterised by ESR
at 270 °C in THF and DME solutions, in which it may be stabilized
7
,8
by the donating solvent. The route from 2 to 3 involves a further
reduction and ring opening which may occur by two possible
mechanisms (Scheme 2). If the second reduction occurs first, the
Scheme 2 Reduction of [2.2]paracyclophane 1.
Scheme 1 Synthesis of dianions 3–5.
1
†
Electronic Supplementary Information (ESI) available: experimental
Fig. 1 H NMR spectra (300 MHz, THF-d
8
) of the reaction mixture obtained
and computational details, and spectral data for the trapping products. See
http://www.rsc.org/suppdata/cc/b3/b311885a/
by reacting 1 with K/Na at a) 250 °C, b) 230 °C, c) 210 °C, d) 20 °C after
12 h.
1
50
C h e m . C o m m u n . , 2 0 0 4 , 1 5 0 – 1 5 1
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

5
.80; d
carbon atoms of the internal phenyls resonate at normal d values (d
6.8–7.2; d = 128.5, 128.8, 139.9, 141.0). The NMR spectra of
C
= 107.9, 110.4, 131.5, 152.3), whereas the protons and
with n = 15–20 which corresponds to molecular weights of several
H
thousands. Poly(xylylenes) are useful materials with applications as
4
=
C
encapsulants in electronic devices. They are prepared from 1
the dianion 5 are very similar to those of 4, so it was impossible to
detect its signals reliably in the NMR spectra of the reaction
mixture (see however Fig. 1b).
according to the Gorham process12 by vapor deposition polymeri-
zation which requires elaborate equipment and temperatures of
around 650 °C. The results presented here suggest that a “living”
polymerization of 1 is possible under mild conditions which might
be useful for the development of an alternative route to poly-
(xylylenes). Further theoretical and experimental studies on the
mechanism responsible for the formation of dianionic p-xylenyl
species will follow.
Quenching the reaction mixture obtained after stirring 1 with a
large excess of K/Na alloy in absolute THF for 30 minutes at 250
°
C with trimethylchlorosilane, resulted in 65% of 1 and afforded
disilyl derivatives 9–11 in a ratio of 2 : 1 : 1 Scheme 3). The
products were separated by preparative GPC and characterized
individually.
When the reaction mixtures from the NMR experiments were
warmed to room temperature, further polymerization took place, as
is evident from the Figs. 1c,d and 3.
This work was financially supported by the COE Program “Giant
molecules and complex systems” of MEXT hosted at Tohoku
University.
Since the spectral region of 5.0–6.0 ppm is characteristic of the
protons of charged terminal aromatic rings, each cross-peak in Fig.
Notes and references
3
corresponds to a certain oligomeric dianion, and the number of
‡ The reaction of 1 with K/Na can also occur at lower temperatures (290 to
270 °C). Under such conditions broad signals of paramagnetic species are
observed together with 3–5.
cross-peaks gives a good estimation of the number of compounds
present in the reaction mixture. Hence, one can conclude from the
Fig. 3 that oligomerization of this sample leads to dianionic species
§
The solvent must be completely anhydrous and contact with air must be
excluded to avoid exclusive formation of 6; see the ESI† for details.
7
8
Ishitani and Pearson suggested that p-xylene is the final product of the
reaction of 1 with K/Na in THF, however, we never observed this product
in our experiments.
¶
All the calculations were performed with the Gaussian 98 software
9
package. Geometry optimizations and vibrational frequency calculations
were performed at the HF/6-311G* level of theory while chemical shifts
were computed using the GIAO method at the single-point B3LYP/
10
6
-311++G(d,p) level of theory.11
Fig. 2 Experimental and computed (in brackets) chemical shifts for the
optimized (C2h) structure of 3. Relevant bond lengths (Å) and bond angles
1 C. J. Brown and A. C. Farthing, Nature, 1949, 164, 915.
2 F. Vögtle, Cyclophane Chemistry, John Wiley & Sons, West Sussex,
1993.
(°) are also reported.
3
J. Zyss, I. Ledoux, S. Volkov, V. Chernyak, S. Mukamel, G. P.
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5
6
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7
S. I. Weissman, J. Am. Chem. Soc., 1958, 80, 6462; V. V. Voevodskii,
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1
29, 1082; A. Ishitani and S. Nagakura, Mol. Phys., 1967, 12, 1; F.
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8
9
5
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1
0 K. Wolinski, J. F. Hilton, and P. Pulay, J. Am. Chem. Soc., 1990, 112,
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8
Fig. 3 1H– H COSY NMR spectrum (300 MHz, THF-d
1
, 298 K) of the
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8
reaction mixture from Scheme 1 stored for 5 days at ambient temperature.
At least 15 cross-peaks attributed to aromatic protons in the terminal anionic
units are observed.
12 W. F. Gorham, US Pat. 3,342,754, Sept. 19, 1967.
C h e m . C o m m u n . , 2 0 0 4 , 1 5 0 – 1 5 1
151