Photoisomerization of perfluoroaryltetrahedranes to perfluoroarylcyclobutadienes

Article abstract of DOI:10.1021/ja208354x

Two perfluoroaryl-substituted cyclobutadiene derivatives, 6 and 7, were prepared as air- and moisture-sensitive red solids by the photochemical isomerization of the corresponding tetrahedranes (4 and 5, respectively). Remarkably, the 9,10-dicyanoanthracen

Full text of DOI:10.1021/ja208354x

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pubs.acs.org/JACS
Photoisomerization of Perfluoroaryltetrahedranes
to Perfluoroarylcyclobutadienes
Yusuke Inagaki, Masaaki Nakamoto, and Akira Sekiguchi*
Department of Chemistry, Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan
S Supporting Information
b
Chart 1. Tetrakis(trimethylsilyl)tetrahedrane (1), Tris-
(trimethylsilyl)tetrahedranyllithium (2), and Tetrakis-
(trimethylsilyl)cyclobutadiene (3)
ABSTRACT: Two perfluoroaryl-substituted cyclobuta-
diene derivatives, 6 and 7, were prepared as air- and
moisture-sensitive red solids by the photochemical isomer-
ization of the corresponding tetrahedranes (4 and 5, re-
spectively). Remarkably, the 9,10-dicyanoanthracene-sensitized
photochemical reaction of 4 also proceeded, giving 6, and
the mechanism of this reaction is also discussed. The first
aryl-substituted cyclobutadienes were characterized by spec-
troscopic data as well as by X-ray crystallography for 6,
showing a distorted rectangular structure with extremely
long CꢀC single bonds.
Scheme 1. Photoisomerization of Perfluoroaryltetra-
hedranes 4 and 5 to Perfluoroarylcyclobutadienes 6 and 7,
Respectively
yclobutadiene (CBD) is a cyclic unsaturated compound
C
with four π-electrons within a rectangular planar four-
membered ring, as found both experimentally and theoretically.1
CBD is also a valence isomer of the highly strained cage com-
pound, tetrahedrane (THD).2 The thermal concerted pericyclic
isomerization of THD to CBD is a symmetry-forbidden process
with a high activation barrier.3 Despite this, Maier et al. were able
to perform the thermal isomerization of tetra-tert-butyltetra-
hedrane to the corresponding CBD at 130 °C.4 However, they
proposed a diradical pathway. The highly strained tetrahedrane
skeleton can be sterically and electronically stabilized if the
tBu groups are replaced by σ-donating trimethylsilyl groups.
Maier and our group succeeded in synthesizing tetrakis(tri-
methylsilyl)tetrahedrane (1) by photochemical isomerization of
tetrakis(trimethylsilyl)cyclobutadiene (3) (Chart 1).5 Unlike
tetra-tert-butyltetrahedrane, 1 is stable up to 300 °C. We have
also reported a new procedure for synthesizing 1 by one-electron
oxidation of 3 with tris(pentafluorophenyl)borane.6 Further-
more, we demonstrated that 1 could be transformed into tris-
(trimethylsilyl)tetrahedranyllithium (2) by reaction with meth-
yllithium (Chart 1).7 Since then, sulfur-substituted derivatives
have been accessible from 2.8 Very recently, we also successfully
synthesized perfluoroaryl-substituted tetrahedranes 4 and 5, the
first stable aryl-substituted tetrahedranes.9 As mentioned, the
thermal isomerization of THD to CBD is known;1,4 however, the
photochemical isomerization of THD to CBD has remained
elusive because of the synthetic difficulty in preparing THD
derivatives with a chromophore.
4 and 5 to the corresponding 6 and 7 by UV irradiation under
mild conditions, as well as the electronic spectra of 6 and 7 and
the crystal structure of 6. Notably, 6 and 7 are the first examples
of aryl-substituted cyclobutadienes. We also found that 4 iso-
merized to 6 by 9,10-dicyanoanthracene (DCA)-mediated oxi-
dation, and the mechanism of this reaction is also discussed.
A C6D6 solution of 4 was irradiated by a high-pressure
mercury lamp (λ > 300 nm) at 10 °C. Upon irradiation, the
color of the solution immediately changed from pale yellow to
red. After 4 h, 4 was quantitatively converted to the correspond-
ing 6 and isolated as air- and moisture-sensitive red crystals in
81% yield (Scheme 1).10 We examined the solvent effects on the
rate of this isomerization. It appears that the reaction is about
twice as fast in polar solvents (THF and CH3CN) as in nonpolar
hexane (see the Supporting Information). Under similar condi-
tions, bis(tetrahedrane) 5 can be transformed into 7, which was
obtained as air- and moisture-sensitive red crystals in 57% yield.10
The photoisomerization of 4 was monitored by UVꢀvis
spectroscopy in hexane. As the intensity of the 271 nm band of
4 decreased, the intensity of a new absorption band at 311 nm
In a previous paper, we reported that X-ray analysis and the
UVꢀvis absorption spectrum of tetrahedranes 4 and 5 suggest
there is a σꢀπ conjugation of the strained tetrahedrane core with
the benzene ring, which causes a considerable bathochromic
shift.9 Herein, we report the first photochemical isomerization of
Received: September 4, 2011
Published: September 16, 2011
r
2011 American Chemical Society
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dx.doi.org/10.1021/ja208354x J. Am. Chem. Soc. 2011, 133, 16436–16439
|

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Figure 2. ORTEP drawing of 6 (30% thermal ellipsoids). Hydrogen
atoms are omitted for clarity. Selected bond lengths (Å) and angles
(deg) for molecule A (right): C1ꢀC2, 1.565(2); C2ꢀC3, 1.365(3);
C3ꢀC4, 1.605(2); C4ꢀC1, 1.350(3); C1ꢀC14, 1.473(3); C2ꢀSi1,
1.8769(19); C3ꢀSi2, 1.8697(19); C4ꢀSi3, 1.8679(19); C4ꢀC1ꢀC2,
93.11(15); C3ꢀC2ꢀC1, 88.62(15); C2ꢀC3ꢀC4, 90.76(14);
C1ꢀC4ꢀC3, 87.51(14); C4ꢀC1ꢀC14, 134.91(17); C14ꢀC1ꢀC2,
131.98(16); C3ꢀC2ꢀSi1, 139.07(15); C1ꢀC2ꢀSi1, 132.20(13);
C2ꢀC3ꢀSi2, 137.63(15); C4ꢀC3ꢀSi2, 131.60(13); C1ꢀC4ꢀSi3,
133.69(15); C3ꢀC4ꢀSi3, 138.65(14). Selected bond lengths (Å) and
angles (deg) for molecule B (left): C20ꢀC23, 1.536(3); C23ꢀC22,
1.394(3); C22ꢀC21, 1.580(3); C21ꢀC20, 1.367(3); C20ꢀC33, 1.471(3);
C23ꢀSi6, 1.877(2); C22ꢀSi5, 1.857(2); C21ꢀSi4, 1.866(2); C21ꢀ
C20ꢀC23, 93.09(16); C22ꢀC23ꢀC20, 88.75(15); C23ꢀC22ꢀC21,
90.20(15); C20ꢀC21ꢀC22, 87.94(16); C21ꢀC20ꢀC33, 134.08(18);
C33ꢀC20ꢀC23, 132.81(18); C22ꢀC23ꢀSi6, 138.05(15); C20ꢀC23ꢀ
Si6, 132.95(14); C23ꢀC22ꢀSi5, 138.50(16); C21ꢀC22ꢀSi5, 130.99(14);
C20ꢀC21ꢀSi4, 133.04(15); C22ꢀC21ꢀSi4, 138.62(15).
Figure 1. UVꢀvis spectroscopy monitoring of the photoisomerization
of 4 to 6 in hexane, measured every minute.
increased, as depicted in Figure 1. The observation of isosbestic
points indicates that this isomerization proceeds very cleanly. In
addition to the absorption maximum at 311 nm (ε = 1900
M
ꢀ1 cmꢀ1), a fairly weak broad band was observed at 462 nm
(ε = 90 Mꢀ1 cmꢀ1).11 The UVꢀvis spectrum of 7 was also
measured. Although the shape of the spectrum was almost the
same as that of 6, the absorption maxima at 502 (ε = 300
M
M
ꢀ1 cmꢀ1), 431 (ε = 900 Mꢀ1 cmꢀ1), and 340 nm (ε = 3900
ꢀ1 cmꢀ1) were red-shifted compared with those of 6, probably
because of the conjugation between the two cyclobutadiene units
in 7.
1.580(3) Å (C22ꢀC21) for B] are significantly stretched
compared with the C(sp3)ꢀC(sp3) single bond lengths (1.554
Å) of cyclobutane, except for C20ꢀC23.14 The skeletal single
bonds of CBD are known to be quite long from theoretical
studies, regardless of the bulkiness of the substituents.1m There-
fore, the exceptional stretching of the skeletal single bonds of 6
may originate from its antiaromaticity. The C(sp2)dC(sp2)
double bond lengths of 6 [1.365(3) (C2ꢀC3) and 1.350(3)
Å (C4ꢀC1) for A, 1.394(3) (C23ꢀC22) and 1.367(3) Å
(C21ꢀC20) for B] are slightly stretched compared with that
of cyclobutene (1.335 Å).14 As a consequence of the different
substituents (Me3Si vs C6F5), the four-membered ring structure
in 6 does not show ideal D2h symmetry but is almost planar. The
folding angle of the CBD ring in 6 is 0.48(2)° for A and 1.42(2)°
for B. The presence of an electron-withdrawing group leads to a
shortening of the CBD skeletal bond lengths: the bonds with two
Me3Si groups are longer than the bonds adjacent to the electro-
negative C6F5 group because of steric and electronic effects.
Thus, C2ꢀC3 (1.365(3) Å) in A is longer than C4ꢀC1 (1.350(3)
Å), and C3ꢀC4 (1.605(2) Å) is longer than C1ꢀC2 (1.565(2) Å).
Molecule B shows a similar tendency. The dihedral angle between
the cyclobutadiene ring and the perfluorophenyl ring is 58.5°, which
is attributable to large steric hindrance.
The characteristic 13C NMR signals of the cyclobutadiene
skeleton were observed at 146.0 (CꢀC6F5), 162.7 (CꢀSiMe3),
and 168.2 ppm (CꢀSiMe3) for 6 and at 147.6 (CꢀC6F4ꢀC),
162.4 (CꢀSiMe3), and 168.3 ppm (CꢀSiMe3) for 7, which are
considerably downfield-shifted compared with those of the THD
skeleton of 4 [ꢀ17.2 (CꢀSiMe3) and ꢀ6.5 ppm (CꢀC6F5)] and
1
5 [ꢀ17.3 (CꢀSiMe3) and ꢀ6.0 (CꢀC6F4ꢀC) ppm]. The H
NMR signals of the trimethylsilyl groups were observed at ꢀ0.02
and 0.11 ppm for 6, and at 0.04 and 0.13 ppm for 7, with the
integration ratio of 2:1 in both cases. In the 29Si NMR spectrum,
the two signals of the trimethylsilyl groups were observed
at ꢀ14.4 and ꢀ13.9 ppm for 6, and at ꢀ14.1 and ꢀ13.9 ppm for
7. The observation of only two signals for the three trimethylsilyl
1
groups in the H and 29Si NMR spectra indicates a fast auto-
merization reaction occurring in solution on the NMR time scale.12
The nucleus-independent chemical shift (NICS) was calculated for
the optimized structure (optimized at the B3LYP/6-31G(d) level)
of 6at the GIAO-B3LYP/6-311+G(3d,p) level. The NICS(0) value
of 29.1 and the NICS(1) value of 14.7 are indicative of the anti-
aromatic character of this compound.13
Single crystals of 6 were obtained by recrystallization from
toluene. The molecular structure of 6 was determined by X-ray
crystallographic analysis, and there are two crystallographi-
cally independent molecules (A and B) of 6 in the unit cell
(Figure 2).10 The cyclobutadiene skeleton has a distorted
rectangular structure with CꢀC bond alternation. The C-
(sp2)ꢀC(sp2) single bond lengths of 6 [1.565(2) (C1ꢀC2)
and 1.605(2) Å (C3ꢀC4) for A, 1.536(3) (C20ꢀC23) and
An electron-transfer photoreaction of 4 using DCA as a singlet
sensitizer was also carried out. It became apparent that chemical
oxidation is also effective for the valence isomerization of 4 to 6.
Because 4 has no absorption bands longer than 350 nm, only
DCA can be excited by using longer-wavelength light.15 A C6D6
solution of 4 and a catalytic amount of DCA (0.8 mol %) was
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COMMUNICATION
Scheme 2. 9,10-Dicyanoanthracene-Sensitized Isomeriza-
tion of 4 to 6 and Proposed Mechanism of This Transfor-
mation via an Electron-Transfer Reaction
Culture of Japan, JSPS Research Fellowship for Young Scientists
(Y.I.). We are grateful to Prof. Hiroshi Ikeda (Osaka Prefecture
University) for helpful discussions on the photo-electron-transfer
reaction.
’ REFERENCES
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(b) Whitman, D. W.; Carpenter, B. K. J. Am. Chem. Soc. 1980, 102, 4272.
Crystallographically characterized cyclobutadienes: (c) Irngartinger, H.;
Rodewald, H. Angew. Chem., Int. Ed. Engl. 1974, 13, 740. (d) Delbaere,
L. T. J.; James, M. N. G.; Nakamura, N.; Masamune, S. J. Am. Chem. Soc.
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(f) Ermer, O.; Heilbronner, E. Angew. Chem., Int. Ed. Engl. 1983, 22, 402.
(g) Irngartinger, H.; Nixdorf, M. Angew. Chem., Int. Ed. Engl. 1983,
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Chem., Int. Ed. Engl. 2001, 40, 1675. Reviews on cyclobutadienes:
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Masamune, S. Tetrahedron 1980, 36, 343. (k) Matsuo, T.; Sekiguchi, A.
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P. v. R. J. Phys. Chem. A. 2000, 104, 1246.
(2) Reviews on tetrahedranes: (a) Maier, G. Angew. Chem., Int. Ed.
Engl. 1988, 27, 309. (b) Maier, G. Pure Appl. Chem. 1991, 63, 275.
(c) Lee, V. Ya.; Sekiguchi, A. In Strained Hydrocarbons; Dodziuk, H., Ed.;
Wiley-VCH: Weinheim, 2009; Chapter 2.
(3) Nemirowski, A.; Reisenauer, H. P.; Schreiner, P. R. Chem.—Eur.
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(4) Maier, G.; Pfriem, S.; Sch€afer, U.; Matusch, R. Angew. Chem., Int.
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(5) Maier, G.; Nuedert, J.; Wolf, O.; Pappusch, D.; Sekiguchi, A.;
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(6) Nakamoto, M.; Inagaki, Y.; Ochiai, T.; Tanaka, M.; Sekiguchi, A.
Heteroatom Chem. 2011, 22, 412.
(7) (a) Sekiguchi, A.; Tanaka, M. J. Am. Chem. Soc. 2003, 125, 12684.
(b) Tanaka, M.; Sekiguchi, A. Angew. Chem., Int. Ed. 2005, 44, 5821.
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irradiated (λ > 420 nm) at 10 °C, and 4 was cleanly converted to
6 (93% yield) (Scheme 2). On the other hand, under the same
photochemical conditions in the absence of DCA, no 6 was
formed. The oxidation potential (EO1/x2) of 4 (+0.700 V vs Fe/Fe+,
+1.08 V vs SCE in acetonitrile) is low enough to promote
oxidation of 4 with the excited singlet DCA in acetonitrile by
electron transfer, as suggested by the calculated free energy
change (ΔGET = ꢀ20.4 kcal/mol).16,17 These results strongly
suggest a mechanism that involves initial generation of the cation
radical 4+•, which rapidly isomerizes to the cation radical 6+•, as
depicted in Scheme 2. This cation radical undergoes back-
electron-transfer to the DCA, which yields neutral 6.18 According
to theoretical calculations at the B3LYP/6-31G(d) level, the
cation radical 6+• is 11.8 kcal/mol more stable than the cation
radical 4+•, which further confirms this mechanism.
Although the mechanism of the photoinduced valence iso-
merization of 4 and 5 to 6 and 7, respectively, shown in Scheme 1,
is not certain, we can consider the photoinduced intramolecular
electron transfer from the tetrahedrane moiety to the perfluor-
obenzene moiety as the general pathway for this type of trans-
formation. Solvent effects on the rate of photoisomerization of 4
to 6 also support this possibility (see the Supporting Infor-
mation).
(9) Nakamoto, M.; Inagaki, Y.; Nishina, M.; Sekiguchi, A. J. Am.
Chem. Soc. 2009, 131, 3172.
(10) For the experimental procedures, spectral data for 6 and 7,
solvent effects on the rate of photoisomerization of 4 to 6, and crystal
data for 6, see the Supporting Information.
(11) TD-DFT calculations at the B3LYP/6-31G(d) level were
carried out for the assignment of the absorptions. A weak absorption
at 462 nm can be assigned to a HOMO (π of cyclobutadiene)ꢀLUMO
(π* of cyclobutadiene) transition, and the absorption at 311 nm can be
assigned to a HOMO (π of cyclobutadiene)ꢀLUMO+1 (π* of perfluo-
robenzene) transition.
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental procedures and
b
spectral data for 6 and 7, solvent effects on the rate of photo-
isomerization of 4 to 6, and tables of crystallographic data,
including atomic positional and thermal parameters, and CIF
files for 6. This material is available free of charge via the Internet
at http://pubs.acs.org.
(12) For theoretical calculations on the automerization reaction of
cyclobutadiene, see:Eckert-Maksiꢀc, M.; Vazdar, M.; Barbatti, M.; Lischka,
H.; Maksiꢀc, Z. B. J. Chem. Phys. 2006, 125, 064310.
(13) (a) Schleyer, P. v. R.; Maerker, C.; Dransfeld, A.; Jiao, H.;
Hommes, N. J. R. v. E. J. Am. Chem. Soc. 1996, 118, 6317. Review on
NICS calculation:(b) Chen, Z.; Wannere, C. S.; Corminboeuf, C.;
Puchta, R.; Schleyer, P. v. R. Chem. Rev. 2005, 105, 3842.
(14) Allen, F. H.; Kennard, O.; Watson, D. G.; Brammer, L.; Orpen,
A. G.; Taylor, R. J. Chem. Soc. Perkin Trans. 2 1987, S1–S19.
(15) For electronic spectra of DCA, see: Olea, A. F.; Worrall, D. R.;
Wilkinson, F.; Williams, S. L.; Abdel-Shafi, A. A. Phys. Chem. Chem. Phys.
2002, 4, 161.
’ AUTHOR INFORMATION
Corresponding Author
sekiguch@chem.tsukuba.ac.jp
(16) Rehm, D.; Weller, A. Isr. J. Chem. 1970, 8, 259.
(17) In the intermolecular electron-transfer process, the change in
Gibbs free energy, ΔG, can be estimated by the following eq:
’ ACKNOWLEDGMENT
This work was financially supported by Grants-in-Aid for Scien-
tific Research program (Nos. 19105001, 23108701, 23550042,
23655027) from the Ministry of Education, Science, Sports, and
Ox
1=2
Red
1=2
ΔGET ¼ ðE ꢀ E Þ ꢀ E0-0 ꢀ ωp
ð1Þ
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COMMUNICATION
Ox
1/2
where E is the half-wave potential of the electron donor’s oxidation
Red
1/2
wave (V), E
is the half-wave potential of the electron acceptor’s
reduction wave (V), E0ꢀ0 is the lowest excitation energy (eV), and ωp
(= e2/εr) is a coulomb term (eV). The cyclicꢀvoltammetry of 4 in
acetonitrile containing 0.1 M [n-Bu4N]+ClO4 was measured. An
irreversible oxidation wave was observed, and the peak potential of
1/2
Red
the oxidation wave was used instead of EO1/x2. The values of E
(ꢀ0.950 V vs SCE), E0ꢀ0 (2.91 eV), and ωp (≈ 0.00 V) in acetonitrile
were taken from the following references: (a) Gould, I. R.; Ege, D.;
Moser, J. E.; Farid, S. J. Am. Chem. Soc. 1990, 112, 4290. (b) Ikeda, H.;
Akiyama, K.; Takahashi, Y.; Nakamura, T.; Ishizaki., S.; Shiratori, Y.;
Ohaku, H.; Goodman, J. L.; Houmam, A.; Wayner, D. D. M.; Tero-
Kubota, S.; Miyashi, T. J. Am. Chem. Soc. 2003, 125, 9147.
(18) For theoretical calculations on the potential energy surface of
ꢁ
the C4H4 cation radical, see: Hrouda, V.; Bally, T.; Cꢀarsky, P.; Jungwirth,
P. J. Phys. Chem. A. 1997, 101, 3918.
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