Dynamic equilibrium between dissociation and regeneration of the C-C bond in trispiro-conjoined cyclopropane compound

Article abstract of DOI:10.1016/j.tetlet.2007.07.175

-The oxidation of 2,2-di(3,5-di-t-butyl-4-hydroxyphenyl)indan-1,3-dione by potassium hexacyanoferrate(III) afforded a trispiro-conjoined cyclopropane compound due to the bond formation between the ipso-carbons. Its cyclopropane ring was found to exist in solution in dynamic equilibrium with biradical species by the dissociation of the C-C bond, which is as long as 1.595 ? as revealed by X-ray crystal analysis.

Full text of DOI:10.1016/j.tetlet.2007.07.175

Tetrahedron Letters 48 (2007) 6877–6880
Dynamic equilibrium between dissociation and regeneration of
the C–C bond in trispiro-conjoined cyclopropane compound
Saiko Kiyohara, Koji Ishizuka, Hidetsugu Wakabayashi, Hiroshi Miyamae,
Makoto Kanazumi, Tatsuhisa Kato and Keiji Kobayashi*
Department of Chemistry, Graduate School of Material Science, Josai University, Sakado, Saitama 350-0295, Japan
Received 29 June 2007; revised 23 July 2007; accepted 26 July 2007
Available online 31 July 2007
Abstract—-The oxidation of 2,2-di(3,5-di-t-butyl-4-hydroxyphenyl)indan-1,3-dione by potassium hexacyanoferrate(III) afforded a
trispiro-conjoined cyclopropane compound due to the bond formation between the ipso-carbons. Its cyclopropane ring was found
˚
to exist in solution in dynamic equilibrium with biradical species by the dissociation of the C–C bond, which is as long as 1.595 A as
revealed by X-ray crystal analysis.
Ó 2007 Elsevier Ltd. All rights reserved.
tBu
tBu
The preparation and magnetic characterization of new
O
OH
tBu
persistent biradicals have been a rapidly growing area
of research for the understanding of the basic strategy
for the design of molecule-based magnets.1 p-Systems
containing two stable phenoxyl radicals such as Yang’s
biradical have been the subject of such research.2 In
the course of our study of the magnetic properties of
bis(phenoxyl)radical compounds, we have been inter-
ested in compound 3 containing two stable phenoxyl
radicals held together as closely as possible, and we
intended to prepare it by oxidizing the precursor phenol
compound 2. However, contrary to our presumption,
trispiro-conjoined cyclopropane 1 was obtained. The
oxidation of compounds in which two phenol moieties
are connected by a common carbon atom, in certain
cases, gives rise to mono- or dispiro-conjoined com-
pounds.3 However, the formation of trispiro-conjoined
compounds is rarely encountered in the oxidation of
such compounds.4 Furthermore, we found a novel
dynamic behavior of ring opening and closing occurring
in the cyclopropane ring of 1. Herein, we report the fun-
damental structural characterization of 1.
O
O
O
O
tBu
K3[Fe(CN)6]
tBu
OH
tBu
tBu
tBu
O
2
1
Scheme 1.
1 in 61% yield (see Scheme 1).6 The structure of 1 was
established by X-ray crystal analysis.7
A single crystal suitable for X-ray studies was obtained
by slow evaporation of solvent from a dichloromethane
solution containing 1. The image obtained by the X-ray
analysis shows the inclusion of solvent molecules
in a 1:2 ratio. The crystal structure unequivocally dem-
onstrates that 1 possesses a cyclopropane ring, two
cyclohexadienone rings and an indan-1,3-dione moiety
(Fig. 1). On the other hand, the 1H and 13C NMR spec-
tra at room temperature are not consistent with such
structure.
Cyclopropane compound 1 was derived from 2, which
was prepared by refluxing ninhydrin and 2,6-di-t-butyl-
phenol in acetic acid.5 The reaction of 2 with aqueous
alkaline K3[Fe(CN)6] in ether afforded insoluble solids,
which were crystallized from dichloromethane to give
Only at low temperatures, such as ꢀ50 °C, either the 1H
or 13C NMR spectrum was observed to be in accord with
the structure of 1, as disclosed by the X-ray analysis. In
1H NMR at room temperature, the four equivalent
*
Corresponding author. E-mail: kobayak@josai.ac.jp
0040-4039/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.07.175

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S. Kiyohara et al. / Tetrahedron Letters 48 (2007) 6877–6880
Figure 1. X-ray structure of 1. The crystal is solvated by CH2Cl2.
protons on the cyclohexadienone ring of 1 appeared as a
remarkably broad signal at 7.83 ppm, which became
broader at higher temperatures but sharper at lower tem-
peratures (Fig. 2). The 13C NMR at room temperature
displayed five distinguishable signals, whereas that at
Figure 3. Temperature dependent 13C NMR spectra of 1 recorded in
CDCl3 solvent.
50 °C showed only three signals (Fig. 3). Eleven signals
were observed at ꢀ50 °C including characteristic signals
for cyclopropane and carbonyl carbons at 52.74 and
54.20 ppm, and at 184.58 and 194.07 ppm, respectively.
In all the above measurements, the NMR signals show
a reversible change depending on temperature.
These observations, that is, the broadening of the signals
at high temperatures and the sharpening of the signals at
low temperatures are readily interpreted in terms of the
intervention of radical species, which enhance spin-lat-
tice relaxation, further indicating that the population
of radical species is high at high temperatures. Bis(phe-
noxyl)radical 3 should be an intermediate for the forma-
tion of 1 by the oxidation of 2 and could also be
incorporated as a counterpart for the dynamic equilib-
rium of 1 (see Scheme 2). The 1H-signal at 7.83 ppm that
disappeared at 50 °C is assigned to the protons on the
cyclohexadienone moiety that have a high spin density
in equilibrium with 3. On the other hand, the signals
due to the indan moiety are intact during the change
in temperature, which can be explained in terms of the
low spin density at this position.
The involvement of radical species was supported also
by the occurrence of an ESR signal for a toluene solu-
tion of 1 and a decrease in its intensity with decreasing
temperature. The 1:2:1 three-line signal (a = 0.18 mT)
implies that an unpaired electron is coupled with two
protons in the two cyclohexadienone ring of 3.
When a solid sample of 1 was heated up to 180 °C, the
solid changed from pale yellow to orange and on cooling
to room temperature reverted to pale yellow. On heat-
ing, an ESR signal appeared as a simple broad singlet
Figure 2. Temperature-dependent 1H NMR spectra of 1 recorded in
CDCl3 solvent.

S. Kiyohara et al. / Tetrahedron Letters 48 (2007) 6877–6880
tBu
6879
tBu
O
O
O
O
tBu
tBu
tBu
tBu
1
O
O
O
O
tBu
tBu
3
Scheme 2.
on the solid sample and its intensity increased with
increasing temperature up to 170 °C.8 These observa-
tions are in line with the event in solution. Thus, the
C–C bond in 1 thermally undergoes homolytic bond dis-
sociation also in the solid state, but the biradical pro-
duced in the solid state is persistent and gradually
regenerates the cyclopropane ring with decreasing tem-
perature. It is conceivable that, in a crystalline environ-
ment, dynamic movement is suppressed and a rather
static equilibrium is achieved.
Small amounts of decomposition products were detected
in the recovered solids. Thus, the solid-state thermo-
chromism described above was not highly efficient.
Above 190 °C, the solid sample decomposed and the
ESR signal faded. The decomposition of 1 proceeded
slowly in chloroform solution to afford 4 and 5 in 48%
and 39% yields, respectively, along with 2 in 11% yield
on standing for two weeks9 (see Scheme 3).
Figure 4. Histogram for the C–C bond distances in cyclopropane
compounds.
X-ray crystal structure of 1 revealed a significantly elon-
gated C–C bond.
It should be noted that the C–C bond for dissociation is
Acknowledgments
˚
significantly elongated as long as 1.595(8) A in contrast
˚
to 1.564(9) A of the other two bonds. We examined crys-
We thank Professor Naoto Hayashi, Department of
Chemistry, Toyama University, Japan, for his assistance
with the CSD analysis.
tal data deposited in the Cambridge Structural Database
(CSD) for the C–C bond lengths of compounds bearing
a cyclopropane ring as a partial structure.10 The distri-
bution of the cyclopropane C–C bond lengths for
12,960 bonds in 2719 compounds are shown in Fig. 4.
The C–C bond in 1 is obviously within 10% of the lon-
gest bond. The high reactivity of 1 to bond dissociation
could be attributed, at least in part, to this long but
weak C–C bond.
References and notes
1. (a) Diradicals; Borden, W. T., Ed.; Wiley: New York,
1982; (b) Borden, W. T.; Iwamura, H.; Berson, J. A. Acc.
Chem. Res. 1994, 109; (c) Rajca, A. Chem. Rev. 1994, 94,
871; (d) Iwamura, H. Adv. Phys. Org. Chem. 1990, 26, 179.
2. (a) Shultz, D. A.; Boal, A. K.; Farmer, G. T. J. Org.
Chem. 1998, 63, 9462; (b) Chandross, E. A. J. Am. Chem.
Soc. 1964, 86, 1263; (c) Kopf, P. W.; Kreilik, W. J. Am.
Chem. Soc. 1969, 19, 6569; (d) Yang, N. C.; Castro, A. J.
J. Am. Chem. Soc. 1960, 82, 6208; (e) Anderson, K. K.;
Schultz, D. A.; Dougherty, D. A. J. Org. Chem. 1997, 62,
7575; (f) Bock, H.; John, A.; Havias, Z.; Bats, J. W.
Angew. Chem., Int. Ed. Engl. 1993, 32, 416; (g) Mukai, K.;
Ishizu, K.; Nakahara, M.; Deguci, Y. Bull. Chem. Soc.
Jpn. 1980, 53, 3363; (h) Van Willigen, H.; Kirste, B.;
Kurreck, H.; Plato, M. Tetrahedron 1982, 38, 759.
In summary, we have demonstrated the dynamic behav-
ior of bond dissociation and regeneration in the trispiro-
conjoined cyclopropane compound 1 in solution. In the
solid state, such dynamic behavior was suppressed and
reversible bond dissociation and recombination were
thermally induced accompanied by color change. The
tBu
tBu
O
O
O
O
1
3. (a) Becker, H.-D. J. Am. Chem. Soc. 1967, 32, 2115; (b)
Becker, H.-D. J. Org. Chem. 1969, 34, 1203; (c) Aleksfuk,
O.; Grynszpan, F.; Biali, S. E. J. Chem. Soc., Chem.
Commun. 1993, 11; (d) Aleksiuk, O.; Cohen, S.; Biali, S. E.
J. Am. Chem. Soc. 1995, 117, 9645; (e) Agbaria, K.;
Aleksiuk, O.; Biali, S. E.; Bohmer, V.; Frings, M.;
O
tBu
tBu
4
5
Scheme 3.

6880
S. Kiyohara et al. / Tetrahedron Letters 48 (2007) 6877–6880
Thondorf, I. J. Org. Chem. 2001, 66, 2891; (f) Chandross,
E. A.; Kreilick, R. J. Am. Chem. Soc. 1963, 85, 2530.
4. Chandross, E. A.; Kreilick, R. J. Am. Chem. Soc. 1964, 86,
7. Crystal data: C39H48O4Cl4 orthorhombic, Pbcm (#57),
˚
˚
˚
a = 9.692(3) A, b = 16.725(4) A, c = 23.261(7) A, V =
3
˚
3770 (1) A Z = 4, R = 0.086, Rw = 0.158, GOF = 1.052.
117.
Crystallographic data for the structural analysis of com-
pound 1 has been deposited with the Cambridge Crystal-
lographic Data Centre (CCDC 650916). These data can be
obtained free of charge via, www.ccdc.cam.ac.uk/conts/
retrieving.html.
1
5. Compound 2: Yellow crystals; mp 184–187 °C. H NMR
(CDCl3, 500 MHz). d 1.34 (36H, s), 5.18 (2H, s), 7.04 (4H,
s), 7.87 (2H, symmetric m of AA0BB0), 8.07 (2H, m of
AA0BB0), EI-MS (70 eV) m/z: 554. Anal. Calcd for
C37H46O4: C, 80.11; H, 8.36. Found: C, 80.09; H, 8.36.
6. Compound 1: Yellow powder; mp 174.2–175.9 °C. 1H
NMR (CDCl3, 500 MHz) d 1.27 (36H, s), 7.81 (4H, br s),
7.88 ( 2H, m of AA0BB0), 8.02 (2H , m of AA0BB0). 13C
NMR (CDCl3, 125 MHz, at ꢀ50 °C) d 29.05, 35.79, 52.74,
54.20, 123.34, 132.26, 136.22, 141.43, 150.05, 184.58,
194.07 ppm. EI-MS (70 eV ) m/z: 552. IR (KBr): mC@O
1705 cmꢀ1. Anal. Calcd for C37H44O4: C, 80.40; H, 8.02.
Found: C, 80.02; H, 7.98.
8. At this temperature, the crystal is not a single crystal;
guest molecules are lost to give opaque solids.
9. Compound 4: Red crystals; mp 157–158 °C, 1H NMR
(CDCl3, 500 MHz) d 1.36 (18H, s), 7.83 (2H, m of
AA0BB0), 7.98 (2H, m of AA0BB0), 8.83 (2H, s) ppm. EI-
MS (70 eV) m/z: 348. Anal. Calcd for C23H24O3: C, 79.28;
H, 6.94. Found: C, 79.42; H, 6.96.
10. Data were retrieved for the crystals in which R is less than
7.5%.