New persistent radicals: Synthesis and electronic spin structure of 2,5- di-tert-butyl-6-oxophenalenoxyl derivatives

Full text of DOI:10.1021/ja000298t

J. Am. Chem. Soc. 2000, 122, 4825-4826
4825
Scheme 1a
New Persistent Radicals: Synthesis and Electronic
Spin Structure of 2,5-Di-tert-butyl-6-
Oxophenalenoxyl Derivatives
Yasushi Morita,*,† Tomohiro Ohba,† Naoki Haneda,†
Suguru Maki,† Junya Kawai,† Kunio Hatanaka,‡
Kazunobu Sato,§ Daisuke Shiomi,§ Takeji Takui,*,§ and
Kazuhiro Nakasuji*,†
Department of Chemistry, Graduate School of Science
Osaka UniVersity, Toyonaka, Osaka, 560-0043, Japan
Department of Structural Molecular Science
The Graduate UniVersity for AdVanced Studies
Myodaiji, Okazaki 444-8585, Japan
Departments of Chemistry and Materials Science
Graduate School of Science, Osaka City UniVersity
Sumiyoshi-ku, Osaka 558-8585, Japan
ReceiVed January 27, 2000
ReVised Manuscript ReceiVed March 29, 2000
a Reagents: (a) cat. concd H2SO4, excess t-BuOH, CF3COOH, 45 °C,
99%; (b) 10 equiv DMF-POCl3, (CH2Cl)2, 90 °C, 99%; (c) 4.2-4.5
equiv LDA-R4CH2COOR5, THF, 0 °C, 97% (for 8), 86% (for 9); (d) (i)
1.5 equiv Et3SiH, 4.5 equiv CF3COOH, CH2Cl2, rt; (ii) Pd/C, H2, EtOH,
rt; (iii) 9.5 equiv KOH, EtOH-H2O, reflux, 82%; (e) (i) 1.5 equiv Et3SiH,
4.5 equiv CF3COOH, CH2Cl2, rt; (ii) Pd/C, H2, EtOH, rt, 85%; (f) (i)
excess (COCl)2, reflux; (ii) 3.5 equiv AlCl3, CH2Cl2, 82% (for 12), 93%
(for 13); (g) 3.3 equiv LiAlH4, THF, rt, 90% (for 14), ∼100% (for 15);
(h) (i) 20 equiv LiI, HMPA, 170 °C; (ii) 2 M HCl aq, rt; (iii) reprecipitated
from hexane-CH2Cl2, 40% (for 3), 62% (for 4).
Neutral radicals such as galvinoxyl, R-nitronyl nitroxide, and
verdazyl play most important roles as spin sources to prepare
magnetically interesting organic materials.1,2 In particular, it is
well-known that galvinoxyl is a pilot open-shell molecule in the
field of organic molecule-based magnetism. The π-spin delocal-
ized nature of galvinoxyl contrasts with other heteroatomic
radicals such as R-nitronyl nitroxide. Novel stable open-shell
molecular systems not only give a testing ground for various
theoretical models, but also they are materials challenges as
building blocks for spin-mediated new molecular functionality.1,3
To expand the variety of such spin sources, we have designed 3-
and 6-oxophenalenoxyl radicals as new neutral radicals, which
possess electronic characteristics similar to those of galvinoxyl.
Although we succeeded in the generation and detection, the
isolation of these radicals was unsuccessful.4 A general strategy
to isolate reactive and open-shell chemical species is the introduc-
tion of tert-butyl groups on appropriate positions.5 In this
contribution, we report on the synthesis and characterization of
the new stable neutral radicals, 2,5-di-tert-butyl-6-oxophenalen-
oxyl derivatives 1 and 2 as studied by ESR and electron-nuclear
multiple resonance (ENDOR/TRIPLE) spectroscopy.
(Scheme 1).6 Although the tert-butylation of 5 by the conventional
method (t-BuCl, AlCl3) failed, the reaction of 5 with t-BuOH in
7
CF3COOH in the presence of concentrated H2SO4 successfully
gave di-tert-butylated product 6 in quantitative yield, which was
formylated by Vilsmeier-Haack reaction. Reaction of the alde-
hyde 7 with lithium enolates generated from tert-butyl acetate or
benzyl tert-butylacetate with lithium diisopropylamide (LDA)
gave the aldol products 8 or 9 as a mixture of diastereomers,
respectively. These compounds were converted to carboxylic acid
derivatives 10 and 11 by reductive removal of the hydroxyl group
and de-esterification. Cyclization into the phenalanone skeletons
was achieved by chlorination of 10 and 11 with (COCl)2 followed
by Friedel-Crafts cyclization. Reduction of 12 and 13 with
LiAlH4 gave the phenalanol derivatives 14 and 15, respectively.
Finally, the methyl ether-protecting groups were removed by LiI,
and the resulting triol was treated with 2 M aqueous HCl, to give
the desired 6-hydroxyphenalenone derivatives 3 and 4 as reddish
orange powders, respectively.8
The radicals 1 and 2 were obtained as brownish black powder
or deep green-colored needles by treatment of the corresponding
9
hydroxyl compounds 3 and 4 with a large excess of active PbO2
The radical precursors, 2,5-di-tert-butyl-6-hydroxyphenalenone
derivatives 3 and 4, were efficiently synthesized from com-
mercially available 2,7-dimethoxynaphthalene (5) in nine steps
(4) (a) Hatanaka, K.; Morita, Y.; Ohba, T.; Yamaguchi, K.; Takui, T.;
Kinoshita, M.; Nakasuji, K. Tetrahedron Lett. 1996, 37, 873-876. (b)
Hatanaka, K.; Morita, Y.; Ohba, T.; Yamaguchi, K.; Takui, T.; Kinoshita,
M.; Nakasuji, K. Tetrahedron Lett. 1996, 37, 877-880.
† Osaka University.
‡ The Graduate University for Advanced Studies.
(5) Goto, K.; Kubo, T.; Yamamoto, K.; Nakasuji, K.; Sato, K.; Shiomi,
D.; Takui, T.; Kubota, M.; Kobayashi, T.; Yakusi, K.; Ouyang, J. J. Am. Chem.
Soc. 1999, 121, 1619-1620.
§ Osaka City University.
(1) For recent overviews, see: Proceedings of the 5th International
Conference on Molecule-Based Magnets, Osaka, Japan, 1996; Itoh, K., Miller,
J. S., Takui, T., Eds.; Gordon and Breach: New York, 1997: Mol. Cryst.
Liq. Cryst. 1997, 305, 1-586; Ibid. 1997, 306, 1-520 and references therein.
(2) (a) Forrester, A. R.; Hay, J. M.; Thomson, R. H. Organic Chemistry of
Stable Free Radicals; Academic Press: London and New York, 1968. (b)
Rozantsev, E. G. Free Nitroxyl Radicals; Plenum Press: New York and
London, 1970. (c) Aurich, H. G. In Nitrones, nitronates and nitroxides; Patai,
S., Rappoport, Z., Eds.; Wiley: Chichester, 1989; Chapter 4. (d) Volodarsky,
L. B.; Reznikov, V. A.; Ovcharenko, V. I. Synthetic Chemistry of Stable
Nitroxides; CRC Press: Boca Raton, FL, 1994.
(6) All new compounds described here gave correct spectroscopic data.
(7) (a) Svanholm, U.; Parker, V. D. J. Chem. Soc., Perkin Trans. 1 1973,
562-566. (b) Boldt, P.; Hilmert-Schimmel, P.; Muller, R.; Heuer, D. Chem.
Ber. 1987, 120, 497-500.
(8) Selected physical data: 3, mp 235 °C; 1H NMR (CDCl3) δ 1.57-1.65
(br, 18), 6.26 (s, 1), 7.50-7.80 (m, 3), 8.35-8.65 (br, 2); EI-MS, m/z 308
(M+, 14%); Anal. Calcd for C21H24O2: C, 81.78; H, 7.84; N, 0.00. Found:
C, 81.43; H, 7.82; N, 0.00. 4, mp 235 °C; 1H NMR (CDCl3) δ 1.44 (s, 9),
1.48 (s, 9), 1.57 (s, 9), 6.10 (s, 1), 7.58 (s, 1), 7.67 (s, 1), 8.40 (d, 1, J ) 1.7
Hz), 8.76 (d, 1, J ) 1.7 Hz); EI-MS, m/z 364 (M+, 20%); Anal. Calcd for
C25H32O2: C, 82.37; H, 8.85; N, 0.00. Found: C, 82.32; H, 8.87; N, 0.00.
(9) Kuhn, R.; Hammer, I. Chem. Ber. 1950, 83, 413-414.
(3) Magnetic Properties of Organic Materials; Lahti, P. M., Ed.; Marcel
Dekker: New York, 1999; pp 1-728.
10.1021/ja000298t CCC: $19.00 © 2000 American Chemical Society
Published on Web 04/29/2000

4826 J. Am. Chem. Soc., Vol. 122, No. 19, 2000
Communications to the Editor
Table 1. Proton hfcc’s and g-Values for 1 and 2a
AH/mT
compd
3, 4
7, 9
8
2,5-t-Bu 8-t-Bu g-value
1
+0.206 +0.078 -0.159 +0.012
(+0.210) (+0.076) (-0.119)
2.0046
2
+0.200 +0.077
(+0.206) (+0.075)
+0.012 (0.006b 2.0049
1
a hfcc’s were determined by H ENDOR spectra in toluene at 290
or 210 K and simulation successfully reproducing the ESR hyperfine
spectra. The relative signs of the hfcc’s were determined in terms of
1H TRIPLE spectroscopy. Values in parentheses were calculated by
HMO-McLachlan method13 using the following parameters: λ ) 1.2,
Q ) -2.6 mT. b The sign was experimentally unknown.
Figure 1. Hyperfine ESR spectra observed for 1 and 2 in toluene at 290
K; microwave frequency used was 9.5192840 for 1 and 9.7799000 for 2
GHz, respectively (a) and simulated one (b).
Figure 3. Canonical resonance structures for 6-oxophenalenoxyl radical
in parentheses, SOMOs (a) of A,5 B, and C, and the corresponding π-spin
density distributions (b) calculated by HMO-McLachlan method.13
Figure 2. 1H ENDOR spectra observed for 1 (at 290 K) and 2 (at 210
heteroatomic perturbation which is attributable to π-spin delo-
calization from the oxygen atom. The spin structure of 6-oxophe-
nalenoxyl in (b) well agrees with the π-electron spin network in
terms of the π-spin density distribution F obtained from the
observed hfcc’s (AR-H) and McConnell’s relationship AR-H ) FQ
(Q < 0).
K) in toluene.
in benzene at room temperature in quantitative yields.10 The two
radicals in the crystal form are stable in the absence of air. Most
of radical 1 decomposes for in a few weeks in air in the solid
state. The radical 2 also decomposes slowly in air in the solid
state, but most of the radical remains unchanged in the air for a
few weeks. Both radicals are stable for a long period of time in
degassed toluene but decompose in the presence of atmospheric
oxygen.11
Figure 1 shows well-resolved hyperfine ESR spectra (a)
observed for 1 and 2 in degassed toluene at 290 K, versus the
simulated ones (b). The ESR measurements were carried out on
a Bruker ESR 300/350 X-band spectrometer at 12.5 kHz field
modulation in order to avoid line shape distortion due to sideband
formation. The spectral simulations were made based on a set of
The effect of the spin delocalization can be understood in terms
of classical canonical resonance structures as shown in Figure 3
(top), in which only representative ones with galvinoxyl features
of spin structure are depicted. To validate the classical picture in
a more quantitative fashion, we calculated contributing weights
of any possible and desired resonance structures in terms of MO
calculations.14 We decomposed the MO-based Slater determinant
1
corresponding to a ground-state electron configuration (S ) /2)
into desired AO-based Slater determinants with the help of the
Moffitt theorem,15 giving contributing weights of any AO-based
determinants which correspond to local spin structures, that is,
resonance ones in terms of a VB picture. The representative
resonance structures, R-γ in parentheses depicted in Figure 3
(top), give dominant contributing weights.
1
isotropic hyperfine coupling constants (hfcc’s) obtained by H
ENDOR/TRIPLE spectroscopies (see Figure 2). Relative signs
of the hfcc’s were determined by invoking 1H electron-nuclear-
nuclear TRIPLE resonance measurements.12 All of the spin
Hamiltonian parameters and proton assignments for 1 and 2 are
summarized in Table 1. Theoretically calculated values for the
hfcc’s in Table 1 were made in terms of the McLachlan method.13
Agreements between the experimental and theoretical values are
satisfactory.
In Figure 3 are depicted SOMOs (a) for phenalenyl neutral
radical (A)5 and 3-oxophenalenoxyl radical (C) as well as
nonsubstituted 6-oxophenalenoxyl radical (B) and the correspond-
ing π-spin density distributions (b). The introduction of oxygen
atoms at 6- or 3-position gives remarkable change in topological
symmetry of π-electron network. This is due to the robust
A remarkable change in topological symmetry of the π-electron
network in oxophenalenoxyl radicals is established via heteroat-
omic perturbation such as the delocalization nature of the
introduced oxygen atoms. In this context, new stable radicals
under study possess electronic characteristics similar to those of
galvinoxyl, which is a pilot open-shell molecule in organic
magnetics and spin chemistry.
Acknowledgment. This work was supported by Grants-in-Aid for
Scientific Research on Priority Area “Delocalized π-Electronic Systems”
(No. 297) from the Ministry of Education, Science, Sports and Culture,
Japan.
Supporting Information Available: Experimental procedures and
data for new compounds (PDF). This material is available free of charge
via the Internet at http://pubs.acs.org.
(10) Selected physical data: 1, mp 70-71 °C dec; TLC Rf 0.41 (5:1 hexane/
ethyl acetate); Anal. Calcd for C21H23O2(H2O)0.7: C, 78.81; H, 7.68; N, 0.00.
Found: C, 78.64; H, 7.51; N, 0.00. 2, mp 253-255 °C dec; TLC Rf 0.75 (5:1
hexane/ethyl acetate); Anal. Calcd for C25H31O2(H2O)0.4: C, 81.00; H, 8.64;
N, 0.00. Found: 81.13; H, 8.48; N, 0.00.
JA000298T
(11) TLC spectroscopy showed no spots from impurities after the radical
is kept for many days in degassed toluene.
(14) Nakazawa, S.; Sato, K.; Kinoshita, T.; Takui. T.; Itoh, K.; Nakamura,
T.; Momose, T.; Shida, T. Synth. Met. 1997, 85, 1735-1736.
(15) Takui, T.; Kalafiloglu, P., unpublished work.
(12) 1H TRIPLE spectroscopic results are shown in Supporting Information.
(13) McLachlan, A. D. Mol. Phys. 1960, 3, 233-252.