Design, synthesis, and properties of conformationally fixed semiquinone monoradical species

Article abstract of DOI:10.1021/jo061502j

(Graph Presented) The design of novel, functionalized semiquinone (SQ) ligands which combine structural rigidity and electron-withdrawing, electron-donating, and electroneutral substituents enables investigation of multiple structure-property relationships and building blocks for new materials, including components of sensors, switches, and molecular spintronics. Along these lines, we report the synthesis of several new SQ ligands containing fused heterocyclic ring systems. Using both electron paramagnetic resonance spectroscopy and quantum chemical calculations, we show how spin density is affected by the fused ring system substituents.

Full text of DOI:10.1021/jo061502j

Design, Synthesis, and Properties of Conformationally Fixed
Semiquinone Monoradical Species
David A. Shultz,† Joseph C. Sloop,*,‡ and Gary Washington‡
Department of Chemistry, North Carolina State UniVersity, P.O. Box 8204, Raleigh, North Carolina
27695, and Department of Chemistry and Life Science, United States Military Academy, 646 Swift Road,
West Point, New York 10996
joseph.sloop@usma.edu
ReceiVed July 19, 2006
The design of novel, functionalized semiquinone (SQ) ligands which combine structural rigidity and
electron-withdrawing, electron-donating, and electroneutral substituents enables investigation of multiple
structure-property relationships and building blocks for new materials, including components of sensors,
switches, and molecular spintronics. Along these lines, we report the synthesis of several new SQ ligands
containing fused heterocyclic ring systems. Using both electron paramagnetic resonance spectroscopy
and quantum chemical calculations, we show how spin density is affected by the fused ring system
substituents.
Introduction
semiquinone (SQ) ligands which combine structural rigidity and
electron-withdrawing, electron-donating, and electroneutral sub-
stituents enables investigation of multiple structure-property
relationships and building blocks for new materials. Along these
lines, we report the synthesis of new SQ ligands shown in
Scheme 1.
New paramagnetic ligands are of general interest in both
organic and inorganic chemistry as components of sensors,
switches, and molecular spintronics.1-8 Previous research in our
group has shown that both conformation and substituents can
modulate spin density and exchange coupling in radical and
biradical ligands.9-11 The design of novel, functionalized
To acquire these new ligand species, the retrosynthetic
strategy shown in Scheme 1 was employed.
Schemes 2-5 describe the preparation of fused, tricyclic
semiquinone precursors. These compounds add both carbocyclic
and heterocyclic, conformationally “locked” species to allow
expansion of our investigation of substituent effects on exchange
coupling between SQ moieties9 to include substituted SQ-
paramagnetic metal ion exchange coupled species.
* To whom correspondence should be addressed. Tel: (845) 938-3904.
† North Carolina State University.
‡United States Military Academy.
(1) Wolf, S. A.; Awschalom, D. D.; Buhrman, R. A.; Daughton, J. M.;
von Molnar, S.; Roukes, M. L.; Chtchelkanova, A. Y.; Treger, D. M. Science
2001, 294, 1488-1495.
(2) Moser, J.-E. Nat. Mater. 2005, 4, 723-724.
(3) Tagami, K.; Tsukada, M. J. Phys. Chem. B 2004, 108, 6441-6444.
(4) Yuasa, J.; Suenobu, T.; Fukuzumi, S. J. Phys. Chem. A 2005, 109,
9356-9362.
(5) Suenaga, Y.; Pierpont, C. G. Inorg. Chem. 2005, 44, 6183-6191.
(6) Evangelio, E.; Ruiz-Molina, D. Eur. J. Inorg. Chem. 2005, 2957-
2971.
(7) Sato, O. J. Photochem. Photobiol. C: Photochem. ReV. 2004, 5, 203-
223.
(8) Dei, A.; Gatteschi, D.; Sangregorio, C.; Sorace, L. Acc. Chem. Res.
2004, 37, 827-835.
(9) Shultz, D. A.; Bodnar, S. H.; Lee, H.; Kampf, J. W.; Incarvito, C.
D.; Rheingold, A. L. J. Am. Chem. Soc. 2002, 124, 10054-10061.
(10) Shultz, D. A.; Fico, R. M., Jr.; Bodnar, S. H.; Kumar, R. K.;
Vostrikova, K. E.; Kampf, J. W.; Boyle, P. D. J. Am. Chem. Soc. 2003,
125, 11761-11771.
(11) Shultz, D. A.; Fico, R. M., Jr.; Lee, H.; Kampf, J. W.; Kirschbaum,
K.; Pinkerton, A. A.; Boyle, P. D. J. Am. Chem. Soc. 2003, 125, 15426-
15432.
10.1021/jo061502j CCC: $33.50 © 2006 American Chemical Society
Published on Web 10/20/2006
9104
J. Org. Chem. 2006, 71, 9104-9113

Conformationally Fixed Semiquinone Monoradicals
SCHEME 1. Planar Semiquinone Retrosynthetic Strategy
SCHEME 2. 3-t-Butyl-9,9-dimethylfluorene Semiquinone Synthesisa
a Conditions: (i) Methyl 2-bromobenzoate (1 equiv), Pd(PPh3)4 (5 mol %), EtOH, Na2CO3, THF, reflux; (ii) (1) CH3MgBr, THF, (2) H30+; (iii) PPA, 20
min; (iv) BBr3, CH2Cl2, -78 °C f rt, 27 h; (v) Fe´tizon’s reagent, Na2SO4, CH2Cl2, 24 h; (vi) 5, KH, THF, 30 min.
Results and Discussion
Attempts to prepare the N-protected carbazole catechol 15
from 1 ultimately resulted in partial deprotection of the amine
during treatment of compound A with BBr3, yielding a large
proportion of compound B (Figure 2).
Synthesis. As shown in Scheme 2, an intramolecular Friedel-
Crafts-type alkylation from 2′(2-hydroxypropyl)-5-t-butyl-3,4-
dimethoxybiphenyl (3) gave rise to 3-t-butyl-1,2-dimethoxy-
9,9-dimethylfluorene (4).12,13 Compound 4 was subsequently
deprotected to yield catechol 5 and then oxidized to quinone 6.
Comproportionation of 5 and 6 provided SQ 7.
An intermediate common to both Schemes 2 and 3, compound
2, served as an intramolecular Friedel-Crafts acylating agent,
yielding the fluorenone derivative 8. However, it was found
that oxidation of 9 with Fe´tizon’s reagent14 did not occur. The
exact reason for this is unknown, but could be due, in part, to
the stability of the catechol when in the keto-enol form shown
in Figure 1.
A second synthesis from 11 avoided this pitfall, as the
conditions for deprotection of the MOM groups are much less
vigorous. Cyclization15 of 5-t-butyl-2′-nitro-2,3-dimethoxym-
ethylenoxybiphenyl (12) to the regioisomeric 4-t-butyl-2,3-
dimethoxymethylenoxycarbazole (13a) and 2-t-butyl-3,4-
dimethoxymethylenoxycarbazole (13b) found in Scheme 4 likely
occurs via a nitrenoid species and does not result in premature
hydrolysis of the methoxymethyl protecting groups. The major
isomer, 13a, was that anticipated based on cyclization away
from the bulky tert-butyl group. Although protection of the
carbazole amine moiety in compounds 13a and 13b was
successful, deprotection of the methylated carbazole derived
from 13b led to an unstable catechol which decomposed before
it could be characterized. For this reason, the remaining synthetic
steps from 13a were employed. Only the stable catechol, 15,
was subsequently oxidized to the quinone, 16. The SQ 17 was
prepared via comproportionation of 15 and 16.
Nevertheless, oxidation of the conjugate base to the SQ 10
was accomplished using ferrocenium, and the resulting EPR
spectrum is consistent with SQ formation.
(12) Shultz, D. A.; Lee, H.; Fico, R. M., Jr. Tetrahedron 1999, 55,
12079-12086.
(13) Shultz, D. A.; Boal, A. K.; Lee, H.; Farmer, G. T. J. Org. Chem.
1999, 64, 4386-4396.
(14) Balogh, V.; Fe´tizon, M.; Golfier, M. J. Org. Chem. 1971, 36, 1339-
(15) Zim, D.; Lando, V. R.; Dupont, J.; Monteiro, A. L. Org. Lett. 2001,
3, 3049-3051.
1341.
J. Org. Chem, Vol. 71, No. 24, 2006 9105

Shultz et al.
SCHEME 3. 3-t-Butyl-9-fluorenone Semiquinone Synthesisa
a Conditions: (i) PPA, 20 min; (ii) BBr3, CH2Cl2, -78 °C f rt, 24 h; (iii) KH, ferrocenium (0.74 equiv), THF, 0.3 h.
SCHEME 4. 3-t-Butyl-9-N-methylcarbazole Semiquinone
oxidation of the conjugate bases using ferrocenium provided
the semiquinones 25a and 25b, permitting EPR spectra to be
recorded.
Synthesisa
Quinone-Semiquinone Cyclic Voltammetry. As indicated
earlier, oxidation of the catechols with Fe´tizon’s reagent gave
the quinones in two cases, 6 and 16. Cyclic voltammetric (CV)
measurements for the quinone-SQ redox couple were per-
formed as described previously,16 and the reduction potentials
relative to ferrocene are listed in Table 1.
Typical quinone-SQ redox potentials are ∼ -0.6 to ∼ -0.85
V.17,18 As shown, the planar quinone-SQ redox potentials are
more negative than similar meta- or para-substituted quinones
bearing alkyl groups which are not conformationally con-
strained.12,18 The planar quinones have functional groups
attached directly to the quinone ring and can therefore donate
electron density effectively into the ring, either inductively (in
the case of 6) or mesomerically (in the case of 16), thereby
moving the reduction to more negative potentials by ca. 250
mV. This is a rather large shift in reduction potential,10,19 which
suggests a strong substituent effect on LUMO energy.
Semiquinone Spin Density Calculations and EPR Spec-
troscopy. Substituents have been shown to alter spin density
distribution in a wide variety of organic radicals.20-27 Since it
is our ultimate goal to explore substituent effects on exchange
a Conditions: (i) 1-Bromo-2-nitrobenzene (1 equiv), Pd(PPh3)4 (5 mol
%), EtOH, Na2CO3, THF, reflux; (ii) P(OC2H5)3, 108 °C, 96 h; (iii) NaH,
DMF; (iv) CH3I, 60 °C, 2 h; (v) CH3OH, reflux, 24 h; (vi) Fe´tizon’s reagent,
Na2SO4, CH2Cl2, 24 h; (vii) 15, KH, THF, 30 min.
(16) Shultz, D. A.; Farmer, G. T. J. Org. Chem. 1998, 63, 6254-6257.
(17) Stallings, M. D.; Morrison, M. M.; Sawyer, D. T. Inorg. Chem.
1981, 20, 2655-2660.
(18) Shultz, D. A.; Boal, A. K.; Lee, H.; Farmer, G. T. J. Org. Chem.
1998, 63, 9462-9469.
The dibenzofuran (Scheme 5) preparation took advantage of
the ability to oxidize dibenzofuran-1-boronic acid, 18, and
1-methoxydibenzofuran, 20, both of which subsequently rear-
range in a fashion reminiscent of the hydroboration-oxidation
of alkenes to alcohols, providing the phenolic compounds
19 and 21. It was necessary to protect the phenol 21 prior
to formation of compound 23 since O-alkylation of 21
occurred to the exclusion of C-alkylation during the butylation
step in route to dibenzofuran 23. Assignments of the tert-
(19) Shultz, D. A.; Vostrikova, K. E.; Bodnar, S. H.; Koo, H. J.;
Whangbo, M. H.; Kirk, M. L.; Depperman, E. C.; Kampf, J. W. J. Am.
Chem. Soc. 2003, 125, 1607-1617.
(20) Neugebauer, F. A.; Fischer, P. H. H. Z. Naturforsch. 1966, 21b,
1036.
(21) Strom, E. T.; Bluhm, A. L.; Weinstein, J. J. Org. Chem. 1967, 32,
3853-3856.
(22) Church, D. F. J. Org. Chem. 1986, 51, 1138-1140.
(23) Zhang, Y. H.; Jiang, B.; Zhou, C. M.; Jiang, X. K. Chin. J. Chem.
1994, 12, 516-523.
(24) Miura, Y.; Kitagishi, Y.; Ueno, S. Bull. Chem. Soc. Jpn 1994, 67,
3282-3288.
1
butyl group position in 23a and 23b were made by H NMR
(25) Jackson, R. A.; Sharifi, M. J. Chem. Soc., Perkin Trans. 2 1996,
775-778.
chemical shifts and coupling constants, as well as 13C chemical
shifts. Deprotection to the catechols 24a and 24b was
effected without incident, but normal oxidative conditions with
Fe´tizon’s reagent failed to produce the quinones. Nonetheless,
(26) Boesch, S. E.; Wheeler, R. A. J. Phys. Chem. A 1997, 101, 8351-
8359.
(27) Shultz, D. A.; Gwaltney, K. P.; Lee, H. J. Org. Chem. 1998, 63,
769-774.
9106 J. Org. Chem., Vol. 71, No. 24, 2006

Conformationally Fixed Semiquinone Monoradicals
SCHEME 5. Dibenzofuran Semiquinone Synthesisa
a Conditions: (i) H2O2, NaOH, 24 h; (ii) (1) K2CO3, 18-crown-6 (2 mol %), (2) CH3I; (iii) (1) t-BuLi, THF, -78 °C, (2) B(OCH3)3, rt, 24 h, (3) H2O2,
NaOH, 24 h; (iv) t-BuOH, PPA, H3PO4, 76 °C, 24 h; (v) BBr3, CH2Cl2, -78 °C f rt, 24 h; (vi) KH, ferrocenium (0.74 equiv), THF, 0.3 h.
TABLE 1. Planar Quinone-Semiquinone Reduction Potentialsa
redox couple
potential (V)
6h7
16h17
-0.94
-0.97
a Cyclic voltammograms are available in Supporting Information.
FIGURE 1. Fluorenone catechol keto-enol equilibrium.
density in compounds 7, 10, 17, 25a, and 25b (see Figure 3).
Proton hyperfine coupling constants (absolute values) obtained
via computation are provided in Table 2.
As shown in Figure 3, while a majority of the spin density is
retained in the SQ ring for these systems, it is apparent that
substituents also affect the distribution of phenyl ring spin
density. Theoretical hyperfine coupling constants reported in
Table 2 provide some quantitative evidence of the extent of
this spin density modulation.
FIGURE 2. Carbazole N-deprotection.
To further evaluate spin density distributions in our planar
SQ solution phase, X-band EPR spectra were collected for each
SQ in THF solvent at room temperature. The spectra and
simulations30 are shown in Figure 4, and the aH values are listed
in Table 2. To facilitate the discussion of our data, SQ ligands
are listed in Table 2 in order of decreasing electron donation
from left to right.
A comparison of the EPR spectra recorded in Figure 4 and
experimental aH values found in Table 2 reveals some similari-
ties with those of SQs in previously reported work. In terms of
general appearance, the spectra are well resolved except for SQs
10 and 17, where line broadening and obscuration of the
hyperfine coupling constants may result from spin exchange
due to higher EPR solution concentration and species instabil-
coupling of our SQ ligands with paramagnetic metal ions, it is
desirable to evaluate spin density distribution in our new SQ
ligands. Theoretical spin density distribution contours for the
semiquinones without solvent (gas phase) generated from
Gaussian 98W density functional computations with full ge-
ometry optimization employing an unrestricted B3LYP method
and the epr-ii basis set28 predict substituent modulation of spin
(28) Spin densities and aH values determined by Gaussian 98 for the
semiquinones without solvent (gas phase): Frisch, M. J.; Trucks, G. W.;
Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski,
V. G.; Montgomery, J. A., Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich,
S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.;
Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.;
Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui,
Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.;
Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.;
Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.; Chen, W.;
Wong, M. W.; Andres, J. L.; Head-Gordon, M.; Replogle, E. S.; Pople, J.
A. Gaussian 98, revision A.9; Gaussian, Inc.: Pittsburgh, PA, 1998.
(29) Spin density contours obtained using the GNU Image Manipulation
Program, 1991. For more information, see www.gimp.org.
(30) Duling, D. EPR Calculations for MS-Windows, NT/95 Software
Tools, version 0.96 ed.; Public EPR Software Tools; National Institute of
Environmental Health Sciences, National Institutes of Health: Triangle Park,
NC, 1996.
J. Org. Chem, Vol. 71, No. 24, 2006 9107

Shultz et al.
TABLE 2. Planar Semiquinone aH Valuesa
17
25a
X ) O
R3 ) t-Bu
R1′ ) H
25b
X ) O
7
10
X ) NCH3
R3 ) t-Bu
R1′ ) H
theord
X ) C(CH3)2
R3 ) t-Bu
R1′ ) H
X ) C ) O
R3 ) t-Bu
R1′ ) H
R3 ) H
Compound
proton
R1′ ) t-Bu
expb,c
expe
theord
expe
theord
expf
theord
expc,g
theord
H4
H4′
H3′
R3
H2′
R1′
N
2.71
1.38
0.36
2.86
0.52
0.16
2.66
0.74
0.02
2.73
0.56
0.04
2.86
0.42
0.09
0.72
0.31
2.65
0.51
0.03
1.86
0.38
2.66
0.76
0.26
2.19
0.64
0.30
0.83
0.84
0.83
0.15
0.96
0.36
1.18
2.03
1.33
2.71
4.97
0.26
0.53
0.92
2.86
1.47
0.45
0.34
0.41
0.26
0.86
0.26
0.83
0.26
2.50
0.83
1.37
0.54
∑aH′ (SQ ring)
∑aH′ (Ph ring)
2.66
1.55
2.73
1.27
3.58
0.82
4.51
0.92
2.66
2.14
2.19
2.03
0.83
5.00
0.15
3.23
a The aH values in Gauss. b Conditions: 0.5 mM SQ in THF, 298 K, X-band frequency ) 9.59 GHz. c EPR simulation line width (set ∼1 G to obtain
satisfactory spectral simulation) may limit the accuracy of experimental aH values; see ref 31. d From ref 28, gas phase. e Conditions: catecholate oxidation
with ferrocenium, 0.1 mM SQ in THF, 298 K, X-band frequency ) 9.59 GHz. f Conditions: 0.1 mM SQ in THF, 298 K, X-band frequency ) 9.77 GHz.
g Conditions: catecholate oxidation with ferrocenium, 0.5 mM SQ in THF, 298 K, X-band frequency ) 9.59GHz.
proton hyperfine coupling constants (∑aH(Ph ring)). These
parameters are proportional to the spin densities in each ring
according to the McConnell relationship (in which aH ∼ FC,
where FC is the carbon spin density)33 and are important in
forming hypotheses regarding exchange coupling of our SQ
ligands to paramagnetic metal ions and regarding exchange
coupling of future SQ ligands that have stable radicals bound
to the phenyl ring.
While experimental ∑aH parameters suggest that the strong
electron-donating substituent (X ) NCH3) increases phenyl ring
spin density relative to SQ ring spin density, the theoretical spin
density contour and ab initio ∑aH results indicate a majority of
the excess spin density in the SQ ring. One possibility for this
discrepancy may be the large simulation line width (∼1 G)
required for a satisfactory fit of the spectrum, resulting in
significant error associated with the smaller experimental aH
values for SQ 17. On the basis of the theoretical results, we
hypothesize that 17 will exhibit stronger exchange coupling with
paramagnetic metal ions (JM-SQ) provided that the major
contribution to exchange is directly related to the product of
spin densities on the SQ and the metal ion. Since the spin density
is the square of the SOMO wavefunction, and the product of
the spin densities is proportional to the overlap integral of the
natural orbitals and therefore to J, see34
FIGURE 3. The 0.001 e/au3 planar SQ spin density distribution
JM-SQ ∼ FM × FSQ
contours.29
ity.31,32 With the exception of SQ 10, the largest proton hyperfine
coupling constant is H4, adjacent to the radical center.10
At the bottom of Table 2 are summations of SQ ring proton
hyperfine coupling constants (∑aH(SQ ring)) and phenyl ring
By the same token, if stable radicals are attached to the phenyl
ring of 17, the exchange coupling between SQ and a radical
(JSQ-radical) will be smaller for 17 than for the other derivatives.
Experimental ∑aH values for the furan semiquinones 25a and
25b are consistent with both theoretically predicted spin density
distributions and aH values. Our results show that oxygen in
(31) Peric, M.; Bales, B. L. J. Magn. Reson. 2004, 169, 27-29.
(32) SQs 10 and 17 began showing signs of degradation within 15 min
upon removal from an argon environment. This instability is of little concern
since we are ultimately interested in transition metal complexes of these
SQs which are, in most cases, inert.
(33) McConnell, H. M.; Chestnut, D. B. J. Chem. Phys. 1958, 28, 107-
117.
(34) Kahn, O. Molecular Magnetism; VCH: New York, 1993.
9108 J. Org. Chem., Vol. 71, No. 24, 2006

Conformationally Fixed Semiquinone Monoradicals
FIGURE 4. Planar SQ X-band EPR and simulations.
the heterocyclic ring systems increases spin density in the SQ
ring relative to compound 7. This is most likely due to
mesomeric electron donation by the furan oxygens. Conversely,
inductive electron-withdrawing properties, as evidenced by
the presence of spin density on the furan oxygens, may be
responsible for the decrease in spin density seen in the
phenyl ring relative to that of compound 7. Differences in
the magnitudes of the SQ and phenyl ring ∑aH values in
25a and 25b may be accounted for by the tert-butyl group
location.
Experimental aH values for dimethyl fluorene SQ 7 are in
agreement with theoretically derived values recorded in Table
2 and consistent with spin densities in Figure 3. As expected,
the spin density is more equally distributed between the SQ
J. Org. Chem, Vol. 71, No. 24, 2006 9109

Shultz et al.
from sodium benzophenone ketyl before use, while CH2Cl2 and
CH3OH were distilled from CaH2.
and Ph rings in SQ 7 than in the remaining planar semiquinones,
a consequence of the slight inductive electron-donating nature
of the -C(CH3)2 group.
Experimental Procedures: 2′-Carbomethoxy-5-t-butyl-3,4-
dimethoxybiphenyl (2). To a 125 mL Kjeldahl flask equipped with
a N2 adapter and stir bar were added methyl-2-bromobenzoate (0.52
mL, 3.70 mmol), 3-t-butyl-4,5-dimethoxyphenylboronic ester (0.85
g, 3.70 mmol), Pd(PPh3)4 (0.21 g, 0.19 mmol), 30 mL of dry THF,
7.4 mL of EtOH, and 11.1 mL of 2 M Na2CO3. A reflux condenser
was added, and the reaction mixture was purged 10 times with N2
and refluxed for 24 h. Following filtration through celite, the solvent
was removed by evaporation under reduced pressure, extracted with
Et2O, washed with saturated brine solution, concentrated, and
subjected to radial chromatography (silica gel w/5-40% ether/pet
ether gradient) to afford 1.03 g (85%) of colorless crystals, 2, mp
62-65 °C. 1H NMR (300 MHz, CDCl3): δ 7.72 (d, J ) 7.62 Hz,
1H), 7.50 (m, 1H), 7.41 (m, 2H), 6.84 (dd, J ) 1.95, 8.25 Hz, 2H),
3.92 (s, 3H), 3.88 (s, 3H), 3.64 (S, 3H), 1.39 (s, 9H). 13C NMR
(300 MHz, CDCl3): δ 165.9, 152.9, 142.9, 130.8, 130.3, 129.4,
129.2, 126.8, 119.1, 110.8, 60.3, 55.7, 52.0, 34.9, 30.4. IR (cm-1):
ν 1719, 1291, 1248. Anal. Calcd for C20H24O4: C, 73.14; H, 7.36.
Found: C, 72.85; H, 7.32.
Semiquinone 10 departs from the general trend described
above. As in the case of 17, the EPR spectrum was poorly
resolved, potentially resulting in uncertainty in the smaller aH
values. Figure 3 clearly predicts significant resonance interaction
between the carbonyl moiety, a strong electron-withdrawing
substituent, and the catecholate oxygen in the semiquinone ring
of 10. The trend indicated in the theoretical and experimental
∑aH magnitudes supports this, illustrating two important
resultssredistribution of spin density away from the SQ ring
relative to compound 7 and a concomitant increase in
phenyl ring spin density. We hypothesize that 10 will exhibit
weaker exchange coupling with paramagnetic metal ions
(JM-SQ), and by the same token, if stable radicals are attached
to the phenyl ring of 10, the exchange coupling between SQ
and a radical (JSQ-radical) will be greater for 10 than for the other
derivatives.
2′(2-Hydroxypropyl)-5-t-butyl-3,4-dimethoxybiphenyl (3). To
an oven-dried 125 mL Schlenk flask equipped with a stir bar and
N2 adapter were added 2 (0.55 g, 1.67 mmol) and 20 mL of distilled
THF. Then, CH3MgI (2.23 mL, 6.7 mmol) was added dropwise, a
reflux condenser added, and the reaction mixture refluxed for 3 h.
The reaction mixture was quenched with deionized water, filtered
through celite, acidified with 1 M HCl, washed with Et2O, and the
layers were separated, and solvent was removed under reduced
pressure. The crude product was chromatographed on SiO2 (deac-
tivated w/1% Et3N) using 20% ether/petroleum ether, yielding 0.43
g (78%) of a yellowish-brown oil, 3, which showed facile
elimination to the alkene and was hence used without further
Conclusions
Planar semiquinone species with electroneutral, electron-
withdrawing, and electron-donating substituents affect hyperfine
coupling constants. General trends are evident. The semiquinone
hyperfine coupling constants are consistent with those previously
reported. Electroneutral groups, such as -C(CH3)2 in SQ 7,
provide for a more uniform distribution of excess spin
density throughout the SQ system. Electron-withdrawing groups
were found to dramatically alter spin density in the planar SQ
system relative to SQ 7 as evidenced by a reduction in the
magnitude of the hydrogen hyperfine coupling in the SQ ring
and an increase in the magnitude of SQ phenyl ring aH values.
Electron-donating groups were generally found to decrease the
magnitude of the phenyl ring aH values. Ab initio methods
support these results, predicting substituent-based variations in
modulation of spin density distribution in conformationally fixed
semiquinone systems. Notwithstanding SQ 10 and SQ 17,
theoretically calculated aH values were found to give good
agreement with experimental hyperfine coupling constants in
most cases. Our results suggest a design criterion for enhancing
paramagnetic metal ion exchange coupling. Since electron-
withdrawing groups increase spin density, the exchange coupling
between the SQ and metal ions should be greater for SQs with
electron-withdrawing groups than for SQs with electron-
donating groups. We will report our findings along these lines
in the future.
1
purification. H NMR (300 MHz, CDCl3): δ 7.59 (d, J ) 7.86
Hz, 1H), 7.34 (t, J ) 7.2 Hz, 1H), 7.24 (m, 1H), 7.12 (d, J ) 7.86
Hz, 1H), 6.85 (s, 1H), 6.79 (s, 1H), 3.92 (s, 3H), 3.83 (s, 3H), 1.74
(br s, 1H), 1.51 (s, 6H), 1.37 (s, 9H). 13C NMR (300 MHz,
CDCl3): δ 152.8, 146.3, 142.9, 140.2, 137.8, 132.2, 127.3, 126.0,
125.7,120.2, 112.3, 74.1, 60.5, 55.8, 35.2, 32.6, 30.6. IR (cm-1):
ν 3436, 3048, 1237.
3-t-Butyl-1,2-dimethoxy-9,9-dimethylfluorene (4). To a beaker
were added 3 (0.25 g, 0.76 mmol) and PPA (∼2.0 g). The reaction
mixture was heated to 40 °C and stirred with a glass rod for 20
min. The resulting brown solution was quenched with ice (∼10 g),
washed with saturated NaCl solution, extracted with Et2O, dried
over Na2SO4, and the solvent removed under reduced pressure. The
crude product was chromatographed on SiO2 (deactivated w/1%
Et3N) using 20% ether/petroleum ether, yielding 0.20 g (80%) of
1
a colorless oil, 4. H NMR (300 MHz, CDCl3): δ 7.64 (d, J )
6.78 Hz, 1H), 7.42 (s, 1H), 7.38 (d, J ) 7.86 Hz, 1H), 7.28 (m, J
) 6.42 Hz, 2H), 3.93 (s, 6H), 1.59 (s, 6H), 1.46 (s, 9H). 13C NMR
(300 MHz, CDCl3): δ 154.2, 152.5, 151.0, 143.3, 143.2, 139.3,
134.7, 126.8, 126.5, 122.1, 119.2, 112.8, 59.8, 59.7, 47.6, 35.3,
31.7, 30.8, 25.8. IR (cm-1): ν 3050, 2953, 1263. Anal. Calcd for
C21H26O2: C, 81.25; H, 8.44. Found: C, 81.04; H, 8.60.
Experimental Section
Instrumentation. NMR data were collected at 300.0 MHz
for 1H and 75.4 MHz for 13C. All 1H and 13C NMR chemical
shifts reported are referenced to deuteriochloroform using the
appropriate chemical shifts unless specifically noted. Reported
melting points are uncorrected. X-band EPR spectra were col-
lected at 9.59-9.77 GHz. Computations were performed on a
computer platform operating at 2.8 GHz, using Gaussian 98W
revision A.9.
Chemicals. Methyl-2-bromobenzoate, o-bromonitrobenzene,
P(OC2H5)3, and dibenzofuran-1-boronic acid (18) are commercially
available and were used without further purification. Compounds
1 and 11 were prepared according to literature methods.15 All
reactions, solvent distillations, and EPR sample preparation were
conducted under nitrogen or argon atmosphere. THF was distilled
3-t-Butyl-1,2-dihydroxy-9,9-dimethylfluorene (5). To a 125 mL
Kjeldahl flask equipped with a N2 adapter and stir bar were added
4 (0.29 g, 0.93 mmol), 10 mL of distilled CH2Cl2, and the mixture
was purged three times with N2 and cooled to -78 °C. Then, a 1.0
M solution of BBr3 (2.7 mL, 2.70 mmol) was added dropwise
slowly to the reaction mixture, which was then allowed to
warm to room temperature and to stir for 10 h under N2,
protected from light. The reaction mixture was quenched with
ice, washed with saturated brine, and extracted with two 10 mL
portions of CH2Cl2. The organic layers were combined and dried
over Na2SO4, and the solvent was evaporated to dryness to afford
1
0.25 g (94%) of a gray solid, 5, mp 54-60 °C (dec). H NMR
(300 MHz, CDCl3): δ 7.62 (d, J ) 6.75 Hz, 1H), 7.36 (dd, J )
10.1, 1.0 Hz, 1H), 7.27 (d, J ) 7.86 Hz, 2H), 7.25 (d, J ) 6.42
9110 J. Org. Chem., Vol. 71, No. 24, 2006

Conformationally Fixed Semiquinone Monoradicals
Hz, 1H), 5.36 (s, 6H), 5.12 (s, 1H), 1.61 (s, 6H), 1.46 (s, 9H). 13
C
with saturated brine solution, concentrated, and subjected to radial
chromatography (silica gel w/0-30% ether/pet ether gradient) to
afford 0.39 g (53%) of a yellow oil, 12. 1H NMR (300 MHz,
CDCl3): δ 7.78 (d, J ) 8.04 Hz, 1H), 7.58 (t, J ) 7.2 Hz, 1H),
7.46 (d, J ) 7.7 Hz, 2H), 7.02 (d, J ) 1.4 Hz, 1H), 6.94 (d, J )
1.47 Hz, 1H), 5.26 (s, 2H), 5.18 (s, 2H), 3.57 (s, 3H), 3.51 (s, 3H),
1.42 (s, 9H). 13C NMR: δ 150.2, 146.1, 143.7, 135.8, 132.6, 131.3,
131.2, 128.6, 127.2, 119.4, 115.4, 114.7, 99.1, 95.6, 57.5, 56.7,
35.4, 30.2. IR (cm-1): ν 1525. HRMS (FAB+): calcd for C20H25-
NO6, 375.1682; found, 375.1668.
4-t-Butyl-2,3-dimethoxymethylenoxycarbazole (13a) and 2-t-
Butyl-3,4-dimethoxymethylenoxycarbazole (13b). To a 25 mL
round-bottom flask equipped with a N2 adapter and stir bar were
added 12 (0.27 g, 0.86 mmol) and P(OC2H5)3 (1.5 mL, 8.75 mmol).
The reaction mixture was purged three times with N2 and refluxed
at 108 °C for 96 h under N2. Filtration through celite, removal of
solvent by vacuum distillation, and radial chromatography (silica
gel w/0-50% ether/pet ether gradient) afforded the colorless oils
13a (0.18 g) and 13b (0.060 g) (conversion rate of 81%). 13a: 1H
NMR (CDCl3, 300 MHz): δ 8.41 (br s, 1H), 7.97 (d, J ) 7.80 Hz,
1H), 7.74 (s, 1H), 7.39 (s, 1H), 7.36 (q, J ) 7.50 Hz, 1H), 7.19 (t,
J ) 7.00 Hz, 1H), 5.28 (s, 2H), 5.25 (s, 2H), 3.68 (s, 3H), 3.60 (s,
3H), 1.76 (s, 9H). 13C NMR: δ 145.5, 139.4, 133.7, 125.7, 124.5,
122.6, 119.8, 110.0, 105.6, 98.6, 58.5, 55.5, 37.1, 32.4. IR (cm-1):
ν 3503. HRMS (FAB+): calcd for C20H25NO4, 343.1784; found,
343.1781. 13b: 1H NMR (CDCl3, 300 MHz): δ 9.07 (s, 1H), 7.99
(d, J ) 7.80 Hz, 1H), 7.76 (s, 1H), 7.41 (t, J ) 8.04 Hz, 1H), 7.36
(t, J ) 7.02 Hz, 1H), 7.19 (t, J ) 7.29 Hz, 1H), 5.29 (s, 2H), 5.26
(s, 2H), 3.73 (s, 3H), 3.69 (s, 3H), 1.52 (s, 9H). 13C NMR: δ 146.8,
139.7, 135.5, 131.1, 125.8, 120.5, 120.1, 112.3, 99.6, 98.6, 58.2,
58.1, 35.3, 31.0. IR (cm-1): ν 3365. HRMS (FAB+): calcd for
C20H25NO4, 343.1784; found, 343.1772.
NMR (300 MHz, CDCl3): δ 153.4, 142.7, 141.2, 141.1, 139.7,
135.9, 132.3, 126.9, 126.2, 122.0,119.1, 110.1, 47.0, 31.0, 30.0,
25.0. IR (cm-1): ν 3509. Anal. Calcd for C19H22O2: C, 80.81; H,
7.85. Found: C, 80.43; H, 7.63.
3-t-Butyl-9,9-dimethylfluorene orthoquinone (6). To a 100 mL
round-bottom flask equipped with a N2 adapter and stir bar were
added 5 (0.116 g, 0.41 mmol), Fe´tizon’s reagent (3.5 g, excess),
Na2SO4 (3.5 g, excess), and distilled CH2Cl2 (20 mL), and the
mixture was purged three times with N2. The reaction mixture
was stirred for 18 h under N2. Filtration through celite and
concentration under reduced pressure afforded 0.10 g (87%) of a
1
red solid (Et2O), 6, mp 182-184 °C. H NMR: δ 7.68 (dd, J )
7.5, 1.8 Hz, 1H), 7.49 (t, J ) 9.3 Hz, 1H), 7.46 (t, J ) 8.4 Hz,
1H), 7.31 (d, J ) 3.1 Hz, 1H), 7.25 (s, 1H), 1.50 (s, 6H), 1.34 (s,
9H). 13C NMR: δ 182.7, 175.9, 158.1, 152.1, 149.6, 145.1, 137.2,
131.0, 127.8, 127.5, 122.9, 122.4, 49.0, 35.9, 29.6, 24.5. IR (cm-1):
ν 1637. HRMS (M + H) (FAB+): calcd for C19H21O3, 281.1542;
found, 281.1551.
3-t-Butyl-9,9-dimethylfluorene semiquinone (7). To a 20 mL
vial equipped with a stir bar in an inert atmosphere were added 5
(0.049 g, 0.17 mmol), 6 (0.048 g, 0.17 mmol), and dry THF (5
mL), and the mixture was stirred for 5 min. Then, KH (2 mg, 0.50
mmol) was added and the reaction mixture stirred for 30 min. The
resulting suspension was filtered to remove excess KH, the
semiquinone 7 was obtained as a green solution, and a 0.1 mM
solution of the semiquinone in THF was prepared for EPR studies.
Electrochemical preparation of 7 was accomplished by placing 6
(0.00155 g, 0.0055 mmol) and electrolyte n-Bu4NPF6 (0.21 g, 0.55
mmol) in 5.5 mL of anhydrous THF. Cyclic voltammetric measure-
ments were then conducted on the solution, with Pt wire serving
as the working and auxiliary electrodes, while the reference
electrode was Ag/AgNO3 in acetonitrile.
9N-Methyl-4-t-butyl-2,3-dimethoxymethylenoxycarbazole (14).
To a 100 mL round-bottom flask equipped with a stir bar were
added 13a (0.292 g, 0.85 mmol) and 4 mL of dry DMF, and the
3-t-Butyl-1,2-dimethoxy-9-fluorenone (8). Using the procedure
for 4, this compound was obtained from 2 in 49% yield as a yellow
1
oil, 8. H NMR (300 MHz, CDCl3): δ 7.60 (t, J ) 7.5, 0.9 Hz,
mixture was heated to 60 °C under N2. To this was added a
1H), 7.45 (t, J ) 1.0 Hz, 1H), 7.44 (dq, J ) 5.7, 0.9 Hz, 1H), 7.23
(dd, J ) 6.6, 1.8 Hz, 1H), 7.20 (s, 1H), 4.09 (s, 3H), 3.93 (s, 3H),
1.41 (s, 9H). 13C NMR (300 MHz, CDCl3): δ 191.5, 153.6, 153.1,
150.6, 144.1, 139.5, 134.9, 134.2, 128.2, 124.0, 123.5, 119.4, 113.7,
61.9, 61.0, 35.9, 30.3. IR (cm-1): ν 1707. HRMS (FAB+): calcd
for C19H20O3, 296.1412; found, 296.1407.
suspension of NaH (0.044 g, 1.85 mmol) in 6.1 mL of dry DMF
(heated at 60 °C). The reaction mixture was stirred for 1 h under
N2. Then, CH3I (0.12 mL, 1.85 mmol) was added dropwise slowly
via syringe and stirred for an additional 1 h. Removal of solvent
by vacuum distillation and chromatography (silica gel w/0-10%
ether/pet ether gradient) afforded 0.172 g (57%) (conversion rate
of 82%) of a pale yellow oil, 14. 1H NMR (CDCl3, 300 MHz): δ
7.93 (d, J ) 7.62 Hz, 1H), 7.68 (s, 1H), 7.40 (q, J ) 6.9 Hz, 1H),
7.37 (t, J ) 7.8 Hz, 1H), 7.21 (t, J ) 7.26 Hz, 1H), 5.30 (s, 2H),
5.13 (s, 2H), 3.79 (s, 3H), 3.68 (s, 3H), 3.60 (s, 3H), 1.74 (s, 9H).
13C NMR: δ 147.4, 146.1, 140.0, 130.5, 126.2, 124.9, 122.5, 120.6,
111.9, 105.2, 100.6, 96.3, 95.6, 81.9, 57.2, 55.9, 41.6, 36.3, 33.6.
IR (cm-1): ν 1472, 1381. HRMS (FAB+): calcd for C21H27NO4,
357.1940; found, 357.1926.
9N-Methyl-4-t-butyl-2,3-dihydroxycarbazole (15). To a 100
mL round-bottom flask equipped with a stir bar were added 14
(0.16 g, 0.45 mmol), 1 mL of concentrated HCl, and 40 mL of
methanol. A reflux condenser was attached, and the mixture was
refluxed for 24 h with stirring under N2, protected from light. The
solvent was removed by evaporation under reduced pressure. The
residue was taken up in CH2Cl2, extracted with saturated brine,
and the organic layer dried over Na2SO4. The solvent was
evaporated to dryness to afford 0.11 g (99%) of a green-gray solid,
15, mp 177 °C. 1H NMR (CDCl3, 300 MHz): δ 7.82 (d, J ) 6.00
Hz, 1H), 7.42 (s, 1H), 7.30 (t, J ) 5.70 Hz, 1H), 7.27 (t, J ) 6.00
Hz, 1H), 7.06 (t, J ) 6.00 Hz, 1H), 6.86 (s, 1H), 3.72 (s, 3H), 1.25
(s, 9H). 13C NMR: δ 144.1, 140.5, 130.0, 128.7, 122.6, 122.2,
117.0, 116.8, 115.2, 108.1, 106.4, 33.1, 28.9, 27.9. IR (cm-1): ν
3241. HRMS (FAB+): calcd for C17H19NO2, 269.1416; found,
269.1398.
3-t-Butyl-1,2-dihydroxy-9-fluorenone (9). Using method B
described in the Supporting Information, this compound was
obtained in 75% yield as a red solid (Et2O), 9, mp 144-146 °C
1
(dec). H NMR (300 MHz, CDCl3): δ 8.07 (br s, 1H), 7.56 (d, J
) 7.08 Hz, 1H), 7.42 (d, J ) 2.46 Hz, 2H), 7.18 (q, J ) 3.66 Hz,
1H), 6.99 (s, 1H), 5.74 (s, 1H), 1.44 (s, 9H). 13C NMR (300 MHz,
CDCl3): δ 194.8, 176.3, 151.3, 148.3, 146.5, 136.9, 134.8, 133.7,
132.0, 126.1, 124.1, 112.9, 110.3, 34.2, 28.9. IR (cm-1): ν 3503,
3389, 1684. HRMS (M + H) (FAB+): calcd for C17H17O3,
269.1178; found, 269.1174.
3-t-Butyl-9-fluorenone semiquinone (10). To a 20 mL vial in
an inert atmosphere were added 9 (0.0082 g, 0.03 mmol, 0.250
mmol), ferrocenium (0.0061 g, 0.022 mmol), 2 mL of degassed
THF, and KH (∼1 mg). The solution was stirred for 20 min. The
resulting green suspension was filtered to remove excess KH, and
a 0.1 mM solution of the semiquinone in THF was prepared for
EPR studies.
5-t -Butyl-2′-nitro-2,3-dimethoxymethylenoxybiphenyl (12).
To a 125 mL Kjeldahl flask equipped with a N2 adapter and stir
bar were added 2-bromonitrobenzene (0.40 g, 1.98 mmol), 3-t-butyl-
4,5-dimethoxymethylenephenylboronic ester (1.23 g, 3.77 mmol),
Pd(PPh3)4 (0.20 g, 0.18 mmol), dry THF (50 mL), EtOH (7.5 mL),
and 2 M Na2CO3 (11.3 mL). A reflux condenser was added, and
the reaction mixture was purged 10 times with N2 and refluxed for
24 h. Following filtration through celite, the solvent was removed
by evaporation under reduced pressure, extracted with Et2O, washed
9N-Methyl-4-t-butylcarbazole orthoquinone (16). Using the
general method described in the Supporting Information, this
J. Org. Chem, Vol. 71, No. 24, 2006 9111

Shultz et al.
compound was obtained in 97% yield as a green solid (CH2Cl2),
16, mp 162-164 °C. H NMR (CDCl3, 300 MHz): δ 7.69 (dd, J
(d, J ) 7.5 Hz), 7.54 (d, J ) 8.0 Hz), 7.49 (d, J ) 8.3 Hz, 1H),
7.38 (t, J ) 7.5 Hz, 2H), 7.31 (t, J ) 7.3 Hz, 2H), 6.97 (d, J ) 8.3
Hz, 1H), 4.29 (s, 3H). 13C NMR: δ 156.6, 147.7, 147.3, 132.3,
126.1, 124.7, 123.1, 120.9, 119.2, 114.7, 111.7, 111.6, 61.2. HRMS
(EI+): calcd for C13H10O3, 214.0630; found, 214.0625.
1
) 8.1, 1.2 Hz, 1H), 7.43 (t, J ) 7.8 Hz, 1H), 7.41 (d, J ) 7.5 Hz,
1H), 7.36 (s, 1H), 7.34 (dt, J ) 9.0, 0.9 Hz, 1H), 7.23 (dt, J ) 7.8,
0.9 Hz, 1H), 4.05 (s, 3H), 1.31 (s, 9H). 13C NMR: δ 198.7, 182.1,
147.7, 140.4, 130.5, 128.4, 122.8, 121.0, 115.8, 114.2, 111.3, 101.4,
54.1, 35.1, 29.8. IR (cm-1): ν 1659. HRMS (M + H) (FAB+):
calcd for C17H19NO2, 268.1338; found, 268.1335.
1,2-Dimethoxydibenzofuran (22). To a 100 mL round-bottom
flask equipped with a stir bar were added 21 (0.71 g, 3.33 mmol),
potassium carbonate (0.92 g, 6.66 mmol), 18-crown-6 (10 mol %),
and 50 mL of acetone, and the mixture was stirred for 15 min.
Then, methyl iodide (0.31 mL, 5.0 mmol) was added dropwise,
and the mixture was refluxed under nitrogen. After 6 h, another
aliquot of methyl iodide (0.31 mL, 5.0 mmol) was added and the
mixture refluxed for 42 h. The solvent was removed under reduced
pressure, the residue taken up in methylene chloride, acidified with
1 M HCl solution (20 mL), separated, and the aqueous layer again
extracted with methylene chloride. The organic portions were
combined, filtered through a silica gel plug, and evaporated to
dryness to afford 0.76 g (99%) of a pale yellow solid, 22, mp 102-
104 °C. 1H NMR: δ 7.86 (dd, J ) 8.7, 1.0 Hz, 1H), 7.55 (dd, J )
8.1, 1.8 Hz, 2H), 7.40 (dt, J ) 7.4, 1.5 Hz, 1H), 7.31 (dt, J ) 7.35,
0.9 Hz, 1H), 6.96 (d, J ) 8.7 Hz, 1H), 4.21 (s, 3H), 3.97 (s, 3H).
13C NMR: δ 156.7, 151.5, 147.7, 136.3, 126.5, 124.6, 123.1, 120.3,
120.0, 114.4, 111.8, 108.7, 61.5, 57.2. HRMS (EI+): calcd for
C13H10O3, 228.0786; found, 228.0785.
9N-Methyl-4-t-butylcarbazole semiquinone (17). Using the
general procedure described in the Supporting Information, this
compound was generated in situ, and EPR measurements were
conducted. Electrochemical preparation of 17 was accomplished
by placing 16 (0.00141 g, 0.0053 mmol) and electrolyte n-Bu4-
NPF6 (0.21 g, 0.55 mmol) in 5.5 mL of anhydrous THF. Cyclic
voltammetric measurements were then conducted on the solution,
with Pt wire serving as the working and auxiliary electrodes, while
the reference electrode was Ag/AgNO3 in acetonitrile.
1-Hydroxydibenzofuran (19). To a 200 mL Schlenk flask
equipped with a stir bar and nitrogen adapter were added diben-
zofuran-1-boronic acid, 18 (1.0 g, 4.72 mmol), 30% hydrogen
peroxide (11.13 mL, 23.58 mmol), 2% sodium hydroxide solution
(18.0 mL), and 30 mL of dry THF. The resulting suspension was
stirred at room temperature for 24 h under nitrogen. The solvent
was removed under reduced pressure, and the residue was taken
up in diethyl ether, acidified with 2% HCl solution (20 mL),
separated, and the aqueous layer extracted with diethyl ether. The
organic portions were combined and evaporated to dryness to afford
3-t-Butyl-1,2-dimethoxydibenzofuran (23a) and 8-t-Butyl-1,2-
dimethoxydibenzofuran (23b). To a 50 mL round-bottom flask
equipped with a stir bar, condenser, and septum were added 22
(0.75 g, 2.33 mmol), H3PO4 (0.81 g, 5.85 mmol), and PPA (1.87
g). The reaction mixture was heated to 76 °C under nitrogen and
stirred for 15 min. Then, t-butanol (0.20 mL, 3.04 mmol) was
added to the reaction mixture, and after 1 h, another 0.20 mL
aliquot of t-butanol was added. The reaction was allowed to
proceed for 24 h. The oily residue was washed with 20%
NaOH solution (20 mL), extracted with CH2Cl2, separated, the
organic layer dried over Na2SO4 and subjected to radial chroma-
tography (0-50% ether/pet ether) to afford 0.20 g (21%) of
colorless crystals, 23a, mp 90-93 °C, and 0.17 g (17%) of a
1
0.82 g (94%) of a pale yellow solid, 19, mp 102-104 °C. H
NMR: δ 7.94 (dd, J ) 1.5, 7.5 Hz, 1H), 7.58 (d, J ) 8.4 Hz, 1H),
7.53 (dd, J ) 1.0, 7.8 Hz, 1H), 7.47 (dt, J ) 0.8, 7.8 Hz, 1H), 7.36
(dt, J ) 1.2, 6.3 Hz, 1H), 7.23 (t, J ) 7.5 Hz, 1H), 7.09 (dd, J )
1.2, 7.9 Hz, 1H), 5.31 (s, 1H). 13C NMR: δ 156.2, 145.6, 127.9,
126.5, 124.0, 123.5, 122.2, 113.5, 112.2, 111.4, 110.4, 109.5. HRMS
(EI+): calcd for C22H8O2, 184.0524; found, 184.0530.
1-Methoxydibenzofuran (20). To a 100 mL round-bottom flask
equipped with a stir bar were added 19 (0.79 g, 4.29 mmol),
potassium carbonate (1.19 g, 8.58 mmol), 18-crown-6 (10 mol %),
and 50 mL of acetone, and the mixture was stirred for 15
min. Then, methyl iodide (0.80 mL, 12.87 mmol) was added
dropwise, a reflux condenser added, and the mixture refluxed
for 24 h under nitrogen. The solvent was removed under reduced
pressure, and the residue was taken up in methylene chloride,
acidified with 2% HCl solution (20 mL), separated, and the
aqueous layer again extracted with methylene chloride. The
organic portions were combined, filtered through a silica gel plug,
and evaporated to dryness to afford 0.78 g (91%) of a pale yellow
1
brownish oil, 23b, as a second fraction. 23a: H NMR: δ 7.85
(dd, J ) 0.6, 7.2 Hz, 1H), 7.56 (d, J ) 8.4 Hz, 1H), 7.46 (d, J )
0.9 Hz, 1H), 7.45 (d, J ) 2.1 Hz, 1H), 6.95 (d, J ) 8.7 Hz, 1H),
4.19 (s, 3H), 3.97 (s, 3H), 1.42 (s, 9H). 13C NMR: δ 152.1, 150.0,
151.3, 148.3, 133.6, 130.4, 128.7, 122.9, 121.0, 117.8, 114.2, 113.0,
112.3, 56.4, 56.1, 34.8, 31.6. HRMS (EI+): calcd for C13H10O3,
1
284.1412; found, 284.1403. 23b: H NMR: δ 7.78 (dd, J ) 0.6,
7.1 Hz, 1H), 7.22 (dd, J ) 0.6, 7.2 Hz, 1H), 7.05 (m, 1H), 6.91
(m, 2H), 4.09 (s, 3H), 3.89 (s, 3H), 1.34 (s, 9H). 13C NMR: δ
156.4, 154.9, 151.3, 146.2, 124.2, 120.6, 116.7, 114.2, 111.1, 108.5,
106.0, 104.9, 61.4, 57.3, 35.0, 32.1. HRMS (EI+): calcd for
C13H10O3, 284.1412; found, 284.1419.
1
solid, 20, mp 52-54 °C. H NMR: δ 7.94 (d, J ) 7.7 Hz, 1H),
7.63 (d, J ) 8.2 Hz, 1H), 7.56 (d, J ) 7.8 Hz, 1H), 7.47 (dt, J )
1.2, 7.5 Hz, 1H), 7.35 (dt, J ) 0.8, 7.4 Hz, 1H), 7.29 (t, J ) 7.7
Hz, 1H), 7.01 (d, J ) 8.1 Hz, 1H), 4.08 (s, 3H). 13C NMR: δ
156.2, 145.6, 127.9, 126.5, 124.0, 123.5, 122.2, 113.5, 112.2, 111.4,
110.4, 109, 56.8. HRMS (EI+): calcd for C23H10O2, 198.0681;
found, 198.0672.
3-t-Butyl-1,2-dihydroxydibenzofuran (24a). Using method B
described in the Supporting Information, this compound was
1
obtained in 94% yield as colorless crystals, mp 61-64 °C. H
NMR: δ 7.85 (s, 1H), 7.44 (s, 2H), 7.38 (d, J ) 7.9 Hz, 1H), 6.93
(d, J ) 8.1 Hz, 1H), 5.50 (br s, 2H), 1.41 (s, 9H). 13C NMR: δ
154.9, 146.4, 143.6, 128.9, 123.9, 116.9, 112.3, 111.6, 110.9, 35.1,
32.1. IR (cm-1): ν 3170. HRMS (EI+): calcd for C16H16O3,
256.1099; found, 256.1099.
2-Hydroxy-1-methoxydibenzofuran (21). To a 200 mL Schlenk
flask equipped with a stir bar and nitrogen adapter were added 20
(2.93 g, 14.8mmol) and 70 mL of dry THF, and the solution was
cooled to -78 °C under N2. Then, t-BuLi (19.1 mL, 32.5 mmol)
was added dropwise slowly with stirring. After 2 h, trimethyl borate
(5.04 mL, 44.4 mmol) was added dropwise and allowed to warm
to room temperature overnight. The solvent and excess trimethyl
borate were removed under reduced pressure, and 70 mL of dry
THF, 30% hydrogen peroxide (11.71 mL, 23.58 mmol), and 2%
sodium hydroxide solution (18.0 mL) were added. The resulting
suspension was stirred at room temperature for 24 h under nitrogen.
The solvent was removed under reduced pressure, the residue taken
up in diethyl ether, acidified with 2% HCl solution (20 mL),
separated, and the aqueous layer extracted with diethyl ether. The
organic portions were combined and evaporated to dryness to afford
8-t-Butyl-1,2-dihydroxydibenzofuran (24b). Using method B
described in the Supporting Information, this compound was
obtained in 94% yield as a brown oil. 1H NMR: δ 7.71 (d, J ) 8.1
Hz, 1H), 7.48 (s, 1H), 7.38 (m, 2H), 6.96 (t, J ) 7.2 Hz, 1H), 6.10
(br s, 2H), 1.38 (s, 9H). 13C NMR: δ 143.5, 129.1, 123.9, 123.1,
120.8, 119.7, 119.1, 118.3, 116.8, 112.1, 111.6, 108.6, 35.4, 31.9.
IR (cm-1): ν 3365. HRMS (EI+): calcd for C16H16O3, 256.1099;
found, 256.1100.
3-t-Butyldibenzofuran semiquinone (25a). Using the procedure
described for compound 10, this compound was generated in situ
(0.5 mM in THF), and EPR measurements were conducted.
8-t-Butyldibenzofuran semiquinone (25b). Using the procedure
1
2.82 g (89%) of a tan solid, 21, mp 84-88 °C. H NMR: δ 7.85
9112 J. Org. Chem., Vol. 71, No. 24, 2006

Conformationally Fixed Semiquinone Monoradicals
described for compound 10, this compound was generated in situ
(0.5 mM in THF), and EPR measurements were conducted.
Supporting Information Available: General experimental
methods, 1H or 13C NMR spectra, cyclic voltammograms, and
density functional theory computational parameters for reported
compounds. This material is available free of charge via the Internet
at http://pubs.acs.org.
Acknowledgment. The authors would like to thank the
USMA Center for Molecular Science for computational support,
and the NCSU Mass Spectroscopy Facility for high-resolution
MS analysis. D.A.S. thanks the NSF (CHE-0345263) for support
of this work.
JO061502J
J. Org. Chem, Vol. 71, No. 24, 2006 9113