Synthesis of a BDPA-TEMPO biradical

Article abstract of DOI:10.1021/ol9001575

The synthesis and characterization of a biradical containing a 1,3-bisdiphenylene-2-phenylallyl (BDPA) free radical covalently attached to a 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) free radical are described. The synthesis of the biradical is a step

Full text of DOI:10.1021/ol9001575

ORGANIC
LETTERS
2009
Vol. 11, No. 9
1871-1874
Synthesis of a BDPA-TEMPO Biradical
Eric L. Dane, Thorsten Maly, Galia T. Debelouchina, Robert G. Griffin, and
Timothy M. Swager*
Massachusetts Institute of Technology and Francis Bitter Magnet Laboratory,
Cambridge, Massachusetts 02139
tswager@mit.edu
Received January 24, 2009
ABSTRACT
The synthesis and characterization of a biradical containing a 1,3-bisdiphenylene-2-phenylallyl (BDPA) free radical covalently attached to a
2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) free radical are described. The synthesis of the biradical is a step toward improved polarizing
agents for dynamic nuclear polarization (DNP).
Biradicals are of considerable interest as polarizing agents
for microwave-driven dynamic nuclear polarization (DNP)
NMR experiments.1,2 Irradiation of the biradical’s electron
paramagnetic resonance (EPR) drives electron-nuclear transi-
tions that transfer the large polarization of the electrons to
the nuclear spins and thereby enhances the signal-to-noise
ratio in NMR experiments. The enhancement factors can
reach a theoretical maximum of ∼660 for electron-1H
polarization transfer and ∼2600 when 13C is the nuclear spin
of interest.3 Thus, optimized biradical polarizing agents can
dramatically decrease acquisition times. These signal en-
hancements are important for a variety of applications
involving solid-state NMR, particularly for systems that are
not amenable to crystallographic studies, such as amyloid4
and membrane5 proteins.
In previous work, we demonstrated that bis-nitroxide
biradicals, which contain two tethered TEMPO moieties,
provide 1H/13C signal enhancements ∼200-fold in solid-state
magic-angle-spinning (MAS) NMR spectra.2 We are inter-
ested in molecules that can produce DNP by a cross effect
(CE) mechanism3that involves three spins, two dipolar
1
coupled electrons, and a nuclear spin (usually H), denoted
by S1 and S2 and I, with electron and nuclear Larmor
frequencies, ω0S1, ω0S2, and ω0I, respectively. In a DNP
experiment, microwave irradiation of the biradical’s EPR
spectrum induces the two electrons to undergo a spin flip-
flop process, during which a nuclear spin is polarized if the
electron Larmor frequencies are separated by the nuclear
Larmor frequency and therefore satisfy the matching condi-
1
tion, ω0S1 - ω0S2 ) ω0I. The high H polarizaton is then
(1) Hu, K.-N.; Bajaj, V. S.; Rosay, M. M.; Griffin, R. G. J. Chem. Phys.
2007, 126, 044512
.
(4) (a) Jaroniec, C. P.; MacPhee, C. E.; Bajaj, V. S.; McMahon, M. T.;
Dobson, C. M.; Griffin, R. G. Proc. Natl. Acad. Sci. U.S.A. 2004, 101,
711. (b) van der Wel, P. C. A.; Hu, K.-N.; Lewandowski, J. R.; Griffin,
R. G. J. Am. Chem. Soc. 2006, 128, 10840. (c) van der Wel, P. C. A.;
Lewandowski, J. R.; Griffin, R. G. J. Am. Chem. Soc. 2007, 129, 5117.
(5) (a) Bajaj, V. S.; Hornstein, M. K.; Kreischer, K. E.; Sirigiri, J. R.;
Woskov, P. P.; Mak-Jurkauskas, M. L.; Herzfeld, J.; Temkin, R. J.; Griffin,
R. G. J. Magn. Reson. 2007, 189, 251. (b) Mak-Jurkauskas, M. L.; Bajaj,
V. S.; Hornstein, M. K.; Belenky, M.; Griffin, R. G. Proc. Natl. Acad. Sci.
U.S.A. 2008, 105, 883.
(2) (a) Hu, K.-N.; Yu, H.-h.; Swager, T. M.; Griffin, R. G. J. Am. Chem.
Soc. 2004, 126, 10844. (b) Song, C.; Hu, K.-N.; Swager, T. M.; Griffin,
R. G. J. Am. Chem. Soc. 2006, 128, 11385
.
(3) (a) Maly, T.; Debelouchina, G. T.; Bajaj, V. S.; Hu, K.-N.; Joo,
C.-G.; Mak-Jurkauskas, M. L.; Sigirhi, J. R.; van der Wel, P. C. A.; Herzfeld,
J.; Temkin, R. J.; Griffin, R. G. J. Chem. Phys. 2008, 128, 052211. (b)
Barnes, A. B.; Pae¨pe, G. D.; Wel, P. C. A. v. d.; Hu, K.-N.; Joo, C.-G.;
Bajaj, V. S.; Mak-Jurkauskas, M. L.; Sirigiri, J. R.; Herzfeld, J.; Temkin,
R. J.; Griffin, R. G. Appl. Magn. Reson. 2008, 34, 237.
10.1021/ol9001575 CCC: $40.75
Published on Web 03/30/2009
 2009 American Chemical Society

transferred via cross-polarization to 13C or 15N, resulting in
an enhanced MAS NMR spectrum.6
biradicals in aqueous solutions that form a rigid glass at T <
90 K. Because of the challenges of making the nonpolar
BDPA and TEMPO radicals water-soluble, we first chose
to synthesize a hydrophobic BDPA-TEMPO biradical as a
model compound. As described in this communication, we
have developed synthetic methods to join the two sensitive
functionalities and studied the biradical’s EPR properties.
In attempting to synthesize a BDPA-TEMPO biradical (9),
our work began by investiging how BDPA was previously
made. Koelsch’s original method9 (Scheme 1, blue) involves
the formation of alcohol 2, the displacement of the hydroxyl
group with a chloride to form 3, and the removal of a chlorine
radical with mercury. The method developed by Kuhn and
Neugebauer10 (Scheme 1, red) involves a conjugate addition
of the fluorene anion to 1, followed by a one-electron
oxidation of the stable carbanion intermediate. The latter
method was pursued to synthesize a functionalized BDPA
derivative because it requires fewer steps and provides higher
yields.
The efficiency of the CE mechanism depends on how
many pairs of electrons satisfy the matching condition. Thus,
the ideal CE polarizing agent would be a biradical with an
EPR spectrum consisting of two sharp lines separated by
ω0I. However, at the high magnetic fields (>5 T) where
contemporary NMR experiments are performed, only a few
known radicals exhibit narrow spectra. Among them are two
stable species, trityl radical derivatives7 and the BDPA
radical8 (Scheme 1), which have similar isotropic g-values
Scheme 1. BDPA Radical Syntheses
Efforts to synthesize BDPA derivatives have been lim-
ited.11 Kuhn10 reported halogenated BDPA derivatives, and
Fox12 reported the synthesis of BDPA derivatives with
methoxy, cyano, and nitro groups at the 4-position of the
phenyl ring. Previously synthesized biradicals containing
BDPA have been limited to molecules containing two BDPA
radicals linked through the phenyl ring.10
The reactivity of the BDPA radical complicates its
incorporation in biradicals. Although the radical is remark-
ably stable to oxygen in the solid state9 and has been reported
to be indefinitely stable to oxygen in solution with the
exclusion of light, its photoreactivity produces a variety of
oxidation products in solution.12 Additionally, solutions of
the radical are reduced to give the corresponding carbanion
when exposed to strong bases, such as hydroxide or alkoxide,
and BDPA also reacts with strong acids.9
The unpaired electron of BDPA is delocalized throughout
the fluorenyl blades, but it is not appreciably delocalized into
the phenyl blade.13 On the basis of this fact, we chose to
connect TEMPO through an amide linkage14 at the para
position of the phenyl ring to minimize the disruption of the
radical’s stability and its propeller-like geometry.
(giso(trityl) ) 2.00307,1 giso(BDPA) ) 2.00264). If trityl or
BDPA serves as one of the lines in the EPR spectrum of a
polarizing agent, then to satisfy the CE matching condition
it is necessary to introduce another radical with a line
separated from the first by ω0I. There are no known stable
radicals that provide a narrow line and meet this condition;
however, TEMPO derivatives have a broad line with
significant spectral density at a frequency separation match-
ing ω0I.
The synthesis of the BDPA-TEMPO biradical is shown in
Scheme 2. Compound 4 was prepared by a condensation of
fluorene and 4-carboxybenzaldehyde. Purification of 4 was
inefficient as a result of the presence of 4-methylbenzoic acid,
which is difficult to remove from commercial 4-carboxyben-
zaldehyde.15 Nevertheless, pure acid 4 was obtained by recrys-
Recently, one of us showed that the enhancements
observed with a trityl-TEMPO mixture are a factor of ∼4
larger than those obtained with TEMPO alone.1 The success
of this experiment provides the rationale for the synthesis
of a biradical that covalently combines a narrow- and broad-
line radical, therefore increasing the dipolar coupling. BDPA
was chosen as the narrow-line species, because it has greater
stability and a narrower line width than trityl at high magnetic
fields. Currently, DNP can only be effectively performed on
(9) Koelsch, C. F. J. Am. Chem. Soc. 1957, 79, 4439.
(10) Kuhn, R.; Neugebauer, F. A. Monatsh. Chem. 1964, 95, 3.
(11) (a) Plater, M. J.; Kemp, S.; Lattmann, E. J. Chem. Soc., Perkin
Trans. 1 2000, 971. (b) Nishide, H.; Yoshioka, N.; Saitoh, Y.; Gotoh, R.;
Miyakawa, T.; Tsuchida, E. J. Macromol. Sci. Pure Appl. Chem. 1992,
A29, 775.
(6) (a) Pine, A.; Gibby, M. G.; Waugh, J. S. J. Chem. Phys. 1972, 56,
1776. (b) Bennett, A. E.; Rienstra, C. M.; Auger, M.; Lakshmi, K. V.;
Griffin, R. G. J. Chem. Phys. 1995, 103, 6951.
(12) Breslin, D. T.; Fox, M. A. J. Phys. Chem. 1993, 97, 13341.
(13) Azuma, N.; Ozawa, T.; Yamauchi, J. Bull. Chem. Soc. Jpn. 1994,
67, 31.
(7) (a) Reddy, T. J.; Iwama, T.; Halpern, H. J.; Rawal, V. H. J. Org.
Chem. 2002, 67, 4635. (b) Bowman, M. K.; Mailer, C.; Halpern, H. J. J.
Magn. Reson. 2005, 172, 254. (c) Liu, Y.; Villamena, F. A.; Sun, J.; Xu,
Y.; Dhimitruka, I.; Zweier, J. L. J. Org. Chem. 2008, 73, 1490.
(8) 1,3-Bisdiphenylene-2-phenylallyl (BDPA) free radical and 2,2,6,6-
tetramethylpiperidine-1-oxyl (TEMPO) free radical will be referred to as
simply BDPA and TEMPO throughout the text.
(14) Sosnovsky, G.; Lukszo, J.; Brasch, R. C.; Eriksson, U. G.; Tozer,
T. N. Eur. J. Med. Chem. 1989, 24, 241.
1
(15) The impurity was identified by H NMR and in our hands could
not be removed from the commercial material through recrystallization,
extraction, or vacuum sublimation.
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Org. Lett., Vol. 11, No. 9, 2009

Scheme 2. Synthesis of BDPA-TEMPO Biradical
tallization (tetrahydrofuran/acetic acid), but with a significant
loss of material. Carrying forward the impure material proved
a better option, because bromination of acid 4 gives compound
5, which has lower solubility and allows isolation of the pure
solid by simple filtration. Dibromide 5 was refluxed in ethanol
with sodium hydroxide to eliminate hydrogen bromide and
produce the conjugate acceptor 6. We found this reaction to be
concentration sensitive. At higher concentrations, bromine (Br2)
instead of hydrogen bromide was eliminated. Subsequent
reaction of 6 with the fluorene anion, which is generated by
deprotonation with sodium tert-butoxide in dimethylacetamide
(DMA), yields a deep blue solution of the stabilized carbanion.
This intermediate is quenched with 2 M HCl and purified to
give the functionalized BDPA precursor 7, which is isolated
as a white, air-stable powder.
spectrum. In solution, one 1H-nucleus resonates at 5.90 ppm.
The upfield shift is caused by the nearby phenyl group’s local
magnetic field. In contrast, the H-nucleus on the opposite
1
side of the fluorene moiety resonates at 8.52 ppm and appears
to interact with the proximate proton on the sp3-hybridized
carbon. This interaction has been confirmed by the observa-
tion of a 20% NOE enhancement in solution. In the process
of characterizing 7 by NMR, we observed a second rotamer,
7b, that appears stable in the solid state but which slowly
converts to the more stable form, 7a, in solution (Figure 1).
Rotamer 7b is observed in the proton NMR of nonrecrys-
tallized 7. The conversion of 7b to 7a was monitored by
NMR and was complete within a week in d8-tetrahydrofuran
at room temperature. Additionally, 7b can be regenerated
from samples of pure 7a by deprotonation followed by an
acid quench.16
To synthesize 9, carboxylic acid 7 was converted to the
acid chloride with oxalyl chloride and catalytic DMF and
then reacted with 4-amino TEMPO. The proton on the sp3-
hybridized carbon of 7 is presumed to be slightly more acidic
than that of the protonated BDPA precursor (pKa ) 14,
DMSO).17 To avoid side reactions, the weak base pyridine
is used to accelerate amide formation and scavenge protons.
A portion of 8 was reduced with ascorbic acid, converting
the TEMPO radical to the hydroxylamine, for characteriza-
tion using 1H and 13C NMR. The biradical 9 was generated
by deprotonating 8, followed by one-electron oxidation.
The 9 GHz liquid-state EPR spectrum of 9 and of precursor
8 are shown in Figure 2. A typical spectrum of a nitroxide
radical consists of three sharp lines. The spectrum of the
biradical differs as a result of the presence of the BDPA
radical and the interaction between the two paramagnetic
centers. Additionally, several new resonances are observed
Figure 1. (a) X-ray ORTEP representation of 7 (50% probability
level). (b) Rotamers of 7.
The X-ray crystal structure of 7 (Figure 1) shows its
preferred geometry, which explains its unique H NMR
(16) See Supporting Information Figures 1 and 2.
(17) Kuhn, R.; Rewicki, D. Liebigs Ann. Chem. 1967, 706, 250.
1
Org. Lett., Vol. 11, No. 9, 2009
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Figure 2. Liquid-state 9 GHz EPR of 9 (black) and 8 (red) in
toluene. The regions with enlarged vertical scales (gray) show the
forbidden singlet-triplet transitions. Experimental parameters: rt,
microwave power 2 mW, sweep width 20 mT, modulation
amplitude 10 mT.
Figure 3. Echo-detected 140 GHz solid-state EPR spectra of 9
(black), TEMPO (red), and BDPA (blue) in toluene. Experiments
are performed at 20 K, with tp(π/2) ) 44 ns, and typically 100
scans are averaged for each point.
well separated from the central region of the spectrum. These
features arise from forbidden singlet-triplet transitions
(S-T) and are common in spectra of biradicals that feature
an intermediate J-coupling. A J-coupling of 140 MHz was
measured based on the separation of the peaks from the
central region.18
Present in the spectrum is an impurity with an EPR signal
characteristic of a TEMPO radical attached to a diamagnetic
fragment (Figure 2, indicated by asterisk), which we attribute
to unreacted 8. We have evaluated the efficiency of the
conversion of 8 to 9 using several pieces of evidence. First,
we obtain nearly quantitative mass recovery (95%) and see
no indication of amide-bond cleavage under the reaction
conditions in TLC. Additionally, the TEMPO moiety is
unaffected by the reaction conditions. Integration of the EPR
signal indicates that on a per molecule basis there is less
than 10% of the monoradical impurity present (see Support-
ing Information). On the basis of this evidence, we estimate
the efficiency of the reaction to be 85%.
indicate the presence of an electron-electron interaction.
Most notable is the shift of the BDPA line to lower field.
In summary, we have developed a general method to
functionalize BDPA by synthesizing a BDPA precursor (7)
with a carboxylic acid functional group. We have used this
precursor to synthesize a BDPA-TEMPO biradical (9) in
good yield and studied the resulting EPR spectrum. The HF-
EPR spectrum, with two peaks, one narrow and one broad,
separated by a value close to the 1H Larmor frequency, has
the desired characteristics for DNP. Future work will focus
on adding water-solubilizing groups so that the biradical can
be tested for DNP-enhancements in aqueous solutions.
Acknowledgment. We thank Dr. Peter Mueller (MIT) for
providing X-ray crystal structures. This work was funded in
part by the National Science Foundation DMR-0706408.
R.G.G. acknowledges the support of the National Institute
of Biomedical Imaging and Bioengineering (EB-002804 and
EB-002026).
For high-field (HF) DNP experiments, the HF-EPR
spectrum in the solid state is of particular interest (Figure
3). The spectrum of the biradical shows features characteristic
of both BDPA and TEMPO, as well as new features that
Supporting Information Available: Experimental and
spectral data for all compounds. This material is available
free of charge via the Internet at http://pubs.acs.org.
(18) (a) Eaton, S., S.; DuBois, D. L.; Eaton, G. R. J. Magn. Reson.
1978, 32, 251. (b) Eaton, G. R.; Eaton, S. S. Spin Labelling, Theory and
Applications; Berliner, L. J.,; Reuben, J., Eds.; Plenum Press: New York,
NY, 1989; pp 339-397.
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