9-Phenylfluorene: A powerful labeling agent

Article abstract of DOI:10.1016/S0040-4039(01)00903-0

A highly efficient method for labeling with deuterium is described using an acidic hydrocarbon as transfer agent. Using 9-[²H]-9-phenylfluorene as the deuterium donor, numerous organic compounds have been labeled in fair yields and good isotopic enrichment.

Full text of DOI:10.1016/S0040-4039(01)00903-0

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 5001–5003
9-Phenylfluorene: a powerful labeling agent
Jean-Christophe Cintrat,* Florence Pillon and Bernard Rousseau
CEA/Saclay, Service des Mole´cules Marque´es, baˆt. 547, De´partement de Biologie Cellulaire et Mole´culaire,
91191 Gif sur Yvette, cedex, France
Received 2 May 2001; accepted 26 May 2001
Abstract—A highly efficient method for labeling with deuterium is described using an acidic hydrocarbon as transfer agent. Using
9-[2H]-9-phenylfluorene as the deuterium donor, numerous organic compounds have been labeled in fair yields and good isotopic
enrichment. © 2001 Elsevier Science Ltd. All rights reserved.
In recent decades, increasing use has been made of
tritiated compounds in biological evaluations, mainly of
metabolism. This use is limited by the difficulty of
obtaining tritiated molecules of high specific activity,
mainly because of the hazards of manipulating radioac-
tive materials. Three standard methods are used to
prepare tritiated compounds of high specific activity.
The first method consists of catalytic reductions, halo-
gen/tritium exchange or hydride reductions1 of a suit-
able precursor. This approach requires prior synthesis
of the derivative, and is labor-intensive. Furthermore,
tritium gas is involved and all reactions must carried
out in a specialized laboratory equipped with glove
boxes and radioactivity hazard evaluation. All these
drawbacks seriously limit the scope of this methodol-
ogy. The second method takes advantage of isotopic
Our aim has been to design a general, environmentally
friendly and safe method, based on the same acid–base
concept (Scheme 1). We needed to find a labeled ‘water
substitute’ that was easy to prepare at high isotopic
enrichment, easy to handle (at least non-volatile),
efficiently transferable to the isotope, easy to remove
from a reaction mixture, and with a pKa far removed
from 14 to avoid rapid exchange with water and result-
ing isotopic dilution. Surprisingly, few research groups
have explored the usefulness of labeled acidic hydrocar-
bons for such purposes.8 Extensive literature searches
revealed that 9-phenylfluorene (9-PhFl) best fitted the
above-mentioned requirements, with a pKa value of
17.9 in DMSO.9
3
The first step was to rationalize an efficient synthesis of
9-[2H]-9-phenylfluorene keeping in mind that this route
has to be transposable to the tritium analogue. Starting
from 9-bromo-9-phenylfluorene, classical hydrogenoly-
exchange using either H2 gas and a transition metal
derivative2 or catalyzed by acids3 or bases.4 In the third
method, an anion is generated by metallation of the
target substrate, which is then quenched with tritiated
water,5 acids6 or even tritium gas7 (Scheme 1).
In the second and third methods, no synthesis of
derivatives is required, but only authorized laboratories
are able to run these experiments since H2 gas is used
3H
A3H = RO3H, RCO23H, 3H2
B M
A
3H
R
H
R
R M
3
to obtain quenching agents of high specific activity.
Scheme 1.
In this paper, we introduce a new approach that has the
advantage of the third method (no need for prior
derivatization of the substrate to be labeled) with none
of the previously mentioned drawbacks (no use of an
2H2
3
3
expensive catalyst, no handling of H2 gas or H2O).
10% Pd/C
Et3N (10 eq.)
EtOAc, RT
2H
Br
Keywords: acidic hydrocarbons; anions; deuterium; labeling.
* Corresponding author. Tel. +33.(0)1.69.08.25.70; fax: +33.(0)1.
69.08.79.91; e-mail: jean-christophe.cintrat@cea.fr
Scheme 2.
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)00903-0

5002
J.-C. Cintrat et al. / Tetrahedron Letters 42 (2001) 5001–5003
9-[2H]-9-PhFl
BuLi (1.1 eq.)
2H
R X
R Li
-
R
-78˚C to RT
THF, -78˚C
X = Br, SnBu3, H
Scheme 3.
Table 1. Labeling experiments using 9-[2H]-9-PhFl
Entry Substrate
Product
Yield (%)
i.e. (%)11
sis of the carbonꢀbromide bond under 2H2 in the presence
of a catalytic amount of Pd0 gave the expected 9-[2H]-9-
PhFl in yields ranging from 82 to 95%, and isotopic
considerably (from 91 to 98.5%) by washing the catalyst
2
with H2 prior to the reduction step.10
enrichment up to 98.5% on a 0.3–5 g scale (Scheme 2).
We then investigated the label-transfer potential of this
compound, starting first from halogeno or stannyl
2
.
The isotopic enrichment of 9-[ H]-9-PhFl was improved

J.-C. Cintrat et al. / Tetrahedron Letters 42 (2001) 5001–5003
5003
derivatives to eliminate the risk of a partial metallation
step (Scheme 3).
G. J. Org. Chem. 1996, 61, 8771–8774; (b) Rasset, C.;
Rousseau, B. J. Label. Compounds Radiophar. 1994, 6,
523–528; (c) Zippi, E. M.; Andres, H.; Morimoto, H.;
Williams, P. G. Synth. Commun. 1994, 24, 1037–1044;
(d) Jaiswal, D. K.; Andres, H.; Morimoto, H.;
Williams, P. G. J. Chem. Soc., Chem. Commun. 1993,
907–909.
As shown in Table 1 (entries 1–3), the method is highly
efficient (isotopic enrichment up to 100%). In terms of
our initial aim (no synthetic derivative needed) it is
simple to start from the target compound, abstract a
proton regioselectively and replace it by its isotope
(entries 5–12). The methodology is quite powerful and
general since aromatic (entries 4–7), benzylic (entry 8),
acetylenic (entry 9) and aliphatic positions (entries 10–
12) have been labeled.
2. Shu, A. Y. L.; Heys, J. R. Tetrahedron Lett. 2000, 41,
9015–9019.
3. Brewer, J. R.; Jones, J. R.; Lawrie, K. W. M.; Saun-
ders, D.; Simmonds, A. J. Label. Compounds Radio-
phar. 1994, 4, 391–400.
4. Brewer, J. R.; Garnes, K. T.; Levinson, S. H.; Saun-
ders, D.; Simmonds, A. J. Label. Compounds Radio-
phar. 1994, 8, 787–794.
In conclusion, this methodology seems very promising
because of its general applicability (access to a wide
range of labeled compounds in a regiospecific fashion).
The only limitation is the metallation process which
requires acidic protons with a pKa above 18. Due to the
easy deuterodehalogenation of 9-bromo-9-phenylfluor-
ene, this approach should be easily transposable to
tritium. This should enable most standard chemistry
laboratories to synthesize tritiated molecules simply and
safely.
5. Kolbe, A.; Schneider, B.; Voigt, B.; Adam, G. J. Label.
Compounds Radiophar. 1998, 2, 131–137.
6. Ciszewska, G.; Pfefferkorn, H.; Tang, Y. S.; Jones, L.;
Tarapata, R.; Sunay, U. B. J. Label. Compounds Radio-
phar. 1997, 8, 651–668.
7. Seltzman, H. H.; Odear, D. F.; Carroll, F. I; Wyrick,
C. D. J. Chem. Soc., Chem. Commun. 1992, 1757–1758.
8. Schlosser, M.; Limat, D. J. Am. Chem. Soc. 1995, 117,
12342–12343.
9. Bordwell, F. G.; Drucker, G. E.; Andersen, N. H.;
Denniston, A. D. J. Am. Chem. Soc. 1986, 108, 7310–
7313.
Acknowledgements
10. Azran, J.; Shimoni, M.; Buchman, O. J. Catal. 1994,
148, 648–653.
The authors are grateful to Dr. Eric Doris for stimulat-
ing discussions.
11. Deuterium incorporation was measured by integration
of the respective 1H NMR signals and by referring to
the signal of another proton or group of protons
within the molecule as internal standard. 1H Spectra of
labeled compounds were in full agreement with those of
the starting materials.
References
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