Regioselective and stereospecific deuteration of bioactive aza compounds by the use of ruthenium nanoparticles

Article abstract of DOI:10.1002/anie.201307930

An efficient H/D exchange method allowing the deuteration of pyridines, quinolines, indoles, and alkyl amines with D₂ in the presence of Ru@PVP nanoparticles is described. By a general and simple procedure involving mild reaction conditions and simple filtration to recover the labeled product, the isotopic labeling of 22 compounds proceeded in good yield with high chemo- and regioselectivity. The viability of this procedure was demonstrated by the labeling of eight biologically active compounds. Remarkably, enantiomeric purity was conserved in the labeled compounds, even though labeling took place in the vicinity of the stereogenic center. The level of isotopic enrichment observed is suitable for metabolomic studies in most cases. This approach is also perfectly adapted to tritium labeling because it uses a gas as an isotopic source. Besides these applications to molecules of biological interest, this study reveals a rich and underestimated chemistry on the surface of ruthenium nanoparticles. Don't just slap a label on it! A regioselective and stereospecific method for the deuteration of nitrogen-containing compounds has been developed on the basis of a C-H activation process triggered by Ru nanoparticles (RuNps). This general and efficient approach to deuterium labeling was applied to 22 compounds, including 8 biologically active substances (see scheme; PVP=polyvinylpyrrolidone). Copyright

Full text of DOI:10.1002/anie.201307930

.
Angewandte
Communications
DOI: 10.1002/anie.201307930
Isotopic Exchange
Regioselective and Stereospecific Deuteration of Bioactive Aza
Compounds by the Use of Ruthenium Nanoparticles**
Grꢀgory Pieters, Cꢀline Taglang, Eric Bonnefille, Torsten Gutmann, Cꢀline Puente, Jean-
Claude Berthet, Christophe Dugave, Bruno Chaudret,* and Bernard Rousseau*
Abstract: An efficient H/D exchange method allowing the
deuteration of pyridines, quinolines, indoles, and alkyl amines
chemistry. Moreover, they are of particular interest for the
development of efficient optical devices, such as optical
polymer fibers and organic material for organic light-emitting
with D in the presence of Ru@PVP nanoparticles is described.
2
[
3]
By a general and simple procedure involving mild reaction
conditions and simple filtration to recover the labeled product,
the isotopic labeling of 22 compounds proceeded in good yield
with high chemo- and regioselectivity. The viability of this
procedure was demonstrated by the labeling of eight biolog-
ically active compounds. Remarkably, enantiomeric purity was
conserved in the labeled compounds, even though labeling
took place in the vicinity of the stereogenic center. The level of
isotopic enrichment observed is suitable for metabolomic
studies in most cases. This approach is also perfectly adapted to
tritium labeling because it uses a gas as an isotopic source.
Besides these applications to molecules of biological interest,
this study reveals a rich and underestimated chemistry on the
surface of ruthenium nanoparticles.
diodes. Recently, the emergence of metabolomics has
increased demand for isotopically labeled compounds. In
particular, their use as internal standards is essential for the
quantitative LC–MS/MS analysis of new drug candidates and
[
4]
of metabolites in biological fluids. For the synthesis of such
molecules, isotopic exchange is clearly preferred over a syn-
thetic pathway from labeled building blocks or involving
a synthetic precursor. As a consequence, the development of
efficient and selective labeling methodologies through cata-
lytic H/D exchange reactions at carbon centers is of great
[
2a]
interest.
Numerous methods based on homogeneous or heteroge-
neous catalysis for H/D exchange have already been de-
scribed. Among these methods, the selective deuteration of
nitrogen-containing molecules has been investigated with
particular intensity because of the presence of this hetero-
atom in many biologically active compounds. For example,
several catalytic systems, such as Rh black, Ru complexes, and
PdNp@PVP colloids (Np: nanoparticle, PVP: polyvinylpyr-
rolidone) have been used to label pyridine-containing com-
pounds with various degrees of deuterium incorporation and
C
ꢀH activation has become a powerful tool for the synthesis
or functionalization of complex organic compounds through
the formation of a large variety of CꢀC, CꢀN, CꢀO, CꢀB, and
[1]
CꢀD bonds. This straightforward method can therefore
afford rapid access to deuterated labeled compounds, which
are vital tools in many areas of academic and industrial
[2]
[5]
science. Deuterated compounds are essential for mechanis-
tic studies in biology and in organic and organometallic
substrate tolerance. For indole derivatives, Schnꢀrch and co-
workers recently described a ruthenium-catalyzed H/D
exchange with a good selectivity and a fair degree of
[
5b]
deuterium incorporation. Concerning alkyl amines, a few
methodologies have been devised, but they have essentially
[
+]
[
*] Dr. G. Pieters, C. Taglang, Dr. T. Gutmann, C. Puente,
Dr. C. Dugave, Dr. B. Rousseau
CEA Saclay, SCBM, iBiTec-S, Building 547, PC # 108
[6]
been applied to nonfunctionalized substrates. Until recently,
labeling procedures for such compounds suffered from major
drawbacks, such as harsh reaction conditions, poor selectivity,
and low deuterium incorporation. A breakthrough was
recently reported by Beller and co-workers, who developed
an a,b deuteration of bioactive secondary and tertiary
91191 Gif sur Yvette (France)
E-mail: bernard.rousseau@cea.fr
Dr. B. Chaudret
LPCNO (Laboratoire de Physique et Chimie de Nano-Objets)
UMR 5215 INSA-CNRS-UPS
[
7]
Institut National des Sciences Appliquꢀes
amines. This remarkable method based on the use of a Ru
complex as a catalyst enables a high degree of deuterium
incorporation. However, it still requires high temperatures
135, Avenue de Rangueil, 31077 Toulouse (France)
E-mail: chaudret@insa-toulouse.fr
Dr. E. Bonnefille
CNRS; LCC (Laboratoire de Chimie de Coordination)
(1508C), which can lead to side reactions and the degradation
of fragile substrates. Moreover, it has never been applied to
chiral compounds. As a consequence, a regioselective, chemo-
selective, and stereospecific labeling method that might be
applicable to a wide diversity of substrates is urgently
required.
Herein, we demonstrate that the use of Ru nanoparticles
allows mild (1–2 bar of deuterium gas at room temperature or
558C), effective, and selective deuteration of a large diversity
of nitrogen-containing compounds, such as pyridines, quino-
lines, indoles, and primary, secondary, and tertiary alkyl
205, Route de Narbonne, 31077 Toulouse (France)
Dr. J.-C. Berthet
CEA Saclay, IRAMIS, SIS2M, CNRS UMR 3299
91191 Gif-sur-Yvette (France)
+
[
] Current address: Eduard Zintl Institute for Inorganic and Physical
Chemistry, Technical University Darmstadt
Petersenstrasse 20, 64287 Darmstadt (Germany)
[**] Support from ERC Ternanomed is acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201307930.
2
30
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Angew. Chem. Int. Ed. 2014, 53, 230 –234

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[
a]
Table 1: Regioselective catalytic deuteration.
[
b]
Compound
Structure
Method
Total D
1.7
1
A
2
B
2.0
3
4
B
A
2.7
2.0
Figure 1. Scope of the deuteration reaction and labeling positions.
amines (Figure 1). The usefulness of this novel methodology
was demonstrated by the labeling of eight bioactive com-
pounds, several of which contain stereogenic centers.
In the course of previous studies, some of us observed that
5
6
A
B
0.7
1.4
the treatment of Ru nanoparticles with D led to deuterium
2
incorporation into the primary-amine ligand (hexadecyl-
[
8]
amine) bound to the Ru surface. This incorporation
occurred under mild reaction conditions (0.8 bar of deuterium
gas, room temperature). This result prompted us to evaluate
the catalytic activity of such Ru nanoparticles towards H/D
exchange processes on nitrogen-containing substrates
through CꢀH activation. These small nanoparticles (1.1 nm)
7
B
C
1.1
1.9
8
9
are readily obtained by decomposition of the organometallic
precursor [Ru(cod)(cot)] (cod = 1,5-cyclooctadiene, cot =
B
B
0.8
0.8
1
,3,5-cyclooctatriene) under an H atmosphere in the pres-
2
[9a]
ence of a stabilizer, such as a polymer or a ligand. These
nanoparticles are particularly attractive because their proper-
ties can be fine-tuned by surface-ligand modification. We
started our experiments with RuNp@PVP, which can be
considered as “naked particles”, to facilitate the interaction
1
1
0
1
[9b]
C
C
C
3.9
4.3
2.8
[
9c]
between the substrate and the reactive Ru surface.
Deuteration experiments were carried out with 3%
RuNp@PVP in THF under a deuterium atmosphere (1 or
12
2
bar). Pyridine derivatives 1, 2, and sterically hindered 2-
1
1
3
4
phenylpyridine (3) were regioselectively deuterated with high
isotopic enrichment (Table 1). For quinolines 4 and 5, the Cꢀ
H activation took place regioselectively with high deuterium
incorporation. However, the concomitant formation of la-
beled tetrahydroquinoline (around 50%) was also observed.
In the indole series (compounds 6–8), RuNp@PVP also
catalyzed H/D exchange with high selectivity and no side-
product formation. With compounds 6 and 7, CꢀH activation
C
5.6
[a] Reaction conditions (all experiments were performed for 36 h in THF
in the presence of 3% RuNp@PVP): method A: D (1 bar), RT;
method B: D (1 bar), 558C; method C: D (2 bar), 558C. Gray disks
show the labeling positions (the number in brackets indicates the
isotopic enrichment). [b] Average number of D atoms per molecule.
2
2
2
took place at the 2- and 3-positions; however, we observed
regioselective labeling at three positions of skatol (8). The
difference in terms of selectivity between indole and skatol
might be explained by distinct modes of coordination on the
Ru surface and is discussed in more detail below. In the case
of hexylamine (9), only 20% H/D exchange at the a-position
was observed at room temperature, whereas the isotopic
enrichment was increased to 40% at 558C. Similar results
were obtained with 3-phenyl-1-propanamine (10). Unidenti-
fied side products were formed with primary amines. Never-
theless, the deuterated amino compounds could be isolated
readily as hydrochloride salts. Highly efficient labeling of
secondary and tertiary amines was observed. Piperidine (11)
was labeled at both the 2- and the 6-position with almost total
replacement of four hydrogen atoms with four deuterium
atoms. 2-Methylpiperidine (12) was also deuterated at the 2-
and 6-positions, along with exchange into the methyl group,
thus leading to the incorporation of up to six deuterium
atoms. Morpholine (13) was deuterated regioselectively with
up to four deuterium atoms. Tributylamine (14) was deuter-
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231

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Communications
a
ated at all C positions to give a highly deuterated compound
(
up to six deuterium atoms).
These results prompted us to apply this new method to the
labeling of biologically active compounds that contain at least
one nitrogen atom. We believed that the mild experimental
conditions might be compatible with the labeling of complex
chiral compounds. Nicotine (15) and anabasine (16), two
nicotinic acetylcholine receptor agonists that both contain
two azacycles (Figure 2), were deuterated regioselectively
with a high level of deuterium incorporation. The exclusive
exchange with the pyridine ring could be explained by
a higher affinity of the pyridine nitrogen atom for the Ru
surface than that of the piperidine or pyrrolidine group. In the
1
Figure 3. H NMR spectra of labeled (top) and unlabeled (bottom)
anabasine (16) in CDCl3.
2
5
deuteration process (the measured ½aꢁ value was + 65.1
D
before and + 65.4 after the labeling experiment (c = 1.0,
CHCl )). Imipramine (20), a tricyclic antidepressant of the
3
dibenzazepine group, incorporated up to eight deuterium
atoms into the tertiary alkyl amine. In these six cases, owing to
the mild conditions used, no side products were detected;
thus, almost quantitative recovery of the product was possible.
Up to five deuterium atoms were regioselectively incor-
porated in protriptyline (21), another tricyclic antidepressant,
by our standard exchange procedure. Interestingly, although
this compound contains a readily reducible disubstituted
double bond, only a limited amount of the reduced compound
(
around 20%) was observed. The probable anchoring of
Figure 2. Structure of labeled biologically active compounds (gray
disks show the labeling positions; the number in brackets indicates
the isotopic enrichment).
protriptyline at the surface of RuNps through an amine–
ruthenium interaction prevents the reduction of the remote
double bond. To the best of our knowledge, no double-bond-
containing compound has previously been labeled with up to
five deuterium atoms by isotopic exchange with deuterium
gas. This result is particularly encouraging because it paves
the way to the labeling of complex and fragile biologically
active molecules. The power of our CꢀH activation process
case of nicotine (15), the enantiomeric purity of the deuter-
2
D
5
ated compound remained unchanged (the measured ½aꢁ
value was ꢀ180.1 before and ꢀ176.5 after the labeling
experiment (c = 0.5, CHCl )). Furthermore, the mild exper-
3
imental conditions led to a pure deuterated sample after
simple filtration through a pad of alumina, as shown in the
case of anabasine (16; Figure 3).
for deuterium labeling is highlighted by the case of paroxetine
(22), an antidepressant drug which belongs to the selective
serotonin reuptake inhibitor series. Deuterated 22 was
synthesized previously by a pharmaceutical company in
a seven-step sequence, whereas our process only required
one labeling step. Once again, this complex compound was
Papaverine (17), an opium-alkaloid antispasmodic drug,
was deuterated at the 2-position in a 79% H/D exchange.
Deuteration of melatonin (18), a hormone that regulates the
circadian rhythms of several biological functions, took place
at the equivalent three positions to those at which deuteration
occurred in the model compound skatol (8). Dextromethor-
phan (19) was selectively deuterated at the N-methyl position,
with the incorporation of three deuterium atoms. This high
regioselectivity could be explained by the low accessibility of
other positions a to the nitrogen atom embedded in the six-
membered ring. Interestingly, this selectivity suggests that the
H/D exchange reaction occurs at the RuNp surface and may
not involve the leaching of Ru into solution. Once again, no
loss of enantiomeric purity was detected after the selective
[
10]
2
5
labeled without significant alteration of chirality (½aꢁ ¼ꢀ78.5
D
(c = 1.0, MeOH)).
The use of deuterated molecules as internal standards in
metabolomic studies requires very high isotopic enrichment.
Such compounds must contain a minimum of two deuterium
atoms and usually less than 0.5% nondeuterated material. In
most cases, the standard conditions described herein yielded
labeled compounds that fulfilled these criteria. In other cases,
we investigated the possibility of increasing deuterium
incorporation in two different ways. In the case of melatonin
(18), doubling of the catalytic charge led to higher isotopic
2
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enrichment without a change in selectivity (at the 2-position
from 70 to 93%, at the 7-position from 40 to 80%, and at the
methylene position from 15 to 25%). Alternatively, in the
case of imipramine (20), repeating the deuteration process led
to a negligible proportion of non- and monodeuterated
molecules (see Table S2 in the Supporting Information).
From a mechanistic point of view, the first interesting
observation is that under the mild conditions of this system,
product, 22 compounds were isotopically labeled in good
yields with high chemo- and regioselectivities. The viability of
this procedure was demonstrated by the labeling of eight
biologically active compounds. Remarkably, conservation of
the enantiomeric purity of the labeled compounds was
observed, even though labeling took place in the vicinity of
the asymmetric center. The level of isotopic enrichment that
can be reached is suitable for metabolomic studies in most
cases. Furthermore, this approach is perfectly adapted to
tritium labeling because it uses gas as an isotopic source.
Extension of the method to other natural compounds, in
particular amino acids and peptides, is currently under
investigation in our laboratory. Besides these applications to
molecules of biological interest, this study reveals a rich and
underestimated chemistry on the surface of Ru nanoparticles,
which can be further exploited as novel reagents in organic
chemistry.
2
3
both C(sp )ꢀH and C(sp )ꢀH bonds can be deuterated at
comparable rates. The second observation concerns the high
regioselectivity of these reactions. No deuteration next to an
oxygen donor group was observed, whereas a strong exchange
occurred at CꢀH bonds next to the nitrogen atom. These
results clearly show that this H/D exchange process arises
from the direct coordination of nitrogen to ruthenium. The
affinity of ethers for the Ru surface is presumably too low to
initiate the deuteration reaction. These results are similar to
those of studies carried out on molecular organometallic
catalysts, in which case only the hydrogen atoms a to
a nitrogen atom could be activated. They are also comparable
to those of older studies on metal complexes, including Ru
complexes, in which case only the accessible ortho hydrogen
atoms of triphenylphosphine or triphenylphosphite could be
Received: September 9, 2013
Published online: November 19, 2013
Keywords: CꢀH activation · deuterium · isotopic labeling ·
.
nanoparticles · ruthenium
[
11]
exchanged as a result of ortho-metalation. However, the
accessibility of the Ru sites on nanoparticles enables much
greater efficiency of these CꢀH activation processes, which
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