C-2 arylation of piperidines through directed transition-metal-catalyzed sp3 C-H activation

Article abstract of DOI:10.1002/chem.201001887

All you need is an open vial! The direct α arylation of cyclic alkylamines (see scheme) requires an open vial, as the hydrogen atom involved in the C(sp³)-H-activation process is ultimately released as hydrogen gas. Reports on the formation of hydrogen gas in direct transition-metal- catalyzed functionalizations are still rare. Open-vial reactions proved crucial to this direct arylation procedure as, upon sealing, catalyst deactivation occurs.

Full text of DOI:10.1002/chem.201001887

COMMUNICATION
DOI: 10.1002/chem.201001887
C-2 Arylation of Piperidines through Directed Transition-Metal-Catalyzed
3
À
sp C H Activation
Hana Prokopcovꢀ,[a] Sheba D. Bergman,[a] Karel Aelvoet,[a] Veerle Smout,[a]
Wouter Herrebout,[b] Benjamin Van der Veken,[b] Lieven Meerpoel,[c] and
Bert U. W. Maes*[a]
In memory of Keith Fagnou
The development of transition-metal-catalyzed methods
cessfully arylated adjacent to nitrogen by using a Ru-cata-
lyzed C-H activation process, directed by a pyrroline group
and mediated by ketone (used also as a solvent). The au-
thors suggested that the key role of the ketone (pinacolone)
is to allow a transmetalation by transforming the initially
formed metal hydride species into a metal alkoxide interme-
diate.[7b] The pyrroline directing group could be removed by
treatment of the a-arylated pyrrolidines with NH2NH2/
AcOH. One example of a piperidine substrate also ap-
peared in this work, which was C-2 p-methoxyphenylated in
moderate yield (38%).[7a] The six-membered piperidine ring
is inherently less reactive than its five-membered analogue
(pyrrolidine) due to its chair conformation.[8] Consequently,
there are very few examples in the literature of direct func-
tionalization of piperidines by means of a transition-metal-
À
for the direct functionalization of C H bonds has attracted
much attention during the past decade.[1] While the direct
2
À
functionalization of sp C H bonds is an active field of re-
search,[2] the corresponding knowledge on sp C H bonds is
3
À
still limited and remains one of the current challenges in or-
ganic chemistry.[3] Within the area of sp C H activation, the
3
À
À
transformation of a C H bond in the a-position to the nitro-
gen atom of saturated cyclic amines is of particular impor-
tance,[4] since such heterocyclic motifs can be found in an
impressive number of natural products and marketed
drugs.[5] The intermolecular, direct transition-metal-cata-
lyzed functionalization of saturated cyclic amines adjacent
to nitrogen offers a simple and efficient synthetic approach
towards valuable building blocks. To the best of our knowl-
edge, only six articles are hitherto published on this topic,
wherein the main focus is on five-membered cyclic sub-
3
[6c,d,7a]
À
catalyzed, sp C H activation process.
We report here
a novel method for the direct C-2 arylation of piperidines
with arylboronate esters, resulting from a mechanistic study
of the transmetalation step of the catalytic cycle.
ACHTUNGTRENNUNG
strates.[6,7a] In 2006, Sames and co-workers reported the first
direct arylation of saturated cyclic amines through transi-
3
À
tion-metal-catalyzed sp C H activation, involving arylboro-
As part of our interest in C-2 functionalized 4-substituted
piperidines, we first applied the conditions described by
Sames and co-workers[7a] to 1-(pyrrolin-2-yl)-4-piperidinone
ethylene ketal (Scheme 1, 1). Unfortunately, the reaction of
1 with phenylboronic acid pinacol ester resulted in a low
conversion to the arylated product, and directing group in-
nate esters as a coupling partner.[7a] Pyrrolidines were suc-
[a] Dr. H. Prokopcovꢀ, Dr. S. D. Bergman, Dr. K. Aelvoet, V. Smout,
Prof. Dr. B. U. W. Maes
Organic Synthesis, University of Antwerp
Groenenborgerlaan 171, 2020 Antwerp (Belgium)
Fax : (+32)32653233
E-mail: bert.maes@ua.ac.be
[b] Prof. Dr. W. Herrebout, Prof. Dr. B. Van der Veken
Cryospectroscopy, University of Antwerp
Groenenborgerlaan 171, 2020 Antwerp (Belgium)
[c] Dr. L. Meerpoel
Johnson & Johnson Pharmaceutical Research & Development
A Division of Janssen Pharmaceutica N.V.
Turnhoutseweg 30, 2340 Beerse (Belgium)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201001887.
Scheme 1. 4-Piperidinone ethylene ketal bearing a pyrroline or pyridine
directing group: substrates designed to explore the C-2 arylation process.
Chem. Eur. J. 2010, 16, 13063 – 13067
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stability was observed (see Supporting Information). We
therefore decided to use pyridine as a directing group
(Scheme 1, 2a). Pyridine is known as an effective and very
entry 1). Reaction in 1,2-dichloroethane (e=10.9), with simi-
lar dielectric constant as tBuOH (e=10.4), resulted in only
15% conversion (entry 2). In 3-ethyl-3-pentanol (5; entry 3),
the same reactivity as in tBuOH was observed, whereas in
tBuOMe, the conversion to the arylated product was very
low (entry 4). These results indicate that a free hydroxyl
group is beneficial for the direct arylation process. As a full
conversion of 2b was still not achieved, we assumed that
this might be caused by catalyst deactivation during the
course of the reaction. Indeed, when fresh catalyst was
added to the reaction mixture of 2b with phenylboronic acid
pinacol ester (6) after 24 h, the reaction conversion in-
[9]
À
stable directing group in C H activation processes, but has
always been considered a nonremovable group when at-
tached to nitrogen.[6b–d] However, our preliminary results
showed that subjecting 2a to Pd-catalyzed hydrogenation
followed by NH2NH2/AcOH treatment smoothly delivered
4-piperidinone ethylene ketal.
In a first attempt to obtain higher conversions, we exam-
ined a variety of ketones using 1-(pyridin-2-yl)-4-piperidi-
none ethylene ketal (2a) as substrate. With pinacolone, only
15% conversion of 2a was obtained. Interestingly, when
acetophenones were applied as solvent, significantly better
conversions were reached (ꢀ49%, see Supporting Informa-
tion). A similar reactivity pattern was observed when C-4
unsubstituted 1-(pyridin-2-yl)pi-
creased further to 83%. We hypothesized that, if the reac-
II
À
tion proceeds through a direct transmetalation of the Ru
H intermediate 7 (Scheme 2) with boronate ester 6, the re-
sulting pinacolborane species has to be scavenged in order
peridine (2b) was used as sub-
strate. These results were not in
accordance with the previously
reported mechanism,[7a] since
we observed that ketones with
nearly similar pKEꢂs (pinaco-
lone, pKE =8.76 and acetophe-
nones, pKE ꢁ8; pKE: keto–enol
equilibrium constant)[10,11] gave
rise to very different conver-
sions. The question arose
whether applying the ketone as
solvent was essential for the re-
action to proceed.[7b]
We selected unsubstituted 2b
as a substrate to test a variety
of solvents and, surprisingly,
when the reaction was per-
formed in tBuOH, 61% conver-
Scheme 2. Proposed mechanism for the direct C-2 arylation of saturated cyclic amines.
sion was achieved (Table 1,
to avoid catalyst poisoning, for example, by oxidative addi-
tion.[12] Then, the role of tBuOH is to scavenge the pinacol-
borane, with formation of the corresponding tert-butylborate
and H2. Up to this point, all reactions were carried out in
closed vials at 1408C, so the in situ created H2 could be re-
sponsible for the loss of catalytic activity (oxidative addition
of H2)[13] (Table 1, entry 1). In order to release the presumed
in situ formed H2-gas, we decided to perform the reaction of
1-(pyridin-2-yl)piperidine (2b) with phenylboronic acid pi-
nacol ester (6) in an open vial under reflux conditions. We
chose the higher boiling 3-ethyl-3-pentanol (5) (b.p.=140–
1438C) as solvent to maintain the temperature around
1408C. In accordance with our hypothesis, we found that the
conversion could be further increased. Moreover, a Raman
spectroscopy measurement confirmed that H2 is formed
during the reaction (see Supporting Information).[14] Addi-
tionally, an independent model experiment showed that
upon heating of pinacolborane with alcohol 5, 1,1-diethyl-
propylborate (8) and H2 were formed (see Supporting Infor-
Table 1. The effect of different solvents on the direct C-2 arylation of 1-
(pyridin-2-yl)piperidine (2b).[a]
Solvent
GC-FID[b] 2b/3b/4b [%]
1
2
3
4
tBuOH
39/47/14
85/15/0
41/45/14
81/19/0
1,2-dichloroethane
3-ethyl-3-pentanol (5)
tBuOMe
[a] The reactions were performed on a 0.5 mmol scale, using [Ru3(CO)12]
(4 mol%), phenylboronic acid pinacol ester (3 equiv), and solvent
(6 equiv) at 1408C for 24 h. [b] Uncorrected GC-FID conversions; the bis
product 4b is a mixture of diastereoisomers.
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Chem. Eur. J. 2010, 16, 13063 – 13067

Homogeneous Catalysis
COMMUNICATION
mation). Compound 8 was also detected in the reaction mix-
further increase of the amount of boronate ester and cata-
lyst favors the formation of bis 4b which is then obtained as
the major product (Table 2, entry 6).
With the optimized conditions in hand (method A,
Scheme 3), we explored the scope of our C-2 arylation
method. Some of the substrates needed four equivalents of
arylboronate ester 10 and 8 mol% of [Ru3(CO)12] in order
to reach full conversion of starting material (method B,
ture of substrate 2b with boronate ester 6. The occurrence
II
À
of a mechanism involving a reaction of Ru H intermediate
II
À
7 with alcohol 5, yielding Ru OR and H2, could be exclud-
ed based on an experiment of 2b with 6 in the absence of al-
cohol,[15] as 3b and 4b were still formed, supporting our
direct transmetalation hypothesis. Raman spectroscopy con-
firmed that H2 is also formed in the absence of alcohol. A
lower conversion of 2b was obtained in comparison with the
experiment in the presence of 5.
The proposed catalytic cycle of our new direct arylation
process is presented in Scheme 2. The mechanism involves
the initial complexation of Ru0 to pyridine, promoting the
3
À
oxidative addition of the sp C H bond adjacent to the pi-
0
II
À
peridine nitrogen to Ru , with formation of a Ru H species
7 (Scheme 2). Subsequently, transmetalation of 7 with the
arylboronate ester yields dialkoxyborane and Ru Ar spe-
cies 9. Reductive elimination of 9 finally gives 3, and the
Ru0 catalyst is regenerated. The dialkoxyborane is destroyed
by reaction with alcohol 5 delivering H2.
II
À
Further optimization showed that one equivalent of 3-
ethyl-3-pentanol (5) could be taken as a standard (Table 2,
entries 1–3). During our research, we experienced some pu-
rification problems caused by remaining phenylboronic acid
pinacol ester (6). This could easily be solved by exchange of
pinacol ester 6 for its neopentylglycol equivalent 10a
(Table 2, entry 4). Ultimately, we found that the reaction
proceeded most effectively by heating 2b with three equiva-
lents of phenylboronic acid neopentylglycol ester (10a) and
6 mol% of [Ru3(CO)12] in one equivalent of 3-ethyl-3-penta-
nol (5) at reflux for 24 h (Table 2, entry 5). In this way, we
obtained 97% total conversion (46% mono-3b, 51% bis-
4b) and 76% isolated yield (38% mono-3b, 38% bis-4b). A
Table 2. Model reaction used to identify the optimal reaction condi-
ACHTUNGTRENNUNG
tions.[a]
6 or 6 or 10a
U
5
GC-FID[b]
Total yield[c]
ACHTUNGTRENUN(NG 3b/4b) [%]
Scheme 3. C-2 arylation reaction of piperidine, pyrrolidine and tetrahy-
droquinoline substrates with a variety of arylboronate esters under opti-
mized conditions. For simplicity, only the structures of mono-arylated
products are represented. [a] Reaction conditions, method A: the reac-
tions were performed in duplicate on a 0.5 mmol scale, using [Ru3(CO)12]
(6 mol%), arylboronic acid neopentylglycol ester (10; 3 equiv), and 3-
ethyl-3-pentanol (5; 1 equiv) at reflux for 24 h. [b] Reaction conditions,
method B: the reactions were performed in duplicate on a 0.5 mmol
scale, using [Ru3(CO)12] (8 mol%), arylboronic acid neopentylglycol
ester (10; 4 equiv), and 3-ethyl-3-pentanol (5; 1 equiv) at reflux for 24 h.
[c] Total isolated yield; isolation performed on a 1.0 mmol scale. [d] 3a–l
represent mono isolated yields. [e] 4a–l represent bis isolated yields.
[f] See the Supporting Information.
10a
N
A
1
2
3
4
5
6
6
6
6
10a
10a
10a
3
3
3
3
3
4
3
2
1
1
1
1
17/51/32
19/52/29
13/51/36
10/49/41
3/46/51
–
–
–
63 (39/24)
76 (38/38)
71 (11/60)
0/14/86
[a] The reactions were performed on a 0.5 mmol scale, using [Ru3(CO)12]
as a catalyst, phenylboronate ester 6 or 10a, and 3-ethyl-3-pentanol at
1408C for 24 h. [b] Uncorrected GC-FID conversions; the bis product 4b
is a mixture of diastereoisomers. [c] Total isolated yield; isolation per-
formed on a 1.0 mmol scale.
Chem. Eur. J. 2010, 16, 13063 – 13067
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
13065

B. U. W. Maes et al.
Scheme 3). 1-(Pyridin-2-yl)piperidine (2b) was successfully
functionalized at the C-2-position with phenylboronic acid
neopentylglycol ester 10a (Scheme 3, 3b/4b), as well as with
arylboronate esters 10 bearing electron-withdrawing sub-
stituents in the meta- or para-position (3e–g/4e–g). The cou-
pling of a-methyl-1-(pyridin-2-yl)piperidine with different
arylboronate esters smoothly yielded the C-2 arylated prod-
ucts (3c, 3k, and 3l), indicating that also substituted piperi-
dines can be applied as substrates in this direct transition-
metal-catalyzed arylation. To our delight, not only were
alkyl substituents well tolerated, as good isolated yields
were also obtained for the C-2 phenylation of 1-(pyridin-2-
yl)piperidine containing a ketal or ester group in the 4-posi-
tion (3a/4a, 3h/4h). To the best of our knowledge, these are
Experimental Section
Two 10 mL vials were each charged with saturated cyclic amine
2
(0.5 mmol), [Ru3(CO)12] (6–8 mol%), arylboronic acid neopentylglycol
ester 10 (3–4 equiv), and 3-ethyl-3-pentanol (5) (58 mg, 0.5 mmol,
1 equiv). The vials were flushed with argon and fitted with a condenser.
The reaction mixtures were then heated to reflux—the temperature of
the oil bath being set at 1538C—under magnetic stirring for 24 h (Ar at-
mosphere). After this time, the reaction mixtures were cooled down and
both combined into a flask using CH2Cl2, and the volatiles were removed
under reduced pressure. A commercially available ruthenium scavenger
(Siliabondꢃ DMT, Silicycle, 1.5–2 g) was added to the residue, along with
CH2Cl2 (100 mL). The resulting suspension was stirred at RT for 16 h.
Subsequently, the solids were removed by filtration through Celite, the
pad was washed with distilled CH2Cl2 (5ꢄ20 mL), and the combined fil-
trate was evaporated to dryness. The residue was purified by reversed-
phase flash chromatography. The desired fractions were combined and
evaporated to dryness to deliver the reaction products 3 and 4.
the first examples of direct functionalization by means of
3
À
transition-metal-catalyzed sp C H activation performed on
piperidines bearing substituents other than simple alkyl
groups. Interestingly, moderate to high yields were also ob-
tained when pyrrolidine or tetrahydroquinoline were used
as substrate (3d/4d, 3i, 3j).
Acknowledgements
As mentioned above, our preliminary results indicated
that the pyridine directing group can be efficiently removed.
Indeed, when the C-2-phenylated product 3b was subjected
to a Pd-catalyzed hydrogenation and subsequent NH2NH2/
AcOH treatment, this delivered the corresponding depro-
tected product 12 in 47% yield (Scheme 4). The crude 1-
This work was financially supported by the University of Antwerp (IOF
and BOF), the Fund for Scientific Research-Flanders (FWO-Flanders),
the Hercules foundation, and J&J PRD. The authors would like to thank
Gaston Diels for valuable remarks.
À
Keywords: C H activation
·
hydrogen
·
homogeneous
catalysis · piperidines · ruthenium
À
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ACHTUNGERTNyNUNG l-
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À
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3
À
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Homogeneous Catalysis
COMMUNICATION
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À
À
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À
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À
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Received: July 5, 2010
Published online: October 27, 2010
Chem. Eur. J. 2010, 16, 13063 – 13067
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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