Iridium-catalyzed oxidant-free dehydrogenative C-H bond functionalization: Selective preparation of N-arylpiperidines through tandem hydrogen transfers

Article abstract of DOI:10.1002/anie.201204582

Relay to the finish: The atom-economical tandem hydrogen autotransfer catalyzed by iridium(III) has been efficiently applied for the preparation of N-arylpiperidines starting from easily accessible anilines, diols, and aldehydes (see scheme). This protocol is also compatible with the use of diethyl carbonate as an ecofriendly solvent. Copyright

Full text of DOI:10.1002/anie.201204582

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Angewandte
Communications
DOI: 10.1002/anie.201204582
Synthetic Methods
À
Iridium-Catalyzed Oxidant-Free Dehydrogenative C H Bond
Functionalization: Selective Preparation of N-Arylpiperidines through
Tandem Hydrogen Transfers**
Kedong Yuan, Fan Jiang, Zeyneb Sahli, Mathieu Achard, Thierry Roisnel, and
Christian Bruneau*
In addition to creating efficient and selective methodologies,
the rise of green chemistry has led organic chemists and
chemical industry to take into account the impact of these
processes on the environment. Among the different eco-
friendly approaches, homogeneous metal-catalyzed hydrogen
autotransfers aimed at developing benign and atom-efficient
protocols for the construction of carbon–heteroatom or
carbon–carbon bonds have attracted considerable interest.[1–3]
In these reactions, the transient formation of unsaturated
carbonyl or imine intermediates, arising from the dehydro-
genation of alcohols or amines acting as alkylating reagents,
has been successfully used for the preparation of a-alkylated
carbonyl derivatives[4] and amines[5,6] accompanied by the
formation of either water or valuable ammonia[7] as the only
side product. Owing to the potential of these transformations,
recent developments have focused on the preparation of well-
defined ruthenium and iridium complexes to allow more
selective and milder reaction conditions,[8] water soluble or
reusable catalysts, and continuous flow approaches.[9] In
addition, the appealing amination from ammonia to afford
alkylated amines has been reported by several groups.[10]
However, to date no tandem methodologies based on hydro-
gen autotransfer involving three different partners have been
reported.[11]
Recently, we achieved the unprecedented oxidant-free
dehydrogenation of cyclic N-benzyl and N-alkylamines using
a ruthenium(II)–arene catalyst (A; Scheme 1) featuring
a phosphinesulfonate chelate.[12–14] This reactivity involves
hydrogen autotransfers and was successfully applied to the
Scheme 1. Key steps of the tandem hydrogen-transfer processes.
preparation of various functionalized cyclic amines, thus
taking advantage of the ability of the in situ generated
ruthenium hydride species, resulting from the dehydrogen-
ation of amines, to reduce the iminium intermediates formed
upon condensation with aldehydes. Herein, we report on the
preparation and catalytic application in hydrogen-transfer
reactions of a new iridium catalyst which promotes dehydro-
genative oxidation of alcohols into aldehydes or ketones, and
of cyclic amines into enamines, and also reduces unsaturated
intermediates. In particular, an efficient atom-economical and
ecofriendly tandem[15] procedure involving multiple hydro-
gen-transfer processes was developed from three easily
accessible components, namely the anilines 2, diols 3, and
aldehydes 4 to produce C3-functionalized N-arylpiperidines
(Scheme 1).[16]
[*] K. Yuan, F. Jiang, Z. Sahli, Dr. M. Achard, Dr. C. Bruneau
UMR6226 CNRS-Universitꢀ de Rennes
Institut des Sciences Chimiques de Rennes
Organometallics: Materials and Catalysis
Campus de Beaulieu, 35042 Rennes Cedex (France)
E-mail: christian.bruneau@univ-rennes1.fr
Homepage: http://www.scienceschimiques.univ-rennes1.fr/en/
home/teams/omc/
Dr. T. Roisnel
UMR6226 CNRS-Universitꢀ de Rennes
Institut des Sciences Chimiques de Rennes
Centre de diffractomꢀtrie X (France)
[**] The authors thank the European Union (FP-7 integrated project
Synflow, NMP-2009-3.2-1 no. 246461; http://synflow.eu) for a fel-
lowship to F.J. Thanks are also due to the Ministry of Higher
Education and Research of Algeria for a PNE fellowship to Z.S.
Thus, aniline (2a) was first reacted with pentan-1,5-diol
(3a) without any bases, in the presence of a catalyst at 1508C
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201204582.
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Table 1: Tandem N,N,C-trialkylation reaction from aniline 2a.[a]
[17]
À
phosphinesulfonate ligand with an Ir O bond of 2.140 ꢀ.
The catalyst B was successfully involved in the tandem
process under the reaction conditions used for A, thus
affording the trialkylated amine 7a as the major product
(entries 4–7). This result demonstrated that B was active for
the base-free N,N-dialkylation of 2a with 3a to give 6, and
that catalytic activity was maintained for the oxidant-free
direct dehydrogenation of 6, thus leading to a reactive
nucleophilic enamine. It is noteworthy that [{IrCl2Cp*}2]
was totally inactive toward C alkylation but led to 5 as a major
compound (entry 3). With B, increasing the concentration
resulted in lower conversion and higher relative amount of 6,
and the best result was obtained at 0.4m (entries 4 and 5). A
slight excess of 4a ensured better selectivity for 7a (entry 6).
Finally, the presence of 1 mol% of camphorsulfonic acid
(CSA) as a Brønsted acid was required to reach complete
conversion and optimized reaction conditions, which afforded
a 70% yield of compound 7a as determined by GC analysis
(entry 8). The tedious separation of the piperidines 7a and 6
by column chromatography using silica gel led to the isolation
of pure 7a in 55% yield.
The overall transformation corresponds to three dehy-
drogenation and three hydrogenation steps, thus six formal
catalytic hydrogen-transfer processes, and also two conden-
sations of the intermediate aldehyde with aniline and one
condensation reaction between an enamine and an aldehyde.
If we assume that the organic reactions proceed quantita-
tively, each individual catalytic step presents an excellent
yield (> 90%). These results show that only one organome-
tallic precatalyst is sufficient for the overall transformation.
Moreover, the reaction allows the first oxidant-free direct
endo dehydrogenation of the cyclic saturated 6, and does not
require more-reactive substrates (i.e. containing activated
benzylic positions as in N-benzyl amines or tetrahydroisoqui-
nolines), for dehydrogenation[14] and thus highlights the
improved activity of this new iridium precatalyst B.
Entry Catalyst
Conc. 2a/3a/4a[c] 5/6/7a Yield
[m][b]
7a [%][d]
1
[{Ru(p-cymene)Cl2}2]
0.6
0.6
0.6
0.9
0.4
0.6
0.4
0.4
1.1:1:1.2
1.1:1:1.2
1.1:1:1.2
1.1:1:1
1.1:1:1
1:1:1.2
1:0:0
7:83:10
82:13:5
3:45:52 42
4:33:63 50
1:22:77 50
3:27:70 58
2:10:88 70(55)
n.d.
5
1
2
A
3
[{Ir(Cp*)Cl2}2]
4
5
6
B
B
B
B
B
7
1.2:1.1:1
1:1:1.1
8[e]
[a] All reactions were carried in toluene. Step 1 was run for 16 h, step 2
for 19h, and step 3 for 2 h under inert an atmosphere using
a thermostated oil bath (1508C). [b] Molar concentration of the limiting
substrate; see column 4. [c] Ratio of the products. [d] Yield of 7a
determined by GC analysis using tetradecane as an internal standard.
Value in parentheses is the yield of the isolated product. [e] 1 mol% of
CSA was added in step 2. n.d.=not determined.
for 16 hours, and after cooling benzaldehyde (4a) was added
and the reaction was heated for an additional 19 hours at
1508C (Table 1). Based on our previous results obtained with
ruthenium catalysts for C3 functionalization of cyclic amines
with aldehydes, formic acid was added as a reducing agent at
the last stage (step 3; 1508C for 2 h) of the procedure to
ensure complete reduction of the unsaturated intermediates.
The expected formation of 3-benzyl-N-phenylpiperidine (7a)
was observed, but N-benzylaniline (5) and N-phenylpiper-
idine (6) were also detected. The {ruthenium(II)(p-cymene)}
dimer did not afford the C-alkylated piperidine 7a but the
presence of 5, arising from N-alkylation of 2a by 4a, was
observed as the main product (entry 1). Surprisingly, our
previously reported ruthenium(II) catalyst A, which exhib-
ited a high efficiency for N or C alkylation of N-benzylpiper-
idine, showed very low activity for the tandem protocol, thus
affording less than 50% conversion of the diol 3a and leading
mainly to the piperidine derivative 6 and only 5% of 7a
(entry 2). We then investigated the reactivity of the new
iridium(III) complex featuring the chelating phosphane
sulfonate. The complex B was obtained in good yield upon
treatment of the deprotonated diphenylphosphinobenzene
sulfonic acid 1 with the cyclopentadienyl dichloride iridium-
(III) dimer (Scheme 2). The solid-state structure of B
revealed a piano-stool geometry around the iridium center
and a six-membered ring arising from the chelation of the
To clarify the difference of reactivity between the
ruthenium catalyst A and the iridium catalyst B, their
efficiencies in the first step were evaluated. Thus, 2a was
reacted with 3a in the presence of 3 mol% of catalyst A at
1508C for 16 hours. A 50% conversion of 2a into the
monoalkylated amine 8 as the major product was observed
(Scheme 3). Addition of 4a, which also acts as hydrogen
acceptor, to this mixture enhanced only the cyclization rate
Scheme 3. Monoalkylation of 2a using the catalyst A versus dialkyla-
tion with the catalyst B.
Scheme 2. Preparation of the catalyst B.
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for the formation of 6. In contrast, the reaction with B was
more efficient and quantitative formation of 6 was obtained
within 16 hours. These results clearly demonstrate that A is an
efficient catalyst for mono N alkylation of aniline with diols,
and explains why the formation of 7a could not take place
efficiently with this catalyst precursor.
With our optimized reaction conditions in hand (Table 1,
entry 8), we next investigated the scope of this tandem
transformation (Scheme 4). The reactions proceeded
smoothly from a variety of aromatic aldehydes (4b–e), thus
affording the N,N,C-trialkylated products 7b–e in yields of up
undergo dehydrogenation or hydrolysis and reaction with
piperonal led to 49% of 7h. As previously observed with the
ruthenium catalytic systems we used for C3 alkylation of
cyclic amines, no dehalogenation occurred with halogenated
aromatic aldehydes and the alkylated anilines 7j and 7k were
obtained in 53 and 54% yield, respectively. The electron-rich
anilines 2b,c, substituted by a methyl or a methoxy group,
afforded the products 7l,m in 54 and 49% yield, respectively.
Finally, the aliphatic formylcyclohexane 4i was also reactive
and gave 7i in 47% yield. The tandem process can also be
applied to substituted pentan-1,5-diols (Schemes 5 and 6).
Scheme 5. 3,5-disubstituted piperidines made by the tandem N,N,C3-
alkylation reactions. The yield is that for the product 7 (3 steps)
isolated by column chromatography. All reactions were carried in
toluene with 2/3b/4/B/HCO2H in a 1:1:1.1:0.03:2 molar ratio. Step 1
was run for 16 h, step 2 for 20h, and step 3 for 2 h under inert an
atmosphere using a thermostated oil bath (1508C).
Scheme 4. Scope of the direct, tandem trialkylation. The yield is that
for the isolated product 7 (3 steps) after purification by column
chromatography. All reactions were carried in toluene with 2/3a/4/B/
HCO2H in a 1:1:1.1:0.03:2 molar ratio. Step 1 was run for 16 h, step 2
for 20h, and step 3 for 2 h under inert an atmosphere using
a thermostated oil bath (1508C).
Scheme 6. Regioselective formation of 2,5-disubstituted piperidine 7t
to 59% after purification by column chromatography. Inter-
estingly, the dimethylamino group of 7d remained intact and
no dealkylation/alkylation[7b] processes occurred under our
reaction conditions, thus leading to 7d in 50% yield upon
purification. The more challenging heteroaromatic aldehydes
were compatible with the experimental conditions and the
best yield, 66%, was obtained for compound 7g having a 2-
thiophene carboxaldehyde. The acetal moiety did not
The treatment of 3b with 2a and subsequent reaction with
various aldehydes afforded a better yield as compared to
those obtained from 3a (Scheme 5). The formation of two
diastereoisomers resulting from the reduction of the enamine/
iminium intermediates by the iridium hydride species was
observed. Diastereoselectivities of up to 3:2 were obtained
even in refluxing toluene when less-hindered, unsubstituted
benzaldehyde and thiophene carboxaldehyde were used, thus
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leading to 7n,o in 63 and 71% yield, respectively, upon
isolation. We expected formation of the cis isomer as the
major product arising from the anti approach of the iridium
hydride species to the enamine intermediate, but interestingly
2D NOESY studies highlighted the formation primarily of
the trans isomers, which seems to indicate prior protonation
of the enamine intermediate and subsequent reduction of the
resulting iminium species. Similar diastereoselectivities were
obtained with naphthaldehyde (see 7q) and o-methylbenzal-
dehyde (see 7s), whereas p-bromobenzaldehyde (see 7p) led
to the formation of the trans isomer in a trans/cis ratio of up to
4:1. Attempts to improve diastereoselectivities by lowering
the temperature led to lower conversions for the C-alkylation
step. To investigate the regioselectivity issues, we also
extended the tandem process to hexan-1,5-diol (3c), which
contains one secondary and one primary alcohol, and could
lead to up to three regioisomers depending on the nature of
the iminium/enamine equilibria (Scheme 6). The reaction of
2a with 3c afforded the expected N-phenyl-2-methylpiper-
idine, which readily reacted with benzaldehyde to regioselec-
tively generate disubstituted piperidines. Complete NMR
analyses revealed the almost exclusive formation of the distal
piperidine 7t, resulting from the formation of the less
hindered enamine, with a diastereoisomeric ratio of 1:1. The
proximal piperidine 7u was also observed in a low 5% yield
upon isolation.
Received: June 12, 2012
Published online: July 29, 2012
Keywords: dehydrogenation · heterocycles · hydrogen transfer ·
.
iridium · synthetic methods
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