Pd/TiO2-Photocatalyzed Self-Condensation of Primary Amines to Afford Secondary Amines at Ambient Temperature

Article abstract of DOI:10.1021/acs.orglett.8b03271

Symmetric secondary amines were synthesized by the self-condensation of primary amines over a palladium-loaded titanium dioxide (Pd/TiO₂) photocatalyst. The reactions afforded a series of secondary amines in moderate to excellent isolated yields at ambient temperature (30 °C, in cyclopentyl methyl ether). Applicability for one-pot pharmaceutical synthesis was demonstrated by a photocatalytic reaction sequence of self-condensation of an amine followed by N-alkylation of the resulting secondary amine with an alcohol.

Full text of DOI:10.1021/acs.orglett.8b03271

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Pd/TiO2‑Photocatalyzed Self-Condensation of Primary Amines To
Afford Secondary Amines at Ambient Temperature
Lyu-Ming Wang,† Kensuke Kobayashi,† Mitsuhiro Arisawa,‡ Susumu Saito,*,† and Hiroshi Naka*,§
†Graduate School of Science, Nagoya University, Chikusa, Nagoya 464-8602, Japan
‡Graduate School of Pharmaceutical Sciences, Osaka University, Yamada-oka 1-6, Suita, Osaka 565-0871, Japan
§Research Center for Materials Science, Nagoya University, Chikusa, Nagoya 464-8602, Japan
S
* Supporting Information
ABSTRACT: Symmetric secondary amines were synthesized by the self-
condensation of primary amines over a palladium-loaded titanium dioxide (Pd/
TiO2) photocatalyst. The reactions afforded a series of secondary amines in
moderate to excellent isolated yields at ambient temperature (30 °C, in
cyclopentyl methyl ether). Applicability for one-pot pharmaceutical synthesis
was demonstrated by a photocatalytic reaction sequence of self-condensation of an amine followed by N-alkylation of the
resulting secondary amine with an alcohol.
Scheme 1. Approaches for Self-Condensation of Primary
Amines to Secondary Amines
ymmetric secondary amino groups (R2N) are fundamental
Sstructures present in various pharmaceuticals, agrochem-
icals, and molecular catalysts.1 Figure 1 illustrates selected
examples of such functional molecules bearing symmetric
secondary amino groups. These molecules have most
frequently been synthesized from the parent secondary amines
(R2NH).2,3
Intermolecular self-condensation of primary amines (RNH2)
is one of the most efficient methods for the synthesis of
symmetric secondary amines (R2NH, Scheme 1a).4−10 The
beauty of this clean reaction scheme is illustrated by the highly
atom-economical and selective production of the target
secondary amine and the coproduction of ammonia as the
only byproduct. The self-condensation process is free from the
problem of organic waste formation involved in the conven-
tional N,N-dialkylation of primary carboxamides or sulfona-
mides followed by cleavage of the amide moieties.11 Moreover,
the selectivity for a secondary amine over a tertiary amine is
generally higher than in the analogous alkylation of primary
amines using alkyl halides12 or alcohols.2,13−16
and Ir complexes have frequently been used to achieve the self-
condensation of amines at 150−185 °C, affording the desired
products in moderate to good yields.5,6 A cobalt complex
catalyzes this reaction at 120 °C.7 Heterogeneous catalysts
such as Raney Ni, Pt−Sn/γ-Al2O3, Cu/Al2O3, NiCuFeOx, and
Pd3Pb/Al2O3 effectively promote this transformation at 120−
145 °C.8 The self-condensation can be carried out at 90 °C
with Pd on a porous graphene oxide catalyst under a H2
atmosphere.9 These methods provide efficient access to various
symmetric secondary amines. However, the development of
self-condensation of primary amines at ambient temperature
has remained elusive.
The self-condensation of primary amines could be effectively
catalyzed by both molecular and heterogeneous catalysts. Ru
During the course of our studies on the photocatalytic
conversion of amines and alcohols by metal-loaded titania-
Figure 1. Functional molecules with symmetric secondary amino
moieties.
Received: October 13, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b03271
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Organic Letters
Letter
based photocatalysts,2,14c,17 we noticed that the self-con-
densation of benzylamines competes with alkylation with
alcohols. In addition, it was reported that the self-condensation
of primary amines proceeds under light irradiation in the
presence of a Pt-modified TiO2 photocatalyst10a or a
polycrystalline CdS photocatalyst,10b albeit the substrate
scope of the intermolecular condensation was limited to
three substrates (ethylamine, propylamine, and benzylamine)
and the yields were poor (20−33%). Inspired by our
observations and these preceding reports, we herein report
the first photocatalytic self-condensation of primary amines at
ambient temperature that enables scalable synthesis of
symmetric secondary amines (Scheme 1b).
water and alcohols.19 In fact, Pt/TiO2 showed good reactivity
for the formation of imine 3a, but almost no catalytic ability for
further hydrogenation to the desired secondary amine 2a
(entry 3). Other photocatalysts such as Au/TiO2,17c Ag/
TiO2,14c Cu/TiO2,2a a Cu/TiO2−Au/TiO2 mixture,2a and
pristine TiO2 poorly promoted the reaction leading to 2a and
3a (entries 4−8). The Pd/TiO2-mediated photocatalytic self-
condensation also proceeded selectively in THF (entry 9).
Other solvents such as acetonitrile, ethyl acetate, and toluene
could be used, but a small amount of 3a was formed (entries
10−12).
Having established the optimized conditions, the scope of
this Pd/TiO2-mediated photocatalytic self-condensation of
primary amines was clarified (Scheme 2). This reaction could
We selected benzylamine (1a) as a model substrate (Table
1). Cyclopentyl methyl ether (CPME) was used as a solvent
Scheme 2. Self-Condensation of Primary Amines to
Secondary Amines Catalyzed by a Pd/TiO2
a
a
Table 1. Photocatalytic Self-Condensation of 1a to 2a
b
photocatalyst
yield (%)
2a
entry
1
2
[metal (mol %)]
solvent
3a
Pd/TiO2 [Pd (1.7)]
Pd/TiO2 [Pd (1.7)]
Pt/TiO2 [Pt (1.0)]
Au/TiO2 [Au (0.9)]
Ag/TiO2 [Ag (1.5)]
Cu/TiO2 [Cu (2.9)]
Au/TiO2 + Cu/TiO2
TiO2
Pd/TiO2 [Pd (1.7)]
Pd/TiO2 [Pd (1.7)]
Pd/TiO2 [Pd (1.7)]
Pd/TiO2 [Pd (1.7)]
CPME
CPME
CPME
CPME
CPME
CPME
CPME
CPME
THF
98 (96)
<1
<1
<1
<1
<1
1
<1
75
88
<1
<1
78
22
<1
20
26
<1
<1
9
c
3
4
5
6
d
7
8
9
10
11
12
CH3CN
AcOC2H5
toluene
88
90
4
3
a
32 W UV-LEDs λ0 = 365 nm. Metal content (Pd: 5 wt %; Pt: 5 wt
%; Cu: 5 wt %; Ag: 4 wt %; Au: 5 wt %) was determined by ICP-AES.
b
GC/MS yields using n-decane as an internal standard, isolated yield
c
d
a
b
in parentheses. In the dark. 10 mg each of photocatalyst [Au (0.5
mol %), Cu (1.5 mol %)].
Conditions were similar to those in Table 1, entry 1. 1a (10 mmol),
Pd/TiO2 (300 mg, 1.3 mol % Pd), CPME (20 mL), 300 W Xe lamp
with a UV-cold mirror (λ = 300−470 nm).
because of its favorable properties in sustainable chemistry.18
Among the tested photocatalysts, Pd/TiO2 proved to be the
best, affording dibenzylamine (2a) in 98% GC yield and 96%
isolated yield under light irradiation (UV-LEDs, λ0 = 365 nm)
for 10 h (entry 1). The Pd/TiO2 photocatalyst was prepared as
previously described using PdCl2(CH3CN)2, TiO2 (Aeroxide
P25), and NaBH4 [see Supporting Information (SI) for
details].2b,17c The presence of palladium nanoparticles ([Pd]n,
mean particle size = 3.3 nm 0.6 nm [based on the analysis of
100 [Pd]n nanoparticles]) on TiO2 was also established.2b GC
monitoring of the reaction progress every hour indicated the
clean formation of 2a from 1a (Figure S1 in SI). We could not
detect any side products such as imine 3a throughout the
reaction. This preliminary kinetic study indicated that the
reaction is subjected to a zero order with respect to the
concentration of 1a (Figure S1). The reaction hardly
proceeded in the dark, indicating that it is driven by light
irradiation (entry 2). Platinum is a privileged cocatalyst on
heterogeneous photocatalysts for hydrogen evolution from
be operated on a 10 mmol scale using a Xe lamp (λ = 300−470
nm), affording 2a in 95% yield (940.1 mg) after 25-h
irradiation. Benzylamines with electron-donating groups, such
as methyl and methoxy groups, gave the corresponding
secondary amines (2b and 2c) in good to excellent yields.
The presence of a fluoro group on the aromatic ring was well
tolerated, and the desired secondary amine 2d was obtained in
96% yield. The reactions of benzylamines bearing more
reducible chloro-, bromo-, or nitro groups on the aromatic
rings were sluggish (see Scheme S1 in SI). A substrate with a
strongly electron-withdrawing CF3 group was less reactive than
2a−d, yet still gave the product 2e in a fair yield. Moreover, the
present method for the first time offers salt-free access to
amines with biphenyl, 1-naphthyl, and 2-/3-pyridyl function-
alities (2f−2i), which are frequently incorporated in the metal
ligands of molecular catalysts (2f and 2h).3b,20 2-Phenethyl-
amines and a long-chain alkyl amine also underwent the self-
condensation reaction, affording the secondary amines 2j−l in
B
DOI: 10.1021/acs.orglett.8b03271
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
high yields. The condensation of a Boc-protected amine took
place selectively at the free amino moiety and gave the desired
Boc-diprotected triamine 2m in 71% yield. Cyclohexanamine
was considerably less reactive than primary amines, but the
sterically congested dicyclohexylamine (2n) was obtained in an
acceptable yield after prolonged irradiation.
of 3a in the presence of Pd/TiO2 under H2 (1 atm) proceeded
both under light irradiation and in the dark (Scheme S2).
Scheme 4. Cross-Condensation
Furthermore, the Pd/TiO2 photocatalyst was found to be
recyclable at least 4 times without losing its photocatalytic
activity (96−98% yields, Table S1).
Coproduction of ammonia (0.19 mmol, 75%) was detected
by 1H NMR analysis in dimethylsulfoxide-d6 on self-
condensation of 1l after trapping with HCl in methanol (see
SI for details).21 This result confirms that the light-induced
conversion of 1 to 2 involves formation of ammonia as a
stoichiometric byproduct (Scheme 1b).
In order to obtain further mechanistic insight, the self-
condensation of 1a to 2a in the presence of impurities was
examined (Table S2). The desired reaction proceeded well
(2a, 82%) in the presence of chlorobenzene, but 17% of imine
3a formed (Table S2, entry 1). This tendency was stronger in
the case of bromobenzene, and 3a (80%) was obtained as a
major product, together with 2a (18%) and recovered
bromobenzene (60%, Table S2, entry 2). More reducible
iodobenzene or nitrobenzene stopped the reaction at the imine
stage without formation of any desired 2a, yet the 3a was
formed in 39% and 77% yields and the impurities were
recovered in 87% and 70% yields, respectively (Table S2,
entries 3 and 4). These results imply the participation of the
borrowing hydrogen mechanism13 promoted by light irradi-
ation of the photocatalyst (Scheme 3).10a The proposed
Next, we tested the cross-condensation of two different
amines.4 Whereas reactions of 1a with n-butyl or n-octylamine
gave complex mixtures, reasonable amounts of cross-con-
densation products were obtained in the reaction of methyl-
amine with 1a; N-methylbenzylamine formed in 43% yield
together with N,N-dimethylbenzylamine (4%, Scheme 4).
Finally, the current protocol was extended to a one-pot
photocatalytic pharmaceutical synthesis (Scheme 5). Alverine
Scheme 5. One-Pot Photocatalytic Synthesis of Alverine at
Ambient Temperature
(4), a drug used for irritable bowel syndrome, was synthesized
through sequential photocatalytic amine self-condensation of
1o to 2o followed by photocatalytic N-ethylation of 2o with
ethanol.2 These two photocatalytic reactions were both
promoted by Pd/TiO2 and coproduce only ammonia and
water as byproducts, respectively. Thus, the present method
for the first time enabled the synthesis of alverine in high yield
without stoichiometric salt-waste formation.
Scheme 3. Proposed Mechanism
In summary, we have developed an efficient photocatalytic
method for the self-condensation of primary amines to
symmetric secondary amines at room temperature. We expect
this method will serve as an economical and environmentally
friendly complement to existing methods for secondary amine
synthesis.
ASSOCIATED CONTENT
* Supporting Information
■
S
mechanism involves (1) formation of an electron−hole pair on
Pd/TiO2 by irradiation and their separation (e− + h+); (2)
production of [Pd]n−H species on the photocatalyst surface
from electrons, protons, and Pd nanoparticles ([Pd]n); (3)
oxidation of primary amine 1a by holes on the photocatalyst
surface leading to benzaldimine and protons; (4) amine−imine
exchange (transamination) between 1a and benzaldimine to
give imine 3a and ammonia; and (5) reduction of imine 3a to
secondary amine 2a by the [Pd]n−H species. The overall
reaction is summarized in the bottom of Scheme 3. The
presence of reducible impurities lowered the yield of 2a and
increased the yield of 3a (Table S2) by retarding the reduction
of 3a. The high reactivity of [Pd]n−H for the reduction of 3a
to 2a accounts for the higher selectivity of Pd/TiO2 for
producing 2a over 3a, compared with other metal/TiO2
analogues (Table 1, entry 1 vs entries 3−7). Hydrogenation
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b03271.
Supplementary tables, figure, and schemes; experimental
procedures, spectroscopic data, and NMR charts (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: saito.susumu@f.mbox.nagoya-u.ac.jp.
*E-mail: h_naka@nagoya-u.jp.
ORCID
Mitsuhiro Arisawa: 0000-0002-7937-670X
Susumu Saito: 0000-0003-0749-2020
Hiroshi Naka: 0000-0002-1198-6835
C
DOI: 10.1021/acs.orglett.8b03271
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors wish to thank Professors R. Noyori (Nagoya U.),
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Science) for fruitful discussions and warm encouragement.
This work was supported by the JGC-S Scholarship
Foundation (to H.N.), Fusion Emergent Research Program
from IRCCS (to H.N. and M.A.), and JSPS KAKENHI Grant
Numbers JP26410115 (to H.N.) and JPA16H010260 and
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