Amino-alcohol cyclization: Selective synthesis of lactams and cyclic amines from amino-alcohols

Article abstract of DOI:10.1039/c3cy00513e

By employing an amination catalyst, previously used in the direct synthesis of amines from alcohol with ammonia, n-amino-alcohols could be selectively cyclized to either the amide or the amine. By the addition of water, the amine could be produced as the major product whereas adding a sacrificial ketone as a hydrogen acceptor resulted in the amide as the major product. Without an additive a mixture of both the amine and the amide was observed. N-substituted amino-alcohols solely gave cyclic amines under these conditions. From 2-(n-alkanol) anilines the cyclic amines were produced, where the n-propanol derivative selectively formed quinoline as the major product.

Full text of DOI:10.1039/c3cy00513e

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Amino-alcohol cyclization: selective synthesis of
lactams and cyclic amines from amino-alcohols†‡
Cite this: DOI: 10.1039/c3cy00513e
*
Dennis Pingen and Dieter Vogt
By employing an amination catalyst, previously used in the direct synthesis of amines from alcohol with ammonia,
n-amino-alcohols could be selectively cyclized to either the amide or the amine. By the addition of water, the
amine could be produced as the major product whereas adding a sacrificial ketone as a hydrogen acceptor
resulted in the amide as the major product. Without an additive a mixture of both the amine and the amide was
observed. N-substituted amino-alcohols solely gave cyclic amines under these conditions. From 2-(n-alkanol)
anilines the cyclic amines were produced, where the n-propanol derivative selectively formed quinoline as the
major product.
Received 17th July 2013,
Accepted 15th August 2013
DOI: 10.1039/c3cy00513e
www.rsc.org/catalysis
Introduction
Amines and amides are versatile building blocks for inter-
mediates and fine chemicals. Much interest has been
addressed to the direct catalytic synthesis of these types of
compounds via alcohols and amines.1–3 The selective conver-
sion of alcohols with amines would provide sustainable and
efficient routes to those building blocks. Cyclic amines and
amides are very important structural features in pharmaceu-
tical chemistry.4 In syntheses of both natural products and
medicinal compounds, alcohol and amine moieties are often
combined to form new cyclic compounds. However, the
alcohol group is often derivatized to create a leaving group.
Therefore it is highly desirable to develop catalysts that are
able to directly perform these reactions without the need for
protective groups.
The group of Milstein recently developed a catalytic system
containing an acridine based diphosphine and RuHCl(CO)
(PPh3)3 which was able to convert primary alcohols to primary
amines with ammonia.5 However, secondary amines
remained untouched. Shortly after that, our group and simul-
taneously the group of Beller7 reported a highly selective Ru
catalyst system, which was able to convert secondary alcohols
with ammonia to primary amines. This transformation follows
the concept of ‘Hydrogen Shuttling’, in which an alcohol is
first dehydrogenated to the corresponding carbonyl compound
that then undergoes condensation with an amine to form an
imine, which is subsequently hydrogenated to the amine
(Scheme 1).
Scheme 1 ”Hydrogen Shuttling” in the intramolecular amino-alcohol cyclization.
Exploring the potential of this catalytic system gave rise to
intramolecular reactions between amines and alcohols.
Examples in which amino-alcohols are cyclized go back to
the late 1940's, where Woods and Sanders reported the cycli-
zation of 5-amino-1-pentanol.8 In the early 1980's, a few
examples are known in which amino-alcohols are cyclized to
the corresponding cyclic amines by applying RuH2(PPh3)4 as
a catalyst.9 Furthermore, Bartók reported RuCl2(PPh3)4 as a
catalyst for N-substituted amino-alcohol cyclization (Scheme 2).10
Scheme 2 A general reaction scheme for amino-alcohol cyclization.
Later, Murahashi showed the cyclization of 1,4- and 1,
5-amino-alcohols to cyclic amides by applying a hydrogen
acceptor. However, only moderate yields and selectivities
were obtained.11 Van Koten's group performed reactions of
diols with aniline employing Ru-pincer complexes. Here it
was found that the cyclization of the amino-alcohol was slow
and incomplete, giving mainly mono-alkylated products.12 In
recent years, more examples have been developed aiming at
the cyclic amide,13 although the reactions often required
significant amounts of base.14–17
Industrial Chemistry, School of Chemistry, University of Edinburgh, King's
Buildings, West Mains Road, Edinburgh, EH9 3JJ, Scotland, UK.
E-mail: D.Vogt@ed.ac.uk
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c3cy00513e
‡ Celebrating 300 years of Chemistry at Edinburgh
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Results and discussion
As Ru3(CO)12/CataCXiumW PCy was shown to be a very effec-
tive catalytic system for alcohol amination,6 we decided to
use this system for the cyclization of amino-alcohols
(Scheme 3). The full conversion of 5-amino-1-pentanol was
achieved giving only 2 different products: piperidine and
piperidone. Table 1 summarizes the results of the cyclization
of several commercially available a,o-amino-alcohols. The
ratio of amine versus amide formation shows a distinct
dependence on the ring-size of the product formed.
Considering the equilibria involved in the reaction, it can
be rationalized how both the cyclic amine as well as the
amide are formed (Scheme 4). Two important steps deter-
mine the products: the loss of hydrogen or the loss of water.
For the amide formation the mechanism of ‘Hydrogen
Shuttling’ is interrupted by the loss of hydrogen.
Some of the intermediates shown in Scheme 4 can indeed
be observed during in the reaction (Fig. 1). Only low amounts
of the intermediate hemi-aminal are observed, as this is a
fairly unstable intermediate, though sufficiently stable in
solution for GC and GC-MS analyses. The cyclic imine is
more stable and can indeed be observed. The imine builds
up quickly in the beginning and then decreases to form the
cyclic amine, which is the most favoured product in the reaction.
Following the substrate pathway, it is expected that the
addition of water would shift the equilibrium towards
the amide (Scheme 4). Remarkably and in contrast with the
Fig.
1
Reaction profile of the cyclization of 5-amino-1-pentanol. 20 mmol
substrate, 0.5 mol% Ru3(CO)12, 3 mol% CataCXiumW PCy (Ru:P = 1:2), 12 mL
cyclohexane, 140 ꢀC, ▪ = 5-amino-1-pentanol, ● = piperidine, ▼ = piperidone,
▲ = cyclic imine, = cyclic hemi-aminal. The reaction composition was determined by
GC and GC-MS, based on amino-alcohol consumption and amine/amide production.
results obtained by Murahashi, the addition of water resulted
in a higher selectivity towards the cyclic amine (Fig. 2).
Table 2 shows that for all of the substrates tested, the cyclic
amine yield and selectivity goes up when water is added; even
obtaining complete selectivity for 5-amino-1-pentanol. For
the substrates resulting in more strained cyclic products, the
selectivity towards the amine also increased.
This interesting result might be due to the use of the very
apolar, aprotic solvent; water can act as a weak acid. Water
might facilitate the dehydration step by hydrogen bonding to
Scheme 3 The cyclization of a,o-amino-alcohols with Ru3(CO)12 and CataCXiumW PCy.
Fig. 2 Reaction profile of the cyclization of 5-amino-1-pentanol with water.
15 mmol 5-amino-1-pentanol, 150 mmol H2O, 9 mL cyclohexane, 0.075 mmol
Ru3(CO)12, 0.45 mmol CataCXiumW PCy, 140 ꢀC, 21 h. ▪ = 5-amino-1-pentanol,
●
= piperidine, ▼ = piperidone, ▲ = cyclic imine. Reaction composition
determined by GC and GC-MS, based on amino-alcohol consumptions and
amine/amide production.
Scheme 4 The equilibria in the cyclization of 5-amino-1-pentanol.
a
Table 1 The cyclization of various commercially available a,o-amino-alcohols catalyzed by Ru3(CO)12 and CataCXiumW PCy
Amino-alcohol
Conversionb
Amine selectivityb (%)
Amide selectivityb (%)
Other (%)
5-Amino-1-pentanol
6-Amino-1-hexanol
4-Amino-1-butanol
3-Amino-1-propanol
100
67.7
94.5
6
69.5
35.4
88
30.5
37.3
0
0
27.2c
12d
6c
0
0
a
1 mmol substrate, 0.5 mol% Ru3(CO)12, 3 mol% CataCXiumW PCy (Ru:P = 1:2), 0.6 mL cyclohexane, 140 ꢀC, 21 h.
Conversions and selectivities determined by GC, based on amino-alcohol consumption and amine/amide production.
Cyclic imine, intermediate towards cyclic amine.
b
c
d
Most likely polymers or oligomers.
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a
Table 2 The cyclization of various a,o-amino-alcohols catalyzed by Ru3(CO)12 and CataCXiumW PCy in the presence of water
Amino-alcohol
Conversionb (%)
Amine selectivityb (%)
Amide selectivityb (%)
Other (%)
5-Amino-1-pentanol
5-Amino-1-pentanold
6-Amino-1-hexanol
4-Amino-1-butanol
3-Amino-1-propanol
100
80
78.5
100
18.6
100
100
81
61.3
68
0
0
19
0
0
0
0
38.7c
30c
0
a
1 mmol substrate, 10 mmol H2O, 0.5 mol% Ru3(CO)12, 3 mol% CataCXiumW PCy (Ru:P = 1:2), 0.6 mL cyclohexane, 140 ꢀC, 21 h.
Conversions and selectivities determined by GC, based on amino-alcohol consumption and amine/amide production.
Most likely polymers or oligomers.
b
c
d
1 mmol phenol added instead of water.
to act as a hydrogen acceptor. The complete selectivity to the
cyclic amide was achieved using propiophenone with 5-amino-
1-pentanol as the substrate (Table 3). No condensation was
observed in this case.
The amide formation appears to be fairly sensitive with
Fig. 3 The proposed synergistic effect of water in the cyclization of amino-alcohols.
regard to the substrate as well as to the hydrogen accepting
ketone. E.g. propiophenone gave good results for 5-amino-
1-pentanol but for 6-amino-1-hexanol the selectivity was not
influenced much. The slightly less sterically demanding
cyclohexanone (Table 3, entry 4) gave the highest selectivity
to the cyclic amide. Variations such as the addition of molecu-
lar sieves or other (less) bulky ketones were applied but did
not result in better selectivities.19 In most of the cases, only
the cyclic amine was observed. Lowering the reaction temper-
ature resulted in intermolecular condensation reactions and
no cyclic products were observed.
In addition, two other excellent catalytic systems used in
the direct amination of secondary alcohols were employed, in
order to find if these could be steered as well. The RuHCl
(CO)(PPh3)3/Xantphos7b system (‘A’) produced only the cyclic
amine, though with a high conversion (100%) and yield
(94%), whereas the recently published Ru3(CO)12/acridine
diphosphine20 (‘B’) only gave around 50% conversion yet with
a high amine selectivity (Scheme 5). However, for both cata-
lytic systems, a ketone additive did not result in any lactam
formation. This emphasizes the uniqueness of the Ru3(CO)12/
CataCXiumW PCy combination.
the intermediate cyclic half-aminal, as depicted in Fig 3. If
this is true, replacing water for a fairly acidic non-reactive
alcohol should give a similar effect. However, most acids will
poison the catalyst or react with the substrates; only a few
possibilities are allowed. Replacing water by phenol, indeed
gave complete selectivity for the cyclic amine. However, in
this case, the conversion was somewhat lower at 80%
(Table 2, entry 2). Additionally, performing the reaction in a
hydrogen atmosphere produces solely the amine in full con-
version. In that case, the resulting imine is consumed even
more rapidly. The beneficial effect of water has been reported
earlier for a related reaction; the metal-catalyzed reductive
amination of ketones but no explanation was given.18
To direct the reaction towards the cyclic amide, several
ketones were tested as hydrogen acceptors. Although the
intramolecular condensation of the intermediate aldehyde
with the amine is highly favoured, it appeared to be impor-
tant to use a slightly bulky ketone to prevent any intermole-
cular competition. Still the ketone has to be reactive enough
a
Table 3 An overview of the results of the a,o-amino-alcohols cyclization with additives
Amino-alcohol
Additiveb
Conversionc (%)
Amine selectivityc (%)
Amide selectivityc (%)
Other (%)
5-Amino-1-pentanol
5-Amino-1-pentanol
6-Amino-1-pentanol
6-Amino-1-pentanol
4-Amino-1-butanol
4-Amino-1-butanol
3-Amino-1-propanol
3-Amino-1-propanol
Water
Propiophenone
Water
Cyclohexanone
Water
Propiophenone
Water
100
100
78.5
100
100
91.6
18.6
100
100
0
81
0
100
19
0
0
0
15
71.3
0
13.7d
38.7e
0
61.3
38.3
68
61.7f
0
30e
100e
Propiophenone
0
0
a
1 mmol substrate, 0.5 mol% Ru3(CO)12, 3 mol% CataCXiumW PCy (Ru:P = 1:2), 0.6 mL cyclohexane, 140 ꢀC, 21 h.
10 mmol for H2O, 2 mmol for ketone.
Conversions and selectivities determined by GC, based on amino-alcohol consumption and amine/amide production.
Condensation products.
Most likely polymers or oligomers.
Totals of the hemi-aminal and amide.
b
c
d
e
f
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Scheme
6
The cyclization pathway of secondary amino-alcohols involving
enamine formation.
Scheme 5 Amination catalysts in the amino-alcohol cyclization.
Fig. 5 2-(n-Alkanol) anilines used in the amino-alcohol cyclization.
would be more likely, as this is much more stable compared
to a zwitterionic species (Scheme 6).
An important class of molecules are the benz-annulated
N-heterocycles. Aniline-derived amino-alcohols were synthe-
sized with different n-alkanol groups (Fig 5).
Fig. 4 Secondary amino-alcohols tested in the cyclization reactions.
The lower conversions for these substrates might be
due to the lower nucleophilicity of aniline. However, a high
selectivity for the cyclic amines was observed, as was already
seen for the a,o-amino-alcohols and the N-substituted
amino-alcohols.
The scope of this transformation was further explored
towards the cyclization of secondary amino-alcohols (Fig 4).
Table 4 shows the results of the cyclization of substrates
S5–S8. However, in this case no amides were formed. Only
substrate S5 gave a small amount of amide. In all other cases,
the cyclic amine was produced with a full selectivity. The
additions of several ketones did not affect the selectivity
at all.
The secondary imine, formed as an intermediate, would
be present as an unstable zwitterionic iminium ion, which
could not be observed. This would suggest that from the
hemi-aminal, the amide is formed more easily. Though imine
formation can occur, rapid isomerization to the enamine
A recent publication by Andersson et al. showed the effi-
cient Ir-catalyzed cyclization of 2-(3-propanol)aniline and 2-
(2-ethanol)aniline.21 They successfully synthesized indoles
and tetrahydroquinolines in high yields. Another recent
report by Cho et al. described a Ru-catalyzed synthesis of
quinoline from aniline and tripropanolamine.22 Employing
our new procedure, indoline can be produced in a high yield.
Unfortunately no selectivity could be induced here to steer
the reaction to the corresponding lactam. The amine was the
only product although employing a propyl spacer resulted in
the aromatization of the resulting amine product (Table 5).
This is not surprising as this six-membered ring only needs
to loose hydrogen from the imine to form the aromatic prod-
uct (Scheme 7). Using propiophenone or cyclohexanone as
the H-acceptor, the quinolone formation could be improved.
Table 4 N-Substituted amino-alcohol cyclization catalyzed by Ru3(CO)12 and
a
CataCXiumW PCy
Amino-
alcohol
Conversionb Amine
(%)
Amide
Other
selectivityb (%) selectivityb (%) (%)
S5d
S5e
S5
100
96.1
100
43.3
58.1
95
0
29.4
0
5
0
0
0
0
0
0
23.4c
Conclusions
100
100
100
100
70
88.3
100
71.9
41.9c
S6d
S6e
S6
0
0
0
0
0
0
0
A simple Ru catalyst system derived from Ru3(CO)12 and
CataCXiumW PCy was employed in the efficient cyclization of
amino-alcohols. By the addition of water or a ketone as the
hydrogen acceptor, the reaction could be steered to either
cyclic amines or cyclic amides, respectively, giving moderate
to high yields and selectivities. To the best of our knowledge,
the selectivities are the highest reported for a,o-amino-
alcohols. The cyclizations of N-substituted amino-alcohols
and 2-(n-alkanol) anilines are very promising for the synthesis
of N-heterocyclic amines and quinoline derivatives. This route
enables various transformations in a single step, which
100
100
100
100
100
100
S7d
S7e
S7
S8
0
a
1 mmol substrate, 10 mmol H2O, 0.5 mol% Ru3(CO)12, 3 mol%
CataCXiumW PCy (Ru:P = 1:2), 0.6 mL cyclohexane, 140 ꢀC, 21 h.
b
Conversion determined by GC, based on the consumption of
amino-alcohol and amine/amide production.
c
Hemi-aminal.
10 mmol H2O added.
2 mmol propiophenone added.
d
e
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a
Table 5 An overview of the results with 2-(n-alkanol) anilines
Amino-alcohol
Additiveb
Conversion (%)c
Amine selectivity (%)c
Amide selectivity (%)c
Other (%)
S9
S9
S9
S9
S10
S10
S10
S11
S11
None
Water
50.4
36.4
71.9
100
77.9
65.9
100
100
100
100
97.9
19.1
47.0
21.2
100
14.7
0
0
0
2.1
0
0
0
0
0
0
0
0
0
Propiophenone
Cyclohexanone
Propiophenone
Water
Cyclohexanone
Water
58.8d
18.9d
78.8d
0
18.8
14.7
Propiophenone
0
a
1 mmol substrate, 0.5 mol% Ru3(CO)12, 3 mol% CataCXiumW PCy (Ru:P = 1:2), 0.6 mL cyclohexane, 140ꢀC, 21 h.
10 mmol for H2O, 2 mmol for ketone.
Conversions and selectivities determined by GC, based on amino-alcohol consumption and amine/amide production.
Quinoline.
b
c
d
Using Ru3(CO)12/acridine diphosphine: a,o-amino-alcohol
(1 mmol) was weighed into a 10 mL stainless steel autoclave
applying a blanket of Ar. Ru3(CO)12 (0.5 mol%, 0.005 mmol,
3.2 mg) and acridine diphosphine (1.5 mol%, 0.015 mmol,
6.6 mg) were added followed by cyclohexane (0.6 mL). For the
reactions using water as an additive, degassed H2O (10 mmol)
was added and the autoclave was closed tightly and heated in
an oil bath for the appropriate time. Reactions using ketone as
the additive were performed using dried, degassed ketone
(2 mmol). The reaction mixture was subjected to GC and
GC-MS analyses.
Scheme 7 The aromatization to quinoline, starting from 2-(3-hydroxypropyl) aniline.
otherwise would require multiple steps and the use of activat-
ing and protecting groups.
Experimental section
Using Ru3(CO)12/CataCXiumW PCy: a,o-amino-alcohol
(1 mmol) was weighed into a 10 mL stainless steel autoclave
applying a blanket of Ar. Ru3(CO)12 (0.5 mol%, 0.005 mmol,
3.2 mg) and CataCXiumW PCy (3 mol%, 0.03 mmol, 10.2 mg)
were added followed by cyclohexane (0.6 mL). For the reac-
tions using water as an additive, degassed H2O (10 mmol)
was added and the autoclave was closed tightly and heated in
an oil bath for the appropriate time. The reactions using
ketone as the additive were performed using dried, degassed
ketone (2 mmol). The reaction mixture was subjected directly
to GC and GC-MS analyses without further workup.
Fig 1 and 2 were produced via a modified procedure: in
an Ar-purged Schlenk tube, Ru3(CO)12 (0.5 mol%, 0.075 mmol,
48 mg) and CataCXiumW PCy (3 mol%, 0.45 mmol, 153 mg)
was dissolved in 9 mL cyclohexane. To this a,o-amino-alcohol
was added. The mixture was then transferred to a 75 mL
stainless steel autoclave purged with Ar. The autoclave was
closed tightly and heated to 140 ꢀC using a heating mantle.
Samples were taken at t = 0.5, 1, 2, 3.75, 5.5, 7.5 10, 21 and
24 h for Fig. 1 and at t = 0.5, 1, 2, 3.75, 5, 7, 12 and 24 h
for Fig. 2.
Chemicals and were purchased from Sigma-Aldrich and
Acros, Ru3(CO)12 was purchased from Strem and all chemicals
were used as received. Substrates S5–S8 were synthesized
according to
a
literature procedure.23 The 2-(n-alkanol)
anilines S9–S11 were synthesized according a literature proce-
dure.24 The synthesis and catalysis reactions were performed
under an inert Ar atmosphere using standard Schlenk tech-
niques. The product distribution and yield analyses were per-
formed on a Shimadzu GC-17 A instrument with an Ultra 2
column (25 m, 0.2 mm id). GC-MS analyses were conducted on
a Shimadzu GCMS-QP2010 SE with a DB-1 MS column (10 m,
0.1 mm id). Amino-alcohol cyclizations were performed in a
10 mL stainless steel autoclave. The reaction profiles were
recorded using a homemade 75 mL stainless steel autoclave
equipped with a manometer and a sampling unit for 50 mL
samples. Samples were subjected directly to GC without
further workup.
The procedures for the cyclization of amino-alcohols:
using RuHCl(CO)(PPh3)3/Xantphos: a,o-amino-alcohol (1 mmol)
was weighed into a 10 mL stainless steel autoclave applying a
blanket of Ar. RuHCl(CO)(PPh3)3 (1.5 mol%, 0.015 mmol,
14.3 mg) and Xantphos (1.5 mol%, 0.015 mmol, 8.7 mg) were
added followed by cyclohexane (0.6 mL). For the reactions using
water as an additive, degassed H2O (10 mmol) was added and
the autoclave was closed tightly and heated in an oil bath for
the appropriate time. The reactions using ketone as the additive
were performed using dried, degassed ketone (2 mmol). The
reaction mixture was subjected to GC and GC-MS analyses.
Acknowledgements
We would like to thank Ton Staring for his technical assis-
tance. This research has been funded by the Netherlands
Ministry of Economic Affairs and the Netherlands Ministry of
Education, Culture, and Sciences within the framework of
the CatchBio program.
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