Polyamide synthesis from 6-aminocapronitrile, part 1: N-alkyl amide formation by amine amidation of a hydrolyzed nitrile

Article abstract of DOI:10.1002/chem.200601897

The synthesis of N-hexylpentanamide from a stoichiometric amount of pentanenitrile and hexylamine has been studied as a model reaction for the synthesis of nylon-6 from 6-aminocapronitrile. The reaction was carried out under mild hydrothermal conditions and in the presence of a homogeneous ruthenium catalyst. For the mild hydrothermal conditions the presence of hexylamine distinctively increases the nitrile hydrolysis compared to the nitrile hydrolysis in the absence of hexylamine. Amine-catalyzed nitrile hydrolysis mainly produces the N-substituted amide. A clear product development is observed, consisting of first the terminal amide formation and second the accumulation of N-hexylpentanamide. With a maximum conversion of only 80 % after 18 h, the nitrile hydrolysis rate at 230°C is still much too low for nylon-6 synthesis. Ruthenium dihydride phosphine was therefore used as a homogeneous catalyst, which significantly increases the nitrile hydrolysis rate. At a temperature of 140°C and with only 0.5 mol% [RuH₂-(PPh ₃)₄]a 60% nitrile conversion is already reached within 2 h. Initially the terminal amide is the sole product, which is gradually converted into N-hexylpentanamide. The reaction has a high initial rate, however, for higher conversions a strong decrease in hydrolysis rate is observed. This is ascribed to product inhibition, which results from the equilibrium nature of the reaction.

Full text of DOI:10.1002/chem.200601897

DOI: 10.1002/chem.200601897
Polyamide Synthesis from 6-Aminocapronitrile, Part 1: N-Alkyl Amide
Formation by Amine Amidation of a Hydrolyzed Nitrile
Adrianus J. M. van Dijk,[a, b] Tom Heyligen,[c] Robbert Duchateau,*[a, b]
Jan Meuldijk,[b, c] and Cor E. Koning[a, b]
Abstract: The synthesis of N-hexylpen-
tanamide from stoichiometric
amount of pentanenitrile and hexyl-
amine has been studied as a model re-
stituted amide. A clear product devel-
opment is observed, consisting of first
the terminal amide formation and
second the accumulation of N-hexyl-
pentanamide. With a maximum conver-
sion of only 80% after 18 h, the nitrile
hydrolysis rate at 2308C is still much
too low for nylon-6 synthesis. Rutheni-
um dihydride phosphine was therefore
used as a homogeneous catalyst, which
significantly increases the nitrile hy-
drolysis rate. At
1408C and with only 0.5 mol% [RuH2-
(PPh3)4]a 60% nitrile conversion is al-
a temperature of
a
AHCTREUNG
G
ready reached within 2 h. Initially the
terminal amide is the sole product,
which is gradually converted into N-
hexylpentanamide. The reaction has a
high initial rate, however, for higher
conversions a strong decrease in hy-
drolysis rate is observed. This is ascri-
bed to product inhibition, which results
from the equilibrium nature of the re-
action.
action for the synthesis of nylon-6
from 6-aminocapronitrile. The reaction
was carried out under mild hydrother-
mal conditions and in the presence of a
homogeneous ruthenium catalyst. For
the mild hydrothermal conditions the
presence of hexylamine distinctively in-
creases the nitrile hydrolysis compared
to the nitrile hydrolysis in the absence
of hexylamine. Amine-catalyzed nitrile
hydrolysis mainly produces the N-sub-
Keywords: amides
· hydrolysis ·
nitriles · polymers · ruthenium
Introduction
Thanks to its excellent physical properties, nylon-6 is one of
the most widely used engineering plastics. However, produc-
tion of the monomer e-caprolactam as the raw material is
cumbersome and environmentally unfriendly.[1] Nylon-6 for-
mation from 6-aminocapronitrile (ACN) as a monomer
(Scheme 1) is an attractive alternative that has received con-
siderable attention, especially from industry.[2] ACN can be
prepared starting from butadiene through consecutive hy-
Scheme 1. The essential chemistry of the nylon-6 synthesis from 6-amino-
capronitrile.
[a]Dr. A. J. M. van Dijk, Dr. R. Duchateau, Prof. C. E. Koning
Laboratory of Polymer Chemistry
Eindhoven University of Technology
PO Box 513, 5600 MB Eindhoven (The Netherlands)
Fax : (+31)40-246-3966
E-mail: r.duchateau@tue.nl
drocyanation and hydrogenation. So far, the knowledge
about the chemistry involved for the combined nitrile hy-
drolysis and amine amidation of ACN is still limited and a
better understanding of the fundamental aspects of each of
the reaction steps involved is required to develop a viable
process for the production of high-molecular-weight nylon-6
from ACN. The chemistry involved in the hydrolysis and the
[b]Dr. A. J. M. van Dijk, Dr. R. Duchateau, Dr. J. Meuldijk,
Prof. C. E. Koning
Dutch Polymer Institute
PO Box 902, 5600 AX Eindhoven (The Netherlands)
[c]T. Heyligen, Dr. J. Meuldijk
Process Development Group
Eindhoven University of Technology
PO Box 513, 5600 MB Eindhoven (The Netherlands)
7664
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FULL PAPER
polycondensation of ACN is described by the reaction of
equimolar amounts of an unactivated nitrile, a primary
amine, and water resulting in the formation of an N-substi-
tuted carboxylic amide (Scheme 1). The process requires
two chemical transformations: 1) the hydrolysis of the nitrile
and; 2) the amidation of the amine.
thermal conditions and with [RuH
lower temperatures.
A
Results and Discussion
Literature on the nitrile hydrolysis is dominated by stud-
ies on the selective formation of the terminal amide[3,4] and
by studies on the mechanism of nitrile hydrolysis.[3v,w,5,6] De-
spite the potential of this transformation, the synthesis of N-
alkyl amides from nitrile and amine reagents has obtained
very little attention so far.
To ensure that the model for the combined hydrolysis and
polycondensation of ACN to nylon-6 is realistic, we restrict-
ed ourselves to a solvent-free system consisting of equimolar
amounts of n-pentanenitrile and n-hexylamine to form N-
hexylpentanamide (Scheme 2). The risk of phase separation
Except for some specific dini-
triles and acetonitrile,[7,8a] the
combined hydrolysis–aminolysis
reaction of a nitrile group with
primary or secondary amines
Scheme 2. Overall reaction for the synthesis of N-hexylpentanamide from pentanenitrile and hexylamine.
requires a catalyst to give a sub-
stituted amide and ammonia as
a high yield product at mild
temperature.[8,9] The most promising results have been ob-
served with ruthenium and platinum catalysts.[8] Murahashi
and co-workers could obtain high yields of N-substituted
amides with different nitrile–amine combinations. However,
with these aliphatic substrates was avoided by limiting the
amount of water used to a maximum of ten equivalents.
Nitrile activation by using hydrothermal conditions: Since
hydrothermal nitrile hydrolysis is a well-known process in
organic chemistry,[11] we first studied the potential of this
this required a 24-hour reaction time at 1608C with no less
[8b]
than 3 mol% of [RuH
A
De Benne-
ville and co-workers demonstrated that an eight-hour reac-
tion time at 1508C is sufficient to obtain the desired product
in 83% yield by using H2S as a
process for the formation of nylon-6 by using pen
and hexylamine as model reagents (I, Scheme 3). Figure 1
A
catalyst.[8a] Although the reac-
tion did not require a solvent,
25 mol% of the highly undesir-
able H2S is needed. The best
catalyst performance so far has
been reported by Cobley and
co-workers.[8c] With a catalyst
loading 30-times lower than
that for the ruthenium system,
the homogeneous platinum
complex used produced 89% of
N-substituted amide under the
Scheme 3. Possible reaction paths for the synthesis of N-hexylpentanamide from pentanenitrile, with R: C4H9
and R’: C6H11.
same conditions as used by
Murahashi. Although promis-
ing, it is clear that these yields
are far from sufficient for
nylon-6 synthesis since complete conversion of the nitrile
and the amine functionalities, and complete selectivity to-
wards the corresponding N-alkylamides are prerequisites for
the formation of high-molecular-weight polyamides.
As a model reaction for the nylon-6 synthesis out of 6-
aminocapronitrile, in this contribution we study the synthe-
sis of N-hexylpentanamide out of a stoichiometric amount
of pentanenitrile and hexylamine. The objective is to eluci-
date the feasibility and bottlenecks for high-yield N-alkyl
amide synthesis from an aliphatic, primary amine and an ali-
phatic nitrile. The combined nitrile hydrolysis and amidation
has been studied, both without a catalyst under mild hydro-
shows the conversion–time history for the reaction of pen-
tanenitrile with ten equivalents of water at 2308C.[12] For the
nitrile–water mixture only, pentanenitrile was hardly hydro-
lyzed. This hydrolysis behavior resembles the literature ob-
servations. For example, a reaction temperature of 2508C re-
sulted in only a 20% propionitrile conversion in a large
excess of water,[5b] while temperatures of at least 3008C
were required to obtain complete nitrile hydrolysis under
hydrothermal conditions.[5,10] Adding a primary amine (n-
hexylamine) to the system clearly promotes the nitrile hy-
drolysis and a steadily and significantly increasing conver-
sion of pentanenitrile with time is observed (Figure 1). Nev-
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7665

R. Duchateau et al.
Figure 1. Conversion–time history for the pentanenitrile hydrolysis with-
~
*
out hexylamine ( ) and with a stoichiometric amount of amine ( ), at
2308C.
ertheless, still rather high temperatures are required. A tem-
perature reduction to 1608C results in only 8% of hydro-
lyzed pentanenitrile after 22 h. Although increasing the tem-
perature seems the most obvious choice to increase the con-
version, a much higher temperature is no option when this
process is used for nylon synthesis. A recent study on the
thermal cyclization of 6-aminocapronic acid amide showed a
dramatic drop in the selectivity for e-caprolactam at high
temperature as a result of amine disproportionation yielding
secondary and tertiary amines.[10] Although amine dispropor-
tionation poses no real threat in organic synthesis, for poly-
condensations, even small amounts of secondary amines
lead to branches and loss of selectivity. Moreover, the stoi-
chiometric balance is disturbed, which is deleterious for the
required high molar mass.[13]
Figure 2. First- (a) and second-order Delplot (b) for the hydrothermal
pentanenitrile hydrolysis in the presence of hexylamine, without NH3 re-
~
*
moval. Reaction products: Pentanamide ( ), N-hexylpentanamide ( ),
^
and pentanoic acid ( ).
A
2%. The maximum amount of the corresponding carboxylic
acid formed is less than 1% based on the initial amount of
nitrile and is only detected at higher conversions. This is re-
markable as the major product of the nitrile hydrolysis at
hydrothermal conditions in the absence of amine is the cor-
responding carboxylic acid,[5a–c,f] and hydrolysis of nitriles
into the terminal amide requires the use of a selective cata-
lyst.[6a,15] The small amount of carboxylic acid formed in the
reaction of pentanenitrile, n-hexylamine and water can be
rationalized by assuming a relatively low hydrolysis rate of
the pentanamide compared to a fast consumption rate of
the formed carboxylic acid by acylation of hexylamine. This
difference in the reaction rates prevents the built up of ap-
preciable amounts of carboxylic acid, as shown by
Scheme 3. As demonstrated in Figure 2a, the development
of the product composition with increasing conversion
shows a clear behavior. At the beginning of the reaction the
terminal amide is the major product, after 25% conversion,
a selectivity of 90% for N-hexylpentanamide is obtained.
Although pentanamide remains present, it only contributes
to less than 10% of the total yield at higher conversions. An
important conclusion that can be drawn from these results is
that even without removal of the formed NH3 the system
has a strong thermodynamic preference for the formation of
the N-substituted amide over the terminal amide and the
carboxylic acid.
Hydrolysis increase due to the reactant hexylamine is in-
teresting, as nitrile hydrolysis without the use of additives
requires high reaction temperatures. Since nitrile hydrolysis
is known to be both acid- and alkaline-catalyzed,[6f,g] the dis-
tinct improved hydrolysis rate of pentanenitrile is most
probably due to the basicity of the amine (I, Scheme 3). The
formation of ammonia counterbalances the consumption of
amine for the combined nitrile hydrolysis and amine amid-
ation.[5c] The result is a more constant nitrile hydrolysis rate.
When primary (or secondary) amines are used, another
plausible origin for the increased reaction rate would be
that the amine acts as a catalyst by formation of an amidine
intermediate.[14] In agreement with the observed composi-
tion development, the hydrolysis of the amidine should lead
to a terminal amide and not directly to an N-substituted
amide as n-hexylamine is a better leaving group than ammo-
nia (route II, Scheme 3). Hence, the direct formation of N-
substituted amides through the hydrolysis of an amidine is
excluded.
The main products of the nitrile hydrolysis in the presence
of amine are the corresponding terminal amide, pentan-
A
(Figure 2). Note that the reactions were performed in a
closed system, so the generated NH3 was not removed. No
loss of selectivity is observed for pentanenitrile and hexyl-
Figure 2a is also considered as the first-rank Delplot,
which allows the assignment of the reaction rank to the dif-
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Polyamide Synthesis: Amine Amidation
FULL PAPER
ferent products involved.[16] The initial selectivity of pen-
A
primary product of the reaction network. The zero selectivi-
ty for N-hexylpentanamide at nitrile conversions approach-
ing zero shows that it is a higher-rank product. Figure 2b
shows the second-rank Delplot for the pentanenitrile hy-
drolysis in the presence of hexylamine at 2308C, which
allows sorting of higher rank products in the reaction net-
work.[16] As the y intercept for pentanamide in the sec-
ondary Delplot goes to infinity for zero conversion, the as-
signment of pentanamide as the primary product is con-
firmed. As N-hexylpentanamide has a finite, positive value
of the y intercept, it is the product of the consecutive amida-
tion of the terminal amide.[17] The observations correspond
with
a consecutive reaction mechanism, as shown in
Scheme 3.[18]
Hexylamine amidation with pentanamide: To assess the fea-
sibilities of routes III and IV in Scheme 3 and to get insight
into the effect of water on the system, the N-alkyl amide
formation from a terminal amide and a primary amine in
the presence of water was studied separately by using the
neat reaction of pentanamide with hexylamine. Although
amine acylation is the most widely used method for prepar-
ing N-substituted amides,[15] the use of terminal amides as
acylation source is hardly reported.[19,20] This is not surpris-
ing as the formation of amides by using amine amidation
with carboxylic acids is already considered as a poor syn-
thetic method.[15,21] Note that amides are even less reactive
towards nucleophilic substitution than carboxylic acids. The
two possible routes for the amine amidation with a terminal
amide in aqueous environment are depicted in Scheme 3.
The first step for reaction III is the hydrolysis of the amide
to the carboxylic acid, followed by the formation of the am-
monium salt. The ammonium salt dehydrates at high tem-
perature. The direct amine amidation using a terminal
amide (IV, Scheme 3) is mainly performed under dry condi-
tions and requires some method of activation.[20b,d]
Figure 3. a) Hexylamine amidation with pentanamide performed at
!
1808C ( ) and at 2308C (3); b) hexylamine acylation of n-hexylamine
~
*
with pentanoic acid performed at 1808C ( ) and at 2308C ( ). Closed
system.
boxylic acid is formed in the model reaction of pentane-
A
Figure 4 shows the results of the amidation of hexylamine
with pentanamide, both dry[22] and in aqueous environment.
N-hexylpentanamide and pentanoic acid comprise the total
The results for the neat hexylamine amidation with pen-
A
for 180 and 2308C. As expected, the amidation rate at
1808C is low. At this temperature it takes more than five
hours to reach 80% conversion. At 2308C the initial rate is
high and the same 80% conversion is already reached
within two hours. However, completion of the reaction does
not occur as equilibrium seems to be reached. Preliminary
experiments showed that stepwise removal of ammonia
from the reaction mixture further increases the N-hexylpen-
tanamide yield. Although no complete ammonia removal
could be achieved with the setup used, the yield increased
from 85% without ammonia removal to 93% with three
ammonia removal cycles. The amine amidation rate with
pentanoic acid is considerably higher than with pentanamide
(Figure 3b). Within five hours a 95% yield is reached at
1808C, and at 2308C the thermodynamic equilibrium of the
system is already reached within one hour. This is in agree-
ment with our observation that only a small amount of car-
Figure 4. Conversion–time histories for the hexylamine amidation with a
stoichiometric amount of pentanamide at 1408C, comparison of the
~
^
effect of water, in the absence of water ( ), 2 equiv of water ( ) and
10 equiv of water (*) compared to pentanenitrile (closed system).
conversion of pentanamide and hexylamine, which excludes
side reactions. The results demonstrate that water influences
the amine amidation positively at higher conversion. For the
water-free reaction conditions
a 76% conversion was
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R. Duchateau et al.
reached, while with ten equivalents of water, over 90% of
amide was converted after 22 h at 1408C. The formation of
1.5% of carboxylic acid suggests that this increased conver-
sion can be assigned to the formation of carboxylic acid as
intermediate product, according to route III in Scheme 3.
During the experiment with ten equivalents of water a maxi-
mum amount of 15% of carboxylic acid was formed, which
was subsequently consumed on progress of the reaction. Al-
though hydrolysis of the amide to the corresponding acid
does not proceed very fast, it definitely contributes to the in-
creased N-hexylpentanamide yield, compared to the water-
free system. Varying the amine concentration (1–8 equiva-
lents) does not influence the amine amidation. This
strengthens the possibility of N-hexylpentanoic amide for-
mation via the pentanoic acid (III, Scheme 3), with amide
hydrolysis being the rate determining step.
Apreliminary conclusion : The observed phenomena for the
synthesis of N-hexylpentanamide from pentanenitrile, n-hex-
ylamine, and water under mild hydrothermal conditions can
be summarized as follows. Nitrile hydrolysis requires harsh
conditions,[5a,b] but the presence of the primary amine re-
duces the activation barrier for hydrolysis leading to consid-
erably higher reaction rates at 2308C. The time constant for
the amine amidation is lower than the time constant for the
nitrile hydrolysis. The terminal amide formed upon nitrile
hydrolysis can either be directly amidated with hexylamine
or first hydrolyzed to the corresponding acid. As the amide
amination is not influenced by the amine, the product N-
hexylpentanamide is probably mainly formed through the
hydrolysis of the terminal amide (III, Scheme 3). This is sup-
ported by the formation of carboxylic acid for higher con-
versions, as the amidation rate is decreased as a result of a
low amine concentration. However, the direct amidation of
hexylamine with pentanamide can not be excluded. Ammo-
nia removal during the amidation reaction proved to be
cumbersome for the used setup. Nevertheless, partial ammo-
nia removal already showed an improved performance. Nev-
ertheless, the reaction rates and conversions obtained by hy-
drothermal nitrile hydrolysis and subsequent amine amida-
tion are too low for the process to be useful for nylon-6 syn-
thesis. In the next section, the results of a study on rutheni-
um-catalyzed nitrile hydrolysis and amine amidation will be
discussed.
Figure 5. a) Concentration–time diagram for the [RuH2(PPh3)4]-catalyzed
(0.5 mol%) n-hexylamine amidation with pentanenitrile and 2 equiva-
lents of water in a closed system. b) The corresponding selectivity–con-
A
*
version plot: Normalized concentration pentanenitrile ( ), pentanamide
~
^
(
), and N-hexylpentanamide ( ).
to a nitrile conversion of 60% is the terminal amide (see
Figure 5b). Distinct amounts of N-hexylpentanamide are
only observed for high nitrile conversions, as is clearly dem-
onstrated. The observed sequential process as depicted in
Scheme 3 is even more pronounced for the ruthenium-cata-
lyzed hexylamine amidation with hydrolyzed pentanenitrile
than for the hydrothermal conditions. Although the same
consecutive reaction steps are observed, a dramatic change
in rate-determining step occurs. Whereas the nitrile hydroly-
sis is rate-limiting for the hydrothermal conditions, this step
is effectively catalyzed by the ruthenium compound render-
ing the amidation reaction to be the rate-limiting step
(Figure 5). An independent experiment has shown that
[RuH
ation rate.
To asses the possible effect of the presence of amines on
R
AHCTREUNG
Ruthenium-catalyzed nitrile hydrolysis and amine amid-
the reaction rate, the nitrile hydrolysis was also carried out
in the absence of hexylamine. Pentanenitrile does not dis-
solve appreciably in water without amine present and a ho-
mogeneous system is required to properly compare the hy-
drolysis rate. Therefore the hydrolysis, both in the absence
and presence of amine, was performed in dimethoxyethane.
ACHTREUNG
AHCTREUNG
at 1408C, in a closed system, is shown in Figure 5a. The
ruthenium catalyst clearly has a strong accelerating effect on
the nitrile hydrolysis compared to the hydrothermal hydrol-
After two hours, at 1408C, with 0.5 mol% of [RuH2(PPh3)4],
A
ysis. With only 0.5 mol% [RuH
A
10% conversion of pentanenitrile is obtained in the pres-
ence of n-hexylamine against 5% conversion without amine.
Although the hydrolysis rate is lower due to dilution and
perhaps solvent effects, a clear increase in rate is observed
for the experiment in the presence of n-hexylamine. There
already reached within two hours at 1408C, while this re-
quires 12 h at 2308C for the uncatalyzed reaction. In analo-
gy with the uncatalyzed reaction the terminal and coupled
amide are the main products. The only reaction product up
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Polyamide Synthesis: Amine Amidation
FULL PAPER
are various conceivable reasons for the increased nitrile hy-
drolysis in the presence of amines, the most probable one
being that the amine acts as a cocatalyst, which promotes
proton exchange that is required to liberate the formed
amide from the catalytic site.[23] Another option might be
the activation of the ruthenium complex by the substitution
of one phosphine ligand by the amine, which results in a
more open and more reactive complex.
Improving the ruthenium-catalyzed nitrile hydrolysis: It was
found that the initial hydrolysis rate is dependent on the
water concentration. When going from one to two equiva-
lents of water, a clear increase in conversion could be ob-
served. A further increase to ten equivalents resulted only
in a slight increase in conversion and above ten equivalents,
no further change in activity could be observed. At 1408C,
the initial rate is about a factor 1.4 higher when two equiva-
lents of water are used instead of one equivalent.[24]
Figure 7. Nitrile conversion for the model reaction at different tempera-
~
!
^
tures. Tr =1008C ( ), 1208C ( ), 1408C ( ), 1608C (3), 1808C ("),
0.5 mol% [RuH2(PPh3)4]in a closed system.
AHCTREUNG
As shown in Figure 6, the ruthenium catalyst concentra-
tion has a strong effect on the final conversion level
tions become significant. It was reported that ruthenium ef-
fectively catalyzes the formation of secondary and tertiary
amines from primary amines at elevated temperatures.[25]
Indeed, part of the discrepancy in amine conversion and N-
hexylpentanamide yield corresponds well to the amount of
dihexylamine formed during the reaction at temperatures
higher than 1608C. As mentioned before, for polycondensa-
tions even small amounts of secondary amines lead to
branches and disturb the stoichiometric balance, which is
detrimental for the required high molar mass of nylon-6.
The results collected in Figure 6 and Figure 7 clearly dem-
onstrate that the nitrile conversion levels off in time. Plausi-
ble reasons for this deactivation can be either catalyst de-
composition or the inhibition of the active metal species by
the formed products. The addition of a mixture of nitrile,
amine, and water to a mixture which had reacted for 19 h re-
sulted in about 60% hydrolysis of the extra added nitrile in
an additional three hours. Hence, the observed renewed ac-
tivity excludes significant decomposition of the active ruthe-
nium species and indicates a reversible inhibition process.
The reversible character of the activity decrease is also sup-
ported by the observation that a stepwise addition of the
catalyst in time results in the same conversion compared to
the experiment, for which the same total amount of catalyst
is initially added. Hence, the process is hampered by prod-
uct inhibition, a well-known phenomenon in catalysis.
To unravel the inhibition mechanism, the influence of the
effect of ammonia, pentanamide, and N-hexylpentanamide
on the hydrolysis activity was studied. Application of a par-
tial ammonia pressure at the start of the reaction did not
affect the hydrolysis activity. It is not surprising that ammo-
nia has no effect, since the strong decrease of the hydrolysis
activity is observed before appreciable amounts of N-substi-
tuted amides and ammonia are formed. The decrease of the
catalyst performance upon addition of the pentanamide
(Figure 8) is much more pronounced than the decrease that
can be expected as a result of merely the dilution of the
neat system as a result of the addition of pentanamide. Ad-
dition of N-hexylpentanamide was found to have a compara-
*
Figure 6. Nitrile conversion for the model reaction with 0.05 mol%( ),
~
!
0.2 mol% ( ), 0.5 mol% ( ), and 1.0 mol% (3) [RuH2(PPh3)4]in a
A
closed system. The shown curves are only guides for the eye.
reached. However, a strong reduction in hydrolysis rate is
observed at higher conversions for all catalyst concentra-
tions ranging from 0.05 to 1.0 mol% (see below). On the
other hand, with 2.5 mol% of ruthenium a 98% nitrile con-
version can be obtained, but for a viable process lower cata-
lyst loadings are required. This reduction in hydrolysis rate
is observed for the ruthenium-catalyzed hydrolysis of nitrile,
both with and without amine present. However, the de-
crease of the nitrile hydrolysis activity is more pronounced
in the presence of amine.
Figure 7 shows the conversion–time history of the nitrile
hydrolysis with 0.5 mol% ruthenium catalyst at different
temperatures. Figure 7 shows an increase of the final conver-
sion with increasing temperature. After two hours of reac-
tion at 1808C the nitrile conversion exceeds 85%, compared
to a 70% conversion at 1608C for the same reaction time.
Unfortunately, at temperatures higher than 1608C, side reac-
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7669

R. Duchateau et al.
sis mechanism has not been extensively studied, as dehydra-
tion is generally not observed.[5b,27] Nevertheless, the exis-
tence of an equilibrium is quite possible, as ruthenium phos-
phines are known to dehydrate amides in the absence of
water, especially when the water that is formed is removed
from the system, for example, by using urea.[28] With
1 mol% of [RuH2(PPh3)4]the formation of the correspond-
G
ing nitrile as the dehydration product is indeed observed at
1408C, see Table 1. The water formed due to the dehydra-
tion is partially involved in hydrolysis of the amide leading
to some carboxylic acid formation. Even in the presence of
one equivalent of water, some pentanenitrile is formed from
U
the corresponding amide. Increasing the amount of water to
ten equivalents is enough to make quantification of the de-
hydration impossible. These results demonstrate that the ni-
Figure 8. Observed reduction in nitrile conversion after 1 h reaction time
with increasing initial amount of pentane amide added, both for a reac-
!
^
tion mixture with ( ) and without ( ) n-hexylamine. With 0.5 mol%
trile hydrolysis catalyzed by [RuH2(PPh3)4]is an equilibrium
A
[RuH2(PPh3)4], T=1408C (closed system).
ACHTREUNG
reaction (Scheme 5) and confirms that pentanamide coordi-
ble effect. So, the terminal and N-hexylpentanamide plays a
dominant role in the reversible catalyst deactivation.[26]
Ruthenium-catalyzed amide dehydration and hydrolysis: Al-
though amide functionalities are rather robust, hydrolysis
into their corresponding carboxylic acids is a common trans-
formation and terminal amides can even be dehydrated to
their nitrile analogues. In Table 1 the results are collected
Table 1. Pentanamide stability towards hydrolysis and dehydration.[a]
[c]
[d]
Entry
[H2O]
U
A
YPN
YPA
[molar equiv]
ACHTREUNG
Scheme 5. Reaction scheme for the ruthenium-catalyzed synthesis of N-
hexylpentanamide from pentanenitrile.
1
2
3
4
5
6
7
0[b]
0[b]
1
1
2
–
1
–
1
–
1
2
0
4
0
2
0
0
0
0
1
3
16
3
nates competitively with nitrile onto the catalyst. Scheme 5
combines all the observations for the system consisting of a
5
5
10
10
nitrile, a primary amine, and water with [RuH2(PPh3)4]as a
A
[a]22 h and 140 8C. [b]Dried pentanamide used. [c]Yield of pentane-
nitrile. [d]Yield of pentanoic acid
catalyst precursor. The system is fully reversible and the
products are in thermodynamic equilibrium under the condi-
tions applied.
ACHTREUNG
for the stability of pentanamide at 1408C against hydrolysis
and dehydration as depicted by Scheme 4. As expected, pen-
tanamide is stable for prolonged periods of time and the ad-
Conclusion
The synthesis of N-hexylpentanamide from pentanenitrile
and hexylamine was studied at mild hydrothermal condi-
tions and with the use of ruthenium catalysts. The reaction
proceeds through two consecutive reactions that are inde-
pendent of the use of a catalyst. The first reaction is the
Scheme 4. Equilibrium reactions for pentanamide.
A
amine amidation of the formed terminal amide. The pres-
ence of amine significantly increases the nitrile hydrolysis as
compared to the nitrile hydrolysis without additional amine.
Still, the hydrolysis remains the rate determining step in the
hydrothermal system and the overall activity of the system
is too low to be of practical use for the production of nylon
polymers out of amino nitriles.
dition of water results in only a small amount of carboxylic
acid formation.
As the amide competes with the nitrile for the coordina-
tion to the ruthenium metal, an equilibrium can also exist
between a ruthenium nitrile species and a ruthenium amide.
The role of amide dehydration as part of the nitrile hydroly-
7670
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Chem. Eur. J. 2007, 13, 7664 – 7672

Polyamide Synthesis: Amine Amidation
FULL PAPER
cooling, the reactors were opened and the content was removed and ho-
mogenized with ethanol, if necessary. Each reactor was rinsed twice with
ethanol. The conversion and yield were determined with GC analysis by
using p-xylene as an internal standard. To investigate the influence of the
concentration on the course of the reaction, the initial concentrations of
the various components were varied.
It was found that [RuH2
nitrile hydrolysis, but has no influence on the amine amid-
ation. The desired reduction of the amount of catalyst re-
A
ACHTREUNG
sults in a decrease of the final yield, which is caused by re-
versible product inhibition of the active species. Preliminary
tests showed that the decrease in hydrolysis rate with lower
catalyst loadings can partly be counterbalanced by using in-
creased temperatures and removing ammonia. On the other
hand, increased temperatures result in side reactions of the
primary amine, which would result in a non-stoichiometric
amount of functional groups in a polymerization reaction of
ACN, and in branched and low-molar-mass nylon-6. Hence,
although very useful for the organic synthesis of N-substitut-
ed amides from nitriles and amines, this ruthenium catalyst
is not the most suitable for the direct polycondensation of
amino nitrile monomers such as ACN to nylon-6. Other cat-
alysts that are less prone to product inhibition and side reac-
tions at high temperatures have been explored and the re-
sults are described elsewhere.[29]
For the catalyst-free experiments at elevated temperature, the reaction
mixture was biphasic at room temperature when 10 equivalents of water
compared to the amount of nitrile were used. The phase behavior for
these experiments is not known and therefore these data give only a
qualitative insight into the global kinetics.
Hexylamine amidation experiments with pentanamide: The reaction be-
tween pentanamide, hexylamine, and water was used to study the amid-
ation. The reaction mixture consisting of pentanamide (7.4 mmol, 0.75 g),
hexylamine (7.4 mmol, 0.99 mL), and water (14.8 mmol, 0.27 mL) was
first brought into a reactor insert in an inert atmosphere. The rest of the
procedure was the same as for the hydrolysis experiments. For the hexyl-
amine amidation with pentanamide under dry conditions, pentanamide
was dried by using azeotropic toluene distillation. Hexylamine was dried
by distillation over sodium hydroxide. The complete reactor was dried in
an oven before use.
N-hexylpentanamide synthesis: Pentanoyl chloride (0.51 mol, 61.2 g) was
added dropwise to a solution of hexylamine (1.02 mol, 103 g) in diethyl
ether (300 mL) and left overnight. The product mixture was distilled
under vacuum, extracted with acidified water and brine, and dried over
Na2SO4. A 97% pure product was obtained and was used for product
identification. Orange, red oil; 1H NMR (300 MHz, CDCl3) d=6.30–6.40
(s, 1H), 3.17 (td, 3Jt =7 and 3Jd =6 Hz, 2H), 2.14 ppm (t, 3J=7 Hz);
13C NMR (300 MHz, CDCl3) d=173.3, 39.3, 36.2, 31.3, 29.4, 27.8, 26.4,
22.3, 22.2, 13.7, 13.6 ppm.
Experimental Section
All reactions and manipulations involving ruthenium complexes were
performed in an argon atmosphere by using standard Schlenk techniques
or in a nitrogen-filled glove box, and the product mixtures were worked
up in open flasks in air. The reaction products were identified by using
1H NMR spectroscopy. Conversions of substrates and selectivities for
product formation were determined by gas chromatography by using p-
xylene as an internal standard. The product mixtures were analyzed by
Acknowledgements
using
a CP 9000 gas chromatograph (GC) equipped with a 30 m”
The authors thank the Dutch Polymer Institute (DPI) for their financial
support. This research forms part of the research program of the Dutch
Polymer Institute (DPI), project #297. Ir. Guido Vinke is gratefully ac-
knowledged for performing the preliminary experiments.
0.32 mm id, capillary CP volamine column and a FID detector. The
sample was injected at 1208C and after a stabilization temperature of
1 min the temperature was raised at 158CminÀ1 to 2908C. All single-
point experiments were performed in duplicate.
Hydrolysis experiments were performed in closed, stainless steel micro-
reactors that contained glass inserts. These autoclaves, with 7-mL internal
volume, were made in-house by the workshop of the Eindhoven Univer-
sity of Technology. Sealing during the reaction was ensured by using
Kalrez o-rings, as these are resistant against the aggressive ammonia that
was formed. The maximum operation temperature and pressure were
2508C and 60 bar, respectively. The internal reactor temperatures were
controlled within Æ28C by a heating mantle. Reaction mixtures were
stirred with magnetic stirring bars.
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ACHTREUNG
AHCTREUNG
A
ACHTREUNG
Standard procedure for the neat hydrolysis of pentanenitrile in the pres-
ence of hexylamine: The reaction mixture of pentanenitrile (8.6 mmol,
0.71 g), hexylamine (8.6 mmol, 0.87 g), water (17.2 mmol, 0.31 g), and
[RuH2(PPh3)4](0.5 mol%, 4.3 mmol, 49 mg) was first prepared in an inert
G
atmosphere. At room temperature the homogeneous reaction mixture
was charged into a 7-mL stainless-steel autoclave containing a glass
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kept constant during the reaction. After the preset reaction time the re-
action vessels were removed from the heating mantle and immersed in
an ice/water bath to quench the reaction. The experiments were checked
for leaks by weighing the autoclave before and after each reaction. After
Chem. Eur. J. 2007, 13, 7664 – 7672
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Received: December 30, 2006
Revised: March 6, 2007
Published online: June 26, 2007
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