Amide directed hydrocarboxylation of N-allylacetamide catalyzed by the aqueous Pd - tppts - Bronsted acid system (tppts = P(C6H4-m-SO3Na)3)1

Article abstract of DOI:10.1139/v01-068

The Pd - tppts - HOTs (tppts = P(C₆H₄-m-SO₃Na)₃, HOTs = p-toluenesulfonic acid) catalyzed hydrocarboxylation of N-allylacetamide in an aqueous medium afforded 4-acetamidobutyric acid and 3-acetamido-2-methylpropanoic acid under mild conditions, with a high regioselectivity towards the linear isomer. During the hydrocarboxylation an acid catalyzed hydrolysis of the amide moieties of both the substrate and the products took place, as well as the formation of acetamide and propanal, presumably via a Pd-catalyzed allylic substitution reaction of N-allylacetamide. The hydrolysis reaction was suppressed by lowering the amount of Bronsted acid cocatalyst (HOTs) or by employing a weaker Bronsted acid such as propanoic acid. The allylic substitution reaction was minimized by increasing the CO pressure but unfortunately this caused a decrease in the regioselectivity. A sudden inhibition took place after ca. 70% conversion, presumably caused by one of the side products. By increasing the tppts concentration to 13.1 mmol L^-1 (20 equiv per Pd) the inhibition was circumvented and a quantitative conversion of N-allylacetamide was achieved.

Full text of DOI:10.1139/v01-068

688
Amide directed hydrocarboxylation of
N-allylacetamide catalyzed by the aqueous
Pd – tppts – Brønsted acid system (tppts = P(C6H4-
m-SO3Na)3)1
Göran Verspui, Gábor Besenyei, and Roger A. Sheldon
Abstract: The Pd – tppts – HOTs (tppts = P(C6H4-m-SO3Na)3, HOTs = p-toluenesulfonic acid) catalyzed
hydrocarboxylation of N-allylacetamide in an aqueous medium afforded 4-acetamidobutyric acid and 3-acetamido-2-
methylpropanoic acid under mild conditions, with a high regioselectivity towards the linear isomer. During the
hydrocarboxylation an acid catalyzed hydrolysis of the amide moieties of both the substrate and the products took
place, as well as the formation of acetamide and propanal, presumably via a Pd-catalyzed allylic substitution reaction
of N-allylacetamide. The hydrolysis reaction was suppressed by lowering the amount of Brønsted acid cocatalyst
(HOTs) or by employing a weaker Brønsted acid such as propanoic acid. The allylic substitution reaction was mini-
mized by increasing the CO pressure but unfortunately this caused a decrease in the regioselectivity. A sudden inhibi-
tion took place after ca. 70% conversion, presumably caused by one of the side products. By increasing the tppts
concentration to 13.1 mmol L–1 (20 equiv per Pd) the inhibition was circumvented and a quantitative conversion of
N-allylacetamide was achieved.
Key words: aqueous media, olefins, palladium, hydrocarboxylation, N-allylacetamide.
Résumé : Le complexe Pd – tppts – HOTs (tppts = P(C6H4-m-SO3Na)3, HOTs = acide p-toluènesulfonique) catalyse
l’hydrocarboxylation du N-allylacétamide, en solution aqueuse et dans des conditions douces, pour conduire à la forma-
tion des acides 4-acétamidobutyrique et 3-acétyl-2-méthylpropanoïque, avec une régiosélectivité élevée en faveur de
l’isomère linéaire. Au cours de la réaction d’hydrocarboxylation, il se produit une hydrolyse acidocatalysée des por-
tions amides tant du substrat que du produit ainsi que la formation d’acétamide et de propanal, probablement par le
biais d’une réaction de substitution allylique du N-allylacétamide, catalysée par le Pd. La réaction d’hydrolyse peut être
supprimée en abaissant la quantité de l’acide de Brønsted utilisé comme cocatalyseur (HOTs) ou en utilisant un acide
de Brønsted plus faible, tel que l’acide propanoïque. On peut minimiser la réaction de substitution allylique en aug-
mentant la pression de CO; toutefois, ceci provoque une diminution de la régiosélectivité. Après environ 70% de
conversion, il se produit une inhibition soudaine de la réaction qui est probablement causée par l’un des produits se-
condaires. En augmentant la concentration de tppts jusqu’à une valeur de 13,1 mmol L–1 (20 équivalents par Pd), on
peut prévenir l’inhibition et obtenir une conversion quantitative du N-allylacétamide.
Mots clés : milieu aqueux, oléfines, palladium, hydrocarboxylation, N-allylacétamide.
[Traduit par la Rédaction] Verspui et al. 692
Introduction
environmentally friendly and economically attractive
process solvent: water (1). Recently, we and others have de-
Organometallic catalysis in aqueous media currently at-
tracts much attention due to the advantages of using the safe,
veloped
a
novel aqueous biphasic system for the
hydrocarboxylation of olefins (2). In the presence of a
Brønsted acid cocatalyst consisting of weakly coordinating
anions, the Pd–tppts catalyst efficiently converts partly water-
soluble olefins, such as ethene, propene, or isobutene, into
their corresponding hydrocarboxylation products: propionic
acid, n- and isobutyric acid and 3-methylbutyric acid, re-
spectively. Unfortunately, when higher olefins were applied,
e.g., 1-octene, due to phase transfer limitations, the reaction
rate decreased dramatically, accompanied by the decomposi-
tion of the catalyst.
We have also shown that a hetero-atom substituted olefin,
N-allylacetamide, was smoothly converted into 4-
acetamidobutanal and 3-acetamido-2-methylpropanal by a
Rh–tppts catalyzed hydroformylation reaction in water (3).
In comparison with the conventional Rh–PPh3 catalyst in an
Received October 10, 2000. Published on the NRC Research
Press Web site at http://canjchem.nrc.ca on June 30, 2001.
Dedicated to Professor Brian R. James on the occasion of his
65th birthday.
G. Verspui and R.A. Sheldon.2 Delft University of
Technology, Laboratory of Organic Chemistry and Catalysis,
Julianalaan 136, 2628 BL DELFT, The Netherlands.
G. Besenyei. Hungarian Academy of Sciences, Chemical
Research Institute Centre, Institute of Chemistry, P.O. box 17,
H-1525 Budapest, Hungary.
1Catalytic conversions in water, part 20.
2Corresponding author (telephone: +31 15 278 2675; fax:
+31 15 278 1415; e-mail: r.a.sheldon@tnw.tudelft.nl).
Can. J. Chem. 79: 688–692 (2001)
DOI: 10.1139/cjc-79-5/6-688
© 2001 NRC Canada

Verspui et al.
689
Scheme 1. The hydrocarboxylation of N-allylacetamide.
Table 1. The Pd–tppts catalyzed hydrocarboxylation of N-allylacetamide.
Yield (%)
Temperature
(°C)
Pressure
(bar)
Time
(h)
1:b
Ratio
Allylic
Hydrolysis
(%)c
Experimenta
1/1
n
iso
substitution (%)b
80
60
40
60
60
60
10
10
10
10
30
50
2
8
8
24
8
24
8
24
8
24
8
21.2
27.8
30.3
41.5
4.0
14.9
37.3
41.6
38.8
44.1
38.4
43.6
0.7
29.1
16.0
14.6
12.5
11.5
13.6
13.4
11.4
6.7
7.7
8.0
9.6
12.8
2.9
7.4
14.0
14.4
9.2
12.1
8.9
0
1.7
0
1.3
0
1.7
2.1
3.3
0.3
1.1
2.8
3.7
5.8
8.3
8.5
11.5
1/2
1/3
0
0
1/4d
1/5d
1/6d
11.6
3.5
10.5
2.9
9.0
5.3
4.5
3.8
24
9.5
aReaction conditions: 50 µmol PdCl2, 0.50 mmol tppts, 0.50 mmol p-toluenesulfonic acid, 10.0 mmol N-allylacetamide, 2.0
mmol n-BuOH (internal standard), 76.5 g total amount of reaction mixture.
bBoth propanal and acetamide, were formed in equal amounts.
cCalculated from the amount of HOAc formed.
d2.50 mmol p-toluenesulfonic acid.
organic solvent, e.g., toluene or THF, the reaction in water
proceeded considerably faster and in a higher selectivity to-
wards the aldehydes. The rapid hydroformylation of N-
allylacetamide in water suggested that the Pd–tppts cata-
lyzed hydrocarboxylation reaction of this water-soluble ole-
fin, affording 4-acetamidobutyric acid and 3-acetamido-2-
methylpropanoic acid (Scheme 1), might proceed smoothly
in a neat aqueous medium as well. The de-acetylated deriva-
tive of the linear product, γ-aminobutyric acid (GABA), is a
well-known neurotransmitter with interesting pharmacologi-
cal properties.
The hydrocarboxylation and alkoxycarbonylation of amide-
functionalized olefins have been sporadically reported.
Becker et al. (4) described the PdCl2(PPh3)2 catalyzed
methoxycarbonylation of N-vinylphtalimide in 2-butanone.
After a 21 h reaction time at 70°C and 100 bar CO pressure,
in the presence of 1% catalyst, 48% of the olefin was con-
verted into the methylester of 2-phthalimidopropanoic acid.
Ojima and Zhang (5) reported the methoxycarbonylation of
N-allylbenzamide in the presence of PdCl2(PPh3)2 in ben-
zene at 80°C and a CO pressure of 100 bar. The products
In this paper we report on the hydrocarboxylation of N-
allylacetamide in water catalyzed by the Pd – tppts –
Brønsted acid system. The high water-solubility of N-
allylacetamide enabled us to study this reaction in a single
phase aqueous medium, i.e., in the absence of phase transfer
limitations.
Results and discussion
In the hydrocarboxylation experiments we started with a
freshly prepared catalyst solution by codissolving PdCl2 and
10 equiv of tppts in water. We previously observed by NMR
that under these conditions a cationic [PdCl(tppts)3]+ inter-
mediate is formed, which is rapidly reduced to [Pd(tppts)3]
in the presence of CO (7). We assume that this reduction is
complete before the autoclave has reached the reaction tem-
perature (within 8 min). We have also established by
multinuclear NMR experiments, that [Pd(tppts)3] is
protonated by the acid cocatalyst to afford the cationic
[PdH(tppts)3]+, which reacts with ethene and CO to give a
relatively stable Pd–acyl intermediate (8). The rate determin-
ing hydrolysis of the Pd—acyl bond affords propanoic acid
and the Pd–hydride which returns to the catalytic cycle.
Since we did not observe any induction period in our
hydrocarboxylation reactions, we assume that the generation
of the active catalyst is complete when the reaction tempera-
ture was reached.
methyl-4-benzamidobutanoate
and
methyl-3-methyl-3-
benzamidopropanoate were obtained in 18% and 71% yield,
respectively. Magnus and Slater (6) studied the Co2(CO)8
catalyzed carbonylation of N-allylacetamide in ethylacetate,
in the presence of 1 equiv of water. Under forcing conditions
(140–160°C), a mixture of 2-acetamidobutyric acid and 2-
acetamido-2-methylpropanoic acid was obtained in a 3.8:1
ratio. The formation of the linear hydrocarboxylation prod-
uct 4-acetamidobutyric acid was not observed. When a com-
bination of Co and Rh carbonyl precursors was used, the
selectivity increased towards 2-acetamidobutyric acid.
When the hydrocarboxylation of N-allylacetamide was car-
ried out under similar reaction conditions to those which we
previously applied for ethene and propene (110°C, 50 bar)
(2a), severe catalyst decomposition took place. In contrast,
at a reaction temperature below 80°C the catalyst remained
© 2001 NRC Canada

690
Can. J. Chem. Vol. 79, 2001
Table 2. The Pd – tppts – Brønsted acid catalyzed hydrocarboxylation of N-allylacetamide.
Yield (%)
Brønsted acid
(mmol)
Time
(h)
1:b
Ratio
Allylic
substitutuion (%)
Hydrolysis
(%)
Experimenta
2/1
n
iso
HOTs (0.5)
8
24
8
24
8
24
8
24
8
24
30.3
41.5
37.3
41.6
37.0
35.3
27.9
38.6
26.4
36.9
2.1
3.3
2.8
3.7
2.8
3.8
2.3
4.8
2.0
3.2
14.6
12.5
13.4
11.4
13.3
9.2
12.4
8.0
13.0
11.4
9.6
0
1.3
0
11.6
7.9
21.2
3.1
10.8
0
12.8
14.0
14.4
11.1
11.4
8.1
11.1
11.6
14.7
2/2
2/3
2/4
2/5
HOTs (2.5)
HOTs (5.0)
HCl (2.5)
HOOCCH2CH3 (2.5)
1.6
aReaction conditions: see Table 1; 60°C, 10 bar CO.
Table 3. The Pd – tppts – Brønsted acid catalyzed hydrocarboxylation of N-allylacetamide.
Yield (%)
Time
(h)
1:b
Ratio
Allylic
substitution (%)
Hydrolysis
(%)
Experimenta
3/1b
Tppts–Pd
5
n
iso
8
24
60
8
24
8
12.5
12.3
55.9
37.3
41.6
44.0
61.7
61.4
1.2
1.7
4.8
2.8
3.7
2.7
3.6
4.0
10.8
7.2
4.3
5.1
2.8
5.3
15.4
0
11.6
3.4
5.4
Extra tppts
10
11.6
13.4
11.4
16.1
17.2
15.9
22.9
14.0
14.4
14.1
21.0
21.3
3/2
3/3
20
12
24
10.6
aReaction conditions: see Table 1; 2.50 mmol p-toluenesulfonic acid, 60°C, 10 bar CO.
b10 equiv of tppts added at t = 24 h.
Fig. 1. The effect of the tppts concentration on the progress of
the reaction (reaction conditions: see Table 3).
mixture. The formation of acetamide and propanal could be
suppressed to some extent by increasing the CO pressure,
but unfortunately, this also resulted in a considerably lower
regioselectivity (1:b = 4 at 50 bar CO, Experiment 1/6). We
suggest that a Pd-catalyzed allylic substitution reaction of N-
allylacetamide takes place, affording acetamide and
allylalcohol, of which the latter is isomerized to propanal
(Scheme 2). In basic media, such an allylic hydrolysis reac-
tion occurs readily at room temperature (9). The reaction ap-
parently also proceeds under our mildly acidic conditions.
Alternatively, N-allylacetamide might undergo an
isomerization reaction to 1-acetamidopropene, followed by a
fast acid-catalyzed hydrolysis, which would also afford
acetamide and propanal. However, the absence of 1-
acetamidopropene in the reaction mixtures and the high sta-
bility of the related enamide, N-methyl-N-vinylacetamide,
under representative reaction conditions are not consistent
with the latter explanation and, thus, would favour the al-
lylic substitution mechanism.
The much higher regioselectivity towards the linear
carboxylic acid in the hydrocarboxylation of N-
allylacetamide, compared to propene (1:b = 1.3–1.6, irre-
spective of the conditions applied) suggests that the amide
moiety has a directing effect. During the insertion of the
C=C bond into the Pd—hydride bond, at which stage the
regioselectivity is determined, the amide might coordinate to
the palladium. We envisage two possible mechanisms that
explain the higher selectivity to the linear isomer compared
to the hydrocarboxylation of the nonfunctionalized olefin
intact and converted N-allylacetamide into 4-acetamidobutyric
acid and 3-acetamido-2-methylpropanoic acid (Table 1).
During the reaction some acetic acid was formed via an
acid-catalyzed hydrolysis of the amide moieties in N-
allylacetamide and both carboxylic acid products. De-
creasing the amount of HOTs to 0.50 mmol led to a slower
hydrolysis (1.3% after 24 h reaction time (Table 2, Experi-
ment 2/1)) with only a small compromise in the initial
hydrocarboxylation rate. The hydrolysis reaction was also
suppressed by using a much weaker Brønsted acid cocatalyst
(we chose propionic acid for analysis reasons).
In addition to hydrocarboxylation and hydrolysis products
we also observed the formation of acetamide, propanal, and
traces of allylalcohol. The formation of these side products
continues at room temperature, in the absence of CO, which
necessitated immediate analysis of the samples (within
5 min) as soon as they were withdrawn from the reaction
© 2001 NRC Canada

Verspui et al.
691
Scheme 3. Possible intermediates in the hydrocarboxylation of
Scheme 2. Possible mechanisms for the formation of propanal
and acetamide.
N-allylacetamide.
propene (Scheme 3): (i) coordination takes place via the ni-
trogen which favours the formation of the anti-Markovnikov
addition product (1) while there is not a large difference in
reactivity of both Pd–alkyl intermediates (1 and 2) towards
CO; (ii) coordination takes place via the oxygen;
Markovnikov addition of Pd-H to N-allylacetamide affords
the six-membered chelate (4), which is more stable but less
reactive towards CO compared to the anti-Markovnikov ad-
dition product (3).
The experiments in Tables 1 and 2 reveal that the 1:b ratio
decreases at higher pressures, indicating that the amide moi-
ety competes with CO for coordination to the palladium.
The rate of hydrocarboxylation is, however, independent of
the pressure, which suggests that the first mechanism is op-
erative, since a higher reactivity at higher pressures would
be expected if inhibition is due to the formation of a putative
stable intermediate such as 4.
In all experiments reported in Tables 1 and 2, conducted
at 60°C, the reaction rates remained constant up to ca. 70%
conversion, after which inhibition was observed. Although
the catalyst decomposes at temperatures >80°C, at 60°C we
did not observe a black precipitate, indicating that this sud-
den decrease in reaction rate is not due to decomposition of
the catalyst. Neither the Brønsted acid concentration, the
type of Brønsted acid, the pressure, nor temperature had a
significant effect on the inhibition. Addition of extra N-
allylacetamide at t = 24 h also gave no further reaction,
which rules out a possible need for a threshold substrate
concentration. Only by increasing the concentration of the
ligand to 13.1 mmol L–1 (20 equiv per Pd) were we able to
achieve full conversion. In the absence of tppts the catalyst
decomposed, while when 5 equiv of phosphine were added,
the reaction was already inhibited after 24% conversion. Ad-
dition of more tppts as a solid to the latter reaction mixture
(Experiment 3/1) led to an almost quantitative conversion.
As previously shown by a number of groups, tppts reacts
with unsaturated compounds, such as acrylic acid esters, un-
saturated alcohols and aldehydes, in the presence of an acid,
to afford phosphonium salts (10). However, under our reac-
tion conditions (60°C, [tppts] = 6.5 mmol L–1, [N-
allylacetamide] = 0.13 mol L–1, [HOTs] = 6.5 mmol L–1) the
tppts ligand remained unchanged; no formation of
phosphonium salts from tppts, N-allylacetamide, and HOTs
or phosphine oxide was observed by 31P NMR spectrometry.
We assume that the observed inhibition of the
hydrocarboxylation is caused by the coordination of byprod-
ucts that can only be displaced by an excess of tppts.
By adding 4-acetamidobutyric acid and 3-acetamido-2-
methylpropanoic acid to the reaction mixture at the begin-
ning of the experiment, we again observed an inhibition af-
ter ca. 70% conversion, just as in the case of the previously
described experiments. We conclude that not the
hydrocarboxylation products, but one (or more) of the side
products formed in the competing hydrolysis and allylic sub-
stitution reaction, are responsible for the lower reaction rate.
Further studies should reveal the identity of the actual inhib-
itor.
Finally, we observed that during the hydrocarboxylation
reaction the 1:b ratio decreases with time. Since the rate of
hydrolysis of both carboxylic acid products is similar, the
decrease in regioselectivity must be due to changes in the
coordination sphere of the palladium. We suggest that a co-
ordinating compound can alter the coordination sphere in
such a manner that the amide substituent of allylacetamide
can no longer fulfil its directing effect during the insertion of
the C=C bond into the Pd—hydride bond (Scheme 2). This
phenomenon is also the subject of further investigations.
Experimental details
All manipulations were done under an nitrogen atmo-
sphere. Tppts (11), N-allylacetamide (12), N-acetyl-4-
aminobutyric acid, and N-acetyl-3-amino-2-methylpropanoic
acid (13) were prepared according to previously described
procedures. All other chemicals are commercially available.
HPLC analyses were done using a Phenomenex organic acid
collumn using a solution of 0.01 M trifluoroacetic acid in
water as eluent.
Hydrocarboxylation reactions
Tppts and PdCl2 were codissolved in water and the result-
ing bright yellow solution was transferred into a 300 mL
Hasteloy C Parr autoclave that contains an aqueous solution
of the Brønsted acid, the substrate, and n-BuOH (standard).
The autoclave was closed, the nitrogen atmosphere was re-
placed by CO, and the autoclave was heated. When the con-
tents reached reaction temperature (t = 0) samples were
taken at regular time intervals (t = 0. 2, 4, 6, 8, and 24 h).
The samples were analyzed by HPLC immediately (within
5 min). The products were identified by LC–MS and by
1
comparing the H and 13C NMR spectra and the retention
times with those of the authentic samples.
© 2001 NRC Canada

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Can. J. Chem. Vol. 79, 2001
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In comparison with the Rh–tppts catalyzed hydro-
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N-allylacetamide in water proceeds considerably less facile.
Nevertheless, in comparison with related Pd chemistry in or-
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high regioselectivity towards 4-acetamidobutyric acid under
mild reaction conditions, but the reaction is complicated by
the hydrolysis of amide moieties in the substrate and the
products, a competing allylic substitution reaction and a sud-
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fortunately
a higher CO pressure led to a lower
regioselectivity. The inhibition could be circumvented by in-
creasing the concentration of the ligand.
Acknowledgements
Financial support by the Netherlands foundation for
Chemical Research (NWO-CW) is gratefully acknowledged.
G.B. thanks the Royal Dutch Academy of Sciences for their
financial support for his stay at the Delft Laboratories.
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© 2001 NRC Canada