Mechanistic studies into metal-catalyzed aldoxime to amide rearrangements

Article abstract of DOI:10.1002/adsc.201100650

The metal-catalyzed rearrangement of aldoximes into primary amides is a completely atom economical synthetic method for the preparation of one of the most important functional groups in chemistry. There have been several reports of various metals successfully catalyzing this reaction, however, there are conflicting views as to the mechanism involved. Herein we report new experimental evidence to support the mechanism and whether this is universal to all catalysts reported or metal specific. We also describe our further studies into the mechanism of the nickel-catalyzed acylation of amines with aldoximes. Copyright

Full text of DOI:10.1002/adsc.201100650

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DOI: 10.1002/adsc.201100650
Mechanistic Studies into Metal-Catalyzed Aldoxime to Amide
Rearrangements
C. Liana Allen,a Ruth Lawrence,a Liam Emmett,a and Jonathan M. J. Williamsa,*
a
Department of Chemistry, University of Bath, Claverton Down, Bath, BA2 7AY, U.K.
E-mail: j.m.j.williams@bath.ac.uk
Received: August 15, 2011; Published online: December 8, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adcs.201100650.
Abstract: The metal-catalyzed rearrangement of al-
doximes into primary amides is a completely atom
economical synthetic method for the preparation of
one of the most important functional groups in
chemistry. There have been several reports of vari-
ous metals successfully catalyzing this reaction,
however, there are conflicting views as to the mech-
anism involved. Herein we report new experimental
evidence to support the mechanism and whether
this is universal to all catalysts reported or metal
specific. We also describe our further studies into
the mechanism of the nickel-catalyzed acylation of
amines with aldoximes.
Research efforts have more recently been focussed
towards finding lower cost metal catalysts and proce-
dures that can be run in the absence of undesirable
organic solvents. This is highlighted in the work re-
ported by Mizuno,[6] Nolan[7] and our own group[8,9]
(Scheme 1). Mizuno and co-workers reported a reusa-
ble supported rhodium hydroxide catalyst to work in
this reaction in aqueous conditions, followed later by
Nolan and co-workers reporting a gold/silver co-cata-
lyzed reaction proceeding under solvent-free condi-
tions. We have shown that relatively inexpensive
copper, zinc and indium salts are as effective as the
precious transition metals at catalyzing aldoxime rear-
rangements.
Keywords: aldoximes; amides; mechanism; rear-
rangement
It has been proposed that the mechanism for this
metal-catalyzed rearrangement proceeds via a dis-
crete nitrile intermediate which is formed through de-
hydration of a coordinated oxime species. This is sup-
ported by the frequent detection of a small quantity
of nitrile by-product on analysis of the crude reaction
The efficient synthesis of amide bonds is of great im-
portance in both research and industrial chemistry
due to the prevalence of this group in biologically
active molecules, agrochemicals and polymer chemis-
try.[1] The metal-catalyzed rearrangement of aldox-
imes into primary amides offers a completely atom
economical synthesis of this important chemical
moiety. This reaction can also be run from the alde-
hyde oxidation level with the formation of the aldox-
ime occurring in situ when mixed with hydroxylamine.
The first catalytic conditions for this rearrangement
were reported by Chang and co-workers in 2003.[2]
They found that Wilkinsonꢀs complex (at a catalyst
loading of 5 mol%) efficiently catalyzed the conver-
sion of a range of aldoximes into their corresponding
primary amides at a temperature of 1508C. Following
this initial publication, reports of iridium,[3] rutheni-
um[4] and palladium[5] catalysts performing the rear-
rangement appeared in the literature detailing proce-
dures at lower temperatures.
Scheme 1. Recently reported conditions for aldoxime rear-
rangements into primary amides.
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Mechanistic Studies into Metal-Catalyzed Aldoxime to Amide Rearrangements
Scheme 2. Proposed mechanism A.
Scheme 3. Proposed mechanism B.
mixture and an observed increase in the rate of reac- belled substrates were carried out for a number of the
tion when a catalytic amount of nitrile is added to the metal complexes reported to catalyze the rearrange-
reaction. Specifically, in the rearrangement of benzal- ment.
doxime into benzamide using a supported rhodium
(i) Experiments using 18O-labelled water: Perform-
catalyst, the reaction profile showed the initial forma- ing the rearrangement in the presence of one equiva-
tion of benzonitrile before subsequent formation of lent of 18OH2 would determine whether water was
benzamide.[10] Additionally, the universal inertness of acting as a nucleophile in the hydration of the nitrile
O-alkylated aldoximes and ketoximes towards rear- intermediate. If water did indeed attack the nitrile in-
rangement suggests that the transformation requires termediate, there would be 18O incorporation into the
the presence of both a hydrogen and a hydroxy amide product 2. As most of the catalysts reported
group.
had been used under anhydrous conditions, it was
The corresponding amide is thought to then be gen- first determined which of these catalysts were still
erated through hydration of this nitrile intermediate. active in the presence of water (Table 1).
The most commonly proposed mechanism involves
nucleophilic attack by water (Scheme 2). Both indi-
vidual metal-mediated steps for this dehydration/hy-
dration process are independently reported within the
Table 1. Catalyst screen in the presence of water.
literature to occur in the presence of a range of metal
complexes.[11]
Whilst this dehydration/rehydration process has
become the generally accepted mechanism for an
oxime into amide rearrangement, inconsistencies have
been reported which suggest that water may not be
acting as the nucleophile in the hydration step, for ex-
ample, we observe that, in completely anhydrous con-
ditions, rearrangement into the amide still proceeded.
Additionally, many of the catalysts used to promote
the oxime rearrangement were shown to not be active
in the hydration of a nitrile with water.
A second proposed mechanism involves another
molecule of aldoxime acting as the nucleophile to
attack the metal-bound nitrile species instead of
water (Scheme 3). This could generate a 5-membered
cyclic intermediate, decomposition of which would
yield the primary amide product and another metal-
bound nitrile to continue the catalytic cycle.[12]
Entry Catalyst
Loading
[mol%]
Conversion[a]
[%]
1
RhCl
Ru
[IrCp*I2]2
Pd(OAc)2
(NO3)3
(OTf)3
G
5
100
48
9
100
18
100
3
2[b]
3
G
R
2.5
5
1
4
5
6
G
In
In
N
E
3
7
ZnCl2
10
10
2
8
NiCl2·6H2O
61
100
9
Cu
N
[a]
[b]
1
Determined by H NMR.
Catalytic system also consisted of dppe (0.1 mol%) and
PTSA (0.4 mol%).
In an attempt to elucidate if one of these proposed
mechanisms was operating, reactions involving 18O-la-
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Scheme 4. Rearrangement in the presence of 18OH2.
Table 1 entries 3, 5 and 7 show catalysts which were
inhibited by the presence of water, hence they were
not used in the 18OH2 study. The other 6 catalysts
were then used in the rearrangement of 4-methylben-
zaldoxime 1 in the presence of one equivalent of
18OH2 (Scheme 4).
Scheme 5. Potential products from the reaction of unlabelled
butyraldoxime in the presence of 18O-labelled 3-phenylpro-
panaldoxime.
Table 2. Change in levels of 18O incorporation upon rear-
rangement of 18O-labelled 3-phenylpropanaldoxime.
Analysis of the mass spectral data for the crude re-
action mixtures showed the presence of a peak with
m/z=136.08, corresponding to the unlabelled 4-meth-
ylbenzamide (conversion into amide was confirmed
Entry Catalyst
Loading Oxime 4
[mol%] atom%
18O[a]
Amide 6
atom%
18O[a]
1
by H NMR spectroscopy). However, a peak with an
m/z=138.08 was not observed for any of the catalytic
systems, indicating that incorporation of 18O into the
amide product had not occurred, supporting an alter-
native hydration pathway where water is not acting as
the nucleophile.
1
RhCl
Ru(PPh3)3(CO)H2
Pd(OAc)2
In(NO3)3
ZnCl2
Cu(OAc)2
(PPh3)3
5
1
5
1
10
2
63
63
67
63
63
67
37
22
39
30
28
33
2[b]
3
A
E
4
5
6
G
(ii) Experiments using 18O-labelled aldoximes: In
order to ascertain whether the aldoxime is acting as a
nucleophile to attack a coordinated nitrile species, a
second 18O-labelling study was carried out using 18O-
labelled 3-phenylpropanaldoxime 4 (synthesized in
accordance with literature procedures).[13] Two batch-
ACHTUNGTRENNUNG
[a]
[b]
Determined by mass spectroscopy.
Catalytic system also consisted of dppe (0.1 mol%) and
PTSA (0.4 mol%).
es of 18O-labelled 3-phenylpropanaldoxime were syn- bling of this label had occurred (Figure 1). Although
thesized, with comparable levels of 18O incorporation butyramide did not ionize under the mass spectrome-
achieved within both: 67 and 63 atom%. The level of try conditions, it was possible to determine by
18O incorporation was determined by comparison of 13C NMR spectroscopy that partial incorporation of
the intensity of the parent ions for 16O and 18O aldox- the 18O label into this amide had occurred. Analysis
ime in the mass spectra.
of the peak attributed to the quaternary carbon
The rearrangement of the synthesized 18O-labelled showed two peaks due to the presence of both isotop-
3-phenylpropanaldoxime 4 was performed in the pres- ic forms.
ence of an equimolar quantity of unlabelled butyral-
These results support proposed mechanism B,
doxime 3. In all cases, conversion into amide was de- attack of another molecule of aldoxime on the coordi-
1
termined by H and 13C NMR spectroscopy. In the ab- nated nitrile species. This is consistent with previous
sence of any catalyst, no conversion into amide was results from our group for when the catalyst NiCl2
seen and the level of 18O label incorporation in 3-phe- was used in the same 18O label crossover experiment.
nylpropanaldoxime remained the same.
The results also show that this bimolecular mecha-
Scrambling of the 18O label between the two pri- nism is universal to all of the catalysts studied. How-
mary amide products 5 and 6 would be expected for a ever, it is possible, for some catalytic systems especial-
mechanism involving attack of the coordinated nitrile ly those which are competent for nitrile hydration
intermediate by an oxime. Following this mechanism with water, that the mechanism may depend on the
(Scheme 3), there is the potential for nucleophilic catalyst being used.6
attack onto a nitrile derived from unlabelled butyral-
doxime by 18O-labelled 3-phenylpropanaldoxime to Cu
afford 18O-labelled butyramide and vice versa to The standard reaction conditions developed within
afford 16O 3-phenylpropanamide (Scheme 5).
our lab for the copper-catalyzed rearrangement are
As shown in Table 2, significantly lower 18O label shown below in Scheme 6.[8]
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
incorporation was observed for 3-phenylpropanamide
In order to examine the catalyst efficiency in the re-
relative to the starting oxime, indicating that scram- arrangement, the conversion of 4-methylbenzaldox-
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Mechanistic Studies into Metal-Catalyzed Aldoxime to Amide Rearrangements
would be initial dehydration of a coordinated aldox-
ime to the active metal-bound nitrile species whilst
hydration of the nitrile species is relatively rapid.
Evidence of this has been reported by Johnson and
Miller who found that the reaction can be described
by two consecutive pseudo-first order rate con-
stants.[14] They report the first step to be cleavage of
À
the aldoxime C H bond, which is seen to be depen-
dent on the concentration of the catalyst. This step is
also assumed to be the rate-limiting step, as evidenced
by the strong kinetic isotope effect seen when a deu-
terated oxime is used as a substrate. Chang et al. have
reported that nitrile additives have a self-acceleration
effect on aldoxime rearrangements.[10] Within Changꢀs
study, reaction profiles were obtained for the rear-
rangement of benzaldoxime into benzamide, which in-
dicated that the initial reaction rate was significantly
increased in the presence of nitrile additives. Addi-
tionally, it was observed that the total concentration
of nitrile remained almost constant throughout the re-
action, with the amount of nitrile derived from the al-
doxime increasing as the amount of nitrile additive
decreased. However, even in the presence of a nitrile
additive the rearrangements typically required 6 h to
Figure 1. Mass spectrum of the products of the crossover re-
action, showing the ratio of 16O ([M+1]=150.0975) to 18O
([M+1]=152.0987) within 3-phenylpropanamide.
reach completion using
a
catalytic system of
Rh(cod)(IMes)Cl in the presence of a Brønsted acid.
A
ACHTUNGTRENNUNG
Based on these previous reports and our own find-
ings we reasoned that, if the rate-determining step is
initial dehydration of the aldoxime, then the rate of
amide formation may be enhanced by performing the
rearrangement in the presence of a nitrile additive.
The reaction was carried out in the presence of vary-
Scheme 6. Standard reaction conditions using a copper cata-
lyst.
ime into 4-methylbenzamide was followed over time ing amounts of octanonitrile 7 (Scheme 7) to see if
1
by H NMR spectroscopy (Table 3).
As shown in Table 3, complete conversion into 4- (Table 4).
methylbenzamide can be achieved in the presence of An increase in conversion of 4-methylbenzaldehyde
2 mol% Cu(OAc)2 after 30 min at 1108C and after 1 oxime into 4-methylbenzamide was seen to accompa-
any change in reaction rate could be observed
ACHTUNGTRENNUNG
hour at the reduced temperature of 808C. Examina- ny an increase in the mol% of nitrile additive
tion of the conversions achieved at 1108C illustrate (Table 4). Therefore, it may be concluded that octano-
that there is a non-linear increase in conversion with nitrile is acting as a rate accelerant in this rearrange-
time and that the initial rate of reaction is compara- ment reaction. Although a further nitrile species is
tively slow. This implies that there is an induction generated in the hydration step to complete the cata-
period for catalyst activation at the beginning of the lytic cycle, octanonitrile is itself not regenerated and
reaction. If initiation of the catalytic cycle is the slow can therefore not be classified as a catalyst. For every
step, it is feasible that the rate-determining step
Table 3. Conversion of 4-methylbenzaldoxime into 4-methyl-
benzamide over time.
Entry
Time [min]
Temperature [oC]
Conversion [%]
1
2
3
4
5
6
30
15
10
240
60
30
110
110
110
80
80
80
100
34
17
100
100
43
Scheme 7. Copper-catalyzed rearrangement run in the pres-
ence of octanonitrile.
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Table 4. Variation of reaction rate with mol% nitrile addi-
tive.
our attention to further probing the mechanism of the
nickel-catalyzed acylation of amines using aldoximes
(Scheme 9).[12] There are several possible mechanisms
where the amine attacks at a different stage of the
catalytic cycle (Scheme 10).
The reaction of benzylamine 10 with butyraldoxime
3 in the presence of 5 mol% NiCl2 was followed over
time to see if any significant reaction intermediates
could be observed (Table 5). The results clearly show
a build up of the primary amide 5 (rearrangement
product of butyraldoxime) before the secondary
Entry Amount of nitrile Conversion
Conversion
[mol%]
into 2 [%]
into 8 [%]
1
2
3
4
0
5
10
25
9
–
14
27
70
100
100
100
molecule of octanamide 8 formed in this competing amide 11 is seen in the reaction mixture. Based on
hydration reaction, one molecule of oxime cannot be this observation it is apparent that the aldoxime is un-
rearranged to the corresponding amide but instead re- dergoing the rearrangement into the primary amide
mains as the nitrile. Therefore, the highest potential and it is this species that is then coupling to the
conversion is reduced by an amount equivalent to the amine. Whether it is the free primary amide or the
amount of nitrile additive hydrated.
In order to achieve the desired rate acceleration
effect without reducing the potential conversion of
oxime into amide attainable, the rearrangement of 4-
methylbenzaldehyde oxime was performed in the
presence of 10 mol% of the corresponding nitrile, 4-
methylbenzonitrile 9 (Scheme 8).
After 10 min under these new conditions the reac-
tion was complete, demonstrating a significant rate in-
Scheme 9. Nickel-catalyzed acylation of amines with aldox-
crease when compared with the reaction run without imes.
the nitrile additive, which showed only a 17% conver-
sion into amide after the same length of time.
The results of the 18O-labelling studies provide fur-
ther evidence in support of an aldoxime rearrange-
ment mechanism that proceeds via nucleophilic attack
of a metal-bound nitrile species by an aldoxime mole-
cule (Scheme 3). It has also been found that such a
mechanism is universal to all of the catalysts investi-
gated. Within this mechanism, initial dehydration of a
coordinated aldoxime to form a metal-bound nitrile
species is thought to be the rate-determining step.
This was evidenced by the comparatively slow initial
reaction rate observed in the conversion vs. time
study in conjunction with the rate enhancement ob-
served for rearrangements run in the presence of cat-
alytic quantities of a nitrile.
(iv) Mechanism studies on the nickel catalyzed
acylation of amines with aldoximes: We then turned
Scheme 10. Proposed mechanisms of secondary amide for-
mation in the nickel-catalyzed acylation of amines with al-
Scheme 8. Rearrangement of 4-methylbenzaldoxime in the
presence of 4-methylbenzonitrile.
doximes.
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Mechanistic Studies into Metal-Catalyzed Aldoxime to Amide Rearrangements
Table 5. Reaction of benzylamine and butyraldoxime over
time.
conversion into the secondary amide was observed
(Scheme 12).
These results clearly indicate that the oxime is in
some way acting catalytically in the reaction between
the primary amide and the amine. An increased rate
of reaction is seen when the aldoxime species is used
in slight excess (compared with the amine), a result
which can be justified by the oxime acting catalytical-
ly to couple the primary amide and amine. Potentially,
this could either be hydrogen bonding activation of
the amide by the oxime or by nucleophilic catalysis.
The mechanism of the nickel-catalyzed acylation of
amines with aldoximes has been shown to proceed via
the primary amide (formed by the rearrangement of
the aldoxime). The subsequent coupling of the pri-
mary amide and amine has been shown to be cata-
lyzed by the aldoxime itself. Further investigation into
this interesting property of oximes is currently under-
way in our laboratory.
Entry Reaction Conversion into 18 Conversion into 28
time [h]
amide 5 [%]
amide 11 [%]
1
3
4
5
6
2
4
6
16
24
35
48
26
2
18
52
74
98
96
4
metal-bound primary amide that is attacked was un- Experimental Section
clear at this stage.
The reaction of butyramide 12 with benzylamine Experimental Procedure I: 18OH2 Additive Study
was run with and without the NiCl2 catalyst to see if
the metal was involved in the primary amide and
oven-dried carousel tube along with the appropriate cata-
4-Methylbenzaldoxime (0.405 g, 3 mmol) was added to an
lyst. 18O-labelled water (18OH2) (0.07 mL, 4 mmol,
97 atom% 18O) and anhydrous toluene (1.5 mL) were
added, then the carousel tube was sealed and if necessary
purged with nitrogen. The reaction mixture was heated at
1108C for 24 h, then allowed to cool to room temperature
before the solvent was removed under vacuum and the
amine coupling. In both reactions, a similar conver-
sion into the secondary amide was seen; 39% with
the nickel catalyst and 32% without (Scheme 11).
This suggests not only that the nickel is not involved
in this step of the mechanism but also that direct
attack of the amine on the free primary amide is not
the major reaction pathway in operation.
1
crude reaction mixtures were analyzed by H and 13C NMR
and mass spectroscopy.
When the same reaction was run with a catalytic
amount of butyraldoxime, a significant increase in
Experimental Procedure II: 18O-Labelled Aldoxime
Study
18O-labelled 3-phenylpropanal oxime (0.038 g, 0.25 mmol)
and unlabelled butyraldehyde oxime (0.024 mL, 0.25 mmol)
were added to an oven-dried carousel tube followed by the
appropriate catalyst. The tube was then sealed and if neces-
sary purged with nitrogen for 10 min. Anhydrous toluene
(0.5 mL) was then syringed into the tube and the reaction
mixture was heated at reflux (1108C) for 24 h. After allow-
ing the reaction mixture to cool to room temperature, it was
filtered through a short plug of Celite, eluting with dichloro-
methane and was then concentrated under vacuum. The iso-
lated material was analyzed by 1H NMR, 13C NMR and
mass spectroscopy.
Scheme 11. Reaction of butyramide and benzylamine.
Experimental Procedure III: Aldoxime-Catalyzed
Transamidation
Butyramide (0.087 g, 1 mmol), benzylamine (0.11 mL,
1 mmol) and 3-phenylpropionaldoxime (0.015 g, 0.1 mmol)
were added to an oven-dried carousel tube. p-Xylene
(1 mL) was added to the tube which was then sealed and
heated at reflux (1558C) for 20 h. The reaction mixture was
then allowed to cool to room temperature before the solvent
Scheme 12. Reaction of butyramide and benzylamine in the
presence of 10 mol% butyraldoxime.
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C. Liana Allen et al.
COMMUNICATIONS
was removed under vacuum and the crude reaction mixture
[6] a) H. Fujiwara, Y. Ogasawara, K. Yamaguchi, N.
Mizuno, Angew. Chem. 2007, 119, 5294–5297; Angew.
Chem. Int. Ed. 2007, 46, 5202–5205; b) H. Fujiwara, Y.
Ogasawara, M. Kotani, K. Yamaguchi, N. Mizuno,
Chem. Asian J. 2008, 3, 1715–1721.
1
was analyzed by H NMR and 13C NMR spectroscopy.
Acknowledgements
[7] R. S. Ramꢂn, J. Bosson, S. Diez-Gonzꢃlez, N. Marion,
S. P. Nolan, J. Org. Chem. 2010, 75, 1197–1202.
[8] S. K. Sharma, S. D. Bishopp, C. L. Allen, R. Lawrence,
M. J. Bamford, A. A. Lapkin, P. Plucinski, R. J. Watson,
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We thank the EPSRC for the award of a studentship (to C.
L. A.) administered through the Doctoral Training Account,
Dr. John Lowe for assistance with NMR studies and Dr.
Anneke Lubben for assistance with mass spectrometry stud-
ies.
[9] C. L. Allen, C. Burel, J. M. J. Williams, Tetrahedron
Lett. 2010, 51, 2724–2726.
[10] M. Kim, J. Lee, H.-Y. Lee, S. Chang, Adv. Synth. Catal.
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