Significant self-acceleration effects of nitrile additives in the rhodium-catalyzed conversion of aldoximes to amides: A New mechanistic aspect

Article abstract of DOI:10.1002/adsc.200900251

It was found that the catalytic activity of rhodium complexes was highly sensitive to the type of N-heterocyclic carbene (NHC) ligands in the conversion of aldoximes to amides. Among those species examined, the (cyclooctadiene) rhodium chloride-carbene complex Rh(cod)(IMes)Cl exhibited the highest reactivity when it was employed in combination with a Bronsted acid, thus allowing mild reaction conditions. A significant rate acceleration effect resulting from the addition of nitrile additives was also observed. With the new protocol, the substrate scope of aldoximes has been widely expanded to include sterically congested and electronically varied derivatives. On the basis of detailed mechanistic studies, it is proposed that the reaction proceeds mainly via intramolecular electrophilic addition of aldoxime to rhodium-bound nitrile, which is different from the generally postulated two-step route: dehydration of aldoxime to nitrile followed by hydration of the latter intermediate.

Full text of DOI:10.1002/adsc.200900251

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DOI: 10.1002/adsc.200900251
Significant Self-Acceleration Effects of Nitrile Additives in the
Rhodium-Catalyzed Conversion of Aldoximes to Amides:
A New Mechanistic Aspect
a
a
a,
a,
Min Kim, Jinwoo Lee, Hee-Yoon Lee, * and Sukbok Chang *
a
Department of Chemistry, Korea Advanced Institute of Science & Technology (KAIST), Daejeon, 305-701, Korea
Fax : (+82)-42-350-2810; e-mail: leeheeyoon@kaist.ac.kr or sbchang@kaist.ac.kr
Received: April 11, 2009; Revised: June 8, 2009; Published online: July 11, 2009
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200900251.
Abstract: It was found that the catalytic activity of
rhodium complexes was highly sensitive to the type
of N-heterocyclic carbene (NHC) ligands in the
conversion of aldoximes to amides. Among those
species examined, the (cyclooctadiene)rhodium
To avoid these drawbacks, a range of improved proce-
dures has been disclosed such as methods using super-
[
5]
[6]
critical water, a vapor-phase process, ionic liq-
[
7]
[8]
uids, or transition metal catalysis. Most recently,
Mizuno et al. revealed a facile catalyst system of sup-
ported rhodium hydroxide for the synthesis of pri-
chloride-carbene complex Rh AHCUTNGREUNNG( cod) ACHTUNGTRNENUNG( IMes)Cl exhib-
[
9]
ited the highest reactivity when it was employed in
combination with a Brønsted acid, thus allowing
mild reaction conditions. A significant rate accelera-
tion effect resulting from the addition of nitrile ad-
ditives was also observed. With the new protocol,
the substrate scope of aldoximes has been widely
expanded to include sterically congested and elec-
tronically varied derivatives. On the basis of de-
tailed mechanistic studies, it is proposed that the re-
action proceeds mainly via intramolecular electro-
philic addition of aldoxime to rhodium-bound ni-
trile, which is different from the generally postulat-
ed two-step route: dehydration of aldoxime to
nitrile followed by hydration of the latter inter-
mediate.
mary amides from aldoximes in water.
We previously reported that aldoximes can be read-
ily dehydrated under mild conditions to afford nitriles
with excellent selectivity when [RuCl ACHTUNGTRENUNG( p-cymene)]
2 2
[
10a]
was used as a catalyst.
In line with this research,
we tried to develop the direct conversion of aldox-
imes to amides, and found that the Wilkinson catalyst,
[10b]
RhCl ACHTUNGTENRUNG( PPh ) , can carry out this important process.
3 3
However, satisfactory product yields in the latter
transformation could be secured only at high temper-
atures (1508C) with large amounts of the catalyst.
The Rh-catalyzed conversion of aldoxime to amide
was assumed to proceed via a two-step route: dehy-
dration of aldoxime to nitrile followed by hydration
of the latter intermediate.
Described herein is our recent development of a
significant improvement of the reaction conditions
Keywords: aldoximes; amides; N-heterocyclic car-
benes; rhodium; self-acceleration effect
using an Rh AHCNUTGTERNNUN(G NHC) catalyst (NHC=N-heterocyclic
carbene). More importantly, a significant self-acceler-
ation effect of nitrile additives was observed leading
to a new mechanistic proposal that the conversion
proceeds mainly via intramolecular attack of aldox-
The interconversion between carbonyl compounds is ime onto metal-bound nitrile, which is different from
one of the most important transformations that have the generally recognized two-step route.
[
1]
attracted great interest of organic chemists. In this
At the outset of our studies, we tried to improve
context, the conversion of readily available oximes to the reaction conditions in the transformation of 4-bro-
[
11]
the corresponding amides has been utilized as a mobenzaldoxime to 4-bromobenzamide (Table 1).
[
2]
highly versatile and practical synthetic tool. In fact, While no conversion was observed in the absence of
e-caprolactam is produced in industry from cyclohexa- metal catalysts at 808C, the combined use of certain
none oxime for the chemical manufacture of Nylon- Rh metal species and Brønsted acids resulted in de-
[3]
6.
However, conventional conditions for the rear- tectable reactivity. For example, when 1.0 mol% of
rangement usually employ high temperatures and/or [Rh(cod)Cl] (cod=1,5-cyclooctadiene) was used
AHCTUNGTRENNUNG
2
[
4]
strong acidic reagents thus generating by-products.
along with p-toluenesulfonic acid (TsOH/H O,
2
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Min Kim et al.
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[a]
Table 1. Screen of reaction conditions.
Scheme 1. ACHUTNRGNENUG( Rh-NHC)-catalyzed reaction of aldoximes.
ime substrates display a significant influence on the
reaction efficiency. In fact, whereas the reaction was
best performed with electron-withdrawing benzaldox-
imes, those with electron-rich substrates displayed
lower efficiency (e.g., 34% yield from 4-methoxyben-
zaldoxime). In addition, steric congestion around the
reacting site gave significantly lower yields (e.g., 9%
from 2-bromobenzaldoxime).
During the course of our subsequent studies, we un-
expectedly found that the addition of catalytic
amounts of nitrile compounds resulted in a significant
increase of the reaction efficiency (Scheme 2). For in-
[b]
Entry
Catalyst
–
Additive
Yield [%]
1
2
3
4
5
6
7
8
9
1
1
–
–
–
<1
<1
5
RhCl ACHTUNGTRENNUNG( PPh )
3 3
[Rh
[Rh
–
ACHTUNGTRENNUNG( cod)Cl]
2
2
A( cod)Cl]
C
H
T
U
N
G
T
R
E
N
N
U
N
G
TsOH/H O
TsOH/H O
TsOH/H O
TsOH/H O
–
–
28
2
<1
2
[c]
Rh
Rh(cod)
Rh(cod)(IMes)Cl
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(cod)
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(IMes)Cl
(IMes)Cl
90
2
[
d]
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
<1
17
2
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
IMes
IMes
<1
<1
0
1
TsOH/H O
TsOH/H O
2
[c]
Rh
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(cod)
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(IPr)Cl
75
2
[
a]
Reaction conditions: A mixture of 4-bromobenzaldoxime
(
(
1.0 mmol), Rh catalyst (1.0 mol%), and additive
10 mol%) in solvent (0.25 mL) was stirred for 6 h at
8
08C.
[
[
[
b]
NMR yield (internal standard: 1,3-benzodioxole).
Isolated yield.
DMF was used as a solvent instead of toluene.
c]
d]
1
0 mol%), 4-bromobenzamide was obtained in 28%
[12]
yield (entry 4). On the other hand, the acid alone Scheme 2. Nitrile effect on the conversion.
did not carry out the reaction (entry 5). A significant
improvement was observed especially when NHC-de-
rived rhodium catalysts were employed in combina- stance, when 4-hydroxybenzaldoxime was treated
[
13]
tion with a Brønsted acid.
The most effective species among those Rh-NHCs the desired amide was obtained in 50% yield after 6 h
examined was Rh(cod)(IMes)Cl in toluene at 808C. In contrast, a dramatically increased yield
entry 6). On the other hand, the same reaction in (99%) was achieved by adding 0.05 equivalents of 4-
polar solvents was almost ineffective (entry 7) and the hydroxybenzonitrile to the reaction mixture (condi-
use of Rh(cod)(IPr)Cl resulted in reduced yield under tions B). Interestingly, the same amount of the added
the otherwise identical conditions (entry 11).
should be noted that the Rh-NHC species utilized in covered after the reaction.
with the above optimized conditions (conditions A),
A
H
U
G
R
N
U
G
ACHTUNGTRENNUNG
[
14]
(
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
ACHTUNGTRENNUNG
[
15]
It nitrile (0.05 equivalents to the amide product) was re-
this study are easily prepared and highly stable to air,
The additive effect of nitrile was quantitatively ana-
and the amide forming-reaction can be carried out lyzed by obtaining reaction profiles in the conversion
under ambient conditions without requiring an inert of benzaldoxime to benzamide under different condi-
atmosphere.
tions (Figure 1). It is noteworthy that the initial rate is
When the optimized conditions were next applied significantly increased by the presence of nitrile addi-
to a range of aldoximes, it was immediately found tive. For instance, while a complete conversion was
that certain types of substrates were rather slowly re- observed within 6 h in the presence of benzonitrile
acting to afford only poor yields (Scheme 1). It turned (5 mol%), the initial reaction rate was much lower
out that both electronic and steric property of aldox- without the nitrile complement giving only 19% con-
1808
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Significant Self-Acceleration Effects of Nitrile Additives
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Figure 1. Reaction profiles in the conversion of benzaldoxime to benzamide under various conditions.
version within the same reaction period (6 h) al-
though it reaches a full conversion after 24 h.
It is first assumed that the neutral rhodium precur-
sor is converted to its cationic species I by the action
[
19]
On the other hand, no conversion was observed of a Brønsted acid, and then the nitrile additive is
with ketoximes or O-alkylaldoximes. Although hydra- associated with the cationic Rh complex giving II to
[
20]
tion of nitriles to amides is known to be catalyzed by release a cod ligand. We propose that the nitrogen
[
9b,12a,16]
[17]
rhodium
inert to the hydration when it was allowed to react center to afford III followed by an intramolecular
with water in the presence of [Rh(cod)(NHC)Cl]. attack of the aldoxime oxygen onto the metal-bound
When benzaldoxime (1a) was subjected to the reac- nitrile leading to an O-(iminophenylmethyl)oximate
or other metal species,
nitrile was atom of the aldoxime coordinates to the Rh metal
[
21]
A
H
U
G
R
N
U
G
ACHTUNGTRENNUNG
[
22]
tion conditions in the presence of 0.05 equivalents of rhodium chelate IV. Alternatively, although aldox-
-bromobenzonitrile (2a), a mixture of two amides of imes can bind to the metal complex prior to nitrile as
benzamide and 4-bromobenzamide was obtained in reported by Leusink, the obserbed rate acceleration
addition to benzonitrile in a quantitative manner effects of nitrile additives should favor the formation
Scheme 3).
The above results in combination with the reaction
4
[
21]
[18]
(
of II followed by oxime coordination to form III.
In fact, it is known that metal-bound nitrile is acti-
profiles suggest that the main operating mechanistic vated to a significant extent making it more electro-
[
23]
path of the present process may be different from the philic,
thus facilitating subsequent oxime addition
[
24]
generally accepted two-step pathway, in which dehy- albeit with a stoichiometric manner. Next, it is pro-
dration of aldoximes to nitriles was assumed to occur posed that a nitrile molecule is released from the imi-
first followed by hydration of the nitrile intermedi- noacyl species IV to provide a putative benzimidic
[9b]
[9]
ACHTUNGTRENNUNGa tes. In order to accommodate the nitrile effects in rhodium chelate V. It can be postulated that two ni-
our present case, the following mechanistic proposal trile ligands then exchange a bidentate benzimidic
is presented using benzaldoxime and benzonitrile as a acid from V releasing benzimidic acid VI that is read-
model
substrate
and
additive,
respectively ily tautomerized to benzamide. The present mechanis-
tic proposal might also be supported by the observa-
(
Scheme 4).
Scheme 3. Cross-over reaction.
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Min Kim et al.
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Scheme 4. Nitrile-mediated mechanistic proposal of the Rh-catalyzed aldoxime conversion.
tion that nitriles were inert to the direct hydration trile-assisted amide formation since the total concen-
[
18]
with water in the absence of aldoximes.
tration of nitriles stays constant only under the cur-
To further verify the new mechanistic pathway, the rent path among other possible mechanistic alterna-
total amount of nitriles in the conversion of an aldox- tives.
ime to an amide in the presence of a different type of
Although we argue that the nitrile effect may origi-
nitrile additive was monitored over the reaction prog- nate from the mechanistic consideration in Scheme 4,
ress (Figure 2). While the amount of the added nitrile other possibilities cannot be completely excluded at
decreased as it was converted to the corresponding least for some types of aldoxime substrates, in which
amide, the amount of the nitrile that corresponds to the conventionally accepted two-step pathway would
the starting aldoxime increased. As a result, the total also be accompanied to operate under either metal-
amount of the two nitriles remained almost constant mediated or metal-free conditions.
along the reaction progress. It can be postulated that
The new protocol of the nitrile-facilitated synthesis
this result strongly suppports our proposal of the ni- of amides could be successfully applied to a range of
Figure 2. Concentration profile of the added and in situ generated nitriles.
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Significant Self-Acceleration Effects of Nitrile Additives
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Table 2. Rh
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(NHC)-Catalyzed conversion of aldoximes to over aliphatic nitriles to result in diminished effects of
[a]
[
21]
amides in the absence and presence of nitriles.
nitrile additives.
In summary, a new efficient procedure for the for-
mation of amides from aldoximes has been developed
using the combined use of Rh ACHUTNGRENNUG( cod) ACHTUNGERTNNUNG( IMes)Cl, a
Brønsted acid, and a complementary nitrile. It is pro-
posed that the conversion proceeds mainly via intra-
molecular electrophilic addition of aldoxime to rhodi-
um-bound nitrile, which is different from the general-
ly postulated two-step route of dehydration of aldox-
ime to nitrile followed by hydration of the latter inter-
mediate.
[
b]
Entry
R
Yield [%]
Nitrile Additive
[c]
No Nitrile
1
2
3
4
5
6
7
8
9
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(4-NO )C H
62
70
92
90
59
34
48
9
96
99
99
99
95
88
95
90
2
6
4
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(4-Cl)C H
6
4
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(4-CF )C H
3
6
4
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(4-Br)C H
6
4
C H5
6
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(4-MeO)C H
6
4
Experimental Section
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(3-MeO)C H
6
4
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(2-Br)C H
6
4
2-Thienyl
63
50
50(92)
99
99
51 (94)
General Procedure for the Rh-Catalyzed Conversion
of Aldoximes to Amides
1
1
0
1
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(4-HO)C H
6
4
[
d]
CH
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(CH )
3
2 5
To an oven-dried screw-top vial were added benzaldoxime
(121 mg, 1.0 mmol), Rh ACHTUNGTREUNNNG( cod) CAHTUNGTRNENUNG( IMes)Cl (5.5 mg, 0.01 mmol),
[
a]
Reaction conditions: aldoxime (1.0 mmol), Rh catalyst
(
(
1.0 mol%), and TsOH/H O (10 mol%) in toluene
0.25 mL) for 6 h at 808C.
benzonitrile (5.2 mg, 0.05 mmol), p-toluenesulfonic acid
monohydrate (19.0 mg, 0.1 mmol) and toluene (0.25 mL).
The reaction mixture was stirred for 6 h at 808C and then
diluted with ethyl acetate. Organic solvents were removed
under vacuum and then benzamide was isolated by a silica
gel column chromatography (ethyl acetate/n-hexane, 1:1);
yield: 115 mg (95%).
2
[
[
b]
Isolated yield.
c]
Complementary nitrile (0.05 equiv. to each aldoxime)
was added.
Yields in parenthesis were obtained from reactions run-
ning for 12 h at 808C under otherwise identical condi-
tions.
[
d]
Supporting Information
substrates (Table 2). In each case, the complementary
nitrile was added in
0.05 equivalents to the corresponding aldoximes). tion.
Experimental details and spectroscopic characterization of
catalytic amount sleceted compounds are given in the Supporting Informa-
a
(
The nitrile additive effect was displayed with most ar-
ylaldoximes examined although the extent of the
effect was dependent on the substituents. For in-
stance, the yield of 2-bromobenzamide was dramati- Acknowledgements
cally increased by adding catalytic amounts of 2-bro-
This research was supported by the KRF grant (KRF-2006-
12-C00587). We acknowledge the Korea Basic Science Insti-
tute for the mass spectrometric analyses.
mobenzonitrile (entry 8). In general, substrates bear-
ing electron-donating substituents showed more pro-
nounced nitrile effects. Heteroaromatic amides could
also be obtained in excellent yield using the devel-
oped protocol (entry 9).
3
Quite interestingly, in contrast to arylaldoximes, ali-
phatic reactants were not much influenced by the
presence of nitrile additives (entry 11). However, it
should be mentioned that satisfactory yields of ali-
phatic amides can be secured by allowing the transfor-
mation to run for a longer reaction time irrespective of
the presence of nitrile additives.
Further studies would be necessary to reason about
the less pronounced nitrile effects in the case of ali-
phatic aldoximes. However, at the moment, it can be
speculated that coordination of an aliphatic nitrile to
the putative aliphatic iminoacyl rhodium species (V in
Scheme 4) would be less facile compared to the aryl
analogues. Alternatively, coordination of aliphatic
oximes to the Rh metal center can be more favored
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Adv. Synth. Catal. 2009, 351, 1807 – 1812