Chlorotropylium Promoted Conversions of Oximes to Amides and Nitriles

Article abstract of DOI:10.1002/ejoc.201901537

Chlorotropylium chloride as a catalyst for the transformations of oximes, ketones, and aldehydes to their corresponding amides and nitriles in excellent yields (up to 99 %) and in short reaction times (mostly 10–15 min). Oximes were electrophilically attacked on the hydroxyl oxygen by chlorotropylium. The produced tropylium oxime ethers were the key intermediates, of which the ketoxime ether led to amide through Beckmann rearrangement, and the aldoxime ether led to nitrile by nitrogen base DBU assisted formal dehydration. This chlorotropylium activation protocol offered general, mild, and efficient avenues bifurcately from oximes to both amides and nitriles by one organocatalyst.

Full text of DOI:10.1002/ejoc.201901537

A Journal of
Accepted Article
Title: Chlorotropylium promoted conversions of oximes to amides and
nitriles
Authors: Jiaxi Xu, Yu Gao, Zhenjiang Li, Jingjing Liu, Tianfo Guo, Lei
Zhang, Haixin Wang, Zhihao Zhang, and Kai Guo
This manuscript has been accepted after peer review and appears as an
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the VoR from the journal website shown below when it is published
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content of this Accepted Article.
To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201901537
Link to VoR: http://dx.doi.org/10.1002/ejoc.201901537
Supported by

European Journal of Organic Chemistry
10.1002/ejoc.201901537
Chlorotropylium promoted conversions of oximes to amides and
nitriles
Dr. Jiaxi Xu, Dr. Yu Gao, Prof. Dr. Zhenjiang Li*, Dr. Jingjing Liu, Dr. Tianfo Guo, Lei Zhang, Dr.
Haixin Wang, Zhihao Zhang and Prof. Dr. Kai Guo*
State Key Laboratory of Materials-Oriented Chemical Engineering, College of Biotechnology and
Pharmaceutical Engineering, Nanjing Tech University, 30 Puzhu Road South, Nanjing 211816, China;
Corresponding Author
Kai Guo and Zhenjiang Li; E-mail: guok@njtech.edu.cn; zjli@njtech.edu.cn. Tel +86 25 5813 9926; Fax
+
86 25 5813 9935.
Abstract
Chlorotropylium chloride as trigger/catalyst promoted transformations of oximes of ketones and
aldehydes to their corresponding amides and nitriles in excellent yields (up to 99%) by short
reaction times (mostly 10–15 min). Oximes were attacked electrophilically on the hydroxyl
oxygen by chlorotropylium, the produced tropylium oxime ethers were the key intermediates, of
which the ketoxime ether led to amide via Beckmann rearrangement, and the aldoxime ether led
to nitrile via nitrogen base DBU assisted formal dehydration. This chlorotropylium activation
protocol offered general, mild, and efficient avenues bifurcately from oximes to both amides and
nitriles by one organocatalyst.
Keywords
Amides; Organocatalysis; Rearrangement; Oximes; Tropylium
1
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European Journal of Organic Chemistry
10.1002/ejoc.201901537
Graphic Abstract
2
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European Journal of Organic Chemistry
10.1002/ejoc.201901537
Introduction
Cycloheptatrienylium (tropylium) ion was one of the most stable carbocations that was firstly
1
prepared in a form of tropylium bromide by Doering in 1954. Tropylium ion possessed
aromaticity and showed remarkable stability due to its 6-electron structure that obeying
2
Hꢀckel’s [4n + 2] rule. Since Doering’s pioneering work on the highly stable tropylium that
3
4-8
ushered non-benzenoid aromatics chemistry, series of derivatives of tropylium, together with
its minimal analog non-benzenoid cyclopropenylium cation (cyclopropenium ion),9, 10 in which n
0, constantly attracted interests in both theoretical and synthetic organic chemistry12, 13. In
1
1
=
14
exploring stable carbocations in promoting or catalyzing organic reactions, triarylmethyliums
emerged as Lewis acidic catalysts15-20 recently; in parallel, another two types of stable
carbocations, cyclopropenium and tropylium (Figure 1), were promising in mediating/catalyzing
organic transformations via both the Lewis acidity of the carbenium carbon centers and the
aromaticity6, 21 of the non-benzenoid aromatic rings.
Figure 1. Two non-benzenoid carbocyclic aromatic ions, cyclopropenium and tropylium.
14
Despite being the ‘first bottled’ stable carbocation, tropylium and its analog ions have rarely
been applied in practical synthetic practice9, 22-24 until Lambert disclosed the first tropylium
25
mediated α-cyanation of amines . Inspired by an insightful mechanism of
dearomatization/rearomatization steps suggested by Lambert in gem-dichlorocyclopropene
furnished nucleophilic substitutions25-28, Nguyen proposed dichlorocycloheptatriene promoted
nucleophilic substitutions that underwent first catalytic tropylium mechanism29-32
.
Oxime undergoes Beckmann rearrangement33 that constructed amide linkage34 vital to
lactam production and provided monomers to nylon industries, i.e. for nylon 6 and nylon 12.35, 36
Beckmann rearrangement usually promoted by acidic catalysts33, 35 such as in the application of
3
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European Journal of Organic Chemistry
10.1002/ejoc.201901537
fuming sulfuric acid and sulfonic acids in commercial routes, however, mild and tolerant organic
catalysis in oxime transformations37-47 were flourished (Figure 2) since Ishihara and Yamamoto
48
disclosed the first organocatalyzed Beckmann rearrangement .
Figure 2. The typical regents in organo-catalyzed Beckmann rearrangements.
49
In contrast to the rearrangements of aldoximes to primary amides or direct dehydration of
aldoximes to nitriles50-53, rare example54 demonstrated organocatalytic transformations of
aldoximes to nitriles without dehydration nor lead to primary amides. Based on our previous
works on organocatalytic Beckmann rearrangement42 and Swern-type oxidation26 that were
governed by aromaticity of the gem-dichloro compounds catalysts, we were interested in
transformations of oximes of ketones and aldehydes to their corresponding amides and nitriles
+
− 32, 55, 56
under promotion of chlorotropylium chloride (ClTp ·Cl )
(Scheme 1).
Scheme 1. Chlorotropylium chloride in promoting transformations of oximes to amides and
nitriles.
(a) Transformations of oximes of ketones and aldehydes to their corresponding amides and
+
−
nitriles under promotion of chlorotropylium chloride (ClTp ·Cl ). (b) The reversible isomerization
between 7,7-dichlorocyclohepta-1,3,5-triene (1, C Cl ) and chlorotropylium chloride (2,
H
7 6
2
+
−
+
−
ClC
7
H ·Cl , ClTp ·Cl ); reversal formation of dichlorocyclohepta-1,3,5-trienes from 2 would
6
produce four isomers which were detailed in Scheme S1.
4
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European Journal of Organic Chemistry
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Results and Discussion
Beckmann rearrangement49, 57-60 was triggered via heterolysis of the N–O bond in the oxime
where an electrophile activated the oxime function. The seemingly straight forward protic acid
catalysis mechanism33 was later revealed much more sophisticated61-64. Considering the
+
electrophilic nature of chlorotropylium (ClTp ) at the carbenium center, we firstly explored the
+
−
activation potential of the ClTp ·Cl (2) in effecting the Beckmann rearrangements (Table 1).
Table 1. The optimization of the Beckmann rearrangement reaction conditions.a
Entry
Solvent
MeCN
MeCN
MeCN
MeCN
Toluene
DMF
2 (mol%) T/°C Time/min yield/%b
1
2
3
4
5
6
7
8
9
10
rt
120
10
40
99
98
80
60
30
23
0
10
80
80
80
80
80
80
80
66
83
80
80
5
10
1
60
5
30
5
30
MeNO
2
5
120
120
30
1,4-dioxane
THF
5
5
13
90
0
1
0
1
DCE
5
0
30
1
MeCN
120
1
2c
MeCN
0 (5%)c
120
0
a
mmol of the acetophenone oxime 3a in solvent (5 mL). b isolated yields. HCl (36−38 wt %, 5
c
1
mol%) was added in place of 2 in promoting the reaction.
Acetophenone oxime (3a) was selected as a model substrate for the preliminary examination
of the performance of chlorotropylium chloride in promoting the reactions. Series trial reactions
to screen the optimal conditions were shown in Table 1. Substoichiometric amount (10 mol %)
+
−
of ClTp ·Cl (2) in acetonitrile (MeCN) promoted transformation of acetophenone oxime (3a) to
N-phenylacetamide (4a) by 40 % yield (Table 1, entry 1) at room temperature by 2 h. Raising the
5
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European Journal of Organic Chemistry
10.1002/ejoc.201901537
temperature to reflux of the solvent (80 °C) received quantitative yield (99%) in remarkably short
reaction time of 10 min (entry 2). Lower loading of chlorotropylium chloride at 5% and 1% (entries
3
and 4) showed excellent yields of 98% (by 10 min) and 80% (by 60 min), respectively.
Encouraged by the promising results, common nonprotic organic solvents were screened by 5%
of 2 at 80 °C (entries 5–8) or around their corresponding boiling temperatures (entries 9 and 10).
Ethers resulted unfavorable yields (entries 8 and 9), however, toluene and 1,2-dichloroethane
(DCE) (entries 5 and 10) exhibited good to excellent outcomes by 30 min.
Scheme 2. Generality and scope of chlorotropylium chloride promoted Beckmann rearrangement.a
a
b
Reactions were performed with 1 mmol of 3 in MeCN (5 mL). Isolated yield. The oxime 3j was formed
as mixtures of E/Z isomers (4:1).
To exclude background reactions, blank control without 2 (entry 11) revealed no conversion
6
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European Journal of Organic Chemistry
10.1002/ejoc.201901537
of the substrate 3a. Possibility of the byproduct HCl in promoting the rearrangement (vide infra,
Scheme 4) was ruled out by using 5% of hydrochloric acid (entry 12) in place of 2, no
+
−
rearrangement of 3a was observed. Hence, optimal conditions of ClTp ·Cl (2) at 5% loading in
reflux MeCN (80 °C) were employed in the following Beckmann rearrangements.
The scope and limitation of chlorotropylium chloride in activating Beckmann rearrangement
was explored by testing typical ketoximes as the substrates under the optimal conditions
(Scheme 2). It was exciting that arrays of aromatic and aliphatic ketoximes were converted
smoothly to the corresponding amides in good to excellent yields (up to 99%). Acetophenone
oximes with electron-donating groups, including methoxy (3b−3d), hydroxyl (3e), and methyl (3h)
on the phenyl ring, all offered the corresponding acetanilides in excellent yields within 10 min.
Acetophenone oximes with electron-withdrawing groups were also conductive under this
protocol. N-(4-Chlorophenyl)acetamide (4f) and N-(4-bromophenyl)acetamide (4g) were
obtained in 33% and 30% yields by 3 h. Furthermore, 2-acetonaphthone oxime 3i featuring an
aromatic fused ring was amenable to furnishing its rearranged product 4i with 88% yield in 20
2
min. In conjunction with methyl, several diversified R groups of oximes 3, including cyclopropyl
2
(
3j), an aliphatic chain (R = MeCO
2
(CH
2
) −, 3k) and phenyl (3m), were studied and engaged in
2
the rearrangements with high yields. One exemplar symmetrical ketoxime 3l was converted into
amides 4l with 96% yields in 10 min.
Commercial relevant oximes were evaluated under the promotion of chlorotropylium chloride.
4
-Acetaminophenol 4e, that is active pharmaceutical ingredient to Paracetamol, received 93%
yield in 10 min. Two aliphatic cyclic oximes 3n and 3o, which were precursors to nylon 6 and
nylon 12, showed sharp contrast. Cyclohexyl oxime 3n failed to obtain acceptable yield (19% by
2
h) of ε-caprolactam 4n, notwithstanding, cyclododecyl oxime 3o was rearranged into the target
ω-laurolactam 4o by 95% yield in 10 min. Finally, to leverage chlorotropylium chloride activation
strategy, we examined its workability in a complex substrate oxime of pregnenolone acetate
(Scheme 2, 3p). One gram of 3p was successfully converted into amide 4p in 90% yield by 2 h.
7
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European Journal of Organic Chemistry
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We conducted the reaction E : Z mixture of (4-hydroxyphenyl)(phenyl)methanone oxime (3q)
1.5 : 1) under these heating reaction condition in Figure S2. As expected, exclusive
(
transmigration was observed, leading to a 1.5 : 1 mixture of amide products in 80% yield. This
result suggests that oxime isomerization does not occur under these reaction conditions.
Intrigued by this chlorotropylium chloride promoted Beckmann rearrangement of ketoxime,
we were interested in aldoximes as substrates under the conditions. To our surprise, no primary
amide usually viable from Beckmann rearrangement of aldoxime65 was observed, nor
dehydration of aldoximes to nitriles51, 52 were detected. To obviate ordinary routes to nitrile by
harsh dehydration steps, we turned our exploration to chlorotropylium chloride furnished
cleavage of the N–O bond in aldoxime that would be facilitated by a precedent proton abstraction
66
53
step by Brønsted bases.
Scheme 3. Generality and scope of chlorotropylium chloride promoted aldoximes to nitriles.a
a
Reactions were performed with 2 mmol of 5 in MeCN (10 mL), aldoxime 5 (1.0 equiv.), tropone (1
8
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European Journal of Organic Chemistry
10.1002/ejoc.201901537
mol%), (COCl) (1.0 equiv.), and DBU (3.0 equiv.); isolated yields.
2
Benzaldoxime (5a) was selected as a model substrate to optimize the reaction conditions
Table S1). Evaluations in terms of Brønsted bases, ratios of base to 5a, solvents, reaction
(
temperatures, the loading of the precatalyst tropone were performed. Optimal conditions
revealed 3 equiv. of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) base in MeCN solvent at 50 °C
under the catalysis of 1 mol% of tropone and 1 equiv. of oxalyl chloride commenced
transformation of 5a to nitrile 6a by quantitative yield in 10 min. Broad scope of the
chlorotropylium chloride catalyzed transformations of aldoximes to nitriles were verified (Scheme
3
) in aromatic, heteroaromatic, and aliphatic aldoximes substrates by excellent to quantitative
yields between 5 to 15 min.
Plausible mechanisms of oxime activations by chlorotropylium chloride were proposed base
on our previous understanding of gem-dichloro compounds catalyzed/promoted reactions26, 42
and seminal works in organocatalyzed Beckmann rearrangements. 39, 40, 44, 47, 67 The activations
were commenced from a common intermediate A (Scheme 4), it was an oxime ether produced
+
from the nucleophilic attack of chlorotropylium (ClTp ) to oxygen of the hydroxyl in ketoxime 3
or aldoxime 5. Spontaneous elimination of a molecule of HCl from A afforded intermediate B,
then B released a chloride anion and delivered the key tropylium oxime ether C.
2
In a Beckmann rearrangement cycle (Scheme 4, a), where the R group was alkyl or aryl
2
1
rather than a hydrogen H (R ≠ H), R migrated to N atom synchronized by cleavage of O–N
bond compensated by carbonyl C=O formation in the released tropone. The intermediate
nitrilium ion D, in equilibrium with its isomer imidoyl chloride E, would be attacked by ketoxime
3
on the nitrilium carbon to form a dimer-like cation F. Intermediate E was regenerated in
accompany to a release of the product amide 4. The D/E isomers were retrieved to undergo the
self-propagating cycles independent of the chlorotropylium.
A counterpart aldoxime to nitrile cycle (Scheme 4, b) was proposed. Aldoxime was not able
to be converted to nitrile or primary amide by simply addition of chlorotropylium chloride, albeit
9
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European Journal of Organic Chemistry
10.1002/ejoc.201901537
tropylium aldoxime ether G was produced. Nitrogen base DBU abstracted a proton from G that
effected collapse of oxime ether moiety to carbon–nitrogen triple bond (C≡N) in product nitrile 6
accompanied by expel of the precatalyst tropone. Equivalent amount of oxalyl chloride reacted
with tropone that regenerated chlorotropylium chloride 2 in situ which will be engaged in the
catalytic cycles.
Scheme 4. Bifurcate mechanisms of oximes to amides and nitriles from common intermediates A, B,
and C. (a) Beckmann rearrangement of ketoxime to amide triggered by catalytic amount of
68
chlorotropylium chloride 2, the key role of intermediate F in the self-propagating cycle was validated ;
(
b) Chlorotropylium chloride catalyzed cleavage of O–N bond in aldoxime assisted by DBU base effected
deprotonation, oxalyl chloride was used to regenerate chlorotropylium chloride from tropone in situ.
Conclusion
+
−
In conclusion, we firstly reported chlorotropylium chloride (ClTp ·Cl ) as trigger/catalyst in
transformations of oximes of ketones and aldehydes to their corresponding amides and nitriles
in high efficiency under mild conditions. This protocol provided rapid access to amides and
nitriles in minutes by excellent to quantitative yields. Seventeen amides and sixteen nitriles were
1
0
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European Journal of Organic Chemistry
10.1002/ejoc.201901537
rapidly synthesized by chlorotropylium protocol in short time (mostly 10-15 min) with excellent
yield (mostly > 90%). Bifurcate mechanisms from tropylium oxime ethers to amides via
Beckmann rearrangement, and to nitriles via formal dehydration by one organocatalyst were
proposed. The present study expanded the scope of tropylium catalysis and hopefully amenable
to wider organocatalysis applications.
Experimental Section
General Remarks
All reactions were conducted with magnetic stirring under argon atmosphere in oven-dried
glassware. Reagents were available from commercial suppliers and used without further
purification unless otherwise noted. All of solvents in the reactions were distilled from proper
drying reagents before using. All the oxime substrates used in this paper were prepared by the
procedure which refluxing a mixture of 1.0 equiv of the corresponding aldehydes or ketones, 2.0
equiv of sodium acetate, and 1.5 equiv of hydroxylamine hydrochloride in aqueous ethyl alcohol.
These oximes are mostly E form unless otherwise specified.
1
13
H NMR and C NMR spectra were conducted on Bruker AV 300 or AV 400 MHz instruments
in CDCl with chemical shifts reported relative to a residual deuterated solvent as the internal
3
standard. High-resolution mass spectra were experimented on Agilent Q-TOF 6520 mass
spectrometer using electron spray ionization (ESI) as the ion source. Gas chromatograms were
performed on an Agilent 7890A instrument using an Agilent DB-WAXETR column (30 m × 0.320
mm × 0.25 μm). Melting points in degrees Celsius(°C) were tested by Shanghai Precision &
Scientific capillary melting point apparatus without corrected.
General procedure A for transformation of ketoximes to amides
To a solution of tropone (5.3 mg, 0.05 mmol) in dry acetonitrile (1 mL) was added oxalyl
chloride (6.35 mg, 0.05 mmol) by dropwise. And the reaction mixture was stirred at room
temperature with gas emission ceased. After 15 min, a solution of ketoxime 3 (1 mmol) in dry
acetonitrile (4 mL) was added to the mixture by dropwise. The reaction solution was heated at
11
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European Journal of Organic Chemistry
10.1002/ejoc.201901537
8
0°C under a nitrogen atmosphere for the appropriate time (see Scheme 2). After monitoring of
the reaction for the completion by TLC, the organic solvent was removed by vacuum. Then the
crude products were purified by silica gel column chromatography (PE/EA) to give the
1
corresponding amide 4. The structure of the compound 4 was confirmed by their mp, TLC, H
13
NMR, and C NMR data.
General procedure B for transformation of aldoximes to nitriles
To a solution of aldoxime 5 (2 mmol) in a flame-dried Schlenk tube was added tropone (1
mol%) in acetonitrile (8 mL). A solution of (COCl) (2 mmol) added to the mixture over 15 min by
2
using a syringe pump. After the slow addition of DBU (6 mmol) in acetonitrile (2 mL), the reaction
was heated at 50°C for the specified time (see Scheme 3). Monitoring completion of the reaction
by TLC, the mixture was diluted with DCM and washed with water. The organic layer was dried
with Na
2
SO
4
and evaporated under reduced pressure to give the crude product. The pure
product 6 was obtained from the crude product purified by flash chromatography (PE/EA). The
1
13
structure of the compound 6 was confirmed by their mp, TLC, H NMR and C NMR data.
1
2
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European Journal of Organic Chemistry
10.1002/ejoc.201901537
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Note. To validate the proposed intermediate F in the self-propagating cycle of the
Beckmann rearrangement, imidoyl chloride H was syn-thesized (see Scheme S3), H
initiated the rearrangement equally ef-fective as that of chlorotropylium chloride.
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