Lewis acidic FeCl3 promoted 2-aza-Cope rearrangement to afford α-substituted homoallylamines in dimethyl carbonate

Article abstract of DOI:10.1039/c9ra03277k

The iron(iii)-catalyzed efficient strategy for the synthesis of α-substituted homoallylamines was accomplished via a cationic 2-aza-Cope rearrangement of aldimines, generated in situ by condensation of commercially available aldehydes and easily synthesizable 1,1-diphenylhomoallylamines. This reaction features a broad substrate scope with high yields and is conducted in an eco-friendly solvent, i.e. dimethyl carbonate.

Full text of DOI:10.1039/c9ra03277k

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Lewis acidic FeCl3 promoted 2-aza-Cope
rearrangement to afford a-substituted
homoallylamines in dimethyl carbonate†
Cite this: RSC Adv., 2019, 9, 18013
Karthik Gadde,
Jonas Daelemans, Bert U. W. Maes
and Kourosch Abbaspour
*
Tehrani
The iron(III)-catalyzed efficient strategy for the synthesis of a-substituted homoallylamines was
accomplished via a cationic 2-aza-Cope rearrangement of aldimines, generated in situ by condensation
of commercially available aldehydes and easily synthesizable 1,1-diphenylhomoallylamines. This reaction
features a broad substrate scope with high yields and is conducted in an eco-friendly solvent, i.e.
dimethyl carbonate.
Received 2nd May 2019
Accepted 29th May 2019
DOI: 10.1039/c9ra03277k
rsc.li/rsc-advances
The use of sustainable catalysts and green solvents are the most basic reagents such as allyl stannanes, allyl silanes, allyl boronates
inuential factors to reduce the environmental impact of chemical and allyl boranes have been used. Despite signicant utility of
synthesis. In this context, abundant base-metal catalysis, especially these reported methods, many of these procedures still show
the eld of iron catalysis has gained signicant attention in drawbacks, including generation of stoichiometric amounts of
organic synthesis.1 Iron has many advantages over other transition metal-containing (toxic) waste. Furthermore, oen harsh depro-
metals, such as low cost, low toxicity, its natural abundance, tection conditions are required to obtain N-unprotected homo-
environmentally benign nature and oen straightforward proto- allylamines, also generating extra waste, which in most of the cases
cols for conversion to its corresponding salts and complexes. cannot be recovered and reused for reagent synthesis.5–7
During the last three decades, there has been a growing interest in
using green solvents in chemical processes. In recent solvent
selection guidelines, dimethyl carbonate (DMC) has been classi-
ed as a “recommended” solvent.2 The green merits of dimethyl
carbonate are fast biodegradability, low toxicity, mild odor, low
evaporation rate, low density and good environmental compati-
bility.3 The latter can be explained by its low photochemical ozone
creation potential (POCP) in comparison to common volatile
organic compounds (VOCs) (2.5; e.g. ethylene ¼ 100) and its recent
production methods using CO2 as a raw material.3f–g
Homoallylamines are valuable intermediates and important
structural motifs in organic chemistry for the synthesis of hetero-
cycles, natural products and pharmaceutical compounds.4 The
most common approach for a-substituted homoallylamine
synthesis is the direct nucleophilic addition of allylic organometal
or metalloid derivatives to imines (Scheme 1A).5 However, the
addition of allyl Grignard reagents to imines are limited and
mainly restricted to non-enolizable imines.6 Aldimines which
contain a-hydrogens generally fail to give acceptable yields of
secondary amines due to the poor electrophilicity of the imine
carbon and competing a-deprotonation.6a–e In this regard, less
Organic Synthesis, Department of Chemistry, University of Antwerp,
Groenenborgerlaan
171,
2020
Antwerp,
Belgium.
E-mail:
kourosch.
abbaspourtehrani@uantwerpen.be; Fax: +32 32653233; Tel: +32 32653226
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra03277k
Scheme 1 Synthesis of a-substituted homoallylamines.
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Table 1 Optimization of the reaction conditionsa
On the other hand, the [3,3]-sigmatropic rearrangement
approach for homoallylamine synthesis is much less developed. In
1950, Horowitz and Geissman rst reported a 2-aza-Cope rear-
rangement.8 This reaction has rarely been utilized in organic
synthesis due to the inherent problem of the reversibility of the
process.9 In pioneering studies, Overman and co-workers devised
an aza-Cope–Mannich reaction sequence to overcome this
problem and have applied this development successfully in
numerous alkaloid syntheses.10 Recently, an aza-Cope rearrange-
ment strategy has been utilized in uorescent probes for imaging
formaldehyde in biological systems.11 In literature, very few reports
of 2-aza-Cope rearrangements are known for the synthesis of
homoallylamines from aldehydes (Scheme 1B).12–15 In 2006,
Kobayashi and co-workers developed a Brønsted acid catalyzed
method for the synthesis of chiral homoallylamines from aldehyde
and chiral camphorquinone derived homoallylamine.12 Unfortu-
nately, highly hazardous dichloroethane was employed as solvent.
In 2008, Rueping and Antonchick reported the rst chiral phos-
phoric acid catalyzed aza-Cope rearrangement for the synthesis of
chiral homoallylamines.13 Although the a-substituted homoallyl-
amines were isolated in moderate to good yield (up to 87%), in
good to high enantiomeric excesses (up to 94% ee) at 50 ꢀC in
methyl tert-butyl ether, this method limited to aromatic aldehydes
and requires longer reaction times (48 h), which is a drawback. In
2011, Wulff et al., developed chiral polyborate – achiral Brønsted
Yieldb
[%]
Catalyst
Entry (10 mol%)
Solvent
CH2Cl2
Temp (ꢀC) Time (h) 3a 4a
1
Fe(OTf)3
50
50
50
50
50
85
50
50
50
82
65
101
78
80
126
110
50
25
50
90
90
90
90
90
90
90
90
18
18
18
18
18
18
18
18
18
18
18
18
18
18
18
18
18
18
18
6
17 78
22 76
54 29
38 44
13 85
2
3
4
5
FeCl3$6H2O CH2Cl2
FeCl2$4H2O CH2Cl2
FeCl2
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
No catalyst
Fe(OTf)3
InCl3
CH2Cl2
CH2Cl2
ClCH2CH2Cl
H2O
DMSO
DMF
MeCN
MeOH
1,4-Dioxane
EtOAc
2-MeTHF
n-BuOAc
Toluene
Neat
DMC
DMC
DMC
DMC
DMC
DMC
DMC
DMC
6
0
65
58
47
0
98
0
7c
8
9
0
0
10
11
12
13
14
15
16
17c
18
19
20
21c
22d
23
24
25
26
27e
94
84 14
0
0
0
0
0
48
9
3
0
1
94
89
94
94
91
0
71
90
99
59
ꢀ
acid pair catalyzed reaction at 60 C in m-xylene to improve the
enantioselection with broader substrate scope for homoallyl-
amines.14 Most recently, Johnson and co-workers reported an
enantioconvergent method for the synthesis of chiral b-amino
amides from racemic b-formyl amides and diphenylhomoallyl-
amines at 60 ꢀC in chloroform.15 To the best of our knowledge, no
examples of Lewis acid catalyzed 2-aza-Cope rearrangement for the
synthesis of homoallyamines from aldehydes in green solvent have
been reported. Inspired by above catalytic methodologies and as
a continuation of our efforts in imine activation reactions,16 herein
we report an efficient and eco-friendly synthesis of homoallyl-
amines via an iron(III)-catalyzed 2-aza-Cope rearrangement of in
situ generated aldimines from readily accessible 1,1-diphenylho-
moallylamines and aldehyde feedstocks. In this developed
protocol, non-toxic and inexpensive iron(III) chloride has been used
as a catalyst and the reaction has been carried out in dimethyl
carbonate.
We commenced the optimization studies using benzaldehyde
(1a) and 1,1-diphenylhomoallylamine (2a) as the model substrates.
In the light of the recent advances in the eld, the choice of
sterically hindered 1,1-diphenylhomoallylamine was crucial to
drive the reaction from the aldimine intermediate to the ketimine
product.13–15 Our preliminary experiments with screening of
various Lewis acids revealed that, the aza-Cope rearrangement on
intermediate 3a can be achieved by using 10 mol% of Fe(OTf)3 (see
the ESI Section-2† for details). Further, we continued our study
from benzaldehyde (1a, 1.0 equiv.) and 1,1-diphenylhomoallyl-
amine (2a, 1.0 equiv.) using iron salts as catalysts in standardly
used dichloromethane in a sealed vial at 50 ꢀC (Table 1). Screening
of a number of iron salts revealed that FeCl3 was the best catalyst in
terms of higher yield and relative cost (entries 1–5). Interestingly
from solvent screening studies, dimethyl carbonate was found to
18
18
18
18
18
18
18
64 24
91
0
0
6
91
94
AlCl3
DMC
70 11
81
BF3$Et2O
DMC
0
a
Reaction conditions: 1a (0.25 mmol, 1.0 equiv.), 2a (0.25 mmol, 1.0
˚
equiv.), catalyst (10 mol%), 4 A MS (100 mg) and solvent (0.5 mL).
b
Yields were determined by 1H NMR analysis with 1,3,5-
c
d
trimethoxybenzene as internal standard. Without MS. 5 mol% of
e
FeCl3 was used. 100 mol% of BF3$Et2O was used. DMC ¼ dimethyl
carbonate. MS ¼ molecular sieves.
be the best alternative for dichloromethane and the most of the
solvents were compatible solvents for the reaction except polar
solvents such as water, DMSO, DMF and MeOH (entries 5–20).
Further, elevating the reaction temperature increased the reaction
rate with complete conversion (entries 18–20). The FeCl3 catalyst
furnished a complete and clean conversion within 6 h at 90 ꢀC in
dimethyl carbonate (99%, entry 20) and no traces of aldimine
intermediate 3a were found in the reaction mixture.
The formation of the product 4a was reduced when the reaction
was carried out in the absence of molecular sieves (entry 21).
Further, upon decreasing the amount of FeCl3 to 5 mol%, the yield
of the product 4a was reduced and unsurprisingly, 3a was observed
as major product without catalyst (entries 22–23). Under the opti-
mized condition, similar yields were achieved with Fe(OTf)3 and
InCl3 (entries 24–25), while other Lewis acids such as AlCl3 and
BF3$Et2O afforded the product in lower yield (entries 26–27). Based
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on our Lewis acid catalyst screening (see the ESI Section-2† for delight, the scope of aldehydes was broad, providing high yields
details), FeCl3, InCl3, BiCl3, Fe(OTf)3, In(OTf)3, Sc(OTf)3 and and showcasing the generality of this method. Aryl aldehydes
Yb(OTf)3 were found to be effective catalysts and furnished the bearing ortho-, meta- and para-substituents at the arene ring
expected rearranged product in above 75% yield, while FeCl2, were all well tolerated under the optimized reaction conditions.
CuCl2, AlCl3, Ni(OTf)2 and other Lewis acids did not lead to good As tabulated, p-tolualdehyde furnished the desired product 4b
yields. These results seems to be in good agreement with litera- in 98% yield. In the case where the aryl ring contains an
ture17 data on classication of Lewis acids with respect to their electron-donating methoxy group at the para-position (4c) and
activities in aldimine reactions. The low-toxic and inexpensive ortho-position (4d), it took 18 h to drive the reaction to
FeCl3 catalyst was selected for further investigation of the scope of completion. In general, we observed that electron rich aromatic
the reaction.
aldehydes require longer reaction time than electron-poor
As depicted in Table 2, the scope of aldehydes was rst aromatic aldehydes. The reaction of 3,5-dimethox-
investigated with 1,1-diphenyl homoallylamine (2a). To our ybenzaldehyde furnished the desired product 4e in 98% yield.
Similarly, benzaldehyde with ouro-, chloro-, bromo- or tri-
uoromethyl substituents at the para-position afforded the
Table 2 Substrate scopea
desired products 4f–i in excellent yields (94%, 90%, 90% and
93% respectively). In addition, substituted benzaldehydes con-
taining a variety of functional groups such as nitro (4j), nitrile
(4k) and ester (4l) were compatible with the reaction conditions.
Interestingly, heteroaromatic aldehydes containing a pyrazole,
pyridine and quinoline ring were also successful and provided
the desired products 4m–q in good yields, although a higher
catalyst loading (25 mol%) was required. The use of a higher
catalyst loading and slow progress of the reaction can be
explained by the strong coordination between the nitrogen from
the heterocyclic aldehyde with the metal catalyst, rendering the
metal catalyst unavailable for participation in the catalytic cycle.
As expected, the reaction of paraformaldehyde afforded 4r in
89% yield. Furthermore, linear, branched, and cyclic aliphatic
aldehydes such as propionaldehyde, butyraldehyde, iso-
butyraldehyde, pivalaldehyde, cyclopropane- and cyclo-
hexanecarboxaldehyde
furnished
the
corresponding
ꢀ
homoallylamines 4s–x, at 50 C in 92%, 92%, 91%, 86%, 89%
and 94% yields, respectively. Interestingly, transformation with
triuoroacetaldehyde hydrate and chloral hydrate provided the
desired products 4y and 4z in excellent yields. In addition, it is
worth noting that a variety of functional groups such as phe-
nylpropargyl (4aa), trans-cinnamyl (4ab) and ester (4ac) were all
well tolerated in the transformation, which would offer the
potential for increasing molecular complexity via further func-
tionalization. In the case of aldehyde substrate containing
a hydroxy group, under the optimized reaction conditions, the
methoxycarbonylated product 4ad0 was obtained in 96% yield
instead of the expected product 4ad.18 To overcome this
particular substrate issue, switching solvent system to
propylene carbonate afforded the desired product 4ad in 94%
yield. Next, we examined the scope of aldehydes with 2-methyl-
1,1-diphenylhomoallylamine (2b). Under the optimized condi-
tions, these reactions furnished E/Z mixtures of the corre-
sponding homoallylamines 4ae–ah in good to excellent yields.
The most of the examples in Table 2 did not require column
chromatography purication.
To further demonstrate the potential of this method, a gram-
scale reaction was performed. The reaction of benzaldehyde (1a,
a
Reaction conditions: 1 (0.25 mmol, 1 equiv.), 2a or 2b (0.25 mmol, 1
˚
equiv.), FeCl3 (10 mol%), 4 A molecular sieves (100 mg) in dimethyl
b
carbonate (0.5 mL, 0.5 M), 90 ꢀC, isolated yield. 25 mol% of FeCl3 5 mmol) and 1,1-diphenyl homoallylamine (2a, 5 mmol) under
c
d
was used. Reactions were carried out at 50 ꢀC. 96% yield of
standard reaction conditions furnished the desired product 4a
in 94% yield (Scheme 2). Further, the ketimine 4a was easily
hydrolyzed under mild acidic conditions to obtain free amine 5a
e
methoxycarbonylation of alcohol product was obtained. In place of
dimethyl carbonate, propylene carbonate was used as solvent at 90 ꢀC.
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`
thank Prof. Filip Lemiere (University of Antwerp) for HRMS
measurements.
Notes and references
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Scheme 2 Scale-up reaction, hydrolysis of the ketimine and recovery
of benzophenone.
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in 90% yield and recovered benzophenone in 91% yield aer
simple acid–base extraction (Scheme 2).
Based on our observations (see the ESI Section-3† for details)
and in light of recent reports,13–15,16a the following reaction
mechanism is proposed (Scheme 3). The reaction of the 1,1-
diphenylhomoallylamine and an aldehyde promoted by iron(III)
chloride generates iminium species A rst, and further subse-
quent a cationic 2-aza-Cope rearrangement of A affords inter-
mediate B through a cyclic transition state, which is less
sterically hindered than the starting sigmatropic isomer A.
Further, the intermediate B results in the formation of the
corresponding a-substituted homoallylamine 4 with concomi-
tant regeneration of the active iron(III) chloride.
In summary, this study demonstrate the rst example of iron
catalyzed 2-aza-Cope rearrangement for the synthesis of a wide
variety of a-substituted homoallylamines from readily acces-
sible starting materials in a green solvent, producing water as
the sole by-product. By this protocol, the synthesis of a-alkyl-,
alkenyl-, aryl- and heteroaryl homoallylamines are achieved
with high yields. Notably, N-unprotected (free NH2) a-
substituted homoallyamines can be easily generated by mild
acidic treatment of the rearranged benzophenone imines.
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