Umpolung synthesis of branched α-functionalized amines from imines via photocatalytic three-component reductive coupling reactions

Article abstract of DOI:10.1039/c6cc09172e

A three component reductive coupling reaction of a (hetero)aromatic amine, a (hetero)aromatic aldehyde and an electron deficient olefin catalysed by eosin Y under green LED light irradiation, for the direct generation of γ-amino acid derivatives, is described. This new umpolung synthesis of amines, which exploits the high nucleophilicity of a putative α-amino radical intermediate, generated via single electron reduction of the in situ generated imine from the Hantzsch ester terminal reductant, is efficient, operationally simple, broad in scope and offers a complementary strategy to existing synthetic approaches.

Full text of DOI:10.1039/c6cc09172e

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Umpolung Synthesis of Branched α-Functionalized Amines from
Imines via Photocatalytic Three-Component Reductive Coupling
Reactions
Received 00th January 20xx,
Accepted 00th January 20xx
Angel L. Fuentes de Arriba, Felix Urbitsch and Darren J. Dixon*
DOI: 10.1039/x0xx00000x
www.rsc.org/
A
three component reductive coupling reaction of
a
α α
-amino acids,5 -amino trifluoroborates6 or hydrogen atom
(hetero)aromatic amine, a (hetero)aromatic aldehyde and an transfer from protected primary and secondary amines.7,8,9
electron deficient olefin catalysed by eosin Y under green LED light Also, α-amino radicals have been generated from electron rich
irradiation, for the direct generation of γ-amino acid derivatives, is tertiary amines by sequential loss of an electron from the
described. This new umpolung synthesis of amines, which exploits nitrogen lone pair, followed by an α-proton. However, the
the high nucleophilicity of
a
putative α-amino radical resulting α-amino radical is readily oxidised by the
second single-electron
intermediate, generated via single electron reduction of the in situ photocatalyst and undergoes
a
generated imine from the Hantzsch ester terminal reductant, is oxidation to generate an iminium intermediate which can be
efficient, operationally simple, broad in scope and offers a subsequently trapped with various nucleophiles.10 Intercepting
complementary strategy to existing synthetic approaches.
the α-amino radical in reactions with, for example, electron
poor alkenes is far less common, and to date has been limited
to N-aryltetrahydroisoquinolines,8 and phenyl methyl amines.9
Photoredox reactions in which imine moieties have been used
as a trap for nucleophilic radicals are limited to a few reports.11
Introduction
Chiral α-branched amines and their derivatives are
commonplace in pharmaceuticals, agrochemicals and
biologically relevant natural products (Figure 1).1 Accordingly,
the development of new and general methods for their
synthesis, through new C-C bond forming processes is
important and timely from both academic and industrial
perspectives. Furthermore, the possibility to carry out such
transformations as multicomponent coupling reactions is
attractive for the ready generation of structural analogues in
discovery processes.2 Such chiral α-branched amine motifs can
be accessed from electrophilic imine substrates, through direct
addition of carbon-centered nucleophiles such as
organometallic reagents and electron rich π-nucleophiles.
However, free radical chemistry offers the possibility to
reverse the polarity of imine derivatives; the formal addition of
Figure 1. Concept of umpolung amine synthesis from imines.
Despite these recent advances we recognised that a direct
method for the generation of ‘free’ α-amino radicals would
offer a wealth of untapped synthetic potential. Our plan was to
identify then develop a new reductive photochemical system
that, directly from an in situ formed imine, would efficiently
generate ‘free’ nucleophilic α-amino radical species capable of
undergoing a range of carbon-carbon bond forming processes
(Figure 1). Through its modular design such a process would be
useful for library and target synthesis alike and herein we wish
to report our findings.
a
hydrogen atom to the C=N π-bond can generate a
nucleophilic α-amino radical able to react with alkenes and
alkynes.3 However, to date these approaches have been
limited by the way the radical is generated, typically requiring
the specific preparation of radical precursors from imines and
subsequent use of, for example, stoichiometric quantities of
tin reagents.4 Recently, under photoredox catalysis, α-amino
radicals have been generated through the decarboxylation of
Results and discussion
As a model reaction we chose the reductive coupling of
benzaldehyde 1a, anisidine 2a and sulfone 3a 12
Following a
.
series of preliminary studies performed to identify new
reactivity, Ru(bpy)3Cl2, [Ir(dF(CF3)ppy)2(dtbbpy)][PF6] and
[Ir(dtbbpy)(ppy)2)][PF6], were found to efficiently promote the
desired reaction in DMSO under blue LED light irradiation
using Hantzsch ester as the terminal reductant. Pleasingly, the
use of organic dyes such as Eosin Y (EO) and Rose Bengal (RB)
under green LED light irradiation, also promoted the desired
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reaction with similarly high yields. After further optimisation influence on the reaction of the p-cyano substituent, a notable
studies (see ESI) the allylated product was isolated in 92% increase in yield was observed for p-methylphenyl (entry 13,
yield.
single crystal X-ray diffraction studies of compound 4m is
shown in the ESI)13 and trifluoromethylphenyl (entry 11).
Importantly, heteroaromatic aldehydes, such as pyridine,
furan, thiophene, imidazole and indole, also successfully
partook in the reaction. As already witnessed with CN
substituent, (entries 10 and 12), the position of the
heteroatom in the aromatic ring influenced the amount of
overreduction observed. While more electron deficient 4- and
2-pyridinecarboxaldehyde produced yields of 50-60% (entries
14 and 16), 3-pyridinecarboxaldehyde improved the product
yield to 83% (entry 15). Additionally, the rate of the
productive reaction diminished for these substrates and
accordingly the reaction time was increased to 3 days (entry
Table 1. Aldehyde 1 scope.a
Entry
1
R
4
4a
4b
4c
4d
4e
4f
Yield (%)b
87
Ph
2
3
2-Naphtyl
4-MeOC6H4
4-Me2NC6H4
2-BrC6H4
85
80
4
97
5
74
14) or
2 days (entry 16) respectively. Electron-rich 5-
6
7c
2-FC6H4
85
membered heterocycles such as furan (entry 17), thiophene
(entry 18) and 1-methyl-1H-imidazole (entry 19) also required
a reaction time of 2 days, yet afforded good yields. Especially
noteworthy is the tolerance of indole (entry 10) as the
heteroaryl substituent. Although the imine had to be
preformed due to the very slow reaction between indole-2-
carbaldehyde and p-anisidine under standard conditions, a
52% yield was obtained after 3 days before the reaction
2-OHC6H4
4g
4h
4i
64
57 (86)c
8
2-MeOC6H4
2-MeC6H4
9
94
54d
0 (82)c
10
11
12
13
14
15
16
17
18
19
20
4-CNC6H4
4j
4-CF3C6H4
4k
4l
3-CNC6H4
85
4-CO2MeC6H4
4-Pyridyl
4m
4n
4o
4p
4q
4r
63e (78)c
65e
3-Pyridyl
83
stalled;
a remarkable outcome given that unprotected
2-Pyridyl
53f
pyrroles or easily abstracted hydrogens often prove
detrimental to photoredox reactions.14
2-Furyl
84f
2-Thiofuryl
1-methyl-1H-imidazol-2-yl
1H-indol-2-yl
87f
4s
4t
79f
Regarding the amine component, reactivity was witnessed in
the presence of both electron-rich and electron-poor
aromatic amines as well as heteroaromatic amines (Table 2,
entries 1-4). Although ortho substituents were well-tolerated,
with O-methoxyphenyl (entry 1) and 2-pyridyl substituents
(entry 4) the preformation of the imine was necessary but the
standard reaction conditions still afforded the desired
products in excellent yields in both cases. Preformation of the
imine was also necessary when electron poor p-nitroaniline
was used as amine component; although in this case the
reaction yield was diminished to 46%.
52e,g
aReactions were carried out with 0.2 mmol of 1 and 2, 0.3 mmol of Hantzsch
ester, 0.5 mmol of 3 and 2 mol% of catalyst under green strip LEDs irradiation (1
W) in DMF. bIsolated yield. cReaction irradiated with green 10W LEDs. dReaction
time: 7 days. eReaction time: 3 days.fReaction time: 2 days.gImine was preformed.
The three-component reductive coupling reaction was found
to tolerate a wide range of aromatic aldehydes to afford 4-
amino 4-aryl-2-methylene carboxylic esters in very good yields
typically within 12 to 15 hours. The best results were achieved
with electron rich aromatic rings such as p-methoxy phenyl
(entry 3) or p-dimethylamino phenyl (entry 4). A wide range of
ortho substituents such methyl (entry 9), bromine (entry 5)
and fluorine (entries 6) were well-tolerated as well. Oxygen
substitution in the ortho position (entries 7-8) increased the
amount of competing reductive amination. For 2-methoxy
substitution (entry 8) this problem could be circumvented by
using more powerful LEDs (10W) in place of normal green LEDs
strip. This preferentially accelerated the productive three-
component reaction resulting in enhanced product yield.
Table 2. Aniline 2 and sulfone 3 scope.
Entry
R1
R2
5
Yield (%)
99a
86
46a
93a
82
1
2
2-MeOC6H4
Ph
COOEt
COOEt
COOEt
COOEt
COOtBu
CN
5a
5b
5c
5d
5e
5f
3
4-NO2C6H4
2-Pyridyl
4-MeOC6H4
4-MeOC6H4
4-MeOC6H4
4
Pleasingly, electron deficient aromatic aldehydes yielded the
desired product (entries 10-13) with acceptable yield although
some competing overreduction was observed in all cases.
5
6
7c
47
PO(OEt)2
5g
49
While the reaction with 4-cyanobenzaldehyde generated the aImine was preformed.
product with 54% yield (entry 10), switching the cyano
Next, we explored the range of allyl-sulfones that were
tolerated in the reaction (Table 2, entries 5-7). A tert-butyl
ester derivative (entry 5) performed in the reaction without
significant loss of yield (82%). However, other carbonyl groups
such as ketones, carboxylic acids or carboxamides all failed to
substituent to the meta position, thus increasing the electron
density of the aldehyde starting material, reduced the reaction
time from 7 days to 2 days and the yield recovered to 85%
(entry 12). Whereas employing more powerful LEDs had no
2 | J. Name., 2012, 00, 1-3
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afford the product (ESI). Interestingly, other electron
withdrawing groups such as cyano (entry 6) and
diethylphosphonate (entry 7) performed in the reaction well,
yielding the expected product with moderate yield. The
developed reaction proved to be robust to scale up; a gram
scale synthesis of 4a proceeded in a yield of 95% which
represents an improvement when compared to the reaction
performed on 0.2 mmol scale.
Three-component coupling product 4a is a versatile and
modifiable building block rich in synthetic potential.15
α-
Methyl-γ-phenyl carboxylic compounds (6a) are easily
Figure 3. Control and cross-over experiments.
accessible through simple hydrogenolysis and some derivatives
have been studied as agonists for antimicrobial peptide
systems.16 The PMP protecting group could be easily removed
An attempted cross-over experiment in which both an imine
and p-tolualdehyde-derived amine 11 were present in the
reaction mixture confirmed that under these conditions only
the imine was capable of giving rise to the reaction product
(Figure 3).
under standard conditions to provide the parent
aminoesters that could find application in medicinal chemistry
programmes as -aminobutyric acid (GABA) derivatives. Also,
-alkylidene -lactam 9a can be formed simply by heating
α-phenyl-γ-
γ
α
γ
compound 4a in the absence of solvent. Such lactams have
already shown potential as inhibitors of homoserine
transacetylase (HTA)17 and have been studied as less toxic
alternatives to
Our new methodology allows a fast and easy approach to the
synthesis of libraries of -methylene -lactams in only two
steps The double bond can be further manipulated through
hydrogenation to give 7a or through addition of nucleophiles
such as ethanol yielding 10a
α-methylene γ
-lactones in cancer treatment.18
α
γ
.
Figure 4. Proposed reaction mechanism.
.
Stern-Volmer quenching experiments (ESI) showed
a
significant quenching of eosin Y fluorescence by Hantzsch
ester, whereas sulfone or imine produced a much less
pronounced effect. On the basis of these observations, we
propose the reaction mechanism as depicted in Figure 4.
Under the reaction conditions, incident light excites EO, which
undergoes a rapid intersystem crossing to the lowest energy
triplet state,22 thus providing an oxidising species (E01/2 = +0.83
V vs SCE in CH3CN)22 which reduces the Hantzsch ester to the
+ 23
-
.
.
radical cation, HE . At this point EO radical-anion, EO
Figure 2. Derivatization of compound 4a.
(E01/2=-1.06 vs SCE in CH3CN)22 is oxidized by the imine (E01/2=-
1.90 V vs SCE in CH3CN)24, but in order for this process to take
place spontaneously, the intervention of transient proton
donor is necessary to significantly lower the redox potential of
To interrogate the reaction pathway to the three-component
coupling product, control and cross-over experiments where
performed. LED irradiation and eosin Y were necessary for the
reaction to proceed. Also, the reaction was inhibited when
TEMPO was added, supporting the participation of radical
intermediates in the transformation (ESI). As phenyl benzyl
amines have been used in photoredox catalysis as reductive
quenchers,19 the appearance of the overreduction product in
the photoredox reactions with the less reactive imines20 raised
the possibility that this compound could be, in fact, a potential
species suffering single electron oxidation by eosin Y to
generate an alpha amino radical21,22 ready to add into
the imine by proton coupled electron transfer (PCET).25
A
possible candidate might be Hantzsch ester radical cation
+
.
(HE ). In agreement with this hypothesis it was observed that
reducing agents other than the Hantzsch ester afforded no
product or very low conversion. The generated α-amino-
radical would then be poised to add to the sulfone C=C to form
the product and benzene sulfinyl radical (E01/2=+0.40 vs SCE)26
+
.
-
.
which can be further reduced by HE, HE or EO . On-off light
experiments demonstrated that no product formation was
occurring in the dark, thus supporting our hypothesis (ESI).
However, a short length radical chain mechanism cannot be
completely excluded (see ESI).27,28
allylsulphone 3a
. However, subjection of a mixture of
reductive amination product 11, allyl sulfone 3a and eosin Y in
DMSO to green LED light irradiation gave no reaction, ruling
out a tandem reductive then oxidative pathway to 4a (Figure
3). Interestingly, a similar reaction set-up but in the presence
Conclusions
[Ir(dF(CF3)ppy)2(dtbbpy)][PF6] as photocatalyst and blue LED In summary, we have developed a new three-component
light irradiation, afforded the reaction product as expected in reductive
63% NMR yield.
coupling reaction of (hetero)arylamines,
(hetero)aromatic aldehydes and electron deficient olefins
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catalysed by eosin Y under green LED light irradiation,
employing Hantzsch ester as the terminal reductant.27 The
reaction is easy to perform, broad in scope and provides a
convenient umpolung alternative to amine synthesis from
imines. The Hantzsch ester was found to play a critical role in
generating the reactive nucleophilic radical intermediates and
cross-over experiments confirmed the imine – and not the
amine resulting from imine reduction – was the source of the
α-amino radical. Work to expand the breadth of
photochemical umpolung imine coupling reactions is ongoing
in our laboratories and the results will be disclosed in due
course.
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We thank Marie Curie Actions-MSCA-IF-EF-ST to A.L.F.A.
(660125), F.U. is supported by the EPSRC Centre for Doctoral
Training in Synthesis for Biology and Medicine (EP/L015838/1).
We also thank Heyao Shi for X-ray structure determination and
Dr. Amber L. Thompson and Dr. Kirsten E. Christensen (Oxford
Chemical Crystallography Service) for X-ray mentoring and
help.
13 Low temperature single X-ray diffraction data were collected for 16
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