Visible-light-mediated utilization of α-aminoalkyl radicals: Addition to electron-deficient alkenes using photoredox catalysts

Article abstract of DOI:10.1021/ja211770y

Synthetic use of α-aminoalkyl radicals formed by single electron oxidation of amines is quite limited. Here we demonstrate addition of α-aminoalkyl radicals to electron-deficient alkenes by visible-light-mediated electron transfer using transition metal polypyridyl complexes as photocatalysts, via a sequential redox pathway.

Full text of DOI:10.1021/ja211770y

Communication
pubs.acs.org/JACS
Visible-Light-Mediated Utilization of α-Aminoalkyl Radicals: Addition
to Electron-Deficient Alkenes Using Photoredox Catalysts
Yoshihiro Miyake, Kazunari Nakajima, and Yoshiaki Nishibayashi*
Institute of Engineering Innovation, School of Engineering, The University of Tokyo, Yayoi, Bunkyo-ku, Tokyo 113-8656, Japan
S
* Supporting Information
Scheme 2. Addition of α-Aminoalkyl Radicals to Electron-
Deficient Alkenes Mediated by Visible Light
ABSTRACT: Synthetic use of α-aminoalkyl radicals
formed by single electron oxidation of amines is quite
limited. Here we demonstrate addition of α-aminoalkyl
radicals to electron-deficient alkenes by visible-light-mediated
electron transfer using transition metal polypyridyl complexes
as photocatalysts, via a sequential redox pathway.
irect functionalization of an sp3 C−H bond adjacent to
Dthe N atom of amines is one of the most important
methods for conventional and efficient synthesis of N-containing
organic compounds.1,2 The generation of iminium ions by two-
electron oxidation of amines and transformations by the addition
of various nucleophiles are extensively studied (Scheme 1a).2,3
On the other hand, α-aminoalkyl radicals4 formed by a single
electron oxidation are expected to work as reactive intermediates,
but successful examples for their use are limited (Scheme 1b)5,6
because they are readily oxidized into iminium ions in the
presence of a stoichiometric amount of oxidants.7
Table 1. Photocatalytic Reaction of Diethyl
Ethylidenemalonate (1a) with Methyldiphenylamine (2a)a
Scheme 1. Oxidation of Amines
Oxidation of amines into iminium ions and α-aminoalkyl
radicals via photoinduced electron transfer has been extensively
studied.8 Although some examples of photolytic use of α-aminoalkyl
radicals as reactive intermediates have been reported, high-
energy UV irradiation is necessary to promote these trans-
formations, and applicable substrates are limited.9−13 Recently,
use of photoredox catalysts such as transition metal polypyridyl
complexes has promoted useful organic transformations under
visible light irradiation, where only the generation of α-aminoalkyl
radicals in situ has been described.3,14−18 We have envisaged
that visible-light-mediated single electron transfer using photo-
redox catalysts is suitable for utilization of α-aminoalkyl radicals
in organic synthesis. Here we report preliminary results on
visible-light-mediated addition of α-aminoalkyl radicals to
electron-deficient alkenes, where the oxidation of amines to
α-aminoalkyl radicals (A) and the reduction of alkyl radicals (B)
generated from the addition of A to alkenes are key steps
(Scheme 2).
a
All reactions of 1a (0.25 mmol) with 2a (0.30 mmol) were carried out
in the presence of photocatalyst (0.0025 mmol) in solvent (2.5 mL) with
14-W white LED illumination at 25 °C for 18 h. Isolated yield.
b
At first, we carried out the reaction of diethyl ethylidenemalo-
nate (1a) with 1.2 equiv of methyldiphenylamine (2a). When
a solution of 1a and 2a in the presence of 1 mol % of [4a][BF4]
in N-methylpyrrolidone (NMP) was illuminated with a 14-W
white LED at 25 °C for 18 h, diethyl 2-(1-diphenylaminopropyl)-
malonate (3a) was obtained in 90% yield (Table 1, entry 1).
Reactions using similar Ir complexes 4b and 4c as catalysts
also proceeded smoothly to give 3a in moderate yields (entries 2
and 3). Use of [Ru(bpy)3][BF4]2 (bpy = 2,2′-bipyridyl) and
Eosin Y as photoredox catalysts dramatically decreased the yield
Received: December 16, 2011
Published: January 26, 2012
© 2012 American Chemical Society
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Communication
of 3a (entries 4 and 5). The choice of solvents is an important
factor to promote the addition. When N,N-dimethylformamide
(DMF) was used in place of NMP, 3a was obtained in 45%
yield (entry 6), while no reaction occurred in other solvents
such as acetonitrile and methanol (entries 7 and 8). Separately,
we confirmed that no formation of 3a was observed in the
absence of visible light or photocatalyst.
Table 3. Photocatalytic Reactions of Diethyl
Ethylidenemalonate (1a) with Amines (2)
a
Other reactions of a variety of alkenes 1 were investigated by
using 4a as a photocatalyst. Typical results are shown in Table 2.
The reactions of 1b−1d bearing an alkyl group at the
β-position proceeded smoothly to give the corresponding alkyl-
ated amines (3b−3d) in good to high yields (entries 1−3). Diethyl
arylmethylidenemalonates (1e−1j) and diethyl ethoxymethyli-
denemalonate (1k) were also applicable to this reaction system,
giving the corresponding alkylated amines (3e−3k) in good
yields (entries 4−10). On the other hand, use of β-ketoesters
(E/Z-1l) in place of diester gave similar yields of 3l as a mixture
of two diastereoisomers (entries 11 and 12), but a lower yield of
3m was observed when the alkene bearing a 1,3-diketone moiety
(1m) was used (entry 13). The reaction of diethyl maleate (1n)
also took place smoothly to give 3n in 81% yield (entry 14).
However, a small amount of 3o was obtained when ethyl
crotonate (1o) was used as a substrate (entry 15). Results for
other alkenes are shown in Scheme S1. These results indicate
that the introduction of two electron-withdrawing groups at the
alkenes is essential to obtain 3 in high yields.
a
All reactions of 1a (0.25 mmol) with 2 were carried out in the
presence of [4a][BF4] (0.0025 mmol) in NMP (2.5 mL) with 14-W
b
c
white LED illumination at 25 °C for 18 h. Isolated yield. 1.2 equiv of
2 was used. 1.5 equiv of 2 was used. For 48 h. [4c][BF4] was used
as photocatalyst. The ratio of isomers is ca. 1/1. The ratio of 3x1 and
3x2 is 3.6/1.
d
e
f
g
h
Table 2. Photocatalytic Reactions of Alkenes (1) with
Methyldiphenylamine (2a)
in a high yield because the oxidizing ability of the excited state
of 4c is higher than that of 4a (entry 6).19 Dialkylarylamines
(2h, 2i) and diisopropylmethylamine (2k) were also applicable,
giving the corresponding amines alkylated at the methyl group
(3v, 3w, 3y) in high yields (entries 7, 8, and 10). When N,N-
dimethylaniline (2j) was used as an amine, a mixture of mono-
alkylated product 3x1 and dialkylated product 3x2 was obtained
with moderate selectivity (entry 9). Cyclic amine N-phenyl-
indoline (2l) was transformed into 3z in 83% yield (entry 11).
Separately, we confirmed that no conversion of a secondary
amine such as N-methylaniline into the corresponding product
was observed.
a
Scheme 3. Reaction of 1a with 2a-d3
a
To obtain information on the reaction pathway, we investi-
gated the reaction of 1a with 2a-d3 (>99% D-enriched at the
methyl protons) under similar conditions (Scheme 3). After
purification by chromatography, the corresponding amine 3a-d3
(51% D-enriched at the activated methine proton and >99%
D-enriched at the α-protons of the amino group) was obtained
in 85% yield. No deuterium incorporation of 3a was observed
in the reaction of 1a with 2a in DMF-d7. These results indicate
that the activated methine proton of 3 is mainly derived from
the α-proton of 2, though hydrogen abstraction from other
species is not excluded completely. Furthermore, we carried out
the reaction of the alkene bearing a cyclopropyl moiety (5) as a
radical clock (Scheme 4).20 The cyclopropyl ring-opening product
6 was obtained in 64% yield. This result strongly supports the
intermediacy of alkyl radicals in this transformation.
All reactions of 1 (0.25 mmol) with 2a were carried out in the
presence of [4a][BF4] (0.0025 mmol) in NMP (2.5 mL) with 14-W
white LED illumination at 25 °C for 18 h. Isolated yield. 1.2 equiv of
2a was used. 1.5 equiv of 2a was used. 2.0 equiv of 2a was used. The
ratio of isomers is ca. 1/1.
b
c
d
e
f
Next, we examined photocatalytic reactions with a variety
of tertiary amines 2. Typical results are shown in Table 3.
Reaction of 1a with ethyldiphenylamine (2b) proceeded
smoothly to give 3p in 89% yield (entry 1). Introduction of a
substituent such as methyl, methoxy, fluoro, or chloro at the
para-position in the benzene ring of 2a did not affect much the
yield of 3 (entries 2−5). The amine bearing a highly electron-
withdrawing ester group (2g) is slightly more difficult to be
oxidized, and use of 4c in place of 4a is important to obtain 3u
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Scheme 4. Reaction of 5 as a Radical Clock
Scheme 5. Plausible Reaction Pathway
abstraction of the α-hydrogen of amines is a kinetically
unfavorable step.4,9c In sharp contrast, our visible-light-
mediated reaction proceeds via a sequential redox pathway as
shown in Scheme 5, and a stoichiometric amount (1.2−2.0
equiv) of amines is sufficient to promote the reaction. The
result described in this paper provides a new entry into the
visible-light-mediated utilization of α-aminoalkyl radicals and a
synthetically useful methodology for the direct functionalization
of an sp3 C−H bond adjacent to the N atom of amines.
Finally, we investigated synthetic applications of the pro-
duced alkylated amines (Scheme 6). Decarboxylation of 3a
proceeded smoothly to give 3o in 92% yield (Scheme 6a).
Intramolecular cyclization of dealkylated secondary amine
proceeded in the reaction of 3v with an excess amount of
trifluoroacetic acid (TFA) to afford the corresponding γ-lactam
(7)24 in 71% yield with a high diastereoselectivity (Scheme 6b).
As a plausible reaction pathway, a radical chain process is
possible, where abstraction of the α-H of 2 by alkyl radical
B generated from the addition of α-aminoalkyl radical A
to 1 affords 3 together with regeneration of A, and this reac-
tion process is independent of photoirradiation. Previously,
Hoffmann’s group reported the addition of α-aminoalkyl
radical to alkenes under UV irradiation, and their reaction system
included a radical chain process as a main reaction pathway,
where the quantum yield is larger than 1.9 To verify the effect of
photoirradiation, we investigated on−off switching of the light
source in the reaction of 1a with 2a in DMF-d7 (Figure 1). When
the LED lamp was switched off at appropriate intervals, the reaction
did not proceed during the “off” periods. In sharp contrast to
Hoffmann’s system,9 the quantum yield in the reaction of 1a with 2a
was estimated to be 0.32, which is in the range common for
molecular transformations via photoinduced electron transfer
mediated by transition metal polypyridyl complexes.8a,b,21 Separately,
we confirmed that use of radical initiators such as AIBN, BEt3/air,
and (tBuO)2 did not promote the reaction of 1a with 2a in NMP at
80 °C at all.22 These results indicate that the dependence of a radical
chain process is negligible in our reaction system.
Scheme 6. Transformations of 3
In summary, we have developed an efficient methodology
for visible-light-mediated utilization of α-aminoalkyl radicals, which
are difficult to be generated directly from amines under ther-
mal reaction conditions. Our reaction system is applicable
for addition of a variety of amines to electron-deficient alkenes.
We also found that this photocatalytic reaction proceeds via a
sequential redox pathway. We believe that the result described
here provides a novel synthetic approach to direct functionaliza-
tion of an sp3 C−H bond adjacent to the N atom of amines.
Further work is in progress to broaden the synthetic applicabi-
lity of the α-aminoalkyl radicals under visible light irradiation.
Figure 1. Time profile of reaction of 1a with 2a: light was swiched off
during the “off” periods.
Considering the experimental results, a plausible reaction path-
way is proposed in Scheme 5. The initial step is the formation of
the α-aminoalkyl radical A from a single electron oxidation of 2
by the excited photocatalyst (*cat) and subsequent deprotona-
tion.8 Addition of A to 1 results in formation of the alkyl radical
species B. Finally, reduction of B by the one-electron-reduced form
of photocatalyst (cat−)19,23 and subsequent protonation occur to
give 3 accompanied by regeneration of photocatalyst (cat).
As mentioned above, Hoffmann’s group reported the high-
energy UV-light-mediated generation of α-aminoalkyl radical
and addition to alkenes.9 In the radical chain process they
proposed, use of a large excess amount of amines (using as a
solvent) is necessary to accelerate the addition reaction because
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental procedures, spectroscopic data, and X-ray data
(CIF). This material is available free of charge via the Internet
at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
ynishiba@sogo.t.u-tokyo.ac.jp
Notes
The authors declare no competing financial interest.
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Communication
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ACKNOWLEDGMENTS
■
This work was supported by the Funding Program for Next
Generation World-Leading Researchers (GR025). K.N. is a
recipient of the JSPS Predoctoral Fellowships for Young
Scientists and acknowledges the Global COE program for
Chemistry Innovation.
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