Regiospecific Intermolecular Aminohydroxylation of Olefins by Photoredox Catalysis

Article abstract of DOI:10.1002/chem.201501590

A simple and regiospecific aminohydroxylation of olefins by photoredox catalysis has been developed. N-protected 1-aminopyridinium salts are the key compounds and serve as amidyl radical precursors by the action of Ir photocatalysts, fac-[Ir(ppy)₃

Full text of DOI:10.1002/chem.201501590

DOI: 10.1002/chem.201501590
Communication
&
Homogeneous Catalysis
Regiospecific Intermolecular Aminohydroxylation of Olefins by
Photoredox Catalysis
Kazuki Miyazawa, Takashi Koike,* and Munetaka Akita*[a]
and selective aminohydroxylation of olefin is still highly desira-
Abstract: A simple and regiospecific aminohydroxylation
ble.
of olefins by photoredox catalysis has been developed. N-
In recent years, photoredox catalysis has opened a new field
protected 1-aminopyridinium salts are the key compounds
in synthetic organic chemistry.[6] It can smoothly promote
and serve as amidyl radical precursors by the action of Ir
single-electron-transfer (SET) processes under mild reaction
photocatalysts, fac-[Ir(ppy)3] and [Ir(ppy)2(dtbbpy)](PF6)
conditions, i.e., at room temperature and under visible light ir-
(ppy=2-pyridylphenyl, dtbbpy=4,4’-di-tert-butyl-2,2’-bi-
radiation, and result in versatile radical reactions through the
pyridine). The present photocatalytic system allows for
redox processes. The reactions involving carbon radicals are
synthesis of vicinal aminoalcohol derivatives from olefins
the center of research. On the other hand, more recently, sev-
with various functional groups under mild reaction condi-
eral examples associated with reaction of N-centered radicals
tions with easy handling.
have been reported, but they are still limited.[7] We expect that
photoredox catalysis can trigger generation of N-centered radi-
cals from well-designed electrophilic amine sources following
Vicinal aminoalcohol skeleton is ubiquitous in biologically
active natural products and drugs.[1] Aminohydroxylation of
olefin has been regarded as one of the simplest strategies to
access vicinal aminoalcohols. Though various types of catalytic
aminooxygenation of olefins have been reported so far,[2,3] one
of the most reliable methods for intermolecular aminohydroxy-
lation is the Os-catalyzed system, which was originally devel-
oped by Sharpless et al. (Scheme 1a).[4,5] However, the reported
methods usually use potentially toxic Os species. In addition,
regioselectivity with respect to addition of the amino and hy-
droxyl groups to internal C=C bonds is frequently unsatisfacto-
ry. Thus, development of new methodologies for easy-to-use
1e reduction and lead to intermolecular aminohydoxylation of
olefins through radical amination (Scheme 1b). Herein we will
first describe the development of new shelf-stable and easy-to-
use amidyl radical sources derived from commercially available
1-aminopyridinium salt (1a). Then, we report on photoredox-
catalyzed regiospecific three-component aminohydroxylation,
which is composed of olefin, the amidyl radical source, and
H2O as a novel protocol for synthesis of vicinal aminoalcohol
derivatives with a range of functionalities.
First of all, we designed electrophilic amine reagents as pre-
cursors for N-centered radicals. Previously, we succeeded in de-
velopment of photoredox-catalyzed trifluoromethylative di-
functionalization of olefin using electrophilic Umemoto’s re-
agent (sulfonium salt) and Togni’s reagent (hypervalent iodine
species) as CF3 radical precursors.[8] These results inspired us to
extend the protocol to aminative difunctionalization of olefins
through N-centered radicals generated from the corresponding
electron-deficient onium reagents. 1-Aminopyridinium salt (1a)
is a commercially available chemical, derivatives of which are
usually used as building blocks for N-containing heterocycles.[9]
Its primary-amine moiety can be modified and tuned through
reactions with acid chloride or acid anhydride. In fact, various
N-protected 1-aminopyridium salts (1b–e)[10] were prepared
from 1a (Scheme 2) and they turned out to be shelf-stable
chemicals. During preparation of the present manuscript, the
group of Studer reported similar N-radical precursors, which
generate secondary-amidyl or imidyl radicals, and photoredox-
catalyzed amidation and imidation of aryl compounds.[7e] On
the other hand, we independently developed new N-protected
1-aminopyridium salts (1b–e), which generate primary-amidyl
radicals, and applied them to photoredox-catalyzed aminohy-
droxylation of olefins.[11]
Scheme 1. Catalytic aminohydroxylation of olefins.
[a] K. Miyazawa, Dr. T. Koike, Prof. Dr. M. Akita
Chemical Resources Laboratory
Tokyo Institute of Technology
R1-27, 4259 Nagatsuta, Midori-ku
Yokohama 226-8503 (Japan)
E-mail: koike.t.ad@m.titech.ac.jp
makita@res.titech.ac.jp
With these reagents in hand, we commenced examination
of photocatalytic aminohydroxylation of styrene (2a) in the
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201501590.
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a 38% NMR yield as a product instead of aminoalcohol. The al-
koxyamine derivative (1g) was less efficient. Furthermore,
other photoredox catalysts, [Ru(bpy)3](PF6)2 and [Ir(ppy)2-
(dtbbpy)](PF6), were also tested. As a result, the Ir catalyst
showed high activity similar to fac-[Ir(ppy)3], whereas the Ru
catalyst turned out to be sluggish (entries 8 and 9, Table 1).[12]
Finally, the present reaction did not proceed at all either in
darkness or in the absence of a photocatalyst (entries 10 and
11, Table 1). These results indicated that a combination of the
Ir photocatalysts, fac-[Ir(ppy)3] and [Ir(ppy)2(dtbbpy)](PF6), and
N-Ts-protected 1-aminopyridinium (1b) is essential for the effi-
cient photocatalytic aminohydroxylation.
Scheme 2. Synthetic scheme for electrophilic amine reagents (1b–e).
The scope of the present photocatalytic aminohydroxylation
is summarized in Table 2. Styrene (2a) and its derivatives bear-
ing OMe (2b), OAc (2c; Ac=acetyl), F (2d), Cl (2e), Br (2 f,g),
Bpin (2h; boronic acid pinacol ester), NHBoc (2i; Boc=tert-bu-
toxycarbonyl), and methylenedioxy groups (2j) on the benzene
ring produced the corresponding aminoalcohols (3ba–bj) in
36–86% yields in a regiospecific manner. Furthermore, the re-
action of a-substituted styrenes, a-methylstyrene (2k) and a-
Table 1. Optimization of the photocatalytic aminohydroxylation of sty-
rene (2a).[a]
Entry
Amine source
Photocatalyst
Yield of 3xa [%][b]
1
2
3[c]
4[c]
5[c]
6[c]
1a
1b
1c
1d
fac-[Ir(ppy)3]
fac-[Ir(ppy)3]
fac-[Ir(ppy)3]
fac-[Ir(ppy)3]
fac-[Ir(ppy)3]
fac-[Ir(ppy)3]
0
98
87
44
7
Table 2. Scope of the photocatalytic aminohydroxylation.[a,b]
1e
TsNNaCl·3H2O (1 f)
0
7[c]
fac-[Ir(ppy)3]
23
1g
1b
1b
1b
1b
8
9
10
11[d]
[Ru(bpy)3](PF6)2
[Ir(ppy)2(dtbbpy)](PF6)
–
35
97
0
3ba: 85%[c]
3bd: 84%
3bb: 82%
3be: 78%
3bc: 86%
3bf: 61%
fac-[Ir(ppy)3]
0
[a] For detailed reaction conditions, see the Supporting Information.
[b] Yields were determined by 1H NMR spectroscopy using SiEt4 as an in-
ternal standard. [c] Reaction time=24 h. [d] In the dark. ppy=2-pyridyl-
phenyl, bpy=2,2’-bipyridine, dtbbpy=4,4’-di-tert-butyl-2,2’-bipyridine,
LED=light-emitting diode.
3bg: 67%
3bh: 68%
3bi: 55%
presence of 2 mol% of fac-[Ir(ppy)3] in a mixed solvent system,
[D6]acetone and D2O, under visible light irradiation with
425 nm blue LEDs. It is well-known that the excited species of
fac-[Ir(ppy)3] serves as a strong reductant (E1/2 =À2.14 V vs.
[Cp2Fe]).[12] Through elaborated exploration of amine sources
(entries 1–7 in Table 1), we found N-Ts-protected 1-aminopyri-
dinium salt (1b; Ts=p-toluenesulfonyl) produced 2-(N-Ts-
amino)-1-phenylethanol (3ba) as a single regioisomer in an ex-
cellent yield and efficiency (98%, 2 h). Unprotected 1-amino-
pyridinium salt (1a) did not react at all (entry 1, Table 1). N-Pfb-
(1c), -Cbz- (1d), and -TFAc- (1e) protected reagents were less
effective (24 h) and afforded the corresponding products in
low yields (3ca: 87%, 3da: 44%, and 3ea: 7%; entries 3–5,
Table 1). It should be noted that classical electrophilic nitrogen
sources such as chloramine-T (1 f) and alkoxyamine derivative
(1g) did not work well under the present photocatalytic condi-
tions (entries 6 and 7, Table 1). The reaction with chloramine-T
(1 f) gave 1-[(4-methylphenyl)sulfonyl]-2-phenylaziridine in
3bj: 36%
3bk: 99%
3bl: 63%[d]
3bm: 40%[e]
3bn: 41%, 4:1 dr[f]
3bo: 51%, 2:1 dr[f,g]
3 bp: 88%, 3:1 dr
3bq: 68%, 1.5:1 dr
3br: 23%, 1:1 dr[h]
[a] For detailed reaction conditions, see the Supporting Information. Dia-
stereomer ratios (dr) were determined by 1H NMR spectra of crude reac-
tion mixtures. [b] Yields of isolated products. [c] Reaction time=2 h.
[d] Reaction time=10 h. [e] 1.5 equiv of 1b with respect to olefin was
used. [f] [Ir(ppy)2(dtbbpy)](PF6) (2 mol%) was used and reaction time was
12 h. [g] 1. 8 equiv of 1b with respect of olefin was used. [h] Reaction
time=8 h.
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pyridylstyrene (2l), afforded the aminoalcohols bearing tertiary
alcohol moieties in good yields (3bk: 99%, 3bl: 63%) exclu-
sively. In addition, an aliphatic alkene, methylene cyclopentane
(2m), could be also applied to the present photocatalytic
system.
tion of cyclohexene (2t) gave the cyclized product 5t in
a 37% yield.[10] This product is reminiscent of HAS (homolytic
aromatic substitution) reaction involving a radical intermediate
(Scheme 3b). These results strongly support that radical inter-
mediates are involved in the present reaction. Furthermore,
aminohydroxylation of E- and Z-b-methylstyrene (E-2n and Z-
2n) afforded the products 3bn with very similar diastereose-
lectivities (Table 2 and Scheme 3c), indicating that the present
photocatalytic aminohydroxylations proceed via the same in-
termediate irrespective of the starting E- and Z-alkenes.
In the case of internal alkenes such as E-b-methylstyrene
(2n), methyl cinnamate (2o), indene (2p), 1,2-dihydro-naph-
tharene (2q), and 1-methylcyclohexene (2r), the reactions also
proceeded in a regiospecific manner but the products were
obtained as mixtures of two diastereomers, (3bn: 41%, 4:1 dr,
3bo: 51%, 2:1 dr, 3 bp: 88%, 3:1 dr, 3bq: 68%, 1.5:1 dr,[10]
3br: 23%, 1:1 dr). It is noteworthy that a variety of functional
groups such as halogens (2d–g), ester (2c,o), boronic acid pi-
nacol ester (2h), N-protected amine (2i), pyridine (2l), and
acetal (2j) groups were tolerable with the present reaction.
Thus, the present aminohydroxylation is a regiospecific reac-
tion for both terminal and internal alkenes regardless of acyclic
and cyclic structure.
On the basis of these observations, as well as electrochemi-
cal and photochemical experiments (see the Supporting Infor-
mation), a plausible reaction mechanism is proposed as shown
in Scheme 4. First, the Ir photocatalyst IrIII is excited by visible
As a demonstration of scalability, aminohydroxylation of 4-
methoxystyrene (2b) with N-Ts-protected 1-aminopyridinium
salt (1b) was carried out on a gram scale. As a result, the prod-
uct 3bb was isolated in a 76% yield (1.22 g) [Eq. (1)].
To gain insight into the reaction mechanism, the photocata-
lytic reaction of 1-phenyl-2-(1-phenylethenyl)cyclopropane (2s)
was examined. It turned out that the linear product 4s was ob-
tained in a 72% isolated yield through a ring-opening process
of the cyclopropane ring (Scheme 3a).[10] In addition, the reac-
Scheme 4. A proposed reaction mechanism.
light to give the excited species *IrIII, which undergoes SET to
aminopyridinium 1 to afford stabilized radical 6 and highly oxi-
dizable Ir species IrIV. Radical 6 fragments into an amidyl radi-
cal and pyridine.[13] The generated amidyl radical reacts with
alkene 2 in a regiospecific manner to yield a radical intermedi-
ate 7, which is oxidized by the strongly oxidizing Ir species IrIV
to afford b-aminocarbocationic intermediate 8 and to regener-
ate the ground state Ir photocatalyst IrIII. Finally, the carbocat-
ionic intermediate 8 is susceptible to nucleophilic attack of
H2O to produce 1,2-aminoalcohol 3.
In conclusion, we have developed a novel and simple strat-
egy for regiospecific synthesis of 1,2-aminoalcohol derivatives
by photoredox catalysis. Well-designed N-protected 1-amino-
pyridinium salts serve both as an electron acceptor and as an
efficient amidyl radical precursor by the action of the photo-
excited iridium photoredox catalysts, fac-[Ir(ppy)3] and [Ir(p-
py)2(dtbbpy)](PF6). The present photocatalytic system enables
regiospecific intermolecular three-component aminohydroxyla-
tion of olefins bearing a wide variety of functionalities through
radical processes. Further development of stereoselective ami-
nohydroxylation by photoredox catalysis is currently underway
in our laboratory.
Acknowledgements
This work was supported by a Grant-in-Aid from the Ministry
of Education, Culture, Sports, Science of the Japanese Govern-
ment (No 26288045 and No 15K13689).
Scheme 3. Control experiments.
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Keywords: amination
homogeneous catalysis · photochemistry
· carbocations · electron transfer ·
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Received: April 23, 2015
Published online on July 14, 2015
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