Visible-light-induced photocatalytic reductive transformations of organohalides

Article abstract of DOI:10.1002/anie.201203599

A photo opportunity: A visible-light-excited iridium catalyst delivers electrons from an amine to an organohalide. The electron transfer then induces reductive scission of the carbon-halogen bond, generating the corresponding alkyl, alkenyl, and aryl radical that can undergo cyclization and hydrodehalogenation reactions. Copyright

Full text of DOI:10.1002/anie.201203599

Angewandte
Chemie
DOI: 10.1002/anie.201203599
Photocatalysis
Visible-Light-Induced Photocatalytic Reductive Transformations of
Organohalides**
Hyejin Kim and Chulbom Lee*
Free-radical generation from organohalides is among the
most useful means to access an open-shell reactive inter-
mediate that has found numerous applications in chemical
synthesis. The halide abstraction, in particular, has been the
mainstay approach in the production of carbon-centered
radicals, although there are some problems associated with
the method (hazardous reagents and specialized appara-
tuses).[1] An alternative is the transition-metal-mediated free-
radical reaction, in which the odd-electron process starts with
the reductive scission of the carbon–halogen bond.[2] This
single-electron transfer (SET) strategy has recently been
demonstrated to be feasible under visible-light photocatalysis
utilizing transition-metal polypyridyl complexes.[3,4] Along
with a novel mechanistic modality, the “green chemistry”
features inherent to these methods hold great promise for the
discovery of new reactions as well as developing practical and
environmentally benign processes on industrial scales.[5]
However, the visible-light-induced radical reaction has to
Table 1: Reductive cyclization of aryl halide 1a.
Entry
Catalyst
(3 mol%)
Reductant
(10 equiv)
Solvent Time Yield
[h]
[%][a]
1[b]
2
3
4
5
NiCl2·DME +Pybox
[Ru(bpy)3]Cl2·6H2O
[Ir(ppy)2(dtbbpy)]PF6
[Ir(ppy)2(dtbbpy)]PF6
[Ir(ppy)2(dtbbpy)]PF6
Zn
MeOH
MeCN
MeCN
MeOH
DMF
48
24
5
–
DIPEA
DIPEA
DIPEA
DIPEA
15
96
96
50
90
98
5
12
12
1.5
6
[Ir(ppy)2(dtbbpy)]PF6 TEA
[Ir(ppy)2(dtbbpy)]PF6 DIPEA
MeCN
MeCN
7[c]
[a] Reaction conditions: 3 mol% catalyst, 10 equiv reductant, solvent
(0.01m), 258C, 20 W CFL. Yields are of isolated product. [b] The reaction
was performed with 5 mol% NiCl2·DME, 6 mol% Pybox, and 3 equiv of
Zn in MeOH (0.2m) at 258C (Ref. [7]). The simple reduction product 1c
was obtained (79%) from a reaction run at 508C for 95 h. [c] A 2 W blue
LED strip was used. Ac=acetyl; DME=dimethoxyethane; Pybox=pyr-
idine-2,6-bis(oxazoline).
date been limited only to suitably activated haloalkanes
3
À
possessing a C(sp ) X bond adjacent to a p system (a-
carbonyl, benzyl) or heteroatom (halogen, oxygen).[6] Despite
the readily conceivable potential, the application to alkenyl,
aryl, and unactivated alkyl halides has not been reported.
Given the widespread utility of organohalide-based radical
processes, it would be of significance to expand the current
scope of the visible-light-harnessing catalytic method to
include a wide range of substrates. Described herein are the
results of our studies on the reductive transformations
(cyclization and hydrodehalogenation) of unactivated orga-
nohalides by visible-light-induced photocatalysis.
in 79% yield after a very sluggish reaction at an elevated
temperature (4 days, 508C, entry 1). The reaction using
[Ru(bpy)3]Cl2·6H2O (bpy = 2,2’-bipyridine) and N,N-diiso-
propylethylamine (DIPEA) under visible-light irradiation
with a 20 W household compact fluorescent lamp (CFL) did
induce cyclization to give 1b albeit in low yield (15%,
entry 2). Changing the catalyst to [Ir(ppy)2(dtbbpy)]PF6
(ppy = 2-phenyl pyridine; dtbbpy = 4,4’-di-tert-butyl-2,2’-
bipyridine),[8] a complex with a higher redox potential (IrIII/
IrII vs RuII/RuI), dramatically enhanced the yield to 96%
(entry 3).[9] While the reaction in methanol gave a similar
result, other conditions employing dimethylformamide
(DMF) or triethylamine (TEA) resulted in a lowered yield
or a longer reaction time (entries 4–6).[10] Interestingly, when
a 2 W blue light-emitting diode (LED, lmax = 454 nm) strip
was used as the light source instead of a CFL, the reaction was
completed in 1.5 h to give 1b in 98% yield (entry 7).[11] It was
noteworthy that these photocatalytic reactions did not form
the simple dehalogenation product 1c in contrast to the Ni
catalysis (entries 1 vs 2–7).
Our initial survey was focused on examining the feasibility
of visible-light photoredox catalysis in the cyclization of aryl
iodide 1a (Table 1). The reaction of 1a under the nickel-
catalyzed conditions failed to afford 1b at 258C,[7] but gave 1c
[*] H. Kim, Prof. C. Lee
Department of Chemistry
Seoul National University
Seoul 151-747 (Republic of Korea)
Fax: (+82)2-875-6636
E-mail: chulbom@snu.ac.kr
Homepage: http://cbleegroup.snu.ac.kr/
[**] This work was supported by the Basic Research Laboratory (BRL)
and Global Ph.D. Fellowship (GPF) programs of the National
Research Foundation (NRF) funded by the Korean government. We
thank Prof. Stefan Bernhard at Carniegie Mellon University and
Jong-In Hong at Seoul National University for valuable suggestions,
and Dr. Ik-Soo Shin and Yongjun Jeon for obtaining cyclic
voltammograms.
The visible-light-induced Ir catalysis established in the
initial studies was tested for a range of aryl and alkenyl halide
reactions (Table 2). Under the standard conditions with either
CFL or LED irradiation, aryl (Table 2; entries 1–4) and
alkenyl (entries 5–7) halides underwent reductive cyclization
to furnish the corresponding carbo- and heterocyclic products
in excellent yield. Consistent with the observations made in
the initial studies, the use of blue LED led to a significant
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201203599.
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Table 2: Visible-light-induced Ir-catalyzed reductive cyclization and
hydrodehalogenation of aryl and alkenyl halides.
reaction of 9a, containing both alkyl and aryl halides, the
reduction of the a-carbonyl chloro and aryl iodo groups
proceeded to provide the fully reduced 9b (entry 8),[12] while
Entry Reactant
Product
Time Yield
[h][a] [%][a,b]
À
the relatively higher reactivity of a C I bond within an aryl
system was noted in the reaction of polyhaloarene 10a
(entry 9).
45
96
1
Having established the applicability of the photochemical
reaction to aryl and alkenyl halides, we next probed the
potential of this Ir catalysis to effect radical reactions of
unactivated alkyl halides. As shown in Table 3, an array of
alkyl iodides devoid of an activating group underwent the
photocatalytic reaction to give the cyclization (entries 1–8)
and hydrodehalogenation (entries 9–10) products. The irra-
diation with blue LEDs, once again, had beneficial effects on
these reactions, resulting in shorter reaction times and higher
(10) (97)
2a
1b
4
(1)
90
(93)
2
3a
3b
4b
54
(6)
82
(94)
3
4a
19
(5)
89
(91)
Table 3: Visible-light-induced Ir-catalyzed reductive cyclization and
hydrodehalogenation of alkyl iodides.
4
5a
5b
Entry Reactant
Product
Time Yield
[h][a] [%][a,b]
19
(6)
92
(93)
5
6a
6b
6b
7
(2)
93
(94)
1
12a
12b
29
90
6
(10) (94)
4
(2)
86
(88)
2
7a
13a
13b[c]
52
88
7
(12) (93)
8a[c]
8b
9
(3)
94
(94)
3
14a
14b
3
87
8
(1)
(90)
7
(2)
81
(83)
4
9a
9b
15a
15b
16b
5
90
9
(1.5) (92)
8
95
10a
10b
5
(3)
(96)
16a
50
89
10
9
(3)
89
(90)
(14) (93)
6
11a
11b
17a
17b
18b
[a] Reaction conditions: 3 mol% [Ir], 10 equiv DIPEA, substrate
(>0.5 mmol) in MeCN (0.01m), 258C, 20 W CFL or 2 W blue LED strip.
The numbers in parentheses refer to the reactions using a blue LED strip.
[b] Yield of isolated product. [c] A 3:1 mixture of E/Z isomers was used.
2
(1)
90
(95)
7
18a
18
(4)
62
(72)
8
improvement in both the reaction time and yield. As
expected, bromide 2a was found to be less reactive than
iodide 1a, and the differential reactivity was also manifested
in the reaction of 3a that gave 3b without reducing the bromo
group (entries 1 and 2). The E and Z isomers of the alkenyl
iodides exhibited similar reactivity, both giving rise to the
same cyclized products (entries 5–7). The feasibility of
cyclization independent of the E/Z configuration is indicative
of a free-radical mechanism for these reactions. This Ir-
catalyzed route also proved to be efficient in the hydro-
dehalogenation of aryl and alkenyl halides, affording the
reduction products in high yield (Table 2, entries 8–10). In the
19a
19b
20b
21b
11
(4)
90
(93)
9
20a
16
(5)
83
(88)
10
21a
[a] Reaction conditions: 3 mol% [Ir], 10 equiv DIPEA, substrate
(>0.5 mmol) in MeCN (0.1m), 258C, 20 W CFL or 2 W blue LED strip.
The numbers in parentheses refer to the reactions with LEDs. [b] Yield of
isolated product. [c] Diastereomeric ratio=59:41. Ts=p-toluenesul-
fonyl.
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yields. Primary as well as secondary iodides with various
alkene and alkyne acceptors participated well in the reaction.
Moreover, the reaction could be carried out on multigram
scales using 1 mol% of the catalyst without decrease in the
yield.[13] As observed from the reactions of alkenyl and aryl
substrates, no uncyclized reduction product was obtained
from the reactions, in contrast to the Ni-catalyzed process
where cyclization was typically accompanied by simple
hydrodehalogenation (3–5%)[7] and with the organotin-medi-
ated processes requiring slow addition of the reagent for the
suppression of the simple reduction.[14]
a free-radical cyclization process through the photoredox
catalytic cycle. Crucial to this mechanism is the reductive
À
scission of the C I bond induced by the single-electron
transfer (SET) from the IrII species (i.e. [IrIII(ppy)2-
(dtbbpyCÀ)]). Whereas the SET to alkyl halides is likely to
À
populate the s*(C X) orbital directly leading to the dissoci-
ation of the halide ion, the reaction of aryl halides may
involve a discrete radical anion intermediate, which then
undergoes electronic reorganization from the p system to the
orthogonal s*(C X) orbital for bond breaking.[17] In the case
À
of alkenyl halides, the SET seems less efficient owing to the
To gain insight into the mechanism of these photoredox
processes, a series of deuterium labeling experiments was
carried out for the hydrodeiodination of 22a (Scheme 1).
high-lying p*/s* orbital, which, together with the high
2
À
dissociation energy of C(sp ) X bonds, may account for the
longer reaction times generally observed in these systems
(Table 2 entries 5–7 and 10). Finally, the reductive process is
completed by a hydrogen-atom abstraction of the carbon-
centered radical from the a-amino position of the aminium
radical cation, as substantiated by the labeling experiment.[18]
In summary, we have described the iridium-catalyzed
reductive cyclization and hydrodehalogenation of organo-
halides induced by visible light. This work shows that a broad
range of alkyl, alkenyl, and aryl halides, not limited to alkyl
substrates with an activating group, are competent partic-
ipants in these photocatalytic free-radical processes and
furnish the products in excellent yield. It has also been
demonstrated that a simple alteration in reaction conditions,
such as changing light sources, can bring about significant rate
acceleration. These findings establish the feasibility of using
structurally diverse organohalides for various free-radical-
mediated reactions through a convenient and environmen-
tally benign catalytic means that makes use of visible light.
Scheme 1. Hydrodeiodination of 22a with deuterated amine 23.
While a kH/kD of 2.1 was estimated from the reaction
employing a 1:1 TEA/[D15]TEA mixture,[15] the reduction
with deuterated amine 23 resulted in 27% deuterium
incorporation. From further examination of the product
mixture by hydrolysis and acylation with 4-phenylbenzoyl
chloride, amides 24–26 were obtained (Scheme 1, yield based
on 22a), which arose presumably from their corresponding
imminium salts generated by the oxidative degradation of
23.[16]
Received: May 9, 2012
Revised: September 16, 2012
Published online: November 4, 2012
Keywords: cyclization · halides · iridium · photochemistry ·
.
radical reactions
A proposed mechanism of the Ir-catalyzed reaction is
depicted in Scheme 2, where the iridium complex mediates
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lyzed reductive cyclization.
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