Visible light-mediated decarboxylative amination of indoline-2-carboxylic acids catalyzed by Rose Bengal

Article abstract of DOI:10.1039/c6ra17524d

A visible light-induced decarboxylative amination of indoline-2-carboxylic acids and azodicarboxylate esters has been developed, in which the oxidation of α-amino acids provides a versatile CO₂-extrusion platform to generate α-aminoalkyl radicals. The corresponding products were obtained with yields of up to 72% catalyzed by a metal-free photocatalyst under mild conditions.

Full text of DOI:10.1039/c6ra17524d

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Visible light-mediated decarboxylative amination of
indoline-2-carboxylic acids catalyzed by Rose Bengal
MengꢀJie Zhanga, Griffin M Schroederb, YanꢀHong Hea,* and Zhi Guana,*
a
Key Laboratory of Applied Chemistry of Chongqing Municipality, School of Chemistry and Chemical
Engineering, Southwest University, Chongqing 400715, PR China
b Chemistry Department, College of Saint Benedict and Saint John’s University, MN 56374, USA
E-mails: heyh@swu.edu.cn (for Y.-H. He); guanzhi@swu.edu.cn (for Z. Guan)
Abstract
A visible lightꢀinduced decarboxylative amination of indolineꢀ2ꢀcarboxylic acids and
azodicarboxylate esters has been developed, in which the oxidation of αꢀamino acids provides a
versatile CO2ꢀextrusion platform to generate αꢀaminoalkyl radicals. Corresponding products were
obtained with yields of up to 72% catalyzed by a metalꢀfree photocatalyst under mild conditions.
Key words: photocatalysis; visible light; decarboxylative amination; indoline; Rose Bengal
Introduction
Visible light as renewable energy source has the advantages of being inexpensive,
nonpolluting and abundant1. Photocatalysis driven by visible light under mild and green
conditions has received widespread attention and has recently become an intriguing field of
research in organic synthesis2ꢀ4. Different types of photocatalysis have been developed. For
example, chiral organocatalysts (such as thioureas) have been used for enantioselective
photoreaction in which substrate is combined to the chiral catalyst through hydrogen bonds
providing a chiral environment5ꢀ12. Metal complexes have been reported as visibleꢀlight
photocatalysts to perform a variety of photoredox reactions13ꢀ15. Many common visible light
2+
photocatalysts are polypyridyl complexes ruthenium and iridium, such as Ru(bpy)3 and
1

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[Ir(ppy)2(dtbbpy)]PF615. However, the ruthenium and iridium salts have numerous disadvantages,
including high cost, potential toxicity, and dwindling availability16. Contrastingly, organic
dyes—such as eosin Y16ꢀ18, MesꢀAcrClO419,20, fluorescein21 and Rose Bengal22ꢀ26—are more
environmentally friendly, inexpensive and more readily available24ꢀ27 than their metallic
counterparts. Recently, organic dyes have also been applied as effective visible light photoredox
organocatalysts. Different types of mechanism for metalꢀfree photoredox catalysis have been
proposed, such as photoinduced electron or energy transfer process28ꢀ35
.
Over the past decade, decarboxylation reactions have widely received attention, largely due
to the ability to change carboxylic acids—which are generally inexpensive and nonꢀtoxic—into
more attractive reactants36. The following methods are often used to realize the CO2ꢀextrusion:
transition metal catalyzed decarboxylation process37,38 and radicalꢀmediated processes, such as
persulfate oxidation decarboxylation method under high temperature39ꢀ41 and photoinduced single
electron transformation36,42ꢀ44. Generally, radical decarboxylation of carboxylic acids has several
advantages for synthetic organic chemistry: the elimination of CO2 as the traceless byproduct does
not influence the reaction system; furthermore, the resulting radical intermediates can be changed
into more attractive, valuable chemical products with relative ease45.
The indoline substructure is considered an advantageous backbone because it is found in
many types of organisms, including pharmaceutical molecules46,47, naturally occurring alkaloids,
and many biologically active molecules48,49. Therefore, the synthesis of indoline derivatives has
attracted much attention50. In 2012, the Nishibayashi group reported amination of protected
indoline via αꢀaminoalkyl radicals by using an iridium complex photocatalyst under visible light51.
Considering the importance of amination products of indoline, it is still desirable to develop more
environmentally friendly methods for the synthesis of these compounds. Herein, we report the first
metalꢀfree decarboxylative amination reaction of N-protected indolineꢀ2ꢀcarboxylic acids and
azodicarboxylate esters catalyzed by an organic dye in the presence of visible light under mild
conditions.
Results and discussion
Initially, the model reaction of NꢀBocꢀindolineꢀ2ꢀcarboxylic acid (1a) with diꢀtertꢀbutyl
azodicarboxylate (2a) at room temperature in presence of a 32W household fluorescent light bulb
2

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was employed to screen the reaction conditions.
At the beginning, a preliminary organic dye screening was performed to determine the most
efficient photocatalyst for the model reaction (Table 1). The model reaction without catalyst only
gave the product 3a in a low yield of 20% under visible light (Table 1, entry 1). When eosin Y
and fluorescein were introduced as photocatalysts, the product 3a was obtained in yields of 34%
and 35%, respectively (Table 1, entries 2 and 3). The best result of 46% yield was obtained by
using Rose Bengal (Table 1, entry 4). The data indicated that the tested organic dyes indeed
promoted the reaction. Therefore, Rose Bengal was chosen as the suitable photocatalyst for the
reaction. To verify the effect of the visible light, the model reaction with Rose Bengal (2 mol%)
was performed in dark, and as expected no reaction was observed (Table 1, entry 5). This control
experiment revealed that visible light is essential for the catalytic operation of this new
decarboxylative amination reaction. Next, the influence of molar ratio on the model reaction was
investigated. When the molar ratio of 2:1 (1a:2a) was employed, the best yield of 52% was
received (Table 1, entry 6). Thus, the molar ratio of 2:1 (1a:2a) was selected as the optimal
condition (for details, please see the supporting information Table S1).
Table 1. Organic dye photocatalyst screeninga.
Entry
Catalyst
None
Yield (%)b
1
2
3
4
5
6
20
34
35
46
0
Eosin Y (5 mol%)
Fluorescein
Rose Bengal
Rose Bengal (in dark)
Rose Bengalc
52
a Reaction conditions: 1a (0.3 mmol), 2a (0.2 mmol), CH3CN (1.0 mL), and catalyst (2 mol%) under irradiation of
32 W fluorescent bulb at rt for 48 h.
b Isolated yield.
c 1a (0.4 mmol).
3

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Figure 1. Structures of the tested organic dyes
The choice of solvent is an important factor to promote this process. Various solvents were
tested with 2 mol % of Rose Bengal as a photocatalyst under visible light (Table 2). Among the
tested solvents, the best yield of 52% was obtained in CH3CN (Table 2, entry 1). The reactions in
CH2Cl2 and ClCH2CH2Cl both resulted in a moderate yield of 50% (Table 2, entries 2 and 3). The
reactions in the other tested solvents did not give better results (Table 2, entries 4ꢀ12). A trace
amount of product 3a was observed in cyclohexane (Table 2, entry 13). Based on the results,
CH3CN was selected as the most suitable solvent for the reaction.
Table 2. Solvent screeninga.
Entry
Solvent
CH3CN
Yield (%)b
1
2
52
50
50
38
44
40
38
33
22
16
9
CH2Cl2
3
ClCH2CH2Cl
CHCl3
4
5
EtOAc
6
n-BuOAc
EtOH
7
8
CH3OH
9
Toluene
DMF
10
11
12
13
DMSO
1,4ꢀDioxane
Cyclohexane
8
trace
a Reaction conditions: 1a (0.4 mmol), 2a (0.2 mmol), solvent (1 mL), and Rose Bengal (2 mol%) under irradiation
4

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of 32 W fluorescent bulb at rt for 48 h.
b Isolated yield.
Next, the influence of solvent volume, catalyst dosage, and wattage of lamp were all
investigated. The optimized reaction conditions were found to consist of the following: a volume
of 0.5 mL (CH3CN), a Rose Bengal dosage of 1 mol%, and a fluorescent lamp wattage of 23 W
(for details, please see the supporting information Tables S2- S4).
With the optimal conditions in hand, we investigated the substrate scope of the visible
lightꢀmediated Rose Bengal catalyzed decarboxylative amination reaction. In general, various
azodicarboxylate esters readily participated in this reaction to generate amination products of
indoline in moderate to good yields. Unfortunately, the 2,2ꢀazobisisobutyric acid dimethyl ester
(2n) failed to give the desired product 3n and the starting material 2n was intact (Table 3, entry
13), probably due to the electronic effect (N connects to alkyl instead of ester carbonyl group like
the others). NꢀBoc, Cbz protected indoline carboxylic acids could be successfully applied and
provided the desired products in moderate to good yields (Table 3, entries 1ꢀ8). When indoline
carboxylate acid with strong electron withdrawing protecting group –sulfone was used, a low yield
of 21% was observed (Table 3, entries 9 and 10). A low yield of 36% was obtained when
Nꢀbenzoyl protected indoline carboxylate acid was employed as a substrate, likely due to its poor
solubility in CH3CN (Table 3, entry 11). However, the presence of the free NH group of indoline
carboxylic acid resulted in the formation of indole 4 and diꢀtertꢀbutyl hydrazineꢀ1,2ꢀdicarboxylate
5 instead of amination product 3n (Scheme 1). The data indicated that the protecting group on
nitrogen atom of indoline carboxylate acid has great influence on the decarboxylative amination
reaction. This reaction can be expanded to other cyclic system that incorporates αꢀoxy group
(2,3ꢀdihydrobenzofuranꢀ2ꢀcarboxylic acid) providing the corresponding amination adduct 3l in a
low
yield
of
21%
(Table
3,
entry
12).
In
addition,
NꢀBocꢀ1,2,3,4ꢀtetrahydroisoquinolineꢀ1ꢀcarboxylic acid (1m) can also participate in the reaction
giving the corresponding product 3m with a yield of 19% (Scheme 2). For all the reactions, 2
equiv of indoline carboxylate acids (1) was used to react with azodicarboxylate esters (2). In the
most cases, nearly all azodicarboxylate esters (2) were consumed, with indoline carboxylate acids
(1) left and unknown byꢀproducts were observed. For the model reaction, a conversion rate of 99%
5

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was determined after 48 h (5 generated from reduction of 2a was investigated as one of the
byꢀprodcts) (Table 3, entry 1). This method represents a general procedure for the amination of
indolines at the Cꢀ2 position. All the products obtained (3aꢀ3m) are new compounds, and their
1
structures have been determined by H NMR, 13C NMR and HRMS (for details, please see the
Supporting information).
Table 3. Substrate scopea.
Entry
Product No.
3a
Product
Time (h)
Yield (%)b
1
48
66 (99c)
2
3
4
5
6
7
3b
3c
3d
3e
3f
48
48
48
48
48
48
72
51
42
66
63
54
3g
6

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8
9
3h
3i
48
48
42
21
HN COO-iPr
N
COO-iPr
N
O S
O
10
11
3j
48
48
22
38
3k
12
13
3l
58
48
22
ꢀꢀ
3n
a Reaction conditions: 1 (0.4 mmol), 2 (0.2 mmol), CH3CN (0.5 mL), and Rose Bengal (1 mol%) under irradiation
of 23 W fluorescent bulb at rt.
b Isolated yield (average of two trials, error: ±4%).
c Conversion rate.
Scheme 1. The reaction of unprotected indoline carboxylate acid.
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Scheme 2. The reaction of 1m and 2m.
COOH
N
1a
Boc
N
Boc
6
N
N
Boc
Boc
H
HN Boc
N
Boc
visible light
Boc
N
Boc
-H
-CO2
N
Boc
N
N
N
N
Boc
Boc
3a
2a*
Boc
8
2a
7
Scheme 3. Possible pathway without catalyst
In order to gain more insight into the mechanism of the visible lightꢀmediated Rose Bengal
catalyzed decarboxylative amination reaction fluorescence experiments with diꢀtertꢀbutyl
,
azodicarboxylate (2a) were performed. The absorption maximum of 2a is centered at λmax = 205
nm with a significant absorption in the UV region. However, 2a exhibits a weak absorption in the
visible light region at λ = 402 nm (ξ= 36 L·mol⁻¹·cm⁻¹) (the spectrum shown in the Supporting
information). Considering the product yield was 20% without the presence of a catalyst (Table 1,
entry 1), we speculated that 2a could be the light absorber. In the absence of Rose Bengal, product
is probably formed via the process shown in Scheme 3. Upon light absorption, 2a accepts a
photon from the visible light to generate excited state 2a*. Reductive quenching of 2a* by
NꢀBocꢀindolineꢀ2ꢀcarboxylic acid (1a) through singleꢀelectron transfer gives the corresponding
radical anion (7). At the same time, αꢀaminoalkyl radical 6 is formed and CO2 is extruded. The
radicalꢀradical coupling of 6 and 7 gives intermediate 8. The protonation of 8 provids the product
(3a). Because 2a has a weak absorption in the visible light region, the reaction without catalyst
only gave a low yield.
8

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Scheme 4. Possible mechanism in the presence of Rose Bengal.
Finally, based on current literature51ꢀ53, a plausible reaction pathway in the presence of Rose
Bengal is proposed in Scheme 4. Upon light absorption, Rose Bengal (RB) accepts a photon from
the visible light to generate excited state RB*. Subsequent singleꢀelectron oxidation of
NꢀBocꢀindolineꢀ2ꢀcarboxylic acid (1a) by the visibleꢀlightꢀexcited photocatalyst generates the
reduced RB(ꢀ) and αꢀaminoalkyl radical 6, and CO2 is extruded. Then the reduction of the
azodicarboxylate ester (2a) by RB(ꢀ) affords radical anion (7). Intermediate 8 is generated by the
radicalꢀradical coupling of 6 and 7, and the αꢀaminated product (3a) is obtained by the protonation
of 8.
Conclusion
In summary, we have developed a visible lightꢀmediated decarboxylative amination of
Nꢀprotected indolineꢀ2ꢀcarboxylic acid and azodicarboxylate esters. This reaction can be expanded
to other cyclic system that incorporates αꢀoxy group and NꢀBocꢀtetrahydroisoquinoline carboxylic
acids. 13 new amination products were prepared with yields of up to 72%. The reaction was
catalyzed by Rose Bengal as a metalꢀfree photocatalyst under mild conditions. This process
provides a general method for the amination of indolines at the Cꢀ2 position.
General procedure for the decarboxylative amination
A roundꢀbottom flask was charged with Rose Bengal (1 mol%), acids 1 (0.4 mmol) and
azodicarboxylate esters 2 (0.2 mmol), to which CH3CN (0.5 mL) was introduced. The resultant
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mixture was stirred at rt under irradiation of 23 W fluorescent bulb for the specified reaction time
and monitored by TLC. The crude mixture was concentrated in vacuo. The residue was purified
by flash column chromatography (petroleum ether/ ethyl acetate10:1ꢀ3:1) to give the products.
Acknowledgements
This work was financially supported by the National Natural Science Foundation of China
(No. 21276211, No. 21472152 and No. 21542014) and the Basic and Frontier Research Project of
Chongqing (cstc2015jcyjBX0106).
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Graphical abstract
A visible-light-induced decarboxylative amination of N-protected indoline-2-carboxylic acids and
azodicarboxylate esters catalyzed by Rose Bengal has been developed.