Organic dye-catalyzed radical ring expansion reaction

Article abstract of DOI:10.1039/c8ra02383b

Herein, we reported an attractive method for synthesizing medium-sized rings that are catalyzed by erythrosine B under fluorescent light irradiation. This synthetic approach featured mild conditions, a facile procedure, a broad substrate scope, and modera

Full text of DOI:10.1039/c8ra02383b

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Organic dye-catalyzed radical ring expansion
reaction†
Cite this: RSC Adv., 2018, 8, 15825
Masato Deguchi, Akitoshi Fujiya, Eiji Yamaguchi, Norihiro Tada,
Bunji Uno
*
and Akichika Itoh
Received 19th March 2018
Accepted 23rd April 2018
Herein, we reported an attractive method for synthesizing medium-sized rings that are catalyzed by
erythrosine B under fluorescent light irradiation. This synthetic approach featured mild conditions,
a facile procedure, a broad substrate scope, and moderate-to-good yields.
DOI: 10.1039/c8ra02383b
rsc.li/rsc-advances
Medium-sized rings are present in numerous important natural
More than 30 years ago, the Beckwith–Dowd ring expansion
products and pharmaceuticals.1 Therefore, several researchers reaction6 was introduced as a novel method to form medium-
have strived to develop various methods, including olen sized rings (Scheme 1(b)). This approach can be an attractive
metathesis,2 fragmentation,3 and pericyclic reactions4 alternative to conventional strategies for synthesizing the
(Scheme 1(a)), for synthesizing these materials. Nevertheless, abovementioned structures. Actually, the fact that no new
unfavorable transannular interactions and entropic factors unsaturated C–C bonds are formed as part of this method
typical of rings of this size make this task quite challenging.5 renders hydrogenation processes unnecessary, thereby mini-
Thus, we believe that new methods need to be developed to mizing the impact of transformation on the substrate.
solve these problems.
Certainly, this approach could be applicable to the synthesis of
natural products.7 However, the original method requires
reagents, e.g., azobisisobutyronitrile (AIBN) or tributyltin
hydride (Bu3SnH), that are difficult to handle. Some related
methods that employ other reagents, including Sm,8 Zn or In,9
B12–TiO2 hybrid catalyst,10 silane,11 and amines,12 have been
reported. Recently, a few synthetic approaches based on photo-
induced reactions have been developed.12b,12d,13 A previous study
reported that even a-(u-carboxyalkyl) b-keto esters could be
employed as relevant substrates.13
In this context, we aimed to establish a more efficient and
facile method than conventional ones to prepare a broad range
of medium-sized rings via a ring expansion reaction. We
continuously investigated various photo-initiated reactions by
employing a photosensitizer and a uorescent lamp as a light
source.14 For example, the CDC cross-coupling reaction14a and
1,3-dipolar cycloaddition/aromatization reaction14d under
photooxidative reaction conditions have been reported. While
investigating these reactions, we found that organic dyes
induced electron transfer from amine substrates to molecular
oxygen.14a,14d Thus, we envisioned that C–halogen bonds can be
cleaved in the presence of a sacricial amine using photo-
sensitized substrates as a springboard. Herein, we reported
Scheme 1 (a) Examples of conventional approaches for the synthesis
of medium-sized rings. (b) Beckwith–Dowd ring expansion reaction.
a
convenient and environmentally friendly method that
employs a photo-induced reaction as part of the Beckwith–
Dowd ring expansion route to synthesize medium-sized rings.
We initiated our study with the optimization of the reaction
conditions (Table 1). Methyl 1-(iodomethyl)-2-oxocyclohexane-
1-carboxylate (1a) was chosen as a substrate. A mixture of 1a
and different types of amines and photocatalysts was irradiated
with four uorescent lamps under a nitrogen atmosphere for
Gifu Pharmaceutical University, 1-25-4, Daigaku-nishi, Gifu 501-1196, Japan. E-mail:
itoha@gifu-pu.ac.jp
† Electronic supplementary information (ESI) available: Experimental procedures,
product characterization, time course of 1a and 2a, emission spectrum of the
uorescent lamp, and detailed information of the DFT calculation. See DOI:
10.1039/c8ra02383b
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Table 1 Optimization of the reaction conditions
Entry
Photocatalyst
Solvent
Amine
2aa (%)
1a0a (%)
1aa (%)
1
2
3
4
5
6
7
8
9
Erythrosine B (EB)
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
MeCN
DMF
iPr2NEt
78(83)
50
12
51
11
36
70
18
0
5
10
0
Eosin Y
AQN-2-Cl
EB
EB
EB
EB
EB
EB
iPr2NEt
18
10
26
15
21
0
iPr2NEt
Et3N
63
22
74
40
26
18
14
1-Methyl imidazole
iPr2NH
iPr2NEt
iPr2NEt
15
79
CHCl3
iPr2NEt
a
Yields are determined by 1H NMR spectroscopy using 1,1,2,2-tetrachloroethane as an internal standard. The number in parentheses denotes
isolated yield.
20 h in DMSO. Aer extensive investigations, we found that standard reaction conditions to substrate 1h, the yield of the
i
a combination of erythrosine B (EB) and Pr2NEt was the most desired product 2a0 was around 10%. However, adding a cata-
efficient combination for this transformation, affording the lytic amount of Ag2CO3 and extending the reaction time
desired product 2a in 83% isolated yield (entry 1).15 This result increased the yield. Additionally, it is worth noting that this
can be explained by two facts: (a) the maximum absorption of reaction could be carried out in an air atmosphere with almost
EB (ꢀ520 nm)16 coincides with the wavelength of the uorescent no decrease in product yield. This result indicates that the
lamp and (b) iPr2NEt is an effective reductive quencher presence of oxygen or water in the atmosphere above the reac-
(Eox(iPr2NEtc+/iPr2NEt) ¼ +0.68 V vs. SCE; e.g., Eox(NEt3c+/NEt3) ¼ tion mixture has a negligible impact on in situ radical genera-
+0.99 V vs. SCE).17a,17b In fact, when we employed other combi- tion and subsequent steps to furnish the ring expanded
nations of photocatalysts, amines, and solvents, 2a was ob- product.
tained in
a
relatively lower yield. Furthermore, in our
Scheme 2 summarizes the results of additional experiments
experiments, we recovered 1a and/or its undesired hydrogena- conducted to investigate the substrate scope. We tested
tion product 1a0 from the reaction mixture in substantial a substrate with an iodopropyl side chain, 1i, and found that the
amounts (entries 2–9).
addition of a silver salt and extension of the reaction time led to
With the optimized reaction conditions in hand, we next the formation of a tertiary alcohol with a fused ring, 3i, instead
explored the substrate scope of this transformation (Table 2). of the corresponding cyclononanone (eqn (1)). Additionally, we
We found that compounds comprising 5–8-membered rings found that this reaction is applicable to substrates with ketones
were suitable substrates for the reaction and gave the desired in open-chain moieties, such as 1j and 1k. Particularly,
products in good yields (2a–2e). When we employed 1,2,3,4- although 2j was volatile, and we needed to treat it carefully
tetrahydro naphthalene-type substrate 1f, the desired reaction during isolation, these substrates gave the rearranged product
proceeded smoothly. On the other hand, indane-based in high yield (eqn (2) and (3)).
substrate 1g gave a mixture of the desired product and the
Next, we performed some experiments to elucidate the
aromatized product (3g). Due to the difficulty of achieving reaction mechanism (Scheme 3). We observed that no ring
complete separation of the desired product and 3g, we further expansion product 2a was obtained when we omitted EB or
investigated the reaction conditions in order to obtain a single iPr2NEt or photo-irradiation from the uorescent lamps. Addi-
product. Although the changes in the reaction time and in the tionally, if we added 1 equiv. of TEMPO (2,2,6,6-tetramethylpi-
composition of the overhead atmosphere did not substantially peridine 1-oxyl free radical), the desired product was obtained
affect the course of the reaction, we found that the addition of 3 in low yield and the presence of a TEMPO-adduct was detec-
equiv. of LiOH led to the formation of the aromatized product ted.18 This result conrms the idea that this reaction proceeds
3g exclusively in moderate yield. Furthermore, we tested via a radical pathway.
whether compounds that contain the C–Br bond could also be
Based on these results, previous reports on the Beckwith–
used as substrates for this reaction. When we applied the Dowd ring expansion reaction,6 and recent reports of radical
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Table 2 Substrate scope
Substrate
Product
Yieldsa (%)
83%
83%
82%
81%
77%
74%
Scheme 2 Applying the reaction conditions on substrates with three-
carbon lateral chain and acyclic b-keto esters.
67%
76%
64%
62%
67%
71%b
Scheme 3 Results of the control experiments.
71%c
this SET step, we calculated the Gibbs energies involved in the
reaction between 1a and EB in a solution environment (solvent:
DMSO; 3 ¼ 46.826) simulated by the IEF-PCM model using the
M06-2X functional.20 The standard Gibbs energy change (DGꢂ)
calculated for the cleavage reaction of 1a with EB2ꢁ (1a + EB2ꢁ
/ 4a + Iꢁ + EBcꢁ) and with EBꢁ (1a + EBꢁ / 4a + Iꢁ + EBc) in
the absence of light irradiation from the uorescent lamp is
1.72 and 1.27 eV, respectively, as shown in Scheme 5. This
indicates that these reactions are thermodynamically uphill
electron-transfer processes. On the other hand, it is well known
that the DGꢂ values for excited EB2ꢁ(EB2ꢁ*)/EBcꢁ and excited
EBꢁ(EBꢁ*/EBc) redox pairs are given by the amended Rehm–
Weller equation with the excited-state energies ꢁ(Ehn) as
DGꢂ(EB2ꢁꢁ*/EBcꢁ) ¼ DGꢂ(EB2ꢁ/EBcꢁ) ꢁ Ehn and DGꢂ(EB */EBc) ¼
DGꢂ(EB /EBc) ꢁ Ehn, respectively.21,22 The reported excited-state
energy of EB is 2.34 eV;16a thus, the DGꢂ values for the two
a
Yields shown in the table are all pure, isolated yields. The underlined
percentages are the yields when the reactions were performed under air
b
c
atmosphere. 3 equiv. of LiOH added. 5 mol% of Ag2CO3 added,
reaction time is 40 h.
reactions set off by C–halogen bond cleavage,17b,19 we inferred
a plausible mechanistic pathway for the reaction, which is
depicted in Scheme 4. The rst step in the C–I bond cleavage
reaction of 1a is photo-induced single electron transfer (SET)
from the excited state of EB. In the experimental conditions
employed, EB is considered to be a monoanionic (EBꢁ) or dia-
nionic (EB2ꢁ) species because its commercially available diso-
dium salt (EBNa2) is added to the reaction mixture containing
DIPEA without acidic treatment. To gain further insight into
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mentioned C–I bond cleavage reactions (1a + EB2ꢁ* / 4a + Iꢁ
+
EBcꢁ and 1a + EBꢁ* / 4a + Iꢁ + EBc) involving photo-induced
SET are ꢁ0.62 and ꢁ1.07 eV, respectively, which indicate that
the Ehn value is sufficiently high for both bond cleavage reac-
tions to be thermodynamically feasible. These results indicate
that the cleavage reaction of 1 is governed by an exergonic C–I
bond cleavage mechanism involving SET from EB2ꢁ* or EBꢁ*.
The primary radical 4 thus generated attacks the ketone, and
then a rapid cyclopropane ring opening occurs. Meanwhile, the
oxidized EB is reduced by the sacricial amine, and the catalytic
cycle of EB is completed. No particular H donor was necessary
for this reaction to proceed; therefore, we assumed that an
“extra” hydrogen atom of the product is derived from iPr2NEt. It
believed that abstraction of a hydrogen atom from the tri-
alkylammounium radical cation could occur,23 which strongly
supports the above mechanism.
Scheme 4 Plausible reaction mechanism.
Conclusions
In conclusion, we have developed an environmentally friendly
ring expansion reaction to synthesize medium-sized rings. This
facile method involves the use of a uorescent lamp as a source
of irradiated light and an easy-to-handle photocatalyst. Attrac-
tively, this reaction proceeds under very mild conditions and is
applicable to a broad spectrum of substrates. We believe that
our method using a photo-triggered reaction course affords
access to various pharmaceuticals and natural products.
Studies to determine the mechanistic details of the reaction and
expand its applicability to other useful substrates are currently
underway in our laboratory.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
The authors would like to thank Enago (www.enago.jp) for the
English language review.
Notes and references
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