Sunlight-induced C–C bond formation reaction: Radical addition of alcohols/ethers/acetals to olefins

Article abstract of DOI:10.1016/j.jphotochem.2021.113263

A sunlight-induced C–C bond formation reactions upon the addition of alcohols/ethers/acetals to olefins proceeded efficiently using di-tert-butyl peroxide (DTBP). The reactions proceeded faster than many of the previously reported sunlight and many conventional lamp photolyses, typically in 3–4 h under irradiation with sunlight, in excellent yield using olefins bearing two electron withdrawing groups (EWGs) (product yield > 95 %) and in good to fair yield with olefins bearing one EWG. The yields observed for some products were ～20 % higher than those obtained using a conventional Xe lamp as the light source, which was confirmed to be due to a light intensity effect. Gram-scale experiments showed similar yields to those observed in their corresponding small-scale experiments.

Full text of DOI:10.1016/j.jphotochem.2021.113263

Journal of Photochemistry & Photobiology, A: Chemistry 413 (2021) 113263
Contents lists available at ScienceDirect
Journal of Photochemistry & Photobiology, A: Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
–
Sunlight-induced C C bond formation reaction: Radical addition of
alcohols/ethers/acetals to olefins
Mamiko Hayakawa, Hisashi Shirota, Souta Hirayama, Ryuusei Yamada, Tadashi Aoyama,
Akihiko Ouchi*
Department of Materials and Applied Chemistry, College of Science and Technology, Nihon University, Kanda-Surugadai, Chiyoda-ku, Tokyo, 101-8308, Japan
A R T I C L E I N F O
A B S T R A C T
Keywords:
–
A sunlight-induced C C bond formation reactions upon the addition of alcohols/ethers/acetals to olefins pro-
ceeded efficiently using di-tert-butyl peroxide (DTBP). The reactions proceeded faster than many of the previ-
ously reported sunlight and many conventional lamp photolyses, typically in 3–4 h under irradiation with
sunlight, in excellent yield using olefins bearing two electron withdrawing groups (EWGs) (product yield > 95 %)
and in good to fair yield with olefins bearing one EWG. The yields observed for some products were ~20 %
higher than those obtained using a conventional Xe lamp as the light source, which was confirmed to be due to a
light intensity effect. Gram-scale experiments showed similar yields to those observed in their corresponding
small-scale experiments.
Sunlight
–
C
C bond formation
Olefin
Alcohol
Ether
Acetal
1. Introduction
oxygen atom using an excited ketone has been used for a long time [10,
11]. Recently, various photocatalysts other than ketones have also been
Sunlight is an infinite energy source and attempts to use this energy
in organic synthesis have been investigated for a long time [1]. How-
ever, sunlight is generally not used in organic synthesis due to its limited
applicability and/or low efficiency, particularly long reaction times, in
the previously reported reactions. Therefore, development of widely
applicable and efficient reactions is critical for using sunlight for synthetic
purpose. After a period of major interest in organic photochemistry
directed toward exotic molecules and reactions, interest is returning to
more commonly used reactions under the concept of green chemistry
used for the same purpose (Scheme 1, condition B). Besides ketones,
tetrabutylammonium decatungstate (TBADT) [12,13], uranyl chloride
and its cation [14], eosin Y [15], and cyclopropenium ion [16] have
been used as photocatalysts for the abstraction of the α-hydrogen atom
(Scheme 2, Path A). On the other hand, the generation of reactive spe-
cies (A’) using organic [17] and metal complex [18] photoredox cata-
lysts via a SET process, and the successive abstraction of an α-hydrogen
atom by A’ has been reported (Scheme 2, Path B). However, these
photocatalytic methods generally require long irradiation times and the
use of organic photocatalysts frequently makes the purification of the
products difficult, which are significant disadvantages for application in
organic synthesis.
–
[2].The formation of new C C bonds is one of the most important
research areas in organic synthesis and many studies have been reported
in organic photochemistry. One of these reactions is the addition of
carbon radicals to olefins (Scheme 1) [3]. Such reactions have also been
The requirement of long irradiation times in photocatalyst induced
reactions is probably due to the deactivation of the excited photocatalyst
(Scheme 2). In the case of hydrogen abstraction, competition between
hydrogen abstraction (Path A) and deactivation probably takes place
and significantly reduces the quantum yield for Path A. A similar
decrease in the quantum yield for the generation of reactive species (A’)
(Path B) is also expected. Therefore, to increase the efficiency of the
–
studied using thermal reactions via C H bond cleavage using various
radical initiators, mostly peroxides [4], but they generally require high
temperature and long reaction times, and usually give low yields for the
desired products (Scheme 1, condition A). Many attempts have been
made to improve these thermal reactions [5–9], but the use of high
temperature and/or long reaction times still remain as problems to be
overcome.
–
photochemical C H bond cleavage, the direct generation of reactive
–
For photochemical C H bond cleavage reactions used for the gen-
species (A’) with a high quantum yield is necessary (Path C).
eration of carbon radicals, the abstraction of an
α-hydrogen atom to an
To use low intensity sunlight as a light source for organic synthesis,
* Corresponding author.
E-mail address: ouchi.akihiko@nihon-u.ac.jp (A. Ouchi).
https://doi.org/10.1016/j.jphotochem.2021.113263
Received 9 February 2021; Received in revised form 16 March 2021; Accepted 19 March 2021
Available online 22 March 2021
1010-6030/© 2021 Elsevier B.V. All rights reserved.

M. Hayakawa et al.
Journal of Photochemistry & Photobiology, A: Chemistry 413 (2021) 113263
Scheme 1. The generation of carbon radicals (type A–C) and their addition to olefins.
Scheme 2. Reaction pathways for hydrogen abstraction used toward the generation of carbon radicals.
Fig. 1. (a) The emission of sunlight (red, 30th August 2020) and transmittance of a Pyrex flask (black), quartz cell (blue), and UV-29 cut-off filter (green). (b) The
absorbance of DTBP (black, 5 × 10ꢀ 5 M in EtOH) and emission of sunlight (red, 30th August 2020). (c) The emission of sunlight: sunny (S-Hi and S-Lo, 9th Sept.,
2020) and cloudy (C-Hi and C-Lo, 14th Sept., 2020). The emission of sunlight was measured in Tokyo (N 35◦, E 139◦).
the reactions must be highly efficient and should proceed within a short
period of time. Herein, we report an efficient sunlight-induced addition
reaction of alcohols/ethers/acetals to olefins using di-tert-butyl peroxide
(DTBP) (Scheme 1, condition C) that corresponds to Path C in Scheme 2
[19]. The reaction proceeded with shorter irradiation times when
compared to the many of the previously reported sunlight and conven-
tional lamp photocatalyzed reactions, and even gave higher yields in
via deactivation and thus reduces the reaction time. Photochemically
•
generated t-BuO abstracts an
α
-hydrogen atom to an oxygen atom [21,
22] to form a carbon radical (2′, Scheme 1), which adds to olefin 1 to
give the desired product 3 via radical 3′ and its hydrogen abstraction. In
addition, t-BuOH that is generated during the course of the reaction can
be easily removed via simple evaporation, which can avoid difficulties in
the purification process that are often caused when using organic
photocatalysts.
•
some cases. The generation of carbon radicals is carried out by t-BuO
generated via the direct photolysis of DTBP using sunlight. The quantum
•
yield for the generation of t-BuO is reported to be almost unity in the
liquid phase [20b], which minimizes the loss of absorbed photon energy
2

M. Hayakawa et al.
Journal of Photochemistry & Photobiology, A: Chemistry 413 (2021) 113263
2.2. Optimization of the reaction conditions
Table 1
Optimization of reaction conditions using 1a-cis and 2Aa.
The reaction conditions were optimized using dimethyl maleate (1a-
cis) and 2-propanol (2A) as a model system (equation 1). The ratio of 1a-
Entry
DTBP
Irradiation
time (h)
Light
Conversion of
Yield of
(mmol)
intensityb
(mW/cm2)
1a-cis c (%)
3aA c(%)
•
cis:DTBP was fixed at 2:1 because two t-BuO are generated from single
1
0.1
0.1
0.1
0
3
4
5
4
4
4
0
0.6ꢀ 1.6
97
94 [91]e
100
DTBP molecule and the reactions were conducted at room temperature.
Table 1 shows an irradiation time of 4 h was sufficient to completely
convert 1a-cis into the desired product (3aA)(Entries 1, 2); prolonged
irradiation did not cause any decomposition of the product (Entry 3).
The reaction also proceeded without DTBP, but a significant decrease in
the conversion of 1a-cis and yield of 3aA was observed (Entry 4). This
reaction was probably initiated via the direct excitation of 1a-cis due to
the slight absorption of sunlight by the substrate (vide supra) and the
consecutive abstraction of the hydrogen atom from 2A by the excited
carbonyl groups in 1a-cis [24]. This result indicates that DTBP is
necessary for an efficient reaction, i.e., reducing the irradiation time and
increasing the yield of the reaction. The reaction did not form the
desired product (3aA) without sunlight irradiation at room temperature
or at 35 ◦C (the maximum reaction tempreature during sunlight irradi-
ation) though a slight conversion of 1a-cis was observed (Entries 5, 6).
However, this conversion was due to the work-up procedure of the re-
action and not by a thermal reaction at room temperature (Entry 7).
2
1.5ꢀ 1.6
100
100
22
3
0.8ꢀ 1.7
97
4
1.5ꢀ 1.6
34 [7]e
0 [0]e
0 [0]e
0 [0]e
5
0.1
0.1
0.1
–
–
–
16
6d
7
13
16
a
Photolysis conditions: 1a-cis (0.2 mmol) and DTBP in 2A (10 mL), light
source: sunlight, N2 atm, room temp. Reactions were conducted using a 30 mL
Pyrex eggplant-shape flask.
b
The weather, the date, and the light intensity of sunlight for each experiment
are described in detail in Supplementary material.
c
Yields are based on the consumed starting material and determined by NMR
spectroscopy. The NMR yields are in good accord with the isolated yields [19].
Result of a single run.
d
e
The reaction was conducted at 35 ◦C.
Yield based on the initial amount of the olefin.
2. Results and discussion
2.1. Emission of sunlight and absorption of di-tert-butyl peroxide (DTBP)
and olefins
•
2.3. Sunlight photolysis of alcohols and olefins
The decomposition of DTBP for the generation of t-BuO have been
reported using a quartz cell and Xe lamp fitted with a UV-29 cut-off filter
[19]. Fig. 1a shows the wavelength of light transmitted through the
quartz cell and UV-29 cut-off filter is almost the same as that of a Pyrex
flask. This result indicated that Pyrex flasks, generally used in organic
synthesis laboratories, have a similar function as UV-29 filters, which
remove light with wavelength < 290 nm from the irradiated light source
to avoid considerable direct absorption of UV light by conjugated ole-
fins. Fig. 1b show the absorption wavelength of DTBP overlaps with the
emission of sunlight at > 290 nm, which could induce the efficient
With the optimized reaction conditions in hand, the scope of the
reaction was investigated using various alcohols and olefins (equation 2,
Table 2). The light intensity indicated in Table 1 was the range between
its maximum and the minimum during the photolysis reaction. The re-
action of 1a-cis and 2A gave 3aA in excellent yield (Entry 1), but the
isolation of 3aA using silica gel colum chromatography was unsuccessful
because of the partial lactonization to 3aA’ during chromatography.
Therefore, 3aA was transformed into lacone 3aA’ upon treatment with
HCl gas prior to its isolation, which proceeded in excellent yield (Entry
1). Dimethyl fumarate (1a-trans), maleic acid (1b-cis), fumaric acid
(1b-trans), and fumaronitrile (1c) all gave their desired products in
excellent yield within 4 h (Entries 2–7). In the case of 1b-cis and –trans,
the generated hydroxyl carboxylic acids underwent lactonization and
formed 3bA during workup (Entries 3–6).
•
cleavage of DTBP to generate t-BuO [20].As for the olefins used in the
experiments (vide infra), olefins bearing one EWG did not exhibit ab-
sorption at > 290 nm, but some of those with two EWGs exhibited a
slight absorption at > 290 nm [19a].
The emission of sunlight is not stable like artificial light sources.
Fig. 1c shows the typical emission spectra of sunlight in Tokyo (N 35◦, E
139◦) on a sunny and cloudy day. S-Hi and S-Lo corresponded to the
highest and lowest intensity of the emission during photolysis on a
sunny day. C-Hi and C-Lo were those on a cloudy day. The emission of S-
Hi, S-Lo, C-Hi, and C-Lo showed light intensities of 1.50, 0.67, 0.40, and
0.25 mW/cm2, respectively; this light intensity was used in Tables 1–4
(vide infra). The intensity of the emission changed with time and
weather. In addition, seasonal deviations in the maximum intensity of
sunlight had to be considered, which was ~40 % in 2019 [23]. Details
on the weather and the dates of each experiment in Tables 1–4 are
shown in the Supplementary material.
The reactivity of olefins bearing one EWG was investigated using t-
butyl acrylate (1d) and t-butyl crotonate (1e). The reaction of 1d with
2A proceeded within 4 h to give adduct 3dA in 34 % yield, which was
cyclized to form 3dA’ in 26 % yield based on the consumed 1d (Entry 8).
The reaction of 1e and 2A proceeded slower than that of 1d (Entries 9,
10). Only a 94 % conversion of 1e was observed after 5 h with 3eA
obtained in 53 % yield (Entry 10). For both 1d and 1e, the yield of the
desired adducts were lower than those obtained from olefins bearing
two EWGs [3c,4a], probably due to the formation of polymers and
oligomers of olefins 1d and 1e [25].
The results obtained for the reaction of 1a-cis and alcohols 2B and 2C
3

M. Hayakawa et al.
Journal of Photochemistry & Photobiology, A: Chemistry 413 (2021) 113263
Table 2
Addition of alcohols to olefinsa.
Entry
Olefin (1)
Alcohol (2)
Irradiation time (h)
4
Light intensityb(mW/cm2)
Conversion of 1 (%)c
Yield of 3 (%)c
Yield of 3′ (%)c,d
1e
1.5ꢀ 2.2
100
1a-cis
2A
2A
3aA 98
3aA’ 97 (93)g
3aA’ 87
2
3
4
4
1.2ꢀ 1.8
0.5ꢀ 2.3
100
100
3aA 98
1a-trans
2A
1b-cis
3bA 97 (86)g
4e
5e
2A
2A
3
2
0.5ꢀ 1.3
1.7ꢀ 1.8
79
60
3bA 92 [73]h
3bA 93 [56]h
6
7
2A
2A
4
4
0.3ꢀ 2.2
1.2ꢀ 1.8
100
100
3bA 97
1b-trans
1c
1d
3cA 96
3dA 34
8
2A
2A
4
0.5ꢀ 2.0
100
3dA’ 26
9
4
5
4
1.1ꢀ 1.6
0.3ꢀ 1.6
1.1ꢀ 1.6
92
94
64
1e
3eA 45 [41]h
3eA 53 [50]h
3eA’ 30 [28]h
10
11
2A
MeOH
1a-cis
1a-cis
2B
3aB 71 [45]h
3aB 75 [65]h
12
13
2B
10
0.6ꢀ 1.5
87
3aB’ 65 [57]h
(62 [54]h)g
EtOH
1a-cis
1a-cis
4
1.6ꢀ 2.1
100
100
2C
2C
3aC 97
3aC 98
3aC’ cis/trans = 35/54
14e
a
11
0.2ꢀ 1.2 f
3aC’ cis/trans = 34/55 (31/54)g
Photolysis conditions, substrate: olefin (1; 0.2 mmol) and DTBP (0.1 mmol) in alcohol (2, 10 mL), light source: sunlight, N2 atm, room temp. The reactions were
conducted using a 30 mL Pyrex eggplant-shape flask.
b
The weather, the date, the light intensity of sunlight, and the number of independent runs for each experiment are described in detail in Supplementary material.
c
Yields are based on the consumed starting material and determined by NMR spectroscopy. The NMR yields are in good accord with the isolated yields [19]. The
yields are the averaged of two to five independent runs unless otherwise stated.
d
Lactonization by treatment with HCl gas at room temperature.
e
Result of a single run.
f
Light source: sunlight (cloudy).
g
h
Isolated yield.
Yield based on the initial amount of the olefin.
are shown in Runs 11–14. The reaction using methanol (2B) was not
completed even after 10 h of irradiation and an 87 % consumption of 1a-
cis and 75 % yield of 3aB was obtained, which was cyclized into lactone
3aB’ in 65 % yield (Entry 12). Ethanol (2C) gave 3aC in 97 % yield
within 4 h with the complete consumption of 1a-cis. Product 3aC was
cyclized into 3aC’ in 89 % yield with a cis/trans ratio of 35/54 (Entry
13). On cloudy days, the reaction required a longer irradiation time, but
the yield and the cis/trans ratio of the expected product was the same as
those obtained on a sunny day (Entry 14). It was surprising that the yield
of product 3aC was 23 % higher than that obtained using a Xe lamp as
the light source (74 %) [19a].
alcohols to olefins in neat alcohols have been conducted only using
conventional lamps and not with sunlight (Table S1) [28]. The intensity
of sunlight is generally more than one order of magnitude smaller than
conventional lamps, but our results show that a comparable yield of the
product can be obtained in the addition reaction of 2A to 1b-cis and
-trans with a shorter irradiation time (Entries 3 and 6) when compared
with that of a benzophenone sensitized reaction using a high-pressure
Hg lamp (18 h, 96 %; Entry S1) [11a], or a higher yield in the addi-
tion reaction of 2B to 1a-cis with a 1.3–3.3-fold longer irradiation time
(Entries 11 and 12) compared with that of a UO2Cl2 sensitized reaction
using a high-pressure Hg lamp (3 h, 65 %; Entry S2) [14a].
To the best of our knowledge, the photocatalyzed addition of
4

M. Hayakawa et al.
Journal of Photochemistry & Photobiology, A: Chemistry 413 (2021) 113263
Table 3
Addition of cyclic ethers to olefinsa.
Entry
Olefin (1)
Ether (2)
Irradiation Time (h)
4
Light intensity b(mW/cm2)
Conversion of 1 (%)c
Yield of 3 (%)c
1
1.3ꢀ 2.4
100
3aD-syn 62 3aD-anti 34
1a-cis
1a-cis
2D
2d
6
0.9ꢀ 1.8
52
3aE-syn 33 [17]e3aE-anti 22 [11]e
2E
3d
4d
1a-cis
1a-cis
2E
2E
9
0.6ꢀ 1.9
0.2ꢀ 1.5
62
40 [25]e 27 [17]e
39 22
16
100
5d
1a-cis
20
0.3ꢀ 1.2
100
2 F
3aF-syn 33 3aF-anti 32
3aD-syn 45 3aD-anti 51
6
7
4
4
1.5ꢀ 2.2
1.2ꢀ 2.0
100
100
1a-trans
1b-cis
2D
2D
3bD-syn 45 3bD-anti 51
3bD-syn 44 3bD-anti 51
8
2D
4
0.5ꢀ 2.0
100
1b-trans
9
2D
2D
2D
4
4
4
1.5ꢀ 2.2
0.5ꢀ 1.7
1.3ꢀ 1.8
100
100
91
1c
1d
3cD-syn 51 3cD-anti 41
3dD 40
10
11
a
1e
3eD-syn 40 [36]e 3eD-anti 41 [37]e
Photolysis conditions, substrate: olefin (1; 0.2 mmol) and DTBP (0.1 mmol) in ether (2, 10 mL), light source: sunlight, N2 atm, room temp. The reactions were
conducted using a 30 mL Pyrex eggplant-shape flask.
b
The weather, the date, the light intensity of sunlight, and the number of independent runs for each experiment are described in detail in Supplementary material.
c
Yields are based on the consumed starting material and determined by NMR spectroscopy. The NMR yields are in good accord with the isolated yields [19]. The
yields are the averaged of two or three independent runs unless otherwise stated.
d
Result of a single run.
e
Yield based on the initial amount of the olefin.
2.4. Effect of light intensity
2.5. Sunlight photolysis of cyclic ethers and olefins
The origin of the significant increase in the yield of product 3aC in the
reaction conducted under sunlight irradiation when compared with that
using a Xe lamp was investigated. The major difference between sunlight
andXelampirradiationwasestimatedtobetheintensityoflight. Therefore,
theeffectofthelightintensityontheyieldof3aCwasinvestigatedusingaXe
lamp (Fig. 2). The yield of3aC was dependent on the light intensity withthe
lowest yield obtained at 18 mW/cm2 and two maxima at 2 and > 24 mW/
cm2. The yield of 3aC at 20 mW/cm2 was the same as the yield obtained in
our previous report using a Xe lamp (74 %) [19] and that at 2 mW/cm2,
which was similar to the intensity of sunlight, was in good agreement with
the yield obtained using sunlight (Table 2, Entry 13). To the best of our
knowledge, such a light intensity effect, increasing the yield by decreasing
the light intensity, has not been reported to date. The origin of the light
intensityeffectisstillnotclearatthemomentbutitisprobablyrelatedtothe
concentration of the intermediates involved in the reaction mixture.
To extend the scope of the reaction, we have investigated the use of
cyclic ethers in the reaction as an alternative to alcohols (equation 3,
Table 3). Most of the products obtained were a mixture of syn and anti
diastereomers [19b]. The addition of 5–7-membered cyclic ethers to
1a-cis proceeded in good to excellent yield (Entries 1–5). However, 6-
and 7-membered cyclic ethers (2E and F) require longer irradiation
times for the reaction to be complete (Entries 2–5). This was probably
due to the slower abstraction of the hydrogen atom from 2E and 2 F by
•
t-BuO when compared to 2D [21] and/or the slower addition of the
6-membered radical of 2E to the olefin when compared to the
5-membered radical of 2D [26]. The addition of THF (2D) to maleic acid
(1b-cis), fumaric acid (1b-trans), and fumaronitrile (1c) proceeded in
excellent yield within 4 h (Entries 7–9). The results on the reactions
between cyclic ethers and olefins bearing two EWGs were comparable to
those using a Xe lamp as the light source [19b].
5

M. Hayakawa et al.
Journal of Photochemistry & Photobiology, A: Chemistry 413 (2021) 113263
Table 4
Addition of cyclic acetals to olefinsa.
Entry
Olefin (1)
Acetal
Light Intensity b(mW/
Yield of 3(%)c
(2)
cm2)
1
2
3
1a-cis
1a-trans
1c
0.8ꢀ 1.8
0.8ꢀ 1.8
0.3ꢀ 1.9
2G
2G
3aG 98
3aG 95
2 G
3cG 86
3dG 35
4d
5
1d
1e
2G
2G
2 G
1.8ꢀ 2.0
0.4ꢀ 1.8
0.5ꢀ 2.2
Fig. 2. The effect of the light intensity on the yield of 3aC and conversion of
1a-cis; Symbols: 1a-cis (◼) and 3aC ( ). Reaction condition: 1a-cis (0.2 mmol)
and DTBP (0.1 mmol) in 2C (10 mL), light source: 500 W Xenon short-arc lamp
fitted with an 18-cm water filter and a UV-29 cutoff filter, irradiation time: 3 h,
N2 atm, room temp. The reactions were conducted using a quartz cylindrical
cell (diameter: 3 cm).
3eG 81 (66)e
3fG 0
6
1f
yield (Entry 2).
3aH-major 90
2.6. Sunlight photolysis of cyclic acetals and olefins
7
1a-cis
2.1ꢀ 2.2
2H
2I
The addition of cyclic acetals to olefins was also investigated
(equation 4, Table 4). The abstraction of the hydrogen atom from cyclic
acetals was faster than that from cyclic ethers [21] so that the reactions
were completed within 3 h when using 2-methyl-1,3-dioxolane (2 G), 1,
3-dioxolane (2H), and 2,2-dimethyl-1,3-dioxolane (2I) (Entries 1–8).
The reactions of 2 G and 2H with olefins bearing two EWGs all pro-
ceeded in high yield (Entries 1–3, 7), but the product from 1d gave only
a fair yield (Entry 4), which was attributed to polymerization and
oligomerization of the olefin [25]. However, 1c and 1e showed a 15 and
29 % increase in the yield of 3 cG and 3eG, respectively, when compared
to that obtained under photolysis with a Xe lamp (Entry 3 and 5,
respectively). In contrast, the expected product (3 fG) was not formed
from the reaction between t-butyl methacrylate (1f) and 2 G (Entry 6),
probably also due to polymerization and oligomerization. In the reaction
between 1a-cis and 2I, the product (3aI) was obtained as a mixture of
syn- and anti-isomers in 60 % yield, which was a > 2-fold increase when
compared to the yield obtained under photolysis with a Xe lamp (Entry
8).
3aH-syn 3aH-
anti
5 3
8
a
1a-cis
0.3ꢀ 1.6
3aI-syn 3aI-anti
41 19
Photolysis conditions, substrate: olefin (1; 0.2 mmol) and DTBP (0.1 mmol)
in acetal (2, 10 mL), light source: sunlight, irradiation time: 3 h, N2 atm, room
temp. N2 atmosphere, room temp. Conversion of 1: 100 %. The reactions were
conducted using a 30 mL Pyrex eggplant-shape flask.
b
The weather, the date, the light intensity of sunlight, and the number of
independent runs for each experiment are described in detail in Supplementary
material.
c
Yields are based on the consumed starting material and determined by NMR
spectroscopy. The NMR yields are in good accord with the isolated yields [19].
The yields are the averaged of two or three independent runs unless otherwise
stated.
d
Result of a single run.
e
Isolated yield.
The yield of 3dD from t-butyl acrylate (1d) and 2D was 40 % (Entry
10), which was comparable to that using a Xe lamp [19b]. In contrast,
the combined yield of 3eD-syn and –anti obtained from the reaction
between t-butyl crotonate (1e) and 2D was ~10 % higher than that
obtained using a Xe lamp (81 % yield) (Entry 11).
The photocatalyzed addition of cyclic acetals to olefins in neat cyclic
acetals have not been conducted using sunlight, but only using con-
ventional lamps (Table S3) [28]. The required sunlight irradiation time
(3 h) in our experiments was comparable to conventional lamp photol-
yses (0.75–16 h) and the yields of the desired products were also
comparable.
Photocatalyzed addition reactions of cyclic ethers to olefins in neat
cyclic ethers have been reported using both sunlight and conventional
lamps (Table S2) [28]. Acetophenone [11b] and TBADT [13b] have
been used for sunlight-induced reactions. Acetophenone was used in the
addition of 2D to diethyl maleate, which required 1d of irradiation
giving the desired product in 80 % yield (Entry S1), whereas our reac-
tion on the addition of 2D to 1a-cis only required 4 h of irradiation with
a 96 % yield of the desired product 3aD-syn/anti (Entry 1). TBADT was
used in the addition of 1,4-dioxane to 1a-cis, which required 5d of
sunlight irradiation to give the desired product in 59 % yield (Entry S8);
our similar reaction using the addition of 2E to 1a-cis required 16 h of
irradiation (which corresponds to 2 d of irradiation in the TBADT
catalyzed reaction) to give the desired product 3aD-syn/anti in 61 %
2.7. Gram-scale synthesis
To test the applicability of this sunlight reaction for use in organic
synthesis, the reaction of 1a-cis and 2A was scaled up 50-fold. The
sunlight irradiation was conducted for 4 and 6 h (0.2ꢀ 0.9 mW/cm2)
using 1a-cis (1.44 g, 10 mmol) and DTBP (0.73 g, 5 mmol) in 2A
(500 mL) using 500-mL eggplant-shape Pyrex flasks without stirring.
The yields of the 3aA were 95 % (75 % conversion) and 99 % (100 %
6

M. Hayakawa et al.
Journal of Photochemistry & Photobiology, A: Chemistry 413 (2021) 113263
conversion) after 4 and 6 h of irradiation, respectively. To investigate
the effect of the volume-to-surface ratio in the reaction, the reaction was
conducted using 1a-cis (2.88 g, 20 mmol) and DTBP (1.46 g, 10 mmol)
in 2A (1 L) using 1-L eggplant-shape Pyrex flasks with 4 h irradiation
(0.4ꢀ 0.9 mW/cm2) without (i) and with (ii) stirring. The yields of 3aA
were almost the same, 91 % (86 % conversion) (condition i) and 89 %
(86 % conversion) (condition ii), which indicated that the volume-to-
surface ratio had no effect on the efficiency of the reaction. The four
conditions all gave similar yields to that observed in the small-scale
experiment (cf. Table 2, Entry 1).
cm water filter and a cut-off filter (Toshiba UV-29) under a nitrogen
atmosphere. The light intensity was measured by an Ushio UIT-150-A
Ultraviolet Radiometer equipped with a UVD-S365 photo detector and
emission spectra of sunlight by an Ushio USR-40D spectral radiometer.
After photolysis, the alcohol/cyclic ether/cyclic acetal was removed in
vacuo at 40–50 ◦C / < 150 Torr (most of the products were volatile
under reduced pressure) and the consumption of the olefin and the
products yield were determined by NMR spectroscopy using a precise
amount of naphthalene as an internal standard; the yields of each
product were calculated based on the consumed starting material. The
isolation of products were conducted using silica gel column
chromatography.
Additional gram-scale experiments on the reaction of 1a-cis (1.44 g,
10 mmol) and DTBP (0.73 g, 5 mmol) in 2D (500 mL), scaled up 50-fold,
were conducted for 4 h under four different conditions using eggplant-
shape Pyrex flasks: (i) 500 mL flask with stirring, (ii) 1 L flask with
stirring, (iii) 500 mL flask without stirring, and (iv) 1 L flask without
stirring. The yields of the 3aD-syn/anti product were 62/36 % (condi-
tion i), 62/36 % (condition ii), 62/33 % (condition iii), and 59/37 %
(condition iv), with 100 % conversion. A considerable difference was not
observed between the four conditions and all gave similar yields to that
observed in the small-scale experiment (cf. Table 3, Entry 1) without
extending the irradiation time.
4.2.2. Lactonization of hydroxyl esters 3aA, 3dA, 3eA, and 3aB
The crude hydroxyl ester and CHCl3 were added to a 30 mL eggplant-
shape flask and HCl gas was bubbled into the resulting solution with
stirring for 25 min [28]. After the reaction, CHCl3 was removed using a
rotary evaporator (<50 ◦C, < 85 Torr). The consumption of the hydroxyl
ester and the yield of the product lactone were determined by NMR
spectroscopy using a precise amount of 2,7-dimethoxynaphthalene
(0.10 mmol) as an internal standard; the yield of the product was
calculated based on the consumption of the starting material. The
isolation of the product was conducted using silica gel column
chromatography.
3. Conclusions
The sunlight-induced addition of alcohols/ethers/acetals to olefins
using DTBP proceeded efficiently, typically within 3–4 h of sunlight
irradiation in excellent yields with olefins bearing two EWGs (> 95 %
yield) and in good to fair yield with olefins bearing one EWG. The yield
observed for some of the products were higher than those obtained using
a Xe lamp as a light source, which was confirmed to be due to a light
intensity effect; the lowest yield was obtained at 18 mW/cm2 and two
maxima were observed at 2 and > 24 mW/cm2 with a ~20 % difference
in the product yield. The reactions proceeded faster than many of the
previously reported sunlight and many conventional lamp photolyses
with similar or better yields. Gram-scale experiments were conducted to
test the applicability of this reaction in organic synthesis, whose results
showed similar yields to those obtained for the small-scale experiments.
4.2.3. 2,2-Dimethyl-5-oxo-tetrahydro-3-furancarboxylic acid methyl ester
(3aA’) [24,29]
Dimethyl maleate (1a-cis, 28.78 mg, 0.20 mmol) and DTBP
(14.97 mg, 0.10 mmol) in 2-propanol (2A, 10 mL). Weather: sunny
(light intensity: 1.5ꢀ 2.2 mW/cm2) [28]. Irradiation time: 4 h. NMR
yield (CDCl3), 3aA: 98 % (conversion: 100 %); 3aA’ (after lactonization
of 3aA): 97 %, (based on the consumed 1a-cis). Eluent for chromatog-
raphy: hexane/ethyl acetate (40/1 → 0/1).
3aA’: 31.94 mg (93 %); colorless oil. 1H-NMR (CDCl3): δ = 1.32 (s,
3 H), 1.61 (s, 3 H), 2.72 (dd, J = 8.8, 18.0 Hz, 1 H), 3.10 (dd, J = 9.6,
18.0 Hz, 1 H), 3.21 (dd, J = 8.8, 9.6 Hz, 1 H), 3.77 (s, 3 H) ppm. 13C-
NMR (CDCl3): δ = 23.3, 28.5, 31.8, 50.4, 52.5, 84.4, 170.3, 173.9 ppm.
IR (KBr disk): 3536, 3461, 2983, 2955, 2852, 1781, 1739, 1601 1559,
1439, 1390, 1377, 1362, 1272, 1227, 1173, 1142, 1120, 1088, 1034,
1001, 966, 930, 911, 888, 851, 837, 793, 764, 738, 678, 599, 544, 540
cmꢀ 1. MS, m/z (relative intensity): 41 (34), 43 (100), 55 (93), 59 (13),
69 (24), 97 (11), 114 (27), 129 (35), 157 (28), 172 (0.01, M+).
4. Experimental
4.1. General remarks
1H and 13C NMR spectra were recorded on a JEOL JNM-ECX400
spectrometer with CDCl3, acetone-d6, or CD3CN as solvent. As internal
standards, TMS (δ 0.0 ppm) in CDCl3, CD3CN (δ 1.94 ppm), or acetone-
d6 (δ 2.05 ppm) were used for 1H NMR, and CDCl3 (δ 77.0 ppm), CD3CN
(δ 118.26 ppm), or acetone-d6 (δ 29.84 ppm) for 13C NMR analyses. IR
spectra were recorded on a JASCO FT/IR-4700. Olefins 1a-cis, 1a-trans,
1b-cis, 1b-trans, 1c, alcohols 2A, 2B, 2C, acetals 2 G, 2H, 2I, and DTBP
were purchased and used as bought. Olefins 1d, 1e, 1f, and ethers 2D,
2E were purchased and distilled before use. Oxepane (2 F) was synthe-
sized according to the reported procedure [27]. See Supplementary
material for experimental details.
4.2.4. Tetrahydro-2,2-dimethyl-5-oxo-3-furancarboxylic acid (terebic
acid, 3bA) [19a,29,30]
Maleic acid (1b-cis, 22.78 mg, 0.20 mmol) and DTBP (14.92 mg,
0.10 mmol) in 2-propanol (2A, 10 mL). Weather: sunny (light in-
tensity:1.0-2.3 mW/cm2) [28]. Irradiation time: 4 h. NMR yield (CDCl3),
97 % (conversion: 100 %). Eluent for chromatography: hexane/ethyl
acetate (100/1 → 60/1).
3bA: 26.6 mg (86 %); white solid. 1H NMR [28], 13C NMR, [28] IR,
and MS spectra were the same as those reported in the literature [19a].
4.2.5. Tetrahydro-5-oxo-3-furancarboxylic acid methyl ester (3aB’) [31]
Dimethyl maleate (1a-cis, 28.67 mg, 0.20 mmol) and DTBP
(14.75 mg, 0.10 mmol) in methanol (2B, 10 mL). Weather: sunny (light
intensity: 0.6ꢀ 1.5 mW/cm2) [28]. Irradiation time: 4 h. NMR yield
(CDCl3), 3aB: 76 % (conversion: 85 %); 3aB’ (after lactonization of
3aB): 64 % (based on the consumed 1a-cis). Eluent for chromatography:
hexane/ethyl acetate (40/1 → 0/1).
4.2. General procedure for the reactions
4.2.1. Photolysis
A solution of alcohol/cyclic ether/cyclic acetal (2) (10 mL), olefin
(1) (0.2 mmol) and DTBP (0.1 mmol) was introduced into a quartz cy-
lindrical cell (diameter: 3 cm) or 30 mL Pyrex eggplant-shape flask
equipped with a three-way stopcock. The three-way stopcock was con-
nected to the cell, a nitrogen source, and small vacuum pump. The so-
lution was evacuated to about 50 mmHg under sonication for 5 s and
nitrogen was then introduced into the cell; this cycle was repeated 10
times. The photolysis was conducted under sunlight irradiation or a
500 W Xe lamp (USHIO Optical Modulex SX-UI500XQ) fitted with an 18-
3aB’: 15.74 mg (62 %); colorless oil; 1H-NMR (CDCl3): δ = 2.76 (dd,
J = 9.6, 17.6 Hz, 1 H), 2.89 (dd, J = 7.4, 17.6 Hz, 1 H), 3.47 (dddd, J =
7.0, 7.4, 8.0, 9.6 Hz, 1 H), 3.78 (s, 3 H), 4.46 (dd, J = 7.0, 9.6 Hz, 1 H),
4.5 (dd, J = 8.0, 9.6 Hz, 1 H) ppm. 13C-NMR (CDCl3): δ = 30.9, 39.9,
52.8, 69.0, 171.5, 175.0 ppm. IR (KBr disk): 2961, 2366, 1774, 1730,
1257, 1201, 1162, 1009, 861, 791, 702, 679, 628, 607, 599, 563, 541,
7

M. Hayakawa et al.
Journal of Photochemistry & Photobiology, A: Chemistry 413 (2021) 113263
528, 514, 504 cmꢀ 1. MS, m/z (relative intensity): 41 (48), 42 (15), 43
(13), 55 (100), 59 (35), 69 (30), 84 (22), 87 (21), 102 (20), 113 (13),
116 (18), 145 (0.02, M+1).
Photobiology, Volume 1, CRC Press, Boca Raton, 2012, pp. 452–455. Chapter 19,
Section 19.2.
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Dimethyl maleate (1a-cis, 28.97 mg, 0.20 mmol) and DTBP
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for chromatography: hexane/ethyl acetate (40/1 → 0/1). 3aC’-cis:
10.0 mg (34 %); colorless oil; 3aC’-trans: 17.1 mg (54 %), colorless oil.
1H NMR [28], 13C NMR, [28] IR, and MS spectra were the same as those
reported in the literature [19a].
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4.2.8. Gram-scale synthesis
Sunlight photolyses were conducted similar to the small scale ex-
periments using a solution of alcohol/cyclic ether (2) (500 mL), olefin
(1) (10 or 20 mmol) and DTBP (5 or 10 mmol) in a Pyrex eggplant-shape
flask (500 mL or 1 L).
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Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgements
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This work was supported by JSPS KAKENHI Grant Number
JP17K05953 (A. O.), the Sasakawa Scientific Research Grant from The
Japan Science Society, and Mayekawa Houonkai Foundation (M. H.).
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