Effect of Reaction Media on Photosensitized [2+2]-Cycloaddition of Cinnamates

Article abstract of DOI:10.1002/open.202000092

The outcome of photosensitized [2+2]-cycloaddition reactions of various cinnamates has been compared in different reaction media, including homogeneous organic solutions under inert conditions, degassed water, and aerated physical gels. The reactions were performed under LED blue light (λ_max=455 nm) irradiation and [Ir{dF(CF₃)ppy}₂(dtb-bpy)]PF₆ (1.0 mol%) as photocatalyst. The processes were optimized taking into consideration solvent, gelator, and substrate. Comparative kinetics analyses, as well as the effect of the reaction media on the diastereoselectivity of the process, were evaluated during this investigation. In a number of cases, carrying out the reaction in a less polar solvent, like toluene or highly polar solvent, like water had a tremendous impact on the diastereoselectivity of the process, pointing towards an effect on the stabilization of the putative diradical intermediate in this medium. Moreover, while for reactions run in homogeneous solution oxygen needs to be excluded, no erosion in yield is observed when the photoadditions were run in aerated gel media.

Full text of DOI:10.1002/open.202000092

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Effect of Reaction Media on Photosensitized [2+2]-
Cycloaddition of Cinnamates
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Alex Abramov,[a] Oliver Reiser,*[a] and David Díaz Díaz*[a, b, c]
8
9
The outcome of photosensitized [2+2]-cycloaddition reactions
of various cinnamates has been compared in different reaction
media, including homogeneous organic solutions under inert
conditions, degassed water, and aerated physical gels. The
reactions were performed under LED blue light (λmax =455 nm)
irradiation and [Ir{dF(CF3)ppy}2(dtb-bpy)]PF6 (1.0 mol%) as pho-
tocatalyst. The processes were optimized taking into consider-
ation solvent, gelator, and substrate. Comparative kinetics
analyses, as well as the effect of the reaction media on the
diastereoselectivity of the process, were evaluated during this
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investigation. In a number of cases, carrying out the reaction in
a less polar solvent, like toluene or highly polar solvent, like
water had a tremendous impact on the diastereoselectivity of
the process, pointing towards an effect on the stabilization of
the putative diradical intermediate in this medium. Moreover,
while for reactions run in homogeneous solution oxygen needs
to be excluded, no erosion in yield is observed when the
photoadditions were run in aerated gel media.
1. Introduction
who synthesized a wide range of cyclobutanes compounds
based on olefins and enones.[22–27] Concurrently with a report by
Wu and coworkers,[28,29] some of us have developed an efficient
photo mediated intermolecular [2+2]-cycloaddition reaction
between simple cinnamates or styrenes based on strong π-π
The bimolecular photocycloaddition reaction[1] have attracted
great scientific attention due to its applications in numerous
fields such as optical data storage,[2–4] material science,[5–7] cell
biology[8–10] and organic synthesis.[11–13] Moreover, stereospecific
cyclobutanes are widespread compounds usually found in
numerous natural products, insects and microbial species
possessing biological activities with potential therapeutic
applications.[14–17] Some of the most prominent examples of
cinnamic acid and styrene derived dimers include, for instance,
truxic acid, piperarborenine D, endiandrin A and magnosalin
(trans).
stacking interactions of the aryl groups and
a diradical
transition state yielding only two diastereomers regioselectively
(head-to-head products).[28] Despite the very good yields
observed for this dimerization, the diastereoselectivity was
found to vary significantly depending on the electronic proper-
ties of the aryl group and the steric effects of the ester group of
the substrates.
Previous work by Pattabiraman’s group on the stereomeric
outcome of the dimerization of cinnamates showed that the
manipulation of the environment by using non-covalent
interactions within macrocyclic cavitands can direct the out-
come of the [2+2]-photocycloaddition reaction.[30] A similar
approach was published by Ramamurthy and co-workers, where
they templated the photodimerization with a water-soluble
palladium nanocage[31] or curcubit[8]uril and γ-cyclodextrin.[32]
Furthermore, the impact of hydrogel and micellar media on the
photodimerization of acenaphthylene has been also studied by
Maitra and co-workers.[33] Interestingly, their results showed that
the syn-to-anti ratio was greater in the gel-bound state
compared to solution, suggesting that the selectivity apparently
is correlated with the rigidity of the gels.
In this work, we hypothesized that a further stabilization of
the benzylic radicals’ transition state by changing the reaction
environment may lead to an improvement of the diastereo-
meric outcome of the [2+2]-cycloaddition reaction. For this
purpose, we designed our study around different solvents for
the reaction. The polarity of the solvent system can have a
stabilizing effect on the transition state species and to push this
concept one step further, also different kinds of low molecular
weight gelators were introduced into the system to investigate
how they can provide additional interactions aiming to perform
the reaction under air without significant losses of yields.
In general, several atom economic methods[18,19] have been
developed for the synthesis of cyclobutane derivatives. Among
those, thermally activated methods[20] have been scarcely
reported, while the photochemical pathway utilizing UV light
has been known for decades.[21] In contrast, photocatalysis using
visible-light was mainly influenced by Yoon and co-workers,
[a] A. Abramov, Prof. Dr. O. Reiser, Dr. D. Díaz Díaz
Institute of Organic Chemistry
University of Regensburg
Universitätstr. 31, Regensburg 93053 (Germany)
E-mail: oliver.reiser@chemie.uni-regensburg.de
[b] Dr. D. Díaz Díaz
Departamento de Química Orgánica
Universidad de La Laguna
Avda. Astrofísico Francisco Sánchez 3, 38206 La Laguna, Tenerife, (Spain)
E-mail: ddiazdiaz@ull.edu.es
[c] Dr. D. Díaz Díaz
Instituto de Bio-Orgánica Antonio González
Universidad de La Laguna
Avda. Astrofísico Francisco Sánchez 2, 38206 La Laguna, Tenerife, (Spain)
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/open.202000092
© 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA. This
is an open access article under the terms of the Creative Commons Attri-
bution Non-Commercial NoDerivs License, which permits use and distribu-
tion in any medium, provided the original work is properly cited, the use is
non-commercial and no modifications or adaptations are made.
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2. Results and Discussion
collapse to the diastereomers 2 and 3. If the latter is slowed
down, A can be populated in its sterically least crowded
arrangement which then leads preferentially to the all-trans δ-
isomer 2. Thus, the stereoselectivity was found to be dependent
on the stabilization of the benzylic radicals: Electron poor
substrates were converted to 2 and 3 unselectively, while
electron rich ones undergo a slower ring closure and thus
allowing an equilibration to the most thermodynamically
favored all-trans arrangements.[28]
In order to evaluate the feasibility of this process using
supramolecular gels as reaction media, aiming at an improve-
ment of the diastereoselectivity for electron deficient cinna-
mates 1b–e, five different LMW gelators G1–G5 were prelimi-
nary selected (Figure 2) based on (1) their ability to gel suitable
solvents for the [2+2]-cycloaddition reactions (e.g. DMF,
toluene, CH3CN) at different concentrations, (2) the stability of
the gels after the incorporation of reactants and catalysts, and
(3) their inertness under irradiation conditions. Moreover, the
selected gelators offer different modes of self-assembly involv-
ing various types of non-covalent interactions (e.g. hydrogen-
bonding involving for example amide (G1, G2), (thio)urea (G3,
G5) and alcohol functions (G4), π-π stacking between aromatic
rings (G5), hydrophobic interactions between long aliphatic
chains (G1–G3, G5)), which not only influence the stability of
the networks, but they can also interfere with the intermolecu-
lar interactions associated to the selectivity observed for the [2
+2]-cycloaddition in solution. Thus, diverse self-assembly
patterns are expected to influence, at least to some extent, the
course of the chemical reactions performed in gel media even
when the solvent is the same.
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Substituted cyclobutanes are accessible via photodimerization
of cinnamates using visible-light triplet sensitization, which was
proposed to proceed through diradical formation[28,29] (Figure 1).
The practical importance of this strategy lies on the prevalence
of strained cyclobutane structures in a variety of natural
products with biological activity.[16] Moreover, a major advant-
age of photocatalyzed [2+2]-cycloadditions of cinnamates by
energy transfer does not depend on the lifetime of the excited
state of the photocatalyst, and not on the redox potential of
the substrates. In particular, the use of 1.0 mol% of [Ir{dF(CF3)
ppy}2(dtb-bpy)]PF6 and blue light (λmax =455 nm; LED455) irradi-
ation under inert gas atmosphere was found to be optimal to
achieve a large variety of substituted cyclobutanes in high
yields under mild conditions without external additives. Only
head-to-head products were obtained with this methodology,
which was rationalized by strong π-π stacking of the arene
moieties.
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The stereoselectivity of the reaction is a consequence of the
reversible dimerization of 1 to A and its subsequent irreversible
Preliminary comparative experiments were carried out at
room temperature (RT) with substrates 1a–b (in DMF solution
(Figure 1 and Table 1, entries 2 and 5) and in gel made of bis-
amide glutamic acid-based gelator G1 (c=15 gLÀ 1) in DMF
(Table 1, entries 1, 4, 5–6). LED blue light (LED455) was used as
irradiation source and [Ir{dF(CF3)ppy}2(dtb-bpy)]PF6 (1.0 mol%)
as photocatalyst.
The results revealed that the [2+2]-cycloaddition reaction
of these cinnamates in gel under aerobic conditions proceeds
with significantly higher yields compared to analogous con-
ditions in DMF solution (Table 1, entry 1 vs. 3). Inert conditions
Figure 1. General scheme of visible light mediated [2+2]-cycloaddition
reaction in different reaction media studied in this work. Ir-catalyst=[Ir{dF
(CF3)ppy}2(dtb-bpy)]PF6.
Table 1. Control experiments.[a]
Entry Substrate Gelator Atm. Conv. [%][b] Yield [%][c] d.r. (2/3)[b]
1
1a
1a
1a
1b
1b
1a
1a
G1
–
–
G1
–
G1
G1
O2
N2
O2
O2
N2
O2
O2
100
100
–
100
100
0
85
85
63
82
85
–
88:12
87:13
90:100
65:35
57:43
–
2
3[d]
4
5
6[e]
7[f]
0
–
–
[a] Reaction conditions: Cinnamate (0.5 mmol), photocatalyst [Ir{dF(CF3)
ppy}2(dtb-bpy)]PF6 (1.0 mol%), dry DMF (1 mL), G1 (15 gLÀ 1), LED455, 24 h,
RT. Abbreviations: Atm.=atmosphere; conv.=conversion; d.r.=diastereo-
Figure 2. LMW gelators selected for this study: N,N’-bis(octadecyl)-L-Boc-
glutamic diamide (G1), N-((1R,2R)-2-undecanamidocyclohexyl)undecanamide
(G2), (+)-(R,R)-dodecyl-3-[2-(3-dodecyl-ureido)cyclohexyl]urea (G3), 1,3:2,4-
dibenzylidene-D-sorbitol (G4), 4-(dodecyloxy)-N-((4-(dodecyloxy)phenyl)car-
bamothioyl)benzamide (G5).
1
meric ratio (2=δ-diastereomer; 3=β-diastereomer). [b] Determined by H
NMR analysis of the reaction crude. [c] Isolated yield. [d] Values taken from
ref. [28], Table 1, entry 1. [e] Experiment in the absence of catalyst. [f]
Experiment in the absence of irradiation.
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are required in solution to rival the results obtained in gel
(Table 1, entry 1 vs. 2; entry 4 vs. 5) in terms of regioselectivity,
kinetics, conversion and yield. No reaction occurred in the
absence of either Ir-catalyst or blue light irradiation (Table 1,
entries 6–7).
(Table 3). Similarly to solution, different solvents in aerated gel
media also affects to some extent the d.r. The lowest
diastereoselectivity was found in DMSO (d.r.=83:17), while
toluene and methylene chloride afforded the highest selectivity
(d.r.=91:9). Ratios within this range were found for
cyclohexane, acetonitrile, DMF and acetone. It should be
emphasized that this behavior is not necessarily extrapolated to
other gelators and/or substrates. Overall, these results demon-
strate that photoredox reactions in supramolecular gel systems
may be modulated by changing the LMW gelator and/or the
solvent for a given substrate.
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With these results in hand, we focused our attention on the
influence of the gel medium on the diastereomeric ratio by
using different LMW gelators (G1–G5) and cinnamate 1a (R1 =
H) as model substrate (Table 2). Both the gelator concentration
and the solvent were modified in order to optimize the reaction
conditions in gel media. The results suggested that both
parameters are important and they should be evaluated
simultaneously during the optimization experiments of photo-
redox processes in gel. Thus, a good balance in terms of lower
gelation concentration, conversion and diastereoselectivity was
obtained with the gel systems increasing the diastereoselectiv-
ity from 87:13 in solution to 89:11 (entry 10) in stable gel
media. However, from Table 2 it is evident that depending on
the parameter of interest there is some flexibility within these
conditions.
Furthermore, due to the presence of substrates and catalyst
which do not take part of the supramolecular gel network, it
should be kept in mind that the minimum gelator concen-
tration suitable for the experiments does not necessarily
correspond to the critical gelation concentration (CGC), being
defined as the minimum gelator concentration required to form
a pure gel in a given solvent. In general, the incorporation of
external additives tends to tilt the metastable equilibrium of a
supramolecular gel towards the most stable thermodynamic
phase (e.g. crystallization, precipitation) over time.[13,15] Such a
reduced gel stability could be compensated, at least to a certain
extent, by increasing the gelator concentration. The effect of
the solvent on the d.r. 2:3 was assessed more in detail taking
substrate 1a (R1 =H) and gelator G1 as a model system
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Applying the best reaction conditions found for 1a, i.e.
highest diastereoselectivity for systems in which a stable gel
can be formed, we evaluated the outcome of the photo-
dimerization for substrates 1b–1f in gel media (Table 4), which
had given poor selectivities in homogeneous solution.[28] For all
substrates the diastereoselectivity could be slightly increased
by carrying out the reactions in gel media rather than in the
respective solvent alone. In general, the use of gel media has a
greater effect in polar solvents like DMF or CH3CN on the
diastereoselectivity than in unpolar solvents (toluene), suggest-
ing a greater stabilization of diradical intermediate A in the
latter. For all substrates the diastereoselectivity could be
increased significantly by carrying out the reactions in a less
polar solvent (toluene) compared to previously used DMF,
suggesting a greater stabilization of the diradical intermediate
A in the first.[28] In comparison to that, gel media showed a
rather small effect on the diastereomeric outcome of the
reaction and tended to have a relatively greater impact in polar
solvents like DMF or CH3CN than in unpolar solvents (toluene).
In agreement with this rational, we found that the reactions
proceed efficiently on water, increasing the amount of the cis-
diastereomer, which is presumably the kinetically preferred one
(Table 5). The most dramatic result was obtained with sub-
strates 1b (R1 =CO2Me) and 1c (R1 =CN), for which the d.r. was
inverted in favor to the cis product 3 (β-diastereomer) (Table 5,
entries 4–5). Moreover, under these conditions the catalyst
loading could be efficiently decreased from 1.0 mol% to
0.02 mol% while maintaining the conversion �95% (Table 5,
entry 3). Surprisingly, although substrate 1e (R1 =NO2) was
poorly converted on water (11%) even after 72 h (Table 5,
entry 7), only the cis diastereomer 3 was obtained. In sharp
Table 2. Screening of solvents, gelators and gelator concentrations for the
[2+2]-cycloaddition of substrate 1a (R1 =H).[a]
Entry
Solvent
Gelator [gLÀ 1
]
Conv. [%][b]
d.r. (2/3)[b]
1
DMF
G1 (15)
G1 (15)
G1 (15)
G1 (30)
G2 (15)
G2 (15)
G3 (5)
G3 (10)
G3 (10)
G4 (5)
G4 (15)
G4 (10)
G4 (20)
G5 (15)
G5 (20)
G5 (20)
G5 (40)
100
100
100
100
100
46
100
100
–
85
100
63
95
88:12
87:13
93:7
88:12
88:12
88:12
88:12
87:13
–
89:11
86:14
89:11
89:11
86:14
88:12
91:9
2
CH3CN
Toluene
DMF
DMF
Toluene
DMF
3[c]
4
5
6
7
8
Table 3. Solvent screening for [2+2]-cycloaddition of 1a in gel made of
G1.[a]
DMF
9[d]
10
11[c]
12
13
14
15[c]
16[c]
17[c]
Toluene
Toluene
CH3CN
Toluene
Toluene
CH3CN
DMF
Entry
Solvent
Gelator
Conv. [%][b]
d.r. (2/3)[b]
1
2
3
4
5
6
7
DMSO
Cyclohexane
CH3CN
DMF
Acetone
Toluene
DCM
G1 (15)
G1 (20)
G1 (15)
G1 (15)
G1 (15)
G1 (20)
G1 (20)
72
100
99
100
100
100
100
83:17
88:12
89:11
88:12
91:9
96
89
84
49
Toluene
Toluene
93:7
91:9
91:9
[a] Reaction conditions: Cinnamate 1a (0.5 mmol), photocatalyst [Ir{dF(CF3)
ppy}2(dtb-bpy)]PF6 (1.0 mol%), solvent (1 mL), LED455, 24 h, RT, air.
Abbreviations: Conv.=conversion; d.r.=diastereomeric ratio (2=δ-dia-
stereomer; 3=β-diastereomer). [b] dDetermined by 1H NMR analysis of the
reaction crude. [c] Gel melted (unstable) [d] No gel formation.
[a] Reaction conditions: Cinnamate 1a (0.5 mmol), photocatalyst [Ir{dF(CF3)
ppy}2(dtb-bpy)]PF6 (1.0 mol%), dry solvent (1 mL), LED455, 24 h, RT, in air.
Abbreviations: Conv.=conversion; d.r.=diastereomeric ratio (2=δ-dia-
stereomer; 3=β-diastereomer); DCM=dichloromethane. [b] Determined
1
by H NMR analysis of the reaction crude.
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These results motivated us to test the reaction also in a
series of aqueous micellar systems. However, the results
indicated no effect of the micelles on the outcome of the
reaction (see ESI, Table S1).
Table 4. Substrate screening in solution and in gel media under different
1
2
3
4
5
6
7
8
9
conditions.[a]
Entry
Substrate
Solvent
Gelator
Conv. [%][b]
d.r. (2/3)[b]
1
2
3
4
5
6
1a
1b
1b
1b
1b
1b
1b
1b
1c
1c
1c
1c
1c
1c
1d
1d
1d
1d
1d
1d
1e
1e
1e
1e
1e
1e
1f
Toluene
DMF
CH3CN
Toluene
DMF
–
–
–
–
100
100
100
100
100
100
–
100
100
100
100
100
100
100
100
86
100
100
72
100
97
92
95
89:11
57:47
68:32
83:17
65:35
64:36
–
Standard kinetics studies using 1a as model substrate
showed second-order reactions in both homogeneous solution
and gel media (Figure 3). Reactions in both DMF solution and in
gel media made of DMF/G1, DMF/G2, DMF/G3 and CH3CN/G5
displayed comparable turn over frequencies (TOF) within the
range 7–8×10À 2 minÀ 1 and half-life times (t1/2) about 6–8 h.
Comparing the kinetics parameters with those from the
reactions in degassed water, we found a much faster reaction
time for the latter with a TOF of 22.9×10À 2 minÀ 1 and a half-life
of 2.9 h. In sharp contrast, reactions performed in toluene/G4 as
gel medium were slowed down to TOF and half-life values of of
1.6×10À 2 minÀ 1 and 29.9 h, respectively.
G1 (15)
G3 (5)
G4 (5–20)
G5 (10)
–
DMF
7[c]
8[d]
9
Toluene
CH3CN
DMF
Toluene
DMF
72:28
60:40
75:25
64:36
64:36
84:16
72:28
68:32
79:21
72:28
69:31
80:20
75:25
54:46
80:20
56:44
56:44
81:19
62:38
75:25
76:26
76:24
–
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
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10
11
12
13
14[d]
15
16
17
18
19
20[d]
21
22
23
24
25
26[d]
27
28
29
30
31[d]
–
G1 (15)
G3 (5)
G4 (5)
G5 (10)
–
DMF
Toluene
CH3CN
DMF
Toluene
DMF
–
G1 (15)
G3 (5)
G4 (5)
G5 (10)
–
Studies of the thermal stability of the gel systems revealed
that those with the highest influence on d.r., i.e. toluene/G4
and CH3CN/G5, also showed the highest gel-to-sol transition
DMF
Toluene
CH3CN
DMF
Toluene
DMF
°
°
temperatures (Tgel) (i.e. 90 C and 76 C, respectively) (Table 6,
entries 4–5). In contrast, DMF/G1, DMF/G2 and DMF/G3 aerated
gels, which caused the least impact on d.r., displayed Tgel values
–
G1 (15)
G3 (5)
G4 (5)
G5 (10)
–
G1 (15)
G3 (5)
G4 (5)
G5 (10)
DMF
96
96
91
23
28
30
Toluene
CH3CN
DMF
DMF
DMF
Toluene
CH3CN
°
�64 C (Table 6, entries 1–3). This apparent correlation is similar
to that previously observed for the photodimerization of
acenaphthylene in hydrogels.[33]
1f
1f
1f
1f
Table 7 and Figure 4 summarize the influence of the
surrounding media (i.e. inert homogeneous solution and
aerated gel) on the d.r. and isolated yields of the [2+2]-
cycloaddition reaction under optimized conditions for each
Traces (<1)
34
68:32
[a] Reaction conditions: Cinnamate (0.5 mmol), photocatalyst [Ir{dF(CF3)
ppy}2(dtb-bpy)]PF6 (1.0 mol%), solvent (1 mL), LED455, 24 h, RT. Reactions in
solution were carried with dry solvents out under strict nitrogen
atmosphere, while those performed in gel were performed under aerobic
conditions. Abbreviations: Conv.=conversion; d.r.=diastereomeric ratio
1
(2=δ-diastereomer; 3=β-diastereomer). [b] Determined by H NMR
analysis of the reaction crude. [c] No gel formation. [d] The gelator
concentration in this case was reduced to 10 gLÀ 1 due to solubility
problems compared to substrate 1a (entry 5).
Table 5. Substrate screening in homogeneous aqueous solution.[a]
Entry
Substrate
Solvent
Conv. [%][b]
d.r. (2/3)[b]
1[c]
2
1a
1a
1a
1b
1c
1d
1e
1f
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
0
–
99
95
100
94
95
11
100
75:25
78:22
38:63
38:63
51:49
0:100
70:30
3[d]
4
Figure 3. Second order kinetics of the [2+2]-cycloaddition model reaction in
different reaction media. Reaction conditions: Cinnamate 1a (0.5 mmol), [Ir
{dF(CF3)ppy}2(dtb-bpy)]PF6 (1.0 mol%), solvent (DMF, toluene, CH3CN or H2O)
(1 mL), LED455, RT. Reactions in solution were carried with dry solvents out
under strict nitrogen atmosphere, while those performed in gel were
5
6
7[e]
8
performed under aerobic conditions. Rate constants, k, are given in (×10À 2
MÀ 1 hÀ 1; half-lives, t1/2, are given in h. Additional kinetics parameters are
available from the authors upon request.
)
[a] Reaction conditions: Cinnamate (0.5 mmol), photocatalyst [Ir{dF(CF3)
ppy}2(dtb-bpy)]PF6 (1.0 mol%), distilled degassed H2O (1 mL), LED455, 24 h,
RT, nitrogen atmosphere. Abbreviations: Conv.=conversion; d.r.=diaster-
eomeric ratio (2=δ-diastereomer; 3=β-diastereomer). [b] Determined by
1H NMR analysis of the reaction crude. [c] Control experiment in the
absence of catalyst. [d] 0.02 mol% catalyst. [e] The same result was
obtained after 72 h. Note: Reactions could be scaled up to 1.1 mmol of
substrate without detriment in the yield and selectivity.
Table 6. Gel-to-sol transition temperature (Tgel) of the gels prepared with
the different gelators. Concentrations and solvents correspond to those
used in the experiments.
°
Entry
Substrate
Solvent [1 mL]
T
gel [ C]
1
2
3
4
5
G1 (15)
G2 (15)
G3 (5)
G4 (5)
G5 (10)
DMF
DMF
DMF
Toluene
CH3CN
50�2
60�2
64�2
90�2
76�2
contrast, the reaction of this substrate in homogeneous DMF
solution afforded the corresponding diastereomers 2:3 with a
d.r. of 57:47.[28]
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