Organophotocatalysis: Insights into the Mechanistic Aspects of Thiourea-Mediated Intermolecular [2+2]Photocycloadditions

Article abstract of DOI:10.1002/anie.201600596

Mechanistic investigations of the intermolecular [2+2] photocycloaddition of coumarin with tetramethylethylene mediated by thiourea catalysts reveal that the reaction is enabled by a combination of minimized aggregation, enhanced intersystem crossing, and

Full text of DOI:10.1002/anie.201600596

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International Edition: DOI: 10.1002/anie.201600596
German Edition: DOI: 10.1002/ange.201600596
Photochemistry
Organophotocatalysis: Insights into the Mechanistic
Aspects of Thiourea-Mediated Intermolecular
[
2 + 2] Photocycloadditions
Nandini Vallavoju, Sermadurai Selvakumar, Barry C. Pemberton,
Steffen Jockusch, Mukund P. Sibi,* and Jayaraman Sivaguru*
Angewandte
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Abstract: Mechanistic investigations of the intermolecular
2+2] photocycloaddition of coumarin with tetramethylethy-
thiourea catalysts influence the excited-state reactivity
[
through a combination of minimized aggregation, enhanced
intersystem crossing, and altered excited state lifetime(s).
The intermolecular [2+2] photocycloaddition of coumar-
lene mediated by thiourea catalysts reveal that the reaction is
enabled by a combination of minimized aggregation, enhanced
intersystem crossing, and altered excited-state lifetime(s).
These results clarify how the excited-state reactivity can be
manipulated through catalyst–substrate interactions and reveal
a third mechanistic pathway for thiourea-mediated organo-
photocatalysis.
[
7,8]
ins
with alkenes in solution is an inefficient process. This is
due to the nature of the excited coumarin chromophores,
which decay both radiatively (emission) and non-radiatively
(internal conversion) from the excited state to the ground
state. Nevertheless, the photochemical reactivity of coumar-
ins could be modulated by 1) confinement within supra-
molecular assemblies/templates that restrict molecular
C
atalytic transformations play a central role in chemical and
[
1]
[9]
biological processes. Environmentally benign catalytic pro-
cesses must not only be attractive from an ecological
perspective but also be sustainable. Organo-photocatalytic
motion and by 2) complexation to Lewis acids that interact
with the carbonyl moiety, thereby altering the nature of the
[4d,8a,c,d]
excited state.
As intermolecular photoreactions are
[2]
transformations based on supramolecular interactions have
rightfully attracted attention owing to their promise in
initiating and propagating catalytic processes by employing
light as the energy source in combination with organic
often challenging owing to the dynamics involved in the
excited state, catalysts that impact photochemical reactivity
are likely to not only affect the rates of the photoreaction but
also alter the rates of other excited-state processes. We
[
2]
[1b,6,10]
templates. Recently, several groups have cleverly exploited
photoredox catalysts with a high degree of versatility for
various chemical transformations. Controlling the photo-
chemistry and photophysics of organic chromophores in
solution to achieve selective photochemical transformations
has been a challenge owing to the short lifetimes of the
employed thioureas 1a–1e (Scheme 1)
and urea deriv-
[11]
atives 1 f–1h (Supporting Information, Chart S1) as effi-
cient photocatalysts for the intermolecular [2+2] cycloaddi-
tion of coumarin (2) with tetramethylethylene (3) as we
envisioned that coumarin aggregation and excited-state
[
7]
deactivation/lifetimes influence its reactivity.
[
3]
reactive excited states. Supramolecular scaffolds have been
employed effectively to control the excited-state processes
[3]
with high efficiency. Compared to thermal reactions, carry-
ing out photochemical transformations with a high degree of
selectivity during large-scale laboratory synthesis has pre-
sented formidable obstacles. Whereas catalyzing photochem-
ical transformations with Lewis acids or hydrogen-bonding
activators is attractive, a mechanistic understanding of these
processes provides an avenue to generalize the overall
[2b, 4]
methodology.
Recently, we reported that organocatalysts
based on atropisomeric thiourea derivatives are superior
templates for mediating enantioselective photoreactions, and
proposed a novel mechanism that proceeded by energy
[5]
sharing mediated through hydrogen-bonding interactions.
We established that two distinct reaction pathways are
feasible depending on the loading of the thiourea catalyst.
Scheme 1. Intermolecular [2+2] photocycloaddition of coumarin (2)
with tetramethylethylene (3) with thiourea organocatalysts 1a–1e.
[5a]
To determine whether the same mechanistic rationale is
applicable to intermolecular reactions, we evaluated thiour-
To evaluate the viability of employing thioureas 1a–1e
and urea derivatives 1 f–1h as photocatalysts, we selected
the symmetric thiourea catalysts 1a and 1c (compounds with
the same aryl units on either side of the thiourea functional
group) and the unsymmetric catalysts 1b, 1d, and 1e
[1b, 6]
[11]
ea
cycloaddition of coumarin (2) with tetramethylethylene (3).
Herein, we report a third mechanistic pathway where
and urea derivatives as catalysts for the [2+2] photo-
[7]
(
compounds with different aryl units on either side of the
[
*] N. Vallavoju, Dr. S. Selvakumar, Dr. B. C. Pemberton,
Prof. Dr. M. P. Sibi, Prof. Dr. J. Sivaguru
Department of Chemistry and Biochemistry
North Dakota State University
Fargo, ND 58108-6050 (USA)
E-mail: mukund.sibi@ndsu.edu
[1b,10a–e]
thiourea functional group).
To compare the efficiencies
of the thiourea catalysts with those of the corresponding urea
catalysts, we synthesized the symmetric urea catalyst 1 f (i.e.,
the urea analogue of 1a) and the unsymmetric urea catalysts
1
g and 1h, which are the urea analogues of 1b and 1d,
jayaraman.sivaguru@ndsu.edu
Homepage: http://sivaweb.chem.ndsu.nodak.edu
[11]
respectively. To assess their effectiveness in promoting the
photocycloaddition of 2 with 3 (10 equiv), 100 mol% of 1a–
1
ation in a Rayonet reactor for 19 h (Table 1). As a control,
the reaction was carried out in the absence of catalyst and
compared with previously reported results (4% conversion
for 68 h irradiation of 2 with 10–20 equiv of 3 with > 310 nm
Dr. S. Jockusch
Department of Chemistry, Columbia University
New York, NY 10025 (USA)
h were employed in combination with 350 Æ 30 nm irradi-
[11]
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under http://dx.doi.org/10.
1
002/anie.201600596.
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Table 1: Catalyst screening for the [2+2] photocycloaddition of 3 and 2 in
Table 2: Influence of the catalyst loading on the intermolecular [2+2]
photocycloaddition of 3 and 2 in CH Cl for various thiourea catalysts.
[
a]
[a]
CH Cl .
2
2
2
2
[
b]
[c]
[c]
[c]
[b]
Entry
Catalyst
2/4
MB [%]
Conv. [%]
Yield [%]
Entry
Catalyst
mol%)
2/4
(
12 h
24 h
36 h
40 h
1
2
3
4
5
6
–
89:11
32:68
38:62
76:24
38:62
22:78
86
96
96
88
93
96
11
65
56
22
58
76
9
62
54
19
54
73
[
c]
1a
1b
1c
1d
1e
1
2
3
4
5
6
7
–
95:05
23:77
56:44
55:45
31:69
75:25
87:13
85:15
01:99
23:77
22:78
01:99
57:43
77:23
77:23
66:34
[d]
[d]
1a (10)
1b (10)
1d (10)
1e (10)
1e (5)
1e (1)
–
–
10:90
10:90
7:93
[
d]
–
–
[d]
[d]
–
[d]
46:54
65:35
31:69
50:50
[
a] Catalyst loading: 100 mol%; irradiation time: 19 h. Averages of
[d]
a minimum of three runs (error: Æ5%) are given. [b] Determined by GC
analysis with a ChirasilDex-CB column. [c] The conversion of 2, the mass
[a] Averages of a minimum of three runs are given (error: Æ5%).
1
balance (MB), and the yield of 4 were determined by H NMR
spectroscopy using triphenylmethane as an internal standard in CDCl or
[b] Determined by GC analysis with a ChirasilDex-CB column. [c] Irradi-
ated for 48 h. [d]>90% conversion observed with shorter irradiation
times. Hence, longer irradiation times were not studied. For further
studies on the catalyst loading, see the Supporting Information.
3
CD CN.
3
transmission using
lamp).
a 450 W medium-pressure mercury
[
7]
was 50:50 (entry 7) after 40 h irradiation. As expected, longer
irradiation times were required for lower catalyst loadings to
achieve a similar level of conversion. Evidently, the thiourea
catalyst 1e efficiently facilitated the reaction with good
turnover even at low catalyst loading. To demonstrate the
versatility of the thiourea catalyst in promoting large-scale
photoreactions, we performed the reaction on gram scale. We
achieved 80% conversion with 30 mol% of 1b, and the
Table 1 reveals that the thiourea-based organocatalysts
are effective in promoting the [2+2] photocycloaddition of
coumarin with tetramethylethylene with moderate to high
conversion and with excellent mass balance. The conversion
was higher with the thiourea than with the corresponding urea
[
11]
catalyst. For example, 65% conversion was observed with
thiourea 1a compared to 30% conversion with the corre-
[
11]
[11]
sponding urea 1 f.
Other symmetric and unsymmetric
desired photoproduct was isolated in 75% yield.
catalysts gave modest conversions (Table 1; see also the
Detailed photophysical investigations were carried out to
understand the role of the thiourea catalysts in promoting the
[2+2] photocycloaddition of 2 with 3. Thioureas 1d and 1e
were selected to understand the role of both the thiourea and
naphthalene chromophores in promoting the photoreaction.
Absorption spectra of the thioureas and 2 were recorded to
determine which of the chromophores is predominantly
absorbing light. Coumarin shows no absorbance above
350 nm, whereas 1d absorbs weakly up to 400 nm (Fig-
ure 1A). It should be pointed out that the 350 nm lamp that
was used for the photocatalytic reactions has a spectral width
of Æ 30 nm. No appreciable fluorescence was observed for 2 at
room temperature in toluene. We thus recorded the lumines-
cence of 2 at 77 K in MCH glass (Figures 1B). We observed
strong fluorescence centered around 408 nm and weak
phosphorescence (overlapping with the fluorescence
[
11]
Supporting Information, Table S1). The most striking result
was achieved with the naphthyl-substituted catalyst 1e, which
gave 76% conversion with excellent (96%) mass balance
(
Table 1, entry 6). As thiourea-based catalysts were effective
in promoting the intermolecular photoreaction of 2 with 3, it
became critical to ascertain the best solvent(s) for this
photocatalytic process. We investigated the efficiency of the
photocycloaddition using 1a, 1b, 1d, and 1e in four different
solvents, namely methylcyclohexane (MCH), toluene, tetra-
[11]
hydrofuran (THF), and dichloromethane. In dichlorome-
thane and toluene, the reaction proceeded with excellent
[11]
conversion, yield, and mass balance (Table S2). In THF, the
conversion varied between 26 to 99%, and the mass balance
[
11]
was moderate. Low conversions and product yields were
observed in MCH.
[11]
Based on the results shown in Table S2, we selected
dichloromethane as the solvent of choice to investigate the
efficiency of the reaction with respect to the thiourea catalyst
loading (Table 2). The catalyst loading was varied from
signal).
The emission characteristics matched the previ-
[
12]
ously reported profile, which was rationalized to be due to
the monomer and the dimeric aggregate of 2 in MCH glass at
77 K. Kinetic decay analysis of the luminescence at 412 nm
showed two lifetimes of 1.2 ns and 4.0 ns. On the other
hand, luminescence measurements with a 1:1 mixture of 1 f
and 2 showed a significant reduction of the coumarin
fluorescence in addition to reduced lifetimes at 412 nm
[11]
7
0 mol% to 10 mol% for thioureas 1a, 1b, and 1d
[11]
(
Table S3, entries 2–13) and from 70 mol% to 1 mol% in
[11]
the case of 1e (Table S3, entries 14–19). To compare the
efficiency of the thioureas at different loadings, the reactant/
photoproduct ratio (2/4) was investigated at various irradi-
ation times (12–48 h) for a given amount of catalyst. Table 2
shows that at a catalyst loading of 10 mol%, the 2/4 ratio for
[11]
(0.6 ns and 3.2 ns). More importantly, for a 1:1 mixture of
2 and 1d, the emission intensity depended on the excitation
wavelength (Figure 1B,C). For the 1:1 mixture of 2 and 1d,
2
4 h irradiation was 1:99 for 1a (entry 2), 23:77 for 1b
with l = 340 nm, weak emission from the coumarin and
ex
(
entry 3), 22:78 for 1d (entry 4), and 1:99 for 1e (entry 5). As
a new emission centered at approximately 523 nm were
observed. Upon changing the excitation wavelength to
360 nm, where 2 does not have any significant absorption,
the new emission band was predominantly observed, which is
likely due to the selective excitation of the coumarin–catalyst
catalyst 1e was found to be optimal for the [2+2] photo-
cycloaddition of 2 with 3, we investigated its efficiency at
loadings of 5 and 1 mol%. With 5 mol% of 1e, the 2/4 ratio
was 31:69 (entry 6), and at 1 mol% loading, the ratio of 2/4
5
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5
0 mm, suggesting that the
naphthalene triplet excited
states of 1e are not directly
involved in the [2+2] cyclo-
addition.
This
finding
implies that the thiourea
unit is likely to be respon-
sible for the enhanced reac-
tivity. The bimolecular rate
constant for the [2+2] pho-
tocycloaddition of the cou-
marin triplet states with tet-
ramethylethylene was deter-
mined by laser flash photol-
[11]
ysis.
Pulsed laser excita-
tion of 2 at 308 nm gave
a weak transient absorption
centered around 400 nm,
which had previously been
assigned to the triplet state
of the coumarin chromo-
Figure 1. A) Absorption spectra of 1d, 2, and a 1:1 mixture of 1d and 2 in toluene. B,C) Steady-state
luminescence spectra of 1d, 2, and a 1:1 mixture of 1 f and 2 in MCH glass at 77 K at l =340 nm (B) and
ex
lex =360 nm (C). D) Steady-state luminescence spectra of 1e, 2, and a 1:1 mixture of 1e and 2 in toluene
[14]
glass at 77 K. E) Transient absorption spectra recorded 0–0.4 ms after pulsed laser excitation (l =355 nm,
phore.
Decay traces at
ex
5
ns pulse width) of argon-saturated toluene solutions of 1e (2 mm) in the absence (blue) and presence (red) different concentrations of
of 2 (2 mm). Inset: Absorbance kinetic traces monitored at 420 nm of 1e (2 mm; blue), 1e and 2 (red), and
d and 2 (green).
3
were recorded and fitted
first-order decay
1
to
a
function. The bimolecular
rate
constant
(k ꢀ 2 ”
q
[
11]
8
À1 À1
complex (Figure S20).
Based on time-resolved lumines-
10 m s ) was obtained from the slope of the plot of the
inverse triplet lifetime versus the concentration of 3.
A few solid-state structures of catalysts bound to sub-
strates have been reported in the literature on photoreac-
tions. Herein, we provide a crystal structure of catalyst 1 f
bound to substrate 2 (Figure 2) to highlight the catalyst–
substrate interactions. Inspection of the single-crystal XRD
structure revealed an NH···O=C distance of approximately
2.05 Š and an N···O=C distance of about 2.87 Š, indicating
[11]
cence studies, this emission was found to originate from the
triplet excited state. The properties of the triplet states of
coumarin and catalyst 1e were investigated by phosphores-
cence spectroscopy in a toluene matrix at 77 K (Figure 1D).
The triplet energy of 2 (62 kcalmol ) was determined from
the first peak of the phosphorescence spectrum with a lifetime
[20]
À1
[21]
[11]
(
tp) of 0.3–0.5 s.
The phosphorescence of catalyst 1e is
slightly red-shifted compared to that of coumarin, which
shows that 1e has a slightly lower triplet energy (61 kcal
weak-to-moderate hydrogen-bonding interactions between
the catalyst and the substrate (Figure 2). As the hydrogen-
À1
[15]
mol ). The phosphorescence lifetime of catalyst 1e in
[11]
toluene glass at 77 K was 1.1 s. Based on the phosphor-
escence lifetimes, it is clear that the lowest triplet excited
states of both catalyst 1e and substrate 2 have pp* config-
uration.
bonding interactions had been ascertained in the crystalline
state, it became critical to evaluate the magnitude of the
interaction that existed between the catalyst and the substrate
in solution to comprehend its effect on photocatalysis. To
address this aspect, thermodynamic binding analysis of both
substrate 2 and photoproduct 4 was performed with thioureas
Laser flash photolysis was performed to investigate the
reactivity of the triplet states (Figure 1E). Pulsed laser
excitation of a 1:1 mixture of 2 (2 mm) and catalyst 1e at
[11,16]
1a, 1b, and 1d (Table S10).
Both substrate 2 and
3
55 nm generated a transient absorption spectrum with
photoproduct 4 were bound to the thiourea catalysts, with
the association constants for the coumarin being slightly
higher than for photoproduct 4. For example, in the case of
a maximum at l = 420 nm (Figure 1E, red spectrum), which
decayed monoexponentially with a lifetime of 1.5 ms. In the
absence of 2, a similar transient absorption spectrum was
observed (blue spectrum). Consistent with literature reports
for other naphthalene derivatives that also show transient
[13]
absorbance around 420 nm, this transient absorption spec-
trum was assigned to the triplet–triplet absorption of 1e.
Furthermore, pulsed laser excitation of a mixture of 2 and 1d,
a catalyst that does not contain a naphthalene substituent, did
not show transient absorption (Figure 1E, inset, green line).
The transient absorption generated from 1e was quenched
neither by 2 (Figure 1E, inset, red line) nor by tetramethyl-
[11]
ethylene (Figure S17),
even at a high concentration of
Figure 2. Single-crystal XRD of catalyst 1 f with coumarin (2).
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À1
1
d, the association constant (K ) for coumarin was 6.3m ,
Our current study has showcased the influence of
thioureas (and ureas) as catalysts to promote intermolecular
photochemical reactions. As urea and thiourea catalysts with
various functionalities are easily synthesized and readily fine-
tuned (in terms of their electronic and steric properties), they
can easily be utilized to influence photocatalytic processes.
We have also demonstrated that both the photochemistry and
the photophysics of individual systems need to be investigated
to provide insights into the mechanism of substrate activation
a
and the corresponding dissociation constant (K ) for the
d
À1 [11]
photoproduct 4 was 0.21m (K = 4.8m ).
a
We suggest that the rate enhancement observed for the
photoreaction using urea and thiourea catalysts is due to the
hydrogen bonding between the NH units of the catalysts and
the carbonyl group of the coumarin, with the [2+2] photo-
[17]
cycloaddition probably occurring from the triplet state of 2.
Having ruled out the involvement of the naphthalene triplet
excited state of catalyst 1e in promoting the photoreaction of
[
19]
during the photoreaction.
2
with 3, the question that needs to be addressed is how the
thiourea catalyst promotes the photoreaction. We suggest that
upon complexation of the thiourea catalyst with coumarin, Acknowledgements
both its ground-state and excited-state properties are
affected. This is reflected in 1) the minimized aggregation of
coumarin (based on luminescence studies in MCH, Fig-
ure 1B), 2) the enhanced intersystem crossing (ISC) in
coumarin (reflected in the phosphorescence signal enhance-
ment, Figure 1D), and 3) altered excited-state lifetimes
J.S. thanks the NDSU for financial support and the National
Science Foundation for the purchase of HPLC and GC
columns (CHE-1213880) and for the purchase of a combiflash
system (CHE-1465075). N.V. and J.S. thank the NDSU for
a Graduate Dissertation Fellowship. We are grateful to Dr.
Angel Ugrinov for solving the single-crystal XRD structures.
[11]
(
derived from luminescence kinetic decay analysis). Alter-
ing excited-state properties by non-covalent interactions is
[18]
well established in the literature, and in the present case, we
have exploited this effect for promoting the efficiency of the
intermolecular [2+2] photocycloaddition of coumarin with an
alkene using catalytic amounts of a thiourea catalyst.
Keywords: asymmetric catalysis · organocatalysis ·
photochemistry · photocycloadditions · thioureas
How to cite: Angew. Chem. Int. Ed. 2016, 55, 5446–5451
Angew. Chem. 2016, 128, 5536–5541
Based on our observations, we propose a likely photo-
catalytic cycle (Figure 3) in which the thiourea/urea catalyst
binds coumarin through hydrogen bonding. Upon light
excitation of coumarin 2, the triplet yield and lifetime are
altered owing to interactions with the thiourea catalysts. This
enables 2 to react efficiently with alkene 3 to form photo-
product 4. Whereas intramolecular photocycloadditions
involving coumarins proceed by two mechanistic pathways
depending on the amount of the thiourea catalyst (via an
exciplex or a ground-state complex), the intermolecular [2+2]
photocycloaddition of coumarins mediated by thioureas
involves minimized aggregation, enhanced intersystem cross-
ing, and altered excited-state lifetime(s).
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Received: January 19, 2016
Published online: March 23, 2016
Angew. Chem. Int. Ed. 2016, 55, 5446 –5451
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5451