Formation of Tetrahydrothiophenes via a Thia-Paternò-Büchi-Initiated Domino Photochemical Reaction

Article abstract of DOI:10.1021/acs.orglett.0c03128

We have established photochemical access to thietane or tetrahydrothiophene compounds from thiobenzophenone derivatives and acrylonitrile, wherein the product selectivity is controlled by a simple adjustment of the reagent concentration in solution. Small libraries of five-membered ring sulfur-containing compounds were prepared through a thia-Paternò-Büchi reaction, followed by a previously unknown regioselective photochemical ring enlargement reaction in a domino process or a stepwise fashion. A mechanism is proposed to rationalize this ring enlargement reaction via a carbene species provided from photoexcited thiocarbonyl compounds.

Full text of DOI:10.1021/acs.orglett.0c03128

pubs.acs.org/OrgLett
Letter
̀
Formation of Tetrahydrothiophenes via a Thia-Paterno−Bu
̈
chi-
Initiated Domino Photochemical Reaction
́
Ahmad F. Kassir, Regis Guillot, Marie-Christine Scherrmann, Thomas Boddaert,* and David J. Aitken*
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ABSTRACT: We have established photochemical access to
thietane or tetrahydrothiophene compounds from thiobenzophe-
none derivatives and acrylonitrile, wherein the product selectivity is
controlled by a simple adjustment of the reagent concentration in
solution. Small libraries of five-membered ring sulfur-containing
̀
compounds were prepared through a thia-Paterno−Buchi reaction,
̈
followed by a previously unknown regioselective photochemical
ring enlargement reaction in a domino process or a stepwise
fashion. A mechanism is proposed to rationalize this ring
enlargement reaction via a carbene species provided from
photoexcited thiocarbonyl compounds.
hotochemical transformations are efficient tools for the
creation of molecular complexity from simple and readily
derivatives and alkene partners, and in the course of this
P
̀
study we discovered an unprecedented thia-Paterno−Bu
̈
chi-
available starting materials,1 and they provide valuable
synthetic access to molecular structures that are difficult to
obtain using other organic synthesis methodologies.2 The
absorption of a photon provides the energy required by a
substrate molecule to undergo a structural transformation in a
chemical reaction in mild and catalyst-free conditions,3
making photochemical reactions attractive commodities in
the pursuit of eco-compatible processes and sustainable
development.4 They are also mutually compatible with
multiple bond-forming transformation processes, which
respond favorably to criteria of step economy.5 Photochemical
reactions can be conveniently combined in domino sequences
with thermal transformations6 or other photochemical
reactions.7,8
initiated domino photochemical reaction, as we disclose
below.
Using as a starting point the work described by Ohno,11a
̀
we carried out the thia-Paterno−Buchi reaction between
̈
thiobenzophenone 1a, as a 130 mM solution in cyclohexane,
and an excess of acrylonitrile 2 using a polychromatic Hg
lamp and a Pyrex immersion reaction vessel (Table 1). The
expected thietane 3a was isolated in a 43% yield after a 1.5 h
reaction time (Table 1, entry 1). We varied the reaction
conditions in an effort to improve the reaction efficiency. The
use of a higher concentration of 1a had little effect on the
yield (Table 1, entry 2), while the use of monochromatic light
sources (300 or 350 nm) in a carousel reactor was less
efficient even after prolonged irradiation (Table 1, entries 3
and 4, respectively). Unexpectedly, when the reaction was
performed using a much lower concentration of 1a, this time
as 6 mM solution in cyclohexane, the initially formed thietane
3a was depleted as the reaction progressed, and the
concomitant formation of a new product was observed.
After a 1.5 h reaction time, thietane 3a was isolated in only a
13% yield while the new product 4a, featuring a
tetrahydrothiophene ring, was obtained in a 29% yield
̀
The light-initiated Paterno−Buchi reaction, which com-
̈
bines a photochemically excited carbonyl compound with an
alkene to give the oxetane, is a well-developed and
synthetically useful process for which many successful
applications are described in the literature.9,10 In stark
contrast, a very limited number of photochemical reactions
of a thiocarbonyl compound with an alkene to give a thietane,
̀
the so-called thia-Paterno−Buchi reaction, have been
̈
described in the literature, and for the most part these
studies date from several decades ago.11 It appeared to us that
this reaction had been underexploited for the preparation of
sulfur-containing four-membered ring compounds, which are
high-value intermediates in organic synthesis for post-
functionalization, such as in ring enlargement or ring opening
reactions.12 We therefore undertook a reinvestigation of the
Received: September 17, 2020
̀
thia-Paterno−Bu
̈
chi reaction between thiobenzophenone
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© XXXX American Chemical Society
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A

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conditions and returning to the polychromatic lamp, benzene
and toluene were assessed as solvents but were less
productive than cyclohexane; 4a was obtained in 44% and
31% yields, respectively (Table 1, entries 12 and 13,
respectively). When the reaction was repeated in tetrahy-
drofuran, neither 3a nor 4a were formed (Table 1, entry 14).
Instead, diphenyl(tetrahydrofuran-2-yl)methanethiol 5 and 2-
(benzhydrylthio)tetrahydrofuran 6 were obtained in 23% and
20% yields, respectively (see Supporting Information). These
two compounds are known to be formed via solvent-derived
free radicals generated from thiobenzophenone in its T1 (n,
π*) state;13 evidently, this process is faster than the reaction
with acrylonitrile.
Table 1. Conditions of the Photochemical Reaction of
Thiobenzophenone and Acrylonitrile
[1a]
entry (mM)
light
source
time yield of 3a yield of 4a
solvent
(h)
(%)
(%)
1
2
3
4
5
6
7
8
130
260
130
130
6
Hg lamp
C6H12
C6H12
C6H12
C6H12
C6H12
C6H12
C6H12
C6H12
C6H12
C6H12
C6H12
C6H6
1.5
1.5
18
18
1.5
3
3
3
3
3
43
41
32
33
13
0
0
7
0
4
0
0
0
Hg lamp
a
300 nm
350 nm
a
0
Hg lamp
Hg lamp
Hg lamp
Hg lamp
Hg lamp
Hg lamp
29
32
62
43
28
29
22
44
31
0
Through these investigations, we established a satisfying
controlled access to either four- or five-membered ring
compounds by simply adjusting the concentration of the
reaction medium.
6
25
50
b
9
25
With optimized experimental conditions for selective access
to tetrahydrothiophene 4a in hand (Table 1, entry 7), we
explored the scope of the reaction on a 5 mmol scale (gram
scale) using several 4,4′-disubstituted thiobenzophenone
derivatives 1a−e. All substrates, regardless of the electronic
effects of their substituents, yielded the target “homo” tetra-
aryl tetrahydrothiophenes 4a−e in 48−62% yields, each
bearing four identical aromatic substituents. The correspond-
ing four-membered ring compounds, thietanes 3a−e, were
not observed in these reactions. Compound 4e was obtained
as single crystals, which were analyzed by X-ray diffraction
(Scheme 1).
We had observed during the optimization studies that
thietane 3a was formed rapidly but was subsequently depleted
as 4a appeared, and it became our working hypothesis that a
domino transformation process was operating. The second
step in this process would be a ring expansion implying, at
c
10
11
12
13
14
25
a
25
25
25
25
350 nm
3
3
3
3
18
0
0
Hg lamp
Hg lamp
Hg lamp
toluene
THF
0
a
b
Performed using the Rayonet apparatus RPR-200. Performed with
c
7 equiv of acrylonitrile. Performed with 46 equiv of acrylonitrile.
(Table 1, entry 5). Single crystals suitable for X-ray diffraction
analysis were obtained for each of them, unambiguously
confirming their molecular structures (Figure 1). The
formation of a tetrahydrothiophene naturally aroused our
curiosity, and our investigations thereafter were oriented
toward improving the yield of compound 4a (Table 1). When
the reaction time was increased to 3 h, 4a was obtained as the
only product in a slightly improved yield of 32% (Table 1,
entry 6). Returning to the effects of concentration, the
reaction was carried out using 1a as a 25 mM solution in
cyclohexane for 3 h, which gratifyingly provided 4a in a 62%
yield (Table 1, entry 7). A further increase of the
concentration to 50 mM had the effect of reducing the
yield of 4a to 43%, and a small amount of the thietane 3a was
isolated as well (Table 1, entry 8). The amount of
acrylonitrile was also adjusted to either 7 or 46 equiv, but
in both cases the yield was not better than 30% (Table 1,
entries 9 or 10, respectivley). The reaction employing a 25
mM solution was carried out using a monochromatic light
source (350 nm) in a carousel reactor for 3 h and yielded
roughly equivalent amounts of 3a and 4a in modest yields
(Table 1, entry 11). Retaining the 25 mM substrate
Scheme 1. Synthesis of “Homo” Tetra-Aryl
Tetrahydrothiophenes 4
Figure 1. X-ray diffraction structures of compounds 3a and 4a.
B
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least formally, the reaction of 3a with one equivalent of 1a.
We carried out some experiments to test this hypothesis using
the polychromatic Hg lamp and a Pyrex reaction immersion
vessel (Table 2). First, a control reaction was run, whereby a
1 mM solution of thietane 3a alone was irradiated for 3 h. No
trace of 4a was found; instead, a retro-[2 + 2] cleavage took
place to furnish 3,3-diphenylacrylonitrile 7a in a 85% yield
(Table 2, entry 1).14 This indicated that 4a could not be
formed from 3a alone. The irradiation of an equimolar
mixture of 3a and 1a gave 55% of the target compound 4a in
addition to 31% of the side product 7a (Table 2, entry 2).
When the reaction was repeated using 2 equiv of 1a, the
target tetrahydrothiophene 4a was obtained in a 76% yield
after only 1 h (Table 2, entry 3). We examined the reaction at
a high concentration (130 mM) but, as expected, this was
deleterious to the ring enlargement reaction, and a high yield
(76%) of 7a was obtained instead (Table 2, entry 4). Two
additional control reactions were performed in which 3a and
1a were heated in the dark with or without a radical initiator
(Table 2, entries 5 or 6, respectively), which induced no
transformation. Collectively, these observations confirm that
the formation of tetrahydrothiophene 4a from thietane 3a is a
photochemically induced ring enlargement that requires the
presence of 1a and proceeds faster than the competitive
unimolecular retro-[2 + 2] cleavage of 3a. They also strongly
support the contention that the overall transformation of 1a
and 2 to tetrahydrothiophene 4a is a domino process
including two independent photochemical reactions.
Scheme 2. Synthesis of Thietanes 3
The “hetero” tetra-aryl tetrahydrothiophenes 4ab−ae, respec-
tively, were obtained in good to excellent yields (66−90%),
each as a unique regioisomer; the regioselectivity was
established by the X-ray diffraction analysis of compound
4ac. Diarylthietanes 3b and 3c each reacted with 1a to furnish
the tetra-aryl tetrahydrothiophenes 4ba and 4ca, respectively,
in good yields (84% and 70%), again as single regioisiomers.
It was apparent that 4ba and 4ca were regioisomers of 4ab
and 4ac, respectively. The tetra-aryl tetrahydrothiophene 4cb
was also prepared from thietane 3c and 4,4′-dimethylth-
iobenzophenone 2b in an 86% yield, again as a single
regioisomer. Diphenylthietane 3a reacted with 3,3′-bis-
(trifluoromethyl)thiobenzophenone 1f to give the tetra-aryl
tetrahydrothiophene 4af, once more as a single regioisomer,
in a more modest yield (38%). Finally, the diastereoselectivity
of the ring enlargement reaction was evaluated. As expected,
the reaction between diphenylthietane 3a and the unsym-
metrical 4-methylthiobenzophenone 1g provided a mixture of
the two diastereoisomers of compound 4ag in a 1:1 ratio
Table 2. Conditions of the Ring Enlargement Reaction
1
(determined by H NMR signal integration) in a 67% yield.
[3a]
entry (mM)
equiv of
1a
yield of 4a yield of 7a
From the early work of Ohno and De Mayo, the thia-
a
conditions
(%)
(%)
̀
Paterno−Buchi reaction between 1a and 2 is known to occur
̈
1
2
3
4
5
6
1
1
1
130
1
1
0
1
2
2
2
2
25 °C, 3 h
25 °C, 3 h
25 °C, 1 h
0
55
76
0
0
0
85
31
0
76
0
from the S2 (π, π*) excited state of the thione and proceed
via a thermally unstable 1,3-dithiane intermediate, which
decomposes to give the thietane 3a.11a,c A number of studies
on thioketone or thioester photodimerization evoke an
important role for the T1 (n, π*) excited state, which evolves
by the formation of a carbene intermediate,15 a process that
25 °C, 4 h
25 to 80 °C, 5 h
25 to 80 °C, 24 h
b
c
0
a
may implicate a hydrogen abstraction reaction.13,16
A
For entries 1−4, no thietane was present at the end of the reaction.
For entries 5 and 6, the thietane and thiobenzophenone starting
mechanistic proposal for the photochemical ring enlargement
reaction is presented in Scheme 4. We suggest that
thiobenzophenone 1a reacts from its T1 (n, π*) excited
state by hydrogen abstraction to provide a carbene
intermediate that then reacts with thietane 3a to form a
sulfonium ylide.17 The latter undergoes a regioselective thia-
Stevens rearrangement in a regioselective manner to give the
product 4a. The regioselectivity of the ring enlargement can
be explained by the geminal diaryl substituent suite, which
weakens the vicinal carbon−sulfur bond.
The formation of compounds 5 and 6 when 1a and 2 were
irradiated in tetrahydrofuran (Table 1, entry 14) is in full
agreement with the implication of thiobenzophenone in its T1
(n, π*) state. The key effect of the concentration of 1a in the
reaction medium on the product selectivity can be also
rationalized by the implication of the T1 (n, π*) state of
thiobenzophenone for the ring enlargement. It has been
demonstrated by Maciejewski, Ramamurthy, and De Mayo18
that low thione concentrations prevent the triplet decay
b
c
materials were recovered entirely. Performed in the dark. Performed
in the dark in the presence of a radical initiator (AIBN).
On the basis of the above results, we reasoned that it
should be possible to employ a different thiobenzophenone in
each of the two photochemical transformations when
conducted separately. This would facilitate substituent
diversification and lead to “hetero” tetra-aryl tetrahydrothio-
phenes. For this purpose, we prepared thietane 3a on 10
mmol scale (about a 2 g scale) with similar efficiency (41%
yield), and we synthesized two further thietanes from 4,4′-
dimethylthiobenzophenone 1b and 4,4′-dichlorothiobenzo-
phenone 1c; compounds 3b and 3c, respectively, were
obtained in 39% and 42% yields (Scheme 2).
Using the optimal conditions from Table 2 (entry 3),
diphenylthietane 3a was reacted with a selection of 4,4′-
disubstituted thiobenzophenones 1b−e bearing diversely
electron-rich and electron-poor substituents (Scheme 3).
C
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induced by self-quenching interactions. Consequently, at a
Scheme 3. Synthesis of “Hetero” Tetra-Aryl
Tetrahydrothiophenes 4
̀
high 1a concentration only the thia-Paterno−Buchi reaction
̈
from the S2 (π, π*) state can occur, whereas at a low 1a
concentration the domino process can be achieved.
To further probe our mechanistic proposal for the
photochemical ring enlargement, we considered the “chem-
ical” version of the thia-Stevens rearrangement reaction.
Thietane 3a and diphenyldiazomethane 8 were heated (80
°C, 48 h, benzene) in the presence of 4 mol %
Rh2(OAc)4.12b,c Satisfyingly, the reaction provided tetrahy-
drothiophene 4a with the same regioselectivity as the
photochemical reaction, albeit in a 13% yield and
accompanied by some starting material and undetermined
side products (Scheme 5). As well as supporting our
mechanistic proposal, this observation suggested that the
photochemical reaction is an efficient alternative to the metal-
initiated transformation.
Scheme 5. Rhodium-Catalyzed Thia-Stevens
Rearrangement
In summary, this work has established an unprecedented
domino photochemical transformation involving a thia-
̀
Paterno−Buchi reaction, followed by a light-induced ring
̈
enlargement reaction. Thiobenzophenone derivatives may be
combined with acrylonitrile, either following a domino
process or in a successive manner via two independent
reactions, to access tetrahydrothiophene derivatives. In this
way the potential of thietanes as useful intermediates in
organic synthesis and the importance of photochemistry in
the development of selective eco-compatible organic method-
ologies are further complemented. Furthermore, this study
supports a long-standing proposed photochemical source of a
carbene species from thiocarbonyl derivatives, which may
justify its investigation for achieving other chemical trans-
formations.
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c03128.
Scheme 4. Proposed Mechanism for the Entire Photochemical Domino Sequence
D
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Letter
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Experimental procedures, spectroscopic data, and
copies of 1H and 13C NMR spectra for all new
compounds (PDF)
Accession Codes
CCDC 2008236−2008239 contain the supplementary
crystallographic data for this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/data_-
request/cif, or by emailing data_request@ccdc.cam.ac.uk, or
by contacting The Cambridge Crystallographic Data Centre,
12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223
336033.
AUTHOR INFORMATION
Corresponding Authors
■
́
Thomas Boddaert − CNRS, Institut de Chimie Moleculaire et
́
́
des Materiaux d’Orsay (ICMMO), Universite Paris-Saclay,
91405 Orsay, France; orcid.org/0000-0002-3939-4700;
Email: thomas.boddaert@universite-paris-saclay.fr
(7) (a) Dittami, J. P.; Nie, X. Y.; Nie, H.; Ramanathan, H.; Buntel,
C.; Rigatti, S.; Bordner, J.; Decosta, D. L.; Williard, P. Tandem
photocyclization-intramolecular addition reactions of aryl vinyl
sulfides. Observation of a novel [2 + 2] cycloaddition-allylic sulfide
rearrangement. J. Org. Chem. 1992, 57, 1151−1158. (b) Booker-
Milburn, K. I.; Baker, J. R.; Bruce, I. Rapid access to azepine-fused
oxetanols from alkoxy-substituted maleimides. Org. Lett. 2004, 6,
1481−1484. (c) Pusch, S.; Schollmeyer, D.; Opatz, T. A light-
induced vinylogous Nazarov-type cyclization. Org. Lett. 2016, 18,
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Conformationally driven two- and three-photon cascade processes in
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protoilludane sesquiterpenes through a photochemical reaction
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2019, 58, 14629−14632.
́
David J. Aitken − CNRS, Institut de Chimie Moleculaire et des
́
́
Materiaux d’Orsay (ICMMO), Universite Paris-Saclay, 91405
Orsay, France; orcid.org/0000-0002-5164-6042;
Email: david.aitken@universite-paris-saclay.fr
Authors
́
Ahmad F. Kassir − CNRS, Institut de Chimie Moleculaire et
des Materiaux d’Orsay (ICMMO), Universite Paris-Saclay,
́
́
91405 Orsay, France
́
́
Regis Guillot − CNRS, Institut de Chimie Moleculaire et des
Materiaux d’Orsay (ICMMO), Universite Paris-Saclay,
91405 Orsay, France
Marie-Christine Scherrmann − CNRS, Institut de Chimie
Moleculaire et des Materiaux d’Orsay (ICMMO), Universite
́
́
́
́
́
Paris-Saclay, 91405 Orsay, France
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c03128
(8) For our own achievements on photochemical domino reactions,
see: (a) Pereira, E.; Faure, S.; Aitken, D. J. Photochemical behaviour
of 5-formyl and 5-acetyl uracils in the presence of ethene.
Tetrahedron Lett. 2008, 49, 1968−1970. (b) Buendia, J.; Chang,
Z.; Eijsberg, H.; Guillot, R.; Xie, J.; Frongia, A.; Secci, F.; Robin, S.;
Boddaert, T.; Aitken, D. J. Preparation of cyclobutene acetals and
tricyclic oxetanes via photochemical tandem and cascade reactions.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
A.F.K. is grateful to the Municipality of Dair Kanoun Al-Nahr,
Lebanon, for the award of a doctoral research scholarship and
also thanks Dr. O. Yazbeck of the Lebanese University for
material support and professional council.
(9) (a) Abe, M. Recent Progress regarding regio-, site-, and
̀
stereoselective formation of oxetanes in Paterno−Bu
̈
chi Reactions. J.
Chin. Chem. Soc. 2008, 55, 479−486. (b) D’Auria, M.; Racioppi, R.
̀
Oxetane synthesis through the Paterno−Buchi reaction. Molecules
̈
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̈
̈
̈
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