Diastereoselective sp3-C–H Functionalization of Arylmethyl Ketones and Transformation of E- to Z-Products Through Photocatalysis

Article abstract of DOI:10.1002/ejoc.201901415

We have developed an efficient metal-free route for the synthesis of 1,4-enedione derivatives under microwave irradiation by reacting easily available arylmethylketones with DMSO or diphenyl sulfoxide in the presence of TBAI and persulfate. The reaction is very clean and completes within very short time. All the reagents and catalysts are cheap and environmentally benign. In addition, the E-isomer of the products can easily be transformed into the Z-isomer by using eosin Y photocatalyst under the irradiation of white CFL.

Full text of DOI:10.1002/ejoc.201901415

DOI: 10.1002/ejoc.201901415
Communication
Photocatalysis
Diastereoselective sp3-C–H Functionalization of Arylmethyl
Ketones and Transformation of E- to Z-Products Through
Photocatalysis
Gaurav K. Rastogi,[a,b] Bhaskar Deka,[a] Mohit L. Deb,*[a] and Pranjal K. Baruah*[a]
Abstract: We have developed an efficient metal-free route for short time. All the reagents and catalysts are cheap and envi-
the synthesis of 1,4-enedione derivatives under microwave irra- ronmentally benign. In addition, the E-isomer of the products
diation by reacting easily available arylmethylketones with can easily be transformed into the Z-isomer by using eosin Y
DMSO or diphenyl sulfoxide in the presence of TBAI and per- photocatalyst under the irradiation of white CFL.
sulfate. The reaction is very clean and completes within very
Introduction
The usual synthetic methods for 1,4-enedione moiety involve
are oxidation of furan and thiophene derivatives,[9] α,ꢀ-en-
ones,[10] oxidative rearrangement of 2-alkynyl alcohols,[11] de-
composition of α-diazo carbonyl compounds,[12] nucleophilic
addition to dibenzoylacetylenes,[13] and reaction of α-halo ket-
ones with sulfinate salt.[14] However, these methods encounter
few shortcomings, for instance use of costly reagents, pro-
longed reaction time, anhydrous reaction conditions, and pre-
functionalized substrates. Wu and Pan reported a reaction route
for the synthesis of 2-(methylthio)-substituted 1,4-enediones
from the reaction of arylmethylketones and dimethyl sulfoxide
(DMSO).[4b] However, the present reaction is more advanta-
geous to the previous report with respect to reaction time, cata-
lyst/oxidant loadings and nature of oxidant. Moreover, the
present reaction also gives inverse diastereoselectivity com-
pared to the previous report (Scheme 1).
C–H Functionalization is a growing field and continues to draw
attention of synthetic organic chemists for last couple of dec-
ades because of their many advantages such as no need for
prefunctionalization of substrates, step and atom economical,
directive, and environment-friendly features.[1] Functionalization
of C–H bonds α- to the carbonyl group is highly important due
to its application in the synthesis of many natural products and
pharmaceuticals and therefore, many research groups are in-
volved to develop new strategy for this functionalization.[2] The
formation of C–C, C–N, C–O bonds α- to the carbonyl group
were recently reported by different research groups.[2] Forma-
tion of C–C double bonds in organic synthesis is crucial. There
are many classical synthetic methods available to synthesize
them.[3] C–H functionalization of two C(sp3)–H bonds to gener-
ate C–C double bond being the most interesting.[4] Generally,
the C–H functionalization reactions are catalyzed by various
transition metal salts.[5] Of late, organic reactions performed un-
der metal-free conditions have received much attention as they
are inexpensive, less hazardous, and not air-sensitive.[6] Herein,
we report a reaction of arylmethylketones and dimethyl sulfox-
ide in presence of tetrabutylammoniumiodide (TBAI) as catalyst
and potassium persulfate as oxidant under microwave irradia-
tion. The product is a 2-(methylthio)-substituted 1,4-enedione
derivative. The 1,4-enedione core is found in diverse bioactive
natural products including marine natural products, steroids,
Scheme 1. Reaction of arylmethylketones with DMSO.
Results and Discussion
sesquiterpenes, antitumor and antifungal agents.[7] They are As 1,4-enedione shows diverse bioactivity and their previous
also used as synthetic precursor of many important reactions.[8] syntheses have some scope for improvement, we planned to
develop an efficient method for this transformation. Therefore,
we started our study with the reaction of acetophenone and
[a] Department of Applied Sciences, GUIST, Gauhati University,
DMSO in presence of various catalysts and oxidants under mi-
crowave irradiation (Table 1). We used variety of catalysts such
as molecular iodine, tetraethylammonium iodide (TEAI), tetra-
butylammonium bromide/iodide (TBAB/TBAI), KI and N-iodo-
succinimide (NIS). However, TBAI was found the best and gave
maximum yield with potassium persulfate as oxidant (entry 4,
Guwahati-781014, Assam, India
E-mail: mohitdd.deb@gmail.com
baruah.pranjal@gmail.com
[b] Department of Applied Organic Chemistry, CSIR-NEIST,
Jorhat-785006, Assam, India
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejoc.201901415.
Eur. J. Org. Chem. 0000, 0–0
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Communication
Table 1). Also different oxidants were tried but K2S2O8 furnished
the best yield (entry 4 vs. 7–9, Table 1). Testing of catalysts/
oxidants loading revealed that 5 mol-% of TBAI was sufficient
along with 0.5 equiv. of K2S2O8 to produce the highest yield
(entry 4, Table 1) and therefore, we considered it was the opti-
mal condition for the reaction. Increase of temperature did not
increase the product yield (entry 14, Table 1). Under thermal
condition the same reaction produced only 65 % of product
after 3 h of heating at 100 °C (entry 15, Table 1). Under the
optimal condition the diastereoselectivity of the reaction was
found 7:1 for E/Z (entry 4, Table 1).
Table 1. Optimization of the reaction.[a]
Entry
Catalyst
[mol-%]
Oxidant
[equiv.]
Time
[min]
Total yield in
[%] & (E/Z)
1
2
3
4
5
6
7
8
I2 (5)
K2S2O8 (0.5)
K2S2O8 (0.5)
K2S2O8 (0.5)
K2S2O8 (0.5)
K2S2O8 (0.5)
–
TBHP (0.5)
H2O2 (0.5)
Oxone (0.5)
K2S2O8 (0.5)
K2S2O8 (0.5)
K2S2O8 (0.7)
K2S2O8 (0.5)
K2S2O8 (0.5)
K2S2O8 (0.5)
8
8
8
7
7
7
7
7
7
7
7
7
62 (4:1)
59 (5:1)
56 (7:1)
88 (7:1)
NR[b]
NR
NR
NR
NR
38 (5:1)
NR
88 (7:1)
88 (7:1)
85 (7:1)
65 (7:1)
TEAI (5)
TBAB (5)
TBAI (5)
–
TBAI (50)
TBAI (5)
TBAI (5)
TBAI (5)
KI (5)
NIS (5)
TBAI (5)
TBAI (10)
TBAI (5)
TBAI (5)
9
10
11
12
13
14
15
Scheme 2. Substrate scope of the reaction. All the reactions were performed
by using arylmethylketones (2 mmol), DMSO (1 mL), TBAI (5 mol-%, 37 mg)
and K2S2O8 (0.5 equiv., 270 mg) under microwave irradiation. Products were
purified by column chromatography using silica gel (100–200 mesh) and
yields are for the isolated products.
7
7[c]
180[d]
[a] Unless otherwise mentioned all reactions were performed under micro-
wave irradiation by using 1a (2 mmol, 240 mg), 2a (1 mL) at 100 °C. Products
were purified by column chromatography by using silica gel (100–200 mesh)
and yields are for the isolated products. [b] NR: No reaction. [c] Reaction was
carried out at 120 °C. [d] Reaction was performed at thermal condition.
suggests that sulfoxide is crucial for the reaction. We then re-
duced the amount of sulfoxide from 1.0 equiv. to 0.5 equiv. and
0.01 equiv. and found that up to 0.5 equiv. of sulfoxide afforded
the best yield. We also recovered the diphenyl sulfoxide almost
completely after the reaction. We then synthesized few other
unsubstituted 1,4-enedione compounds 4 by changing the
arylmethylketones and obtained very good yield (Scheme 3).
With the optimized condition in hand we then explored the
substrate scope of the reaction. A variety of arylmethylketones
were used for this purpose (Scheme 2). It was observed that
heterocyclic ketones produced better yield with greater E/Z ra-
tio. Electron-donating or -withdrawing groups on the phenyl
ring did not have much effect on the reaction yield. Aliphatic
ketones did not give the reaction at all. In all the cases E, Z
isomers were easily separated due their difference in polarity,
and we determined their ratio by isolating both in pure form.
With the curious mind we then used 1.0 equiv. of diphenyl
sulfoxide (as thioalkyl source) instead of DMSO in the standard
reaction. But we did not get the anticipated product. Instead
it gave exclusively the E-isomer of unsubstituted 1,4-enedione
product in very good yield (Scheme 3). The geometry of the
product was confirmed from single-crystal X-ray analysis (Fig-
ure 1 in supporting information). We thought diphenyl sulfox-
ide has no use in the reaction and therefore, we tried the reac-
tion without adding any thioalkyl source in solvent-free condi-
tion or by using solvents like DMF, toluene, p-xylene. But, inter-
estingly no reaction was observed in each of the cases. This
Scheme 3. Reaction of arylmethylketones with diphenyl sulfoxide.
2 © 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Communication
It was noticed that the E-product of compound 3 (the polar
isomer) is slowly transformed to the Z-product (nonpolar iso-
mer) in a solution when kept in outside. We strongly believed
that the reaction was catalyzed by light and therefore, a freshly
prepared solution of E-isomer was kept in dark condition to
check the extent of conversion. The structure was confirmed by
single crystal structure of 3b (Z) (Figure 1). But no transforma-
tion was observed even after 48 h at room temperature. This
interesting behaviour encouraged us to study it thoroughly un-
der different light sources (Table 2). Recently photocatalyzed
reactions have attracted interest in synthetic organic chemis-
try.[17] Photocatalyzed reactions do not require harsh condi-
tions, radical initiators and stoichiometric oxidants/reductants.
Figure 1. Single crystal structure of 3b-(Z).
The mechanism for the formation of 3 and 4 is proposed
based on some literature reports and our study (Scheme 4).
Both the reactions for the formation of 3/4 proceed through a
radical pathway which is confirmed by performing a reaction in
presence of BHT or TEMPO (1.5 equiv. of each). In each case we
obtained only traces of product. First, iodination occurs at the
α-position of carbonyl group of 1 generating A.[15] Intermediate
B is then obtained from A through Kornblum oxidation involv-
ing DMSO or DPSO. Then B undergoes aldol reaction with an-
other molecule of 1 giving 4. On the other hand, DMSO produ-
ces methanethiol on heating[16] which then reacts with iodine
radical (comes from homolysis of molecular iodine) giving
methylthio radical (MeS·). Molecule 4 then reacts to MeS· and
iodine radical giving D which subsequently affords the product
3 after eliminating a molecule of HI. As DPSO cannot decom-
pose to thiol (due to resonance C-S bond attains partial double
bond character), the reaction stops at the alkene stage and
compound 4 becomes the final product. The formation of
E-isomer as major product in our case is completely opposite
to the previous report[4b] which may be due to the presence of
metal ion (CuII in the previous report) which binds to the sulfur
atom and oxygen atom of one carbonyl group through a six-
membered cyclic transition state resulting the Z-isomer as ma-
jor product. However, in the present report we did not use tran-
sition metal catalyst/oxidant and therefore, no possibility of for-
mation of such transition state. Consequently, E-isomer is the
major product.
Table 2. Optimization for the 3-(E) to 3-(Z) transformation.[a]
Entry
Light source
Catalyst
Solvent
Time
[h]
Yield
[%]
1
2
3
4
5
6
7
8
Sun light
Sun light
Sun light
Sun light
–
Toluene
Toluene
36
12
12
12
12
12
8
8
8
8
8
30
40
35
33
52
50
65
76
82
87
85
67
60
54
Eosin Y
Rose bengal Toluene
Eosin B
Eosin Y
Eosin Y
Eosin Y
Eosin Y
Eosin Y
Eosin Y
Eosin Y
Eosin Y
Eosin Y
Eosin Y
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
CH3CN
DCM
Blue LED (8 W)
Green LED (8 W)
CFL (8 W)
CFL (15 W)
CFL (18 W)
CFL (20 W)
CFL (25 W)
CFL (20 W)
CFL (20 W)
CFL (20 W)
9
10
11
12
13
14
8
8
8
EtOH
[a] All the reactions were performed by using 3a-(E) (0.5 mmol, 141 mg) at
room temperature under different light sources. 5 mol-% of all the catalysts
were used. Products were purified by column chromatography using silica
gel (100–200 mesh) and yields are for the isolated products.
Table 3. Substrate scope of 3-E/Z transformation.[a]
Entry
Substrates
Products
Yield [%]
1
2
3
4
5
6
7
8
3b (E)
3d (E)
3e (E)
3g (E)
3i (E)
3k (E)
3m (E)
3n (E)
3o (E)
3p (E)
3q (E)
3r (E)
3s (E)
3b (Z)
3d (Z)
3e (Z)
3g (Z)
3i (Z)
3k (Z)
3m (Z)
3n (Z)
3o (Z)
3p (Z)
3q (Z)
3r (Z)
3s (Z)
85
88
84
91
87
80
82
89
91
80
85
85
78
9
10
11
12
13
[a] All the reactions were performed by using 3(E) (0.5 mmol) in presence of
eosin Y (5 mol-%, 16 mg) in toluene (2 mL) at room temperature under CFL
(20 W) light source for 8 h. Products were purified by column chromatogra-
phy using silica gel (100–200 mesh) and yields are for the isolated products.
Scheme 4. Tentative mechanism for the reaction.
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Communication
the solvent under reduced pressure and purified the crude product
by column chromatography (silica gel, 100–200 mesh; ethyl acetate/
hexane as eluent) to obtain the desired product 4a.
We found that eosin Y as photoredox catalyst (5 mol-%) un-
der the irradiation of white CFL (20 W) in toluene at room tem-
perature gave the maximum yield (entry 10, Table 2). Other
solvents like acetonitrile, DCM, ethanol did not afford good re-
sult. We then screened few other E-substrates for the E/Z trans-
Representative Procedure for the Conversion of 3a-(E) to 3a-(Z)-
Isomer: To a 10 mL round-bottomed flask equipped with a mag-
formation and noted the results in Table 3. We observed that netic stir bar were added 3a-(E) (0.5 mmol, 141 mg), eosin Y (5 mol-
%, 16 mg) and toluene (2.0 mL). After this, the reaction mixture was
stirred in open air under the irradiation of 20 W CFL lamp (kept at
a distance of ≈ 8 cm from the flask) at room temperature for 8 h.
After the completion of the reaction (monitored by TLC) the solvent
was removed under reduced pressure. The crude product was puri-
fied by column chromatography using silica gel (100–200 mesh;
ethyl acetate/hexane as eluent) to obtain 3a-(Z) as pure product.
almost all E-isomers gave very good yield of Z-products.
To study the mechanistic aspect of photo-isomerisation, we
performed the reaction in presence of 2 equiv. of TEMPO. Pres-
ence of radical scavenger hampers the formation of Z-product,
which confirms that the conversion proceeds through radical
formation. Based on the literature reports,[18] we believe that
first eosin Y is excited by absorbing white light. It then interacts
with the E-isomer producing the biradical 3X, which subse-
quently transformed to Z-product (Scheme 5).
CCDC 1962788 [for 3b-(Z)] contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre.
Acknowledgments
MLD is thankful to the Science and Engineering Research Board
(SERB), India, (Grant No. SB/FT/CS-073/2014) for the financial
support. BD acknowledges MHRD, Govt of India, for research
fellowship under the TEQIP-III Project. We acknowledge the So-
phisticated Analytical Instrumentation Facility (SAIF), GU, for the
use of single-crystal X-ray diffractometer. The authors acknowl-
edge Dr Ranjit Thakuria, Dept. of Chemistry, Gauhati University
for the X-ray structure analysis.
Scheme 5. Tentative mechanism for the photo rearrangement.
Conclusions
Keywords: Arylmethylketones · C–H functionalization ·
Diastereoselectivity · 1,4-Enediones · Photocatalysis
In summary, we have developed an efficient metal-free route
to synthesize 1,4-enedione derivatives under microwave irradia-
tion from easily available arylmethylketones. The reactions com-
plete within very short time in comparison to earlier reported
methods. All the reagents and catalysts are cheap and environ-
mentally benign. Furthermore, the E-isomers of the products 3
can be easily transformed to Z-isomers by metal-free visible
light photoredox catalysis.
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Experimental Section
Representative Procedure for the Synthesis of 3a: Acetophenone
(2 mmol, 240 mg), DMSO (1 mL), TBAI (5 mol-%, 37 mg) and K2S2O8
(0.5 equiv., 1 mmol, 270 mg) were taken in a 10 mL microwave
reaction vessel. Sealed with a Teflon septum, the mixture was
heated under microwave at 100 °C for 7 minutes. After completion,
the reaction mixture was cooled to room temperature and poured
into water (10 mL). Then extracted with cold ethyl acetate (2 ×
10 mL) and dried with anhydrous Na2SO4. Removed the solvent
under reduced pressure and purified the crude product by column
chromatography (silica gel, 100–200 mesh; ethyl acetate/hexane as
eluent) to obtain the desired product 3a.
Representative Procedure for the Synthesis of 4a: Acetophenone
(2 mmol, 240 mg), Diphenyl sulfoxide (1 mmol, 202 mg), TBAI
(5 mol-%, 37 mg) and K2S2O8 (1 mmol, 270 mg) were taken in a
10 mL microwave reaction vessel. Sealed with a Teflon septum, the
mixture was heated under microwave at 100 °C for 10 minutes.
After completion, the reaction mixture was cooled to room temper-
ature and poured into water (10 mL). Then extracted with cold ethyl
acetate (2 × 10 mL) and dried with anhydrous Na2SO4. Removed
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Received: September 25, 2019
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
Photocatalysis
G. K. Rastogi, B. Deka, M. L. Deb,*
P. K. Baruah* ....................................... 1–6
Diastereoselective sp3-C–H Func-
tionalization of Arylmethyl Ketones
and Transformation of E- to Z-Prod-
ucts Through Photocatalysis
A Metal-free synthetic route for 1,4- sulfoxide was developed. E-isomers of
enedione derivatives under microwave the products can be transformed into
irradiation from the reaction of aryl- Z-isomers through visible light pro-
methylketones with DMSO or diphenyl moted photocatalysis.
DOI: 10.1002/ejoc.201901415
Eur. J. Org. Chem. 0000, 0–0
www.eurjoc.org
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim