Photocatalytic C(sp3)?H Activation towards α-methylenation of Ketones using MeOH as 1 C Source Steering Reagent

Article abstract of DOI:10.1002/adsc.201801431

Unprecedented direct access to terminal enones via α-methylenation of aryl ketones to form C=C bond is achieved under visible-light conditions using methanol as one carbon source substrate and solvent as well. The reaction involves Cu@g-C₃N₄-catalysed in situ oxidation of methanol into formaldehyde followed by dehydrative cross aldol type reaction. Various aryl ketones react efficiently with MeOH, producing α,β-unsaturated carbonyl compounds only in 4–8 h at room temperature in excellent yield (84–97%). Operational simplicity, wide substrate scope, ambient reaction conditions, visible-light photocatalysis and novel application of MeOH as methylene donor substrate are the salient features making the envisaged protocol mild, efficient and green alternative to the existing methods for synthesis of such fine chemicals. (Figure presented.).

Full text of DOI:10.1002/adsc.201801431

Title: Photocatalytic C(sp3)–H activation towards α-methylenation of
ketones using MeOH as 1C source steering reagent
Authors: Fooleswar Verma, Prashant Shukla, Smita R. Bhardiya,
Manorama Singh, Ankita Rai, and Vijai K. Rai
This manuscript has been accepted after peer review and appears as an
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content of this Accepted Article.
To be cited as: Adv. Synth. Catal. 10.1002/adsc.201801431
Link to VoR: http://dx.doi.org/10.1002/adsc.201801431

10.1002/adsc.201801431
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: 00.0000/0000.000 ((will be filled in by the editorial staff))
Photocatalytic C(sp3)–H activation towards α-methylenation of
ketones using MeOH as 1C source steering reagent
Fooleswar Verma,[a] Prashant Shukla,[b] Smita R. Bhardiya, [a] Manorama Singh, [a]
Ankita Rai, [b] Vijai K. Rai[a]*
a
Department of Chemistry, Guru Ghasidas Vishwavidyalaya (Central University), Bilaspur (C.G.)-495009, INDIA
[Email: vijaikrai@hotmail.com; Phone.: +91-75-87178627]
School of Physical Sciences, Jawaharlal Nehru University, New Delhi, 110067, INDIA
b
Received: ((will be filled in by the editorial staff))
paraformaldehyde[13a] (Scheme 1b) as one-carbon
Abstract. Unprecedented direct access to terminal
enones via α-methylenation of aryl ketones to form C=C
bond is achieved under visible-light conditions using
methanol as one carbon source substrate and solvent as
well. The reaction involves Cu@g-C3N4-catalysed in
situ oxidation of methanol into formaldehyde followed
by dehydrative cross aldol type reaction. Various aryl
ketones react efficiently with MeOH, producing α,β-
unsaturated carbonyl compounds only in 4-8 h at room
temperature in excellent yield (84-97%). Operational
simplicity, wide substrate scope, ambient reaction
conditions, visible-light photocatalysis and novel
application of MeOH as methylene donor substrate are
the salient features making the envisaged protocol mild,
efficient and green alternative to the existing methods
for synthesis of such fine chemicals.
source through Eschenmoser’s salt promoted
Mannich-type reaction followed by elimination.[14,15]
Thus, in spite of being highly reactive,[12,13] toxicity of
HCHO has been the main limitation and led to search
for new substrate scope for α-methylenation of
ketones. Accordingly, literature reports DMA,[16]
DMF,[17] and DMSO[18] as alternative one carbon
donating
substrates
for
oxidative
α-C-H-
methylenation (Scheme 1c-e). However, poor
reactivity of these alternatives 1C-source[15-18] towards
α-methylenation essentially demand oxidising agents
and forcing refluxing temperature with slow
conversion rate.[15-18] Thus, search for efficient and
mild one-carbon source for improved α-methylenation
of ketones is still crucial and highly demanding target
in organic synthesis.
Keywords: Visible-light; Terminal enone; Ketone; 1 C
source; g-C3N4.
To fill this literature gap for α-methylenation of
ketones, we were fascinated to exploit the
unparalleled high electrophilicity of HCHO for cross-
aldol type reaction with ketones having α-hydrogen,
but its toxicity and environmental impact was our
Visible light-induced photoredox catalysis has
emerged as state-of-the-art alternative in modern
organic synthesis for activation of organic
molecules.[1,2] To design a photocatalyst with high
selectivity and efficiency for accelerating the visible
light-mediated organic transformations would
advance the goal. The polymeric graphitic C3N4 (g-
C3N4) is one of the most charming materials[3] and is
of particular interest as visible light photocatalyst,[4]
although it has some limitations due to rapid charge
recombination.[5] To overcome this problem, several
strategies have been developed including doping of
[5-9]
metal/non-metal in the cavity of g-C3N4
as metal-
doped g-C3N4, such as Cu/Co/Ni@g-C3N4 shows
extremely high catalytic activity for organic reactions
along with its reusability.[10]
Direct functionalization of Csp3-H bond has received
a great attention as it offers more efficient step and
atom economical approach for synthesising complex
organic framework.[11] Recently, synthesis of various
α,β-unstaurated carbonyl compounds has been
reported via installation of terminal alkenes to
aldehydes and ketones.[12-15] The strategy involves α-
methylenation of carbonyl compounds with
formaldehyde[12]
(Scheme
1a)
and
Scheme 1. Literature reports on α- methylenation.
1
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10.1002/adsc.201801431
Advanced Synthesis & Catalysis
main concern. Alternatively, we anticipated to utilize
methanol as 1C-source for in situ generation and
utilization of HCHO for α-Csp3-H methylenation. In
continuation
of
our
interest
in
organic
transformations,[19] we report herein, the first visible
light-induced α-methylenation of ketones at room
temperature using MeOH as 1-C source and Cu@g-
C3N4 as new and efficient photocatalyst without use
of any oxidant, which is hitherto unreported and not
accessible through any classical or modern approach
reported till date for α-methylenation of ketones
(Scheme 1f).
To realize our idea, acetophenone 1a and
methanol 2 was taken as model substrate to probe the
feasibility of novel catalytic α-methylenation under
visible light. The reaction mixture was stirred under
the visible light at room temperature, and the results
are summarized in Table 1. When, a mixture of
acetophenone 1a (1.0 mmol), K2CO3 (1 mmol), PPh3
(0.1 mmol) and MeOH (5 mL) was stirred for 8 h in
the presence of g-C3N4 (20 mg), and we were happy
to isolate the desired product 2a albeit in very low
yield (Table 1, entry 1). Then, we turned our attention
to use metal-doped @g-C3N4 as catalyst, and our
strategy worked well as the product 2a was isolated in
6 h using Ni@g-C3N4 (15 wt%, 20 mg) with improved
yield of 2a (Table 1, entry 2). After screening
diff erent catalysts, Cu@g-C3N4 (15 wt%, 20 mg) not
only improved the yield up to 97% but it increased
the conversion rate also and reaction got completed
Scheme 2. Control experiment for methylenation of ketone
1a.
only in 4h (Table 1, entries 2-5). The reason for
increasing efficiency of the reaction by doping Cu
may presumably be due to the presence of vacant d-
orbital, which polarizes carbonyl oxygen of
formaldehyde for increasing its reactivity. Next, a
variety of bases were screened (Table 1, entries 5-13)
and K2CO3 (1.0 mmol) was found the most efficient
base in presence of PPh3 (0.1 mmol) as additive for
present transformation showing preeminent results
with 97% yield in 4 h (Table 1, entry 5). The amount
of catalyst was found 20 mg optimum; decreasing the
catalyst loading from 20 to 15 mg, yield of the
product 2a was also decreased (Table 1, entries 5 and
14), while there was no change in yield on increasing
the catalytic amount (Table 1, entries 5 and 15).
However, in the absence of either catalyst or additive
or base, we could not isolate the any product 2a
(Table 1, entries 16-18). Also, we performed the
reaction under inert atmosphere. However, we did not
isolate any product which confirms that oxygen from
air is essential as oxidant for oxidation of MeOH into
HCHO. Thus, the optimized condition for the present
reaction was concluded as: Cu@g-C3N4 (20 mg),
PPh3 (0.1 mmol), K2CO3 (1 mmol) with MeOH (5
mL) in presence of LED light at room temperature,
and this outcome was applied as preeminent condition
for construction of product 2a.
Furthermore, six sets of controlled experiments
were performed to observe the catalytic effect of
visible light in the present investigation by taking 1a
(1.0 mmol), Cu@g-C3N4 (20 mg), PPh3 (0.1 mmol)
and K2CO3 (1 mmol) (Scheme 2a-f). First experiment
was performed using HCHO as 1-C source and water
as solvent in presence of visible light and we isolated
the desired product 2a in 6 h with satisfactory yield
(Scheme 2a). Next, three separate experiments were
run in without using visible light taking methanol as
one carbon source as well as solvent, (i) in the dark
condition (Scheme 2b), (ii) at room temperature
(Scheme 2c) and at 80 ⁰ C temperature (Scheme 2d).
However, yield of the product 2a was lower in dark
condition (35%) and good (73%) at room temperature
as well as no change in yield at 80 ⁰ C temperature.
Table 1. Optimization of reaction conditions for synthesis of
terminal enone 2a.[a]
Entry Catalyst (mg)
Base
Time (h)
08
Yield (%)[b, c]
1
@g-C3N4 (20)
K2CO3
K2CO3
42
69
88
81
97
86
78
69
79
76
70
51
55
92
97
-
2
3
Ni@g-C3N4 (20)
Co@g-C3N4 (20) K2CO3
Fe@g-C3N4 (20) K2CO3
06
06
4
06
5
Cu@g-C3N4 (20) K2CO3
Cu@g-C3N4 (20) Cs2CO3
Cu@g-C3N4 (20) CsOH
04
6
04
7
04
8
Cu@g-C3N4 (20) Cs2OAc 04
9
Cu@g-C3N4 (20) Na2CO3
Cu@g-C3N4 (20) Li2CO3
Cu@g-C3N4 (20) KOH
Cu@g-C3N4 (20) K3PO4
Cu@g-C3N4 (20) KOtBu
Cu@g-C3N4 (15) K2CO3
Cu@g-C3N4 (25) K2CO3
Cu@g-C3N4 (20) K2CO3
04
04
04
04
04
04
04
04
04
04
10
11
12
13
14
15
16[d]
17
18
Cu@g-C3N4 (20)
-
-
-
-
K2CO3
[a]Reaction conditions: Acetophenone 1a (1 mmol), MeOH (5 mL), PPh3
(0.1 mmol), base (1 mmol), 7-volt LED light, r.t., in air. [b]Yield of For the confirmation of radical formation, we have
isolated and purified product. [c]All compounds gave C, H and N analyses
checked a set of experiment with 1 eq. of TEMPO (a
stable free radical) and we observed that the very
within ± 0.35%, and satisfactory spectral (1H NMR, 13C NMR and EIMS)
data. [d]Without PPh3.
2
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10.1002/adsc.201801431
Advanced Synthesis & Catalysis
experiment (Table 1, entry 5) concluded that visible
light not only improves the yield of product but also it
increases the conversion rate showing co-operative
effect with catalyst in the present reaction due to the
higher triplet energy of acetophenone.[20]
With these optimized reaction conditions in hand,
a variety of aromatic (1a-t) as well as heteroaromatic
(1u-x) ketones were screened and the corresponding
terminal enone 2a-x were isolated in good to excellent
yield (Scheme 3). However, there was no specific
effect of substituents on phenyl/heterocyclic ring was
noticed and ketone 1 tolerated both electron
withdrawing (F, Cl, Br, NO2, CN and CO2Me) as well
as electron releasing (Me, OMe and OEt) groups to
afford the corresponding enone 2 within 4-8 h under
mild reaction conditions (Scheme 3). In case of
aliphatic ketones, taking acetone as substrate, we did
not isolate the corresponding product, presumably due
to high volatility of acetone as the reaction was not
performed in closed vessel instead in presence of light
in open flask. But, using another aliphatic ketone, for
example, heptanone, which is relatively less volatile
(boiling point 151 ⁰ C), corresponding enone was
isolated albeit in low yield (31%) and hence we did
not continue using other aliphatic ketones in the
envisaged method. All the synthesized compounds
were characterized by their 1H and 13C NMR spectra.
The formation of enone 2 from ketone 1 involves
in situ conversion of MeOH into HCHO via initial
formation of MeOK using K2CO3 followed by
PPh3/Cu@g-C3N4.[21] Catalysts such as Ir, Rh and Co
having vacant d-orbital in combination with
phosphine are reported to catalyze the oxidation of
MeOH into HCHO through abstraction of Csp3-H
proton of MeOH.[21,22] Optimization experiment
reveals that neither Cu@gC3N4 nor PPh3 alone
oxidizes the MeOH (Table 1, entry 16 and 18), which
further concludes that PPh3/Cu@g-C3N4 binds with
oxygen of MeOK and thus facilitating the oxidation
of methanol (Scheme 4a). Tentative mechanism
presumably proceeds through Cu@g-C3N4 induced
Scheme 3. Substrate scope for the formation of terminal
enones 2.
small amount (37%) of desired product 2a obtained
(Scheme 2e). We also used the deuterated methanol
and observed that the deuterated phenyl vinyl ketone
(93%) obtained (Scheme 2f). Finally, model
photocatalytic excitation of enolate
4
into
corresponding radical 5, which further assist the
carbon-carbon bond formation with electrophilic
formaldehyde 3 producing corresponding alcohol 6,
which finally underwent K2CO3-induced dehydration
to afford the product 2a (Scheme 4). The reaction
mechanism in the absence of light may be explained
by simple base-catalyzed aldol reaction. The reaction
presumably involves base-catalyzed activation of
acetophenone to react with in situ generated HCHO,
followed by dehydration of the aldol product.
To check the reusability of catalyst, five
sequential experiments were run (Fig. 1) using
acetophenone 1 (1.0 mmol), PPh3 (0.1 mmol), K2CO3
(1.0 mmol) in MeOH (5 mL), with Cu@g-C3N4 (20
mg) under visible light condition at room temperature.
After completion of each reaction, Cu@g-C3N4 was
recovered using centrifuge, washed with water and
reused for the next reaction of fresh batch of
acetophenone. The Cu@g-C3N4 catalyst could be
recycled and reused five times with good yield
Scheme 4. Tentative mechanism for formation of enone 2.
3
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10.1002/adsc.201801431
Advanced Synthesis & Catalysis
spectroscopy
(EDX),
transmission
electron
microscopy (TEM) and X-ray photoelectron
spectroscopy (XPS).
Characterization of Cu@g-C3N4
SEM images exposed the two-dimensional
arrangement and fine agglomeration in g-C3N4 and
Cu@g-C3N4 (Fig. 2a and b).[23c] The Carbon and
nitrogen in g-C3N4 (Fig. 4a) and carbon, nitrogen and
a small amount of Cu metal in Cu@g-C3N4 were
studied by EDX spectra (Fig. 4b). The X-ray
diffraction (XRD) pattern has shown the graphitic-
like phase of C3N4 in g-C3N4 and Cu@g-C3N4. The
strong peaks at 27.6° attributed to the typical
characteristic (002) interlayer stacking peak and the
weak diffraction (100) peak at 13.2° attributed to the
in-plane repetitive units in the g-C3N4 (JCPDS No.
85-1326) and confirmed the dissimilarity in g-C3N4
and Cu@g-C3N4 (Fig. 4c).[24] A transmission electron
microscopy (TEM) image of g-C3N4 and Cu@g-C3N4
has also confirmed the graphitic- like structure and
copper in Cu@g-C3N4 and it also supported the XRD
analysis very well (Fig. 2c and d).[23c] Blue
luminescence was observed in photoluminescence
spectra of g-C3N4 and Cu@g-C3N4 because of
transition among lone pair (LP) states in the valence
band and the π* antibonding states in the conduction
band. The PL spectrum of g-C3N4 and Cu@g-C3N4
(Fig. 4d) shows strong blue luminescence at ∼465 nm
and ∼460 nm, respectively. The states consisting of
the sp3 C−N σ band, the sp2 C−N π band, and the LP
state of the bridge nitride atom are accountable for the
luminescence from g-C3N4. The absorption in the
visible region for g-C3N4 helps in investigation of its
visible light photocatalysis. The Fourier transform
infrared (FTIR) spectrum exhibits evidently exposed
broad bands in the 3000−3500 cm−1 region that could
be recognized for O−H and N−H stretching. The
absorption in the range of 1200−1700 cm−1 is
consigned to the characteristic stretching modes of g-
C3N4 heterocycles. At ∼806 cm−1 absorption band is
Fig. 1. Recyclability of Cu@g-C3N4 for the synthesis of 1-
Phenylprop-2-en-1-one 2a.
without any significant loss of its activity (Fig. 1).
Furthermore, SEM (ESI, Fig. 2, S2) and XRD pattern
(ESI, Fig. 3, S3) confirms that there was no
substantial change in morphology and structure of
catalyst after reused 5 times in consecutive reactions.
Thus, we disclose an original visible light induced
mild and efficient method for methylenation of
C(sp3)-H in aryl alkyl ketones using MeOH with
Cu@g-C3N4 catalyst under ambient reaction
conditions. Operational simplicity, wide substrate
scope, excellent yield (84-97%) of enones, ambient
reaction conditions and visible-light photocatalysis
are the salient features of the envisaged protocols.
Application of MeOH as new one carbon donor
substrate for methylenation process as well as solvent
for the reaction makes the envisaged method
advantageous than earlier reported method for
synthesis of such fine chemicals and would be the
best alternative to be further exploited for academia
and industries.
Experimental:
Preparation of Cu@g-C3N4:
For preparation of photocatalyst, urea was
calcinated at 500 °C temperature under air free
condition in muffle furnace for 2 h, to get converted
as yellowish graphitic carbon nitride (g-C3N4) and
then it was used as support for Cu-metal
encapsulation.[23] The solution (sol) of g-C3N4 (Sol A,
425 mg) and Cu(OAc)2.H2O (Sol B, 238 mg, 15 wt%
of g-C3N4, Cu 75 mg) was prepared in aqueous
methanol (50%) individually through sonication.
Then, Sol B was added in Sol A and stirred for 6 h at
room temperature (rt). Prepared nanoparticle was
centrifuged and washed with water and methanol and
o
dried under vacuum at 50 C. The prepared Cu@g-
C3N4 catalyst was then characterized by FTIR, X-ray
diffraction (XRD), photoluminescence (PL), scanning
electron microscopy (SEM), energy-dispersive X-ray
Fig. 2. (a) SEM image of g-C3N4, (b) SEM image of Cu@g-
C3N4, (c) TEM image of g-C3N4 and (d) TEM image of
Cu@g-C3N4.
4
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10.1002/adsc.201801431
Advanced Synthesis & Catalysis
shown can be deconvoluted into four peaks. The
peaks cantered at 398.1 and 398.7 eV are derived
from the sp2-hybridized nitrogen atoms in -C=N-C
groups and the amino function groups, respectively.
The peak at around 400.2 eV was attributed to the
charging effects or positive charge localization in
heterocycles, while that at 399.4 eV is attributed to
the tertiary nitrogen in N-C3 groups or in H-N-C2
(Fig. 3d).[23c]
General method for the synthesis of enones 2:
A flame-dried round bottom flask was charged with a
mixture of acetophenone 1 (1.0 mmol), K2CO3 (1.0
mmol), PPh3 (0.1 mmol), Cu@g-C3N4 (20 mg) in
MeOH (5 mL) and stirred under the visible light
condition at room temperature for 4-8 h. After
completion of the reaction (as monitored by TLC),
water was added to the reaction mixture and the
catalyst was filtered and added EtOAc. Then, the
organic layer was washed with brine (2 × 10 mL) and
water (2 × 10 mL), dried under vacuum evaporator to
obtain the crude isolates, which were further purified
by flash chromatography, EtOAc/n-hexane to obtain
the analytically pure sample of product 2.
Characterization data of all the synthesized
compounds are given in ESI.
Fig. 3. XPS spectra of Cu@g-C3N4; (a) Scan survey, (b) Cu
2p, (c) C 1s and (d) N 1s data.
considered to be the out-of-plane skeletal bending
modes of triazine and the band at ∼1640 cm−1 is
recognized to the C=N stretching vibration mode (ESI,
Fig. 1, S2).[25]
Moreover, the X-ray photoelectron spectroscopy
(XPS) measurements were performed to observe the
chemical composition and chemical position of the
constituent elements in the Cu@g-C3N4 sample (Fig.
3). The XPS survey spectra depicts that C, N, and Cu
elements were detected on the Cu@g-C3N4 sample
(Fig. 3a), indicating the loading of Cu on g-C3N4. The
peaks of Cu 2p3/2 and Cu 2p1/2 were located at 932.8
and 946.5 eV, respectively, along with the presence of
the two strong satellite peaks of copper at 938.3 and
962.3 eV, indicating the presence of Cu(II) in Cu@g-
C3N4, without the formation of any other oxidation
state (Fig. 3b). The C 1s spectra peaks at 285.7 and
287.1 eV were assigned to the C=N coordination and
sp2-hybridized carbon in N-containing aromatic rings
(-N-C=N), respectively (Fig. 3c). The N 1s spectra
Acknowledgements
We sincerely thank the SERB, New Delhi, India (File No.
SR/FT/CS-99/2010) for partial financial support. We are also
thankful to AIRF, JNU, New Delhi, NIT Raipur and IIT Kanpur
for providing microanalyses and spectra.
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10.1002/adsc.201801431
Advanced Synthesis & Catalysis
COMMUNICATION
Photocatalytic C(sp3)–H activation towards
α-methylenation of ketones using MeOH as
1C source steering reagent
Adv. Syn. Catal. 2018, Volume, Page–Page
Fooleswar Verma, Prashant Shukla, Smita R.
Bhardiya, Manorama Singh, Ankita Rai, Vijai
K. Rai
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