Efficient and recyclable solid acid-catalyzed alkylation of active methylene compound via oxonium intermediate for atom economical synthesis of organic compounds

Article abstract of DOI:10.1007/s11164-021-04499-3

In the present work, we report the catalytic reaction of active methylene compounds with cyclic enol ethers and aryl acetals through oxonium intermediate under solvent-free conditions using heterogeneous solid acid catalysts. Among studied solid acid catalysts, Amberlyst-15 gave excellent yields (35–85%) of alkylated products. The catalyst showed broader substrate scope, and a recyclable catalytic cost-efficient approach of the alkylation was examined on the different types of cyclic enol ethers and aryl acetal.

Full text of DOI:10.1007/s11164-021-04499-3

Research on Chemical Intermediates
https://doi.org/10.1007/s11164-021-04499-3
Efcient and recyclable solid acid‑catalyzed alkylation
of active methylene compound via oxonium intermediate
for atom economical synthesis of organic compounds
Dhanaji R. Naikwadi1,2 · Balasaheb D. Bankar1,2 · Krishnan Ravi1,2
Ankush V. Biradar1,2
·
Received: 17 February 2021 / Accepted: 18 May 2021
© The Author(s), under exclusive licence to Springer Nature B.V. 2021
Abstract
In the present work, we report the catalytic reaction of active methylene compounds
with cyclic enol ethers and aryl acetals through oxonium intermediate under sol-
vent-free conditions using heterogeneous solid acid catalysts. Among studied solid
acid catalysts, Amberlyst-15 gave excellent yields (35–85%) of alkylated products.
The catalyst showed broader substrate scope, and a recyclable catalytic cost-efcient
approach of the alkylation was examined on the diferent types of cyclic enol ethers
and aryl acetal.
Keywords Alkylation · Solid acid · Cyclic enol ether · Acetal · Active methylene
compound
Introduction
The “atom-economy” is exceptionally important in manufacturing organic com-
pounds (C–C bond) adopting green synthetic routes. In this context, the develop-
ment of efcient catalytic methodologies with the least consumption of energy,
time, and better waste management is desirable. [1–3] However, there are many
challenges to synthesize these chemicals at an industrial scale due to stringent envi-
ronmental rules. Notably, for the synthesis of value-added chemicals, alkylation of
active methylene compounds (AMCs) with olefns, benzyl halide, and condensa-
tion with carbonyl compound is regarded as one best way to construct C–C single
and double bonds [4, 5]. Though primarily for the activation of AMCs with various
*
Ankush V. Biradar
ankush@csmcri.res.in
1
2
Inorganic Materials and Catalysis Division, CSIR-Central Salt and Marine Chemicals Research
Institute, Bhavnagar, Gujarat 364 002, India
Academy of Scientifc and Innovative Research (AcSIR), Ghaziabad, Uttar Pradesh 201002,
India
Vol.:(0123456789)
1 3

D. R. Naikwadi et al.
intermediates and unsaturated molecules like alkenes, [6] alkynes, [7–9], and 1,
3-dienes [8–10], diferent transition metal-based catalysts were carried out, remark-
ably, very few reports are available on the reaction of AMCs and various cyclic enol
ethers (CEE) and acetal, excludes Knoevenagel condensation, alkylation on allylic
alcohol/halide, and benzylic alcohols for the construct of a C–C bond [11–17]. For
instance, Bihovsky et al. used strong acid (trifuoroacetic acid) for the alkylation
of CEE for condensing with the dicarbonyl compound through oxonium ion [18],
whereas Nguyen et al. reported alkylation of AMC on CEE using AuCl3–AgOTf
catalysts [19]. In another report, Li et al. carried out a similar alkylation using cop-
per (II) trifate as the catalyst in an ionic liquid solvent [20]. Motokura et al. used
HY zeolite-supported Au gold clusters (Au (0) HY) for the addition of acetylacetone
(AcAc) to the alkene. In the more recent year, iminoiodane and Brønsted base-medi-
ated coupling of CEE with 1, 3-dicarbonyl compounds was reported [22].
Moreover, Davies et al. recently reported the acetalbenzaldehyde alkylation
through oxonium intermediate by using a telescopically synthesized nanoporous
aluminosilicate heterogeneous catalyst for direct aldol condensation reactions to the
construction of a C–C and C=C bond formation [21]. However, the applications of
these aldol adducts and Knoevenagel have been reported in previous literature as
valuable intermediates and used in the total synthesis of natural products [36]. Nev-
ertheless, these reported methods use expensive metal salts, bases, and additives;
also it is non-selective and generates enormous waste. To overcome these draw-
backs, we have adopted a sustainable approach using a recyclable heterogeneous
solid acid catalyst like Amberlyst-15 (Ab-15) under solvent-free conditions [23, 24].
However, this heterogeneous catalyst is widely used in the sustainable transforma-
tion of organic reactions. Ab-15 was found as the best heterogeneous catalyst for
manufacturing organic compounds due to its selectivity and reusability. [23] Hence,
the combination of such industrially graded catalysts and AMCs to their products is
an important class of substances in organic and pharmaceutical. [16] Consequently,
AMCs are highly acidic and can be deprotonated easily under acidic or basic con-
ditions because AMC acts as a multi coupling reagent with both electrophilic and
nucleophilic sites. [25–39] In Scheme 1, diferent types of AMC compounds are
highly used in various pharmaceutical, fragrance, favor, and fragrance industries to
produce food supplements [16]. Herein, we reported a simple and greener protocol
for alkylation of AMCs with CEE and aryl acetals using a solid acid catalyst. The
Scheme 1 Various active methylene compounds
1 3

Efcient and recyclable solid acid‑catalyzed alkylation…
Scheme 2 The model reaction of alkylation AMCs on CEE
Scheme 3 The alkylation reaction of AMCs with the CEE
reported approach of alkylation on aryl acetals to aldol and Knoevenagel product
will be benefcial for synthesizing C–C and C=C double bonds in one-pot Scheme 2.
Results and discussion
We began our investigation for C–C bond formation through the alkylation of vari-
ous types of AMC with CEE using inexpensive and eco-friendly solid acid catalysts.
In a typical reaction protocol, electron-rich oxygen-containing furan and pyran com-
pounds were used for alkylation with various AMCs. CEE is fascinating through
oxonium intermediate and the results are detailed below. However, the oxonium ion
was produced by protonating using the Brønsted solid acid catalyst. Furthermore,
oxonium ion is stabilized by AMCs and produces the C–C bond between AMC and
CEE. Owing to the acidity of the catalyst, the regioselective functionalization of
AMCs generally occurs at the C2 position of CEE. In addition to this, the alkylated
CEE and acetal benzaldehyde on acetoacetic ester possess diastereoisomer and geo-
metrical isomerism. In our attempt to study the process of major products, we sepa-
rately reported in both parts with stable isomer Scheme 3.
Brønsted acid‑catalyzed alkylation of active methylene compound on CEE
via oxonium ion.
Initially, our investigation began with to get the desired product by performing the
reaction of 3, 4-dihydro-pyran with AcAc as a model substrate. It is a known fact that
the AMCs possess the acidic proton, and hence to explore its reactivity, the reaction
1 3

D. R. Naikwadi et al.
Table 1 Result of catalyst
variation for alkylation of AMC
on CEE.a
Entry
Catalyst
Conv. (%)
Isolated
Yield
(%)
1
2
3
4
5
6
Blank
p-Toluenesulfonic acid
Indion-190
Amberlite-IR-120
Ab-15
Amberlyst-15(powder)
5
Nd
65
45
59
78
80
99
50
69
99
>99
aReaction conditions: Substrate: 3,4-dihydro-pyran (4.5 mmol),
AcAc (4.5 mmol), Temp. 80 °C, time: 4 h, catalyst: 80 mg, Nd=not
detected, (Amberlyst-15powder-Ab-15p)
was carried out without using any catalyst, and the conversion of starting materials
was negligible (5%) (Table 1, entry 1). Then, the reaction was tested using diferent
(-SO3H functionalized) homogeneous and heterogeneous Brønsted acidic catalysts to
fnd out the best catalyst. In the presence of homogeneous Brønsted acid, like para-tol-
uene sulfonic acid (PTSA) as a catalyst, the reaction gave 99% conversion with a 65%
yield of the desired product (Table 1, entry 2). However, PTSA being a homogeneous
catalyst, it could not be recovered and reused. Then, we tested diferent heterogeneous
Brønsted acids (Indion-190, and AM-120) catalysts, which aforded the alkylation of
AcAc with 3, 4-dihydro-pyran showed 50, 69% conversion with 45, 59% yield of the
desired product (Table 1, entries 3 and 4) in 4 h. However, the strong solid acid Ab-15
having SO3H-functionality, ionic form-hydrogen, Max. operating temp. 393 K, parti-
cle size-0.600–0.685 μm, internal porosity-0.36, concentration of acid sites-≥4.70 eq/
kg, surface area-53 m2/g). Ab-15 in actual, sufcient acidic and thermally stable, ear-
lier published reports found excellent catalysts for such reactions like esterifcation,
alkylation, acylation, and condensation used [36–42]. The Ab-15 as catalyst gave the
desired product in excellent yields of (1a) 78% with almost conversion (Table 1, entry
5). Notably, the powder form of Amberlyst-15 (Ab-15p) possesses a strong acidity,
which has given almost complete conversion and (1a) 80% yield (Table 1, entry 6).
After reaction completion, the catalyst was separated by a simple fltration method.
Further investigation was carried out using diferent solvents at their refuxed
temperature (Tables 2). In polar solvents like methanol, it was observed that the
Table 2 Solvent variation of
Entry
Solvent
Yield%
alkylation on CEEa
1
2
3
4
Methanol
Tetrahydrofuran(THF)
Toluene
29
15
5
Sol-free
80
aReaction conditions: Substrate: AcAc (5 mmol), 3, 4-dihy-
dro-2H-pyran (5 mmol), Temp. 80 °C, 80 mg Ab-15, time 4 h.
(Yield=Isolated yield)
1 3

Efcient and recyclable solid acid‑catalyzed alkylation…
Table 3 Substrate scope of alkylation on CEE.a
Entry Substrate
AMC Product
Temp.ºC Time, (h)
Yield (%)
N1
80
4
80
1.
1a
N2
N1
N1
80
80
80
4
60
65
35
2.
1b
4
3.
1c
1d
4b.
12
N6
80
6
60
5.
1e
N7
N5
N1
80
80
50
6
4
8
55
70
74
6.
7.
8.
1f
1g
1h
N3
50
8
70
9.
1i
N4
50
12
50
10.
1j
N2
N6
50
50
8
8
60
62
11.
12.
1k
1l
N7
1m
50
8
60
13.
1 3

D. R. Naikwadi et al.
Table 3 (continued)
aReaction conditions: 2,3-Dihydro-furan (4.5 mmol), AMC (4.5 mmol), Temp.: 80 °C, 80 mg Ab-15,
Catalyst. b 1:2 molar ratio (pyran and AcAc)
methoxy group self-acts as a nucleophile to attack the oxonium ion intermediate and
formed the ether as a by-product. In the polar aprotic solvent, to some extent, satis-
fying results were obtained because of the absence of nucleophilicity, whereas sig-
nifcantly less conversion was observed in the case of non-polar solvent. From these
results, the solvent-free reaction condition is the best route for these alkylation reac-
tions of AMC. After optimum condition, we explored the amount of catalyst; the
less amount of catalyst gave 55% conversion with 98% selectivity toward the desired
products (Table S1 entry 1). However, higher loading of catalyst showed almost
complete conversion and 80% selectivity toward the desired product (Table S1 entry
5). Furthermore, the catalyst was separated by fltration, dried at 80 °C for 4 h and
recycled up to 6 times without losing catalytic activity in both schemes (see SI,
Table S2).
After optimizing the various parameters, we have extended this protocol for
the alkylation of various CEE with diferent AMCs at their refuxed temperature.
First, the 3,4- dihydro-pyran was tested with AcAc to yield (1a) 80%; on the other
hand, a lower product yield of (1b) 60% was observed on changing nucleophiles
methyl acetoacetate (Table 3, entry 2). In the case of 2-substituted dihydro-pyran,
the yield was decreased to 65% (1c) of mono-alkylation (Table 3, entry 3). Fur-
ther increasing the time from 4 to 12 h with the increasing equivalence of AcAc,
the double alkylated product was observed (1d) (Table 3, entry 4). However,
the changing of various AMCs with 2, 3-dihydro-pyran aforded corresponding
desired products in the range of (1e, 1f, and 1 g) 55–70% yields (Table 3, entries
5–7). Moreover, the reaction of cyclic pyran and 2-substituted pyran could also
be performed on a gram-scale with Ab-15 to give in good yields of alkylated
products. To further demonstrate the utility of this transformation on cyclic furan
Table 4 Catalyst
Entry
Catalyst
Conv. (%)
Isolated
variation of alkylation on
Yield. (Prod-
A) (%)
acetalbenzaldehyde. a
1
2
3
4
5
6
7b
Blank
nd
99
40
45
99
>99
99
nd
70
45
55
75
78
p-Toluenesulfonic acid
Indion-190
Amberlite-IR-120
Ab-15
Ab-15p
Ab-15
(Prod-B)78
aReaction conditions: 1:1 equivalence of acetal benzaldehyde
(4.5 mmol) and AcAc (4.5 mmol) with 80 mg, Time 20 min, Temp.
b
80 ˚C. Nd-not detected. acetal benzaldehyde (4.5 mmol) and AcAc
(4.5 mmol) with 80 mg, Time 4 h, Temp. 80 ˚C.(Product A=Aldol
and B=Knoevenagel product)
1 3

Efcient and recyclable solid acid‑catalyzed alkylation…
Scheme 4 Model reaction for the alkylation of AMC with acetal benzaldehyde
with diferent AMCs, however, using AcAc with 2,3-dihydro-furan gives the 74%
yield (1h) of the desired product (Table 3, entry 8), whereas using various AAE
(acetoacetic ester) the yield was slightly decreased (1i, 1j, and 1k) (Table 3,
entries 9–11). Likewise, the method was also applicable on benzylic diketone
(AMCs) functional to convert into the desired product (1l) and (1m) (Table 3,
entries 12 and 13). The present methodology was also applicable to AMCs and
benzylic acetal reactions for desired alkylated products Scheme 3. Notably, the
reaction pathway also works through oxonium ions to corresponding products.
A similar reaction protocol was applied to get the desired product by perform-
ing the reaction of benzacetal with AcAc as a model substrate shown in Scheme 4.
Notably, the acetals are highly moisture sensitive and converted into benzalde-
hyde as a by-product. The blank reaction was performed; the reactant was not
converted to product (Table 4, entry 1). Using the homogeneous PTSA as a cata-
lyst, the reaction gave 70% yield of the desired product (Table 4, entry 2). Then,
we used solid acidic Indion-190 and Amberlite-120 catalysts, which aforded the
alkylation and showed a 45, 55% yield of the desired product (Table 4, entries 3
and 4). Similarly, the strong solid acid Ab-15 as catalyst gave the highest yield
75, 78% (Table 4, entries 5 and 6). Extending the time from 20 min to 4 h, then
Knovenagel product was obtained in the alkylation 2 h (Table 4, entry 7).
We interpret this outcome to indicate that the one-pot two diferent products
can be synthesized. Subsequently, the solvents were screened, and sol-free showed
favorable for this alkylation on benzyl acetals (Table 5, entries 1–5).
The substrate scope of the Brønsted acid‑catalyzed alkylation of active methylene
compound on acetal derivatives
Benzyl acetal compounds are derivative of benzaldehyde and have electrophilic-
ity similar to aldehydic carbonyl carbon; According to previous literature reports,
acetals’ reactions with AMCs to give Aldol product in the somewhat mild reaction
1 3

D. R. Naikwadi et al.
Table 5 Solvent variation of
Entry
Solvent
Yield%
Prod-A
alkylation on benzacetal.a
1
2
3
4
5
Methanol
Tetrahydrofuran
Toluene
Sol-free
Sol-free
–
20
15
75
78(B)
aReaction conditions: (1:1 equivalence)acetal benzaldehyde
(5.5 mmol) and AcAc (5.5 mmol) with 80 mg Ab-15, 80 °C
20 min. bReaction condition: 1:1 equivalence of acetal benzalde-
hyde (4.5 mmol) and AcAc (4.5 mmol) with 80 mg Ab-15,Time
4 h, Temp. 80 ˚C (Yield=Isolated yield) (Product A=Aldol and
B=Knovenagel product)
conditions and short reaction times hired, in addition to the solid acid catalysts, are
sufcient to catalyze the direct Knoevenagel products. However, both products were
analyzed by GC–MS analysis technique with comparative reaction time. Acetal
(-OMe) is shown as a good leaving group like halogens. However, the reactivity
causes conversion in benzaldehyde as a by-product. With the previously optimized
reaction conditions, various substrates were screened for AMC’s alkylation with
benzyl acetals.
The product (2a) was obtained in high yield within 20 min, and upon extension of
reaction time, the product (2a) further undergoes elimination to form (2h) (Table 6,
entries 1 and 8). Notably, time is key to the reaction product; the aldol products
occurred only 20 min (Table 6, entries 1–7), whereas after 20 min, the eliminated
product (Knoevenagel product) was obtained in 4 h (Table 6, entries 8–14). In the
case of aldol product, (2a) was obtained in the highest yield. However, the high-
est yield of Knoevenagel product was observed in the case of (2j) as 85% (Table 6,
entry 10). Moreover, alkylation of AMC bearing both electron-donating substituents
EDG and electron-withdrawing substituents (EWG) could give the desired products
moderate to excellent yields. In hydroxyl and cyanide, substituted acetal benzalde-
hyde gave (2b) 74 and (2c) 68% yields within 20 min (Table 6, entries 2 and 3).
However, after 4 h, the yields were (2i) 64% of E-product and (2j) 85% of corre-
sponding desired products (Table 6, entries 9 and 10). Similar results were obtained
in ortho-substituted acetal benzaldehyde (2f and 2n) (Table 6, entries 6 and 14).
We have also attempted with esters, cyanide, and phenyl ring containing AMCs to
convert into corresponding alkylated products, the reaction aforded good yields of
the desired aldol and Knoevenagel products. This works’ novelty lies in the compe-
tence of solid acid catalysts in the excellent conversion of acetals to desired products
under solvent-free reaction conditions with good yields.
The plausible mechanism of alkylation on CEE and acetals is proposed in
Scheme 5 and 6. The pyran–furan containing electron-rich oxygen gives lone pair to
proton and formed the oxonium intermediate. Similarly, the generated enol of AMC
attacks the oxonium carbon (C-2) carbon and stabilizes the oxygen to produce the
desired product.
1 3

Efcient and recyclable solid acid‑catalyzed alkylation…
Table 6 Substrate scope alkylation on acetalbenzaldehyde.a-b
Entry
Substrate
AMC
Product
Yield%
1a
75
N1
2a
2a
3a
74
N2
N1
2b
68
2c
4a
5a
52
64
N2
N6
2d
2e
6a
7a
N1
N4
72
2f
52
2g
2h
8b
9b
78
N1
N3
64(E)
2i
10b
11b
12b
13b.
85
N1
2j
80
N5
N2
2k
57(E)
68(E)
2l
N6
N1
2m
14b
70
2n
1 3

D. R. Naikwadi et al.
Table 6 (continued)
aReaction condition: 1:1 equivalence of acetal benzaldehyde (4.5 mmol) and AMC (4.5 mmol) with
80 mg Ab-15, Time-20 min, Temp.: 80 °C. bReaction condition: 1:1 equivalence of acetal benzaldehyde
(4.5 mmol) and AMC (4.5 mmol) with 80 mg Ab-15, Time 4 h, Temp.:80 °C. (E)=Trans product
Scheme 5 The plausible mechanism of alkylation of AMC on CEE
Scheme 6 Probable mechanism of alkylation on acetal via Oxonium ion
1 3

Efcient and recyclable solid acid‑catalyzed alkylation…
Also, the plausible mechanism by alkylation on aryl acetal is proposed in
Scheme 6. The benzalacetal-containing oxygen abstracts the proton from the
catalyst. The lone pair of another oxygen is forced to depart a protonated group
forming the oxonium intermediate. Similarly, the generated enol of AMC will
attack the benzylic carbon, which leads to forming the desired alkylated product
A. Once the prod-A was formed, it was converted into B via bimolecular elimina-
tion reaction after some extended time.
Recycle and reproducibility studies
In a heterogeneous Ab-15 catalytic reaction, the catalyst was separated by a simple
fltration technique and furthered catalyst was oven-dried at 80 ºC 4 h for the next
cycle.
Conclusion
In summary, the use of recyclable solid acid catalysts demonstrates the greener and
atom economic protocol for the formation of carbon–carbon bonds between AMCs
with CEE and aryl acetal. The synthetic utility of these alkylation reaction proto-
cols has been investigated and is observed to an efcient and sustainable approach.
Moreover, the catalytic protocol is recyclable, scalable, and can be extended to pre-
pare alkylation products in multigram scales. A plausible mechanism is also pro-
posed for the formation of the alkylated product via oxonium intermediate.
Experimental section
All reactions were performed in oven-dried round-bottom fasks, under an inert
atmosphere at ambient temperatures, unless otherwise stated. Unless specifed,
all chemicals and catalysts were purchased from commercial sources and used as
received. Amberlyst-15 crushed in Mortars & Pestles and converted into pow-
der form. In a typical reaction protocol, AcAc (4.5 mmol), of the 3, 4-dihydro-
2H-pyran, 80 mg catalyst (Ab-15) in 50-mL round-bottom fask (RB), acetal benza-
ldehyde (4.5 mmol) and AcAc (4.5 mmol) with 80 mg Ab-15. The reaction mixture
was stirred at 50–80 °C, 0.20–12 h. After reaction completion catalyst was sepa-
rated by normal fltration, and the solution was extracted in ethyl acetate and con-
centrated. For the 10 g scale reaction, similar analysis and procedure were followed
using 1:1 equivalence of cyclic enol ethers and acetal substrate, with AcAc and
200 mg Ab-15. The pure product was obtained by column chromatography using
silica gel (100–200 mesh) as the stationary phase and hexane/ethyl acetate as an elu-
ent. The product was analyzed by GC, GC–MS (see ESI, fg S2 to S25), LC-HRMS
1
(see SI, Fig S25-S46), and NMR spectrum (see SI, Fig S47-S56). The H and 13C
spectra were recorded on Bruker, Advance II (600 MHz) and (500 MHz). Chemical
shifts were recorded in CDCl3, or DMSO-d6 solutions referenced to tetramethylsi-
1
lane (TMS) (0.00 ppm) or CDCl3 (7.26 ppm), DMSO-d6 (2.50 ppm), for H-NMR,
CDCl3 (77 ppm) and DMSO-d6 (39.5 ppm) for 13C NMR. The proton NMR peak of
1 3

D. R. Naikwadi et al.
water changes for the solvent, e.g., (1.56) in CDCl3, (3.33) in DMSO-d6. Multiplici-
ties are given as s (singlet), d (doublet), t (triplet), q (quartet), dt (doublet of triplets),
or m (multiplet).
Supplementary Information The online version contains supplementary material available at https://doi.
org/10.1007/s11164-021-04499-3.
Acknowledgements BDB acknowledges to UGC Government of India for the senior research fellow-
ship, and KR acknowledges to CSIR Government of India for the senior research fellowship and AVB
acknowledges to Science & Engineering Research Board (SERB/F/2139/2017-2018), India, for fnan-
cial support. The analytical division provides the centralized instrumentation facility with all requisite
instrumental analysis of CSIR- Central Salt and Marine Chemicals and Research Institute, Bhavnagar.
Funding Science and Engineering Research Board (SERB/F/2139/2017–2018), India.
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