Polystyrene resin supported palladium(0) (Pd@PR) nanocomposite catalyzed synthesis of β-aryl and β,β-diaryl unsaturated scaffolds following tandem approaches

Article abstract of DOI:10.1039/c5ra00228a

A one pot general tandem procedure is described for β-aryl and β,β-diaryl alkenes synthesis following an alternative to the classical approaches by using aryl aldehyde as one of the starting materials. The developed polystyrene resin supported palladium(0) (Pd@PR) nanocomposite has been applied in an unprecedented sequential condensation-decarboxylation-Heck (CDH) and condensation-Heck (CH) strategies to generate the substituted alkenes (C₆(C₆)CC-C, C₆-CC-C₆, and C₆(C₆)CC-CO-C₆ units) under ligand, and additive free milder basic reaction conditions. The added momentous benefit over the classical methodologies is in terms of its multi-component approach to achieve the desired products without tedious step wise purifications. This journal is

Full text of DOI:10.1039/c5ra00228a

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ARTICLE TYPE
Polystyrene resin supported palladium(0) (Pd@PR) nanocomposite
catalyzed synthesis of β–aryl and β,β–diaryl unsaturated scaffolds
following tandem approaches†
a,b
Arun K. Shila,b and Pralay Das
*
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
they are expensive, required additives and inert reaction
An one pot general tandem procedure is described for β-aryl
and β,β-diaryl alkenes synthesis following an alternative to
the classical approaches by using aryl aldehyde as one of the
conditions.8 Recently, a one pot CDH sequence has been
50 attempted for the synthesis of 4ꢀhydroxy stilbenes under the
additive and homogeneous palladiumꢀphosphine complex
catalyzed condition.9 In this report authors took electroꢀlabile 4ꢀ
hydroxy aromatic aldehyde as starting material which enabled an
easy decarboxylation of the intermediate condensate. The choice
55 of the substrate has greatly restricted the wider scope of this
process. Moreover, use of much higher equivalents of active
methylene component (14 equiv. malonic acid) and secondary
nitrogeneous base (15 equiv. piperidine) which might have
participated in byꢀproduct formation, have limited the outreach of
60 the methodology. But still design based one pot condensative
approaches for Pdꢀcatalyzed βꢀarylation starting from
10 starting materials. The developed polystyrene resin supported
palladium(0) (Pd@PR) nanocomposite has been applied in an
unprecedented sequential condensation-decarboxylation-
Heck (CDH) and condensation-Heck (CH) strategies to
generate the substituted alkenes (C6(C6)C=C-C, C6-C=C-C6,
15 and C6(C6)C=C-CO-C6 units) under ligand, and additive free
milder basic reaction conditions. The added momentous
benefit over the classical methodologies is in terms of its
multi-component approach to achieve the desired products
without tedious step wise purifications.
20 1. Introduction
Pd catalyst/ ligand
Additive, nitrogeous base
CHO
Ar
R1
HO
R1
HO
(a)
In recent era, βꢀaryl or β,βꢀdiaryl unsaturated compounds such as
Malonic acid, ArX
cinnamates,1 stilbenoids,2 chalcones and neoflavonoids have
attracted huge attention due to their extremely privileged
25 biological profile and natural abundance. The conventional
synthetic procedures of these compounds consist of acid or base
catalyzed condensation,5a,b Wittig reaction5c and Lewis acid
mediated cyclization5d with tedious work up, and step wise
purifications. To surmount the limitations, the introduction of
30 tandem or one pot sequential strategy is virtually more beneficial
and has been shown significant interest in the area of
contemporary organic synthesis especially methodology
development5e,f and target oriented synthesis. In recent years, the
transition metal catalyzed tandem processes have been highly
35 nourished due to its minimal atom wastage, economic feasibility
and efficiency.6 In this respect heterogeneous palladium(0)
nanocomposite catalyzed dominoꢀHeck approach might be an
attractive alternative to furnish the valuable alkene scaffolds.
Although the employment of transition metal nanocomposite as
40 heterogeneous catalyst in such reactions is a challenging task
mainly due to catalytic stability, reactivity and byꢀproduct prone
side reactions in a variety of combined substrate/ reagent
conditions.7
3
4
Previous work
R2
R3
R1
A= monoꢀEthyl malonate,
Malonic acid, Cyanoꢀ
acetic acid, Acetophenone
R2= H, Ar
Pd@PR
ArI
CHO
NH4COOH
(b)
R1
K2CO3
A
R3= CO2Et, CO2H, CN, Ar,
COPh
Ar
Ligand and additive
free approaches
O
O
A= monoꢀEthylmalonate
R1= 2ꢀOH
Present work
Scheme 1. a) Previously reported method for the synthesis of 4ꢀ
65 hydroxystilbenoids utilizing CDH strategy; b) Present work
arylaldehyde remains untouched. To counteract the existing
problems there is an intense urge to develop an efficient and
facile single pot strategy for the synthesis of functionalized βꢀaryl
or β,βꢀdiaryl alkenes.
70 In this context herein, we describe ligand free Pd@PR
nanocomposite (formerly reported as SSꢀPd, prepared from
commercially available Amberlite IRA 900 resin (chloride
form))10 catalyzed expeditious one pot sequential CDH and CH
approaches using aromatic aldehyde, methylene compound and
Synthesis of β,βꢀdiarylalkenes through double Heck coupling is
45 much difficult owing to the presence of less reactive βꢀhydrogen
in 1,2ꢀdisubstituted olefin. Often palladiumꢀphosphine and
carbene complexes have shown good results, but unfortunately
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aryl halide to cinnamate, stilbene, chalcone and neoflavonoid 25 three component reactions for the synthesis of diaryl substituted
synthesis.
olefin scaffolds following CDH and CH strategies. To select the
best reagents and condition several optimization studies were
perform to achieve the highest yield of the products (Table 1). 4ꢀ
Methoxybenzaldehyde (150 mg), monoethyl malonate (1.2
2. Results and Discussion
5
The Pd@PR nanocomposite was prepared following our earlier 30 equiv.), 4ꢀiodoanisole (1.2 equiv.), Pd@PR (3 mol% Pd), and
reports (supporting information) through in situ reductionꢀ
deposition strategy using palladium(II) salt as Pd precursor.10a
The heterogeneous Pd@PR nanocomposite was morphologically
NH4OOCH (1 equiv.) followed by K2CO3 (2 equiv.) in DMF at
135 oC was found to be the optimum condition to give 1a in 72%
yield (Table 1, entry 9). Considering other reagents
10 characterized by transmission electron microscopy (TEM)
analysis before its application (Fig. 2). Inspired by our recent
results,10,11 we applied Pd@PR catalyst for one pot sequential
Table 2 Pd@PR nanocomposite catalyzed tandem synthesis of β,βꢀ
35 diarylacryl derivativesa
R1
i. Pd@PR [3 mol% Pd],
HCOONH4 [2 equiv.],
DMF [2 ml.], 135 oC, 2 h
ii. K2CO3 [2 equiv.],
135 oC, 18 h
I
CHO
+
R2
R1
R3
+
R2
CO2H
R3
1b-1n
HO
CO2Et
MeO
MeO
CO2Et
CO2Et
OMe
Me
R1= 4ꢀOMe, R3= H
1b: 73%; E/Z= 59.5:40.5
R1= 4ꢀOH, R3= 4ꢀOMe
R1= 4ꢀOMe, R3= 4ꢀMe
1c: 80%; E/Z= 54:46
1d: 80%; E/Z= 100:0
MeO
Me
CO2Et
CO2Et
CO2Et
Figure 2. A) Image of Pd@PR catalyst; B) Transmission electron
15 microscopy image of Pd@PR catalyst
OMe
OH
OMe
Table 1 Optimization of the reaction condition for CDH reaction
R1= 4ꢀOMe, R3= 4ꢀOH
1g: 65%; E/Z= 83.5:16.5
R1= H, R3= 4ꢀOMe
R1= 4ꢀMe, R3= 4ꢀOMe
1e: 76%; E/Z= 66:34
1f: 69%; E/Z= 100:0
OMe
OMe
i. PdꢀCatalyst,
I
CHO
+
OMe
MeO
CO2Et
CO2H
Condesing reagent,
+
MeO
Solvent, 135 oC, 2 h
ii. Base, 135 oC, 18 h
CO2Et
CO2Et
MeO
CO2Et
CO2Et
OMe
MeO
1a
Pd Catalyst
Condensing
reagent[equiv.]
1a % Yield a
Entry
Base [equiv.]
Solvent
[mol% of Pd]
Pd@PR [3]
Pd@PR [3]
OMe
R1= 4ꢀOMe, R3= 2ꢀOMe
OMe
K2CO3 [2]
K2CO3 [2]
1
2
NH4OAc [2]
47
66
DMF
DMF
R1= 4ꢀOMe, R3= 2ꢀOMe
R1= 2ꢀOMe, R3= 4ꢀOMe
1j: 65%; E/Z= 100:0
1h: 68%; E/Z= 100:0
1i: 54%; E/Z= 72:28
NH4OOCH [1]
3c
4
DMF
DMF
K2CO3 [2]
K2CO3 [3]
trace, 39b
68
NO2
OMe
NH4OOCH [1]
NH4OOCH [1]
Pd@PR [3]
Pd@PR [4]
O2N
CO2H
CO2Et
CO2Et
K2CO3 [3]
trace, 42b
trace, 31b
trace, 37b
60
5
Pd@PR [3]
Pd@PR[3]
DMF
6
KOOCH [2]
Et3N [2]
K2CO3 [2]
K2CO3 [2]
K2CO3 [2]
DMF
DMF
DMF
7
8
Pd@PR [2]
Pd@PR [2]
OMe
OMe
R1= 3ꢀNO2, R3= 4ꢀOMe
R1= 2ꢀNO2, R3= 4ꢀOMe
NH4OOCH [2]
R1= 4ꢀOMe, R3= 4ꢀOH
1k: 71%; E/Z= 100:0
1l: 70%; E/Z= 100:0
1m: 61%; E/Z= 100:0
9
NH4OOCH [2]
NH4OOCH [2]
NH4OOCH [2
NH4OOCH [2]
Pd@PR [3]
Pd@PR [3]
Pd@PR [3]
Pd@PR [4]
Pd(OAc)2 [3]
K2CO3 [2]
DMF
72
MeO
52
10
K2CO3 [2]
Dioxane
Toluene
53
46
30
K2CO3 [2]
K2CO3 [2]
K2CO3 [2]
11
12
13
CN
PEGꢀ400
NH4OOCH [2]
DMF
DMF
DMF
14
15
NH4OOCH [2]
NH4OOCH [2]
Pd(allyl)2Cl2 [3] K2CO3 [2]
Pd2dba3 [3] K2CO3 [2]
37
41
OMe
R1= 4ꢀOMe, R3= 4ꢀOMe
1n: 56%
a Yields of isolated products; Reaction condition: A mixture of substrates
(4ꢀmethoxybenzaldehyde (150 mg.), monoethyl malonate (1.2 equiv.) and
20 4ꢀiodoanisole (1.2 equiv.)) and palladium catalyst were magnetically
a
All are isolated yields; Reaction conditions: Aryl aldehyde (150 mg),
o
stirred at 135 C in presence of condensing agent for 2 hours and then
active methylene carboxylic acid (1.2 equiv.), aryl iodide (1.2 eq.),
40 Pd@PR (3 mol% Pd), NH4OOCH (2 equiv.), K2CO3 (2 equiv.), DMF (2
ml), 135 oC , the reaction was continued for 2 h before addition of K2CO3
followed by 18 h; E/Z ratio was determined by 1H NMR spectra.
base was added into the reaction mixture; b Yield of isolated byꢀproduct,
c
4,4’ꢀdimethoxybiphenyl, Both NH4OOCH (1 equiv.) and K2CO3 (2
equiv.) were added simultaneously into the reaction mixture.
2
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(specially NH4OAc), NH4OOCH was found to be performing
dual roles for facile condensationꢀdecarboxylation reaction and as
a reducing agent to facilitate conversion of Pd(II) to Pd(0), and in
situ repairing/ regeneration of Pd@PR catalyst which further
confirmed by ICPꢀAES analysis (overall Pd leaching 0.14 ppm
after three cycles, supporting information).
The optimized condition was further applied to investigate the
broader substrates scope as outlined in Table 2. Under the
standard reaction condition, both aryl aldehydes and aryl iodides
Aldol condensation of acetophenone with aromatic aldehydes and
Heck reaction afforded the desired βꢀaryl chalcones 2a and 2b in
moderate yields (Scheme 4).
55
RCH2CO2H
Nucleophilic
5
addition
Knovenagel
condensation
HCOONH4
ArCHO
R
OH2
Ar'
R
O
10 containing electron releasing substituents (ꢀOMe, ꢀOH and ꢀMe)
participated in CDH sequence to afford corresponding βꢀaryl
ethylcinnamates (C6(C6)C=CꢀC unit) (1b-g) in good to moderate
yields. No significant effect of steric hindrance was observed for
2ꢀOMe substituted aryl aldehydes or iodides for the same reaction
15 and gave corresponding CDH products 1hꢀ1k in good yields.
More challenging electron withdrawing 2ꢀNO2 and 3ꢀNO2
substituted benzaldehydes were smoothly reacted with both mono
ethyl malonate and malonic acid respectively in combination with
4ꢀiodoanisole to furnish 1l and 1m in moderate yields. Similarly,
20 3,3ꢀbis(4ꢀmethoxyphenyl)acrylonitrile 1n was obtained in 56%
when 4ꢀanisaldehyde was treated with cyanoacetic acid and 4ꢀ
iodoanisole under the same reaction condition. Most of the
products in Table 2 were found to be 100% E configuration
(confirmed by NMR studies and the reason might be the Eꢀ
25 configured dehydroꢀdecarboxylation (I) as described in
mechanism section).
In continuation, we applied the set tandem protocol for the
synthesis of hydroxystilbenes which are among the most
privileged scaffolds owing to their diversified biological and
30 unrivalled physicochemical activities. The cascade Knoevenagel
condensationꢀdouble decarboxylationꢀHeck process resulted
hydroxy stilbene derivatives (1o and 1p) in 61% and 65% yields
(E/Z= 100:0) respectively through the formation of corresponding
styrene intermediates (Scheme 2).
Ar
Ar'I
Ar
C
O
Pd(0)@PR
I
V
Oxidative
addition
KI
Dehydroꢀ
decarboxylation
Reductive
elemination
R = CO2Et, CO2H, CN
Ar'
PR@Pd (II)
CO2 + H2O
I
Ar'
R
III
Insertion
K2CO3
R
Ar
Ar
(II)
PR@Pd
II
I
IV
Scheme 3. Plausible mechanistic pathway for the one pot
60 synthesis of β, βꢀdiaryl alkenes
O
CHO
R3
CH3
R1
O
i. Pd@PR [3 mol% Pd]
+
HCOONH4 [2 equiv.]
I
DMF [2 mL], 135 oC, 2 h
ii. K2CO3 [2 equiv.],
135 oC, 18 h
R3
+
R1
OMe
O
O
35
MeO
CHO
+
2a: 65%
2b: 64%
CO2H
R2
i. Pd@PR [3 mol% Pd],
HCOONH4 [2 equiv.],
DMF [2 mL.], 135 oC, 2 h
ii. K2CO3 [2 equiv.], 135 oC, 18 h
R1
CO2H
I
R1
Scheme 4. Pd@PR nanocomposite catalyzed tandem synthesis of
65 βꢀaryl chalcones following CH approach (E/Z ratio determined by
1H NMR spectra)
+ R2
OMe
X
R1
CHO
CO2Et
i. Pd@PR [3 mol% Pd]
HCOONH4 [2 equiv.]
R1
+
HO
HO
1o: 61%; E/Z= 100:0
CO2H
1p: 65%; E/Z= 100:0
DMF [2 mL], 135 oC, 2 h
ii. K2CO3 [2 equiv.], 135 oC,
18 h
I
O
O
+
Scheme 2. Pd@PR nanocomposite catalyzed tandem synthesis of
hydroxy stilbenoids (E/Z ratio determined by 1H NMR spectra)
OH
OH
N
40
Mechanistically it was presumed that the aromatic aldehyde
first undergoes Knoevenagel condensation with αꢀcarboxy active
methylene component leading to the formation of most
favourable Eꢀconfiguration of the cinnamyl intermediate (II) after
dehydroꢀdecarboxylation of (I). The intermediate (II) is then
O
O
O
O
3a: 66%
3b: 58%
45 coupled with aryl iodide in presence of Pd@PR catalyst through
the oxidative addition, insertion and reductive elimination steps,
to afford the desired β,βꢀdiaryl alkene (V) (Scheme 3).
Scheme 5. Pd@PR nanocomposite catalyzed tandem synthesis of
neoflavonoids following CDH approach
70 Whereas, the palladium catalyzed Heck arylation at βꢀposition of
in situ produced ethyl cinnamate intermediate with 2ꢀiodophenol
followed by cyclization gave neoflavonoids 3a and 3b in
comparable yields (Scheme 5).
To our great interest, the set reaction condition was further
extended for one pot tandem approach synthesis of βꢀ
50 arylchalconoids (C6(C6)ꢀC=CꢀCOꢀC6 unit) (Scheme 4) and
neoflavonoids (C6(C6)ꢀC=CꢀC unit) (Scheme 5). The sequential
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3. Conclusions
65
70
In summary, we have developed Pd@PR nanocomposite
catalyzed proficient multicomponent strategies for the synthesis
of βꢀaryl cinnamates, stilbenes, chalcones and neoflavonoids. The
additive free one pot sequential process for the synthesis of βꢀaryl
substituted alkene derivatives employing combined substrate/
reagent compatible heterogeneous palladium nanocatalyst have
imparted its potential future practical utility. In addition, we have
10 explored the dual roles of NH4OOCH which governs the
efficiency of condensation and stabilization of palladium(0)
nanoparticles over the solid surface. Overall, the developed
methodology comprises of the application of heterogeneous
transition metal catalyst in multicomponent reactions, additive
15 and ligand free process, and catalyst recyclability.
5
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75
80
6
4. Experimental Section
85
General experimental procedure for one pot sequential
20 condensation-decarboxylation-Heck (CDH) approach: To a
mixture of arylaldehyde (150 mg), aryl iodide (1.2 equiv.), active
methylene carboxylic acid (1.2 equiv.), ammonium formate (1.5
equiv.) and Pd@PR (3 mol% Pd) in 40 mL reaction vial was
added 2 ml of dry DMF. The reaction mixture was stirred in
25 preheated silicone oil bath at 135 oC for 2 hours and then K2CO3
(2 equiv.) was added into it. After addition of K2CO3 the reaction
was again continued at the same temperature. The progress of the
reaction was monitored by TLC. On completion, the cooled
reaction mixture was extracted with ethyl acetate (3×5 ml) by
30 addition of 2 ml of water and dried over anhydrous Na2SO4.
Evaporation of the combined organic layer followed by column
chromatography (Hexane and ethyl acetate gradient) over silica
gel (mesh 60ꢀ200) afforded the desired products (1a-1p).
7
8
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35 Acknowledgements
10
This work was financially supported by the CSIR (project
ORIGINꢀCSC0108). We also gratefully acknowledge the
Director CSIRꢀIHBT, Palampur, for providing necessary facilities
40 during the course of work. The authors thank AIRF, JNUꢀNew
Delhi, for TEM and SAIF, IIT Bombay for ICPꢀAES analysis.
AKS thanks CSIR for fellowship.
110 11 (a) N. R. Guha, C. B. Reddy, N. Aggarwal, D. Sharma, A. K. Shil,
Bandna and P. Das, Adv. Synth. Catal., 2012, 354, 2911ꢀ2915; (b) A.
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Notes and references
a Natural Product Chemistry and Process Development, CSIRꢀInstitute of
45 Himalayan Bioresource Technology, Palampurꢀ176061, (H. P.), India,
Fax:+91ꢀ1894ꢀ230433,
Eꢀmail: pdas@ihbt.res.in, pdas_nbu@yahoo.com
bAcademy of Scientific & Innovative Research, New Delhi, India
†Electronic Supplementary Information (ESI) available: [Typical
50 experimental procedures, recyclability experiment, Hg poisoning
experiment, spectral data and copies of 1H, 13C NMR and ESIMS spectra
of synthesised products ]. See DOI: 10.1039/b000000x/
†IHBT communication no. 3686
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