[Bmim][PF6] Catalyzed Synthesis of C3-Symmetric

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Amberlyst-15/[Bmim][PF6] Catalyzed Synthesis of C3Symmetric Triarylbenzenes via Cyclotrimerization of Alkynes Kishor V. Wagh, and Bhalchandra M. Bhanage ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b00729 • Publication Date (Web): 15 Jun 2016 Downloaded from http://pubs.acs.org on June 18, 2016

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ACS Sustainable Chemistry & Engineering

Amberlyst-15/[Bmim][PF6] Catalyzed Synthesis of C3-Symmetric Triarylbenzenes via Cyclotrimerization of Alkynes Kishor V. Wagh and Bhalchandra M. Bhanage* Department of Chemistry, Institute of Chemical Technology Matunga, Mumbai-400019. India. Tel: +91 2233612601; Fax: +91 2233611020. E-mail: [email protected], [email protected]

Abstract: The construction of C3-symmetric triarylbenzenes via cyclotrimerization of alkynes using Amberlyst-15/1-butyl-3-methylimidazolium hexafluorophosphate [Bmim][PF6] has been developed. The catalytic system works well for various substrates under milder reaction conditions. The developed methodology is operationally simple, metal-free, solvent-free and additive-free reactions with excellent yields make it good for synthetic utility. The recyclability of the Amberlyst-15/[Bmim][PF6] as well as 100 % atom efficiency makes the developed protocol more greener. Keywords: Ionic liquid, Cyclotrimerization, Heterogeneous catalysis, Green chemistry, 100% Atom economy.

Introduction Aromatic organic moieties have attracted huge popularity from the synthetic community in the past decade1-2 as they are key building blocks in the fields of medicinal, material chemistry and many coal chemical industries.3-11 Among different aromatic compounds, 1,3,5-triarylbenzene and its derivatives has gained significant attention, which can be used to design organic light emitting diodes,12-14 electrode devices,15 resistance materials16 and 1

ACS Paragon Plus Environment


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conducting polymers.17-19 In addition, 1,3,5-triarylbenzenes have been employed as a key conciliator in the synthesis of bulky ligands,20-22 fullerene fragments,23 synthetic dendrimers,24 polycyclic aromatic hydrocarbons (PAHs)25-26. Accordingly, in the past much research has been done for the regioselective synthesis of these moieties. Condensation of aryl methyl ketones using various Lewis and Bronsted acids has been one of the most classical ways for the synthesis of 1,3,5-triarylbenzenes.27-37 However, the protocols have one or more shortcomings such as time consuming, lower reaction yield, tedious handling and work-up, to maintain inert condition, costly, toxic and non-recyclable catalysts. The coupling of 1,3,5-trihaloarenes with various arylating

agents using transition-metals have

been

another powerful way to construct 1,3,5-triarylbenzenes.38-42 These processes suffer from one or more drawbacks such as requirement for functionalized starting materials, transition metals, stoichiometric salt formation, reactions occurring in various steps, production of toxic waste materials. Further, transition-metal-catalyzed cyclotrimerization of alkynes is of great synthetic importance in view of atom economy. Reppe cyclotrimerization43 and many transition metal complex systems such as Pd, Ru, Rh, Co, Ni have been described for this transformation.44-50 However, metal catalyzed cyclotrimerization of alkynes originates the synthesis of mixture of 1,3,5- and 1,2,4-trisubstituted arenes. To overcome these drawbacks Xu and coworkers reported InCl3/2-Iodophenol catalyzed synthesis of triarylbenzenes from alkynes.51 Although, this protocol was efficient for regioselctive cyclotrimerization of alkynes but it also suffers from certain limitations with respect to the use of expensive catalyst, additives, organic solvent, generation of toxic waste without recovery of the catalyst. Obviously, investigation of green routes by using renewable catalytic system to synthesize 1,3,5-triarylbenzenes from alkynes is highly desirable. In recent years, catalysis and organic synthesis using ionic liquids (ILs) have experienced a remarkable development, and a range of catalytic processes have been successfully 2


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accomplished in such media.52-53 ILs often referred to as a “designable solvent” because of its unique chemical and physical properties such as negligible vapor pressure, good thermal stability, good solvating ability, non-volatility and non-flammability.54-59 In addition, ILs have been attractive alternative for a number of organic transformations due to their chemical nature and ionic characters which provide the distinctive ionic environment that helps in stabilizing the catalytically active species or intermediates that obviously affects the reaction activity and selectivity.60-63 Here, we developed Amberlyst-15/[Bmim][PF6] recyclable catalytic system for the synthesis of C3-symmetric triarylbenzenes via cyclotrimerization of alkynes (Scheme 1).

Scheme 1 Cyclotrimerization of alkynes. Results and Discussion The cyclotrimerization of phenylacetylene (1a) was taken for the optimization study. The study of the catalysts, catalyst loading, reaction media, time and temperature were done for this model reaction (Table 1). The model reaction for the synthesis of 1,3,5-triarylbenzene (2a) was performed in the presence of different catalysts such as proton acids, metal salts and acidic ionic liquids (Table 1, entries 1-9). The results show that the Amberlyst-15 was the best catalyst for the synthesis of 2a (Table 1, entry 8). The activity of amberlyst-15 is good because it has higher surface area (42 m2/g) and better H+ exchange capacity (4.2 meq/g). Therefore, it was selected as a catalyst for further investigations. However, the product 2a formation was not seen in the absence of catalyst (Table 1, entry 9). Hence, it was indicated that Bronsted acidity is essential for this reaction. The screening of solvents was also carried 3



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out for cyclotrimerization of phenyleacetylene (Table 1, entries 10-16). The model reaction was efficient in toluene furnishing 2a in 42% yield (Table 1, entry 8). However, ionic liquids [Bmim][BF4] and [Bmim][PF6] was found to be more favorable reaction media rather than in conventional organic solvents as it provided 2a in 56% and 68% yields respectively (Table 1, entries 15, 16). This is possibly because an ionic liquid increases the activity of Amberlyst15. Hence, further reaction conditions were optimized using [Bmim][PF6] as a reaction media. It was observed 3 equiv. of catalyst is optimum for this transformation as decreasing the catalyst loading to 2 equiv. decreases the yield from 90% to 76% (Table 1, entries 1718). Next, the study of reaction temperature was explored. The temperature at 80 °C was the optimum temperature for 2a formation (Table 1, entries 19-20). Consequently, the effect of reaction time was also investigated and 12 h is found to be the sufficient time for 2a synthesis (Table1, entries 21-22). Furthermore, it was found that reaction provided 2a in 40% yield in the absence of air (Table 1, entry 23). From these screening studies, the optimal reaction conditions for 2a: 1a (3 mmol), Amberlyst-15 (3 equiv.), [Bmim][PF6] IL (2 mL), 80 °C, 12 h, normal air atm. Table 1 Optimization Table for synthesis of 2aa

Entry

solvent

time (h)

temp (°C)

yield (%)b

p-TSA

toluene

14

Reflux

22

H2SO4

toluene

14

Reflux

10

Cu(OTf)2

toluene

14

Reflux

Trace

catalyst

Effect of catalyst 1c 2c 3c

4


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4c

FeCl3

toluene

14

Reflux

N.D.

5d

-SO3H functionalized IL

toluene

14

Reflux

12

6d

[NMP]+HSO4-

toluene

14

Reflux

23

7d

[HMIM]+HSO4-

toluene

14

Reflux

28

8

Amberlyst-15

toluene

14

Reflux

42

-

toluene

24

Reflux

N.D.

10

Amberlyst-15

dioxane

14

Reflux

30

11

Amberlyst-15

cylohexane

14

Reflux

10

12

Amberlyst-15

CH3CN

14

Reflux

06

13

Amberlyst-15

CHCl3

14

Reflux

12

14

Amberlyst-15

DCE

14

Reflux

29

15

Amberlyst-15

[Bmim][BF4]

14

100

56

16

Amberlyst-15

[Bmim][PF6]

14

100

68

9 Effect of solvent

Effect of catalyst loading 17e

Amberlyst-15

[Bmim][PF6]

14

100

90

18f

Amberlyst-15

[Bmim][PF6]

14

100

76

Effect of temperature 19e

Amberlyst-15

[Bmim][PF6]

14

90

90

20e

Amberlyst-15

[Bmim][PF6]

14

80

90

Effect of time 21e

Amberlyst-15

[Bmim][PF6]

12

80

90

22e

Amberlyst-15

[Bmim][PF6]

10

80

81

23e,g

Amberlyst-15

[Bmim][PF6]

12

80

40

a

Reaction conditions: 1a (3 mmol), Amberlyst-15 (1equiv.), Solvent (2 mL). b GC Yield. c 10 mol %. d 2 mmol, eAmberlyst-15 (3equiv.). f Amberlyst-15 (2equiv). g Without air. N.D. (not detected).

5



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Next, various alkynes were also examined for the synthesis of 1,3,5-triarylbenzene derivatives (Table 2). The reaction of 1a in Amberlyst 15/[Bmim][PF6] provided the 2a in 88% yield (Table 2, entry 1). Furthermore, 1a having electron-donating substituents such as -CH3, -C3H7 and -OCH3 furnishes the respective triarylbenzenes 2b-2d in good to excellent yields (Table 2, entries 2-4). It was found that 1a having -ortho, -meta, -para halo substituents also giving the required 1,3,5-trihaloarenes 2e-2i in excellent yields under the optimized reaction condition (Table 2, entries 5-9). Furthermore, the reaction also worked on disubstituted, bulky and internal alkynes was also affording the respective triarylbenzenes 2j-2l in good yields (Table 2, entries 10-12). Table 2 Synthesis of various triarylbenzene derivativesa entry

alkynes

yield (%)b

products

1

88 2a

2

82

3

80

4

75

6


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5

85

Cl

6

88 Cl

Cl

2f

7

85

Cl

8

Cl

81 Cl 2h

9

80

Cl

10

84 Cl

2j

11

Cl

77 2k

7



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Me

12

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Me

80 Me 2l

a

Reaction conditions: Alkynes (3.0 mmol), Amberlyst-15 (3 equiv.), [Bmim][PF6] (2 mL), air, 80 ˚C, 12 h. N.D. (not detected),b Isolated yield. The recyclability of the catalytic system Amberlyst-15/[Bmim][PF6] for the model reaction was investigated (Figure 1).The product yield remain unchanged as the catalytic system was reprocessed for five time, signifying that the catalytic system was effective for recyclability. However, the activity of catalytic system was gradually decreasing might be due to deactivation as well as handling loss. Next, we also studied the model reaction on a gram scale, which provides desired product 2b in significant yield.

Fig 1 Catalyst recyclability study. Reaction conditions: 1a (3 mmol), Amberlyst-15 (3equiv.), [Bmim][PF6] (2 mL), air, 80 ˚C, 12 h. GC yield. According to the experimental result and previous reports,29,33 a possible reaction mechanism has been illustrated (Scheme 2). Initially, alkyne 1 in the presence of acidic catalyst and air or might be due to moisture present in catalytic system rapidly hydrated and converted into ketone 2 which was confirmed by GCMS. Next, the corresponding enol 4 was formed in situ from protonated ketone 3 which is in equilibrium with each other. 8


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Subsequently, cyclotrimerization of enol form 5 gave substituted cyclophloroglucinol 6. Finally, cyclophloroglucinol 6 gets activated under the influence of Amberlyst15/[Bmim][PF6] and undergoes dehydration with providing the corresponding product 7.

Scheme 2 Reaction pathway for the synthesis of C3-symmetric triarylbenzenes. Conclusions In summary, synthesis of C3-symmetric triarylbenzenes via cyclotrimerization of alkynes has been firstly developed using Amberlyst-15/[Bmim][PF6] as a recyclable catalytic system. This new method would provide a simple, compatible, and potentially powerful method for the construction of 1,3,5-triarylbenzene derivatives with higher yields. Furthermore, short reaction time, simple experimental procedure, mild reaction conditions, hazardous organic solvent free and transition metal free condition makes the developed protocol more greener. The reusability of catalytic system makes this waste-free process willing for scale-up.

Experimental Section All chemicals and reagents were acquired with their highest purity from various firms and were utilized as it is. GC with capillary column (Elite-1, 30 m × 0.32 mm × 0.25 µm) and flame ionization detector was used for analysis. GC-MS-QP 2010 instrument (Rtx-17, 30 m × 25 mm ID, film thickness 0.25 µm df) (column flow 2 mLmin-1, 80 °C to 240 °C at 10 9



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°C/min rise). The triarylbenzenes were purified by column chromatography on 60-120 mesh silica gel. The 1H and 13C NMR spectra were studied at 500 and 100 MHz in CDCl3 solvent and TMS as an internal standard. Ionic liquids were prepared as per the procedure from literature.64 Procedure for the synthesis of C3-symmetric triarylbenzenes via cyclotrimerization of alkynes: The Amberlyst-15 [H+ exchange capacity (4.2 meq/g) and high surface area (42 m2/g)] (3 equiv.), [Bmim][PF6] (2 mL) and 1a (3 mmol) was taken in reaction vessel. The reaction at 80 °C for 12 h carried out and was monitored by GC/TLC. After the completion of reaction, 5 mL of di-isopropyl ether was used for extraction of product in the ether phase and the extraction procedure (2 × 5 mL) was repeated. The organic layers containing products were mixed and dried over anhydrous Na2SO4. The crude product was purified on column chromatography [silica gel, 60-120 mesh; PE:EA] and gave the corresponding product 2a in 88% yield. The 2a formation was confirmed by various spectroscopic techniques. The purity of product was studied by GC-MS analysis. Recyclability study: After extraction, the reaction vessel bearing the recovered Amberlyst-15/[Bmim][PF6] was dried at 60 °C in vacuo for an hour and then studied with 1a directly for next cycles.

Corresponding Author *E-mail: [email protected] ; [email protected]. Tel: +91 2233612603; Fax: +91 2233611020. Notes The authors declare no competing financial interest.

Acknowledgements 10


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The author (KVW) is highly thankful to the UGC (University Grant Commission, India) for providing the financial assistance.

Supporting Information Copies of 1H,

13

C NMR and CHNS data of the products. This material is available free of

charge via the Internet at http://pubs.acs.org.

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temperature ionic liquids incorporating the imidazolium cation. Green Chem. 2001, 3, 156.

Table of Content: Amberlyst-15/[Bmim][PF6] Catalyzed Synthesis of C3-Symmetric Triarylbenzenes via Cyclotrimerization of Alkynes Kishor V. Wagh and Bhalchandra M. Bhanage* Department of Chemistry, Institute of Chemical Technology, Matunga, Mumbai- 400019, India

A novel and simple methodology for the synthesis of C3-symmetric triarylbenzenes via cyclotrimerization of alkynes using Amberlyst-15/[Bmim][PF6] as an efficient catalytic system with higher atom efficiency has been developed. The reaction works smoothly under metal-, solvent-free and mild conditions and afford desire products in good to excellent yield.

R

Amberlyst -15 [Bmim] [PF6] H R

12 h, 80 °C

R = H, -Me, -OMe, -Cl, -Br, -F

R

R

Metal -free Solvent -free 100 % Atom economy Green protocol Gram scale Recyclable catalytic system

18


[Bmim][PF6] Catalyzed Synthesis of C3-Symmetric

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