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. 2019 Jul 19;5(7):e02000. doi: 10.1016/j.heliyon.2019.e02000

Synthesis of ynones at room temperature catalyzed by copper chloride cryptand complex under solvent free conditions

Sankar Jyoti Bora 1, Bolin Chetia 1,∗
PMCID: PMC6656957  PMID: 31372526

Abstract

An air-stable highly efficient reusable CuCl2-cryptand-[2.2.Benzo] catalyst is reported first time for coupling reaction of terminal alkynes with different acyl chlorides in the presence of triethylamine acting both as base and solvent at room temperature to give the corresponding ynones. Easy-going, short reaction time, cost-effective, palladium-, phosphorus- and solvent-free, high yield and recyclability up to 5th times make this method green procedure for ynones’ synthesis with wide substrate variety.

Keywords: Analytical chemistry, Solvent free, Catalysis, Cryptand, Ynones

1. Introduction

In recent years, Ynones (α, β-Acetylenic ketones) have got tremendous importance due to their numerous applications in organic synthesis, click chemistry, pharmaceutical and natural products. Ynones are the constructive building blocks in organic chemistry that are used as synthetic intermediates for the preparation of many biologically-active compounds, heterocyclic derivatives [1] etc. Ynones are also precursors for the synthesis of various valuable organic functional groups like isoxazoles [2a], pyrimidines [2b], quinolones [3], furans [4], pyrazoles [5], flavones [6], oximes [7], polyfunctionalized pyrroles [8], chiral propargylic alcohols [9] and many more. There are various reaction protocols that have been reported for the synthesis of ynones which include oxidation of propargylic alcohols [10], coupling reactions of terminal alkynes with organic halides in the presence of carbon-monoxide gas [11], oxidation of alkynes [12], reaction of terminal alkynes with nitriles [13], and the most important method is the reaction between terminal alkynes and carboxylic acid derivatives in the presence of various strong bases [14] and elements such as silver [15], zinc [16], tin [17], silicon [18], lithium [19], copper [20], magnesium [21], cadmium [22], aluminum [23], thallium [24], gallium [25], stibium [26], indium [27], etc. Recently the synthesis of ynones catalyzed by palladium catalysts like palladacycle complexes [28a], Pd/C [28b], Pd(PPh3)Cl2/CuI [28c], PdNPs-PPS [28d], PdCl2(PPh3)2, [28e] Pd(OAc)2, [28f] palladium (II)acyclicdiaminocarbene complex[28g] Pd(PPh3)4/ZnCl2[28h] and polystyrene-supported palladium (0) complex[28i] palladiumchloride–cryptand-22 complex[28j] Pd-Ph1-phoxide[28k] gets much more attention. On the other hand, palladium free methodologies get copious interest because of the high cost of palladium and difficulty to separate from the product. Generally, in industrial practice palladium catalysts are avoided whenever possible in the last step of synthesis of complex molecule [20b].

Recently Wang et al. reported synthesis of ynones using copper nanoparticles supported on mesoporous polymer [20e] and a very few copper catalysed methodologies were reported for the synthesis of ynones [20]. The above mentioned reported protocols suffers from various disadvantages such as long reaction time, high temperatures, use of air and moisture sensitive catalysts, phosphorous containing catalysts, necessity of additives, use of hazardous organic solvents, use of toxic carbon-monoxide gas, harsh reaction conditions etc. In practice the best method from environmental and economic point of view is the reaction without solvent and thus solvent-free reactions undeniably have become frequent and specialized over recent years.

Cryptands have numerous applications in various fields like sensing [29a], biological activity [29b] etc. Recently it has been shown that cryptand has great catalytic activity in various reactions [30,28j,20e] because of many advantages like owing to their versatile processing capabilities, separation and recycling etc. The chelating effect of nitrogen and oxygen present in macrocycles along with its flexibility give support in stabilizing the complex.

To overcome the drawbacks of the reported protocols we have developed an efficient, environmentally benign and operationally simple CuCl2-cryptand [2.2.Benzo] complex catalyst for coupling of acyl chloride and terminal alkynes (Scheme 2) in presence of triethyl amine (TEA) acting both as a base and solvent. In comparison to the other methods this catalyst catalysed the reaction in a very low amount of catalyst loading without solvent. High yield, solvent-, phosphorus-, palladium free conditions and recyclability up to fifth cycle along with no heat requirement makes this protocol efficient greener and cost effective and may be applied in large scale industrial synthesis. The catalyst is air stable and remains effective for a long period without loss of its catalytic activity. This protocol gives high yield at short reaction time and to the best of our knowledge this is a novel catalyst used for the synthesis of ynones.

Scheme 2.

Scheme 2

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Synthesis of ynones by CuCl2-cryptand [2.2.Benzo] catalyst.

2. Experimental

2.1. Synthesis of cryptand and its copper complex

The cryptand derivative L1 was produced by the reaction of 1,2-bis(2-iodoethoxy)ethane (1a) and m-phenylenediamine (1b) as shown in Scheme 1. The CuCl2-cryptand [2.2.Benzo] (C1) complex was synthesised by adding CuCl2.2H2O solution to the cryptand solution in ethanol. The synthetic procedure, characterization data and explanation of complexation were provided supporting information and were found to be coincide with the literature [31].

Scheme 1.

Scheme 1

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(A) Na2CO3 CH3CN (dry) reflux 48 h. 42% yield. (B) CuCl2.2H2O ethanol reflux and 7h.

2.2. General procedure for synthesis of ynones by CuCl2-Cryptand [2.2.Benzo] complex

For synthesis of ynones by using CuCl2-Cryptand [2.2.Benzo] catalyst (Scheme 2) mixture of terminal alkyne (1.0 mmol), acid chloride (1.0 mmol), Et3N (2.5 mmol) and 0.76 mol% CuCl2-Cryptand [2.2.Benzo] was stirred at room temperature (26 °C) without any solvent under aerobic conditions. The progress of the reaction was monitored by TLC and after completion of the reaction 20 mL diethyl ether was added to the reaction mixture for two times and filtered it off. The residue was separated out and used for further reaction. The filtrated organic layers were washed with 50 mL of water for two times and dried with anhydrous Na2SO4, filtered and concentrated in rotary evaporator. The crude product thus obtained was further purified by preparative TLC using hexane and ethyl acetate in 9:1 ratio to obtain the desired product. The residual part obtained after separation of organic layer was used for another reaction. All the synthesized derivatives of ynones were characterized by 1H and 13C NMR spectroscopy.

3. Results and discussion

Initially, we selected benzoyl chloride and phenylacetylene as a model substrate for the optimization of reaction conditions (Fig. 1). To evaluate the best condition for synthesis of ynones, we studied the effect of various parameters like catalyst loading, solvent, base and temperature. First, we optimized the reaction for catalyst loading, the reaction was carried out without catalyst (Table no. 1, entry 1) and realized that no conversion of the reaction. Next, we carried out the model reaction with various amount of catalysts loading (Table 1, entry 2–7) and was observed that 0.76 mol % of the catalyst was the best amount to catalyse the reaction and provided 94% yield of 2a. Subsequently we performed solvent optimization (Table 1, entry 8–13) on the model reaction and found that solvent less condition gave the best yield. Among the solvents toluene (Table 1, entry12) gave yield up to 42 %. Next, we carried the reaction only with CuCl2.2H2O (Table 1, entry 14) and Cryptand [2.2. Benzo] (Table 1, entry 15) and it was found that the reaction gave trace amount of yield in former but in the later case the reaction did not proceed. Thereafter, we studied the reaction without using any base in toluene solvent and it was observed that the reaction did not proceed (Table 1, entry 16). After that, we studied the effect of various bases like trimethylamine, K2CO3, Cs2CO3, Na2CO3, KOH on the model reaction (Table 1, Entry 17–24) also study the temperature and solvent effect with these bases in Table 1 entry 18–19, observed that these bases did not catalyse the reaction even in presence of solvent while TEA gives excellent yield of the desired product (Table 1 entry 7) without any solvent.

Fig. 1.

Fig. 1

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Reaction picked for optimization.

Table 1.

Optimization conditions for the synthesis of ynones.a

Entry Catalyst (mol %) Base Temp (°C) Solvent Yieldb %
Catalyst loading
1 - TEA rt - N.R.
2 0.10 60
3 0.21 TEA rt - 82
4 0.42 TEA rt - 89
5 0.63 TEA rt - 91
6 0.76 TEA rt - 94
7 1.0 TEA rt - 94
Solvent effect
8 1.0 TEA rt Water Trace
9 1.0 TEA rt DMF Trace
10 1.0 TEA rt DCM 29
11 1.0 TEA rt CH3CN 40
12 1.0 TEA rt Toluene 42
13 1.0 TEA rt THF trace
Test before complexation
14 CuCl2.2H2O TEA rt - Tracec
15 Cryptand [2.2.Benzo] TEA rt - N.R.d
Base effect
16 1.0 - rt toluene N.R.
17 1.0 K2CO3 rt - N.R.
18 1.0 K2CO3 60 - N.R.
19 1.0 K2CO3 60 toluene 12
20 1.0 Cs2CO3 rt - N.R.
21 1.0 Na2CO3 rt - N.R.
22 1.0 Na2CO3 rt toluene Trace
23 1.0 KOH rt - N.R.
24 1.0 KOH rt Toluene 10
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Bold significance is to point out the amount of catalyst loading for the smooth progress of the reaction.

a

Reaction condition: benzoyl chloride 140 mg (1 mmol), phenylacetylene 102 mg (1 mmol), CuCl2-cryptand [2.2.Benzo] (mol%), Base (2.5 mmol), solvent 2 mL; N.R. = no reaction; All reactions were carried out for 2 h.

b

Isolated yield.

c

10 mg of CuCl2.2H2O.

d

10 mg of cryptand [2.2.Benzo].

Thus, TEA (2.5 mmol), 0.76 mol % of CuCl2-cryptand [2.2.Benzo] catalyst and stirred at room temperature for 2 h are the best conditions for synthesis of ynones. With this optimized conditions of newly adopted efficient protocol we extended our studies to different substrates of acyl chloride and terminal alkyne (Fig. 2). The reaction with various derivatives of arylacetylene with substituents at the para-positions proceeded smoothly (Fig. 2). Moreover, benzoyl chloride substituted with –Me, –OMe, –Cl, –Br and –NO2 also provided good yields of the corresponding ynones. Aliphatic acyl chloride derivatives which generally give very low yields, this catalyst pushed these reactions to give good yields. The outcome in Fig. 2 established that the coupling of various derivatives of acyl chloride and terminal alkyne catalyzed by the cryptand complex gives very good yield in a very short time period.

Fig. 2.

Fig. 2

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CuCl2-Cryptand[2.2.Benzo] catalysed synthesis of various Ynones.ab

In Fig. 3, we demonstrated the comparison of our CuCl2-cryptand [2.2.Benzo] complex catalyzed method for the synthesis of ynones over existing protocols. As appeared in Fig. 3, current condition has the extraordinary preferred position over the existing methods.

Fig. 3.

Fig. 3

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Comparison of previous reported works with our work.

XPS analysis was also executed to know the oxidation state of the CuCl2-cryptand [2.2. Benzo] complex (Fig. 4). In the survey scan Fig. 4 (a), all index peak corresponds to Cu, O, N, C and Cl, which confirms the presence of all these elements in the in the synthesized complex. The survey scan spectrum shows photoelectron peak of Cu 3s, Cu 2p, Cu 3p, Cu LLM auger for copper, O 1s for oxygen, N 1s for nitrogen, Cl 2s, Cl 3p, Cl 2p for chlorine and C1s for carbon. High resolution XPS spectra shows characteristic main peak at 934.8 eV and two satellite peak at 942.1 eV and 943.5 eV for Cu 2p3/2, another main peak and satellite peak observed for Cu 2p1/2 at 954.8 eV and 962.55 eV respectively [32]. From all of these data it is confirmed that copper metal present in the complex C1 is in +2 oxidation state.

Fig. 4.

Fig. 4

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(a) XPS survey scan of complex C1, (b) High resolution XPS spectra of Complex C1 before and after catalysis.

There are several copper catalysed methodologies for the synthesis of ynones20 but the mechanism is not sure till-date. From GC-MS analysis it is evident that no homo-coupling of terminal alkynes were observed in the reaction mixture catalysed by CuCl2-cryptand [2.2.Benzo] as the same was observed in the reaction of catalyzed by PdCl2 (for GC-MS spectra see ESI Figs. S9 and 10). For the search role of the catalyst, we performed the XPS analysis of the complex after catalysis [Fig. 4 (b)], comparison with the high-resolution XPS spectra of cryptand complex (C1) before catalysis showed that the oxidation state of the complex did not change after catalysis, which remain same as Cu2+.

As the catalyst is heterogeneous, the catalyst was separated out from reaction mixture easily by centrifuging at 1200 rmp and washed with water for several times. This catalyst was reused in coupling reaction and established that it could be recycle up to 5th times. A small decrease in each catalytic cycle was observed may be due to loss of catalyst during the process of recovery as it confirmed by weighing the catalyst after each cycle [Fig. 5].

Fig. 5.

Fig. 5

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Recyclability test of the catalyst.

4. Conclusion

In conclusion a mild and efficient palladium-, phosphorus-, solvent-free protocol was developed successfully for synthesis of ynones via coupling of terminal alkyne and acyl chloride catalyzed by a novel CuCl2-cryptand [2.2.Benzo] complex at room temperature without any additives. The advantages of the current method include, mild reaction conditions, air stable catalyst, short reaction time, broad substrate scope and high yields. A wide-ranging substrate of acyl chlorides and terminal alkynes were converted to their corresponding Ynones with excellent yield. Moreover, the catalyst can be easily separated and can be reused up to five times without any significant loss of its catalytic activity.

Declarations

Author contribution statement

Bolin Chetia: Conceived and designed the experiments; Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data.

Sankar J Bora: Performed the experiments; Analyzed and interpreted the data; Wrote the paper.

Funding statement

This work was supported by the Department of Science and Technology, Govt. of India under DST-SERB (No. SB/FT/CS-161/2012), UGC-SAP.

Competing interest statement

The authors declare no conflict of interest.

Additional information

No additional information is available for this paper.

Acknowledgements

The authors acknowledge the Department of Science and Technology, Govt. of India for financial assistance under DST-SERB (No. SB/FT/CS-161/2012), UGC-SAP and also thankful to SAIF NEHU-Shillong, SAIF-Cochin-Kerala, SAIF-IITM, SAIC-TU for spectral data.

Appendix A. Supplementary data

The following is the supplementary data related to this article:

SI_heliyon_17062019
mmc1.doc (4.3MB, doc)

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