Direct α-Alkylation of Acetophenones with ... - ACS Publications

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Direct α‑Alkylation of Acetophenones with Benzhydrols as Well as 1‑Phenylethanols Using Amberlyst-15/Ionic Liquid as an Efficient Catalytic System Kishor V. Wagh and Bhalchandra M. Bhanage* Department of Chemistry, Institute of Chemical Technology, Matunga, Mumbai 400019, India S Supporting Information *

ABSTRACT: The direct α-alkylation of acetophenones with benzhydrols as well as 1-phenylethanols using Amberlyst-15/ [Bmim][PF6] ionic liquid as an affordable catalytic system has been developed. This protocol provides a simple, mild, metalfree method for the synthesis of alkylated carbonyl compounds in good to excellent yields. Further, the developed methodology is moisture-stable and operationally simple and can be recycled up to four cycles without much significant loss in its catalytic activity. KEYWORDS: Ionic liquid, Alkylation reaction, Heterogeneous catalysis, Green chemistry, Atom economy

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at 130 °C.39 Balamurugan and co-workers have developed two methods for the direct α-alkylation of unactivated ketones with alcohols via in situ formation of acetals using TfOH40 and AgSbF641 catalysts. Even though fundamentally these protocols are simple and efficient for α-alkylation of ketones, the process suffers from certain limitations with respect to the use of expensive metal-based catalysts, hazardous reagents, and volatile organic solvents. More importantly, there is no recovery of the catalysts. Therefore, the development of a convenient and environmentally benign protocol for α-alkylation of ketones is in high demand. At the moment, traditional catalysis with ionic liquids (ILs) has become a promising research area.42−48 ILs possess unique chemical and physical properties such as low vapor pressure, nonvolatility, nonflammability, good thermal stability, and solvating ability.49−51 In addition, protocols containing ILs are an attractive alternative for a number of organic transformations due to catalyst recycling and improved activities and selectivities of the reaction.52−54 Herein, we report a practical and mild route for the direct α-alkylation of acetophenones with benzhydrols as well as 1-phenylethanols using Amberlyst-15/[Bmim][PF6] (1-butyl-3- methylimidazolium hexafluorophosphate) recyclable catalytic system (Scheme 1).

INTRODUCTION Carbon−carbon bond formation is an essential link in synthetic organic chemistry.1−3 Over the years, transition-metal-catalyzed cross-coupling reactions have been one of the most powerful tools for C−C bond formation.4−6 Traditionally, C−C bonds have been synthesized by the coupling of an electrophilic coupling partner (C−X, X = halide, tosylate, mesylate, triflate, etc.) with a nucleophilic coupling partner, normally an organometallic reagent (C−M, M = Zn, Mg, Sn, etc.).7 However, the use of these building blocks has limits in their application due to moisture sensitivity, need of additional synthetic steps, and generation of stoichiometric amounts of waste products. Nowadays, environmentally benign and readily available unactivated precursors such as carboxylates,8−13 ethers,14−18 and alcohols19−22 are used as coupling partners in cross-coupling reactions. In comparison, the direct use of alcohols would be preferable in cross-coupling reactions because they are abundant in nature and easy to store and handle and are an economical alternative for various functionalized species. R. Kumar and E. V. Van der Eycken published a review on the direct reaction of alcohols with various nucleophiles to construct C−C bonds in which water is the only byproduct.23 Further, the direct α-alkylation of ketones with alcohols is of great synthetic importance in view of a new synthetic protocol for C−C bond formation.24−26 Due to low acidity of α-hydrogen, the alkylation reactions described were mostly performed under harsh conditions.27 Hence, for the αalkylation of ketones, most of the reports use ketones in the activated form such as enol acetates,28 silyl-enol ethers,29 1,3dicarbonyls,30−36 and enamines.37,38 Furthermore, very few methods are known for the direct α-alkylation of ketones with nonactivated carbonyl compounds. Gu and co-workers have reported the α-alkylation of aryl methyl ketones with diarylmethanols using Fe(OTf)3 as a catalyst in chlorobenzene © XXXX American Chemical Society

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RESULTS AND DISCUSSION Initially, we began our study to investigate the optimum reaction conditions by screening solvents, reagents, reagent loading, substrate mol ratio, reaction time, and temperature (Table 1). For this purpose, acetophenone (1a) and 4Received: October 12, 2015 Revised: November 27, 2015

A

DOI: 10.1021/acssuschemeng.5b01282 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering Scheme 1. Direct α-Alkylation Reactions of Ketones

Table 1. Optimization of Reaction Conditions for α-Alkylation Reactions of 1a with 2ba

entry

solvent

reagent

time (h)

temp (°C)

yieldb (%)

1 2 3 4 5 6 7 8 9c 10c 11 12d 13e 14e,f 15e,g 16e,g 17e,g 18e,g 19e,g 20e,g,h 21e,g,i 22e,g 23e,g

MeCN toluene dioxane CHCl3 DCE [Bmim][PF6] [Bmim][PF6] [Bmim][PF6] [Bmim][PF6] [Bmim][PF6] [Bmim][PF6] [Bmim][PF6] [Bmim][PF6] [Bmim][PF6] [Bmim][PF6] [Bmim][PF6] [Bmim][PF6] [Bmim][PF6] [Bmim][PF6] [Bmim][PF6] [Bmim][PF6] [Bmim][BF4] [Bmim][Cl]

Amberlyst-15 Amberlyst-15 Amberlyst-15 Amberlyst-15 Amberlyst-15 Amberlyst-15 mont. k-10 H3PW12O40 FeCl3 Cu(OTf)2

12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 10 8 10 10 10 10

reflux reflux reflux reflux reflux 100 100 100 100 100 100 100 100 100 100 80 70 80 80 80 80 80 80

08 10 26 36 44 56 33 27 16 26 N.D. 60 68 74 78 78 66 78 70 09 64 61 40

Amberlyst-15 Amberlyst-15 Amberlyst-15 Amberlyst-15 Amberlyst-15 Amberlyst-15 Amberlyst-15 Amberlyst-15 Amberlyst-15 Amberlyst-15 Amberlyst-15 Amberlyst-15

a

Reaction conditions: 1a (1.1 mmol), 2b (1.0 mmol), reagent (2 equiv), HC(OEt)3 (1 equiv), solvent (2 mL). bGC yield. c10 mol %. dAmberlyst-15 (3 equiv). eAmberlyst-15 (4 equiv). f1a:2a (1.2:1). g1a:2a (1.3:1). hWithout (HC(OEt)3). iWith EtOH (4 equiv). N.D. (not detected).

providing desired product 3b in 56% yield (Table 1, entry 6). The plausible cause for its higher activity can be explained on the basis of physical properties like superior H+ exchange capacity (4.2 mequiv/g) and higher surface area (42 m2/g). Subsequently, when the reaction was performed in the absence of reagent, formation of 3b was not observed (Table 1, entry 11). To further improve the yield of 3b, we have also studied the effect of Amberlyst-15 concentration, and it was observed that yield of 3b increases with increasing the reagent concentration from 2 equiv to 4 equiv (Table 1, entries 12− 13). The ratio of model substrates 1a/2b was also examined, and it was noted that 1.3:1 is an optimum ratio to promote the desired transformation (Table 1, entries 14−15). During temperature study, it was found that 80 °C was the optimum reaction temperature, and a further decrease in temperature led to decrease in the yield of 3b (Table 1, entries 16−17). The influence of reaction time was also investigated, and it was found that the minimum time required for reaction to give a higher yield of 3b is 10 h (Table 1, entries 18−19). However, when the reaction was performed without HC(OEt)3, very low

methoxybenzhydrol (2b) were chosen as model substrates for the synthesis of representative compound 3-(4-methoxyphenyl)-1,3-diphenylpropan-1-one (3b). Initially, we performed the reaction using model substrates and Amberlyst-15 as reagent in various solvents under reflux condition (Table 1, entries 1−5). It was observed that, among the solvents screened, DCE was found to be an effective solvent providing 3b in 44% yield (Table 1, entry 5). Furthermore, when the reaction was performed in ionic liquid [Bmim][PF6] as a reaction medium, a significant increase in the yield of 3b was noted, i.e., 56% (Table 1, entry 6). This is possibly because of enhancement in the acidity of Amberlyst-15 due to the ionic liquid. As [Bmim][PF6] provided higher yield of 3b, it was used for further studies. We observed that the reaction was more favorable in ionic liquid, but the self-coupled dimeric ether of alcohol was observed as a side product. Next, various Bronsted and Lewis acidic reagents such as montmorillonite K-10, H3PW12O40, FeCl3, and Cu(OTf)2 were also studied (Table 1, entries 7−10). Out of the reagents screened, Amberlyst-15 was found to be the best as compared to that of other reagents, thus B

DOI: 10.1021/acssuschemeng.5b01282 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering Table 2. Substrate Study for α-Alkylation Reactions of Ketones with Alcoholsa

Reaction conditions: ketones (1.3 mmol), alcohols (1.0 mmol), Amberlyst-15 (4 equiv), [Bmim][PF6] (2 mL), HC(OEt)3 (1 equiv), 80 °C, 10 h. N.D. (not detected), bIsolated yield. a

amount of 3b formation was observed which shows its necessity for the reaction (Table 1, entry 20). The use of triethyl orthoformate helps in generation of acetal in situ and maintains its good concentration over the progress of reaction. Importantly, when the reaction was performed in the presence of ethanol as a replacement for triethyl orthoformate, the desired product 3b was obtained in 64% yield (Table 1, entry 21). Furthermore, when reaction was performed in other ionic liquids such as [Bmim][BF4] and [Bmim][Cl] as reaction media, 3b was obtained in 61% and 40% yield, respectively (Table 1, entries 22−23). From these screening studies, the optimal reaction condition for the synthesis of 3b is 1a (1.3

mmol), 2b (1.0 mmol), Amberlyst-15 (4 equiv), [Bmim][PF6] IL (2 mL), and HC(OEt)3 (1 equiv), 80 °C, 10 h. With the optimized reaction condition in hand, the utility of the developed protocol was explored with a range of ketones and alcohols for the synthesis of various α-alkylated carbonyl compounds (Table 2). The reaction of 1a with 2a provided the desired product 1,3,3-triphenylpropan-1-one (3a) in 64% yield (Table 2, entry 1). The reactions of 1a with benzhydrols having electron donating (−OMe) and chloro substituents gave the desired products 3b and 3c in 75% and 66% yields, respectively (Table 2, entries 2−3). Furthermore, it was found that sterically hindered o-Me-benzhydrol (2d) also worked well providing the C

DOI: 10.1021/acssuschemeng.5b01282 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

The reusability of the catalyst is a significant advantage particularly for the commercial application of the protocol. Hence, in this work, the reuse of the reagent and reaction medium was investigated using the model reaction (Figure 1). It was found that Amberlyst-15/[Bmim][PF6] was utilized four times without much loss in activity. To show the synthetic utility of this protocol, scale-up reactions were carried out with the model reaction on a 5.0 mmol scale, and it was found that the catalyst was also effective and provided the corresponding product 3b in 66% yield. According to the experimental result and previous reports,40,41 a possible mechanism is proposed, as illustrated in Scheme 2. Initially, alcohol 1 in the presence of acidic catalyst is rapidly converted into self-coupled dimeric ether 7. Intermediate 7 was isolated and characterized by GC−MS. Subsequently, the dimeric ether 7 gets activated under the influence of Amberlyst-15/[Bmim][PF6] catalytic system and generates carbocation 6. Next, the acetal 4 was formed in situ from ketone which is in equilibrium with ethyl vinyl ether 5. Finally, alkylation of 5 with the carbocation 6 results in desired product 3. At this stage, another possible pathway cannot be fully ruled out, which involves generation of carbocation 6 directly from alcohol 1 under the influence of Amberlyst-15/ [Bmim][PF6]. Next, the alkylation of 5 with 6 provides corresponding product 3. Subsequently, we have also carried out the controlled experiment by reaction of intermediate 7 with 2 under optimized reaction conditions, and it was noticed that reaction offered 3 in good yield.

Figure 1. Catalyst recyclability study. Reaction conditions: 1a (1.3 mmol), 2b (1.0 mmol), Amberlyst-15 (4 equiv), [Bmim][PF6] (2 mL), HC(OEt)3 (1 equiv), 80 °C, 10 h. GC yield.

corresponding 1,3-diphenyl-3-(o-tolyl)propan-1-one (3d) in 69% yield (Table 2, entry 4). Next, 1-phenylethanol and their derivatives with electron donating (−Me, −OMe) and halo (−Cl, −Br) substituents also furnished the desired products 3e−i in good yields (Table 2, entries 5−9). Subsequently, we have investigated the reaction of various acetophenone derivatives with 2a having (−Me, −OMe, −N(CH3)2, −Cl) substituents which furnished the corresponding products 3j−m in good yields (Table 2, entries 10−13). Next, we screened the aliphatic ketone 1f which provided the corresponding product 3n in 26% yield (Table 2, entry 14). However, the reaction of 2a with cyclohexanone provided the desired product 3o in a trace amount (Table 2, entry 15). Furthermore, the reaction of electron withdrawing ketone 1h with 2a (Table 2, entry 16) and the reaction of aliphatic as well as allylic alcohol with 1a did not furnish the desired products under the optimized reaction conditions. This can be attributed to low reactivity (Table 2, entries 17−18).

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CONCLUSIONS In summary, we have developed a metal-free, inexpensive, and practical method for the direct α-alkylation of acetophenones with benzhydrols as well as 1-phenylethanols using Amberlyst15/ionic liquid. The synthetic protocol proceeds in an atom-

Scheme 2. Plausible Mechanism for α-Alkylation Reaction of Ketone with Alcohol

D

DOI: 10.1021/acssuschemeng.5b01282 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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and step-economic fashion to construct a C−C bond in which water is the only byproduct. In addition, the catalytic system is moisture-stable and has greater substrate compatibility and tolerability. The present protocol has advantages in the form of a good yield of products, milder conditions, simple operational procedure, and reusability of the catalyst.

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EXPERIMENTAL SECTION

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ASSOCIATED CONTENT

REFERENCES

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All chemicals and reagents were obtained from firms of repute with their highest purity available and were used without further purification. GC equipped flame ionization detector with a capillary column (Elite-1, 30 m × 0.32 mm × 0.25 μm) was carried out for gas chromatography analysis, with GC−MS-QP 2010 instrument (Rtx-17, 30 m × 25 mm ID, film thickness 0.25 μm df) (column flow 2 mL min‑1, 80 to 240 °C at 10 °C/min rise). Products were purified by flash chromatography on 60−120 mesh silica gels, SiO2. 1H and 13C NMR spectra were recorded at 500 and 100 MHz in CDCl3 as the solvent and with TMS as an internal standard. For this study, ionic liquids were synthesized according to the procedure reported in the literature.55 Typical Experimental Procedure for the Direct α-Alkylation Reactions of Acetophenones with Benzhydrols as Well as 1Phenylethanols. To a well-stirred mixture of Amberlyst-15 [H+ exchange capacity (4.2 mequiv/g) and high surface area (42 m2/g)] (4 equiv) in [Bmim][PF6] (2 mL) were added HC(OEt)3 (1 equiv), 1a (1.3 mmol), and 2b (1 mmol). The reaction mixture was stirred at 80 °C for 10 h, and the progress of the reaction was monitored by GC/ TLC. After the completion of reaction, it was cooled to room temperature, and 5 mL of di-isopropyl ether was added with vigorously shaking. The product was extracted in the ether phase, and the extraction procedure (3 × 4 mL) was repeated. The combined organic layer was dried over anhydrous Na2SO4, evaporated under reduced pressure. The residue obtained was purified by column chromatography [silica gel, 60−120 mesh; PE:EA] to give the corresponding product 3b in 75% yield. The structure of the product was confirmed by GC−MS, 1H NMR, and 13C NMR spectroscopic techniques. The purity of the compound was determined by GC−MS analysis. After extraction, the reaction vessel containing the recovered Amberlyst-15/ [Bmim][PF6] was dried in vacuo for an hour and then charged with 1a and 2b directly for subsequent cycles.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b01282. Copies of 1H, 13C NMR and HR-MS data of the products (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; bm.bhanage@ictmumbai. edu.in. Phone: +91 2233612603. Fax: +91 2233611020. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The author (K.V.W.) is greatly thankful to the UGC (University Grant Commission, India) for providing the Senior Research Fellowship (SRF). We are thankful to Rahul B. Yewale from Central Instrumentation Facility, Savitribai Phule Pune University (formerly known as Pune University), for providing HR-MS analysis of products. E

DOI: 10.1021/acssuschemeng.5b01282 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acssuschemeng.5b01282 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX