Introduction

Carbon–carbon bond formation is the essence of organic synthesis. The development of general strategies for the formation of carbon–carbon and carbon–heteroatom bonds is of great interest to scientists working in the fields of medicinal chemistry, agrochemical production, and natural product synthesis. Common methods to elongate the molecular skeleton involve additions of organometallic reagents to carbonyl compounds1. Among these methods, the most notable reactions involve the use of Grignard or alkyllithium reagents to react with ketones for the generation of the corresponding alcohols2,3,4. Nevertheless, competitive reduction products and aldol adducts are often produced5,6. Improved results for the alkylation of carbonyl compounds with Grignard reagents can be obtained when metal salts are added1,5, including CeCl3, LiCl, LnCl3·2LiCl, and ZnCl2, among others. Li et al. reported the environmentally friendly Barbier–Grignard type alkylation and arylation of carbonyl compounds in water using metal catalysts such as Rh(acac)(CO)2/Zn7, Zn/CuI8, and RuCl3/In(OAc)39. Capriati et al.10,11,12 applied the Barbier–Grignard type reactions to the synthesis of substituted tetrahydrofurans from γ-haloketones. The same research group explored the lithium-induced alkylative ring opening of tetrahydrofurans, which leads to the synthesis of primary alcohols12,13.

Several reliable reactions exist for the preparation of epoxides, which are versatile intermediates in organic synthesis14,15. Often alkenes and carbonyl compounds serve as the starting materials. A different approach to obtain epoxides with an α-ester functionality (i.e., glycidic ester) is through the Darzens reaction16, in which an α-haloester reacts with an aldehyde or a ketone in the presence of a base. Thus C–C and C–O bond formations take place in sequence to give an α, β-epoxy ester. This reaction has a limited scope due to it being a time-consuming process with relative low yields of the products17. Recently, a phase-transfer catalyzed Darzens reaction has been reported18. Technical problems associated with this reaction include the requirement of separation of the catalysts from the reaction mixture and their subsequent reuse or disposal. This gives rise to negative environmental impacts. Accordingly, it is in need of a new method that can overcome these problems.

Alkali metals on inert supports19 or as solutions in amines20 have been widely applied for reductions. Notable examples include the Benkeser reaction, the Birch reduction21, the calcium reduction22,23,24, and the Wurtz reaction20. Silica gel is an ideal support for heterogeneous catalysts due to its excellent thermal and chemical stability. Its use is also cost-effective. Recently, solid alkali metal-based reductions in organic solvents have become easier than before with the finding that these metals can be made significantly less pyrophoric by thermal absorption into nanostructured silica to form the metal–silica gel reagents19. Such an approach provides a green chemistry pathway by reducing the dangers associated with the handling of naked alkali metals19. Recently, applications of alkali metals and their salts (Na, Na2K, and K2Na) in silica gel are demonstrated in the Birch reduction21, desulfurization19, desulfonation25, and ester reduction26.

By considering the advantages associated with the metal-silica gel reagents, we developed new C–C bond formation methods involving the use of sodium metal impregnated with silica gel (i.e., Na@SiO2). This free-flowing powder can be handled easily in an open atmosphere. Its applications enabled the effective alkylation and arylation of carbonyl compounds 1 with organohalides 2 to give alcohols 3 under mild conditions (Fig. 1). It also assisted the epoxide formation by the reaction of α-haloesters 4 with aldehydes or ketones 1 with high efficiency. To the best of our knowledge, these are the first Na@SiO2-mediated syntheses of alcohols and epoxides.

Figure 1
figure 1

Application of Na@SiO2 in the C–C and C–O bond formations.

Carbonyl compound 1 underwent C–C and C–O bond formations with organohalides 2 and α-haloesters 4 to give alcohols 3 and epoxides 5, respectively.

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Results

Reaction Optimization and Scope

Our initial attempts included the reaction of cyclopropyl phenyl ketone (1f, 1.0 equiv) with 4-bromotoluene (2b, 1.2 equiv) in the presence of Na@SiO2 (10 equiv) in a mixture of THF and methanol (1:1, Fig. 2). The desired alkylated product 3f was obtained in 19% yield along with the by-product alcohol 6 (28%) through reduction at 25 °C for 2.0 h. The major product was the self-coupled biphenyl 7 (47% yield). The reaction was found to be dependent chiefly on the equivalent of Na@SiO2 used and whether the solvent was aprotic. Increased amount of the desired adduct 3f to 48% yield was obtained when 5.0 equivalents of Na@SiO2 were used in THF without any co-solvent. The best result was obtained when the reaction was carried out in the presence of 1.4 equivalents of Na@SiO2 in THF at 25 °C for 2.0 h. To our satisfaction, the desired tertiary alcohol 3f was produced in 89% yield. This newly developed reaction was found tolerable with moisture as shown by the results in the last row of Fig. 2. In a control experiment, arylation of phenone 1f was performed by addition of ~2.0% of water in THF. The product distribution was found almost the same as that with dry THF.

Figure 2
figure 2

Optimization of the alkylation reaction.

Reaction conditions were optimized by use of THF, methanol, and water as solvents with different molar equivalents of Na@SiO2. The best results were obtained in the presence of 1.4 equivalents of Na@SiO2 in THF.

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With the optimized conditions in hand, the scope and applicability of this new C–C bond formation was explored. Different ketones and aldehydes were found to be adaptable to the new reaction, which included alkanones 1a and 1b, four- to six-membered cycloalkanones 1c–e (entries 1–5 of Table 1), and phenones 1f–h (entries 6–8). Aryl aldehydes 1i and 1j (entries 9 and 10) were also transformed in success to the corresponding secondary alcohols 3i and 3j, respectively. Naturally-occurring steroid pregnenolone 1k (entry 11) underwent smooth alkylation with n-butyl bromide 2d to afford the corresponding alcohol 3k in 78% yield. Furthermore, treatment of the N-protected alkaloid, indole-3-carboxyaldehyde 1l (entry 12), with allyl bromide 2e in presence of Na@SiO2 produced the homoallylic alcohol 3l in 80% yield through a 1,2-addition pathway.

Table 1 Reaction of ketones, aldehydes, and Schiff bases with organohalides in the presence of Na@SiO2 to give the corresponding adducts by formation of C–C bonds.
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Schiff bases are often inert toward alkylation under mild conditions27. By utilization of the Na@SiO2 reagent, Schiff bases 8a and 8b reacted with allyl bromide (2e) or tert-butyl bromide (2c) to give the amines 9a–c in very good yields (87–90%, entries 13–15 of Table 1). In contrast, when the sterically hindered tert-butyl bromide is employed in the Grignard reaction, the adducts are usually generated in moderate yields28.

The C–C bond formation methods shown in Figs 1 and 3 were adaptable to various organohalides. They included aryl chloride 2a, aryl bromide 2b, sterically hindered bromide 2c, alkyl bromide 2d, and allyl bromide 2e. The corresponding alcohols and amines were obtained in good to excellent yields (77–91%, Table 1).

Figure 3
figure 3

Alkylation of Schiff bases with a sterically hindered bromide.

tert-Butyl bromide was found reactive for alkylation of the Schiff bases 8 in the presence of Na@SiO2.

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Epoxide Formation and Antiviral Activity

Encouraged by the success on the Na@SiO2-mediated formation of alcohols and amines, we expanded its applicability to the Darzens reaction for epoxide synthesis. Treatment of an α-halo ester 4a, lactone 4b, or ketone 4c with aldehydes 1i–o (entries 1–5 of Table 2) in the presence of Na@SiO2 (1.8–2.0 equiv) in THF provided glycidic esters 5i–q in 80–90% yields in 30 min. The moieties attached to the glycidic esters could be substituted phenyl, naphthyl, and benzodioxole groups. In comparison with the results obtained from the standard conditions associated with the Darzens reaction, the yields are much higher (80–91% versus 19–82%) and the reaction time is shorter (30 min versus 16–70 h)29,30 for the newly developed method shown in Fig. 1.

Table 2 Epoxide formation by reaction of aldehydes with α-haloesters or 4-(chloromethyl)coumarins with the aid of Na@SiO2.
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During the epoxide formation, the ester moiety in compounds 4 (see Fig. 1) could be replaced by an α, β-unsaturated lactone moiety. Efficient formation of epoxides 11a–c was accomplished by reaction of aldehydes 1j and 1n with chlorocoumarins 10a–c (1.2 equiv) and Na@SiO2 (1.8–2.0 equiv) at room temperature (see entries 10–12 of Table 2).

Compounds containing a coumarin moiety in connection with benzothiazole, benzoxazole, or imidazopyridine exhibit antiviral activity31,32,33,34. Thus the biological activity of coumarin epoxides 11a–c (see entries 10–12 of Table 2) was evaluated in the hepatitis C virus (HCV) subgenomic replicon system in huh 5-2 cells. All of these three new compounds showed positive results. For the coumarin epoxide 11b, its 50% inhibitory concentration for virus replication (EC50) and host cell growth (CC50) were 0.90 and 91.9 μM, respectively. The SI value (CC50/EC50) was as high as 102. To the best of our knowledge, this compound exhibited the most appealing anti-HCV activities in comparison with all others in the coumarin family.

Discussion

In the Na@SiO2-mediated alkylation reactions, the self-coupled dimer 7 from p-bromotoluene was isolated as the by-product (Fig. 2)35. In a control experiment, this reaction was carried out in the presence of (2,2,6,6-tetramethyl-1-piperidinyl)oxy (TEMPO), which acted as a radical scavenger. Consequently, the desired adduct 3f was not detected at all. The same radical quenching phenomenon was also observed in the examples for the formation of hindered amines as shown in Fig. 3. These results indicate a radical pathway involved when Na@SiO2 was employed to initiate the alkylation.

A plausible mechanism for the glycidic ester formation is presented in Fig. 4. After Na@SiO2 removes the chlorine atom from α-chloroacetates 12, the carboradicals 13 adds to aldehydes 14 to give the radical intermediates 15. It has been reported that metal-silica gel reagents can adsorb substrates36. Adsorption of the alkoxyl radicals 15 to silica gel impregnated with sodium metal could bring the substrates and the electron donors into proximity36. Use of silica gel as support also increases the effective surface area and constrain both the substrates and the metals in pores for decreasing the entropy of activation for electron transfer37. As a consequence, at least six possible isomers 16a–f can be generated as shown in Fig. 4. The isomers 16a and 16c with internal steric hindrance would transform to the thermodynamically more stable conformers 16b; likewise, isomers 16d and 16e would transform to conformers 16f. After a diastereotopic methylene hydrogen in the isomers 16b or 16f is removed by Na@SiO2, an epoxide formation reaction takes place to give primarily the trans epoxides 18 (not the cis epoxide 17) with high diastereoselectivity. The trans configuration of the epoxide 18 originates from the thermodynamically more stable anti conformation between the R and the COOR’ groups in 16b and 16f (cf. the Newman projection of 16f).

Figure 4
figure 4

A plausible mechanistic for formation of glycidic esters from α-chloroacetates and carbonyl compounds in the presence of Na@SiO2.

The epoxide products 18 were generated in the trans form; their formation originates from the thermodynamically more stable anti conformation between the R and the COOR’ groups in the conformers 16b and 16f.

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The mechanism shown in Fig. 4 indicates that abstraction of diastereotopic protons by Na@SiO2 triggers the formation of epoxides. Nevertheless, we did not observe the similar phenomenon for allyl bromide 2e, which also has two diastereotopic protons. Allyl bromide 2e underwent alkylation with carbonyl compound 1l and imine 8a to form secondary alcohol 3l or amine 9c, respectively, instead of an epoxide or aziridine. Therefore, the presence of an electron-withdrawing group (such as –COR and –COOR) in an organohalide is the borderline between the alcohol and epoxide formation.

The Darzens reaction is the anionic condensation of an α-haloester with an aldehyde in the presence of base to produce a glycidic ester. It goes through an intramolecular SN2 attack process to generate an α-chloro-β-oxide as the intermediate. In the newly developed epoxide formation reaction, the same starting materials were used and the base was replaced by Na@SiO2. α-Chloro-β-hydroxy esters were not detected in any reaction mixtures. Therefore, our glycidic ester formation reaction deviates from the conventional Darzens reaction with respect to their reaction mechanisms16.

In conclusion, Na@SiO2 was developed as a powerful reagent to solve one of the most important themes in organic synthesis — carbon to carbon bond formation. In the presence of Na@SiO2, the addition of organohalides to ketones and aldehydes gave the corresponding adducts (i.e., alcohols) in good to excellent yields. The Na@SiO2 reagent also assisted the reaction of α-halocarbonyl compounds with aldehydes in the formation of glycidic esters through an unprecedented radical pathway. In comparison with the myriad other available methods, the four advantages associated with the newly developed reactions include: (1) The desired adducts are often obtained in very good yields. (2) Reactions are completed at room temperature within 2.0 hours. (3) The requirement for anhydrous solvents is unnecessary. (4) The manipulation of the heterogeneous reactions is simple. Application of the environment-benign reagent Na@SiO2 in organic synthesis fits into five of the twelve guidelines of green chemistry38. Furthermore, a new coumarin-containing epoxide was generated by this newly developed method; it possessed high potency and selectivity against the hepatitis C virus.

Methods

The Standard Procedure 1 for the Syntheses of Alcohols and Amines

A reaction flask equipped with a magnetic stirring bar, rubber stopper, and nitrogen balloon was charged with Na@SiO2 (1.3–1.7 equiv). To this reaction mass was added THF (2.0–4.0 mL) via syringe at room temperature. Then an aldehyde, ketone, or imine (1 or 8, 1.0 equiv) was premixed with an organohalide (2, 1.2 equiv). The mixture was diluted with THF (0.50 mL) and the resultant solution was injected into the reaction mass via syringe. After the reaction mixture was stirred at 25 °C for 1.0–3.0 h, the inorganic residue was filtered. The filtrate was concentrated under reduced pressure and then purified by use of column chromatography packed with silica gel and eluented with a mixture of EtOAc and hexanes to give the desired alcohols 3 or amines 9.

The Standard Procedure 2 for the Synthesis of Epoxides

A reaction flask equipped with a magnetic stirring bar, rubber stopper, and nitrogen balloon was charged with Na@SiO2 (1.8–2.0 equiv). To this reaction mass was added THF (1.0–2.0 mL) via syringe at room temperature. Then an aldehyde 1 was premixed with an organohalide 4 or 10 (1.2 equiv). The mixture was diluted with THF (0.50 mL) and the resultant solution was injected into the reaction mass via syringe. After the reaction mixture was stirred at 25 °C for 0.50–2.0 h, the inorganic residue was filtered. The filtrate was concentrated under reduced pressure and then purified by use of column chromatography packed with silica gel and eluented with a mixture of EtOAc and hexanes to give the desired trans-epoxides 5 or 11.

Additional Information

How to cite this article: Kapoor, M. and Hwu, J. R. Na@SiO2-Mediated Addition of Organohalides to Carbonyl Compounds for the Formation of Alcohols and Epoxides. Sci. Rep. 6, 36225; doi: 10.1038/srep36225 (2016).

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