Bu₄NI-catalyzed construction of tert-butyl peresters from alcohols

Abstract

A new method for the synthesis of tert-butyl peresters directly from available alcohols catalyzed by Bu₄NI at room temperature in an aqueous system was developed. Additionally, allylic esters could also be obtained by combing this method and Kharasch–Sosnovsky reaction via a two-step one-pot procedure.

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The synthesis of tert-butyl peresters is an important task to the research community due to their importance as a core structure in organic chemistry.¹ However, methods for the synthesis of tert-butyl peresters are limited. The traditional method for the synthesis of tert-butyl peresters relied on the reaction of a carboxylic acid and its derivative with tert-butyl hydroperoxide.² In 2011, an alternative method for the synthesis of tert-butyl peresters from aldehydes was disclosed by Wan et al.³ However, versatile and practical methods for the synthesis of tert-butyl peresters are still desirable.

Tetrabutylammonium iodide (TBAI) in combination with tert-butyl hydroperoxide (TBHP) has been recognized as high effective and environmentally begin catalyst in most coupling and radical reactions in recent years.⁴ For example, construction of C–O,⁵ C–N,⁶ C–C^7,4f bonds through cross-dehydrogenation coupling (CDC) reaction catalysed by TBAI have been well established. In continuation of our interests in green chemistry,⁸ herein, we disclose the synthesis of tert-butyl peresters from the reaction of alcohols with TBHP using Bu₄NI as the catalyst.

At the beginning of our studies, a model reaction of phenylmethanol and TBHP was chosen for optimization of the reaction conditions. It was found that the solvent can greatly effect the yield of the reaction. When water was used as solvent, the highest yield of the desired product was obtained (Table 1, entry 1). High yield was also obtained when DMF was used (Table 1, entry 2). However, other solvents such as DMSO, toluene, CH₃OH etc. showed lower yields (Table 1, entries 3–6). Further study indicated that other catalysts including KI, CuI, FeCl₂ etc. lead to relatively low yields (Table 1, entries 7–14). When the reaction time was reduced to 12 h, a lower yield of the product was obtained (Table 1, entry 15). However, prolonging the reaction time showed almost the same yield (Table 1, entry 16). The effect of the temperature on the reaction was also investigated. Decreasing the reaction temperature to RT led to a slightly lower yield (Table 1, entry 17). But there was no significant changes on the yield when the reaction temperature was elevated to 60 °C (Table 1, entry 18).

Table 1 Optimization of the reaction conditions^a

Entry	Catalysts	Solvent	t (h)	T (°C)	Yield (%)
a Reaction conditions: 1a (0.5 mmol), 2 (2.5 mmol), TBAI (0.25 mmol), solvent (2 mL), 16 h. Isolated yields. EG: ethylene glycol.
1	Bu4NI	H2O	16	40	98
2	Bu4NI	DMF	16	40	83
3	Bu4NI	DMSO	16	40	40
4	Bu4NI	CH3OH	16	40	39
5	Bu4NI	Toluene	16	40	64
6	Bu4NI	EG	16	40	50
7	KI	H2O	16	40	68
8	CuI	H2O	16	40	67
9	CuCl2	H2O	16	40	30
10	FeCl2	H2O	16	40	32
11	FeCl3	H2O	16	40	31
12	I2	H2O	16	40	40
13	Bu4NCl	H2O	16	40	35
14	CuBr	H2O	16	40	28
15	Bu4NI	H2O	12	40	80
16	Bu4NI	H2O	24	40	97
17	Bu4NI	H2O	16	RT	95
18	Bu4NI	H2O	16	60	97

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After completion of the search for the optimized reaction conditions, we chose a variety of structurally diverse phenylmethanol possessing a wide range of functional groups to understand the scope and generality of the reaction. It should be noted that a variety of functional groups, including methyl, ether, sulfide, halide and ester ran well in this reaction. Because of smaller hindrance of 4-substituted phenylmethanol and 3-substituted phenylmethanol compared with its analogues 2-substituted phenylmethanol, 4-substituted phenylmethanol and 3-substituted phenylmethanol gave a higher yield than that of 2-substituted phenylmethanol (Table 2, 3d–3l). Additionally, naphthalen-1-ylmethanol and naphthalen-2-ylmethanol were also good substrates in this reaction affording the desired products in high yield (Table 2, 3p, 3r). Heteroarenes, thiophen-2-ylmethanol could also providing the corresponding product in high yields (Table 2 and 3q). It is interesting to note that when aliphatic alcohols 2-(4-methoxyphenyl)ethanol was subjected to this reaction, tert-butyl 4-methoxybenzoperoxoate (3s) was obtained in 36% yield (Scheme 1). A decarbonylation process may occurred in this reaction. However, other aliphatic alcohols such as 2-(4-chlorophenyl)ethanol, 2-phenylethanol were not suitable for this reaction. The same excellent results were also obtained when this method was performed at a gram scale using (4-chlorophenyl)methanol as substrate (Scheme 2). This example clearly demonstrates the preparative utility of this newly developed method.

Table 2 Synthesis of tert-butyl peresters catalysed by Bu₄NI^a

a Reaction conditions: 1 (0.5 mmol), 2 (2.5 mmol), Bu₄NI (20 mol%), water (2 mL), RT, 16 h. Isolated yield.

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Scheme 1 Reactions between aliphatic alcohol 2-(4-methoxyphenyl)ethanol and TBHP under standard conditions.

Scheme 2 A gram-scale preparation of cyclohex-2-en-1-yl 4-chlorobenzoate via Kharasch–Sosnovsky reaction.

To investigate the synthetic application of the methodology in organic synthesis. The Kharasch–Sosnovsky reaction between the tert-butyl peresters and cyclohexene was conducted in two-step one-pot process. As it was shown in Table 3, both electron-withdrawing groups (F, Br, Cl) (Table 3, 4a, 4b, 4g) and electron-donating groups (CH₃S, PhO) (Table 3, 4c, 4e) are all well tolerated in this system. Besides substituted phenylmethanol, thiophen-2-ylmethanol and naphthalen-2-ylmethanol (Table 3, 4d, 4f) also worked well in this system.

Table 3 Synthesis of allylic ester in two-step one-pot process^a

Entry	Substrate 1	Product 4	Yield (%)
a Step 1: 1 (0.5 mmol), 2 (2.5 mmol), Bu4NI (20 mol%), CH2Cl2 (2 mL), RT, 16 h; step 2: CuBr (5 mol%), cyclohexene (10 equiv.), 40 °C for 36 h.
1			80
2			76
3			73
4			85
5			79
6			60
7			82

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In order to elucidate the mechanism of this reaction, control experimental was carried out. When radical scavengers like TEMPO was added to the reaction, the yield of the product was decreased dramatically (Scheme 3). This suggested that the transformation probably proceeded through a radical progress.

Scheme 3 Investigation of the reaction mechanism.

According to the above results as well as the known literature,^3,5 a plausible reaction mechanism of the reaction which is similar with the catalytic cycle proposed by Wan etc.³ is proposed in Scheme 4. Firstly, the tert-butoxyl B and tert-butylperoxy A radicals were generated from the reaction between TBHP and TBAI. Then aldehyde D which is generated the oxidation of alcohol C reacted with tert-butoxyl B to afford the acyl radical E. Finally, the desired perester were formed from the reaction between the acyl radical E and the tert-butylperoxy A.

Scheme 4 Possible reaction pathways.

Conclusions

In summary, we have discovered a new method for the synthesis of tert-butyl peresters catalysed by TBAI directly from commercially available and low-cost alcohols and TBHP at room temperature in aqueous system. This method and it combination with Kharasch–Sosnovsky reaction have good potential in synthetic chemistry. Mechanism investigation indicated that a radical process probably included. The further investigation of the reaction mechanism and its applications are underway in our lab.

Acknowledgements

Financial support from the National Natural Science Foundation of China (21402103), the China Postdoctoral Science Foundation (150030), the Scientific Research Foundation of Shandong Province Outstanding Young Scientist Award (BS2013YY024) and the research fund of Qingdao Agricultural University's High-level Person (631303) were gratefully acknowledged.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra27500h

‡ These authors contributed equally to this article.

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