Morpholinium bisulfate [morH][HSO₄]-promoted O, S, and Nacylation at room temperature

Abstract

Morpholinium bisulfate [morH][HSO₄] found to be highly inexpensive, efficient, and reusable catalyst to promote the acylation of phenols, thiols, alcohols, and amines in presence of acetic anhydride under solvent-free condition with excellent yields by stirring at room temperature. Present methodology deals with remarkable features of mild reaction condition, high yielding, shorter reaction time, easy workup, and more importantly, environmentally benign.

Keywords:

• acylation
• morpholinium bisulfate
• solvent-free

Introduction

When used in place of classical organic solvents, room temperature ionic liquids offer a new and environmentally benign approach toward modern synthetic chemistry 1–3. This approach has been successfully applied in several classical organic transformation 4 5. Ionic liquids have interesting advantages, such as extremely low vapor pressure, excellent thermal stability, reusability, and the ability to dissolve many organic and inorganic substrates 6.

Acylation is a fundamental process in organic chemistry, that provides an efficient route for protecting groups during oxidation, peptide coupling, and glycosidation reactions 7 8. Protection of –OH, –SH and –NH₂ group is usually achieved by acylation with acetic anhydride, due to the ease of deprotection 9 10. The various catalysts developed for acylation include nucleophilic agents 11 12 such as DMAP and Bu₃P, Lewis acids such as metal halides 13–15, metal triflates 16–20, metal perchlorates 21–26, ionic liquids 27, and several solid acids, such as nafion-H 28, zeolites 29, clays 30, HBF₄–SiO₂ 31, HClO₄–SiO₂ 32, zirconocene bis(perfluorooctanesulfonate)s 33 , ruthenium(III) chloride 34, and alumina supported MoO₃ 35.

However, industrial applications of some of the above mentioned catalytic systems suffer from limitations, such as non-recoverability, stringent conditions, use of halogenated solvents, and hazardous materials. Also lack of reusability, high catalyst loading, and high cost of some of these reported catalysts determine us think about developing a mild and cost-effective methodology.

As a part of our research work 36–38 to develop economical and environment friendly routes in organic transformation, we have tried to implement [morH][HSO₄] 39 (Figure 1) as a cost-effective and efficient catalyst for the acylation of alcohols, phenols, thiols, and amines at room temperature.

Figure 1.  Morpholinium bisulfate ([morH][HSO₄]).

Results and discussion

Herein, we wish to report the acylation of alcohols, thiols, phenols, and amines using acetic anhydride in the presence of catalytic amount of morpholinium bisulfate by stirring at room temperature (Scheme 1). Preparation of morpholinium bisulfate was found to be highly cost effective 39.

short-legendScheme 1. Acylation of O, Sand N at room temperature

In order to explore the scope of the methodology, a wide range of structurally diverse and functionalized phenols, thiols, alcohols, and amines that underwent acylation with excellent yield in most of the cases were examined. During the experiment various electron-donating and withdrawing substituents were taken into consideration (Table 1). It is noted that [morH][HSO₄] not only accelerates the rate of the reaction, but also increases the yield.

Table 1. Acylation of phenols, alcohols, thiols, and amines using [morH][HSO₄] as a catalyst.

During acylation, halogenated phenols (Table 1, Entries 6, 7, and 12) and nitro-phenols (Table 1, Entries 10 and 11) showed some influence on the resultant time and yield which may be due to electronic effect of substituent. Aliphatic secondary alcohols (Table 1, Entry 16 and 19) require more time than the primary, which could be due to the steric effect exerted on the chain. This catalytic system also works well with tertiary alcohols (Table 1, Entry 20). Same catalytic system also acylates thiols with good yield (Table 1, Entries 13 and 14). Rapid acylation of amines was observed during experiments (Table 1, Entries 21–25). From results, acylation of amino group was more rapid than hydroxy groups, whereas the acylation of thiols was considerably slower than hydroxy groups.

Subsequently, an attempt has been made for the reusability of [morH][HSO₄] considering (Table 1, Entry 2) as model reaction. The yields after the catalyst were found to be from 97 to 94%, which was obtained within time periods ranging from 5 to 12 min after 1–5 runs (Table 2).

Table 2. Recovery and reusability of catalyst.

Run	Time (min)	Yield (%)^a
1	5	97
2	5	96
3	8	96
4	8	95
5	10	94
^aYields were isolated.

From those results, it can be noticed that we have successfully reused the catalyst in subsequent runs and the catalytic activity of catalyst did not show any significant decrease even after five runs.

Experimental

All reagents used were from Merck and Aldrich and used without further purification. IR spectra were recorded on JASCO FT-IR 4100 spectrometer in KBr with absorption in cm^–1. ¹HNMR was measured on Bruker DPX 400-MHz spectrometer. Mass spectra [ES-MS] were recorded on a Water-Micro mass Quattro-II spectrophotometer.

General procedure for acylation

4-Anisidine (1 mmol) was treated with acetic anhydride (1.5 mmol) under neat condition. Morpholinium bisulfate [morH][HSO₄] (0.05 mmol) was added to the reaction mask which was stirred for an appropriate time. The course of the reaction was monitored by TLC. After completion of reaction, the mixture was extracted with ethyl acetate to afford 4-methoxyphenyl acetate which was in full agreement with spectral data.

During acylation of 2-aminothiophenol and ethylene glycol (Table 1, Entry 14, and 17), three equivalent amounts of AC₂O were used and in case of tert-butanol (Table 1, Entry 16), 4.5 equivalent amounts of AC₂O were used.

p-Methoxyphenyl acetate

IR (KBr): 1758 (C = O) cm^–1; ¹H NMR (CDCl₃): δ ppm 2.24 (s, 3H, –COCH₃), 3.72 (s, 3H, –OCH₃), and 6.82–7.03 (m, 4H, Ar–H); MS (m/z): 166 [M⁺] (Table 1, Entry 2).

Ethane-1,2-diyl di-acetate

IR (KBr): 1730 (C = O) cm^–1; ¹H NMR (CDCl₃): δ ppm 2.07 (s, 6H) and 4.25 (s, 4H); MS (m/z): 146 [M⁺] (Table 1, Entry 17).

p-Tolyl-acetamide

IR (KBr): 3301 (–NH) and 1665 (C = O) cm^–1; ¹H NMR (CDCl₃): δ ppm 2.18 (s, 3H, –COCH₃), 2.27 (s, 3H, –CH₃), and 7.01–7.12 (m, 4H, Ar–H). MS (m/z): 149 [M⁺] (Table 1, Entry 24).

Thioacetic acid S-(2-acetylamino-phenyl)ester

IR (KBr): 3320 (–NH), 1730 (S–COCH₃), and 1690 (NH–COCH₃) cm^–1; ¹H NMR (CDCl₃): δ ppm 2.12 (s, 3H, –COCH₃), 2.40 (s, 3H, –COCH₃), 7.11–7.69 (m, 4H, Ar–H), and 7.78 (s, 1H, –NH); MS (m/z): 209 [M⁺] (Table 1, Entry 14)..

t-Butyl acetate

IR (KBr): 1785 (C = O) cm^–1, ¹H NMR (CDCl₃): δ ppm 1.42 (s, 9H, (CH₃)₃), 2.14 (s, 3H, –OCH₃); MS (m/z): 116 [M⁺] (Table 1, Entry 20).

Reusability of catalyst

After completion of reaction, the mixture was extracted with ethyl acetate and placed for a while for the formation of two phases. The aqueous layer of ionic liquid phase was simply separated with a separating funnel, washed with ether, dried under reduced pressure, and directly reused for further analysis.

Conclusion

Introduction of [morH][HSO₄] can be considered as an interesting new alternate route for acylation. The merits of this method includes solvent-free, highly inexpensive, ease of handling, easily recycling, and reuse of the catalyst. Therefore, with increasing “green” concern, the solvent-free conditions employed in the present method will make it environment friendly and useful for industrial applications. Further application of [morH][HSO₄] for other reaction systems are under investigation.

Acknowledgements

The authors are thankful to The Head, Department of Chemistry, Dr. Babasaheb Ambedkar Marathwada University, Aurangabad, India, for providing laboratory facilities.