Hydrocyanation of sulfonylimines using potassium hexacyanoferrate(II) as an eco-friendly cyanide source

ARTICLE

Hydrocyanation of sulfonylimines using potassium hexacyanoferrate(II) as an eco-friendly cyanide source

Zheng Li^*; Rongzhi Li; Huanhuan Zheng; Fei Wen; Hongbo Li; Junjun Yin; Jingya Yang

Key Laboratory of Eco-Environment-Related Polymer Materials for Ministry of Education, College of Chemistry and Chemical Engineering, Northwest Normal University, 730070 Lanzhou, Gansu, P. R. China

ABSTRACT

An efficient and eco-friendly method for hydrocyanation of sulfonylimines via one-pot two-step procedure using potassium hexacyanoferrate(II) as a cyanide source, benzoyl chloride as a promoter, and potassium carbonate as a base is described. This protocol has the features of using nontoxic, nonvolatile and inexpensive cyanide source, high yield, and simple work-up procedure.

Keywords: sulfonylimine, hydrocyanation, potassium hexacyanoferrate(II), Strecker-type reaction, green chemistry

RESUMO

Um método eficiente e ecologicamente amigável de hidrocianação de sulfoniliminas por um procedimento de duas etapas em uma única operação usando hexacianoferrato(II) de potássio como fonte de cianeto, cloreto de benzoíla como promotor e carbonato de potássio como uma base é descrito. Este método tem como característica o uso de fonte de cianeto atóxica, não volátil e barata, alto rendimento e um procedimento simples.

Introduction

The Strecker reaction, nucleophilic addition of cyanide ion to imines, is of great importance to modern organic chemistry as it offers one of the most direct and viable methods for the synthesis of α-aminonitriles.¹α-Aminonitriles are significantly important intermediates for the synthesis of a wide variety of amino acids, amides, diamines, and nitrogen-containing heterocycles.² The recent advance of Strecker reaction has been reviewed by Feng and coworkers.³ The hydrocyanation of sulfonylimines to synthesize α-sulfonylimidonitriles is one of the most important Strecker-type reactions. Ooi⁴ reported the Strecker reaction of sulfonylimines using aqueous potassium cyanide as a cyanide source. Kim⁵ studied the hydrocyanation of sulfonylimines using ethyl cyanoformate as a cyanide source. Nakamura,⁶ Feng⁷ and Kantam⁸ investigated the Strecker-type reaction of sulfonylimines using trimethylsilane cyanide as a cyanide source. In addition, the reported Strecker reactions also utilize HCN,⁹ Zn(CN)₂,¹⁰ (EtO)₂P(O)CN,¹¹ Et₂AlCN,¹² Bu₃SnCN,¹³ MeCOCN,¹⁴ and acetone cyanohydrin¹⁵ as cyanide sources. However, some problems including toxicity, volatility or high cost are still associated with these cyanating agents. To overcome these problems, attention has been given to the development of alternative cyanide sources that are relative cheap, less toxic, and easier to handle.

Potassium hexacyanoferrate(II), K₄[Fe(CN)₆], is mainly used as carburizing agent in the iron and steel industry. And it is also used in the food industry for metal precipitation. In addition, it has been described as an anti-agglutinating auxiliary for table salt (NaCl). K₄[Fe(CN)₆] is a by-product of the coal chemical industry and commercially available on a ton scale, and it is even cheaper than KCN. Recently, K₄[Fe(CN)₆] has been used as a cyanide source for some substitution reactions to synthesize benzonitriles,¹⁶ aroyl cyanides,¹⁷ benzyl cyanides,¹⁸ cinnamonitriles,¹⁹ dihaloacrylonitriles²⁰ and cyano substituted heterocycles.²¹ Our recent research interests focused on the cyanation of unsaturated compounds by nucleophilic addition reactions using K₄[Fe(CN)₆] as an eco-friendly cyanide source, which included the cyanation of aldehydes and ketones to cyanohydrins,²² the cyanation of aldimines and ketimines to α-aminonitriles,²³ and the cyanation of α,β-unsaturated ketones to β-cyano ketones.²⁴ In this work, we report an efficient method for the hydrocyanation of sulfonylimines to α-sulfonylimidonitriles using K₄[Fe(CN)₆] as an eco-friendly cyanide source.

Results and Discussion

Initially, N-benzylidene-4-methylbenzenesulfonamide (1a) was selected as a substrate to investigate the feasibility of hydrocyanation of sulfonylimines using K₄[Fe(CN)₆] as an eco-friendly cyanide source (Scheme 1). The reaction was attempted under different conditions such as using Lewis acids, Lewis bases, and organometallic compounds as catalysts at different temperature in various solvents. However, no product could be obtained for this reaction, the possible reason is that K₄[Fe(CN)₆] is too stable to release cyanide ions in the studied conditions. In the later study, it was found that benzoyl chloride could efficiently promote the reaction to yield corresponding hydrocyanation product through an intermediate, benzoyl cyanide, which could be isolated and identified from the reaction system. Actually, the hydrocyanation reaction for 1a directly utilized benzoyl cyanide as a cyanating agent could effectively give the same product by a similar way in high yield. This further demonstrated the fact of benzoyl cyanide as a reaction intermediate. It was also found that only 0.2 equiv of K₄[Fe(CN)₆] was needed for 1 equiv of 1a, which indicated that six CN ^- of K₄[Fe(CN)₆] could be fully utilized in this reaction. In addition, no by-products were observed in this reaction, which implied the high chemoselectivity of the reaction.

It was found that bases played a key role in the studied reaction. The reaction gave very low yield in the absence of a base (Table 1, entry 1). However, the reaction could be proceeded smoothly in the presence of some bases, such as Et₃N, DMAP, K₂CO₃, KOH, and NaOH (Table 1, entries 2-6). Among them, K₂CO₃ gave the highest yield within the shortest reaction time (Table 1, entry 4).

The solvents also played an important role in the hydrocyanation of 1a using K₄[Fe(CN)₆] as an eco-friendly cyanide source, benzoyl chloride as a promoter, and potassium carbonate as a base (Table 2). A set of anhydrous solvents was tested for the reaction. Polar solvents such as MeOH, EtOH, MeCN, THF, and CH₂Cl₂ gave the desired product 2a in moderate to high yields (Table 2, entries 1-5). Especially the reaction in EtOH afforded 2a in the highest yield (Table 2, entry 2). However, it was found that the reaction in polar aprotic solvents such as DMSO and DMF gave unidentified by-products (Table 2, entries 6-7). In addition, no product was observed in nonpolar solvents such as toluene and n-hexane.

Based on the above promising findings, a series of sulfonylimines formed from aldehydes and sulfonamides were examined for the hydrocyanation in EtOH using K₄[Fe(CN)₆] as a cyanating agent, benzoyl chloride as a promoter, and K₂CO₃ as a base (Table 3, Scheme 1). It was found that sulfonylimines including electron-donating groups on aromatic rings of R² gave the corresponding products in very high yields (Table 3, entries 1-5). In contrast, sulfonylimines including electron-withdrawing groups on aromatic rings of R² gave slightly lower yields (Table 3, entries 6-10). In addition, ortho-substituted sulfonylimines exhibited large sterical hindrance and gave lower yields than the para-ones (Table 3, entries 7-8). Sulfonylimines with aliphatic R², such as ethyl and isopropyl, could also participate in the hydrocyanation reactions to give the corresponding products in high yields although they needed more reaction time (Table 3, entries 12-13). Sulfonylimines including heterocycle, such as furan-2-yl, could also smoothly react with K₄[Fe(CN)₆] (Table 3, entry 11). R¹ groups on sulfonylimines had no obvious effect on the yields of products. In addition, the sulfonylimines formed from ketones and p-toluenesulfonamide were also attempted for the hydrocyanation reactions using K₄[Fe(CN)₆]. However, very low isolated yields were obtained under the similar conditions. The possible reason is the large sterical hindrance of the substrates.

A plausible mechanism for the hydrocyanation of sulfonylimines to synthesize α-sulfonylimidonitriles using K₄[Fe(CN)₆] as a cyanide source, according to Hunig's review,²⁵ is shown in Scheme 2. Firstly K₄[Fe(CN)₆] reacts with benzoyl chloride to form benzoyl cyanide as an intermediate. Then benzoyl cyanide is attacked by ethanol in the presence of potassium carbonate to yield nucleophilic additional intermediate A. Intermediate A undergoes the loss of ethyl benzoate to produce hydrogen cyanide in situ. Then the nucleophilic additions of hydrogen cyanide to sulfonylimines 1 yield α-sulfonylimidonitriles 2 as products.

Conclusions

In summary, an efficient method has been developed for the hydrocyanation of sulfonylimines to synthesize α-sulfonylimidonitriles by one-pot two-step procedure using K₄[Fe(CN)]₆ as an original eco-friendly cyanide source, benzoyl chloride as a promoter, and potassium carbonate as a base. The protocol compared with literature methods has advantages of using non-toxic, nonvolatile and inexpensive cyanide source, high yield, and simple work-up procedure.

Experimental

IR spectra were recorded using KBr pellets on an Alpha Centauri FTIR spectrophotometer. ¹H NMR and ¹³C NMR spectra were recorded on a Mercury-400BB instrument using CDCl₃ as solvent and Me₄Si as internal standard. Melting points were observed in an electrothermal melting point apparatus. Potassium hexacyanoferrate(II) was dried at 80 ^oC under vacuum for 24 h and finely powdered prior to use. Sulfonylimines were generally prepared according to modified literature methods by refluxing of 4-methylbenzenesulfonamide or benzenesulfonamide with various aldehydes in toluene using anhydrous magnesium sulfate as a dehydrating agent and acetic acid as a catalyst.²⁹

The general procedure for the hydrocyanation of sulfonylimines

The mixture of K₄[Fe(CN)₆] (0.1 mmol) and benzoyl chloride (0.6 mmol) was stirred at 160 ^oC for 3 h, then the reaction system was cooled to room temperature, and sulfonylimines (0.5 mmol) and potassium carbonate (0.1 mmol) in ethanol (8 mL) were added. The mixture was further stirred at room temperature for appropriate time indicated in Table 3. After completion of the reaction monitored by TLC, the resulting mixture was filtered to remove the solids. The liquor was concentrated, and the residues were isolated by column chromatography using petroleum ether and ethyl acetate (6:1) as eluent to give pure product.

Acknowledgements

The authors thank the National Natural Science Foundation of China (21162024) and Provincial Natural Science Foundation of Gansu (1107RJZA189) for the financial support of this work.

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Submitted: July 5, 2013

Published online: September 4, 2013

Supplementary Information

Data spectra and spectra of synthesized compounds are available in pdf: [Supplementary material]

Publication Dates

• Publication in this collection
  19 Nov 2013
• Date of issue
  Nov 2013

History

• Received
  05 July 2013
• Accepted
  04 Sept 2013