Efficient one pot synthesis of aromatic nitriles from aromatic aldehydes using ammonia water and iodine in ionic liquid

Luo Huimou, Li Yiqun
(Department of Chemistry, Jinan University, Guangzhou 510632, China)

Received Oct.16, 2004; Supported by the National Natural Science Foundation of China (20272018) and the Guangdong Natural Science Foundation (021166, 04010458)

Abstract Treatment of aromatic aldehydes with iodine and ammonia water using room temperature ionic liquids 1-butyl-3-methylimidazolium tetrafluoroborate at room temperature afford the corresponding nitriles in high yields. The notable advantages of this protocol are such as no need of catalyst, mild conditions, simple operation, short reaction time, high yields and recycling of the ionic liquid.

The remarkable synthetic properties of the nitrile group have ensured long standing studies of their utilization in organic synthesis[1]. There are only a few methods known for the conversion of aldehydes to nitriles in one-pot procedure[2]. Unfortunately, most of these methods suffer from serious drawbacks which include use of hazardous (selenium dioxide)[3] and expensive (hydroxylamine o-sulfonic acid)[4], (o-2,4-dinitrophenylhydroxylamine)[5] or commercially nonavailable reagents, long reaction time, low yields, drastic reaction conditions, and tedious workup procedure. Therefore, there is a need for the development of protocols using readily available and safer reagents which lead to high yields of nitrile compounds. Recently, NaN/AlCl[6], ClSi(N[7], HNOHHCl/phathalic anhydride/Et[8], I/ammonia/THF[9], Graphite/HNOHHCl/MeSOCl[10] and HNOHHCl/dry Al/MeSOCl[11], etc. have been reported as the new protocol for this one-pot conversion. Synthetic chemists continue to explore new methods to carry out the direct synthesis of nitriles from aldehydes.
Room temperature ionic liquids (RTILs) have aroused increasing interest for their promising role as alternative media in synthesis, separation, and electrochemistry as a result of their unique chemical and physical properties[12]. RTILs can dissolve a wide spectrum of organic, organometallic, and inorganic compounds. Also, they have no detectable vapor pressure and are relatively thermal stable. So, there is no loss of solvent through evaporation with RTILs. This will avoid environmental and safety problems due to volatilization, as is the case in traditional organic solvents. Therefore, they are proposed as novel solvent systems to replace traditional solvents that are generally toxic, flammable, and volatile. RTILs are regarded to have the potential to be alternative reaction media for “Green chemistry”[13]
We herein report the use of 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim]BF) ionic liquid for the synthesis of nitriles from aromatic aldehydes with iodine and ammonia water under mild conditions (Scheme 1). And ionic liquids could be recovered and reused for more than five times without losing any activity.

Scheme 1

    We found that aromatic aldehydes were readily converted to give the corresponding nitriles in good yields. The results obtained were compiled in Table 1.

Table 1 Conversion of aldehydes into nitriles using [Bmim]BF

Products were characterized by their melting points, IR spectra according to the reported literature, known samples obtained from the commercially available or prepared in current method.
Yields refer to pure isolated products.
Yield in parenthesis obtained from the ionic liquid in the case of the forth run.

Typical procedure for synthesis of nitriles in the presence of iodine and ammonia water in [Bmim]BFIonic liquid [Bmim]BF (5mL), ammonia water (10mL) and aromatic aldehyde (2.5mmol) were previously charged into the 25 ml round-bottom flask equipped with a magnetic stirrer. Under vigorous stirring, iodine (2.75 mmol) was added slowly in portion during 60 minutes. The mixture was stirred vigorously at room temperature for the appropriate time (Table 1). The dark solution became colorless(or light gray in some cases) after stirring for 10-25 min per portion, an indication that the reaction was complete. After completion of the reaction, as indicated by TLC, the reaction mixture was washed with diethyl ether (3×10 mL). The combined ether extracts were concentrated in vacuo after quenched with saturated NaHSO solution. The products were confirmed by melt point, IR spectra. The ionic liquid was recovered by extracting the ammonia water phase with dichloromethane and used in subsequent runs after removing dichloromethane.