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INTERFINE CHEMICALS > Chemicals > Ionic Liquids > Applications

Solvent-free microwave-assisted synthesis of ionic liquids and their application as catalysts for benzoin condensation reaction

Audrey Aupoix, Giang Vo-Thanh* - Laboratoire de Catalyse Moléculaire, ICMMO,Université Paris-Sud 11, 91405 Orsay Cedex, FRANCE. E-mail: audrey.aupoix@icmo.u-psud.fr

Objectives: To develop eco-friendly chemical processing for the synthesis of ionic liquids using solvent-free conditions under  microwave activation.To provide a highly efficient procedure for benzoin synthesis using ionic liquid both as solvent and catalyst.

Solvent-Free Microwave-Assisted Synthesis of Imidazolium and Pyridinium-based Ionic Liquids

Classical synthesis > Disadvantages : slow (2-3 days), large excess of RX, halide contamination, conventional solvent (DMF, acetonitrile, acetone,…).
Microwave synthesis > One-pot two-step microwave procedure : cleaner and greener.

Microwave synthesis of 1-alkyl-3-methylimidazolium

8

 

R Time (min) Temperature (°C) Yield (%)
N-alkylation Metathesis N-alkylation Metathesis X=OTf X=NTf2 X=PF6 X=BF4
C4H9 8 15 90 100 98 98 96 99
C8H17 8 10 120 99 95 97 99
C10H21 8 10 130 88 89 90 92
C12H25 8 10 140 88 90 91 94

R Time (min) Temperature (°C) Yield (%)
N-alkylation Metathesis N-alkylation Metathesis X=OTf
C4H9 10 10 130 120 99
C8H17 10 10 130 130 91
C10H21 10 10 140 120 96

Advantages > slight excess of alkyl halide,  solvent-free synthesis, short reaction times (18-30 minutes).

Microwave effects Specific

Substrate Time (min) Temperature (°C) Yield (%)
N-alkylation Metathesis MW Δ
1-Methylimidazole 8 10 120 99 73
Pyridine 10 10 130 91 74
1,2-dimethylimidazole 8 10 120 96 81
Experiments conducted under identical reaction conditions, Δ: conventional heating
R : C8H17, MX : KOTf

Benzoin condensation reaction
Conditions: benzaldehyde : DBU: IL (1 : 0,2 : 0,2). Temperature: 80°C. Time: 60min

Alkyl chain length effects

Entry Xa Conversionb (%) Yieldb (%)
1 OTf 88 72
2 NTf2 83 59
3 BF4 86 59
4 PF6 83 51
a) IL : [Omim][X], Ar : Ph
b) Conversion and yield estimated by GC using an internal standard


Electronic effects

Entry Ar Conversiona (%)
1 p-NO2-C6H4 0
2 NTf2 83
3 p-CI-C6H4 93
4 p-CH3-C6H4 69
5 p-OMe-C6H4 47
6 m-NO2-C6H4 0
7 m-CH3-C6H4 84
8 m-OMe-C6H4 89
a) Conversion estimated by NMR
     IL : [Omin][OTf] ;  Ar : R-Ph

Alkyl chain length effects

Entry R Conversiona (%) Yielda (%)
1 C4H9 87 54
2 C8H17 88 72
3 C10H21 87 65
4 C12H25 85 59
a) Conversion and yield estimated by GC using an internal standard :
ethyl benzoate. (Ar : Ph)

→ ILs : new catalysts for benzoin condensation
→ Good yields obtained in short reaction time (1 hour)

Conclusion
A rapid and highly efficient procedure for the synthesis of ionic liquids using solvent-free microwaves activation under green chemistry conditions was described.
Good to excellent yields (86-99%) were obtained in very short reaction times (18 -30 minutes).  Specific microwave effects were investigated and this is in agreement with reaction mechanism. These ionic liquids are used efficiently as catalysts for benzoin condensation reaction.

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