The heck reactions of aryl bromides in ionic liquids medium of n-butyl-n-methylpyrrolidinium trifluoromethanesulfonate

Abstract

The application of the ionic liquids in the palladium catalyzed Heck reactions have been reported extensively, however, without proper evidence addressing the role of ionic liquids in the catalytic process. In this thesis, a new series of N-alkyl-Nmethylpyrrolidinium trifluoromethanesulfonate salts (46-52) have been synthesized to study their potential as the alternative solvents for the Heck reaction, instead of using conventional molecular solvents. These salts were synthesized through quaternization reaction between N-methylpyrrolidine (38) with several alkyl iodides, followed by metathesis reaction with silver trifluoromethanesulfonate, to give the desired products (46-52) (yield: 72-96%) with melting points ranging from room temperature to 300°C. All the salts obtained were characterized by using 1H and 13CNMR spectroscopies, CHN elemental analysis, melting point, density and molar conductivity. The effects of alkyl chains towards the melting points and molar conductivities of these salts (46-52) have been investigated. The N-butyl-Nmethylpyrrolidinium trifluoromethanesulfonate, [Bmplim]CF3SO3 (49), has been chosen as the solvent in the Heck reactions between methyl acrylate (61) and several types of aryl bromides (56-60) to give Heck adducts (62-66) with satisfactory yield between 37 to 97%. Parameters such as the types of bases, the amount of Pd catalyst loadings and the reaction temperatures were also studied in order to optimize the percentage conversion of respective Heck adducts (62-66). Results show that, the optimum condition to enhance the percentage conversion for this catalytic system is by using Et3N as base, Pd catalysts loading at 1.5mmol% and reaction temperature at 120°C, to achieve the calculated TONs of ~6667. With these conditions, the Heck adducts of reactive aryl bromides; 4-bromonitrobenzene (56), 4-bromoacetophenone (57) and bromobenzene (58) have achieved an extremely high percentage conversion (~100%). As for the unreactive aryl bromides; 4-bromoanisole (59) and 4- bromoaniline (60), the addition of PPh3 was proved to be useful; however, leads to contamination from the by-product which results to problematical separation of the desired products (62-66). The ionic liquid of [Bmplim]CF3SO3 (49), can be recycled up to three runs, without showing any distinct losses in its activities