Dehydration reactions of benzylic alchohols and preparation of palladium nanocrystals in imidazolium and phosphonium ionic liquids

Asadollahi Kalviri, Hassan (2012) Dehydration reactions of benzylic alchohols and preparation of palladium nanocrystals in imidazolium and phosphonium ionic liquids. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Palladium catalyzed dehydrative etherification of benzyl alcohols in imidazolium ionic liquids (ILs) were studied. Several hydrophobic and hydrophilic imidazolium ionic liquids were employed and as expected, higher yields were obtained when hydrophobic ionic liquids were used as the reaction media. Among the transition metal complexes studied, Pd(CH₃CN)₂ Cl₂ demonstrated the highest catalytic activity. [BMMIm]PF₆([BMMIm]= 1-butyl-2,3-dimethylimidazolium) was also used to study the possibility of N-heterocyclic carbene (NHC)-palladium complex formation. A catalytic cycle is proposed that supports the important roles of the ionic liquids and Pd in this reaction, in which ionic liquids stabilize the proposed ionic intermediates. -- Phosphonium ILs were found to catalyze the above-mentioned reactions and an added metal was not needed. Benzyl ethers and alkenes were obtained from primary and secondary benzylic alcohols in good to excellent yields. Commercially available hydrophobic phosphonium ionic liquids containing the trihexyl(tetradecyl)phosphonium cation paired with six different anions were used for the reactions under microwave irradiation. NOESY NMR, in addition to some other NMR techniques, were used to study the interaction of substrates and ILs in order to gain insight into the reaction mechanism. The acidic properties of the phosphonium ionic liquids were also investigated in order to potentially correlate the pH of the ionic liquids and the reaction yield. There was no correlation. -- Trihexyl(tetradecyl)phosphonium ILs were also found to support the formation of Pd(0) nanoparticles without the addition of reducing agents such as NaBH₄. Upon varying the Pd(II) sources and the anions of the phosphonium ILs, crystalline Pd nanoparticles grew as truncated octahedrons, octahedrons and trigonal plates with a narrow size distribution. No nanoparticle formation was observed when phosphonium ionic liquid with chloride anions were used. The presence of chloride anions is proposed to lead to oxidative etching of the particles and this process is reduced in the presence of certain anion partners including bis(2,4,4-trimethylpentyl)phosphinate. Two possible reduction mechanisms based on reactions of the phosphonium cation were suggested for the formation of the Pd(0) species. Different techniques such as TEM, XRD, FT-IR and TGA were employed to characterize the prepared nanocrystals and assess the role of the phosphonium ILs in their synthesis. The in situ prepared Pd nanocrystals in phosphonium ILs were used in a Suzuki coupling reaction and high yields were obtained.

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