Iron catalysts for the epoxidation of olefins and C-C Cross coupling

Hasan, Kamrul (2012) Iron catalysts for the epoxidation of olefins and C-C Cross coupling. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

This thesis describes the synthesis, structure and catalytic activity of aminebis(phenolate)Fe(llI) complexes. Fe(III) halide complexes were synthesized supported by tetradentate amine-bis(phenolate) ligands. The general formulae of these ligands are abbreviated as [O₂N₂]RR’ and [O ₂NO’]RR’R”, where [O₂N₂] and [O₂NO’] to describe the four donor atoms of the ligands, R and R'('Bu or Me) represent the substituents at the 2-and 4- positions on the phenolate rings and R" defines the pendant donor functionality such as methoxyethyl (Meth) or tetrahydrofurfuryl (Furf). These paramagnetic complexes were characterized by a variety of methods including elemental analysis, MALDI-TOF mass spectrometry, UV-vis spectroscopy, and magnetic moment measurements. Three iron complexes, FeCl[O₂N₂]BuBu (1), FeBr[O₂N₂]BuBu (2) and FeBr[O₂NO’]BuMeMeth (3) were structurally characterized by single crystal X-ray diffraction. Magnetic moment measurements of these complexes show high-spin d⁵ Fe(lll) centers. Also, variable temperature magnetic susceptibility measurements of the three structurally characterized complexes were conducted using a SQUID magnetometer. -- For epoxidation of olefins, H₂O₂ was chosen as terminal oxidant because it is convenient and produces water as a by-product. The catalytic activity of these aminebis(phenolate)Fe(III ) halide complexes were investigated for the epoxidation of trans-stilbene. The complex FeCl[O₂N₂]BuBu (1) gave modest yield (36%) of trans-stilbene oxide after 19 h stirring of the reaction mixture at 25°C. However, a practical method for the epoxidation of olefins was discovered by the combination of FeCl₃·6H₂O and 1-methylimidazole in acetone. Heating the reaction mixture to 62°C for 19-21 h significantly improved the product yields. The optimized catalytic conditions were applicable for both aromatic and aliphatic internal and terminal olefins. Also, in-situ generated Fe(III) complexes, made by adding to FeCl₃·6H₂O solutions of selected amine-bis(phenol) ligands, were investigated for the epoxidation of trans-stilbene to see whether any enhancement of reactivity could be obtained. Tridentate amine-bis(phenol) ligands (abbreviated H₂[O₂N]RR’) improved the product yields up to 64% but these yields were less than the epoxidation activity (93%) while using FeCl₃·6H₂O and I-methylimidazole alone. -- Finally, six amine-bis(phenolate)Fe(IlI)(acac) (acac = acetylacetonate) complexes were synthesized by employing previously reported tetradentate amine-bis(phenolate) ligands including [O₂NN’]RR’R”, where R” is the pendant donor functionality such as N,N-dimethylaminoethyl (NMe2). These paramagnetic complexes were also characterized by a variety of methods including elemental analysis, MALDI-TOF mass spectrometry, UV-vis and lR spectroscopy, cyclic voltammetry, and magnetic moment measurements. Four of these complexes, Fe(acac)[O₂N₂]BuBu (9), Fe(acac)[O₂NN’]BuBuNMe2 (10), Fe(acac)[O ₂NN’]BuMeNMe2 (11), and Fe(acac)[O₂NO’]BuMeMeth (13) were structurally characterized by single crystal X-ray diffraction. Magnetic moment measurements of these complexes indicate high-spin d⁵ Fe(III) centers. Cyclic voltammetry of these complexes show ligand-centered reversible redox processes. Preliminary catalytic activity of these complexes was investigated for the C-C cross-coupling of aryl Grignard reagents with alkyl halides. Secondary cyclic halides such as cyclohexyl chloride and bromides gave high yields, however, benzyl chloride and bromide gave moderate and acyclic 2-chloro- and 2-bromobutanes gave poor yields of cross-coupled product.

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