Iron-catalyzed cross-coupling reactions

• Main Author: Qian, Xin, 1985-
• Other Authors: Memorial University of Newfoundland. Dept. of Chemistry
• Format: Thesis
• Language: English
• Published: 2010
• Subjects: Iron compounds > Synthesis; Ligands; Catalysts; Grignard reagents; Allyl halides; Newfoundland studies; University of Newfoundland
• Online Access: http://collections.mun.ca/cdm/ref/collection/theses5/id/33594

Thesis (M.Sc.)--Memorial University of Newfoundland, 2011. Chemistry Includes bibliographical references. This thesis concerns the synthesis and study of iron(III) halide complexes supported by amine-bis(phenolate) ligands. These paramagnetic molecules were characterized by a variety of methods including MALDI-TOF mass spectrometry and UV-vis spectroscopy. Two dimeric iron complexes, {FeCl[O 2N]BuMenPr }2 (1), {FeCl[O2N]BuMeBn }2 (5) and one monomeric FeBr2[O2NH] BuMenPr (8), where [O2N]BuMenPr = n-propylamine-N,N-bis(2-methylene-4-methyl-6-tert-butylphenolate) and [O2N]BuMeBn = benzylamine-N,N-bis(2-methylene-4-methyl-6-tert-butylphenolate), were characterized in the solid state by single crystal X-ray diffraction. The abbreviated nomenclature for these ligands can be generalized by the form [O2N]RR'R",where [O2N] describes the three donor atoms (two oxygens are phenolic and one nitrogen is aminic) of the ligand, R and R' define the substituents at the 2- and 4- positions, respectively, on the phenol group, and R" represents the alkyl group on the central amine donor (such as nPr for n-propyl or Bn for benzyl). Variable temperature magnetic susceptibility data for {FeCl[O2N]BuMenPr}2 (1) was also obtained using a SQUID magnetometer. -- The easily synthesized, inexpensive, and relatively air-stable novel iron complex {FeCl[O2N]BuMenPr}2 can be used as a mild and efficient catalyst for C-C cross-coupling of aryl Grignards with alkyl halides. Compared to other iron-catalyzed sp3-sp2 C-C cross-coupling reactions, it displays several advantages: i) the complex is easy to handle and has the potential for large-scale applications, ii) by employing microwave conditions, hindered Grignard reagents can react with primary alkyl halides bearing β-H atoms and gives excellent yields, iii) secondary alkyl chlorides and benzyl halides can be effectively used as electrophilic substrates, iv) a number of functional groups are tolerated. Cross-coupling products are obtained in good to excellent yield as shown by GC-MS and 1H NMR analysis. Screening of cross-coupling reactions for over thirty substrate combinations and the effect of microwave heating on reaction yields are described. Mechanistic studies suggest a radical-mediated route to cross-coupling as shown by "radical clock" experiments. -- We also present the ability of FeCl3 to cleave both C-Cl bonds in CH2Cl2 and two C-Cl bonds in CDCl3 in the presence of Grignard reagents. Different conditions for the catalytic system are explored, and the products are obtained from low to excellent yields. The results are monitored by GC-MS and 1H NMR. To our knowledge, this is the first time that such transformations have been observed using an iron pre-catalyst, leading to the first efficient double aryl-alkyl coupling of CH2Cl2. Hypotheses regarding the mechanism of cross-coupling are presented. -- Keywords: Iron; Homogeneous catalyst; Green chemistry; C-C formation, Grignard reagent