Authors: Ban Wang
Source: TopScholar (Western Kentucky University) (article, 2019-01-01)
Field: Chemistry — Organic Chemistry

Synthesis of Bis(imino)pyridine Iron(II) Complexes and Development of Bis(imino)pyridine Iron(II) Catalyzed Carbene Transfer Reactions Metal catalysis of symmetric and asymmetric carbene transfer reactions has been widely applied in natural product synthesis and material science over years. Metal carbene can be easily generated from the extrusion of nitrogen under the catalysis of metal complexes to further undergo various organic reactions, O/N/C-H insertions, cycloadditions, and ylide formations. Currently, the dominant effective catalysts for carbene reactions are built with expensive precious metal, for example, rhodium, ruthenium, palladium, gold. Notably, the effective reactivity and enantioselectivity of the dirhodium(II) catalysts are researched and established over the decades. However, the use of precious metal catalysts is the major source of metal residues in pharmaceutical products; thus, it becomes a concerning safety factor towards the environment. Iron, instead, to our interest, is an economical and ecofriendly element. Iron has been used in different catalytic reactions but achieved moderate reactivity and low enantioselectivity towards carbene transfer reactions. Within, the electronic environment and the mechanism of iron catalysts are underdeveloped. A new series of ligands named bis(imino)pyridine family has been found to be able to offer coordinate sites for transition metals to build effective metal complexes can be used for different organic reactions. This type of ligand can be easily synthesized in relatively short steps and the structure of the substituents can be facially tuned. These advantages show the great potential of bis(imino)pyridine ligands in organic catalysis. In this project, bis(imino)pyridine ligands were applied as the backbone structure to construct a series of achiral and chiral iron catalysts that were investigated in catalytic metal carbene reactions in terms of reactivity and selectivity. By manipulating the structure of the ligands, the high reactivity of the achiral iron(II) complexes towards various carbene reactions was achieved, while moderate enantioselectivity was observed by the catalysis of chiral iron(II) complexes. To our delight, the bis(imino)pyridine iron(II) complex, for the first time, is shown as an effective metal carbene catalyst for carbene transfer reactions of donor–acceptor diazo compounds. Its broad catalytic capability is demonstrated by a range of metal carbene reactions, from cyclopropanation, cyclopropenation, epoxidation, and Doyle–Kirmse reaction to O–H insertion, N–H insertion, and C–H insertion reactions. The asymmetric cyclopropanation of styrene and methyl phenyldiazoacetate was successfully achieved by the new chiral bis(imino)pyridine iron catalyst, which delivers a new gateway for the development of chiral iron catalysis for metal carbene reactions. Pyridine ; Carbene ; Chemistry ; Catalysis ; Medicinal chemistry ; Polymer chemistry ; Organic chemistry N-Heterocyclic Carbenes in Organic and Inorganic Chemistry ; Asymmetric Hydrogenation and Catalysis ; Catalytic Cross-Coupling Reactions