Iron catalysts as applied in organic synthesis: (1) hydrosilylation of carbonyl compounds (2) aldol-condensation and cyclotrimerization of aldehydes (3) dimerization of cycloolefins towards synthesis of high energy-density fuels

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2016-12

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Abstract

The demand for energy and chemical products by the ever-increasing world population has put enormous pressure on a few important elements in the periodic table. While catalysis is the key technology for the conversion of raw material into valuable products, many catalysts are based on the least abundant, expensive and sometimes toxic precious metal elements found in the second and third rows of the transition metal series. In particular, extensive research effort has been invested in the development of platinum group metals (e.g. Ru, Rh, Ir, Pd and Pt). This is not sustainable for future applications of catalysis and new approaches need to be developed. In this dissertation, we describe our strategies for exploring the chemistry of base metals such as Fe. The field of Fe-catalyzed transformations remains in its infancy despite its terrestrial abundance, low cost and the environmentally benign nature of iron in general. In the first part of this dissertation, we describe the reduction of carbonyl moieties via hydrosilylation technology. Hydrosilylation protocols have recently emerged as attractive complements to traditional methods that employ molecular hydrogen and hydride transfer reagents. We report the synthesis, characterization and catalytic applications of a series of iron complexes of general formula BIAN-Fe(arene) (BIAN = bis-(arylimino) acenaphthene and arene = trifluoromethylbenzene (CF3C6H5), trifluorobenzene (F3C6H3), benzene (C6H6), toluene (meC6H5), cumene (iPrC6H5), and 2-methylnaphthelene (2MeNap)). These complexes were prepared by treatment of the BIAN-FeCl2 intermediate with sodium amalgam or hydrogenation of the BIAN-Fe(COD) (COD = cyclooctadiene) in the presence of the arene and characterized by 1H and 13C NMR spectroscopy and single crystal X-ray diffraction. The molecular structures of BIAN-Fe(COD), BIAN-Fe(CF3C6H5), BIAN-Fe(F3C6H3), BIAN-Fe(C6H6), BIAN-Fe(meC6H5), BIAN-Fe(iPrC6H5), BIAN-Fe(OMeC6H5) and BIAN-Fe(2MeNap) all show -6-coordination of the arene to the iron center. The iron-arene complexes are active pre-catalysts for the hydrosilylation of carbonyl moieties. Aldehydes and ketones were transformed into their respective alcohols and amides to amines in excellent yields with turnover frequencies up to 200 h-1 at room temperature. The identity of the arene plays a key role in determining catalyst activity. Introduction of steric bulk or electron-withdrawing substituents weakens the iron-arene interaction and leads to more facile entry into the catalytic cycle. In the second part of this dissertation, we describe C-C bond-forming reactions catalyzed by simple salts. Using FeCl3 as catalyst, we discovered that we could effect the dimerization of cyclic olefins. When hydrogenated, products of this transformation afford valuable bicyclic hydrocarbons that are potential jet-fuel additives. We extended this chemistry to mono terpenes such -pinene to access potential jet-fuel additives from bio-renewable sources. In an attempt to extend this chemistry to C-C bond-forming reactions between cyclic olefins and aldehydes, we discovered that we could convert simple aldehydes into -unsaturated carbonyl compounds via aldol-condensation reaction. Interestingly, we also discovered that we could divert the course of the reaction to form cyclotrimerized products (1,3,5-trioxanes) by adding water to the reaction mixture.

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Keywords

Hydrosilylation, Iron, Cyclotrimerization, Carbonyl Componounds, Aldehydes, Ketones, Amides, Alcohols, Amines, Trioxanes, Terpenes, Pinenes, Olefins, Aldol-Condensation, Dimerization, Iron-Arene Complexes, Earth-Abundant Metals, Synthesis, C-C Coupling

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