Iron-catalyzed transformations of 1,3-dienes

The research described in this thesis was focused on developing new iron catalysts for the production of useful synthetic building blocks. Iron is one of the most abundant elements in the Earth's crust, but it is underutilized in organic synthesis. Electron-rich iron, in particular, is not well understood despite its ability to form important carbon–carbon bonds from commodity chemicals such as hydrocarbons. The goal of the research in this thesis was to develop new reactions to gain a better understanding of the reactivity of iron catalysts, the mechanisms by which they operate, and their synthetic capabilities.;Electron-rich iron, also known as low-valent iron, can be stabilized by redox-active ligands that also confer unique reactivity upon the metal center. Iminopyridine ligands, for example, are redox-active in well-defined iron complexes and, in this thesis, are used to support iron catalysts to effect the selective functionalization of 1,3-dienes. Specifically, four regio- and stereospecific 1,4-addition reactions of 1,3-dienes are described: 1,4-addition of α-olefins, 1,4-hydroboration, 1,4-hydrosilylation, and 1,4-polymerization of isoprene. The first three reactions can selectively generate synthetic building blocks to provide products that are difficult to access by other methods. The isoprene polymerization reaction is an affordable alternative to more expensive metal-catalyzed polymerization procedures and can selectively provide two types of synthetic rubber. In each of these reactions, a ligand-dependent selectivity was observed and, in some cases, selecting the right ligands accessed both 1,4-addition isomers. Isolation of a structurally well-defined bis(iminopyridine) iron catalyst supported the hypothesis that the iminopyridine ligands are redox active during catalysis.;A well-defined biaryl iron complex was developed as a new method to access low-valent iron catalysts via controlled reductive elimination. This biaryl iron complex can be used as a soluble in situ low-valent iron source for the catalysis of chemo-, regio-, and stereoselective reactions with iminopyridine iron catalysts. The iron precursor was also used as a starting material in the synthesis of two well-defined iron complexes, an iminopyridine complex and a well-defined phosphoramidite iron complex that was previously inaccessible. The reactivity and selectivity of the iron catalysts discussed in this thesis demonstrate that iron is capable of effecting highly selective transformations from commodity chemicals such as 1,3-dienes.