1,4-Functionalization of 1,3-dienes with low-valent iron catalysts.

Iron-catalyzed or -mediated transformations of organic substrates have been important throughout the development of organic chemistry due to iron's abundance, low cost, and favorable toxicity profile. Highly reduced iron species, although difficult to isolate and characterize, have proven valuable as catalysts for a variety of C-C and C-heteroatom bond forming processes, as well as cyclization and cycloisomerization reactions. We have developed iminopyridine-ligated low-valent iron catalysts that facilitate selective 1,4-hydrovinylation, hydoboration, hydrosilylation, and polymerization of 1,3-dienes. The catalysts are generated in situ from iron(II) precursors in the presence of activated magnesium metal or trialkylaluminum reductants. The 1,4-addition processes provide access to valuable products such as 1,4-dienes, allylboronic esters, allylsilanes, and highly regioregular polyisoprene. In these transformations, addition is stereoselective, providing (E)-alkene isomers selectively, and (1,2)-addition products are generally not observed. Moreover, modification of steric bulk on the iminopyridine ligand can be used to change selectivity for (1,4)- versus (4,1)-addition to dienes with nonsymmetric substitution. Access to low-valent iron precursor complexes is limited, and we have developed a diaryliron(II) precursor that undergoes smooth reductive elimination in the presence of iminopyridine ligands to provide easy access to low-valent iron catalysts without the use of heterogeneous reductants, which complicate the isolation and study of low-valent iron complexes. We obtained crystal structures of our iron(II) catalyst precursor and an iminopyridine-ligated reduced iron species generated from it. Spectroscopic analysis suggests that although this species is formally iron(0), the redox-active iminopyridine ligands accept electron density from the metal and the complex is more properly formulated as iron(II) coordinated by two radical-anion ligands. We believe that a closely-related set of reaction manifolds is responsible for the 1,4-functionalization reactivity displayed by the iron(iminopyridine) complexes (see text). Kinetics experiments and deuterium-labeling studies provide evidence for the proposed catalytic cycle. The geometry of the double bond remaining after 1,4-addition is set by the requirement that the diene bind to the iron center in an s-cis geometry, and the regioselectivity of addition can be rationalized by the location of steric bulk on the iminopyridine ligand. The transformations presented in this Account utilize iron catalysts to provide access to valuable diene 1,4-addition products such as 1,4-dienes, allylboronate esters, and allylsilanes, as well as highly regioregular polyisoprene. The development of a stable diaryliron(II) precatalyst, structural characterization of an iminopyridine-ligated iron(0) complex, and mechanistic insights into the selective nature of this transformation provide a window into the reactivity profile of low-valent iron.