Acetylacetone (acac) catalytic systems are some of the earliest examples of base metal catalysis. Examples include Kumada (Fe), Mukaiyama (Co), Ullmann (Cu) reactions, as well as dozens of other examples, some recent. Sterically hindered β-diketonates remain largely unexplored due to synthetic and geometric limitations, i.e. “backbone-only” substitution. As a result, β-diketone ligands were largely left out of significant advancements in low-coordinate metal chemistry stabilized by bulky ligands over the past few decades. Dipivaloylmethane remains the hindered acac-type ligand of choice, for 60 years, prepared in about the same yield. We have prepared a family of increasingly hindered ligands capable of restricting the tendency for exhaustive ligation, instead forming novel metal(acac)(halide) complexes in many cases. Coordination with copper(II) was used to gauge the impact of the steric modifications for each ligand, given the well-documented rigidity of these complexes. The crystal-phase structural consequences of increasing the ancillary bulk of the ligands has been directly probed, giving insight into the factors which perturb the electronically preferred coordination environment of copper(II) acetylacetonates. Additionally, metal complexes derived from these compounds are insulated from typical decomposition pathways, allowing for reactive intermediates to be isolated for the first time. For example, these ligands afford steric protection to the carbonyl carbons—typically sensitive to bases or nucleophiles—permitting isolation of acac-supported organometallic species. Given these qualities, it is anticipated that the complexes will show improved catalytic efficiency in milder reaction conditions, perhaps offering novel reaction pathways. Furthermore, the relevant mechanistic cycles (a matter of much dispute) can be directly probed by isolating, characterizing, and directly employing putative intermediates in catalysis. Preliminary results in both of these directions will be discussed.

Sterically hindered β-diketones: Synthesis and applications in catalysis

2019 Fall National Meeting

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