Ruthenium's affinity for p-bond formation lends it great utility as a catalyst for carbon-carbon bond formation, i.e. ring closing metathesis. One such reaction is Bergman cyclization (BC), wherein a z-enediyne core proceeds via a diradical intermediate to make a new, intramolecular carbon-carbon bond, ultimately forming an aromatic ring. Due to this reactive diradical intermediate, BC offers a powerful chemical tool by which biological substrates may be cleaved or unusual aromatic systems may be synthesized. This reaction can be thermally induced, and chemical modification of the enediyne can be used to alter the reaction temperature. Metals have been utilized to induce this cyclization via s-coordination to heteroatom-decorated enediynes. This affords finer control of reactivity and introduces a reaction trigger, so that BC can be induced on demand. Recently, ruthenium arenes have been shown to induce BC by p-coordination to enediynes. This p-binding strategy involves direct coordination of the metal to the enediyne carbons, and affords triggerable room temperature BC. This room temperature reactivity is not shared by the bulk of s-binding BC systems. However, the s-binding strategy lends these systems water and air stability, a trait not shared by the p-coordinated analogues. In an effort to combine the complementary strengths offered by each of these strategies, a novel class of BC ligands has been developed to encourage both binding modes simultaneously. These hybrid s/p-systems offer heteroatoms for chelation, giving robust metal coordination, while their macrocyclic form simultaneously directs the metal to the enediyne p-system. Because the binding pocket is reduced in size upon BC, these systems also present the possibility of catalytic behavior. In such a catalytic system, BC induces metal ejection from the ligand and an uncyclized enediyne is subsequently bound. The development and thermal reactivity of these s/p-enediyne systems, as well as their metal binding behavior will be discussed.

Macrocyclic enediynes for metal-mediated Bergman cyclization

246th National Meeting (2013)

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