Hangman Salen Platforms Containing Two Xanthene Scaffolds

A synthetic strategy for the construction of chiral salen ligands bearing two rigid xanthene spacers
functionalized with carboxylic acid and ester groups is presented. Suzuki cross-coupling methodology is
used to furnish the appropriately functionalized xanthene spacers to a salicyladehyde, which is subsequently
condensed with (1R,2R)-(−)-1,2-diaminocyclohexane to produce salen ligands featuring an expandable
molecular cleft capable of multiple hydrogen-bonding interactions in addition to metallosalen oxidation
chemistry. The ability of these “Hangman” platforms to support multielectron chemistry mediated by
proton-coupled electron transfer (PCET) is established by their proclivity to promote the catalytic
disproportionation of hydrogen peroxide to oxygen and water via a high-valent metal oxo. Within this
functionalized Hangman framework, the stereochemistry of the cyclohexyl backbone of the salen platform
is revealed in the epoxidation of 1,2-dihydronaphthalene by the metal oxo.