Photocatalytic Eosin Y Disproportionation as Driving Force for Terpene Cyclizations

A mechanistic paradigm for terpene cyclization driven by photocatalytic disproportionation of eosin Y in fluorinated alcohol media was discovered. By employing a 1:1 mixture of HFIP and PFTB, the neutral form of eosin Y (EH₂) undergoes efficient intersystem crossing to its triplet state, initiating hydrogen atom transfer (HAT) with ground-state EH₂. This generates two radicals, EH• and EH₃•, with EH• identified as the key species triggering polyene cyclizations. Spectroscopic and computational analyses revealed that hydrogen bonding with fluorinated alcohols stabilizes the otherwise unfavored open carboxylic acid form of eosin Y, enabling visible-light absorption and reactivity. The study clarifies the long-debated mechanism of eosin Y-mediated cyclizations, introducing a dual-radical generation strategy from a single photocatalyst molecule. These findings introduce a new paradigm in photoredox catalysis, offering a mild, selective, and sustainable approach to complex terpene synthesis and paving the way for broader applications in synthetic organic chemistry.