Energy Transfer (EnT) Catalysis of Non-Symmetrical Borylated Dienes: Origin of Reaction Selectivity in Competing EnT Processes.

Energy transfer catalysis (EnT) has had a profound impact on contemporary organic synthesis enabling the construction of higher in energy, complex molecules, via efficient access to the triplet excited state. Despite this, intermolecular reactivity, and the unique possibility to access several reaction pathways via a central triplet diradical has rendered control over reaction outcomes, an intractable challenge. Extended chromophores such as non-symmetrical dienes have the potential to undergo [2+2] cycloaddition, [4+2] cycloaddition or geometric isomerisation, which, in combination with other mechanistic considerations (site- and regioselectivity), results in chemical reactions that are challenging to regulate. Herein, we utilise boron as a tool to probe reactivity of non-symmetrical dienes under EnT catalysis, paying particular attention to the impact of boron hybridisation effects on the target reactivity. Through this, a highly site- and regioselective [2+2] cycloaddition was realised with the employed boron motif effecting reaction efficiency. Subtle modifications to the core scaffold enabled a [4+2] cycloaddition, while a counterintuitive regiodivergence was observed in geometric isomerisation versus [2+2] cycloaddition. The observed reactivity was validated via a mechanistic investigation, determining the origin of regiodivergence and reaction selectivity in competing EnT processes.