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Free keywords:
boron, photochemistry, energy transfer catalysis, 2+2 cycloaddition, divergent reactivity
Abstract:
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 isomerization, which, in combination with other mechanistic considerations (site- and regioselectivity), results in chemical reactions that are challenging to regulate. Leveraging spin density as a predictive tool, in combination with the use of a core functionality that can be adequately tuned to potentially modulate reactivity, would be highly enabling in revealing the intimate links between core structure and EnT induced reactivity. Herein, we utilize boron as a tool to explore reactivity of non-symmetrical dienes under EnT catalysis, paying particular attention to the impact of boron hybridization effects on the target reactivity. Through this, a highly site- and regioselective 2+2 cycloaddition was realized with the employed boron motif effecting reaction efficiency. Reaction divergence to enable 4+2 cycloaddition was achieved, while a counterintuitive regiodivergence was observed in geometric isomerization versus 2+2 cycloaddition. The observed reactivity was validated via an in-depth mechanistic investigation determining the origin of reactivity and regiodivergence in competing EnT processes and revealing the intimate links between structure and reactivity.
