Enantioselective Photochemical Reactions of Cyclopropene

Objective

In this proposal, we aim to explore the photochemistry of cyclopropene derivatives to access enantioenriched cyclopropenes and polycyclic fused rings with multiple stereocenters. Cyclopropenes can be photochemically excited to form intermediates, depending on the mode of excitation: triplet excitation leads to a diradical intermediate, while singlet excitation leads to a vinyl carbene intermediate. This project is divided into two parts. In the first part, we will utilize deracemization through selective singlet excitation of cyclopropenes using chiral Lewis acid catalysts. Deracemization converts a racemic mixture into a single enantiomer with 100% efficiency, unlike classical resolution techniques. Cyclic deracemization involves selectively racemizing one enantiomer, leading to the accumulation of the opposite enantiomer. The chiral Lewis acid, coordinated to the ester group of the cyclopropene via two-point binding, will enable selective excitation of one enantiomer into a singlet state. This excited singlet state will collapse into an achiral vinyl carbene, which can cyclize to reform the racemic starting material, driving the mixture toward enantioenriched cyclopropene. In the second part, we will explore the triplet excitation of cyclopropenes. Triplet excitation activates the olefin rather than the σ-bond, forming a triplet (1,2-diradical) intermediate. We plan to generate this intermediate using a chiral thioxanthone catalyst developed by the Bach group. The chiral environment provided by the catalyst will block one face, allowing selective reaction from one face, leading to enantioenriched bicyclo[2.1.0]pentanes.