Radical polar crossover in the synthesis of complex molecules

Photocatalysis employs visible light as a cheap, abundant and renewable source of photons and offers mild and functional group tolerant pathways to complex scaffolds unattainable by ground state chemistry.

Many photochemical processes formally proceed either via electron transfer (operative in photoredox catalysis) or via triplet energy transfer (TET).

TET employs a photocatalyst that absorbs light, becomes excited and can subsequently transfer energy to another molecule to reach the triplet state.

In these processes, the photocatalyst (or 'sensitizer') often contains precious metals such as iridium - which limits sustainability.

This proposal outlines a strategy to enable a new reaction manifold - radical-polar crossover (RPC) from the triplet state - through the design and implementation of photocatalysts (PC) based upon earth-abundant metals or organic dyes.

We propose that an alkene can be activated to a triplet diradical (via TET) and can subsequently react with a (tethered) alkene or arene.

If that alkene is substituted with a group such as a halogen or sulfur, addition-elimination to liberate a radical can occur.

Single electron transfer from that liberated radical - the redox shuttle - may then generate a cation, which can be trapped in a two-electron process: this exemplifies radical polar crossover from the triplet state.

We aim to exploit this process in the synthesis of complex molecules including polycyclic heterocycles and natural product-like scaffolds This project falls within the EPSRC synthetic organic chemistry and catalysis research areas.

The development of transition metal-free photocatalysts is conducive to the development of new methodologies that are mild, safe and sustainable.

This work will provide access to structural motifs found biologically active molecules and demonstrate the potential of this new reaction manifold for the generation of complex scaffolds.