Mechanistic investigation reveals radical-stabilization-driven hydrogen atom transfer in paracyclophane ketoesters under visible light, suggesting new avenues for metal-free photopolymerization.
We report a radical-stabilization-driven photoreactivity of [2,2]paracyclophane (PCP) ketoesters that enables site-selective hydrogen atom transfer (HAT) under visible-light irradiation. The PCP ketoester enables free-radical photopolymerization under air, deep curing, and low toxicity. In combination with an iodonium salt, a photooxidative SET mechanism takes place, allowing dual radical-cationic initiation and access to interpenetrating polymer networks. In the three-component PCP/iodonium/amine system, a photoredox catalytic mechanism significantly enhances polymerization rates. Upon excitation, PCP-based ketoesters undergo 1,5-HAT rather than the Norrish type-I pathway observed for conventional glyoxylates. Detailed spectroscopic, isotopic labeling, electron spin resonance (ESR) spin-trapping, and computational studies reveal that stabilization of the radical promotes a radical-polar crossover cascade cyclization. The HAT process is leveraged to develop metal-free photoinitiators that operate efficiently under 405 nm LED irradiation.
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Michelas et al. (2026) studied this question.
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