Driving Forces and Spin-Gated Reactivity of N5-Hydropyrimidopteridinetetraone Radicals in Photocatalysis

Pyrimidopteridinetetraones (PPTs) are potent
excited-state
oxidants that also function as hydrogen atom transfer (HAT) catalysts.
Here, we investigate the photobasicity and mechanistic role of N-hydropyrimidopteridinetetraone
radicals (PPTH•) in catalyst turnover.
Optical and electron paramagnetic resonance (EPR) spectroscopy, quantum
chemical calculations, and thermochemical analysis reveal that excited-state
protonation of PPTs occurs at the N5 position of the
heterocycle, generating PPTH• as a
key catalytic intermediate. Formation of the exocyclic N–H
bond from the excited state is associated with a bond dissociation
free energy (BDFE) of 131 kcal mol–1 from the singlet
and 106 kcal mol–1 from the triplet excited state,
while subsequent homolytic N–H cleavage (BDFE ≈ 57 kcal
mol–1) enables catalyst regeneration. Spin-resolved
mechanistic analysis shows that singlet radical encounters proceed
uniformly via a direct HAT (dHAT) pathway across aliphatic, benzylic,
and redox-active substrates with low activation free energies. In
contrast, triplet encounters access substantially higher-energy reaction
manifolds and can give rise to substrate-dependent mechanistic trajectories,
including dHAT, concerted proton-coupled electron transfer (cPCET),
or stepwise electron and proton transfer sequences. These findings
establish a spin-state–dependent framework for HAT in PPT photoredox catalysis, highlighting how electron affinity
and (photo)basicity govern excited-state reactivity and positioning PPTs as a platform for proton-coupled electron transfer chemistry.