Triplet Energy Transfer as a Handle to Tune 1,2-Dialkyldiazene Fragmentation in Radical C(sp3)–C(sp2) Cross-Coupling

TL;DR

This paper explores how light-induced energy transfer can control the fragmentation of 1,2-dialkyldiazenes in a chemical reaction, improving the efficiency of cross-coupling processes.

Contribution

The study introduces a tunable energy-transfer mechanism for activating 1,2-dialkyldiazenes in radical cross-coupling reactions.

Findings

Triplet energy transfer from photocatalysts to 1,2-dialkyldiazenes was confirmed as the activation pathway.

The triplet energy levels of two model 1,2-dialkyldiazenes were determined to be around 2.3 eV.

Energy-transfer-based catalytic systems improved cross-coupling efficiency with electron-rich aryl bromides.

Abstract

Mechanistic investigation of light-induced processes
is paramount
as it not only offers an overall mechanistic picture but also provides
information about the efficiency and associated rate constants of
the different reaction steps. In some cases, study of systematic series
of photosensitizers or quenchers also allows to determine ground-state
redox potentials or triplet energy levels of unknown species. Herein,
through a combination of steady-state and time-resolved spectroscopic
techniques, we elucidate the mechanism of geminate triplet radical
pair formation from 1,2-dialkyldiazenes operating via energy transfer
from excited photocatalysts. Stern–Volmer and Rehm–Weller
analyses confirmed the energy-transfer pathway and provided access
to the triplet energy level of two model 1,2-dialkyldiazenes, which
were found to be around 2.3 eV. Further evidence was gained by mediator-enhanced
triplet energy transfer to an anthracene substrate, showcasing that
the excited diazene can serve as an energy shuttle that can be intercepted
before bond fragmentation to release N2 and the corresponding
radicals. This activation mechanism confers clear advantages over
conventional high-energy UV photofragmentation as triplet sensitization
was shown to promote a more efficient solvent-cage escape of the resulting
geminate radical pairs relative to direct excitation. Additionally,
the structure of the 1,2-dialkyldiazenes was found to profoundly influence
the kinetics of fragmentation following energy transfer. These mechanistic
insights were leveraged to improve C­(sp3)–C­(sp2) cross-coupling efficiency with challenging electron-rich
aryl bromides by slowing alkyl radical generation through photocatalyst
selection to match the rate of Ni oxidative addition, thereby demonstrating
the tunability of energy-transfer-based dual catalytic systems.

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