posted on 2016-03-31, 00:00authored byRobert
F. Higgins, Steven M. Fatur, Samuel G. Shepard, Susan M. Stevenson, David J. Boston, Eric M. Ferreira, Niels H. Damrauer, Anthony K. Rappé, Matthew P. Shores
A combined experimental
and theoretical investigation aims to elucidate
the necessary roles of oxygen in photoredox catalysis of radical cation
based Diels–Alder cycloadditions mediated by the first-row
transition metal complex [Cr(Ph2phen)3]3+, where Ph2phen = bathophenanthroline. We employ
a diverse array of techniques, including catalysis screening, electrochemistry,
time-resolved spectroscopy, and computational analyses of reaction
thermodynamics. Our key finding is that oxygen acts as a renewable
energy and electron shuttle following photoexcitation of the Cr(III)
catalyst. First, oxygen quenches the excited Cr3+* complex;
this energy transfer process protects the catalyst from decomposition
while preserving a synthetically useful 13 μs excited state
and produces singlet oxygen. Second, singlet oxygen returns the reduced
catalyst to the Cr(III) ground state, forming superoxide. Third, the
superoxide species reduces the Diels–Alder cycloadduct radical
cation to the final product and reforms oxygen. We compare the results
of these studies with those from cycloadditions mediated by related
Ru(II)-containing complexes and find that the distinct reaction pathways
are likely part of a unified mechanistic framework where the photophysical
and photochemical properties of the catalyst species lead to oxygen-mediated
photocatalysis for the Cr-containing complex but radical chain initiation
for the Ru congener. These results provide insight into how oxygen
can participate as a sustainable reagent in photocatalysis.