Photoredox Catalysis with Spin Magnetic Field Effects: A Scheme for Chiral Resolution

Quantities that break both mirror symmetry and time-reversal
symmetry,
such as the orbital angular momentum, are known to connect molecular
chirality with an applied magnetic field. This concept has led to
observations such as magneto-chiral dichroism and chirality-induced
spin selectivity (CISS). However, being small, these effects often
require additional amplification procedures such as flow chemistry
to achieve bulk enantioseparation. In this work, we demonstrate how
the magnetic field effect on photogenerated radical pairs, which also
breaks time-reversal symmetry, can be harnessed for enantiopurification.
Fundamental to this process is the collective decay of the singlet
and triplet radical-pair states made possible by an applied magnetic
field. Because opposite enantiomers exhibit spin–orbit coupling
matrix elements of opposite signs, the singlet and triplet decay channels
interfere constructively in one enantiomer. Meanwhile, molecules of
the other enantiomer are funnelled into the first enantiomer through
excited-state chirality inversion, achieving (dynamic kinetic) chiral
resolution. Using an axially chiral binaphthyl derivative and a borane
photosensitizer as prototype, we predict an appreciable enantiomeric
excess (e.e.) of 90% to be possible at steady state, attained within
hundreds of milliseconds when irradiated by a laser. Importantly,
our analytical results showcase regimes of perfect enantioselectivity
(100% e.e.), accessible by further chemical optimization of the photosensitizer
for which general strategies are discussed. Overall, this work illustrates
a so-far untapped but powerful control knob for photoredox catalysis
based on spin chemistry principles.