Multifaceted View on the Mechanism of a Photochemical Deracemization Reaction

Upon irradiation
in the presence of a chiral benzophenone catalyst
(5 mol %), a racemic mixture of a given chiral imidazolidine-2,4-dione
(hydantoin) can be converted almost quantitatively into the same compound
with high enantiomeric excess (80–99% ee).
The mechanism of this photochemical deracemization reaction was elucidated
by a suite of mechanistic experiments. It was corroborated by nuclear
magnetic resonance titration that the catalyst binds the two enantiomers
by two-point hydrogen bonding. In one of the diastereomeric complexes,
the hydrogen atom at the stereogenic carbon atom is ideally positioned
for hydrogen atom transfer (HAT) to the photoexcited benzophenone.
Detection of the protonated ketyl radical by transient absorption
revealed hydrogen abstraction to occur from only one but not from
the other hydantoin enantiomer. Quantum chemical calculations allowed
us to visualize the HAT within this complex and, more importantly,
showed that the back HAT does not occur to the carbon atom of the
hydantoin radical but to its oxygen atom. The achiral enol formed
in this process could be directly monitored by its characteristic
transient absorption signal at λ ≅ 330 nm. Subsequent
tautomerization leads to both hydantoin enantiomers, but only one
of them returns to the catalytic cycle, thus leading to an enrichment
of the other enantiomer. The data are fully consistent with deuterium
labeling experiments and deliver a detailed picture of a synthetically
useful photochemical deracemization reaction.