Photoactive Chiral Metal–Organic Frameworks for Light-Driven Asymmetric α‑Alkylation of Aldehydes

Chiral metal–organic frameworks (MOFs) with porous
and tunable
nature show promise as heterogeneous asymmetric catalysts. Through
incorporating the stereoselective organocatalyst l- or d-pyrrolidin-2-ylimidazole (PYI) and a triphenylamine
photoredox group into a single framework, we have developed two enantiomeric
MOFs, Zn–PYI1 and Zn–PYI2,
to prompt the asymmetric α-alkylation of aliphatic aldehydes
in a heterogeneous manner. The strong reductive excited state of the
triphenylamaine moiety within these MOFs initiated a photoinduced
electron transfer, rendering an active intermediate for the α-alkylation.
The chiral PYI moieties acted as cooperative organocatalytic
active sites to drive the asymmetric catalysis with significant stereoselectivity.
Control experiments using the lanthanide-based metal–organic
frameworks Ho–TCA and MOF–150, assembled from 4,4′,4″-nitrilotribenzoic acid, as
catalysts suggested that both the photosensitizer triphenylamine moiety
and the chiral organocatalyst d-/l-PYI moiety were necessary for the light-driven α-alkylation reactions.
Further investigations demonstrated that the integration of both photocatalyst
and asymmetric organocatalyst into a single MOF makes the enantioselection
superior to that of simply mixing the corresponding MOFs with the
chiral adduct. The easy availability, excellent stereoselectivity,
great separability, and individual components fixed with their well-defined
porous and repeating structures make the MOF a versatile platform
for a new type of tandem catalyst and cooperative catalyst.