Regioselective Ring-Opening of Spiro-Epoxyoxindoles by a Dual-Ligand Zinc-Based Metal–Organic Framework as an Efficient Heterogeneous Catalyst

Developing
a heterogeneous catalyst for alcoholysis/hydrolysis
of spiro-epoxyoxindoles to form β-alkoxy/β-hydroxy alcohols
is an emerging and promising research area in the academia and industry
due to its importance in the pharmaceutical industry. In this work,
a chiral-phase pure Zn-based metal–organic framework (MOF),
{[Zn2(D-CAM)2(L)]·MeOH·2H2O}n (ADES-6), has
been constructed from mixed organic ligands involving D­(+)-camphoric
acid (D-H2CAM) and an amide-functionalized
N-donor ligand ((E)-N′-(pyridin-4-ylmethylene)­isonicotinohydrazide
(L)) through versatile synthetic modes and characterized
by different analytical techniques along with single-crystal X-ray
diffraction (SXRD) analysis. The catalytic activity reveals that the
prepared ADES-6 MOF could be exploited as a heterogeneous
catalyst for regioselective ring-opening of different spiro-epoxyoxindoles
to form β-alkoxy/β-hydroxy/β-chloro alcohols efficiently
in the presence of appropriate nucleophiles. Furthermore, ADES-6 displays excellent activity for diverse substrates and can
be reused in at least five successive cycles without significant reduction
in activity. Mechanistic explorations indicate that the high efficiency
of ADES-6 is attributed to the presence of a vacant Lewis
acidic metal center, and an amide group of the ligand L in the framework as a Lewis basic site can synergistically activate
the catalytic reaction. Importantly, ADES-6 behaved as
one of the rare first main-group MOF-based catalysts reported in the
ring-opening of spiro-epoxyoxindoles efficiently and few catalytic
products are validated unambiguously by SXRD analysis.