2 research outputs found
Chiral 3D Covalent Organic Frameworks for High Performance Liquid Chromatographic Enantioseparation
In spite of their
great promise for enantioselective processes
due to the rich host–guest chemistry, it remains a challenge
to construct covalent organic frameworks (COFs) with chiral three-dimensional
(3D) structures. Here we report bottom-up synthesis of the first example
of 3D chiral COFs by imine condensation of an enantiopure 2-fold symmetric
TADDOL-derived tetraaldehyde with a tetrahedral tetraÂ(4-anilyl)Âmethane.
After postsynthetic oxidation of imine linkages, the framework is
transformed into an amide-linked COF with retention of crystallinity
and permanent porosity as well as enhanced chemical stability. The
resultant isostructural COFs feature a 4-fold interpenetrated diamondoid
open framework with tubular channels decorated with chiral dihydroxy
auxiliaries. Both COFs can be used as chiral stationary phases for
high performance liquid chromatography to enantioseparate racemic
alcohols, and the oxidized COF shows superior separation performance
compared to the pristine framework
Chiral Covalent Organic Frameworks with High Chemical Stability for Heterogeneous Asymmetric Catalysis
Covalent
organic frameworks (COFs) featuring chirality, stability,
and function are of both fundamental and practical interest, but are
yet challenging to achieve. Here we reported the metal-directed synthesis
of two chiral COFs (CCOFs) by imine-condensations of enantiopure 1,2-diaminocyclohexane
with <i>C</i><sub>3</sub>-symmetric trisalicylaldehydes
having one or zero 3-<i>tert</i>-butyl group. Powder X-ray
diffraction and modeling studies, together with pore size distribution
analysis demonstrate that the ZnÂ(salen)-based CCOFs possess a two-dimensional
hexagonal grid network with AA stacking. Dramatic enhancement in the
chemical stability toward acidic (1 M HCl) and basic (9 M NaOH) conditions
was observed for the COF incorporated with <i>tert</i>-butyl
groups on the pore walls compared to the nonalkylated analog. The
ZnÂ(salen) modules in the CCOFs allow for installing multivariate metals
into the frameworks by postsynthetic metal exchange. The exchanged
CCOFs maintain high crystallinity and porosity and can serve as efficient
and recyclable heterogeneous catalysts for asymmetric cyanation of
aldehydes, Diels–Alder reaction, alkene epoxidation, epoxide
ring-opening, and related sequential reactions with up to 97% ee