Steric Effect Of Layered Double Hydroxides In Asymmetric Catalysis

Asymmetric catalysis is an important strategy to obtain single enantiotopic chiral compounds. Recently, utilization of steric effect to improve the enantioselectivity has attracted the researchers' concern in heterogeneous asymmetric catalysis. The enhancement of chiral induction has been observed in the rigid pores of microporous zeolites, mesoporous materials and nanocarbons. This work focuses on how to utilize the steric effect of bi-dimensional flexible nanospace and the rigid planar nanosheets of anionic layered double hydroxides to improve enantioselectivity in asymmetric catalysis. The main innovative research findings and conclusions are shown as follows:1. The enantioselectivity is proposed to be improved benefiting from the confinement effect of the interlayer nanospace, via intercalating the chiral catalyst titanium tartrate complex into the interlayer space of layered double hydroxides. The titanium tartrate complexes (Ti(Ⅳ)TAm) have been intercalated within the gallery spaces of MⅡ/Al layered double hydroxides (LDHs) (MⅡ=Mg2+, Zn2+, Co2+) by ion-exchange. The complex anions are arranged in the interlayer gallery in an interdigitated bilayer with the coordinated carboxylates pointing to the brucite-like layer and the alkoxy group adjacent to the equivalent in another complex through hydrophobic interaction. The titanium tartrate complex constrained in the LDH interlayer region shows obviously enhanced asymmetric induction in the heterogeneous sulfoxidation of pro-chiral methyl phenyl sulfide (e.e.5% homo,48% hetero). The composition of the brucite-like layer makes impact on the host-guest interaction. Zn/Al layer exhibits stronger interaction with interlayer Ti(Ⅳ)TA2, resulting in the decreased electron density of Ti center and higher catalytic activity. However, host-guest interaction hardly imposes effect on the enantioselectivity.2. The interlayer spacing of titanium tartrate intercalated LDHs (Mg/Al-Ti(Ⅳ)TAm) was further adjusted by tuning the layer charge density (Dc) to promote the enantioselectivity of interlayer catalytic reaction. The interlayer spacing of Mg/Al-Ti(Ⅳ)TAm increases with the elevation of Dc. However, there seems to be an optimized interlayer spacing for the enantioselectivity of asymmetric sulfoxidation. Further studies prove that the bidimensional interlayer space could be swollen in proper solvents, and thus accommodate the reactants in the interlayer space. The enantioselectivity of asymmetric sulfoxidation increases with the decrease of final interlayer spacing of Mg/Al-Ti(Ⅳ)TAm. The bi-dimensional nanospace exhibits obvious steric confinement effect.3. According to the characteristic of LDHs with interlayer anion arrangement restricted by the positive brucite-like layer, the restriction of LDH layers on the coordination modes between L-tartrate and Ti has been studied. The interlayer orientation of L-tartrate anions in either perpendicular-lying or flat-lying has been controlled. The interlayer tartrate in perpendicular orientation chelates with Ti center in [2.2.1] bicyclic strain with both C-O-Ti and C=O-Ti coordination, and not only displays higher enantioselectivity for sulfoxide but also gives higher catalytic activity in the asymmetric sulfoxidation. The interlayer tartrate in flat-lying orientation holds only the C-O-Ti coordination mode, which militates against the improvement of enantioselectivity.4. To efficiently utilize the catalysts in the interlayer space, according to the characteristic of LDHs with swellable bi-dimensional nanospace to obtain high dispersion LDH nanosheets by delamination, the single LDH nanosheet has been utilized as ester-like substituent of chiral amino acid to improve enantioselectivity. Nanosheet attached L-amino acids to substitute the carboxylic hydrogen has been obtained by delamination of L-amino acids anions intercalated LDHs in formamide. In direct asymmetric aldol reaction of 4-nitrobenzaldehyde and cyclohexanone, the nanosheet attachedα-amino acids demonstrate remarkable promotion of enantioselectivity whether they are used alone or together with zinc center, resulting in the enrichment of trans product (2S,1'R). The catalytic position of the metal center has been monitored by both energy dispersive X-ray (EDX) and confocal laser scanning microscopy (CLSM) characterizations. The results support that once the substituent effect of the nanosheet is valid, the enantioselectivity in asymmetric aldol reaction could be enhanced.