Modification of chiral dimethyl tartrate through transesterification: Immobilization on POSS and enantioselectivity reversal in sharpless asymmetric epoxidation

Abstract

Modification of dimethyl tartrate has been investigated through transesterification with aminoalcohols to provide reactive functionalities for the covalent bonding of chiral tartrate to polyhedral oligomeric silsesquioxanes. The transesterification of dimethyl tartrate has been widely studied using different catalytic systems and reaction conditions. Through the proper selection of both the catalytic system and the reaction conditions, it is possible to achieve monosubstituted or bis-substituted tartrate derivatives as sole products. All the intermediate chiral tartrate-derived ligands were successfully used in the homogeneous enantioselective epoxidation of allylic alcohols providing moderate enantiomeric excess over the products. Attached amine groups have been used to support the modified tartrate ligands on to a haloaryl-functionalized silsesquioxane moiety. This final chiral tartrate ligand displays reverse enantioselectivity in the asymmetric epoxidation of allylic alcohols with regard to the starting dimethyl tartrate ligand, both molecules having the same chiral sign. However, the POSS-containing ligand can be easily recovered in almost quantitative yield and reused in asymmetric epoxidation reactions. In addition, recovered silsesquioxane-pendant ligand, though displaying decreasing catalytic activity in recycling epoxidation tests, showed very stable enantioselective behavior.

Stereodivergent Catalysis

“…This reproducible inversion of enantioselectivity was explained as a consequence of the Ti-ligand complexes, which can be monomeric and dimeric depending on the chain length of the PEG: long chain polymer prevents the formation of dimers. More recently, a similar effect has been observed in the epoxidation of cinnamyl alcohol with l -tartrate derived from N -Boc- N -methyl-2-aminoethanol, which led to the formation of the (2 S ,3 S )-epoxide in 72% yield and 70% ee, whereas the ligand supported on silsesquioxane led to the formation of the corresponding enantiomer in 51% yield and 40% ee …”

“…More recently, a similar effect has been observed in the epoxidation of cinnamyl alcohol with L-tartrate derived from N-Boc-N-methyl-2-aminoethanol, which led to the formation of the (2S,3S)-epoxide in 72% yield and 70% ee, whereas the ligand supported on silsesquioxane led to the formation of the corresponding enantiomer in 51% yield and 40% ee. 367 In the case of the asymmetric cyclopropanation of styrene with ethyl diazoacetate catalyzed by clay-immobilized bis-(oxazoline)-copper 428 catalysts, a reversal of enantioselectivity has been observed by reduction of the solvent polarity. 368,369 Just in styrene as solvent a 70/30 cis/trans mixture of cyclopropanes 429 was obtained and the major (1R,2S)diastereomer was formed in 34% ee.…”

Chiral catalysts immobilized on achiral polymers: effect of the polymer support on the performance of the catalyst

“…Similarly, García et al have also reported that when using for this chiral epoxidation a supported tartrate derivative 340, immobilised onto polyhedral oligomeric silsesquioxanes (POSS), a reversed enantioselectivity was observed with regard to the use of the related non-supported ligand 339. 389 In both cases, the change in enantioselectivity was proposed to occur as a result of the coexistence of two Ti-ligand complexes which differ in the molecularity of the ligand, one with a monomeric ligand and the other with a dimeric ligand. The POSS support and the longer PEG chains seem to favor 2 : 1 instead of 2 : 2 Ti : ligand complexes.…”

Self-Assembled Polyhedral Oligosilsesquioxane Dendrimers for Circularly Polarized Luminescence Energy Transfer

“…Shape-persistent molecular nanoparticles including derivatives of polyhedral oligosilsesquioxanes (POSSs), polyoxometalates, fullerenes, and proteins have been widely applied to construct nanosized building units (for instance, POSSs have a nanoscopic size: Si–Si distance = 0.5 nm, CH 2 –CH 2 distance = 1.5 nm) with the desired geometries and functions. − POSSs have specific geometric shapes and symmetries, which are popularly exploited as building units in biomaterials, catalysis, molecular electronics, optics, and sensors. − POSSs connected with functional groups, such as perylene bisimides, have been demonstrated to afford quasicrystalline and crystalline phases, where a strong π–π stacking interaction induces crystallization to give various phases including lamellar, cubic, hexagonal, A15, and dodecagonal quasicrystalline phase, demonstrated by Cheng and others. − Design, synthesis, and functionalization of chiral POSS derivatives provide opportunities to construct nanosized chiral building blocks with atomic precision, whereby new functions and applications in chiroptical activity and supramolecular chirality are expected. Nevertheless, connecting POSSs and their derivatives with chiral units to form macroscopic chiral structures has rarely been reported. − Recently, our group reported the first case of POSS-participated chiral nanoassemblies, which serve as a matrix to enable charge-transfer complexation . However, chiral functions and structure–property correlation of nanoassemblies from POSS derivatives are majorly unexplored.…”