Kinetic resolution | EPFL Graph Search

In organic chemistry, kinetic resolution is a means of differentiating two enantiomers in a racemic mixture. In kinetic resolution, two enantiomers react with different reaction rates in a chemical reaction with a chiral catalyst or reagent, resulting in an enantioenriched sample of the less reactive enantiomer. As opposed to chiral resolution, kinetic resolution does not rely on different physical properties of diastereomeric products, but rather on the different chemical properties of the racemic starting materials. The enantiomeric excess (ee) of the unreacted starting material continually rises as more product is formed, reaching 100% just before full completion of the reaction. Kinetic resolution relies upon differences in reactivity between enantiomers or enantiomeric complexes. Kinetic resolution can be used for the preparation of chiral molecules in organic synthesis. Kinetic resolution reactions utilizing purely synthetic reagents and catalysts are much less common than t

Mass transport of water vapor and ions from Å-scale graphene nanopores

Wan-Chi Lee

Hydrodynamics at the nanoscale involves both fundamental study and application of fluid and mass transport phenomena in nanometer-sized confinements. Nanopores in single-layer graphene can be highly attractive for exploring the molecular transport of gas and water molecules and hydrated ions at the ultimate scales of pore size and pore length. However, the experimental data is limited, and the state-of-the-art artificial nanopores still underperform compared to biological channels in cellular membranes. This dissertation focuses on developing ultimate graphene nanopore devices to study mass transport phenomena under controlled spatial confinement. We first investigated the kinetics of liquidâvapor transport from nanoscale confinements which is attractive for novel evaporation and separation applications; however, it is not explored at the ultimate confinement limit, i.e., at the atomic-thick and Ã-scale nanopore placed at the liquidâvapor interface. We show that the evaporation flux from such nanopores increases with decreasing pore size by up to one order of magnitude relative to the bare liquidâvapor interface. Molecular dynamics simulations reveal that oxygen-functionalized nanopores render rapid rotational and translational dynamics to water molecules by reducing and shortening the lifetime of waterâwater hydrogen bonds. Graphene nanopores also enable the study of ion transport across sub-nanometer-scale 2D confinements. We produce tailor-made nanopores approaching the size of hydrated ions by decoupling the pore nucleation and expansion. Monovalent metal ions are efficiently sieved from divalent ions, with K+/Mg2+ selectivity up to 70 and Li+/Mg2+ selectivity up to 50, corresponding to a sieving resolution of 1 Ã. Mitigating the non-selective pore formation further enhance the ion-sieving performance, reaching K+/Mg2+ selectivity up to 350 and Li+/Mg2+ selectivity up to 260. The pore size and structure allow adjusting the diffusion of ions across the nanopores, suggesting that the sterically induced partial dehydration process may play an important role in the observed cation selectivities. These selectivities were realized from centimeter-scale suspended graphene membranes, prepared in crack-free fashion by using dual layer reinforcement strategy where the first layer is 200-nm-thick nanoporous carbon (NPC) film hosting 20 nm pores which ensures a conformal contact and reinforcement of the graphene film and the second (top) layer is Nafion.Finally, a dual layer reinforcement is also demonstrated for preparing crack-free centimeter-scale gas separation membranes to utilize the full potential of graphene nanopores for energy-efficient applications. The bottom layer of the composite film is NPC film while the top layer is made of a 500-nm thick multi-walled carbon nanotube (MWNT) film with a pore size ranging from 200 to 300 nm. The obtained selectivities from crack-free centimeter-scale graphene membranes for H2/CH4 and H2/CO2 are 11â23 and 5â8, respectively, which is significantly higher than the corresponding Knudsen selectivities. Overall, this dissertation presents a graphene nanopore toolkit for studying fluid mechanics at the ultimate scales. The findings of enhanced water evaporation rate and ion selectivity using the nanopore platform could enrich our understanding of mass transport under extreme confinement and open new opportunities for a range of separation applications.

EPFL2022

Noniterative approach to the total asymmetric synthesis of 15-carbon polyketides and analogs with high stereodiversity

Sandrine Gerber, Kai Torsten Meilert, Marc-Etienne Schwenter, Pierre Vogel

Starting from inexpensive furan and furfuryl alcohol, a noniterative approach to the synthesis of pentadeca-1,3,5,7,9,11,13,15-octols and their derivatives has been developed. The method relies upon the double [4+3]-cycloaddition of 1,1,3-trichloro-2-oxylallyl cation with 2,2'-methylenedifuran and conversion of the adducts into meso and (+/-)-threo-1,1'-methylenebis (cis- and trans-4,6-dihydroxycyclohept-1-ene) derivatives. The latter undergo oxidative cleavage of their alkene moieties, generating 5-hydroxy-7-oxoaldehydes that are reduced diastereoselectively into either syn or anti-5,7-diols. Asymmetry is realized using either chiral desymmetrization with Sharpless asymmetric dihydroxylation or by kinetic resolution of polyols using lipase-catalyzed acetylations. All of the possible stereomeric pentadeca-1,3,5,7,9,1 1,13,15-octols and derivatives can be obtained with high stereoselectivity applying simple operations, thus demonstrating the high stereodiversity of this new, noniterative approach to the asymmetric synthesis of long-chain polyketides.

2005

Mass transport of water vapor and ions from Å-scale graphene nanopores

Wan-Chi Lee

EPFL2022