Molecular aggregation in liquid water: Laplace spectra and spectral clustering of H-bonded network

Application of the spectral clustering method based on the analyses of the Laplace matrix is an acceptable indicator of the global properties of H-bonded network. The first peak of the Laplace spectra contains six eigenvalues. These results suggest that six communities are always formed in our simulated systems independently of the number of molecules in the cubic box. We showed that the H-bonded environment on the surface of the clusters is different from what can be found inside of the clusters. The fraction of four-coordinated molecules is significantly larger in the case of surface molecules. Our work emphasizes that the periodic boundary conditions always cause clustering in the system

The influence of cations on the dipole moments of neighboring polar molecules

It is shown that the dipole moment of polar (water, methanol, formamide, acetone and acetonitrile) molecules in the neighborhood of a cation is increased primarily by polarization from the bare electrostatic charge of the cation, although the effective value of the latter is somewhat reduced by "back donation" of electrons from neighbouring polar molecules. In other words, the classical picture may be viewed as if a point charge slightly smaller than the nominal charge of the cation would be placed at the cation site. It was found that the geometrical arrangement of the polar molecules in the first solvation shell is such that their mutual polarization reduces the dipole moments of individual molecules, so that in some cases they become smaller than the dipole moment of the free protic or aprotic molecule. We conjecture that this behavior is essentially a manifestation of the Le Chatellier-Braun principle

Hydrogen bonding and percolation in propan-2-ol -- water liquid mixtures: X-ray diffraction experiments and computer simulations

Synchrotron X-ray diffraction measurements have been conducted on aqueous mixtures of propan-2-ol (a.k.a. isopropanol, or 2-propanol), for alcohol contents between 10 and 90 molar %, from room temperature down to 230 K. Molecular dynamics simulations, by using an all-atom parametrization of the propan-2-ol molecule and the well-known TIP4P/2005 water model, were able to provide semi-quantitative descriptions of the measured total structure factors. Various quantities related to hydrogen bonding, like hydrogen bond numbers, size distribution of cyclic entities and cluster size distributions, have been determined from the particle co-ordinates obtained from the simulations. The percolation threshold at room temperature could be estimated to be between isopropanol concentrations of 62 and 74 molar %, whereas at very low temperature, calculations yielded a value above 90 molar %

Synthesis of (−)-cytisine using a 6-endo aza-Michael addition

An asymmetric synthesis of (-)-cytisine has been achieved. The piperidine C-ring was formed using a stereodivergent intramolecular 6-endo aza-Michael addition. The B-ring was established by intramolecular pyridine N-alkylation. The absolute stereochemistry was established by an Evans acyl oxazolidinone enolate alkylation reaction that proceeded with an unexpected stereochemical outcome due to participation of the pyridine nitrogen lone pair.MOE (Min. of Education, S’pore)Accepted versio

Conformationally Locked Pyramidality Explains the Diastereoselectivity in the Methylation of trans-Fused Butyrolactones

A stereoselectivity model inspired by the total synthesis of stemona alkaloids is developed to explain why enolate-derived 3,4-fused butyrolactones are methylated with a preference for syn alkylation. The model shows how conformational locking present in nonplanar enolate structures favors syn over anti methylation, due to less significant structural distortions in the syn pathway. The developed model was also successfully used to rationalize selectivities of previously documented methylation reactions.peerReviewe

Bilateral aromatic extension of corannulene nucleus

Herein we show that a dibromo imide derivative of corannulene could be subjected to a palladium-catalyzed Heck reaction in order to install aryl vinylene groups in a bilateral fashion on the C5-symmetric aromatic nucleus. A photochemically-induced oxidative cyclization then affords π-extended structures in isolated overall yields of 35–44 %. Due to the aromatic area extension and the presence of electron withdrawing groups, the synthesized molecules exhibit an anodic shift of ≈1 V in their first reduction potential as compared to pristine corannulene. The bilateral growth of the aromatic scaffold, therefore, represents a viable strategy to prepare π-extended corannulenes with an electron deficient character.Agency for Science, Technology and Research (A*STAR)Ministry of Education (MOE)Nanyang Technological UniversityFinancial support from the Ministry of Education Singapore under the AcRF Tier 1 (2018-T1-001-176, M4012047), Agency for Science, Technology and Research, (A*STAR)-AMEIRG A1883c0006, and NTU (M4081566) is gratefully acknowledged

How an early or late transition state impacts the stereoselectivity of tetrahydropyran formation by intramolecular oxa-Michael addition

The intramolecular oxa-Michael addition giving tetrahydropyrans has been examined experimentally using both acidic and basic catalysis. With acidic catalysis, the diequatorial product is exclusively obtained in a kinetically controlled reaction in all cases. Under basic conditions at low temperature, the reaction is again under kinetic control, but formation of the axial-equatorial isomer is generally favoured with an (E)-Michael acceptor, although isomerisation to the diequatorial isomer is observed at higher temperatures. Computationally, it is found that the acid catalysed reaction has a late transition state and the kinetic favouring of the diequatorial isomer has a steric explanation. In contrast, under strongly basic conditions, an early transition state is found. Electrostatic effects are likely to be the main contributor to the stereoselectivity for the (E)-isomer and steric interactions for the (Z)-isomer.Accepted versio

The total synthesis of raistrickindole A

The total synthesis of raistrickindole A has been achieved, thereby confirming the proposed structure as an N-hydroxylated DKP. In the first but less selective approach, the DKP was built up by cyclization of a diastereoisomerically mixed N-hydroxylated dipeptide. In the second approach, the same DKP was constructed stereoselectively by the intramolecular Mitsunobu reaction of a hydroxamic acid. The synthesis was completed by a stereoselective oxidative cyclization.Nanyang Technological UniversityNational Research Foundation (NRF)Submitted/Accepted versionWe thank NTU for financial support of this work. This research was partially supported by the National Research Foundation (NRF) Singapore, NRF Competitive Research Programme (CRP), Grant Award Number NRF-CRP18-2017-01

Stereoselective Synthesis of Fluoroalkanes via FLP Mediated Monoselective C─F Activation of Geminal Difluoroalkanes

Abstract A method of desymmetrization of geminal difluoroalkanes using frustrated Lewis pair (FLP) mediated monoselective C–F activation where a chiral sulfide is the Lewis base component is reported. The stereoselective reaction provides generally high yields of diastereomeric sulfonium salts with dr of up to 95:5. The distribution of diastereomers is found to be thermodynamically controlled via facile sulfide exchange. The use of enantiopure chiral sulfides allows for high stereospecificity in nucleophilic substitution reactions and the formation of stereoenriched products