A templating approach for monodisperse self-assembled organic nanostructures

The precise structural control is known for self-assembly into closed spherical structures (e.g., micelles), but similar control of open structures is much more challenging. Inspired by natural tobacco mosaic virus, we present the use of a rigid-rod template to control the size of a one-dimensional self-assembly. We believe that this strategy is novel for organic self-assembly and should provide a general approach to controlling size and dimension.     

Phase behavior of a simple dipolar fluid under shear flow in an electric field

Nonequilibrium molecular dynamics simulations are performed on a dense simple dipolar fluid under a planar Couette shear flow. Shear generates heat, which is removed by thermostatting terms added to the equations of motion of the fluid particles. The spatial structure of simple fluids at high shear rates is known to depend strongly on the thermostatting mechanism chosen. Kinetic thermostats are either biased or unbiased: biased thermostats neglect the existence of secondary flows that appear at high shear rates superimposed upon the linear velocity profile of the fluid. Simulations that employ a biased thermostat produce a string phase where particles align in strings with hexagonal symmetry along the direction of the flow. This phase is known to be a simulation artifact of biased thermostatting, and has not been observed by experiments on colloidal suspensions under shear flow. In this paper, we investigate the possibility of using a suitably directed electric field, which is coupled to the dipole moments of the fluid particles, to stabilize the string phase. We explore several thermostatting mechanisms where either the kinetic or configurational fluid degrees of freedom are thermostated. Some of these mechanisms do not yield a string phase, but rather a shear-thickening phase; in this case, we find the influence of the dipolar interactions and external field on the packing structure, and in turn their influence on the shear viscosity at the onset of this shear-thickening regime.     

Humic acid sorption onto a quartz sand surface: A kinetic study and insight into fractionation

A kinetic study of Aldrich humic acid sorption onto a quartz sand surface has revealed an initial rapid uptake of humic acid molecules followed by a much slower sorption. The humic acid molecular weight and chemical fractionation resulting from adsorption onto the simple quartz sand surface were investigated for the two kinetic steps by coupled asymmetric flow-field flow fractionation-UV/visible absorption spectrophotometry. The molecular weight distribution of residual humic acid in solution after adsorption deviated from the original molecular weight distribution, showing preferential adsorption of certain molecular weight components. This fractionation is different after the two kinetic steps. Humic acid molecules characterised by a molecular weight below 4800 Da and with a weight-average molecular weight (M(w)) of 1450 Da were adsorbed after the fast kinetic step, whereas humic acid molecules in the molecular weight range 1400-9200 Da and of M(w) 3700 Da were adsorbed after the slower uptake. Therefore, the adsorption of low molecular weight humic components takes place initially, and is then followed by the adsorption of higher molecular weight components. Chemical adsorptive fractionation, investigated by studying the 253 nm/203 nm absorbance ratio over time, shows that aromatic components are preferentially adsorbed during the fast kinetic step. The fractionation pattern may be explained by the physicochemical characteristics of the Aldrich humic acid and the underlying sorption processes. The trend for the sorption kinetics of europium onto the quartz sand surface in the presence of humic acid is similar to that of the humic acid itself.     

Total synthesis and proof of relative stereochemistry of (-)-aureonitol

Two trisubstituted epimeric tetrahydrofurans, 1 and 2, have been synthesized in order to confirm the relative stereochemistry in the natural product aureonitol. The key step in the synthesis of 1 and 2 involved a stereoselective intramolecular allylation of an allylsilane with an aldehyde, which introduced the stereotriad in the five-membered ring. The major tetrahydrofuran diastereoisomer 18 from this cyclization reaction was subsequently elaborated to tetrahydrofuran 1. Its 3-epimer (2) was then prepared from 1 via an oxidation-reduction sequence. Compound 1 exhibits identical (1)H NMR data to those reported for aureonitol, which was isolated from Helichrysum aureonitons by Bohlmann in 1979, whereas the (1)H NMR data for 2 are markedly different. The (1)H NMR data (in CDCl3, CD3OD, and C6D6) and (13)C NMR data (in CDCl3) for 1 are also identical with those reported for a natural product isolated from various Chaetomium sp. by Abraham, Seto, and Teuscher. These findings support Abraham's conclusion that the structure of aureonitol should be revised from 2 to 1. The enantioselective synthesis of 1 has also confirmed that (-)-aureonitol isolated by Abraham contains the (2S,3R,4S) absolute configuration of stereocenters on the tetrahydrofuran ring.     

Diastereoselection of chiral acids in a cylindrical capsule

The cylindrical dimeric capsule binds two chiral carboxylic acids with modest selectivity for homochiral or heterochiral pairs.     

A Rare Alder-ene Cycloisomerization of 1,6-Allenynes

Thermally induced cycloisomerization reactions of 1,6-allenynes gives α-methylene-γ-lactams via intramolecular Alder-ene reactions. The mechanism is supported by computational and deuterium labelling studies. This thermal, non-radical method enables the discovery of a hitherto unknown route that proceeds via a divergent mechanism distinct from the previous [2+2] cycloisomerization manifold.     

Reactions in viscous media: potential and free energy surfaces in solvent–solute coordinates

We propose a universal definition of reaction-specific solvent coordinate, described in terms of molecular-dynamic trajectories of the solvent and suitable for a variety of reactions in viscous solvents. We then use molecular dynamics simulations to obtain two-dimensional potential energy and free energy surfaces of model liquid phase isomerisation reactions in solvent–solute coordinates and discuss factors determining their topography.     

Facile asymmetric construction of a functionalized dodecahydrobenz[a]indolo[3,2-h]quinolizine template

We report a highly diastereoselective approach for the synthesis of a functionalized dodecahydrobenz[a]indolo[3,2-h]quinolizine ring system that is present as the heterocyclic core of the manadomanzamine alkaloids. We have achieved complete control over the relative and absolute stereochemistries at the three contiguous stereocentres at ring positions 1, 10 and 24 in only two linear synthetic steps. The introduction of useful functionality to the heterocyclic skeleton is significant as this may allow for future derivatization, and application of this route in an asymmetric synthesis of the manadomanzamine natural products.