Template-directed synthesis of mechanically interlocked molecular bundles using dynamic covalent chemistry

Mixing the dipyrido[24]crown-8 derivatives carrying one or two formyl group(s) on the 4 position(s) of their pyridine ring(s) with a 3-fold symmetrical trisammonium ion template in a 3:1 ratio in CD3NO2 results in the formation of thermodynamically stable [4]pseudorotaxanes which, upon addition of a 1,3,5-trisaminobenzene cap, form mechanically interlocked molecular bundles with one and two caps, respectively, by virtue of dynamic imine bond formation.     

Supramolecular self-assembly of dendronized polymers: reversible control of the polymer architectures through acid-base reactions

Acid-base switchable supramolecular dendronized polyacetylenes (DPAs) with increasing steric bulk on going from generation one [G1] to three [G3], were constructed using multiple self-assembly processes between Fréchet-type [G1]-[G3]-dendritic dialkylammonium salts and a dibenzo[24]crown-8-containing polymer. The formation of the supramolecular systems is acid-base switchable to either an ON (rodlike dendronized polymers) or an OFF (flexible polymers) state. Thus, by controlling the superstructures of the supramolecular polymers with the [G1]-[G3] dendrons, it is possible to induce conformational changes within the polymer backbones. The supramolecular dendronized polymers, as well as their threading-dethreading properties, were characterized by (1)H NMR and UV absorption spectroscopies, gel permeation chromatography (GPC) and light scattering (LS). Independent measures of molecular weight (GPC, LS) indicate that DPAs behave as increasingly rigid macromolecules with each generation in solution. Molecular dynamics simulations of each DPA suggest that the lengths of the polymer backbones increase accordingly. Atomic force microscopy of the [G3]-dendronized polystyrene (DPS), as well as the DPAs, reveal surface morphologies indicative of aggregated superstructures.     

Vinylcyclobutane-cyclohexene rearrangement: theoretical exploration of mechanism and relationship to the Diels-Alder potential surface

[graph: see text] The vinylcyclobutane-cyclohexene rearrangement has been studied computationally with density functional theory and complete active space SCF calculations. The rearrangement proceeds through a diradical that exists on a very flat potential energy surface. Transition structures for conformational processes, only slightly higher in energy than the minimum energy reaction path, account for the stereochemistries of products observed in the thermal rearrangements of vinylcyclobutane derivatives. The connection of this rearrangement to the Diels-Alder reaction of butadiene with ethylene is discussed.     

Why 6-methylpentacene deconjugates but avoids the thermally allowed unimolecular mechanism

[reaction: see text] Tautomeric equilibria involving hydrogen migration in methylacenes were explored computationally using DFT methods. As the aromatic system becomes more extended, the methylene isomer is predicted to become favored. Reasonable-looking pericyclic sigmatropic hydrogen shifts are found to be energetically prohibitive, and bimolecular mechanisms involving radical pair intermediates are energetically feasible.