Coaxial Helices in Chiral Supramolecular Aggregates from Highly Hindered Chiral Allenes.

Chiral allenes self-assembly following a cooperative mechanism into a supramolecular chiral aggregate consisting of two coaxial helices: the internal helix described by the allene stack and the external helix which consist in a 4-helix described by the four allene substituents. More precisely, this supramolecular aggregate possesses six axially chiral elements within its structure-the allene, the allene stack (internal helix) and the stacks of the four allene substituents (external 4-helix)-. Interestingly, slight variations in the magnitude of the tilting degree while keeping its P- or M- orientation (internal helix) can vary the orientation of the 4-axial motifs at the external helix.

Macromolecular Switch Based on Conformational Control Exerted by an Achiral Side Chain within an Axially Chiral Locked Pendant.

Molecular switches, supramolecular chemistry, and polymers can be combined to create stimuli-responsive multichiral materials. Therefore, by acting on the extended/bent conformational composition of an achiral arm, it is possible to create a macromolecular gear, where different supramolecular interactions can be activated/deactivated to control the helical sense of a polymer containing up to five different chiral axial motifs. For this, a chiral allene with a flexible achiral arm was introduced as a pendant in poly(phenylacetylene).

Dynamic Axial-to-Helical Communication Mechanism in Poly[(allenylethynylenephenylene)acetylene]s under External Stimuli.

Helix inversion in chiral dynamic helical polymers is usually achieved by conformational changes at the pendant groups induced through external stimuli. Herein, a different mechanism of helix inversion in poly(phenylacetylene)s (PPAs) is presented, based on the activation/deactivation of supramolecular interactions. We prepared poly[(allenylethynylenephenylene)acetylene]s (PAEPAs) in which the pendant groups are conformationally locked chiral allenes.

2-Hydroxychalcone-β-Cyclodextrin Conjugate with pH-Modulated Photoresponsive Binding Properties.

Stimuli-responsive supramolecular receptors are important building blocks for the construction of self-assembled functional materials. We report the design and synthesis of a pH- and light-responsive 2-hydroxychalcone-β-cyclodextrin conjugate () and its characterization by spectroscopic and computational methods. follows the typical reaction network of -chalcone-flavylium photoswitches.

Accessible triplet excited states in the photoisomerization of allenes with extended conjugation.

A series of bidentate allene- and enyne-containing ligands have been synthesized and the photochemical properties of their rhenium(I) complexes have been studied. These complexes exhibit facile isomerization of the conjugated double bonds upon ambient light exposure. Simulations unveiled a very efficient intersystem crossing and the consequent key role of the triplet states in the observed photochemistry of these substrates upon rhenium(I) complexation.

A Chiral Molecular Cage Comprising Diethynylallenes and N-Heterotriangulenes for Enantioselective Recognition.

Chirality, a characteristic tool of molecular recognition in nature, is often a complement of redox active systems. Scientists, in their eagerness to mimic such sophistication, have designed numerous chiral systems based on molecular entities with cavities, such as macrocycles and cages. In an attempt to combine chirality and redox-active species, in this contribution we report the synthesis and detailed characterization of a chiral shape-persistent molecular cage based on the combination of enantiopure diethynylallenes and electron-rich bridged triarylamines, also known as N-heterotriangulenes.

NMR spectral fingerprint patterns as diagnostics for the unambiguous configurational analysis of the classic organo-gelator 1,3:2,4-dibenzylidene-d-sorbitol (DBS) and its derivatives.

On the basis of experimental data and density functional theory (DFT) chemical shift and scalar coupling predictions, simple spectral nuclear magnetic resonance (NMR) fingerprint patterns have been established for the determination of the configuration in 1,3:2,4-dibenzylidene-d-sorbitol (DBS), a classic low molecular weight gelator, and its derivatives. The results rigorously prove the orientation of the phenyl rings in DBS that had been previously assumed in the literature on the basis of thermodynamic arguments.

Photochemically Driven Tandem Process in the Construction of a Biscyclopropylcage from 2,5-Dimethoxy--benzoquinone and Terminal Acetylenes.

A photochemical two-step one-pot synthesis of novel biscyclopropyl-box-shaped compounds via the reaction of 2,5-dimethoxy--benzoquinone and monosubstituted alkynes is reported. The reaction mechanism for a process in which six new C-C bonds are formed is explored by means of experimental and computational techniques. The whole process occurs with complete selectivity, and only one densely decorated diastereomer is obtained; such a degree of control and substitution makes for a rather powerful and complexity-building process.

Double Protonation of a -Bipyridoallenophane Detected via Chiral-Sensing Switch: The Role of Ion Pairs.

We prove that the confinement of the conformational space of pyridoallenophanes leads to intense chiroptical responses. Unlike the cyclic dimer [14], single-conformation [14]pyridoallenophanes isomerize under thermal and photochemical conditions. Yet, less-strained [14]-bipyridoallenophanes are stable and are prepared successfully.

Opening access to new chiral macrocycles: from allenes to spiranes.

Chiral macrocycles offer great potential and versatility regarding their applications. They have been employed in asymmetric catalysts, as chiral sensors, and as chiral supramolecular frameworks. For these reasons, they have been attracting increasing interest over the years.

Disodium cromoglycate: exploiting its properties as a NMR weak-aligning medium for small organic molecules.

Chromonic phases are a family of lyotropic liquid crystals (LC) formed by ionic aromatic mesogens such as disodium cromoglycate (cromolyn), sunset yellow and others. It is well known that chromonic phases are oriented in the presence of external magnetic fields, leading to the observation of anisotropic NMR observables such as quadrupolar splittings or residual dipolar couplings. Despite the fact that the cromolyn nematic LC phase (N) presents important advantages like great homogeneity, small line broadening, and easy sample preparation, it has been scarcely used as a water-compatible NMR orienting medium, in part due to a too strong induced degree of alignment on the guest molecules.

Preparation and characterization of a halogen-bonded shape-persistent chiral alleno-acetylenic inclusion complex.

A chiral bidentate inclusion complex has been formed by halogen-bond interaction between the pyridyl moieties of a pyridoallenoacetylenic host and octafluorodiiodobutane. X-ray crystallography showed that the guest adopts a chiral conformation inside the molecular channels formed by stacking of the host units. A 10 ppm shielding of the (15)N NMR resonance for the pyridil units provided evidence of the formation of the halogen-bond complex in solution.

Theoretical and experimental exploration of the photochemistry of resveratrol: beyond the simple double bond isomerization.

The photochemical isomerization of resveratrol has been the subject of recent studies in which contradictory results were reported. The photoproduct mixture of this reaction needs to be considered more complex than the coexistence of cis and trans isomers. An unidentified third product, at least, has been detected in various studies although its nature was unknown.

A stepwise retro-imino-ene as a key step in the mechanism of allene formation via the Crabbé acetylene homologation.

The mechanism of the acetylene homologation procedure accidentally discovered and further developed by Crabbé and coworkers is unknown. Kinetic isotope effect (KIE) experiments, however, suggest that an intramolecular hydrogen shift is the key step of the transformation. In this work, we present a computational study of this mechanism.

Experimental and computational exploration of indolizinyl carbene generation. A route to biindolizines.

In previous work, (E)-2-enynyl pyridines were reported to yield indolizinyl singlet carbenes through base-catalyzed E/Z isomerization followed by a 5-exo-dig pseudocoarctate cyclization. We report herein that in the presence of ethyl acrylate these carbenes undergo stereoselective cis-cyclopropanation due mainly to electrostatic interactions in the transition state. The scope of this carbene generation scheme has been further explored through the preparation of a symmetric bis(pyridylenyl)alkyne which spontaneously furnished the biindolizine core in a one-pot reaction.

Mechanistic investigation on the formation of indolizines from 2-enynylpyridines.

2,3,7-Trisubstituted indolizines were obtained from E- or Z-2-enynyl-4-substituted pyridines. The mechanistic pathway involves a base-catalyzed double-bond isomerization, if the E-isomer is the starting material, followed by a concerted pseudocoarctate cyclization.