P/M 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). Through flexible arm control between extended and bent conformations, it is possible to selectively induce either a P or M helical sense in the polymer, while the relative spatial distribution of the substituents in the allene remains unaltered in two perpendicular planes (configurationally locked). These results show that complex dynamic multichiral materials can be obtained by the polymerization of appropriate monomers that combine chirality, switching properties, and the ability to generate chiral supramolecular assemblies.

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. Therefore, their substituents are placed in specific spatial orientations. As a result, the screw sense of a PAEPA is fixed by the allenyl substituent with the optimal size/distance relationship to the backbone. This helical sense command can be surpassed by supramolecular interactions between another substituent on the allene and appropriate external stimuli, such as amines. So, a helix inversion occurs through a novel axial-to-helical communication mechanism, opening a new scenario for taming the helices of chiral dynamic helical polymers.

2-Hydroxychalcone-ß-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-ß-cyclodextrin conjugate (1-Ct) and its characterization by spectroscopic and computational methods. 1-Ct follows the typical reaction network of trans-chalcone-flavylium photoswitches. Upon light irradiation, 1-Ct can be photochemically converted into the cis-chalcone/hemiketal forms (1-Cc/1-B) under neutral pH conditions or to the flavylium cation (1-AH+) at acidic pH values. This stimuli-responsive ß-cyclodextrin host, 1-Ct, was found to form stronger intramolecular self-inclusion complexes (Kintra = 14) than 1-AH+ (Kintra = 3) and weaker than 1-Cc/1-B (overall Kintra = 179), allowing control over their stability and binding properties by combinations of pH and light stimuli.

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. Its ability for chiral recognition in solution was revealed through UV/vis titrations with enantiopure helicenes.

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-p-benzoquinone and Terminal Acetylenes.

A photochemical two-step one-pot synthesis of novel biscyclopropyl-box-shaped compounds via the reaction of 2,5-dimethoxy-para-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 cis-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 [142], single-conformation [141]pyridoallenophanes isomerize under thermal and photochemical conditions. Yet, less-strained [141]-bipyridoallenophanes are stable and are prepared successfully. They, unexpectedly, undergo double protonation as a result of cooperative ion-pairing and hydrogen bonding. The complex formation forces a single configuration of the axis connecting both pyridyl rings recognized by a diagnostic circular dichroism (CD) signal at 330 nm.

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. Despite all of the work developed in this area, most of the reported chiral macrocycles are not conformationally stable and present weak chiroptical responses. Such features substantially limit the scope of applications for these compounds. On the other hand, we have shown that axially chiral allenes can be introduced into macrocycles, conferring conformational stability and outstanding chiroptical responses. However, these allenes photoisomerize when conjugated with electron-donating groups, hampering the possibility of synthesizing systems with tuned optical properties. To overcome all of these limitations with a single structural motif, we propose the use of spiranes to construct new stable, conformationally rigid, and chemically functionalizable macrocyclic structures with strong chiroptical responses. As a first step in this new direction, we theoretically predict the chiroptical responses for macrocycles bearing spiranes to be as strong as with their allenic counterparts. As a side product, we also test the popular Minnesota functional, M06-2X, and compare it with cam-B3LYP, which has been previously analyzed with respect to experimental data in our laboratory. Thus, we hereby propose that spiranes are a good alternative to allenes for the construction of new chiral macrocycles.

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.

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. However, the use of cromolyn-brine mixtures led to the optimum degree of alignment allowing to record (1)H-(13)C dipolar couplings with good accuracy.

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. In this work, we aim to provide a thorough description of the photochemical course of this reaction through experimental and computational approaches working in a synergetic association.

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. We found that the reaction proceeds via an unexpected stepwise retro-imino-ene rearrangement. This mechanism justifies the role of Cu(I) as a reaction catalyst and is also compatible with the KIE experiments reported.

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. Computational characterization of this transformation suggests a highly asynchronous double cyclization.

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.