Steric Scale of Common Substituents from Rotational Barriers of <i>N</i>‑(<i>o</i>‑Substituted aryl)thiazoline-2-thione Atropisomers

A steric scale of 20 recurrent groups was established from comparison of rotational barriers on <i>N</i>-(<i>o</i>-substituted aryl)­thiazoline-2-thione atropisomers. The resulting energy of activation Δ<i>G</i>^⧧_rot reflects the spatial requirement of the ortho substituent borne by the aryl moiety, electronic aspects and external parameters (temperature and solvent) generating negligible contributions. Concerning divergent rankings reported in the literature, the great sensitivity of this model allowed us to show unambiguously that a methyl appears bigger than a chlorine and gave the following order in size: CN > OMe > OH. For the very bulky CF₃ and <i>i</i>Pr groups, constraints in the ground state decreased the expected Δ<i>G</i>^⧧_rot values resulting in a minimization of their apparent sizes

Atropisomerism in Amidinoquinoxaline <i>N-</i>Oxides: Effect of the Ring Size and Substituents on the Enantiomerization Barriers

The atropisomerism of novel 2,3-dihydro-1<i>H</i>-pyrimido­[1,2-<i>a</i>]­quinoxaline 6-oxides <b>1</b> bearing dissymmetric (<i>ortho</i>-substituted) 5-aryl residues and the homologous 1,2-dihydroimidazo­[1,2-<i>a</i>]­quinoxaline 5-oxides <b>2</b> was investigated. The existence of a chiral axis was demonstrated for compound <b>1a</b> by X-ray diffraction and by DFT calculations of the ground state geometry. The resolution of the atropisomeric enantiomers on chiral stationary phases is reported. The barriers to enantiomerization were determined by off-line racemization studies and/or by treatment of the plateau-shaped chromatograms during chromatography on chiral support. A clear ring size effect was evidenced. In all cases, six-membered amidine derivatives <b>1</b> showed higher barriers than the corresponding lower homologues <b>2</b>, which also display lower sensitivity to the substituent size. Transition states for the interconversion of the atropisomers were located using DFT calculations, and involved the interaction of the <i>ortho</i> substituent with the formally sp² nitrogen in the amidine moiety. In contrast, in the most favored enantiomerization transition state of the 2-nitro derivative the <i>ortho</i> substituent is close to the <i>N</i>-oxide group

Atropisomerization in <i>N</i>‑aryl-2(1<i>H</i>)‑pyrimidin-(thi)ones: A Ring-Opening/Rotation/Ring-Closure Process in Place of a Classical Rotation around the Pivot Bond

Uncatalyzed racemization processes in atropisomeric diphenyl-like frameworks are classically described as the result of the rotation around the pivotal single bond linking two planar frameworks. Severe constraints leading to more or less distorted transition states account for the experimental barrier to atropenantiomerization. In 1988, one of us hypothesized that, in <i>N</i>-aryl-2­(1<i>H</i>)-pyrimidin-(thi)­ones, a ring-opening/ring-closure process was contributing to the observed racemization process accounting for the lower barriers in the sulfur analogues than in oxygen analogues. Now, a series of six novel 6-amino-5-cyano-1,4-disubstituted-2­(1<i>H</i>)-pyrimidinones <b>5a</b>–<b>5f</b> and two 6-amino-5-cyano-4-<i>p</i>-tolyl-1-substituted-2­(1<i>H</i>)-pyrimidinethiones <b>6a</b> and <b>6b</b> were synthesized and characterized through spectroscopic and X-ray diffraction studies. Semipreparative HPLC chiral separation was achieved, and enantiomerization barriers were obtained by thermal racemization. The rotational barriers of 6-amino-5-cyano-1-<i>o</i>-tolyl-4-<i>p</i>-tolyl-2­(1<i>H</i>)-pyrimidinone (<b>5b</b>) and 6-amino-5-cyano-1-(naphthalen-1-yl)-4-<i>p</i>-tolyl-2­(1<i>H</i>)-pyrimidinone (<b>5e</b>) were found to be 120.4 and 125.1 kJ·mol^–1 (<i>n</i>-BuOH, 117 °C), respectively, and those of the corresponding thiones were 116.8 and 109.6 kJ·mol^–1 (EtOH, 78 °C), respectively. DFT calculations of the rotational barriers clearly ruled out the classical rotation around the pivotal bond with distorted transition states in the case of the sulfur derivatives. Instead, the ranking of the experimental barriers (sulfur versus oxygen, and <i>o</i>-tolyl versus 1-naphthyl in both series) was nicely reproduced by calculations when the rotation occurred via a ring-opened form in <i>N</i>-aryl-2­(1<i>H</i>)-pyrimidinethiones

Atropisomerization in <i>N</i>‑aryl-2(1<i>H</i>)‑pyrimidin-(thi)ones: A Ring-Opening/Rotation/Ring-Closure Process in Place of a Classical Rotation around the Pivot Bond

Uncatalyzed racemization processes in atropisomeric diphenyl-like frameworks are classically described as the result of the rotation around the pivotal single bond linking two planar frameworks. Severe constraints leading to more or less distorted transition states account for the experimental barrier to atropenantiomerization. In 1988, one of us hypothesized that, in <i>N</i>-aryl-2­(1<i>H</i>)-pyrimidin-(thi)­ones, a ring-opening/ring-closure process was contributing to the observed racemization process accounting for the lower barriers in the sulfur analogues than in oxygen analogues. Now, a series of six novel 6-amino-5-cyano-1,4-disubstituted-2­(1<i>H</i>)-pyrimidinones <b>5a</b>–<b>5f</b> and two 6-amino-5-cyano-4-<i>p</i>-tolyl-1-substituted-2­(1<i>H</i>)-pyrimidinethiones <b>6a</b> and <b>6b</b> were synthesized and characterized through spectroscopic and X-ray diffraction studies. Semipreparative HPLC chiral separation was achieved, and enantiomerization barriers were obtained by thermal racemization. The rotational barriers of 6-amino-5-cyano-1-<i>o</i>-tolyl-4-<i>p</i>-tolyl-2­(1<i>H</i>)-pyrimidinone (<b>5b</b>) and 6-amino-5-cyano-1-(naphthalen-1-yl)-4-<i>p</i>-tolyl-2­(1<i>H</i>)-pyrimidinone (<b>5e</b>) were found to be 120.4 and 125.1 kJ·mol^–1 (<i>n</i>-BuOH, 117 °C), respectively, and those of the corresponding thiones were 116.8 and 109.6 kJ·mol^–1 (EtOH, 78 °C), respectively. DFT calculations of the rotational barriers clearly ruled out the classical rotation around the pivotal bond with distorted transition states in the case of the sulfur derivatives. Instead, the ranking of the experimental barriers (sulfur versus oxygen, and <i>o</i>-tolyl versus 1-naphthyl in both series) was nicely reproduced by calculations when the rotation occurred via a ring-opened form in <i>N</i>-aryl-2­(1<i>H</i>)-pyrimidinethiones

Ruthenium-Vinylhelicenes: Remote Metal-Based Enhancement and Redox Switching of the Chiroptical Properties of a Helicene Core

Introducing metal-vinyl ruthenium moieties onto [6]­helicene results in a significant enhancement of the chiroptical properties due to strong metal–ligand electronic interactions. The electro-active Ru centers allow the achievement of the first purely helicene-based redox-triggered chiroptical switches. A combination of electrochemical, spectroscopic, and theoretical techniques reveals that the helicene moiety is a noninnocent ligand bearing a significant spin density

Ruthenium-Vinylhelicenes: Remote Metal-Based Enhancement and Redox Switching of the Chiroptical Properties of a Helicene Core

Introducing metal-vinyl ruthenium moieties onto [6]­helicene results in a significant enhancement of the chiroptical properties due to strong metal–ligand electronic interactions. The electro-active Ru centers allow the achievement of the first purely helicene-based redox-triggered chiroptical switches. A combination of electrochemical, spectroscopic, and theoretical techniques reveals that the helicene moiety is a noninnocent ligand bearing a significant spin density

Atropisomerism in a 10-Membered Ring with Multiple Chirality Axes: (3<i>Z</i>,9<i>Z</i>)‑1,2,5,8-Dithiadiazecine-6,7(5<i>H</i>,8<i>H</i>)‑dione Series

For the first time, chirality in (3<i>Z</i>,9<i>Z</i>)-1,2,5,8-dithiadiazecine-6,7­(5<i>H</i>,8<i>H</i>)-dione series was recognized. Enantiomers of the 4,9-dimethyl-5,8-diphenyl analogue were isolated at room temperature, and their thermal stability was determined. X-ray crystallography confirmed the occurrence of a pair of enantiomers in the crystal. Absolute configurations were assigned by comparing experimental and calculated vibrational/electronic circular dichroism spectra of isolated enantiomers. A distorted tesseract (four-dimensional hypercube) was used to visualize the calculated enantiomerization process, which requires the rotation around four chirality axes. Conformers of higher energy as well as several concurrent pathways of similar energies were revealed

Anisotropic Organization and Microscopic Manipulation of Self-Assembling Synthetic Porphyrin Microrods That Mimic Chlorosomes: Bacterial Light-Harvesting Systems

Being able to control in time and space the positioning, orientation, movement, and sense of rotation of nano- to microscale objects is currently an active research area in nanoscience, having diverse nanotechnological applications. In this paper, we demonstrate unprecedented control and maneuvering of rod-shaped or tubular nanostructures with high aspect ratios which are formed by self-assembling synthetic porphyrins. The self-assembly algorithm, encoded by appended chemical-recognition groups on the periphery of these porphyrins, is the same as the one operating for chlorosomal bacteriochlorophylls (BChl's). Chlorosomes, rod-shaped organelles with relatively long-range molecular order, are the most efficient naturally occurring light-harvesting systems., They are used by green photosynthetic bacteria to trap visible and infrared light of minute intensities even at great depths, e.g., 100 m below water surface or in volcanic vents in the absence of solar radiation. In contrast to most other natural light-harvesting systems, the chlorosomal antennae are devoid of a protein scaffold to orient the BChl's; thus, they are an attractive goal for mimicry by synthetic chemists, who are able to engineer more robust chromophores to self-assemble. Functional devices with environmentally friendly chromophoreswhich should be able to act as photosensitizers within hybrid solar cells, leading to high photon-to-current conversion efficiencies even under low illumination conditionshave yet to be fabricated. The orderly manner in which the BChl's and their synthetic counterparts self-assemble imparts strong diamagnetic and optical anisotropies and flow/shear characteristics to their nanostructured assemblies, allowing them to be manipulated by electrical, magnetic, or tribomechanical forces

Anisotropic Organization and Microscopic Manipulation of Self-Assembling Synthetic Porphyrin Microrods That Mimic Chlorosomes: Bacterial Light-Harvesting Systems

Being able to control in time and space the positioning, orientation, movement, and sense of rotation of nano- to microscale objects is currently an active research area in nanoscience, having diverse nanotechnological applications. In this paper, we demonstrate unprecedented control and maneuvering of rod-shaped or tubular nanostructures with high aspect ratios which are formed by self-assembling synthetic porphyrins. The self-assembly algorithm, encoded by appended chemical-recognition groups on the periphery of these porphyrins, is the same as the one operating for chlorosomal bacteriochlorophylls (BChl's). Chlorosomes, rod-shaped organelles with relatively long-range molecular order, are the most efficient naturally occurring light-harvesting systems., They are used by green photosynthetic bacteria to trap visible and infrared light of minute intensities even at great depths, e.g., 100 m below water surface or in volcanic vents in the absence of solar radiation. In contrast to most other natural light-harvesting systems, the chlorosomal antennae are devoid of a protein scaffold to orient the BChl's; thus, they are an attractive goal for mimicry by synthetic chemists, who are able to engineer more robust chromophores to self-assemble. Functional devices with environmentally friendly chromophoreswhich should be able to act as photosensitizers within hybrid solar cells, leading to high photon-to-current conversion efficiencies even under low illumination conditionshave yet to be fabricated. The orderly manner in which the BChl's and their synthetic counterparts self-assemble imparts strong diamagnetic and optical anisotropies and flow/shear characteristics to their nanostructured assemblies, allowing them to be manipulated by electrical, magnetic, or tribomechanical forces