Engineering Azobenzene Derivatives to Control the Photoisomerization Process

In this work, we show how the structural features of photoactive azobenzene derivatives can influence the photoexcited state behavior and the yield of the trans/cis photoisomerization process. By combining high-resolution transient absorption experiments in the vis-NIR region and quantum chemistry calculations (TDDFT and RASPT2), we address the origin of the transient signals of three poly-substituted push-pull azobenzenes with an increasing strength of the intramolecular interactions stabilizing the planar trans isomer (absence of intramolecular H-bonds, methyl, and traditional H-bond, respectively, for 4-diethyl-4'-nitroazobenzene, Disperse Blue 366, and Disperse Blue 165) and a commercial red dye showing keto-enol tautomerism involving the azo group (Sudan Red G). Our results indicate that the intramolecular H-bonds can act as a "molecular lock" stabilizing the trans isomer and increasing the energy barrier along the photoreactive CNNC torsion coordinate, thus preventing photoisomerization in the Disperse Blue dyes. In contrast, the involvement of the azo group in keto-enol tautomerism can be employed as a strategy to change the nature of the lower excited state and remove the nonproductive symmetric CNN/NNC bending pathway typical of the azo group, thus favoring the productive torsional motion. Taken together, our results can provide guidelines for the structural design of azobenzene-based photoswitches with a tunable excited state behavior.

Diaryl-Pyrano-Chromenes Atropisomers: Stereodynamics and Conformational Studies

The dynamic scenario of di-aryls-pyrano-chromenes was investigated using DFT calculations. The symmetry of the chromene scaffold and the presence of two ortho-substituted aryls substituents can generate two syn/anti diastereoisomers and conformational enantiomers with different rotational barriers. The relative conformations and configurations were derived using NOESY-1D experiments. Depending on the energies related to the conformational exchange, the experimental energy barriers were determined through Dynamic NMR, Dynamic HPLC or kinetic studies. The atropisomeric pairs were resolved in the latter scenario, and their absolute configuration was assigned using the ECD/TD-DFT method.

Atropostatin: Design and Total Synthesis of an Atropisomeric Lactone–Atorvastatin Prodrug

Atorvastatins play an important role in the inhibition of HMG-CoA reductase, an enzyme present in the liver that takes part in the biosynthesis of cholesterol. In this article, we report the total synthesis of a lactone–atorvastatin prodrug with additional atropisomeric features. Conformational and experimental studies of model compounds were designed to test the stability of the chiral axis. Docking calculations were performed to evaluate the constant inhibition of a library of atorvastatins. Full synthesis of the best candidate was achieved and thermally stable atropisomeric lactone–atorvastatin was obtained. The absolute configuration of the chiral axis of the atropisomers was assigned by means of chiroptical ECD spectroscopy coupled with TD-DFT calculations.

A Pyridyl-1,2-azaborine Ligand for Phosphorescent Neutral Iridium(III) Complexes

A novel 1,2-azaborine (i.e., 4-methyl-2-(pyridin-2-yl)-2,1-borazaronaphthalene, 1a) has been synthesized and used for the first time as a B-N alternative to common cyclometalating ligands to obtain neutral phosphorescent iridium(III) complexes (i.e., 2a, 3, and 4) of general formula [Ir(C^∧N)₂(N^∧NB)], where C^∧N indicates three different cyclometalating ligands (Hppy = 2-phenylpyridine; Hdfppy = 2-(2,4-difluoro-phenyl)pyridine; Hpqu = 2-methyl-3-phenylquinoxaline). Moreover, the azaborine-based complex 2a was compared to the isoelectronic C═C iridium(III) complex 2b, obtained using the corresponding 2-(naphthalen-2-yl)pyridine ligand 1b. Due to the dual cyclometalation mode of such C═C ligand, the isomeric complex 2c was also obtained. All new compounds have been fully characterized by NMR spectroscopy and high-resolution mass spectrometry (MS), and the X-ray structure of 2a was determined. The electronic properties of both ligands and complexes were investigated by electrochemical, density functional theory (DFT), and photophysical methods showing that, compared to the naphthalene analogues, the azaborine ligand induces a larger band gap in the corresponding complexes, resulting in increased redox gap (basically because of the highest occupied molecular orbital (HOMO) stabilization) and blue-shifted emission bands (e.g., λ_max = 523 vs 577 nm for 2a vs 2b, in acetonitrile solution at 298 K). On the other hand, the ³LC nature of the emitting state is the same in all complexes and remains centered on the pyridyl-borazaronaphthalene or its C═C pyridyl-naphthalene analogue. As a consequence, the quantum yields of such azaborine-based complexes are comparable to those of the more classical C═C counterparts (e.g., photoluminescence quantum yield (PLQY) = 16 vs 22% for 2a vs 2b, in acetonitrile solution at 298 K) but with enhanced excited-state energy. This proves that such type of azaborine ligands can be effectively used for the development of novel classes of photoactive transition-metal complexes for light-emitting devices or photocatalytic applications.

Synthesis and Stereodynamic and Emission Properties of Dissymmetric Bis-Aryl Carbazole Boranes and Identification of a CPL-Active B-C Atropisomeric Compound

We synthesized bis-aryl carbazole borane derivatives having emissive properties and axial chirality. The resolution of a thermally stable atropisomeric pair (compound 1b), due to a B-C chiral axis, was achieved by chiral stationary-phase high-performance liquid chromatography (CSP-HPLC). Complete photophysical properties of all compounds were measured and simulated by time-dependent density functional theory (TD-DFT) calculations of the complete fluorescence cycle, and circularly polarized luminescence spectra were obtained for the atropisomers of compound 1b, whose absolute configuration was derived using a TD-DFT simulation of the electronic circular dichroism (ECD) spectra.

Sulfoxonium Ylides in Aminocatalysis: An Enantioselective Entry to Cyclopropane-Fused Chromanol Structures

The 1,1a,2,7b-tetrahydrocyclopropa[c]chromene, arising from fusion of chromane and cyclopropane rings is the core of medicinally relevant compounds. Engaging sulfoxonium ylides in enantioselective aminocatalytic reactions for the first time, a convenient entry to this scaffold is presented. Several ring-fused derivatives were obtained in moderate-to-good yields and enantioselectivities and with perfect diastereoselectivity at the cyclopropane, using an α,α-diphenylprolinol aminocatalyst. The versatility of the hemiacetal moiety in the products was leveraged to effect various synthetic manipulations.

Catalytic Enantioselective Access to Dihydroquinoxalinones via Formal α-Halo Acyl Halide Synthon in One Pot

An enantioselective one-pot catalytic strategy to dihydroquinoxalinones, featuring novel 1-phenylsulfonyl-1-cyano enantioenriched epoxides as masked α-halo acyl halide synthons, followed by a domino ring-opening cyclization (DROC), is documented. A popular quinine-derived urea served as the catalyst in two out of the three steps performed in the same solvent using commercially available aldehydes, (phenylsulfonyl)acetonitrile, cumyl hydroperoxide and 1,2-phenylendiamines. Medicinally relevant 3-aryl/alkyl-substituted heterocycles are isolated in generally good to high overall yield and high enantioselectivity (up to 99 % ee). A rare example of excellent reusability of an organocatalyst at higher scale, subjected to oxidative conditions, is demonstrated. Mechanistically, labile α-ketosulfone has been detected as the intermediate involved in the DROC process. Theoretical calculations on the key epoxidation step rationalize the observed stereocontrol, highlighting the important role played by the sulfone group.

GC/MSn analysis of the crude reaction mixtures from Friedel-Crafts acylation: Unambiguous identification and differentiation of 3-aroylbenzofurans from their 4- and 6-regioisomers

RATIONALE

3-Aroylbenzofurans and their 2-nitrophenyl derivatives constitute fundamental intermediates for the synthesis of target compounds with pharmaceutical properties. However, their preparation via the Friedel-Crafts acylation of 2-phenylbenzofurans, using Lewis acid as catalyst, often leads to mixtures of regioisomeric aroylbenzofurans that can be challenging to distinguish, thus preventing the reaction characterization.

METHOD

We report a method for the unambiguous identification and differentiation of the desired 3-benzoyl isomers from their 4- and 6-regioisomers in a crude reaction mixture using gas chromatography coupled to multiple-stage mass spectrometric (GC/MSⁿ ) analysis performed in collision-induced dissociation (CID) mode.

RESULTS

Upon electron ionization, each set of isomers displayed nearly identical mass spectra. MSⁿ revealed fragmentation patterns that varied in the location of the benzoyl group on the benzofuran scaffold: CID experiments performed on the molecular ion allowed the distinction of the 3-acyl isomers from the 4- and 6-regioisomers; CID experiments on the [M - Ar]⁺ ion allowed the distinction of the 4-benzoyl from the 6-benzoyl regioisomer, when the nitro group is located on the 2-phenyl ring. Moreover, the unusual loss of OH^• radical allowed ascertaining the position of the nitro group in 3-acyl regioisomers bearing the NO₂ group. The origin of the diagnostic OH^• loss was investigated through MSⁿ experiments using ¹⁸ O-labelled 3-benzoyl derivatives.

CONCLUSIONS

The method allows the rapid characterization of crude reaction mixtures of benzoylbenzofurans using solely GC/MSⁿ analysis, simplifying the workflow of extensive isolation and purification for structure elucidation.

Highly twisted carbazole-borane derivatives: B–N stereodynamic analysis and consequences on their emission properties

The manuscript describes the stereodynamic features of amino bis-mesityl-boranes bearing carbazole and benzocarbazole as donor heterocycles, and how they influence the photophysical properties.

A new ionone derivative from Lycium intricatum Boiss. (Solanaceae)

Lycium intricatum Boiss also called 'Awsadj' is a subshrub belonging to the Solanaceae family and producing red berries. In the present work, we report the isolation of a new compound, ((1 R,3aR,7aS)-3a,7-dimethyl-1-((E)-prop-1-en-1-yl)-1,3a,4,7a-tetrahydroisobenzofuran-5(3H)-one (1)). Three known compounds were also isolated for the first time from L. intricatum, and identified as isoscopoletin (2), 3,4,5-trimethoxybenzyl alcohol (3), and (+)-isolariciresinol (4). Compounds 1-4 were isolated from the extract of leaves using chromatographic techniques, and the structure of compound 1 was fully elucidated by NMR, MS and ECD experiments.

Microwave-Assisted Synthesis of 2-Substituted 2-Thiazolines and 5,6-Dihydro-4H-1,3-thiazines

An efficient and general method for the synthesis of 2-substituted thiazolines and 5,6-dihydro-4H-1,3-thiazines is developed via microwave-assisted ring closure of ω-thioamidoalcohols promoted by ethyl polyphosphate (PPE). The cyclization reaction involves an SN2-type mechanism and features the advantages of very short reaction times, high yields and a predictable stereochemical outcome. The acyclic precursors are prepared in high overall yields by an improved diacylation–thionation–saponification sequence from commercially available ω-amino­alcohols. The whole process is metal-free and operationally simple.

Metal and Oxidant-Free Brønsted Acid-Mediated Cascade Reaction to Substituted Benzofurans

Substituted hydroxy-benzofurans are easily accessible by treatment of resorcinols and 1,2-diaza-1,3-dienes (DDs) under acidic conditions. The reaction happens through an uncommon Michael reaction between aromatic derivatives as aromatic C(sp²)-H nucleophiles and DDs as acceptors. Also, the behavior of different phenols and 2-naphthol was investigated.

Predictive chirality sensing via Schiff base formation

Among the large number of chiroptical sensors that have been developed to date, few allow rational determination of the absolute configuration of chiral substrates together with quantitative ee analysis. We have prepared and tested stereodynamic N-aryl aminobenzaldehyde sensors that bind chiral amines via Schiff base formation. The covalent binding of the amine substrate generates a conformational bias in the chromophoric sensor moiety which results in characteristic CD signals. Computational analysis revealed that CD prediction of the sign of the Cotton effect and thus determination of the absolute configuration of the substrate becomes practical with a sterically crowded sensor design because the number of conformations to be considered is largely reduced and the chiroptical sensor response is less sensitive to conformational equilibria. The amplitude of the measured CD signal can be used for quantitative ee analysis of nonracemic amine samples with the help of a calibration curve.

Conformational and Stereodynamic Behavior of Five- to Seven-Membered 1-Aryl-2-iminoazacycloalkanes

The stereodynamic behavior of 1-arylpyrrolidin-2-imines, having a C_aryl-N stereogenic axis, has been studied by means of dynamic nuclear magnetic resonance and density functional theory calculations, evaluating the steric effect of ortho-aryl substituents. The rotational barrier due to E/Z isomerism about the -C=N-H bond was also determined. The dynamic stereochemistry of homologous six- and seven-membered iminoazacycloalkane rings and their oxo-analogues was also comparatively investigated, evidencing a ring size effect. It was found that the seven-membered heterocycle shows additional dynamic features because of ring inversion.

Determination of the absolute configuration of conformationally flexible molecules by simulation of chiro-optical spectra: a case study

The assignment of the absolute configuration (AC) of two conformational flexible organic molecules by means of TD-DFT simulation of the electronic circular dichroism (ECD) spectra is presented. The factors leading to a reliable assignment were evaluated in the various steps of the process. The effects of different functionals and basis sets in the geometry optimization step is very limited in terms of the resulting optimized geometries, whereas the inclusion of the solvent in the calculations has a much larger effect on the correct evaluation of the conformational ratio. B3LYP and M06-2x were found to be the most accurate functionals for geometry optimization. CAM-B3LYP and ωB97X-D provided the best results in the TD-DFT simulations.

The absolute configuration (AC) assignment of two very flexible organic molecules by TD-DFT simulation of ECD spectra is evaluated by modifying the calculation level in the various steps of the AC assignment process.

Synergistic Catalysis: Highly Enantioselective Acetyl Aza-arene Addition to Enals

A novel catalytic enantioselective methodology based on synergistic catalysis for the synthesis of chiral 2-acyl pyridines and pyrazines is reported. The strategy involves the metal-Lewis acid activation of acetyl aza-arenes and the secondary-amine activation of enals. The proposed mechanism is supported by DFT calculations.

Stereodynamic Analysis of New Atropisomeric 4,7-Di(naphthalen-1-yl)-5,6-dinitro-1H-indoles

A series of atropisomeric molecules containing the indole ring and two stereogenic axes were prepared. The four atropisomers were resolved by enantioselective HPLC. The rotational barriers of the indole–naphthyl axes were evaluated by means of kinetic analysis either by NMR or enantioselective HPLC. The absolute configuration of the ­atropisomers was determined by a combination of X-ray spectroscopy and TD-DFT simulation of electronic circular dichroism spectra.

Atropisomerism in 3-arylthiazolidine-2-thiones. A combined dynamic NMR and dynamic HPLC study

Sterically hindered 3-arylthiazolidine-2-thiones were prepared by a solvent-free reaction with arylisothiocyanates and 1,4-dithiane-2,5-diol. Atropisomerism was observed in two compounds (3 and 4, aryl = 1-naphthyl and 2-methylnaphth-1-yl), whose rotational energy barriers were measured using dynamic NMR and dynamic HPLC. The experimental analyses were supported by DFT calculations. Thermally stable atropisomers were obtained by dehydration of compounds 3 and 4 and the absolute configuration of the atropisomers of compound 6 was determined by theoretical simulation of the ECD and VCD spectra.

Tetrasubstituted cyclopentadienones as suitable enantiopure ligands with axial chirality

A series of thermally stable atropisomeric phencyclones has been successfully synthesized as ligands in new ruthenium-based enantioselective hydrogenation catalysts.

High-frequency electro-optic measurement of strained silicon racetrack resonators

The observation of the electro-optic effect in strained silicon waveguides has been considered a direct manifestation of an induced χ(2) nonlinearity in the material. In this work, we perform high-frequency measurements on strained silicon racetrack resonators. Strain is controlled by a mechanical deformation of the waveguide. It is shown that any optical modulation vanishes, independent of the applied strain, when the applied voltage varies much faster than the carrier effective lifetime and that the DC modulation is also largely independent of the applied strain. This demonstrates that plasma carrier dispersion is responsible for the observed electro-optic effect. After normalizing out free-carrier effects, our results set an upper limit of (8±3) pm/V to the induced high-speed effective χeff,zzz(2) tensor element at an applied stress of -0.5 GPa. This upper limit is about 1 order of magnitude lower than previously reported values for static electro-optic measurements.

Catalytic highly enantioselective transfer hydrogenation of β-trifluoromethyl nitroalkenes. An easy and general entry to optically active β-trifluoromethyl amines

In the presence of a thiourea catalyst, β-CF3 nitroalkenes react with Hantzsch esters in a highly enantioselective fashion, giving a broad range of β-CF3 amine precursors with a tertiary stereocentre at the β-position. This reaction represents the first general catalytic enantioselective approach to this important class of β-CF3 amines.

Chaotic dynamics in coupled resonator sequences

Optically induced thermal and free carrier nonlinearities in silicon micro-ring resonator influence their behavior. They can be either deleterious by making them instable and by driving their resonances out of the designed wavelengths, or enabler of different applications. Among the most interesting one, there are optical bistability and self induced oscillations. These lead to all optical logic, signal modulation, optical memories and applications in neural networks. Here, we theoretically and experimentally demonstrate that when many resonators are coupled together, thermal and free carrier nonlinearities induce also chaos. The chaotic dynamics are deeply analyzed using experimentally reconstructed phase space trajectories and the tool of Lyapunov exponents.

N-Heterocyclic carbene rhodium(i) complexes containing an axis of chirality: dynamics and catalysis

Novel rhodium(i) complexes [RhCl(NBD)(NHC)] [NHC = 1-benzyl-3-R-imidazolin-2-ylidene; R = Me, Bz, Tr, ^tBu]: determination of the rotation barriers about the Rh-carbene and catalytic activity in the hydrosilylation of terminal alkynes.

Enantioselective gold-catalyzed synthesis of polycyclic indolines

The synthesis of architecturally complex polycyclic fused indolines is achieved in a chemo-, regio-, and stereodefined manner, via an enantioselective gold-catalyzed cascade hydroindolination/iminium trapping synthetic sequence. Highly functionalized tetracyclic fused furoindolines (2) and dihydropyranylindolines (4) are synthesized in moderate to good yields and enantiomeric excesses of up to 87%.