Xiangai Yuan

Visible-Light-Promoted Radical C–H Trifluoromethylation of Free Anilines

The trifluoromethyl-substituted anilines are biologically active compounds and useful building blocks. In this communication, we have developed the first visible-light-induced radical trifluoromethylation of free anilines with the commercially available and easily handled Togni reagent at room temperature. The resulting products were successfully transformed into a variety of valuable fluorine-containing molecules and heterocyclic compounds. This protocol provides an economical and powerful route to trifluoromethylated free anilines.

Direct Aromatic CH Trifluoromethylation via an Electron-Donor–Acceptor Complex

A novel electron-donor-acceptor (EDA) complex-mediated direct C-H trifluoromethylation of arenes with Umemotos reagent has been developed. This transformation has been enabled by an unprecedented EDA complex formed by Umemotos reagent and an amine, which was supported by experiments and theoretical calculations. The radical-based methodology presented here allows to access highly-functionalized trifluoromethyl arenes in up to 81 % chemical yield.

Role of Planar Conformations in Aggregation Induced Spectral Shifts of Supermolecular Oligofluorenols in Solutions and Films: A Combined Experimental and MD/TD-DFT Study

The supramolecular approach of fluorenol polymers brings about excellent self-assembly behavior to fabricate organogels and superstructured thin films through highly directional noncovalent interactions. To understand the aggregation effects on electronic structures, the packing structures and the UV/vis absorption spectra of oligofluorenols (PFOHn, n = 1/3-8), with and without OC8H17 side chains, were studied experimentally and theoretically in crystal, amorphous solids, and solutions, respectively. For the ground state in vacuum the steric repulsion between two adjacent fluorenol units renders the PFOH oligomers twisted in a helix conformation, while the molecular aggregation favors the appearance of planar π-conjugated structures. In comparison with the crystal packing, the content of planar conformation (with the torsion angle less than 20°) is increased in amorphous solids. The hydroxyl groups in oligofluorenols facilitate the formation of hydrogen bonding networks. The red shift in absorption spectra was observed in a systematic experimental study of unsubstituted and substituted oligofluorenols with the increasing concentration both in toluene and chloroform solutions. The subsitituted oligofluorenol R-PFOH1 with only one OC8H17 side chain exhibited a shoulder peak at 430-440 nm, which is different from PFOH1 without side chain and 3R-PFO1 with three OC8H17 side chain. Time-dependent density functional theory (TDDFT) calculations, which were carried out on conformation ensembles taken from a series of molecular dynamics (MD) simulations, revealed that the increase in the content of planar π-conjugated conformations is correlated to the red shift in the absorption spectra upon increasing the solution concentrations. The aggregation-induced red-shift in absorption spectra of oligofluorenols, as well as the blue-shift for oligothiophenes, was rationalized in a unified way from the increased (and reduced) content of planar conformations in molecular aggregates.

Synergistic Photoredox Catalysis and Organocatalysis for Inverse Hydroboration of Imines

The first catalytic inverse hydroboration of imines with N-heterocyclic carbene (NHC) boranes has been realized by means of cooperative organocatalysis and photocatalysis. This catalytic combination provides a promising platform for promoting NHC-boryl radical chemistry under sustainable and radical-initiator-free conditions. The highly important functional-group compatibility and possible application in late-stage hydroborations represent an important step forward to an enhanced α-amino organoboron library.

Unconventional O–H···C Hydrogen Bonding and Effects of Conformational Changes on Infrared Spectroscopy of o-Cresol in Solutions

The unconventional O-H···C intramolecular hydrogen bonding and the effect of conformational changes on IR spectra of o-cresol in aqueous solutions were investigated by using molecular dynamics (MD) simulations, density functional theory (DFT), and experiments. A facial rotational isomerization between global minimum with trans conformation and the cis isomer is predicted to take place in gas phase with a low barrier of about 3.7 kcal/mol through a vertical-like transition state. Upon solvation in aqueous solution, the contents of energetically high-lying vertical and cis conformations of neutral o-cresol are increased to 19% and 57%, respectively, in comparison with those (vertical, 0%; cis, 27%) in vacuum. The IR spectra of aqueous solution are closely related to the relative population of the different conformations, especially for the cis conformation with hydroxyl group facing to alkyl group. The appearance of cis conformations and unconventional O-H···C intramolecular hydrogen bond (HB) caused the low-frequency shift in OH stretching vibration of the IR spectra, which was also correlated with cation-like charge distribution and the decrease in s-component of oxygen hybridization orbital. However, the intermolecular HB between the o-cresol and surrounding water (o-cresol) molecules gave rise to more evident shifts in IR spectra than that caused by the intramolecular HB contributions in cis isomer. Further identification of intramolecular HB effect is performed through the comparison of spectrum changes that occurs on passing from aqueous solution to less interacting (carbon tetrachloride and cyclohexane) solvents. The absence of the intermolecular HB interaction between o-cresol and carbon tetrachloride (and cyclohexane) solvents leads to the weaker intensity and narrower width of OH stretching vibration region (around 3407 cm-1) in the IR spectra than that in aqueous solution.

Unexpected solvent effects on the UV/Vis absorption spectra of o-cresol in toluene and benzene: in contrast with non-aromatic solvents

Cresol is a prototype molecule in understanding intermolecular interactions in material and biological systems, because it offers different binding sites with various solvents and protonation states under different pH values. It is found that the UV/Vis absorption spectra of o-cresol in aromatic solvents (benzene, toluene) are characterized by a sharp peak, unlike the broad double-peaks in 11 non-aromatic solvents. Both molecular dynamics simulations and electronic structure calculations revealed the formation of intermolecular π-complexation between o-cresol and aromatic solvents. The thermal movements of solvent and solute molecules render the conformations of o-cresol changing between trans and cis isomers. The π-interaction makes the cis configuration a dominant isomer, hence leading to the single keen-edged UV/Vis absorption peak at approximately 283u2009nm. The free conformation changes between trans and cis in aqueous solution rationalize the broader absorption peaks in the range of 260–280u2009nm. The pH dependence of the UV/Vis absorption spectra in aqueous solutions is also rationalized by different protonation states of o-cresol. The explicit solvent model with long-ranged interactions is vital to describe the effects of π-complexation and electrostatic interaction on the UV/Vis absorption spectra of o-cresol in toluene and alkaline aqueous (pHu2009>u200910.3) solutions, respectively.

Simulations of absorption spectra of conjugated oligomers: role of planar conformation and aggregation in condensed phase

ABSTRACT This Review highlights the structure/property relationship underlying the morphology modulation through various factors towards the exploration of light-absorbing materials for efficient utilisation of solar power. Theoretical study using a combination of molecular dynamics imulations and the time-dependent density functional theory demonstrated that the planarity plays an important role in tuning spectral properties of oligomer aggregates. The aggregation-induced blue-shift in absorption spectra of oligothiophenes and the red-shift for oligofluorenols were rationalised in a unified way from the reduced (and increased) content of planar conformations in molecular aggregates. The planarity versus non-planarity of oligomers can be modulated by introduction of alkyl side chain or steric bulky substituents. The substitution with various groups in the ortho-position of azobenzene leads to the distorted backbone, breaking symmetry, and hence the red-shift in spectra, expanding the application in biological systems with visible light absorption. The donor–acceptor substituent groups in conjugated oligomers can increase the degree of planarity, electron delocalisation and polarisation, and charge separation, giving rise to the red-shift in spectra and enhancement in polarisability and charge mobility for device applications. The solvent dependent and pH-sensitive properties and intramolecular hydrogen bonds also caused the shift of absorption spectra with the appearance of planar conformers.

BONDING FEATURES AND REACTION MECHANISM OF THE OXIDATION OF 2-PROPANOL BY A Cp*Ir AMIDO COMPLEX

The mechanistic study on the oxidation of 2-propanol by the model complex CpIr[κ2-(N,C)-(NHCMe2–2-C6H4)] (R) is performed using density functional theory (DFT) calculations. It is found that the rate-determining step is the hydrogen migration from 2-propanol to R via a six-membered transition state. The reaction is calculated to be favorable thermodynamically. To further understand the reaction mechanism, some bonding features are discussed, such as the correlation of the geometry of R and the Ir–N π bond involved, the transformation of the nitrogen hybridization, the variation of Ir–N bond distance, and so on.