Kexian Chen

Metal-free allylic/benzylic oxidation strategies with molecular oxygen: recent advances and future prospects

The selective oxo-functionalization of hydrocarbons under mild conditions with molecular oxygen as the terminal oxidant continues to be a hot topic in organic synthesis and industrial chemistry. Though many oxidation protocols in combination with transition metal salts, enzymes, organometallic catalysts, or organocatalysts have been summarized recently, a review that focuses solely on the metal-free allylic/benzylic oxidation strategies with molecular oxygen is still unavailable. This critical review will summarize recent significant advances achieved in this important field under the scope of green chemistry, which covers the promising applications and brief mechanistic profiles involving three kinds of efficient catalysts, namely N-hydroxyimides, homogeneous/heterogeneous light-sensitive molecules, and heteroatom-doped carbon materials, and concerns the sustainability of these methods, as well as predicts the potential utilization of available but unreported analogous catalysts or catalytic systems in this field. Special emphasis will also be placed on the burgeoning metal-free strategies with visible light irradiation from the long-term greenness and sustainability of these oxidation processes due to their established appealing performances under ambient conditions.

Toward understanding macrocycle specificity of iron on the dioxygen-binding ability: a theoretical study.

In an effort to examine the interaction between dioxygen and iron-macrocyclic complexes, and to understand how this interaction was affected by those different macrocyclic ligands, dioxygen binding with iron-porphyrin, iron-phthalocyanine, iron-dibenzotetraaza[14]annulene, and iron-salen complexes is investigated by means of quantum chemical calculations utilizing Density Functional Theory (DFT). Based on the analysis of factors influencing the corresponding dioxygen binding process, it showed that different macrocyclic ligands possess different O-O bond distances, and different electronic configurations for the bound O(2) and non-aromatic macrocyclic ligands favor dioxygen activation. Furthermore, the smaller the energy gap between the HOMO of iron-macrocyclic complexes and the LUMO of dioxygen, the more active the bound O(2) becomes, with a longer O-O bond distance and a shorter Fe-O bond length.

Aerobic oxidation of β-isophorone catalyzed by N-hydroxyphthalimide: the key features and mechanism elucidated

Due to the insufficient understanding of the selective oxidation mechanism of α/β-isophorones (α/β-IP) to ketoisophorone (KIP), the key features in the β-IP oxidation catalyzed by N-hydroxyphthalimide (NHPI) have been explored via theoretical calculations. β-IP is more favourable to being activated by phthalimide-N-oxyl radical (PINO˙) and peroxyl radical (ROO˙) than α-IP owing to the different C-H strengths at their reactive sites, thereby exhibiting selective product distributions. It was found that NHPI accelerates β-IP activation due to the higher reactivity of PINO˙ than ROO˙ and the equilibrium reaction between them, yielding considerable hydroperoxide (ROOH) and ROO˙. In addition, the ROOH decomposition is more favourable viaα-H abstraction by radicals than its self-dehydration and thermal dissociation. The strong exothermicity of this α-H abstraction, along with that from H-abstraction by co-yielded hot HO˙, is in favor of the straightforward formation of KIP, simultaneously leading to the isomerization of a few β-IP to α-IP and production of 4-hydroxyisophorone (HIP) and water. The proposed mechanisms, consistent with the experimental observations, allow for the deeper understanding and effective design of oxidation systems involving similar substrates or NHPI analogues that are of industrial importance.

Unexpected oxidation of β-isophorone with molecular oxygen promoted by TEMPO

A novel and efficient protocol for the oxidation of β-isophorone (β-IP) using molecular oxygen without any additives catalyzed by 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) has been established. The generally accepted catalytic mechanism of alcohols by the oxoammonium cation (TEMPO+) derived from TEMPO indeed fails to explain our experimental observations, because a favorable radical-based process is confirmed by electron spin resonance measurements. Our results show that a plateau of the time-dependence curve is observed in the oxidation of β-IP with TEMPO at low temperature, which is quite different from that of N-hydroxyphthalimide (NHPI). The proposed mechanism of this catalytic process is also compared with that of NHPI. The theoretically characterized reaction pathways show that unlike the phthalimide N-oxyl radical, TEMPO promotes the oxidation via its interaction with the active intermediate hydroperoxide (ROOH) rather than its initial interaction with β-IP, and byproduct water also assists the α-H atom transfer from ROOH to TEMPO. In addition to the intensive oxidation of alcohols catalyzed by TEMPO, the present study widens its specific applications in the active C–H bonds of hydrocarbons, and also provides new insights into its promoted metal-free oxidation.

Recent progress in studies on polarity of ionic liquids

Although many ionic liquids have been reported, their polarity is not completely understood. Different empirical polarity scales for molecular solvents always lead to different polarity orders when they are applied on ionic liquids. Based on a literature survey, this review summarizes the recent polarity scales of ionic liquids according to the following 4 classes: (1) equilibrium and kinetic rate constants of chemical reactions; (2) empirical polar parameters of ionic liquids; (3) spectral properties of probe molecules; (4) multiparameter approaches. In addition, their interrelations are presented. A systematic understanding of the relationship between different polarity parameters of ionic liquids is of great importance for finding a universal set of parameters that can be used to predict the polarities of ionic liquids quantitatively. The potential utilization of the electron paramagnetic resonance in this field is also addressed.

Structures and Electronic Properties of Lithium Chelate-Based Ionic Liquids

The conformations, electronic properties, and interaction energies of four chelate-based ionic liquids [Li(EA)][Tf2N], [Li(HDA)][Tf2N], [Li(DEA)][Tf2N], and [Li(DOBA)][Tf2N] have been theoretically explored. The reliability of the located conformers has been confirmed via the comparison between the simulated and experimental infrared spectra. Our results show that the N-Li and O-Li coordinate bonds in cation are elongated as the numbers of coordinate heteroatoms of alkanolamine ligands to Li(+) increased. Also the binding energies between Li(+) and ligands are increased and the interaction energies between cations and Tf2N anion are decreased. The cation-anion interaction energies follow the order of [Li(DOBA)][Tf2N] < [Li(HDA)][Tf2N] < [Li(DEA)][Tf2N] < [Li(EA)][Tf2N], which fall within the energetic ranges of conventional ionic liquids. Interestingly, the strongest stabilization orbital interactions in these ionic liquids and their cations revealed by the natural bond orbital analysis lie in the interaction between the lone pair (LP) of the coordinate heteroatoms in ligands or anion as donors and the vacant valence shell nonbonding orbital (LP*) of Li(+) as acceptors, which are very different from that of conventional ionic liquids. Moreover, the charges transferred from cations to anion are quite similar, and the charge of Li(+) is proposed for possibly predicting the order of the interaction energies of ionic liquids in series. The present study allows for the deeper understanding the differences between chelate-based ionic liquids and conventional ionic liquids.

Elucidating Interactions between DMSO and Chelate-Based Ionic Liquids

The C-D bond stretching vibrations of deuterated dimethyl sulfoxide ([D6 ]DMSO) and the C2 -H bond stretching vibrations of 1,1,1,5,5,5-hexafluoropentane-2,4-dione (hfac) ligand in anion are chosen as probes to elucidate the solvent-solute interaction between chelate-based ionic liquids (ILs) and DMSO by vibrational spectroscopic studies. The indirect effect from the interaction of the adjacent S=O functional group of DMSO with the cation [C10 mim](+) and anion [Mn(hfac)3 ](-) of the ILs leads to the blue-shift of the C-D stretching vibrations of DMSO. The C2 -H bond stretching vibrations in hfac ligand is closely related to the ionic hydrogen bond strength between the cation and anion of chelate-based ILs. EPR studies reveal that the crystal field of the central metal is kept when the chelate-based ILs are in different microstructure environment in the solution.

Aerobic Oxidation of Secondary Alcohols to Ketones Catalyzed by Ionic Liquid Functionalized TEMPO

Protocols of aerobic oxidation of secondary alcohols to ketones by using TEMPO modified by bifunctionalized imidazolium-based ionic liquids have been developed. [Imim-TEMPO][X] (X= FeCl4, CuCl2) has been proved to be efficient and recyclable catalysts with modest or excellent yields and selectivity. Therefore, relevant literatures are summarized herein.

Luminescence Properties and Application of Versatile Coordination Compounds Based on Modified Platinum(II) Terpyridyl Complexes

A series of square-planar Platinum(II) terpyridyl modified by alkynyl, crown ether pendants, or supramolecular triblock copolymers have versatile luminescence behaviors. In this field, the utilization of the Platinum(II) terpyridyl coordination compounds have received great attention in the past few decades. This paper will summarize several relevant literatures about luminescence properties and application in responsing pH, conformational change, microenviromental change and detection of heparin quantification.