Professor LIU YouCheng, a well-known organic chemist and one of the pioneers of free-radical chemistry in China, has made numerous outstanding contributions to the development of free-radical chemistry. His research group has investigated a series of free-radical reactions and elucidated the mechanisms. The areas he has studied cover the decomposition mechanism of diacyl peroxides in benzene, revealing for the first time the carboxyl conversion reaction; electron transfer reaction of vinyl Grignard reagents; and the structure and mechanism of C–C bond initiators. In particular, his group has carried out an extensive study on the electron transfer reaction of aminoxyl radicals with biomolecules such as cysteine, glutathione, ascorbic acid and Vitamin E. Liu’s group has also studied radical ion chemistry, photoinduced electron transfer as well as the electron transfer reaction in cyclodextrins and biomimetic systems. His group has clearly demonstrated the electron transfer and concerted hydride transfer and the existence of the intermediate mechanism of coenzyme NADH models. These pioneering studies not only enrich the fundamental theory of free-radical chemistry, but are also of significance to interdisciplinary science such as free-radical biology as well as having practical value in green chemistry. The picture on the cover page illustrates a carbon-centered radical, an oxygen-centered radical, an aminoxyl radical with an electron transfer pathway, and a coenzyme NADH model to exemplify Professor Liu’s research achievements.
Professor LIU YouCheng is a pioneer of free radical chemistry in China. Born in Shucheng, Anhui Province in 1920, he received his undergraduate education at the National Central University, China and studied as a graduate student on a scholarship from the British Council at the University of Leeds, England, where he received his PhD degree in organic chemistry in 1948 under the guidance of Professor F. Challenger. He then went to the United States and took the position of research associate with Professor B. Riegel in the chemistry department of Northwestern University, Evanston, Illinois until 1951. He then worked at the University of Chicago as a postdoctoral fellow with Professor M. S. Kharasch and thereby began his career in free radical chemistry research. Because of his strong desire to serve his mother country, Dr. Liu made strenuous efforts to return to China, finally suc- ceeding with the help of the Chinese Government in 1954. He accepted the offer from the Ministry of Education to become a chemistry professor at Lanzhou University, Lanzhou, Gansu Province in the northwest region of China in 1955. There, he started his professional career of teaching and research in organic chemistry. After only 2 years, he became the chairman of the chemistry department and remained at that post until 1983, a period of 26 years. During this period, Lanzhou University underwent strong development to become an important educational institution in China. Professor Liu resumed his research in free radical chemistry with only a small group of young col-leagues and graduate students and meager facilities in the middle fifties. Through stupendous effort and hard work, they had estab- lished an excellent center for free radical chemistry research by the early 1980s. ........
Professor LIU YouCheng is a pioneer of free radical chem-istry in China. He was elected as a Member of the Chinese Academy of Sciences in 1980 and as a Fellow of the Royal Society of Chemistry in 2008. Professor Liu set up the first laboratory of free radical chemistry in China at Lanzhou University, Lanzhou, Gansu Province and has made out-standing contributions to many aspects of free radical chemistry. Therefore, he is generally known as the founder of free radical chemistry in China. I was fortunate enough to be a student of Professor Liu for my MS and PhD qualifica-tions and had the privilege of being educated and guided by him. I have been deeply impressed and influenced by his dedication and enthusiasm to science, his rigorous attitude towards scientific research, his hard-working and modest personality, and the great care and attention he pays to his students and colleagues. ........
This review article summarizes the electron transfer reactions of piperidine aminoxyl radicals. Electrochemical studies revealed the single electron transfer nature of piperidine aminoxyl radicals. In solution, piperidine aminoxyl radicals serve as single electron transfer oxidation reagent to react with various biologically interesting molecules such as glutathione, cysteine, ascorbic acid, and amines. The reaction product distribution, reaction kinetics, intermediates, and the reaction features in biological mimic environments including micelles and cyclodextrins were investigated. Oxoammonium salts, the one-electron transfer oxidation products of piperidine aminoxyl radicals, are agents of organic synthesis to selectively generate ketones or di-ketones from alcohols or ketones bearing α-methylene group under mild conditions. The new reactions of oxoammonium salts with aromatic amines, phenols, heterocycles including phenothiazines, papaverine, and bilirubin are also illustrated.
Cyclopalladated ferrocenylimines as a novel kind of highly efficient palladium catalyst precursors, which are air-stable and easy to handle, have been successfully applied in a wide variety of catalytic reactions ranging from classical reactions, such as Heck, Suzuki, Sonogashira and Buchwald-Hartwig couplings, to cyanation, addition reactions of arylboronic acids, and coupling reactions involving terminal alkynes. The preliminary achievements regarding their applications to homogeneous catalysis are concisely described herein.
While stable in CH2Cl2, hexane or THF, in the presence of MeOH, self-promoted dimerization of the triarylphosphine-alkene 1, a ligand for Pd-catalyzed reactions, produced an unusual racemic bis(phosphine) 2 in high yield. The reaction of 2 with Pd(dba)2, followed by oxidative addition of p-IC6H4NO2, yielded a trans-chelated Pd(II) aryl iodide complex.
A very simple molecular cation, 4-(4-dimethylaminophenyl)-2,6-diphenylpyrylium, has been demonstrated to have a function of molecular half-adder and half-subtractor according to the detectable spectroscopic changes of the molecular system in response to the inputs of acid and base. Distinct algebraic operations can be performed in this reconfigurable molecular logic system.
An effective, catalytic method has been developed to remove TBS and THP groups from protected alcohols. TBS and THP ethers were selectively cleaved using a catalytic amount of NO+BF4– (5 mol%) in methanol at room temperature.
A unique transformation to realize the allylic amination from vinylic bromides was described and an unexpected C–Pd migration was observed from sp2 carbon to adjacent allylic sp3 carbon initiated from vinyl bromide. Various 3-aryl-2-bromopropenes and secondary amines were surveyed and the allyl amination products were obtained in moderate isolated yields. The primary amine was not fit for this transformation. Mechanistic studies indicate that this migration went through β-hydride elimination and reverse C=C bond insertion.
The protecting group free synthesis of Anomala Osakana Pheromone isomer has been achieved with high enantioselectivity (92% ee). A chiral γ-hydroxy-α, β-acetylenic ester was used as the key intermediate, which was obtained via asymmetric alkynylation of aldehyde. This was followed by readily handled selective hydrogenation and lactonization in three steps with a high overall yield (86%).
A library consisting of a series of O,O′-diaryzoyl-L-(–)-tartaric acids (2) was designed and synthesized. The substituent on the aromatic ring of 2 significantly affects the diastereomeric excess and efficiency of the resolution of racemic albuterol (1). Excellent resolving reagent 2a was selected for the resolution of rac-1 via the parallel approach. However, a family of three resolving reagents failed to improve the resolution efficiency of rac-1.
An efficient and simple protocol for the synthesis of o-hydroxyarylketones has been developed. The reaction proceeds smoothly with catalytic amounts of copper catalyst under air atmosphere without ligands. This study indicates that the o-hydroxy plays an important role in the oxidation of alcohol to the corresponding carbonyl compounds.
The catalytic potential of carbon nucleophiles has seldom been disclosed due to their reactivity toward carbon electrophiles to form stable carbon-carbon bonds, which are too strong to be cleaved for the expected catalytic cycles. We have developed an efficient catalytic cyanosilylation of carbonyl compounds with an in situ generated alkene/phosphine adduct, which is tuned not to couple with a carbonyl compound or an electron-deficient alkene under the reaction conditions when its nucleophilicity is translated into catalytic activity. In the presence of 3 mol% of methyl acrylate and 3 mol% of triphenylphosphine, a broad range of alkyl, alkenyl, and aryl ketones and aldehydes undergo cyanosilylation reaction with trimethylsilyl cyanide at room temperature to yield structurally diversified cyanohydrin silyl ethers in excellent yields. By using methyl acrylate/triphenylphosphine as a highly effective nucleophilic catalyst system for the cyanosilylation of carbonyl compounds, this study demonstrates a new concept for the development of useful organocatalysis utilizing the nucleophilicity of alkene/phosphine adducts generated in situ.
A polyethylene glycol (PEG)-supported NADH model as a novel organic reductant was designed and synthesized. The reductions of various α, β-unsaturated ketones by the PEG-supported NADH model were examined, and the results showed that the reductions completely and quickly proceed with no catalyst at ambient temperature. The main advantages of this liquid-like PEG-supported NADH model are easy workup, and an optimal potential for recycling use and solvent-free for use in reactions.
The interaction of lumichrome (LC) as an endogenous fluorescence probe with nucleobases, nucleosides and nucleic acids has been studied by UV-visible absorption, fluorescence spectroscopy, polarized fluorescence and viscosity. The fluorescence of LC was strongly quenched by a series of nucleic acids and their precursors. The influences of the medium, temperature and salt effects on LC-nucleobase and LC-nucleic acid complexes were investigated. The influences of polarized fluorescence, thermal denaturation and viscosity on LC-ctDNA interaction were examined. The results demonstrate that the main binding model of LC-ctDNA includes strong intercalating into the DNA helix chain.
Water-soluble β-cyclodextrin (β-CD)-modified carbon nanocrystals (CNCs) have been synthesized by the reaction of CNCs with mono(6-diethylenetriamino-6-deoxy)-β-CD (DETA-β-CD) and comprehensively characterized by 1H NMR, FT-IR, transmission electron microscopy (TEM) and UV-Vis. DETA-β-CD-modified CNCs (DETA-β-CD-CNCs) are luminescent, and the quantum yield is 8.41%. The photoinduced electron transfer (PET) process between DETA-β-CD-CNCs and (ferrocenylmethyl) trime-thylammonium iodide (Fc+) was investigated by means of fluorescence spectroscopy.
A new type of chiral bisoxazoline ligands 1 based on spiro[4,4]-1,6-nonadiene backbone was easily prepared in six steps from racemic spiro[4,4]-nonane-1,6-dione, with the Pd-catalyzed coupling of the enol triflates with CO and amino alcohols as the key steps for the construction of the oxazoline moiety. The structure of the ligand (R,S,S)-1b was unambiguously established by X-ray crystallographic analysis. The chiral Cu(II) complex generated in situ from the combination of spiro bisoxazoline ligand (S,S,S)- 1c and Cu(OTf)2 was effective in the catalysis of asymmetric chlorination of the β-ketoester, methyl 1-oxo-2,3-dihydro-1H- in-dene-2-carboxylate, affording the corresponding chlorinated derivative in 99% yield with 17% ee.
Highly diastereoselective addition of lithium enolate of γ-substituted α-diazoacetoacetate to chiral N-sulfinyl imines has been developed. The addition products were further subjected to photo-induced Wolff rearrangement or Rh(II)-catalyzed intramolecular N–H insertion to afford chiral 4,5-disubstituted 2-oxo and 3-oxo pyrrolidines, respectively.
Photooxidation of Hantzsch 1,4-dihydropyridines (1,4-DHP, 1a–1d) by direct irradiation (λ > 300 nm) under an oxygen atmosphere has been carefully examined in this work. Spectroscopic and electrochemical studies demonstrate that photoinduced singlet electron transfer from 1,4-DHP to molecular oxygen occurs. The generated superoxide radical anion (O2–•) is responsible for this typical photochemical oxidation.
Efficient synthesis and resolution of a series of meta-substituted inherently chiral aminocalix[4]arene derivatives have been described. Consequently, the meta-nitro, bromo and chloro substituted inherently chiral calix[4]arenes could be directly synthesized by the nitration, bromination, and chlorination of the acylating product of aminocalix[4]arene. meta-Cyano and phenyl substituted inherently chiral aminocalix[4]arenes were readily obtained by the nucleophilic substitution reaction or Suzuki coupling reaction from the meta-bromo substituted one under the Pd-catalyzed conditions. For kinetic resolution of the racemic inherently chiral aminocalix[4]arenes, it was found that the electron-withdrawing ability of substituent was helpful to improving the resolution efficiency of the acylation process, and the kinetic resolution could be efficiently applied to the resolution of meta-nitro substituted inherently chiral aminocalix[4]arene, providing (cS)- or (cR)-isomer in up to 95% or 99.9% ee value, respectively, with the corresponding chiral acylating reagent. Moreover, by introduction of the chiral auxiliary, enantiopure antipodes of meta-cyano and phenyl substituted inherently chiral aminocalix[4]arenes could also be readily obtained. These enantiopure aminocalix[4]arenes are potential building blocks for constructing novel chiral receptors and asymmetric catalysts.
Three arylamide-bridged biscoumarin derivatives 1–3 have been designed and prepared. Compounds 1 and 2 are induced by the intramolecular N—H…O and N—H…F hydrogen bonding to possess a helical conformation, and 3 is induced to have an extended conformation. A comparison of their absorption and fluorescent spectra in a variety of solvents of a wide range of polarity with those of control compound 4 reveals that, for foldamers 1 and 2, the intramolecular hydrogen bonding and the helical conformations exist in most solvents, but do not exist or are very weak in DMF and DMSO.
Substituted triazolinones were prepared by a three-component reaction of aldehydes, hydrazines and azodicarboxylates using TFA as a catalyst. The procedure was convenient and efficient, utilizing readily available substrates. A plausible mechanism for the cascade process is proposed.
4-Hydroxycinnamic acid derivatives, including ferulic acid (FA) (1), sinapic acid (SA) (2) and caffeic acid (CA) (3), are widely distributed in the plant kingdom, and undergo oxidative cross-coupling leading to the corresponding dehydrodimers, trimers and even higher oligomers in plants. In order to evaluate the antioxidative ability of these 4-hydroxycinnamic acid derivatives and their oligomers, we synthesized 8-8′-bis-lactone-dimers (8-8′-DiFA (4), 8-8′-DiSA (5) and 8-8′-DiCA (6)), as well as a new trimer (7), by the reaction of the corresponding monomers (1–3) with Ag2O in methanol, and assayed their free radical-scavenging activity by the reaction with 2,2-diphenyl-1-picrylhydrazyl radical (DPPH•). It was found that CA (3) and 8-8′-DiCA (6) bearing o-dihydroxyl groups exhibited significantly higher radical-scavenging activity than those bearing no such groups, and oxidative coupling of CA (3) resulted in remarkable enhancement in the activity.
The property of the lowest excited triplet state of fluorenone (FL) in acetonitrile, ethyl acetate, toluene, cyclohexane has been investigated using time-resolved laser flash photolysis. The appearances of the corresponding cation radicals and anion radicals suggest that the electron transfer reaction occurs between 3FL* and amines. Experimental results show a decrease in the quenching rate constants along with reducing solvent polarity. The change of quenching rate constants of the FL excited triplet with the three N,N-dimethylaniline derivatives are explained using the Hammett constant.
A series of lithium and aluminum complexes bearing [OC(But)CHP(Ph2)=NBut]– ligand were synthesized and characterized. Re-action of ButC(O)CH2Br with Ph2PNHBut afforded [Ph2P(NHBut)CH2C(O)But]+Br– (1). Treatment of 1 with excess of NaH in THF generated ligand precursor Ph2P(CH2C(O)But)=NBut (2). Reaction of 2 with AlR3 (R = Me, Et) gave N,O-chelate aluminum complexes [R2Al{OC(But)CHP(Ph2)=NBut}] (3, R = Me; 4, R = Et). Lithiation of 2 with an equiv. of LiBun formed lithium com-plex [Li{OC(But)CHP(Ph2)=NBut}] (5). Reaction of the lithium complex with an equiv. of AlCl3 yielded [Cl2Al{OC(But) CHP(Ph2)=NBut}] (6). Complex 6 was also obtained by reaction of 3 with an equiv. of AlCl3. Compounds 2–6 were characterized by NMR spectroscopy, elemental analysis and single crystal X-ray diffraction techniques (for 2, 3 and 6). Catalysis of complexes 3 and 4 as well as [Me2Al{1-{OC(Ph)CH}-3-R1-5-MeC3HN2}] (R1 = Me, But) toward the ring-opening polymerization of ε-caprolactone was investigated.
A quantum chemistry method was developed to calculate the Hammett substituent constants of various organic radicals. These newly obtained constants allow, for the first time, the quantitative analysis of the electron demand of organic radicals. Calculations reveal that the electron demand of organic radicals varies dramatically. It was demonstrated that the Hammett relationship of bond dissociation energies is determined only by the change of electron demand during the homolysis process.
The manganese(III) acetate-mediated radical reaction of [60]fullerene (C60) with three carboxylic acids, that is, bromoacetic acid, 3-chloropropionic acid, and 1-naphthylacetic acid, was investigated. The reaction of C60 with bromoacetic acid in the presence of 4-(dimethylamino)pyridine afforded the anticipated C60-fused lactone, while the reaction with 3-chloropropionic acid unexpectedly led to the formation of a novel C60-fused lactone with the loss of one molecule of HCl. Interestingly, the reaction with 1-naphthylacetic acid under similar conditions gave both C60-fused lactone and 1,4-adduct, yet each of them could be selectively obtained by controlling reaction conditions. It was also observed that the C60-fused lactone bearing a bulky naphthyl group existed as two isomers.
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