- Neutral Heteroleptic Lanthanide Complexes for Unravelling Host-Guest Assemblies in Organic Solvents: The Law of Mass Action Revisited
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The binding of lanthanide containers [Ln(β-diketonate)3dig] [dig = 1-methoxy-2-(2-methoxyethoxy)ethane] to aromatic tridentate N-donor ligands (L) in dichloromethane produces neutral nine-coordinate heteroleptic [LLn(β-diketonate)3] complexes, the equilibrium reaction quotients of which vary with the total concentrations of the reacting partners. This problematic drift prevents the determination of both reliable thermodynamic stability constants and intrinsic host-guest affinities. The classical solution theory assigns this behavior to changes in the activity coefficients of the various partners in nonideal solutions, and a phenomenological approach attempts to quantitatively attribute this effect to some partition of the solvent molecules between bulk-innocent and contact-noninnocent contributors to the chemical potential. This assumption eventually predicts an empirical linear dependence of the equilibrium reaction quotient on the concentration of the formed [LLn(β-diketonate)3] complexes, a trend experimentally supported in this contribution for various ligands L differing in lipophilicity and nuclearity and for lanthanide containers grafted with diverse β-diketonate coligands. Even if the origin of the latter linear dependence is still the subject of debate, this work demonstrates that this approach can be exploited by experimentalists for extracting reliable thermodynamic constants suitable for analyzing and comparing host-guest affinities in organic solvents.
- Babel, Lucille,Baudet, Karine,Besnard, Céline,Kale, Vishal,Mirzakhani, Mohsen,Naseri, Soroush,Nozary, Homayoun,Piguet, Claude
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supporting information
p. 62 - 75
(2020/02/20)
-
- Reductive Electrochemical Activation of Molecular Oxygen Catalyzed by an Iron-Tungstate Oxide Capsule: Reactivity Studies Consistent with Compound i Type Oxidants
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The reductive activation of molecular oxygen catalyzed by iron-based enzymes toward its use as an oxygen donor is paradigmatic for oxygen transfer reactions in nature. Mechanistic studies on these enzymes and related biomimetic coordination compounds designed to form reactive intermediates, almost invariably using various "shunt" pathways, have shown that high-valent Fe(V)=O and the formally isoelectronic Fe(IV) =O porphyrin cation radical intermediates are often thought to be the active species in alkane and arene hydroxylation and alkene epoxidation reactions. Although this four decade long research effort has yielded a massive amount of spectroscopic data, reactivity studies, and a detailed, but still incomplete, mechanistic understanding, the actual reductive activation of molecular oxygen coupled with efficient catalytic transformations has rarely been experimentally studied. Recently, we found that a completely inorganic iron-tungsten oxide capsule with a keplerate structure, noted as {Fe30W72}, is an effective electrocatalyst for the cathodic activation of molecular oxygen in water leading to the oxidation of light alkanes and alkenes. The present report deals with extensive reactivity studies of these {Fe30W72} electrocatalytic reactions showing (1) arene hydroxylation including kinetic isotope effects and migration of the ipso substituent to the adjacent carbon atom ("NIH shift"); (2) a high kinetic isotope effect for alkyl C - H bond activation; (3) dealkylation of alkylamines and alkylsulfides; (4) desaturation reactions; (5) retention of stereochemistry in cis-alkene epoxidation; and (6) unusual regioselectivity in the oxidation of cyclic and acyclic ketones, alcohols, and carboxylic acids where reactivity is not correlated to the bond disassociation energy; the regioselectivity obtained is attributable to polar effects and/or entropic contributions. Collectively these results also support the conclusion that the active intermediate species formed in the catalytic cycle is consistent with a compound I type oxidant. The activity of {Fe30W72} in cathodic aerobic oxidation reactions shows it to be an inorganic functional analogue of iron-based monooxygenases.
- Bugnola, Marco,Shen, Kaiji,Haviv, Eynat,Neumann, Ronny
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p. 4227 - 4237
(2020/05/05)
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- Synthesis and antitumor activity evaluation of compounds based on toluquinol
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Encouraged by the promising antitumoral, antiangiogenic, and antilymphangiogenic properties of toluquinol, a set of analogues of this natural product of marine origin was synthesized to explore and evaluate the effects of structural modifications on their cytotoxic activity. We decided to investigate the effects of the substitution of the methyl group by other groups, the introduction of a second substituent, the relative position of the substituents, and the oxidation state. A set of analogues of 2-substituted, 2,3-disubstituted, and 2,6-disubstituted derived from hydroquinone were synthesized. The results revealed that the cytotoxic activity of this family of compounds could rely on the hydroquinone/benzoquinone part of the molecule, whereas the substituents might modulate the interaction of the molecule with their targets, changing either its activity or its selectivity. The methyl group is relevant for the cytotoxicity of toluquinol, since its replacement by other groups resulted in a significant loss of activity, and in general the introduction of a second substituent, preferentially in the para position with respect to the methyl group, was well tolerated. These findings provide guidance for the design of new toluquinol analogues with potentially better pharmacological properties.
- Cheng-Sánchez, Iván,Torres-Vargas, José A.,Martínez-Poveda, Beatriz,Guerrero-Vásquez, Guillermo A.,Medina, Miguel ángel,Sarabia, Francisco,Quesada, Ana R.
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-
- o-xylylene bis(triethyl ammonium tribromide) as a mild and recyclable reagent for rapid and regioselective bromination of anilines and phenols
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Background: o-Xylylene bis(triethyl ammonium tribromide) (OXBTEATB) as a recyclable and high bromine containing di-(tribromide) reagent has been employed for the bromination of various organic substrates such as phenol and aniline or its derivatives. This catalyst can be recovered and reused several times. Methods: Aryl bromides shown in Table 1, were easily produced from bromination of aromatic compounds by OXBTEATB. This high-yield process lets the reagents to be recycled and reused. Results: As shown in Table 1, substituted anilines, phenols and β-naphthol were found to be the most reactive and immediately converted to the corresponding mono-brominated products by OXBTEATB. Conclusion: OXBTEATB can be considered a solidified bromine. This novel reagent has variable solubility in different polar protic and aprotic solvents but insoluble in non-polar aprotic solvent. Subsequently, OXBTEATB can be recognized as a more useful brominating and regioselective catalyst than the liquid bromine.
- Hemati, Roya,Shahvelayati, Ashraf S.,Yadollahzadeh, Khadijeh
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p. 682 - 687
(2018/07/14)
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- The synthesis of dendroflorin
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The first synthesis of dendroflorin has been achieved in 10 steps with an overall yield of 5.5%. The key step in the synthesis features the biphenyl structure is built through Suzuki–Miyaura reaction. In addition, the ortho-localization effect induced by aromatic substituent during the bromination of intermediate 8 is also observed and discussed.
- Deng, Yi,Jiang, Kun,Cai, Mao-Jun,Qu, Shi-Jin,Dai, Yi-Ru,Tan, Chang-Heng
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p. 602 - 609
(2017/05/26)
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- Chemical Potential of the Solvent: A Crucial Player for Rationalizing Host–Guest Affinities
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Access to reliable values of the thermodynamic constants βH,G1,1, which control simple host–guest ([HG]) association, is crucial in medicine, biology, pharmacy, and chemistry, since the optimum concentration of an effector (i.e., a drug) acting on a receptor is set to 1/βH,G1,1. Intermolecular association between charged species in polar solvents, for which water is the archetype, largely obeys this principle. Any deviation from ideality, which alters the speciation in solution, is mastered by the Debye–Hückel theory of ionic atmosphere. Much less is known for related association reactions involving neutral species in non-polar (lipophilic) media such as membranes, bilayers, or organic polymers. Taking the intermolecular association between [La(hfa)3dig] guest (hfa=hexafluoroacetylacetonate, dig=2-{2-methoxyethoxy}ethane) and tridentate polyaromatic host receptors L1–L3 in dichloromethane as a proof-of-concept, we show that the progress of the association reactions, as measured by the increase in the mole fraction of occupied sites of the receptors, disrupt the chemical potential of the solvent to such an extent that βH,G1,1 may seemingly be shifted by two orders of magnitude, thus leading to erroneous dose-response prescriptions. A simple chemical model, which considers a subset of solvent molecules in surface contact with the partners of the association reaction, restores reliable access to true and interpretable thermodynamic constants. The concomitant emergence of a concentration-dependent corrective parameter reestablishes satisfying dose-dependent response under real conditions. This “complement” to the law of mass action offers a simple method for safely taking care of the non-predictable variations of the activity coefficients of the various partners when host–guest reactions are conducted in non-polar media.
- Baudet, Karine,Guerra, Sebastiano,Piguet, Claude
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supporting information
p. 16787 - 16798
(2017/10/07)
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- Total synthesis of (±)-ganocins B and C
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The first total synthesis of structurally unique polycyclic phenolic meroterpenoids, ganocins B and C is reported. The synthesis features gold-catalyzed intramolecular cascade cyclization to construct the C/D ring bearing an angular methyl group, diastereoselective Michael addition, and acid-mediated one-pot Robinson cyclization/deprotection/isomerization.
- Liu, Yao,Zhou, Chu-Jun,Li, Qingjiang,Wang, Honggen
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supporting information
p. 10362 - 10365
(2016/11/18)
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- Isoquinolinium Dichromate and Chlorochromate as Efficient Catalysts for Oxidative Halogenation of Aromatic Compounds under Acid-Free Conditions
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Isoquinolinium dichromate and isoquinolinium chlorochromate were found as efficient catalysts to trigger oxidative bromination and iodination of aromatic hydrocarbons with KBr/KI and KHSO4 under acid-free conditions. Reaction times reduced highly significantly under sonication, followed by corresponding mono bromo derivatives in very good yield with high regioselectivity.
- Rao, A. Sambashiva,Rajanna,Reddy, K. Rajendar,Kulkarni, Subhash
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p. 832 - 837
(2016/02/12)
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- A dinuclear iron(II) complex bearing multidentate pyridinyl ligand: Synthesis, characterization and its catalysis on the hydroxylation of aromatic compounds
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A dinuclear iron(II) complex Fe2L2(μ2-Cl)2Cl2 (L = N,N-bis(pyridin-2-ylmethyl)prop-2-yn-1-amine) was prepared and fully characterized by UV–Vis spectroscopy, elemental analysis, electrochemical analysis and X-ray single crystal diffraction analysis. The catalytic activity of the complex was assessed for the hydroxylation of aromatic compounds by using aqueous H2O2 as an oxidant in acetonitrile. The catalytic system was applicable in a wide range of substrates including aromatic compounds with both electron-donating and electron-withdrawing substituents and showed moderate to good catalytic activity and selectivity in the oxidation reactions. Particularly, in the case of benzene the selectivity of phenol achieve to 74% with the reaction conversion of 24.8%.
- Gu, Erxing,Zhong, Wei,Ma, Hongxia,Xu, Beibei,Wang, Hailong,Liu, Xiaoming
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p. 159 - 165
(2018/03/29)
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- Biocatalytic One-Pot Synthesis of l-Tyrosine Derivatives from Monosubstituted Benzenes, Pyruvate, and Ammonia
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l-Tyrosine derivatives were obtained in >97% ee via a biocatalytic one-pot two-step cascade using substituted benzenes, pyruvate, and NH3 as starting materials. In the first step, monosubstituted arenes were regioselectively hydroxylated in the o-position by monooxygenase P450 BM3 (using O2 as oxidant with NADPH-recycling) to yield the corresponding phenols, which subsequently underwent C-C coupling and simultaneous asymmetric amination with pyruvate and NH3 using tyrosine phenol lyase to furnish l-DOPA surrogates in up to 5.2 g L-1. Instead of analytically pure arenes, crude aromatic gasoline blends containing toluene were used to yield 3-methyl-l-tyrosine in excellent yield (2 g L-1) and >97% ee.
- Dennig, Alexander,Busto, Eduardo,Kroutil, Wolfgang,Faber, Kurt
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p. 7503 - 7506
(2015/12/11)
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- Trifluoromethanesulfonyloxy-group-directed regioselective (3 + 2) cycloadditions of benzynes for the synthesis of functionalized benzo-fused heterocycles
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Highly regioselective (3 + 2) cycloadditions of (trifluoromethanesulfonyloxy)benzynes [(triflyloxy)benzynes] with 1,3-dipoles followed by cross-coupling reactions provided multisubstituted benzo-fused heterocycles. The triflyloxy group at the 3-position of benzynes, and even that at the remote 4-position, greatly affected the regiocontrol of the cycloaddition. These groups also served to install other substituents at their ipso-positions.
- Ikawa, Takashi,Kaneko, Hideki,Masuda, Shigeaki,Ishitsubo, Erika,Tokiwa, Hiroaki,Akai, Shuji
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supporting information
p. 520 - 526
(2015/02/05)
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- A Metal-Free and Ionic Liquid-Catalyzed Aerobic Oxidative Bromination in Water
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A metal-free aerobic oxidative bromination of aromatic compounds in water has been developed. Hydrobromic acid is used as a bromine source and 2-methylpyridinium nitrate ionic liquid is used as a recyclable catalyst. Water is used as the reaction mediate. This is the first report of aerobic oxidative bromination using only catalytic amount of metal-free catalyst. This system shows not only high bromine atom economy, but also high bromination selectivity. The possible mechanism and the role of the catalyst in this system have also been discussed.
- Wang, Jian,Chen, Shu-Bin,Wang, Shu-Guang,Li, Jing-Hua
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p. 513 - 517
(2015/03/30)
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- Candida antarctica lipase B-catalyzed regioselective deacylation of dihydroxybenzenes acylated at both phenolic hydroxy groups
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Candida antarctica lipase B proved to be highly active in the deacylation of substituted hydroquinones and resorcinols acylated at both phenolic hydroxy groups. The deacylation reactions were much faster than the corresponding direct acylations of these dihydroxybenzenes catalyzed by the same lipase. More importantly, they took place generally in a markedly regioselective manner: the acyloxy group remote from the substituent was preferentially cleaved. The main or exclusive products obtained were the regioisomers of those produced through the direct acylation of the dihydroxybenzenes. In the case of alkyl-substituted hydroquinone derivatives, the regioselectivity increased with an increase in the bulk of the substituent. In the case of 4-substituted diacylated resorcinols, the 3-O-monoacyl derivatives were obtained generally as the sole products. Quite interestingly, some secondary alcohols proved to act as better acyl acceptors than the corresponding primary alcohols in these enzymatic deacylations.
- Miyazawa, Toshifumi,Hamada, Manabu,Morimoto, Ryohei
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- Bromination of arenes using I2O5-KBr in water
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An efficient and environmentally benign bromination of various aromatic compounds using aN aqueous I2O5-KBr system at room temperature has been developed in this work. A series of aromatic compounds such as acetophenones, benzaldehydes, benzoic acids, anilines, amides, and aminopyridine have been successfully brominated in excellent regioselectivities and good yields under the typical reaction conditions. The features of KBr as brominating reagent, water as solvent, and mild conditions make this system an attractive synthetic procedure. [Supplementary materials are available for this article. Go to the publisher's online edition of Synthetic Communications for the following free supplemental resource(s): Full experimental and spectral details.]
- Hou, Jieping,Li, Zejiang,Jia, Xiao-Dong,Liu, Zhong-Quan
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supporting information
p. 181 - 187
(2013/12/04)
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- Copper(II)-catalyzed aromatization followed by bromination of cyclohexenones leading to phenols and bromophenols
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Conversion of substituted cyclohexenones into the corresponding phenols can be achieved using copper acetate as the catalyst in the presence of LiBr and CF3COOH under oxygen. With the use of excess LiBr, electrophilic aromatic bromination afforded the corresponding bromophenol under similar catalytic conditions. Copyright
- Tong, Hung-Chun,Reddy, K. Rajender,Liu, Shiuh-Tzung
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p. 3256 - 3261
(2014/06/09)
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- Polymer-anchored peroxo compounds of molybdenum and tungsten as efficient and versatile catalysts for mild oxidative bromination
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A polymer supported peroxomolybdate(VI) compound of the type [MoO 2(O2)(CN)2]-PAN [PAN = poly(acrylonitrile)] (PANMo) was obtained by reacting H2MoO4 with 30% H 2O2 and the macromolecular ligand, PAN at near neutral pH. The macrocomplex has been characterized by elemental analysis (CHN and EDX analysis), spectral (IR, UV-Vis and 13C NMR, 95Mo NMR), thermal (TGA-DTG) as well as SEM studies. The catalytic activity of PANMo and its previously reported tungsten containing analog PANW, in oxidative bromination of organic substrates has been explored. The supported complexes could serve as efficient heterogeneous catalysts for the oxidative bromination of a variety of structurally diverse aromatic compounds, with H 2O2 as terminal oxidant, to afford bromo organics in impressive yields under environmentally clean conditions. The catalysts afforded regeneration and could be reused for a minimum of six reaction cycles.
- Boruah, Jeena Jyoti,Das, Siva Prasad,Borah, Rupam,Gogoi, Sandhya Rani,Islam, Nashreen S.
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p. 246 - 254
(2013/05/23)
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- Synthesis of a focused chemical library based on derivatives of embelin, a natural product with proapoptotic and anticancer properties
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The synthesis of new derivatives of embelin, a natural inhibitor of X-linked inhibitor of apoptosis protein (XIAP) is described. The design of these new molecules involved introduction of aromatic groups directly linked to the benzoquinone core. To allow a large flexibility in the nature and the length of the added chain, the strategy involves first aSuzuki-Miyaura reaction with functionalized aromatics, yielding a first generation of molecules. Then, by appropriate use of the functional groups, a second generation of representative embelin derivatives was prepared.
- Viault, Guillaume,Gree, Danielle,Das, Saibal,Yadav, Jhillu Singh,Gree, Rene
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experimental part
p. 1233 - 1241
(2011/04/17)
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- An efficient copper-catalysed aerobic oxybromination of arenes in water
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An aerobic oxybromination of arenes catalysed by Cu(NO3) 2 was achieved using HBr as a bromine source, molecular oxygen as the oxidant and water as solvent with high selectivity. The catalyst shows not only high chemoselectivity for monobromination, but also a remarkable regioselectivity for para-isomers.
- Wang, Jian,Wang, Wei,Li, Jing-Hua
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supporting information; experimental part
p. 2124 - 2126
(2011/02/24)
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- Synthesis and application of poly(diallyldimethylammonium tribromide) as a novel polymeric brominating agent
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In this study, the synthesis and applications of a new supported tribromide reagent based on poly(diallyldimethylammonium chloride) is reported. This supported tribromide is used in α-bromoacetalization of ketones, bromination of alkenes and regioselective bromination of activated aromatic compounds. This method is mild and no Br2 and HBr were used. Other advantages of this reagent are stability, high efficiency, simple filtera- bility and reusability. In this study, the synthesis and applications of a new supported tribromide reagent based on poly(diallyldimethylammonium chloride) is reported. This supported tribromide is used in α-bromoacetalization of ketones, bromination of alkenes and regioselective bromination of activated aromatic compounds. This method is mild and no Br2 and HBr were used. Other advantages of this reagent are stability, high efficiency, simple filterability and reusability. Copyright
- Hossein, Mahdavi,Zahra, Kachoei
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experimental part
p. 2221 - 2225
(2011/10/12)
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- BENZOFURAN-4,5-DIONES AS SELECTIVE PEPTIDE DEFORMYLASE INHIBITORS
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The instant invention provides novel benzofuran-4,5-diones and pharmaceutical compositions thereof useful for inhibiting PDF and for treating proliferative and infectious diseases. Compounds may be selective for eukaryotic (e.g., human) PDF or prokaryotic PDF
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Page/Page column 111
(2010/11/18)
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- Bioconversion of aromatic compounds by Escherichia coli that expresses cytochrome P450 CYP153A13a gene isolated from an alkane-assimilating marine bacterium Alcanivorax borkumensis
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The cytochrome P450 CYP153 family has been isolated from alkane-assimilating bacteria. CYP153 has been shown to mediate terminal hydroxylations of linear alkanes or alkyl aromatics. We here performed the biotransformation of various aromatic compounds by Escherichia coli cells that expressed the CYP153A13a (P450balk) gene, which was isolated from an alkane-degading marine bacterium Alcanivorax borkumensis. Aromatic compounds including a short alkyl moiety or methyl ether moiety, and phenolic compounds were converted to their respective hydroxylated products, whose structures were determined by HRMS and NMR analyses. The present study revealed that the catalytic function of CYP153A13a is multifunctional, i.e., it can hydroxylate not only the terminal of short alkyl groups that attached to aromatic rings but also the p-position of phenolic compounds substituted with a halogen or the acetyl group. CYP153A13a was also shown to demethylate methylether-including aromatic compounds.
- Otomatsu, Toshihiko,Bai, Liming,Fujita, Naoya,Shindo, Kazutoshi,Shimizu, Keiko,Misawa, Norihiko
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experimental part
p. 234 - 240
(2011/01/03)
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- Synthesis of bromo-conduritol-B and bromo-conduritol-C as glycosidase inhibitors
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For the synthesis of bromo-conduritol-B skeleton, bromo-1,4-benzoquinone was subjected to bromination followed by the reduction of the carbonyl groups with NaBH4. Substitution of bromides bonded to sp3-hybridized carbon atoms with AgOAc gave the bromo-conduritol-B tetraacetate in high yield. For the construction of bromo-conduritol-C skeleton, 2,2-dimethyl-3a,7a-dihydro-1,3-benzodioxole was used as the starting material. Photooxygenation of the diene unit gave an unsaturated bicyclic endoperoxide. Bromine was incorporated into the molecule by the addition of bromine to the double bond. Opening of the peroxide linkage followed by HBr elimination and reduction of the carbonyl group provided the conduritol-C structure in good yield. Bromo-conduritol-B exhibited strong enzyme-specific inhibition against α-glycosidase.
- Cantekin, Seda,Baran, Arif,Caliskan, Rasit,Balci, Metin
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scheme or table
p. 426 - 431
(2009/05/31)
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- Synthesis of arylbromides from arenes and Nbromosuccinimide bromosuccinimide (NBS) in acetonitrile - A convenient method for aromatic bromination
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Regioselective and chemoselective electrophilic bromination of a wide series of activated arenes using N-bromosuccinimide (NBS) in acetonitrile occurs readily. Environmentally friendly conditions, large substrate scope, and ease of synthesis enhance the utility of this method over other electrophilic bromination conditions.
- Zysman-Colman, Eli,Arias, Karla,Siegel, Jay S.
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experimental part
p. 440 - 447
(2009/12/01)
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- Regioselective, photochemical bromination of aromatic compounds using N-bromosuccinimide
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Regioselective nuclear bromination of aromatic compounds is investigated with N-bromosuccinimide as the brominating agent under UV irradiation to afford the corresponding brominated compounds. The reaction proceeds at ambient temperature (30 ± 2 °C) without any catalyst. In most of the reactions, regioselectively mono-brominated products are obtained in good to high yields. The conversion and selectivity for bromination depend on the nature of the substituent on the aromatic ring.
- Chhattise, Prakash K.,Ramaswamy,Waghmode, Suresh B.
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p. 189 - 194
(2008/03/30)
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- A convenient and regioselective oxidative bromination of electron-rich aromatic rings using potassium bromide and benzyltriphenylphosphonium peroxymonosulfate under nearly neutral reaction conditions
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Regioselective oxidative bromination of electron-rich aromatic rings has been studied using potassium bromide as a bromine source in the presence of benzyltriphenylphosphonium peroxymonosulfate as oxidant under nearly neutral reaction conditions. In most cases we obtained monobrominated derivatives regioselectively and in good to high yields without the aid of strong acids.
- Adibi, Hadi,Hajipour, Abdol R.,Hashemi, Majid
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p. 1255 - 1259
(2007/10/03)
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- Simple, mild, and efficient method for the reduction of 1,4-benzoquinones to hydroquinones by the action of NaN3
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A simple, mild, and efficient method is presented for the reduction of 1,4-benzoquinones to hydroquinones by the action of NaN3 under neutral conditions in the presence of water. Copyright Taylor & Francis Group, LLC.
- Algi, Fatih,Balci, Metin
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p. 2293 - 2297
(2007/10/03)
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- Supramolecular ordering, thermal behavior, and photophysical, electrochemical, and electroluminescent properties of alkoxy-substituted yne-containing poly(phenylene-vinylene)s
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The synthesis and the characterization of defect-free alkoxy-substituted yne-containing poly(phenylene-vinylenes) was investigated. It was found that a degree of 7 was obtained for all the polymers enabling a reliable comparison of their properties as a function of the length and geometry of the grafted side chains. A formation of crystalline superstructures of either spherulitic-like or rodlike morphology was identified as following typical nucleation and growth process for polymer having a symmetric substitution pattern. Results show that the high absorption coefficients and high quantum yields in the solid state combined with enhanced oxidation stability made polymers as potential donor materials in the design of organic solar cells.
- Egbe, Daniel Ayuk Mbi,Carbonnier, Benjamin,Ding, Liming,Muehlbacher, David,Birckner, Eckhard,Pakula, Tadeusz,Karasz, Frank E.,Grummt, Ulrich-Walter
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p. 7451 - 7463
(2007/10/03)
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- Formal total synthesis of shikonin via D?tz benzannulation
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The D?tz benzannulation reaction provides mild conditions for the construction of highly functionalized aromatic compounds. We have utilized this method for the formal total synthesis of shikonin. The key step in our approach is the application of the D?tz benzannulation reaction between Fischer a chromium carbene complex and an optically active alkyne for the construction of the aromatic skeleton. This is followed by protecting group manipulation and formation of the epoxide moiety using the Sharpless procedure.
- Pulley, Shon R.,Czakó, Barbara
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p. 5511 - 5514
(2007/10/03)
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- Influence of the conjugation pattern on the photophysical properties of alkoxy-substituted PE/PV hybrid polymers
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Alkoxy-substituted phenylene-ethynylene (PE)/phenylene-vinylene (PV) hybrid polymers of general constitutional structure (-Ar-C≡C-Ar-CH=CH-)n (2), (-Ar-C≡C-Ar-CH=CH-Ar-C≡C-)n (3), and (-Ar-C≡C-Ar-C≡C-Ar-CH=CH-Ar-CH=CH-)n (4) have been synthesized and characterized. Comparisons of their photophysical properties in solution as well as in solid state (photoconductivity, absorption, emission, and electroluminescence) with those of their corresponding alkoxy-substituted poly(phenylene-ethynylene) (1) and poly(phenylene-vinylene) (5) have been carried out. Large octadecyloxy groups were selected as side chains in order to reduce the effect of π-π stacking interaction on the properties of the polymers in the solid state. This resulted in easy detection of photoconductivity and higher fluorescence quantum yields in solid state. The hybrid conjugation pattern in 4 is more favorable for photoconductivity than that in 2. A red shift of the absorption and emission spectra in solution is observed from 1 over 2b, 3, and 4 to 5. LED devices of ITO/PEDOT/polymer/Ca configuration have been fabricated with all compounds, except 2a. The phenylvinylene side groups in 3 not only give rise to a red shift of the solid-state photoluminescence spectrum and electroluminescence spectrum (relative to 2b) but also bring about a decrease of the turn-on voltage and improve the electroluminescence efficiency for more than 100 times.
- Egbe, Daniel Ayuk Mbi,Bader, Cornelia,Klemm, Elisabeth,Ding, Liming,Karasz, Frank E.,Grummt, Ulrich-Walter,Birckner, Eckhard
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p. 9303 - 9312
(2007/10/03)
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- Excimer formation in oligo[2,5-bis(hexadecyloxy)-1,4-phenylene]s followed by fluorescence spectroscopy
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Using the steady-state and time-resolved fluorescence spectroscopy, the behavior of "hairy-rod" oligo- and poly[2,5-bis(hexadecyloxy)-1,4-phenylene]s in tetrahydrofuran solutions was investigated. The materials were prepared by the Yamamoto coupling reaction using zinc as a reducing metal, the nickel(II)/triphenylphosphine complex as a catalyst, and 2,2′-bipyridine as a coligand. The appropriate oligomer fractions were separated by fractional precipitation and characterized by GPC and end group analysis. The fluorescence quantum yield of oligomers and polymers increased with their increasing conjugation length. The fluorescence emission spectra of polymers and longer oligomers exhibited one emission maximum at 390 nm with a single-exponential decay and fluorescence lifetimes (τ) around 1 ns. The substitution in positions 2 and 5 forces the adjacent backbone benzene units out of the plane, which results in twist angles 60-80°, and the bulky substituents exclude the cofacial sandwich-type configuration necessary for excimer formation. However, with shorter oligomers, another emission band at 460 nm appeared. Fluorescence decays at 460 nm were found to be double-exponential with longer excited-state lifetimes [e.g. τ1 = 6.9 ns (76%), τ2 = 2.4 ns (24%)]. With shorter oligomers (dimer, trimer), we assume a sandwich-type configuration with sufficiently close interchain distance and hence the excimer can form. Hydrophobic interactions of long aliphatic side chains in a polar medium play an important role in the excimer formation.
- Vyprachticky, Drahomir,Cimrova, Vera,Machova, Lund'ka,Pokorna, Veronika
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p. 1473 - 1489
(2007/10/03)
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- Investigations into the bromination of substituted phenols using diethyl bromomalonate and diethyl dibromomalonate
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Substituted 4-bromophenols can be synthesised efficiently by heating the corresponding phenol in either neat diethyl bromomalonate or diethyl dibromomalonate. We discuss the regioselectivity of such reactions and comment on the scope and limitation of this procedure.
- Coumbarides,Dingjan,Eames,Weerasooriya
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p. 179 - 180
(2007/10/03)
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- New routes toward drimanes and nor-drimanes from (-)-sclareol
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New synthetic strategies to prepare natural drimanes and monocarbocyclic terpenoids from (-)-sclareol (2) are reported. The preparation of natural 9,11-drimen-8α-ol (12) and the first enantiospecific synthesis of marine metabolite fulvanin 2 (1) are described.
- Barrero,Alvarez-Manzaneda,Chahboun,Gonzalez Diaz
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p. 1561 - 1564
(2007/10/03)
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- Synthesis and antitumoral activities of marine ent-chromazonarol and related compounds
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Efficient syntheses of ent-isozonarol (6a), ent-isozonarone (7a) and ent-chromazonarol (8) from (-)-sclareol (12) are described. 6a and 7a show a significative antitumoral activity.
- Barrero, Alejandro F.,Alvarez-Manzaneda, Enrique J.,Herrador, M. Mar,Chahboun, Rachid,Galera
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p. 2325 - 2328
(2007/10/03)
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- Synthesis of trifluoromethyl-substituted conjugated enynes including a fluorinated siccayne
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Trifluoromethyl-substituted conjugated enynes have been synthesized in good to excellent yield through palladium-catalyzed condensations of 2-bromo-3,3,3-trifluoropropene with 1-alkynes.A novel fluorinated siccayne has been prepared by this methodology.
- Hu, Chang-Ming,Hong, Feng,Xu, Yuan-Yao
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- Mechanism of the Oxidation of NADH by Quinones. Energetics of One-Electron and Hydride Routes
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The kinetics of NADH oxidation by 7 o-benzoquinones and 14 p-benzoquinones were studied by using buffered aqueous solutions and UV/vis spectroscopy.For each quinone the rate law was first order in NADH and first order in quinone.The rate constants varied from 0.0745 to 9220 M-1s-1.Variation of the pH from 6 to 8 gave no change in rate.The use of 4-D and 4,4-D2NADH revealed kinetic isotope effects.The dideutero data gave kH/kD in the range 1.6-3.1 for p-quinones and 4.2 for 3,5-di-tert-butyl-o-quinone.When p-quinones were used, the log k was a linear function of Eo for the quinone/hydroquinone monoanion (Q/QH(1-)) couple with a slope of 16.9 V-1. o-Quinones reacted about 100 times more rapidly, but the same linear relationship with a slope of 16.4 V-1 was observed.Comparisons to data for one-electron-transfer reactions indicate that such mechanisms are not involved.A hydride-transfer mechanism accommodates all the data, and rate-limiting hydrogen atom transfer followed by electron transfer cannot be ruled out.
- Carlson, Brian W.,Miller, Larry L.
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p. 479 - 485
(2007/10/02)
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- The Kinetics and Mechanisms of Additions to Olefinic Substances. Part 16. Addition of Halogens to 1,4-Benzoquinone and to 1,4-Naphthoquinone, and Dehydrohalogenation of the Resulting Adducts
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The products of acid catalysed halogenation of 1,4-benzoquinone and 1,4-naphthoquinone in acetic acid have been investigated under conditions which could lead to the formation of dichloride, dibromide, and bromochloride.The variation in ratio of bromochloride to dibromide with relative availability of bromine and chlorine as electrophile and nucleophile suggests that the first entering halogen atom is attached nucleophilically as the result of '1,4'- (rather than then '1,2'-) addition, and that the second halogen atom is then attached electrophilically.Acetate ion does not compete effectively with chloride ion in the nucleophilic stage of the reaction; and chlorine acetate does not add, even with sulphuric acid as catalyst, to 1,4-benzoquinone.The mechanistic significance of these results is discussed.The kinetics and products of solvolytic eliminations from the dihalides derived from 1,4-benzoquinone have been studied in 95percent ethanol; the effects of structure on the rate make it probable that the reactions lie towards the E1cB end of the spectrum of elimination mechanisms.
- Atkinson, Romily C.,Mare, Peter B. D. de la,Larsen, David S.
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p. 271 - 280
(2007/10/02)
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