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Acid Catalysis (acid + catalysis)
Kinds of Acid Catalysis Selected AbstractsOne-Pot Synthesis of Quinoline Derivatives Directly from Terminal Alkynes via Sequential Ruthenium(II) and Acid CatalysisADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 11-12 2010Min Zhang Abstract A convenient one-pot synthesis of 2,3-disubstituted, 2,3,6-trisubstituted, and 2,3,6,7-tetrasubstituted quinoline analogues from terminal alkynes via sequential ruthenium(II) and para -toluenesulfonic acid (p -TSA) co-catalyzed reactions is described. The catalytic process is shown to take place first via intermediate formation of an allyl ketone and then addition of an aniline derivative to the allyl ketone. The p -TSA is a catalyst for both allyl ketone and quinoline synthetic steps. The method allowed us to synthesize a wide range of quinoline derivatives and introduce different substituents by employing various simple starting materials. The reaction allows the synthesis of halogen-containing products. [source] Unexpected Acid Catalysis in Reactions of Peroxyl Radicals with Phenols,ANGEWANDTE CHEMIE, Issue 44 2009Luca Valgimigli Dr. Schwache organische Säuren in millimolaren Konzentrationen erhöhen unerwartet die Reaktivität von Peroxylradikalen gegenüber üblichen Phenol-Antioxidantien drastisch. Der Grund dafür ist, dass sich der Reaktionsmechanismus stark von dem ohne Säure unterscheidet: Der geschwindigkeitsbestimmende Elektronentransfer findet vom H-verbrückten Phenol zum Hydroperoxid-Radikalkation statt, das unter diesen Bedingungen im Gleichgewicht mit dem Peroxylradikal vorliegt (siehe Schema). [source] ChemInform Abstract: Asymmetric Broensted Acid Catalysis: Catalytic Enantioselective Synthesis of Highly Biologically Active Dihydroquinazolinones.CHEMINFORM, Issue 22 2009Magnus Rueping Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] ChemInform Abstract: Contact Ion Pair Directed Lewis Acid Catalysis: Asymmetric Synthesis of trans-Configured ,-Lactones.CHEMINFORM, Issue 45 2008Thomas Kull Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] ChemInform Abstract: A Strategy for C,H Activation of Pyridines: Direct C-2 Selective Alkenylation of Pyridines by Nickel/Lewis Acid Catalysis.CHEMINFORM, Issue 29 2008Yoshiaki Nakao Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] ChemInform Abstract: VAPOL Phosphoric Acid Catalysis: The Highly Enantioselective Addition of Imides to Imines.CHEMINFORM, Issue 11 2008Yuxue Liang Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] The First Simple Method of Protection of Hydroxy Compounds as Their O-Boc Derivatives under Lewis Acid Catalysis.CHEMINFORM, Issue 49 2006Giuseppe Bartoli Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF. [source] Enantiodiscrimination and Enantiocontrol of Neutral and Cationic PtII Complexes Bearing the Tropos Biphep Ligand: Application to Asymmetric Lewis Acid Catalysis.CHEMINFORM, Issue 10 2006Koichi Mikami Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF. [source] Broensted Acid Catalysis: Organocatalytic Hydrogenation of Imines.CHEMINFORM, Issue 5 2006Magnus Rueping Abstract For Abstract see ChemInform Abstract in Full Text. [source] A Survey of Acid Catalysis and Oxidation Conditions in the Two-Step, One-Flask Synthesis of Meso-Substituted Corroles via Dipyrromethanedicarbinols and Pyrrole.CHEMINFORM, Issue 41 2004G. Richard III Geier Abstract For Abstract see ChemInform Abstract in Full Text. [source] Highly Efficient Asymmetric Lewis Acid Catalysis with Platinum Group Complexes of Conformationally Flexible, 1,3-Butadiene-Bridged Diphosphines, NUPHOS.CHEMINFORM, Issue 7 2004Simon Doherty Abstract For Abstract see ChemInform Abstract in Full Text. [source] ChemInform Abstract: Computational Studies on the BF3 -Catalyzed Cycloaddition of Furan with Methyl Vinyl Ketone: A New Look at Lewis Acid Catalysis.CHEMINFORM, Issue 4 2001Martin Avalos Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] Unifying Metal and Brønsted Acid Catalysis,Concepts, Mechanisms, and ClassificationsCHEMISTRY - A EUROPEAN JOURNAL, Issue 31 2010Magnus Rueping Prof. Abstract Asymmetric catalysis is a key feature of modern synthetic organic chemistry. Traditionally, different combinations of ligands and metals are used to perform highly enantioselective reactions. Since the renaissance of organocatalysis in the early 2000s, tremendous improvement in the field of metal-free catalysis has been achieved. Recently, the combination of transition metals and organocatalysts has allowed the development of new protocols enabling transformations that could not previously be realized. This article aims to present the latest contributions in the field of combined chiral Brønsted acid and metal catalyzed reactions, highlighting the advantages of these catalytic systems as well as describing the uncertainties regarding the molecular structure of the catalytically active species and the reaction mechanisms. [source] The Formation of Silylated ,-Lactams from Silylketenes through Lewis Acid Promoted [2+2] Cycloaddition: A Combined Theoretical and Experimental StudyEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 12 2005Béatrice Pelotier Abstract The stereoselective formation of silylated cis -,-lactams from (trimethylsilyl)ketene and an ,-imino ester by Lewis acid catalysis is described. Theoretical results suggest that the reaction between (trimethylsilyl)ketene and trans -(methoxycarbonyl)- N -methylformaldimine would proceed most favourably with the BF3 catalyst coordinated to the ketene. Moreover, the calculated energy barriers account for the cis:trans ratio found experimentally. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] The Mechanism of Hydrolysis of Aryl Ether Derivatives of3-HydroxymethyltriazenesEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 10 2005Emília Carvalho Abstract 1-Aryl-3-aryloxymethyl-3-methyltriazenes hydrolyse to the corresponding anilines and phenols by specific-acid-catalysed, general-acid-catalysed and pH-independent mechanisms. All compounds studied exhibit specific- and general acid catalysis, though for 5a general acid catalysis was not observed below a pH of approximately 4, while for compounds 5e,f, such catalysis was absent above a pH of approximately 5. The pH-independent pathway is observed only for those compounds, 5d,f, that contain good aryloxy nucleofugic groups. The specific-acid-catalysed pathway is supported by a solvent deuterium isotope effect (SDIE) of 0.64, consistent with a mechanism involving protonation of the substrate followed by rate-determining unimolecular decomposition of the protonated species. The kH+ values gave rise to a Hammett , value of ,0.93, reflecting the competing effect of the substituents on the protonation of the substrate and the cleavage of the aryl ether. Correlation of kH+ with the pKa of the phenol leaving group affords a ,lg of 0.3. Decomposition of the protonated intermediate proceeds via a triazenyliminium ion that can be trapped by methanol. The general-acid-catalysed process exhibits an SDIE of 1.43 and Hammett , values of 0.49, 0.84 and 1.0 for reactions catalysed by chloroacetic, formic and acetic acids, respectively. Correlation of kA with the pKa of the acid gave Brønsted , values that diminish from 0.6 for O -aryl systems that are poor nucleofuges (5a,b) to 0.2 for the best nucleofuge (5f), reflecting the different extents of proton transfer required to expel each phenol. Compounds containing powerful nucleofuges exhibit a pH-independent reaction that has an SDIE of 1.1, a Hammett , value of 3.4 and a Brønsted ,lg value of 1.4. These imply a mechanism involving displacement of the aryloxide leaving group to form a triazenyliminium ion intermediate that again was trapped as a methyl ether. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] Benzo[a]heptalenes from Heptaleno[1,2- c]furans.HELVETICA CHIMICA ACTA, Issue 4 2007Abstract It is shown in this ,Part 2' that heptaleno[1,2- c]furans 1 react thermally in a Diels,Alder -type [4+2] cycloaddition at the furan ring with vinylene carbonate (VC), phenylsulfonylallene (PSA), , -(acetyloxy)acrylonitrile (AAN), and (1Z)-1,2-bis(phenylsulfonyl)ethene (ZSE) to yield the corresponding 1,4-epoxybenzo[d]heptalenes (cf. Schemes,1, 5, 6, and 8). The thermal reaction of 1a and 1b with VC at 130° and 150°, respectively, leads mainly to the 2,3- endo -cyclocarbonates 2,3- endo - 2a and - 2b and in minor amounts to the 2,3- exo -cyclocarbonates 2,3- exo - 2a and - 2b. In some cases, the (P*)- and (M*)-configured epimers were isolated and characterized (Scheme,1). Base-catalyzed cleavage of 2,3- endo - 2 gave the corresponding 2,3-diols 3, which were further transformed via reductive cleavage of their dimesylates 4 into the benzo[a]heptalenes 5a and 5b, respectively (Scheme,2). In another reaction sequence, the 2,3-diols 3 were converted into their cyclic carbonothioates 6, which on treatment with (EtO)3P gave the deoxygenated 1,4-dihydro-1,4-epoxybenzo[d]heptalenes 7. These were rearranged by acid catalysis into the benzo[a]heptalen-4-ols 8a and 8b, respectively (Scheme,2). Cyclocarbonate 2,3- endo - 2b reacted with lithium diisopropylamide (LDA) at ,70° under regioselective ring opening to the 3-hydroxy-substituted benzo[d]heptalen-2-yl carbamate 2,3- endo - 9b (Scheme,3). The latter was O -methylated to 2,3- endo -(P*)- 10b. The further way, to get finally the benzo[a]heptalene 13b with MeO groups in 1,2,3-position, could not be realized due to the fact that we found no way to cleave the carbamate group of 2,3- endo -(P*)- 10b without touching its 1,4-epoxy bridge (Scheme,3). The reaction of 1a with PSA in toluene at 120° was successful, in a way that we found regioisomeric as well as epimeric cycloadducts (Scheme,5). Unfortunately, the attempts to rearrange the products under strong-base catalysis as it had been shown successfully with other furan,PSA adducts were unsuccessful (Scheme,4). The thermal cycloaddition reaction of 1a and 1b with AAN yielded again regioisomeric and epimeric adducts, which could easily be transformed into the corresponding 2- and 3-oxo products (Scheme,6). Only the latter ones could be rearranged with Ac2O/H2SO4 into the corresponding benzo[a]heptalene-3,4-diol diacetates 20a and 20b, respectively, or with trimethylsilyl trifluoromethanesulfonate (TfOSiMe3/Et3N), followed by treatment with NH4Cl/H2O, into the corresponding benzo[a]heptalen-3,4-diols 21a and 21b (Scheme,7). The thermal cycloaddition reaction of 1 with ZSE in toluene gave the cycloadducts 2,3- exo - 22a and - 22b as well as 2- exo,3- endo - 22c in high yields (Scheme,8). All three adducts eliminated, by treatment with base, benzenesulfinic acid and yielded the corresponding 3-(phenylsulfonyl)-1,4-epoxybenzo[d]heptalenes 25. The latter turned out to be excellent Michael acceptors for H2O2 in basic media (Scheme,9). The Michael adducts lost H2O on treatment with Ac2O in pyridine and gave the 3-(phenylsulfonyl)benzo[d]heptalen-2-ones 28a and 3- exo - 28b, respectively. Rearrangement of these compounds in the presence of Ac2O/AcONa lead to the formation of the corresponding 3-(phenylsulfonyl)benzo[a]heptalene-1,2-diol diacetates 30a and 30b, which on treatment with MeONa/MeI gave the corresponding MeO-substituted compounds 31a and 31b. The reductive elimination of the PhSO2 group led finally to the 1,2-dimethoxybenzo[a]heptalenes 32a and 32b. Deprotonation experiments of 32a with t -BuLi/N,N,N,,N,-tetramethylethane-1,2-diamine (tmeda) and quenching with D2O showed that the most acid CH bond is HC(3) (Scheme,9). Some of the new structures were established by X-ray crystal-diffraction analyses (cf. Figs.,1, 3, 4, and 5). Moreover, nine of the new benzo[a]heptalenes were resolved on an anal. Chiralcel OD-H column, and their CD spectra were measured (cf. Figs.,8 and 9). As a result, the 1,2-dimethoxybenzo[a]heptalenes 32a and 32b showed unexpectedly new Cotton -effect bands just below 300,nm, which were assigned to chiral exciton coupling between the heptalene and benzo part of the structurally highly twisted compounds. The PhSO2 -substituted benzo[a]heptalenes 30b and 31b showed, in addition, a further pair of Cotton -effect bands in the range of 275,245,nm, due to chiral exciton coupling of the benzo[a]heptalene chromophore and the phenylsulfonyl chromophore (cf. Fig.,10). [source] Kinetic evidence for the occurrence of kinetically detectable intermediates in the cleavage of N -ethoxycarbonylphthalimide under N -methylhydroxylamine buffersINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 2 2002M. Niyaz Khan The kinetics of the aqueous cleavage of N -ethoxycarbonylphthalimide (NCPH) in CH3NHOH buffers of different pH reveals that the cleavage follows the general irreversible consecutive reaction path NCPH ENMBC AB, where ENMBC, A, and B represent ethyl N -[o -(N -methyl- N -hydroxycarbamoyl)benzoyl]carbamate, N -hydroxyl group cyclized product of ENMBC, and o -(N -methyl- N -hydroxycarbamoyl)benzoic acid, respectively. The rate constant k1 obs at a constant pH, obeys the relationship k1 obs = kw + knapp [Am]T + kb[Am]T2, where [Am]T is the total concentration of CH3NHOH buffer and kw is first-order rate constant for pH-independent hydrolysis of NCPH. Buffer-dependent rate constant kb shows the presence of both general base and general acid catalysis. Both the rate constants k2 obs and k3 obs are independent of [Am]T (within the [Am]T range of present study) at a constant pH and increase linearly with the increase in aOH with definite intercepts. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 34: 95,103, 2002 [source] Quantum chemical study of leaving group activation in T. vivax nucleoside hydrolaseINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 3 2006Stefan Loverix Abstract General acid catalysis is a powerful and widely used strategy in enzymatic nucleophilic displacement reactions. However, in the nucleoside hydrolase of the parasite Trypanosoma vivax, crystallographic and mutagenesis studies failed to identify a general acid. The only groups in the vicinity of the leaving group that contribute to catalysis are (i) the indole side chain of Trp260, and (ii) the 5,-group of the substrate's ribose moiety. The x-ray structure of the slow Asp10Ala mutant of nucleoside hydrolase with the substrate inosine bound in the active site displays a face-to-face aromatic stacking interaction between Trp260 and the purine base of the substrate, as well as a peculiar C4,-endo ribose pucker that allows the 5,-OH group to accept an intramolecular hydrogen bond from the C8 of the purine. The first interaction (aromatic stacking) has been shown to raise the pKa of the leaving purine. Here, we present a DFT study showing that the 5,-OH group of ribose fulfills a similar role, rather than stabilizing the oxocarbenium-like transition state. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006 [source] Cover Picture: (Adv. Synth.ADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 1 2008Catal. The cover picture, provided by Sh,, Kobayashi, shows an example of chiral Lewis acid catalysis based on Lewis acid,surfactant combined catalysts (LASCs). Chiral hydrophobic environments made up of scandium(III) dodecylsulfate and a chiral bipyridine ligand catalyze the asymmetric aldol-type reactions of silyl enol ethers in water as the sole solvent. [source] First-principle studies of intermolecular and intramolecular catalysis of protonated cocaineJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 10 2005Chang-Guo Zhan Abstract We have performed a series of first-principles electronic structure calculations to examine the reaction pathways and the corresponding free energy barriers for the ester hydrolysis of protonated cocaine in its chair and boat conformations. The calculated free energy barriers for the benzoyl ester hydrolysis of protonated chair cocaine are close to the corresponding barriers calculated for the benzoyl ester hydrolysis of neutral cocaine. However, the free energy barrier calculated for the methyl ester hydrolysis of protonated cocaine in its chair conformation is significantly lower than for the methyl ester hydrolysis of neutral cocaine and for the dominant pathway of the benzoyl ester hydrolysis of protonated cocaine. The significant decrease of the free energy barrier, ,4 kcal/mol, is attributed to the intramolecular acid catalysis of the methyl ester hydrolysis of protonated cocaine, because the transition state structure is stabilized by the strong hydrogen bond between the carbonyl oxygen of the methyl ester moiety and the protonated tropane N. The relative magnitudes of the free energy barriers calculated for different pathways of the ester hydrolysis of protonated chair cocaine are consistent with the experimental kinetic data for cocaine hydrolysis under physiologic conditions. Similar intramolecular acid catalysis also occurs for the benzoyl ester hydrolysis of (protonated) boat cocaine in the physiologic condition, although the contribution of the intramolecular hydrogen bonding to transition state stabilization is negligible. Nonetheless, the predictability of the intramolecular hydrogen bonding could be useful in generating antibody-based catalysts that recruit cocaine to the boat conformation and an analog that elicited antibodies to approximate the protonated tropane N and the benzoyl O more closely than the natural boat conformer might increase the contribution from hydrogen bonding. Such a stable analog of the transition state for intramolecular catalysis of cocaine benzoyl-ester hydrolysis was synthesized and used to successfully elicit a number of anticocaine catalytic antibodies. © 2005 Wiley Periodicals, Inc. J Comput Chem 26: 980,986, 2005 [source] Hydrolysis of 2-(p -nitrophenoxy)tetrahydropyran: solvent and ,-deuterium secondary kinetic isotope effects and relationships with the solvolysis of simple secondary alkyl arenesulfonates and the enzyme-catalyzed hydrolysis of glycosides,JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 6-7 2004Imran A. Ahmad Abstract The effect of solvent composition in aqueous ethanol, trifluoroethanol and hexafluoropropan-2-ol on the rate constant and activation parameters for the uncatalysed hydrolysis of 2-(p -nitrophenoxy)tetrahydropyran (1) was investigated, and the m(YOTs) value is 0.60. This appreciable but less than maximal value is in accordance with an SN1 mechanism with rate-limiting ionization. The ,-deuterium secondary kinetic isotope effect (,-kie) for the uncatalysed hydrolysis of 1 is 1.17 in water (46°C), 1.15 in aqueous trifluoroethanol (50% mole fraction, 70.6°C) and 1.13 in aqueous ethanol (50% mole fraction, 70.6°C). These values correspond to about 1.19 at 25°C, which is characteristic of rate-limiting ionization in an SN1 reaction and appreciably higher than values for enzyme-catalysed glycolysis. The ,-kie is smaller under aqueous acidic conditions (1.07, 0.1,mol,dm,3 hydrochloric acid, 20.2°C) when 1 hydrolyses with acid catalysis. The previously reported ,-kie for the hydrolysis of 1 in buffered aqueous dioxan (1.063, 25°C) is now seen to correspond to acid-catalysed hydrolysis. These new results for 1 indicate that transition structures in enzyme-catalysed glycolyses with ,-kie values of less than about 1.15 at 25°C involve a lower degree of carbenium ion character than has hitherto been assumed. Copyright © 2004 John Wiley & Sons, Ltd. [source] Specific acid catalysis and Lewis acid catalysis of Diels,Alder reactions in aqueous mediaJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 3 2004Egid B. Mubofu Abstract A comparative study of specific acid catalysis and Lewis acid catalysis of Diels,Alder reactions between dienophiles (1, 4 and 6) and cyclopentadiene (2) in water and mixed aqueous media is reported. The reactions were performed in water with copper(II) nitrate as the Lewis acid catalyst whereas hydrochloric acid was employed for specific acid catalysis. At equimolar amounts of copper(II) nitrate and hydrochloric acid (0.01,M, for example) and under the same reaction conditions, the reaction rate for 1a with 2 is about 40 times faster with copper catalysis than with specific acid catalysis. Moreover, at 32°C and 0.01,M HCl, the reaction of 1b with 2 is about 21 times faster than the same uncatalyzed reaction in pure water under the same reaction conditions. The inverse solvent kinetic isotope effect shows that these Diels,Alder reactions undergo specific acid catalysis. Copyright © 2004 John Wiley & Sons, Ltd. [source] Characterization of ring-opening polymerization of genipin and pH-dependent cross-linking reactions between chitosan and genipinJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 10 2005Fwu-Long Mi Abstract In this study, a novel chitosan-based polymeric network was synthesized by crosslinking with a naturally occurring crosslinking agent,genipin. The results showed that the crosslinking reactions were pH-dependent. Under basic conditions, genipin underwent a ring-opening polymerization prior to crosslinking with chitosan. The crosslink bridges consisted of polymerized genipin macromers or oligomers (7 , 88 monomer units). This ring-opening polymerization of genipin was initiated by extracting proton from the hydroxyl groups at C-1 of deoxyloganin aglycone, followed by opening the dihydropyran ring to conduct an aldol condensation. At neutral and acidic conditions, genipin reacted with primary amino groups on chitosan to form heterocyclic amines. The heterocyclic amines were further associated to form crosslinked networks with short chains of dimmer, trimer, and tetramer bridges. An accompanied reaction of nucleophilic substitution of the ester group on genipin by the primary amine group on chitosan would occur in the presence of an acid catalysis. The extent in which chitosan gels crosslinked with genipin was significantly dependent on the crosslinking pH values: 39.9 ± 3.8% at pH 5.0, 96.0 ± 1.9% at pH 7.4, 45.4 ± 1.8% at pH 9.0, and 1.4 ± 1.0% at pH 13.6 (n = 5, p < 0.05). Owing to the different crosslinking extents and different chain lengths of crosslink bridges, the genipin-crosslinked chitosan gels showed significant difference in their swelling capability and their resistance against enzymatic hydrolysis, depending on the pH conditions for crosslinking. These results indicated a direct relationship between the mode of crosslinking reaction, and the swelling and enzymatic hydrolysis properties of the genipin-crosslinked chitosan gels. The ring-opening polymerization of genipin and the pH-dependent crosslinking reactions may provide a novel way for the preparation and exploitation of chitosan-based gels for biomedical applications. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1985,2000, 2005 [source] Organic and polymer chemistry of electrophilic tri- and tetrasubstituted ethylenesJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 12 2004H. K. Hall Jr. Abstract A survey of the spontaneous reactions of electrophilic olefins and nucleophilic olefins is presented as an area in which organic chemistry merges with polymer chemistry. The products include both small molecules and polymers, arising via tetramethylene biradical zwitterions that can cyclize or initiate polymerizations. Electrophilic tri- and tetrasubstituted olefins are particularly useful in delineating the transition from radical chemistry to ionic chemistry. A periodic table embodying these results enables predictions. Charge-transfer complexes, although observed in many of these reactions, play no significant role. Various aspects arising from these investigations include new cationic initiators, Lewis acid catalysis, quinodimethane chemistry, and photochemistry. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2845,2858, 2004 [source] Organocatalysis Mediated by (Thio)urea DerivativesCHEMISTRY - A EUROPEAN JOURNAL, Issue 21 2006Stephen J. Connon Dr. Abstract Over the last decade the potential for N,N -dialkyl(thio)urea derivatives to serve as active metal-free organocatalysts for a wide range of synthetically useful reactions susceptible to the influence of general acid catalysis has begun to be realised. This article charts the development of these catalysts (with emphasis on the design principles involved), from early "proof-of-concept" materials to contemporary active chiral (bifunctional) promoters of highly selective asymmetric transformations. [source] | |||||||||||||||||||||||||