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Allylic Positions (allylic + position)
Selected AbstractsDirect Substitution of the Hydroxy Group at the Allylic/propargylic Position with Carbon- and Heteroatom- centered Nucleophiles Catalyzed by Yb(OTf)3CHINESE JOURNAL OF CHEMISTRY, Issue 4 2008Wen HUANG Abstract An efficient and highly selective Yb(OTf)3 -catalyzed direct substitution of the hydroxy group at the allylic and propargylic positions with a variety of heteroatom- and carbon-centered nucleophiles, such as alcohols, thiols, amines, amides and active methylene compounds has been developed. The advantages of the present catalytic system are wide availability of the starting materials, especially for tolerance to thiols, no need for dried solvents and additives, mild conditions, short time of reaction, simple manipulation and environmentally friendly catalyst that can be recovered and reused at least ten times without significant reduction of activity. [source] Kinetics and mechanism of the reaction between maleic anhydride and fatty acid esters and the structure of the productsEUROPEAN JOURNAL OF LIPID SCIENCE AND TECHNOLOGY, Issue 5 2008Florina Stefanoiu Abstract Alkenyl succinic anhydrides (ASA) were obtained by reaction between maleic anhydride and high-oleic sunflower oil (HOSO) esters. A kinetics study of the maleinization of alkyl esters indicated that the maleinization reaction was second order overall and first order with respect to the individual reactants, and the activation energy was 77.2,±,3.3,kJ/mol in the investigated temperature range (185,225,°C). These results showed that the cis configuration and the central position of the double bond in HOSO esters facilitate the maleinization of the latter. On the contrary, the length of the linear ester moiety had no influence on the course of the maleinization reaction. Moreover, new evidence demonstrates that there are two different reaction mechanisms: ene-reaction and addition in allylic position with a 2,:,1 ratio, respectively. This ratio was constant throughout the reaction, thus indicating that these mechanisms are independent. [source] Effect of the Leaving Group on the Rate and Mechanism of the Palladium-Catalyzed Isomerization of Cyclic Allylic Benzoates in Allylic SubstitutionsEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 5 2006Christian Amatore Abstract In chloroform, the reaction of cis -5-phenylcyclohex-2-enyl 4-Z-benzoate (cis - 1Z, Z = NO2, Cl, H, Me, MeO) with Pd0 complexes ligated to PPh3 is reversible and proceeds with isomerization at the allylic position. The rate of isomerization of cis - 1Z to trans - 1Z depends on the catalytic precursor: Pd0(PPh3)4 > {Pd0(dba)2 + 2PPh3} in agreement with an SN2 mechanism in the rate-determining isomerization of the cationic (,3 -allyl)palladium complexes formed in the oxidative addition. For a given precursor, the rate of isomerization of cis - 1Z to trans - 1Z also depends on the substituent Z, i.e., on the leaving group. The isomerization rate follows the same order as the leaving group properties: 4-NO2,C6H4 -CO2,> 4-Cl,C6H4 -CO2, > C6H5 -CO2, > 4-Me,C6H4 -CO2, > 4-MeO,C6H4 -CO2,. The same tendency is found for the equilibrium constant between the neutral cis - 1Z and the cationic (,3 -allyl)palladium complex in DMF. The higher the concentration of the cationic (,3 -allyl)palladium complex, the faster the isomerization of cis - 1Z to trans - 1Z is. The isomerization of cis - 1Z to trans - 1Z and that of the cationic (,3 -allyl)palladium complexes are at the origin of the lack of stereospecificity observed in catalytic nucleophilic allylic substitutions. These isomerizations are affected by both the leaving groups and the Pd0 precursors, which therefore are not "innocent" but may play an important role in palladium-catalyzed nucleophilic substitutions. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] Regioselective Reactions on a Chiral Substrate Controlled by the Configuration of a Chiral CatalystADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 2-3 2010Raju Ranjith Kumar Abstract A racemic mixture may be partially transformed in the presence of a chiral catalyst by kinetic resolution and formation of products with new structural features. If the starting material is fully consumed the products may still be enantiomerically enriched. The situation is summarized in the Introduction. A brief discussion on the regioselective transformations occurring on a racemic mixture under the influence of a chiral catalyst is presented in Section 2. Often stereo-differences occur, each enantiomer of the starting material resulting in a different product. It allows one to predict what the behaviour of some enantiopure substrates should be in presence of each of the enantiomers of a chiral catalyst. Many examples are presented in Section 3. The chiral substrates under consideration have two different reacting sites, usually of the same nature (OH, CC, allylic positions, CH for carbene insertion, epoxide fragment, etc.). In some cases the absolute configuration of the catalyst allows an excellent control of the regioselectivity. This approach is promising for the selective transformation of chiral molecules. [source] Polymerization of linseed oil with phenolic resinsJOURNAL OF APPLIED POLYMER SCIENCE, Issue 2 2010Gökhan Çayl Abstract In this study, linseed oil was directly polymerized with different oil soluble resoles. p- Ethyl (PEP), p-tertiary butyl (PTB), p-tertiary octyl (PTO), and p- phenyl (PPP) phenols were separately reacted with formaldehyde to give linseed oil soluble resoles. These were then reacted with linseed oil to give transparent rubbery polymers. A model reaction was performed with methyl oleate and PTB phenol resole to clarify the reaction mechanism. Reaction products were characterized with 1H-NMR and IR techniques. Spectral examination of the model reaction showed that polymerization reaction proceeded via ene reaction of the quinone methide formed at the end group of the resole with the allylic positions of the fatty ester. Rubbery polymers were obtained with linseed oil using 10 to 40% of the different resoles. Hard, load bearing and tough materials were obtained at 40% phenolic resin loading. Mechanical properties of the materials were characterized by dynamic mechanical analyzer (DMA) and stress,strain tests. The best mechanical and thermal properties were obtained with PEP resole which showed a storage modulus of 350 MPa and a tan , peak at 65°C. Storage moduli of 275, 250, and 30 were obtained for PPP, PTB, and PTO resoles-linseed oil polymers, respectively. At the same phenolic resin loading, elongation at break and swelling ratios in CH2Cl2 increased with the longer alkyl substitution on the resole resins. The highest thermal stability was observed by PEP resole,linseed oil polymer which has a 5% weight loss temperature of 340°C as determined by TGA. The lowest thermal stability was observed for PTB resole-linseed oil polymer at 220°C. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010 [source] Biobased polyisocyanates from plant oil triglycerides: Synthesis, polymerization, and characterizationJOURNAL OF APPLIED POLYMER SCIENCE, Issue 5 2008Gökhan Çayl Abstract In this study, an easy and efficient synthesis of unsaturated plant oil triglycerides having isocyanate groups is reported. In the first step of the synthesis, the triglyceride was brominated at the allylic positions by a reaction with N -bromosuccinimide, and in the second step, these brominated species were reacted with AgNCO to convert them to isocyanate-containing triglycerides. At the end of the reaction, approximately 60,70% of the bromine was replaced by NCO groups, and the double bonds of the triglyceride were not consumed. When the amount of AgNCO was increased, the yield also increased. The final products were characterized with IR and 1H-NMR, and polyurethanes and polyureas were obtained from these fatty isocyanates with alcohols and amines, respectively. The polymers were characterized by differential scanning calorimetry and thermogravimetric analysis. Differential scanning calorimetry curves showed that glycerin polyurethane showed a glass-transition temperature at 19°C, castor oil polyurethane showed two glass-transition temperatures at ,43 and 36°C, and triethylene tetraamine polyurea showed a glass-transition temperature at 31°C. Some properties of the polymers, such as the tensile strength and swelling ratios, were also determined. The swelling rate of glycerin polyurethane was higher than that of castor oil polyurethane in dichloromethane. The equilibrium swelling ratio was highest for the castor oil polyurethane. The polyurethanes synthesized in this study had a Young's modulus around 50 kPa and a tensile strength around 0.01 N/mm2 (100 kPa). The tensile strength of glycerin polyurethane was higher than that of castor oil polyurethane. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008 [source] | ||||||||||