This Review aims at a comprehensive overview on the recent stereospecific control polymerizations by group 3 metal complexes based catalytic systems in combination with aluminum alkyls and ...organoborates etc. co‐catalysts. The involved monomers include styrene and substituted styrenes, and the conjugated dienes like isoprene and butadiene, and their derivatives. The syndio‐ and iso‐selective polymerizations of styrene, unmasked polar styrenes and the copolymerizations of styrene with conjugated dienes and polar styrenes are summarized. The factors of influencing the catalytic activity and selectivity are discussed. The regio‐ and stereo‐ selective polymerizations of conjugated dienes, polar diene derivatives and their copolymerizations are reviewed. The relationship between catalyst structure and polymerization activity, the mechanism of stereo‐selective polymerization and the influence of different cocatalysts are discussed.
A solo performance: In this Review, a variety of group 3 single‐site catalysts for the precise control of regio‐ and stereo‐ selective (co)polymerizations of styrene and conjugated dienes are systematically introduced. The activities, selectivities, molecular weights and molecular weight distributions dependent on the structure of the catalysts, cocatalysts, and reaction conditions etc. are detailed discussed.
Mononuclear amidinate yttrium complex C4H9C(NR)2Y(o‐CH2C6H4NMe2)2 (R=2,6‐iPr2C6H3) and a series of binuclear rare‐earth metal complexes bearing a bridged amidinate ligand ...(RN)2C(CH2)4C(NR)2RE{CH2C6H4(o‐NMe2)}22 (R=2,6‐iPr2C6H3, RE=Y, Lu, Sc) were synthesized and fully characterized. The catalytic behavior of these complexes for (co)polymerization of conjugated dienes such as isoprene and myrcene in the presence of co‐catalyst Ph3CB(C6F5)4 was investigated. These catalytic systems show impressively high activity and 3,4‐regioselectivity in living (co)polymerization. The binuclear bridged amidinate yttrium catalytic system not only exhibits the highest activity among the reported catalytic systems for 3,4‐polymerization of isoprene but also allows the steady polymerization in a living manner from −20 to 80 °C. Compared with the dramatic drop of 3,4‐selectivity for the mononuclear analogue, the binuclear catalytic system still shows moderate 3,4‐selectivity at 80 °C. Moreover, a facile one‐pot synthetic strategy for a polymer blend containing 3,4‐ and 1,4‐polyisoprene (PIP) was established through the in situ modification of the active amidinate yttrium species by addition of an excess amount of AlMe3.
Amidinate complexes, a mononuclear amidinate yttrium complex and a series of binuclear rare‐earth metal complexes, have been synthesized. They are excellent catalyst precursors for living (co)polymerization of conjugated dienes such as isoprene and myrcene. In contrast to the mononuclear analogue, a synergistic effect of the binuclear complex has been observed. Moreover, a polymer blend of 3,4‐ and 1,4‐polyisoprene (PIP) was obtained through one‐pot synthesis.
The asymmetric 1,2-Markovnikov hydrosilylation of conjugated dienes with primary silanes catalyzed by a quinoline-oxazoline cobalt complex has been described. This protocol provides an efficient ...approach to chiral allyl dihydrosilanes with high regioselectivity and enantioselectivity (up to 96% ee). The catalyst system is effective for a wide array of conjugated dienes, including mono- and 1,2-disubstituted dienes with aryl and/or alkyl substituents. Further, the products are applied to the synthesis of polyorganosiloxanes (organo-silicone copolymers) containing side chains of enantioenriched allylic functionalities using a pyridine-oxazoline cobalt-catalyzed step-growth polymerization with terephthalaldehyde. The result of a deuterium-labeling experiment involving the reaction of PhSiD3 and 1,3-pentadiene suggests that the hydrosilylation most likely proceeds through a modified Chalk–Harrod mechanism involving the 1,2-insertion of the terminal double bond of the diene into the Co–Si bond.
Herein, a concise, effective, and scalable strategy is reported that the introduction of polar molecules (PMs) (e.g., anisole (PhOMe), phenetole (PhOEt), 2‐methoxynaphthalene (NaphOMe), thioanisole ...(PhSMe), and N,N‐dimethylaniline (PhNMe2)) as continuously coordinated neutral ligand of cationic active species in situ generated from the constrain‐geometry‐configuration–type rare‐earth metal complexes A–F/AliBu3/Ph3CB(C6F5)4 ternary systems can easily switch the regio‐ and stereoselectivity of the polymerization of conjugated dienes (CDs, including 2‐subsituted CDs such as isoprene (IP) and myrcene (MY), 1,2‐disubstituted CD ocimene (OC), and 1‐substituted polar CD 1‐(para‐methoxyphenyl)‐1,3‐butadiene (p‐MOPB)) from poor selectivities to high selectivities (for IP and MY: 3,4‐selectivity up to 99%; for OC: trans‐1,2‐selectivity up to 93% (mm up to 90%); for p‐MOPB: 3,4‐syndioselectivity (3,4‐ up to 99%, rrrr up to 96%)). DFT calculations explain the continuous coordination roles of PMs on the regulation of the regio‐ and stereoselectivity of the polymerization of CDs. In comparison with the traditional strategies, this strategy by adding some common PMs is easier and more convenient, decreasing the synthetic cost and complex operation of new metal catalyst and cocatalyst. Such regio‐ and stereoselective regulation method by using PMs is not reported for the coordination polymerization of olefins catalyzed by rare‐earth metal and early transition metal complexes.
The introduction of polar molecules (PMs) (e.g., PhOMe, PhOEt, NaphOMe, PhSMe, and PhNMe2) to the CGC‐type rare‐earth metal complexes A–F/AliBu3/Ph3CB(C6F5)4 ternary catalytic systems can easily switch the regio‐ and stereoselectivity of the polymerization of conjugated dienes (CDs) (e.g., IP, MY, OC, and p‐MOPB) from poor selectivities to high selectivities.
The use of antioxidants in biodiesels, especially the synthetic TBHQ, has been an efficient way of maintaining the oxidative stability of biofuel during storage for a longer period. Considering other ...types of products, the use of several natural antioxidants, such as Eugenol, has been promising to increase oxidation stability and conservation of foods, with few studies on biodiesel conservation. This study aimed to evaluate the antioxidant action of Eugenol and TBHQ in commercial biodiesel produced from soybean oil and animal fat blend in order to evaluate the efficiency of these antioxidants. Thus, the samples of pure commercial biodiesel (without antioxidant), enriched with the antioxidant TBHQ and Eugenol were prepared and stored in an oven at constant temperature of 43 °C for 150 days. Periodically, the samples were characterized by ultraviolet absorption spectroscopy and oxidation stability (Rancimat method). They were also chemically characterized concerning the acidity, peroxide and iodine amounts. The results showed higher efficiency of the antioxidant Eugenol in relation to the TBHQ, considering that the oxidation stability was within the standard legislations.
The development of stereoselective and efficient reactions for the construction of conjugated dienes and polyenes has remained at the forefront of organic chemistry, due to their key roles in ...medicinal chemistry, organic synthesis, and material science. The synthesis of conjugated dienes and polyenes is typically accomplished in a multistep manner by the sequential installation of individual CC bonds because it allows for control of stereoselectivity and efficiency of formation of each double bond. A conceptually distinct dienylation approach entails a stereoselective appendage of a four-carbon unit, shortcutting diene synthesis. Dienylation with sulfolene provided a direct route to E-dienes, but the synthesis of substantially more challenging Z-dienes remained elusive. Here, we report that a highly Z-selective dienylation can now be achieved by a simple adjustment of a ligand, enabling stereodivergent synthesis of E- and Z-dienes from one reagent and in one step. A detailed mechanistic investigation of the E- and Z-selective dienylation provided insight into the divergent behavior of the two catalytic systems and revealed that differences in relative stabilities of catalytically active palladium phosphine complexes have a major impact on the stereochemical outcomes of the dienylation.
•Simultaneous monitoring of lipid hydroperoxides (LOOH) and conjugated dienes (CD).•Clarifying the mechanism of CD generation over lipid peroxidation.•Kinetically differentiation between the values ...of LOOH and CD.
The concentration of total lipid hydroperoxides (LOOH) and total conjugated dienes (CD) were simultaneously monitored during peroxidation of canola, sunflower, and olive oils at 90 °C. Significantly different kinetic parameters differentiating the two analytical variables as a function of the fatty acid compositions as well as the constituting minor components of the oils were obtained. This was attributed to the fact that LOOH represent essentially the fundamental primary oxidation products, including non-conjugated (nCDOOH) and conjugated (CDOOH) diene hydroperoxides, whereas CD stand actually for CDOOH and conjugated diene non-hydroperoxides (CDnOOH) as the primary and secondary, respectively, oxidation products. The simultaneous study on the kinetics of change in LOOH and CD provided invaluable insights into lipid peroxidation with respect to the different species of conjugated/non-conjugated diene hydroperoxides/non-hydroperoxides.
Stereoconvergent transformation of E/Z mixtures of olefins to products with a single steric configuration is of great practical importance but hard to achieve. Herein, we report an iron‐catalyzed ...stereoconvergent 1,4‐hydrosilylation reactions of E/Z mixtures of readily available conjugated dienes for the synthesis of Z‐allylsilanes with high regioselectivity and exclusive stereoselectivity. Mechanistic studies suggest that the reactions most likely proceed through a two‐electron redox mechanism. The stereoselectivity of the reactions is ultimately determined by the crowded reaction cavity of the α‐diimine ligand‐modified iron catalyst, which forces the conjugated diene to coordinate with the iron center in a cis conformation, which in turn results in generation of an anti‐π‐allyl iron intermediate. The mechanism of this stereoconvergent transformation differs from previously reported mechanisms of other related reactions involving radicals or metal‐hydride species.
An iron‐catalyzed stereoconvergent 1,4‐hydrosilylation reaction enables the highly effective synthesis of Z‐allylsilanes from easily available E/Z mixtures of conjugated dienes.
Unravelling the relationship between food and health requires a more in-depth knowledge of the various changes occurring in the gastrointestinal tract during digestion and which may ultimately affect ...the nutritional quality and safety of ingested food lipids before absorption into the bloodstream. In this context, this review deals with the oxidation process of food lipids under digestive conditions and the studies carried out on this topic using different digestion models: in vitro, in vivo or ex vivo, static or dynamic, and including one, two and/or three digestive phases (oral, gastric and duodenal). These studies have contributed to clarifying the occurrence and extent of lipid degradation under such a particular environment, many of them also highlighting the factors affecting the advance or delay of the oxidation of dietary lipids during digestion, like: food lipid content, unsaturation degree and initial oxidative status; the presence in the food bolus of compounds showing antioxidant activity (polyphenols, tocopherols...) either added or naturally present; the presence in the food bolus of proteins (including iron or not); food technological or culinary processings (salting, smoking, cooking...), among others. Likewise, the methodologies employed to study lipid oxidation under digestive conditions are also summarized and future research perspectives are discussed.
Coordination polymerization of 1-phenyl-1,3-butadiene (1PB) has been carried out using diiminophosphinato lutetium complexes 1 and 2, pyridyl-methylene-fluorenyl complexes 3 (Sc 3a; Lu 3b; Y 3c) and ...4 (Sc 4a; Lu 4b; Y 4c), β-diketiminato yttrium complex 5, bis(imino)aryl gadolinium complex 6, bis(phosphino)carbazolide yttrium complex 7, and bis(phosphinophenyl)amido yttrium complex 8. Complexes 1, 2, 3a,b, and 4a,b, under the activation of Ph3CB(C6F5)4 and Al i Bu3, show high activities as well as higher than 96.6% 3,4-regioselectivities, of which 1 and 2 are nonstereoselective, giving atactic 3,4-poly(1PB), while 3a,b and 4a,b exhibit distinguished syndioselectivities (≥95.3% rrrr). However, 3c and 4c are inactive. In addition, complexes 5–7 are also completely inert although they are highly active and cis-1,4-selective for isoprene polymerization. On the contrary, the analogous cis-1,4-selective complex 8 exhibits syndio- (≥96.1% rrrr) and 3,4-regio- (>99%) selectivity. Hydrogenation of a syndiotactic 3,4-poly(1PB) affords a syndiotactic poly(4-phenyl-1-butene) elastomer with a T g of 17 °C. The cationic cyclization transfers a syndiotactic 3,4-poly(1PB) into a rigid cyclized polyolefin with a T g as high as 327 °C. Density functional theory (DFT) simulations are employed to illustrate the mechanisms for the control of activity and stereoselectivity from thermodynamic and geometric viewpoints.
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