Lewis base-bridged intermetallic compounds comprising the reaction product of a transition metal alkoxide with a reducing metal having a higher oxidation potential than the transition metal in the presence of a Lewis base and said reaction product being associated by coordinate covalent bridges formed from the Lewis base and composed of donor atoms selected from the group consisting of oxygen, nitrogen, sulfur and phosphorous; and catalyst components and systems for the polymerization of alpha olefins.
A Lewis acid is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any species that has a filled orbital containing an electron pair which is not involved in bonding but may form a dative bond with a Lewis acid to form a Lewis adduct. For example, NH3 is a Lewis base, because it can donate its lone pair of electrons. Trimethylborane (Me3B) is a Lewis acid as it is capable of accepting a lone pair. In a Lewis adduct, the Lewis acid and base share an electron pair furnished by the Lewis base, forming a dative bond.[1] In the context of a specific chemical reaction between NH3 and Me3B, the lone pair from NH3 will form a dative bond with the empty orbital of Me3B to form an adduct NH3•BMe3. The terminology refers to the contributions of Gilbert N. Lewis.[2]. The terms nucleophile and electrophile are more or less interchangeable with Lewis base and Lewis acid, respectively. However, ...
A Lewis base catalyzed ring expansion of isatin with 2, 2, 2-trifluorodiazoethane (CF3CHN2) is developed. It is characterized that the merge of tetramethylethylenediamine and CF3CHN2 generates reactive triazene intermediates, which construct substituted 3-hydroxy-4-(trifluoromethyl)quinolinones with high efficiency. Synthetic application of the procedure is broadened by 3-trifluormethylpyrazole fused 3-hydroxy-4-(trifluoromethyl)quinolinone synthesis ...
A nitrogen-containing Lewis base mediated regioselective [4 + 2] cyclization strategy between allenic esters and α,β-unsaturated ketimines was discovered, affording a series of hydropyridine derivatives in moderate to good yields under mild conditions.
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A complex is a species in which the central atom is surrounded by a group of Lewis bases that have covalent bonds to the central atom. The Lewis bases that surround the central atom are generally referred to as ligands. Complexes are so named because when they were first studied, they seemed unusual and difficult to understand. Primarily, transition metals form complexes and their most observable property is their vivid color. The color of transition metal complexes is dependent on the identity and oxidation state of the central atom and the identity of the ligand.. Acids and bases were originally defined by Arrhenius as substances that donated protons (positively charged hydrogen ions) and hydroxide ions (consisting of one hydrogen atom bonded to one oxygen atom and having an overall charge of minus one) respectively. Subsequently, Bronsted and Lowry redefined acids and bases as being proton donors and proton acceptors, respectively. This broadened definition made it possible to include ...
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33] Mashima K., Kusano K.-h., Sato N., Matsumura Y.-i., NozakiK., Kumobayashi H., Sayo N., Hori Y., Ishizaki T., AkutagawaS., Takaya H., Cationic BINAP-Ru(II) Halide Complexes:Highly Efficient Catalysts for Stereoselective AsymmetricHydrogenation of alpha- and beta-Functionalized Ketones, J.Org. Chem., 1994, 59, 3064-3076 ...
Commonly used triethyl aluminum and tri-isobutyl aluminum cocatalysts may be replaced in olefin polymerizations with a cocatalyst conforming to the formula AlR z (X z ) n L z m wherein R z may be a linear or branched organic moiety having at least 5 carbons and X z may be a linear or branched organic moiety having at least 5 carbons or heteroatom substituted organic moiety or a heterocyclic moiety having at least 4 atoms and may be anionic (n=2) or dianionic (n=1). One X z may also be hydrogen. The aluminum complex may also be in the form of an adduct complex where L z may be a Lewis base and m=1−3. In the absence of a Lewis base, m=0.
A component of a polymerization catalyst is prepared by reacting an organo-magnesium compound, or a complex with an organo-aluminium compound, with an halogenating agent such as silicon tetrachloride or hydrogen chloride, and then reacting the product with a Lewis Base compound, particularly an ester and finally with titanium tetrachloride. The product obtained is combined with an organo-aluminium compound preferably together with a Lewis Base and used to polymerize an olefine monomer. The catalyst has a high activity and is stereospecific.
The catalyst is effective with a wide variety of TMS enolates and ketones, generally providing the tertiary aldol adducts in good to excellent yields. Two notable exceptions were when the phenyl ester enolate was used, the β-lactone was produced in ,99% yield (entry 7), and when acetophenone was used, the aldol product was generated in 57% yield (entry 9). The low yield in the case of acetophenone is likely due to the interaction of the basic catalyst with the enolizable alpha proton on the substrate.. Surprisingly, the catalyst was also effective in Mannich-type reactions when protected aldimines were used as substrate (Scheme 2). No apparent deactivation was observed, despite the increased basicity of the products, and yields were generally good to excellent in all cases. Deprotection of the N-Boc product in entry 2 generated the primary amine in quantitative yield.. ...
Amines and boranes are the archetypical Lewis bases and acids, respectively. The former can readily undergo one-electron oxidation to give radical cations, whereas the latter are easily reduced to afford radical anions. Here, we report the synthesis of a neutral tricoordinate boron derivative, which acts as a Lewis base and undergoes one-electron oxidation into the corresponding radical cation. These compounds can be regarded as the parent borylene (H-B:) and borinylium (H-B+.), respectively, stabilized by two cyclic (alkyl)(amino)carbenes. Ab initio calculations show that the highest occupied molecular orbital of the borane as well as the singly occupied molecular orbital of the radical cation are essentially a pair and a single electron, respectively, in the p(π) orbital of boron. ...
Answer: This is due to different electronegativities of H and F bonded to nitrogen. In (CH3)3 N, the lone pair on N is concentrated on N and so it can act as an electron pair donor (Lewis base). However in (CF3)2 N, the electron density on N is decreased due to strongly electronegative F atom attached to C. Hence, it cannot act as a Lewis base ...
Abstract. β-Functionalized sulfonamides were produced in good yields by the regioselective ring opening of N-tosylaziridines with trimethylsilylated nucleophiles, catalyzed by N,N,N,N-tetramethylethylenediamine (TMEDA).. ...
Halogen bonding has emerged at the forefront of advances in improving ligand: receptor interactions. In particular the newfound ability of this extant non-covalent-bonding phenomena has revolutionized computational approaches to drug discovery while simultaneously reenergizing synthetic approaches to the field. Here we survey, via examples of classical applications involving halogen atoms in pharmaceutical compounds and their biological hosts, the unique advantages that halogen atoms offer as both Lewis acids and Lewis bases.
A brief technical meaning of acidity (from the Latin acidus which means sour) is often described as the number of H+ ions that are neutralized by a base. According to a Swedish chemist, Svante Arrhenius, an acid is a substance that releases hydrogen ions (H+) in water, while a base (or alkali) is a substance that gives up hydroxide ions (OH-) when dissolved in water. You will notice from the above definition that it limits acids and bases to substances that can be dissolved in water. Subsequently, acid was defined by Bronsted and Lowry as a proton donor and base to be a proton acceptor. This definition envisaged that even substances that are insoluble in water can be acids and bases. Then there is another definition of acids that was given by Lewis, which is of a generic nature and defines a Lewis acid as an electron acceptor, and a Lewis base as an electron donor. The behaviour of acid-base systems is unlike redox reactions in that there is no change in the oxidation state or number. Acidity ...
On the basis of NMR studies, the authors believe that 9b acts as a bifunctional chiral ligand where the phosphine atom acts as a Lewis base and the phenolic OH acts as a Lewis acid. The key factor is the intramolecular hydrogen bonding between the phenolic OH and the oxygen atom of carbonyl group to stabilize the key enolate intermediate 18 formed in situ. In addition, the intramolecular hydrogen bonding between the phenolic OH and the nitrogen anion stabilized by the sulphonyl group can facilitate a relatively stable or rigid transition state for achieving high enantioselectivity in the aza-BH reaction (Scheme 6). ...
A transition metal compound containing at least one π-bonded arene is contacted with an inorganic oxide, hydroxide, oxyhalide, hydroxyhalide or halide. The transition metal compound can be a compound such as titanium(O)ditoluene or TiCl2.2AlCl3 durene. The inorganic compound is preferably of high surface area and this may be inherent in the compound or can be achieved by a grinding procedure. The inorganic compound can be treated with a halogen containing compound either before, during or after the contacting with the transition metal compound. The product of the contacting is useful either as catalyst for the polymerization of unsaturated hydrocarbon monomers or as a component of such a catalyst. When used as a component of an olefine polymerization catalyst, the transition metal product can be mixed with an organo-aluminium compound and a Lewis Base compound such as an ester. The catalysts can be used to polymerize or copolymerize olefine monomers to obtain a high yield of a polymer having good
Alkene chlorination and bromination are very general reactions, and mechanistic study of these reactions provides additional insight into electrophilic addition reactions of alkenes.. Much less detail is known about fluorination and iodination. The order of reactivity is F2 , Cl2 , Br2 , I2.. The differences between chlorination and bromination indicate the trends for all the halogens, but these differences are much more pronounced for fluorination and iodination. Fluorination is strongly exothermic and difficult to control, whereas for iodination the reaction is not normally favorable thermodynamically and easily reversible so often produces very low yields. But all of these reactions take place with a similar mechanism.. Iodine is a highly polarizable molecule that behaves as electrophilic iodine (I+) in the presence of a suitable Lewis base, such as an alkene reacts with molecular iodine, a characteristic iodonium ion is formed, and subsequent reaction of addition the nucleophilic ion (I-) ...
Mechanisms of reactions of the frustrated Lewis pairs (FLPs) with carbon dioxide (CO2) and hydrogen (H2) are studied by using quantum chemical modelling. FLPs are relatively novel chemical systems in which steric effects prevent a Lewis base (LB) from donating its electron pair to a Lewis acid (LA). From the main group of the periodic table, a variety of the electron pair donors and acceptors can create an FLP and the scope of the FLP chemistry is rapidly expanding at present. Representative intermolecular FLPs are phosphines and boranes with bulky electron-donating groups on phosphorus and bulky electron-withdrawing groups on boron - e.g., the tBu3P/B(C6F5)3 pair. The intramolecular FLPs feature linked LB and LA centers in one molecule.. Investigations of the FLP reaction mechanisms were carried out using the transition state (TS) and the potential energy surface (PES) calculations plus the Born-Oppenheimer molecular dynamics (BOMD) as an efficient and robust implementation of general ab initio ...
0033] In the formulae (8) and (9), L1 represents a Lewis base and L2 represents M2, R5R6M3, R73C or R8M3. [Z].sup.- represents a non-coordinating anion [Z1].sup.- or [Z2].sup.-. Herein, [Z1].sup.- represents an anion in which a plurality of groups are bonded to an element, namely, [M1G1G2 . . . Gf]31 (in which M1 represents an element in Group 5 to Group 15 of the periodic table, preferably an element in Group 13 to Group 15. G1 to Gf each represent a hydrogen atom, a halogen atom, an alkyl group having 1 to 20 carbon atoms, a dialkylamino group having 2 to 40 carbon atoms, an alkoxy group having 1 to 20 carbon atoms, an aryl group having 6 to 20 carbon atoms, an aryloxy group having 6 to 20 carbon atoms, an alkyl aryl group having 7 to 40 carbon atoms, an aryl alkyl group having 7 to 40 carbon atoms, a halogen-substituted hydrocarbon group having 1 to 20 carbon atoms, an acyloxy group having 1 to 20 carbon atoms, an organic metalloid group or a hetero-atom-containing hydrocarbon group having 2 ...
base: However, there are other definitions. The Brønsted-Lowry definition of a base states that bases absorb the H+ ions that are given off by acids, and the Lewis definition states that bases are electron-pair donators. This means that the Arrhenius definition given above is correct, but incomplete. Interestingly, it also means that compounds are neither inherently Brønsted-Lowry nor Lewis bases, because they may function as either depending on what theyre reacting with. (Though, in practice you probably wont see hydrochloric acid behave as a base by any definition ...
Bis(perfluorocatecholato)silane Si(cat( F) 2 was prepared, and stoichiometric binding to Lewis bases was demonstrated with fluoride, triethylphosphine oxide, and N,N-diisopropylbenzamide. The potent Lewis acidity of Si(cat( F) 2 was suggested from catalytic hydrosilylation and silylcyanation reactions with aldehydes. Mechanistic studies of hydrosilylation using an optically active silane substrate, R-(+)-methyl-(1-naphthyl)phenylsilane, proceeded with predominant stereochemical retention at silicon, consistent with a carbonyl activation pathway. The enantiospecificity was dependent on solvent and salt effects, with increasing solvent polarity or addition of NBu 4BAr( F) 4 leading to a diminished enantiomeric ratio. The medium effects are consistent with an ionic mechanism, wherein hydride transfer occurs prior to silicon-oxygen bond formation. ...
The binding behavior of the bifunctional Lewis acid (775-C5H5Fe)(2)[mu-(C5H3(BPh))(2)] (3) toward different pyridine derivatives as the Lewis base has been investigated. The 1: 1 and 1:2 adducts, 3 center dot D and 3 center dot D2 (donor, D = 4-tert-butylpyridine (tBupy) and 4-dimethylaminopyridine.... Full description. ...
The reaction proceeds through generation of an acylium center. The reaction is completed by deprotonation of the arenium ion by AlCl4−, regenerating the AlCl3 catalyst. However, in contrast to the truly catalytic alkylation reaction, the formed ketone is a moderate Lewis base, which forms a complex with the strong Lewis acid aluminum trichloride. The formation of this complex is typically irreversible under reaction conditions. Thus, a stochiometric quantity of AlCl3 is needed. The complex is destroyed upon aqueous workup to give the desired ketone. For example, the classical synthesis of deoxybenzoin calls for 1.1 equivalents of AlCl3 with respect to the limiting reagent, phenylacetyl chloride.[14] In certain cases, generally when the benzene ring is activated, Friedel-Crafts acylation can also be carried out with catalytic amounts of a milder Lewis acid (e.g. Zn(II) salts) or a Brønsted acid catalyst using the anhydride or even the carboxylic acid itself as the acylation agent. ...
You may also wish to search for items by Stone, J. and Stone. 10 matching references were found. Stone, J.A.; Splinter, D.E., A high-pressure mass spectrometric study of the binding of (CH3)3Sn+ to lewis bases in the gas phase, Int. J. Mass Spectrom. Ion Processes, 1984, 59, 169. [all data] Stone, J.A.; Wytenberg, W.J., A high pressure mass spectrometric study of the formation and chemistry of the pentafluorosulphur cation (SF5+), Int. J. Mass Spectrom. Ion Processes, 1989, 94, 269. [all data] Stone, J.A., Binding energies of (CH3)3Si+ to neutrals, Mass Spectrom. Rev., 1997, 16, 25. [all data] Stone, J.M.; Stone, J.A., A high pressure mass spectrometric study of the binding of (CH3)3Si+ and (CH3)3C+ to toluene and benzene, Int. J. Mass Spectrom. Ion Processes, 1991, 109, 247-264. [all data] Zhang, W.; Beglinger, C.; Stone, J.A., High-pressure mass spectrometric study of the gas-phase association of Cl- with alpha,omega-diols, J. Phys. Chem., 1995, 99, 30, 11673, ...
Al2Br6 dissociates readily to give the strong Lewis acid, AlBr3. Regarding the tendency of Al2Br6 to dimerize, it is common for heavier main group halides to exist as aggregates larger than implied by their empirical formulae. Lighter main group halides such as boron tribromide do not show this tendency, in part due to the smaller size of the central atom. Consistent with its Lewis acidic character, water hydrolizes Al2Br6 with evolution of HBr and formation of Al-OH-Br species. Similarly, it also reacts quickly with alcohols and carboxylic acids, although less vigorously than with water. With simple Lewis bases (L), Al2Br6 forms adducts, such as AlBr3L. Aluminium tribromide reacts with carbon tetrachloride at 100 °C to form carbon tetrabromide: ...
A series of metal-organic frameworks (MOFs) M2(dobpdc) (M=Mn, Co, Ni, Zn; H4dobpdc=4,4′-dihydroxy-1,1′-biphenyl-3,3′-dicarboxylic acid), with a highly dense arrangement of open metal sites along hexagonal channels were prepared by microwave-assisted or simple solvothermal reactions. The activated materials were structurally expanded when guest molecules including CO2 were introduced into the pores. The Lewis acidity of the open metal sites varied in the order Mn,Co,Ni,Zn, as confirmed by C=O stretching bands in the IR spectra, which are related to the CO2 adsorption enthalpy. DFT calculations revealed that the high CO2 binding affinity of transition-metal-based M2(dobpdc) is primarily attributable to the favorable charge transfer from CO2(oxygen lone pair acting as a Lewis base) to the open metal sites (Lewis acid), while electrostatic effects, the underlying factor responsible for the particular order of binding strength observed across different transition metals, also play a role. The ...
This work aims to highlight the underrated role played by pure phosphotriester (or phosphate) ligands in catalysis, when compared to other phosphorus-containing donors such as phosphane oxides or phosphites. To probe this and to enlarge the very narrow catalytic scope of these Lewis bases, easily accessible mono-and bidentate phosphotriesters were tested as donors in two important transition metal-based catalytic transformations: the zinc-catalyzed hydrosilylation of ketones and the titanium-promoted diethylzinc addition to aldehydes. In both cases, the reactions were successful and the corresponding alcohols were obtained in high yields.
The atom-efficient reaction of CO2 with a variety of epoxides has been efficiently achieved employing iron pyridylamino-bis(phenolate) complexes as bifunctional catalysts. The addition of a Lewis base co-catalyst allowed significant reduction in the amount of iron complex needed to achieve high epoxide conversions. The possibility of controlling the selectivity of the reaction towards either cyclic carbonate or polycarbonate was evaluated. An efficient switch in selectivity could be achieved when cyclic epoxides such as cyclohexene oxide and the seldom explored 1,2-epoxy-4-vinylcyclohexane were used as substrates. The obtained poly(vinylcyclohexene carbonate) presents pending vinyl groups, which allowed post-synthetic cross-linking by reaction with 1,3-propanedithiol. The cross-linked polycarbonate displayed a substantial increase in the glass transition temperature and chemical resistance, thus opening new opportunities for the application of these green polymers.. ...
The generation and isolation of two novel borenium cations has been described. The observation that the reaction of the Lewis acid B(C6F5)3 and the Lewis base diazabicyclo[2.2.2]octane (DABCO) with pinacol borane (HBpin ...
The present invention provides a method of treating of subterranean formation using a gel of a graft copolymer of a hydroxyalkyl cellulose, guar or hydroxypropyl guar prepared by a redox reaction with vinyl phosphonic acid. The gel is formed by hydrating the graft copolymer in an aqueous liquid containing at least a trace amount of at least one divalent cation. The gel is crosslinked by the addition of a Lewis base or Bronsted-Lowry base, which is substantially free of polyvalent metal ions, to the gel in an amount sufficient to effect crosslinking of the graft copolymer.
We explain the high bps of alcohols with the existence of hydrogen bonds (H-bonds), this is the same explanation we see for the incredibly high boiling point of water (water has about 4 H-bonds per molecule, vs. about 2 for alcohols). Hydrogen bonds involve a very strong polar attraction combined with very small atomic radii, this results in a partial covalent bond being formed. This bond is much weaker than the covalent bonds common in organic molecules (i.e. 100 kcal/mol for O-H bonds vs. about 5 kcal/mol for a H-bond), but is significantly stronger than the weak van der Waals forces holding alkanes together (about 0.1 - 0.2 kcal/mol).. Note that as MWs increase the differences in properties diminish - all of the molecules begin to behave more like alkanes as the alkane portion become predominant.. Acidity of Alcohols: Alcohols are weakly acidic, as we might expect due to their similarity to water. They can also function as weak Lewis bases, becoming protonated to give oxonium ions:. ...
A component for an olefin polymerisation catalyst which component is the product of treating a particulate support material with (a) an organomagnesium compound, (b) a transition metal compound of Groups IVA, VA or VIA, e.g. bis-butoxy titanium dichloride, (c) a pacifying agent, e.g. HCl and (d) optionally an aluminium compound, e.g. ethyl aluminium dichloride, an organometallic compound, e.g. zirconium tetrabenzyl, a halogenating agent e.g. silicon tetrachloride, or a Lewis Base compound, e.g. ethyl benzoate.
BF3 would have an sp2 hybridization, but it has an empty p-orbital (can accept a lone pair and have an octet). When BF3 undergoes an acid-base reaction, Boron can accept a lone pair from a Lewis base, and that bond is called a coordinate covalent bond? And that coordinate covalent bond would be a sigma bond ...
Diborane reacts with Lewis bases to form adducts.. I did some research on my own, and found that ammonia breaks it unsymmetrically but $\ce{CO}$ and $\ce{PF3}$ break it symmetrically. I cannot find the reason. ...
Thus, the two substrates of this enzyme are ATP and [[[isocitrate dehydrogenase (NADP+)]]], whereas its two products are ADP and [[[isocitrate dehydrogenase (NADP+)] phosphate]]. This enzyme belongs to the family of transferases, specifically those transferring a phosphate group to the sidechain oxygen atom of serine or threonine residues in proteins (protein-serine/threonine kinases). The systematic name of this enzyme class is ATP:[isocitrate dehydrogenase (NADP+)] phosphotransferase. Other names in common use include [isocitrate dehydrogenase (NADP+)] kinase, ICDH kinase/phosphatase, IDH kinase, IDH kinase/phosphatase, IDH-K/P, IDHK/P, isocitrate dehydrogenase kinase (phosphorylating), isocitrate dehydrogenase kinase/phosphatase, and STK3. This enzyme participates in mapk signaling pathway. ...
0029] The group I-B metal and the chalcogen element-containing organic compound may be chemically bonded to each other. Further, the group III-B metal and the chalcogen element-containing organic compound may be chemically bonded to each other. Still further, the chalcogen element-containing organic compound and the Lewis base organic solvent may be chemically bonded to each other. Through the chemical bonds as described above, a higher-concentration material solution of 8 wt % or higher can be prepared more easily. Examples of the above-mentioned chemical bonds include coordinate bond between respective elements. Such chemical bond can be confirmed by, for example, a nuclear magnetic resonance (NMR) technique. In the NMR technique, the chemical bond between the group I-B metal and the chalcogen element-containing organic compound can be detected as a peak shift of multinuclear NMR of a chalcogen element. The chemical bond between the group III-B metal and the chalcogen element-containing ...
Purpose. The complex interaction between the multiple neural, mechanical and intrinsic muscular factors responsible for muscle damage during voluntary movements has not been well elucidated. Specifically, the association between neural recruitment strategies used to activate muscles during voluntary exercise and the occurrence of muscle damage has not been examined. The purpose of this study was to explore the role of motor unit activation strategies in the initiation of exercise induced muscle damage. In addition, this study was undertaken to examine electromyographic signals (EMG) from the knee extensor muscles during concentric, eccentric and isometric contraction in order to examine possible differences in neural activation strategies between contraction types and contraction velocities. Methods. Isovelocity knee extensor strength tests were performed before and 24 hours after low level electrically evoked eccentric exercise of the quadriceps muscle (n = 6). Surface EMG was measured from the ...
The effect of ligand and Al cocatalysts in ethylene polymerization, using V(N-1-adamantyl)Cl2(L) [L = 2-(2,6-Me2C6H3)NCH2(C9H6N), 8-(2,6-Me2C6H3)NCH2(C9H6N)] and V(N-2-MeC6H3)Cl2[2-(2,6-R2C6H3)NCH2(C5H4N)] (R = Me, iPr), has been explored. The reaction products in the presence of Et2AlCl or Me2AlCl cocatalyst were polyethylene whereas the reaction product of the 2-methylphenylimido analogues in the presence of MAO cocatalyst was 1-butene with high selectivity, suggesting that the catalyst/cocatalyst nuclearity effect plays a role in this catalysis.
The present invention provides a high-activity solid main catalyst component for ethylene polymerization, a process for preparing the same and a catalyst containing the same. The main catalyst component consists essentially of a magnesium compound, a titanium compound, an electron donor, an alkyl aluminum compound and fumed silica, and is prepared by a process comprising the step of: reacting powdered magnesium with an alkyl halide to form a magnesium compound in the nascent state; adding the magnesium compound in the nascent state together with a titanium compound and an alkyl aluminum compound into an electron donor to form a complex; adding and well dispersing a fumed silica thereto to form a homogeneous thick mixture; molding and drying the mixture to form main catalyst component particles with good particle morphology; and optionally, dispersing the main catalyst component particles into a hydrocarbon solvent or a mineral oil to form a slurry. The main catalyst component has high magnesium
In this thesis, the synthesis of enantioenriched compounds using novel methodologies that employ metal- and biocatalysis is described.. In the first part, the synthesis of enantioenriched cyanohydrins, which are highly versatile synthetic intermediates, is described. A minor enantiomer recycling methodology, which uses a catalytic system consisting of a titanium salen dimer and a lipase, was highly successful in yielding the desired products, often in essentially enantiopure form. Alternatively, when the minor enantiomer recycling method gave unsatisfactory results, the same two catalysts were used in a sequential two-step process. The minor enantiomer recycling procedure was used to synthesize three different β-adrenergic antagonists with very high enantiomeric excesses via the corresponding O-acetylated cyanohydrins. With the same cyclic process, O-(α-bromoacyl) cyanohydrins were synthesized and subsequently transformed to aminofuranones via an intramolecular Blaise reaction. In addition, ...
Whitehead Institute researchers are using a modified CRISPR/Cas9-guided activation strategy to investigate the most frequent cause of intellectual disability in males.. February 15, 2018 ...
Clearly we are at an early stage in our understanding of the molecular mechanisms by which TA-specific mAb-based immunotherapy mediates effective clinical responses. Although not conclusive, the information we have reviewed has focused attention on the potential role of TA-specific adaptive immunity in patients treated with TA-specific mAb-based immunotherapy. Such responses would be desirable, because they provide a mechanism for long-term protection and immunologic memory. Moreover, the generation of TA-specific adaptive immunity provides a mechanism to explain the improved clinical responses in at least some patients treated with repeated administrations of some TA-specific mAb.. If T cells do play a role in the clinical efficacy of TA-specific mAb-based immunotherapy, one might ask why T cells activated in this setting are effective at controlling tumor cell growth but not when patients are vaccinated with T-cell-based activation strategies. Several possibilities can be envisioned to ...
Hi Juanma, You make many good points. However, I have replies to each of them. :-) Juanma Barranquero ,[email protected], wrote: , Youd have to define practical I define it as the benefits to new Emacs users with no .emacs file outweigh the costs. IMO, it is important that new users enjoy Emacs as soon as possible. If they enjoy it, they will continue using it forever. , Not all keyboards have F11 and F12 Thats OK. People without those keys can use C-x C-s and C-x b. , I dont think theres much agreement about the best way to switch , buffers (I dont use iswitchb-buffer, for example, but bs-cycle-next, , so your proposed binding is of no use to me) We should pick one method. People who dont like it can rebind the key. , next-buffer and previous-buffer are already in C-x ,left,right, iswitchb-buffer is not bound to anything :-) , save-buffer is in C-x C-s On second thought, C-x C-s isnt that hard to type. OK, so lets bind M-x recompile to F11 instead? Buffer switching can be bound to ...
You cannot adequately understand the Lewis acid theory with respect to main group elements if you only draw simple Lewis structures (which, per definition, should have complete octets on all main group elements) and then attempt to find electron pair acceptors. Well, to be perfectly honest, you might get lucky with boron compounds but the game will end there.. It is relatively clear to most people that metal cations can act as Lewis acids: they are positively charged because they lost at least one electron. They also have relatively low-lying orbitals (e.g. the one that lost an electron) with which the Lewis bases can interact. One has to apply more thought to molecules such as $\ce{CO2}$, though.. To analyse a molecule such as $\ce{CO2}$, you should start by thinking about the polarity of the bonds in question. The periodic table or whichever data page you wish to consult will tell us that oxygen is much more polar than carbon; therefore, the bonds are polarised towards oxygen. This can be ...
Description. UCI Chem 51A Organic Chemistry (Fall 2009) Lec 07. Organic Chemistry -- Lewis Acids, Lewis Bases, and Organic Reaction Mechanisms View the complete course: http://ocw.uci.edu/courses/chem_51a_organic_chemistry.html Instructor: James Nowick, Ph.D. License: Creative Commons BY-NC-SA Terms of Use: http://ocw.uci.edu/info. More courses at http://ocw.uci.edu Description: Fundamental concepts relating to carbon compounds with emphasis on structural theory and the nature of chemical bonding, stereochemistry, reaction mechanisms, and spectroscopic, physical, and chemical properties of the principal classes of carbon compounds. Organic Chemistry (Chem 51A) is part of OpenChem: http://ocw.uci.edu/collections/open_chemistry.html This video is part of a 27-lecture undergraduate-level course titled Organic Chemistry taught at UC Irvine by Professor James Nowick. Recorded October 9, 2009. Index of Topics: -1:15 Lewis Acids and Bases -2:21 BF3 Example -8:49 NH4+ Example -12:41 Tert-Butyl ...
TY - JOUR. T1 - Mechanistic Model for Enantioselective Intramolecular Alkene Cyanoamidation via Palladium-Catalyzed C-CN Bond Activation. AU - Frost, Grant B.. AU - Serratore, Nicholas A.. AU - Ogilvie, Jodi M.. AU - Douglas, Christopher J.. N1 - Funding Information: We thank National Institutes of Health for funding this work (R01 GM095559). We thank Prof. Steve Kass (UMN) for constructive feedback.. PY - 2017/4/7. Y1 - 2017/4/7. N2 - We studied key aspects of the mechanism of Pd-catalyzed C-CN bond activation and intramolecular enantioselective alkene cyanoamidation. An Abboud-Abraham-Kamlet-Taft (AAKT) linear solvation energy relationship (LSER) model for enantioselectivity was established. We investigated the impact of Lewis acid (BPh3), Lewis base (DMPU), and no additives. BPh3 additive led to diminished enantioselectivity and differing results in 13CN crossover experiments, initial rate kinetics, and natural abundance 12C/13C kinetic isotope effect measurements. We propose two catalytic ...
Abstract: The synthesis of three adducts of ferrocene boroxine (C10H9Fe)3B3O3 with dative bonds to amines (pyridine, 4-picoline, 4-aminomethyl pyridine, and p-bromoaniline), as well as to two diamines (4,4-bipyridine and 1,4-diazabicyclo[2.2.2]octane) to form bis-complexes which have the potential to be simple molecular rotors was completed. In a related project, complexes of either diphenylborinic acid or 9-borabicyclononane with Lewis base/ hydroxyl group pairs (picolinic acid, nicotinic acid, isonicotinic acid, picolinic acid N-oxide, nicotinic acid N-oxide, isonicotinic acid N-oxide, benzoic acid, 3,5- dibromobenzoic acid, 2-pyridinmethanol, L-Leucine, and L-Phenylalanine) were formed. These primarily intramolecular reactions formed rings of either 5 or 6 atom members. There was also evidence that some of the more geometrically challenged systems could form dimers. The characterization of all of these complexes was performed by 1H NMR and, when possible, LCMS and single crystal x-ray ...
Wenjuan Zhang,Shaoli Wang, Shizhen Du, Cun-Yue Guo, Xiang Hao, and Wen-Hua Sun, 2-(1-(2,4-Bis((bis(4-fluorophenyl)methyl)-6-methylphenylimino)ethyl)-6-(1- (arylimino)ethyl)pyridylmetal (iron or cobalt) Complexes: Synthesis, Characterization and Ethylene Polymerization Behavior, Macromol. Chem. Phys., 2014, 215 (18), 1797-1809. DOI: 10.1002/macp. ...
On the basis of temperature-dependent UV-vis and circular dichroism (CD) spectroscopy measurements, we observed that C3-symmetrical discotic molecules, chiral (R)-1 and achiral 2, both self-assemble in a highly cooperative fashion. Chiral (R)-1 shows a higher degree of cooperativity, meaning it requ …
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