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Phosphinooxazolines
Phosphinooxazolines (often abbreviated PHOX) are a class of chiral ligands used in asymmetric catalysis. Their complexes are particularly effective at generating single enatiomers in reactions involving highly symmetric transition states, such as allylic substitutions, which are typically difficult to perform stereoselectively. The ligands are bidentate and have been shown to be hemilabile with the softer P‑donor being more firmly bound than the harder N‑donor. Synthesis The synthesis of phosphinooxazolines is modular and it is not normally necessary to introduce the phosphine and oxazoline moieties in any particular order. However while examples exist of the phosphine being introduced first, it is more common to see the synthesis of a phenyloxazoline which is subsequently combined with a source of diphenylphosphine. Methods for doing this depend on the nature of the substituent in the X position: * When X = fluorine coupling involves anionic displacement with a dipheny ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Tsuji–Trost Reaction
The Tsuji–Trost reaction (also called the Trost allylic alkylation or allylic alkylation) is a palladium-catalysed substitution reaction involving a substrate that contains a leaving group in an allylic position. The palladium catalyst first coordinates with the allyl group and then undergoes oxidative addition, forming the -allyl complex. This allyl complex can then be attacked by a nucleophile, resulting in the substituted product. This work was first pioneered by in 1965 and, later, adapted by Barry Trost in 1973 with the introduction of phosphine ligands. The scope of this reaction has been expanded to many different carbon, nitrogen, and oxygen-based nucleophiles, many different leaving groups, many different phosphorus, nitrogen, and sulfur-based ligands, and many different metals (although palladium is still preferred). The introduction of phosphine ligands led to improved reactivity and numerous asymmetric allylic alkylation strategies. Many of these strategies ar ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Asymmetric Hydrogenation
Asymmetric hydrogenation is a chemical reaction that adds two atoms of hydrogen to a target (substrate) molecule with three-dimensional spatial selectivity. Critically, this selectivity does not come from the target molecule itself, but from other reagents or catalysts present in the reaction. This allows spatial information (what chemists refer to as chirality) to transfer from one molecule to the target, forming the product as a single enantiomer. The chiral information is most commonly contained in a catalyst and, in this case, the information in a single molecule of catalyst may be transferred to many substrate molecules, amplifying the amount of chiral information present. Similar processes occur in nature, where a chiral molecule like an enzyme can catalyse the introduction of a chiral centre to give a product as a single enantiomer, such as amino acids, that a cell needs to function. By imitating this process, chemists can generate many novel synthetic molecules that intera ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Oxazoline
Oxazoline is a five-membered heterocyclic organic compound with the formula . It is the parent of a family of compounds called oxazolines (emphasis on plural), which contain non-hydrogenic substituents on carbon and/or nitrogen. Oxazolines are the unsaturated analogues of oxazolidines, and they are isomeric with isoxazolines, where the N and O are directly bonded. Two isomers of oxazoline are known, depending on the location of the double bond. Oxazoline itself has no applications however oxazolines have been widely investigated for potential applications. These applications include use as ligands in asymmetric catalysis, as protecting groups for carboxylic acids and increasingly as monomers for the production of polymers. Isomers Synthesis The synthesis of 2-oxazoline rings is well established and in general proceeds via the cyclisation of a 2-amino alcohol (typically obtained by the reduction of an amino acid) with a suitable functional group. The overall mechanism is usua ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Regioselectivity
In chemistry, regioselectivity is the preference of chemical bonding or breaking in one direction over all other possible directions. It can often apply to which of many possible positions a reagent will affect, such as which proton a strong base will abstract from an organic molecule, or where on a substituted benzene ring a further substituent will be added. A specific example is a halohydrin formation reaction with 2-propenylbenzene: : Because of the preference for the formation of one product over another, the reaction is selective. This reaction is regioselective because it selectively generates one constitutional isomer rather than the other. Various examples of regioselectivity have been formulated as rules for certain classes of compounds under certain conditions, many of which are named. Among the first introduced to chemistry students are Markovnikov's rule for the addition of protic acids to alkenes, and the Fürst-Plattner rule for the addition of nucleophiles ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
BINAP
BINAP (2,2′-bis(diphenylphosphino)-1,1′-binaphthyl) is an organophosphorus compound. This chiral diphosphine ligand is widely used in asymmetric synthesis. It consists of a pair of 2-diphenylphosphinonaphthyl groups linked at the 1 and 1′ positions. This C2-symmetric framework lacks a stereogenic atom, but has axial chirality due to restricted rotation (atropisomerism). The barrier to racemization is high due to steric hindrance, which limits rotation about the bond linking the naphthyl rings. The dihedral angle between the naphthyl groups is approximately 90°. The natural bite angle is 93°. Use as ligand in asymmetric catalysis BINAP is used in organic synthesis for enantioselective transformations catalyzed by its complexes of ruthenium, rhodium, and palladium. As pioneered by Ryōji Noyori and his co-workers, rhodium complexes of BINAP are useful for the synthesis of (–)-menthol. Silver complexes are also important; BINAP- AgF can be used to enantiose ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Intramolecular Heck Reaction
The intramolecular Heck reaction (IMHR) in chemistry is the coupling of an aryl or alkenyl halide with an alkene in the same molecule. The reaction may be used to produce carbocyclic or heterocyclic organic compounds with a variety of ring sizes. Chiral palladium complexes can be used to synthesize chiral intramolecular Heck reaction products in non-racemic form. Introduction The Heck reaction is the palladium-catalyzed coupling of an aryl or alkenyl halide with an alkene to form a substituted alkene. Intramolecular variants of the reaction may be used to generate cyclic products containing ''endo'' or ''exo'' double bonds. Ring sizes produced by the intramolecular Heck reaction range from four to twenty-seven atoms. Additionally, in the presence of a chiral palladium catalyst, the intramolecular Heck reaction may be used to establish tertiary or quaternary stereocenters with high enantioselectivity.Overman, L. E. ''Pure Appl. Chem.'' 1994, ''66'', 1423. A number of tande ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Heck Reaction
The Heck reaction (also called the Mizoroki–Heck reaction) is the chemical reaction of an unsaturated halide (or triflate) with an alkene in the presence of a base and a palladium catalyst (or palladium nanomaterial-based catalyst) to form a substituted alkene. It is named after Tsutomu Mizoroki and Richard F. Heck. Heck was awarded the 2010 Nobel Prize in Chemistry, which he shared with Ei-ichi Negishi and Akira Suzuki, for the discovery and development of this reaction. This reaction was the first example of a carbon-carbon bond-forming reaction that followed a Pd(0)/Pd(II) catalytic cycle, the same catalytic cycle that is seen in other Pd(0)-catalyzed cross-coupling reactions. The Heck reaction is a way to substitute alkenes. History The original reaction by Tsutomu Mizoroki (1971) describes the coupling between iodobenzene and styrene in methanol to form stilbene at 120 °C (autoclave) with potassium acetate base and palladium chloride catalysis. This work was an ex ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Heck Reaction Scheme
Heck may refer to: * HECK (band), a British rock band * Heck (surname) * Heck, Dumfries and Galloway, Scotland * Heck, North Yorkshire, England * Heck cattle * Heck horse * Heck reaction, a chemical reaction that forms a substituted alkene * Parnall Heck, a 1930s British four-seat cabin monoplane * NOAAS ''Heck'' (S 591), originally USC&GS ''Heck'', a survey ship * A minced oath for "hell" Hecke may refer to: * Hecke algebra (other) * Hecke character In number theory, a Hecke character is a generalisation of a Dirichlet character, introduced by Erich Hecke to construct a class of ''L''-functions larger than Dirichlet ''L''-functions, and a natural setting for the Dedekind zeta-functions an ... * Hecke operator * Hecke (surname) {{disambiguation, geo ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Palladium
Palladium is a chemical element with the symbol Pd and atomic number 46. It is a rare and lustrous silvery-white metal discovered in 1803 by the English chemist William Hyde Wollaston. He named it after the asteroid Pallas, which was itself named after the epithet of the Greek goddess Athena, acquired by her when she slew Pallas. Palladium, platinum, rhodium, ruthenium, iridium and osmium form a group of elements referred to as the platinum group metals (PGMs). They have similar chemical properties, but palladium has the lowest melting point and is the least dense of them. More than half the supply of palladium and its congener platinum is used in catalytic converters, which convert as much as 90% of the harmful gases in automobile exhaust (hydrocarbons, carbon monoxide, and nitrogen dioxide) into nontoxic substances (nitrogen, carbon dioxide and water vapor). Palladium is also used in electronics, dentistry, medicine, hydrogen purification, chemical applications, groun ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Symmetric Vs Asymmetric
Symmetry (from grc, συμμετρία "agreement in dimensions, due proportion, arrangement") in everyday language refers to a sense of harmonious and beautiful proportion and balance. In mathematics, "symmetry" has a more precise definition, and is usually used to refer to an object that is invariant under some transformations; including translation, reflection, rotation or scaling. Although these two meanings of "symmetry" can sometimes be told apart, they are intricately related, and hence are discussed together in this article. Mathematical symmetry may be observed with respect to the passage of time; as a spatial relationship; through geometric transformations; through other kinds of functional transformations; and as an aspect of abstract objects, including theoretic models, language, and music. This article describes symmetry from three perspectives: in mathematics, including geometry, the most familiar type of symmetry for many people; in science and nature ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Racemic
In chemistry, a racemic mixture, or racemate (), is one that has equal amounts of left- and right-handed enantiomers of a chiral molecule or salt. Racemic mixtures are rare in nature, but many compounds are produced industrially as racemates. History The first known racemic mixture was racemic acid, which Louis Pasteur found to be a mixture of the two enantiomeric isomers of tartaric acid. He manually separated the crystals of a mixture by hand, starting from an aqueous solution of the sodium ammonium salt of racemate tartaric acid. Pasteur benefited from the fact that ammonium tartrate salt that gives enantiomeric crystals with distinct crystal forms (at 77 °F). Reasoning from the macroscopic scale down to the molecular, he reckoned that the molecules had to have non-superimposable mirror images. A sample with only a single enantiomer is an ''enantiomerically pure'' or ''enantiopure'' compound. Etymology From racemic acid found in grapes; from Latin ''racemus'', me ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Back Bonding
In chemistry, π backbonding, also called π backdonation, is when electrons move from an atomic orbital on one atom to an appropriate symmetry antibonding orbital on a ''π-acceptor ligand''. It is especially common in the organometallic chemistry of transition metals with multi-atomic ligands such as carbon monoxide, ethylene or the nitrosonium cation. Electrons from the metal are used to bond to the ligand, in the process relieving the metal of excess negative charge. Compounds where π backbonding occurs include Ni(CO)4 and Zeise's salt. IUPAC offers the following definition for backbonding: A description of the bonding of π-conjugated ligands to a transition metal which involves a synergic process with donation of electrons from the filled π-orbital or lone electron pair orbital of the ligand into an empty orbital of the metal (donor–acceptor bond), together with release (back donation) of electrons from an ''n''d orbital of the metal (which is of π-symmetry with re ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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