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MODULE No. 19, Ambident Nucleophile and Regioselectivity 1. Ht Weblinks : 1. http://www.chem.ucalgary.ca/courses/351/Carey5th/Ch06/ch6-0-1.html 2. www.chem.ucla.edu/harding/IGOC/A/ambident_nucleophile.html 3. www.adichemistry.com/inorganic/.../hsab/hard-soft-acid-base-theory.htm 4. www.nature.com/nrmicro/journal/v11/n6/fig.../nrmicro3028_F1.html 5. www.masterorganicchemistry.com/.../addition-reactions-regioselectivity/ CHEMISTRY PAPER No. 5: Organic chemistry-2 (Reaction Mechanism-1) MODULE No. 19, Ambident nucleophile and regioselectivity Suggested readings Atkins, P. W. & Paula, J. de Atkin’s Physical Chemistry 9th Ed., Oxford University Press (2012). Jerry March, Advanced Organic Chemistry, Fourth Edition. P. S. Kalsi, Organic Reactions And Their Mechanisms CHEMISTRY PAPER No. 5: Organic chemistry-2 (Reaction Mechanism-1) MODULE No. 19, Ambident nucleophile and regioselectivity Named Organic Reactions By Thomas Laue, Andreas Plagens William Brown, Christopher Foote, Brent Iverson, Eric Anslyn, Organic Chemistry, Enhanced Edition Glossary A CHEMISTRY PAPER No. 5: Organic chemistry-2 (Reaction Mechanism-1) MODULE No. 19, Ambident nucleophile and regioselectivity Ambident nucleophile: An ambident nucleophile is an anionic nucleophile whose negative charge is delocalized by resonance over two unlike atoms or over two like but non-equivalent atoms. Aprotic Solvent: Polar aprotic solvents are solvents that will dissolve many salts, but lack an acidic hydrogen. These solvents generally have intermediate dielectric constants and polarity. Although discouraging use of the term "polar aprotic", IUPAC describes such solvents as having both high dielectric constants and high dipole moments, an example being acetonitrile. Other solvents meeting IUPAC's criteria include DMF, HMPA, and DMSO D Dields –Alder reaction: The Diels–Alder reaction is an organic chemical reaction (specifically, a [4+2] cycloaddition) between a conjugated diene and a substituted alkene, commonly termed the dienophile, to form a substituted cyclohexene system. It was first described by Otto Paul Hermann Diels and Kurt Alder in 1928, for which work they were awarded the Nobel Prize in Chemistry in 1950. H HSAB: The HSAB concept is an initialism for "hard and soft (Lewis) acids and bases". Also known as the Pearson acid base concept, HSAB is widely used in chemistry for explaining stability of compounds, reaction mechanisms and pathways. It assigns the terms 'hard' or 'soft', and 'acid' or 'base' to chemical species. Hydroboration: In chemistry, hydroboration refers to the addition of a hydrogen-boron bond to C-C, C-N, and C-O double bonds, as well as C-C triple bonds. This chemical reaction is useful in the organic synthesis of organic compounds. Hydroboration produces organoborane compounds that react with a variety of reagents to produce useful compounds, such as alcohols, amines, alkyl halides. K Kolbe nitrile synthesis: The Kolbe nitrile synthesis is a method for the preparation of alkyl nitriles by reaction of the corresponding alkylhalide with a metal cyanide. A side product for this reaction is the formation of an isonitrile because the cyanide ion is an ambident nucleophile and according to Kornblum's rule is capable of reacting with either carbon or nitrogen. The reaction is named after Hermann Kolbe. CHEMISTRY PAPER No. 5: Organic chemistry-2 (Reaction Mechanism-1) MODULE No. 19, Ambident nucleophile and regioselectivity N Nef Reaction: The Nef reaction is an organic reaction describing the acid hydrolysis of a salt of a primary or secondary nitroalkane (1) to an aldehyde or a ketone (3) and nitrous oxide. P Protic Solvent: A protic solvent is a solvent that has a hydrogen atom bound to an oxygen (as in a hydroxyl group) or a nitrogen (as in an amine group). In general terms, any solvent that contains labile H+ is called a protic solvent. The molecules of such solvents readily donate protons (H+) to reagents. R Regioselectivity: In chemistry, regioselectivity is the preference of one direction of chemical bond making or breaking 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 add. Did you know? Application of the Hard and Soft, Acids and Bases (HSAB) theory to toxicant--target interactions: Many chemical toxicants and/or their active metabolites are electrophiles that cause cell injury by forming covalent bonds with nucleophilic targets on biological macromolecules. Covalent reactions between nucleophilic and electrophilic reagents are, however, discriminatory since there is a significant degree of selectivity associated with these interactions. Over the course of the past few decades, the theory of Hard and Soft, Acids and Bases (HSAB) has proven to be a useful tool in predicting the outcome of such reactions. This concept utilizes the inherent electronic characteristic of polarizability to define, for example, reacting electrophiles and nucleophiles as CHEMISTRY PAPER No. 5: Organic chemistry-2 (Reaction Mechanism-1) MODULE No. 19, Ambident nucleophile and regioselectivity either hard or soft. These HSAB definitions have been successfully applied to chemical-induced toxicity in biological systems. Thus, according to this principle, a toxic electrophile reacts preferentially with biological targets of similar hardness or softness. The soft/hard classification of a xenobiotic electrophile has obvious utility in discerning plausible biological targets and molecular mechanisms of toxicity. The purpose of this perspective is to discuss the HSAB theory of electrophiles and nucleophiles within a toxicological framework. In principle, covalent bond formation can be described by using the properties of their outermost or frontier orbitals. Because these orbital energies for most chemicals can be calculated using quantum mechanical models, it is possible to quantify the relative softness (σ) or hardness (η) of electrophiles or nucleophiles and to subsequently convert this information into useful indices of reactivity. This atomic level information can provide insight into the design of corroborative laboratory research and thereby help investigators discern corresponding molecular sites and mechanisms of toxicant action. The use of HSAB parameters has also been instrumental in the development and identification of potential nucleophilic cytoprotectants that can scavenge toxic electrophiles. Clearly, the difficult task of delineating molecular sites and mechanisms of toxicant action can be facilitated by the application of this quantitative approach. Time-line Year Description Image 1894 The Nef reaction was reported in 1894 by the chemist John Ulric Nef, who treated the sodium salt of nitroethane with sulfuric acid resulting in an 85–89% yield of nitrous oxide and at least 70% yield of acetaldehyde. However, the reaction was pioneered a year earlier in 1893 by Konovalov, who converted the potassium salt of 1-phenylnitroethane with sulfuric acid to acetophenone CHEMISTRY PAPER No. 5: Organic chemistry-2 (Reaction Mechanism-1) MODULE No. 19, Ambident nucleophile and regioselectivity 1953 The Irving-Williams stability series (1953) pointed out that for a given ligand the stability of dipositive metal ion complexes increases: Ba2+ < Sr2+ < Ca2+ < Mg2+ < Mn2+ < F e2+ < Co2+ < Ni2+ < Cu2+ < Zn2+ . 1960 In the nineteen sixties, Ralph Pearson developed the Type A and and Type B logic by explaining the differential complexation behaviour of cations and ligands in terms of electron pair donating Lewis bases and electron pair accepting Lewis acids: Lewis acid + Lewis base Lewis acid/base complex Pearson classified Lewis acids and Lewis bases as hard, borderline or soft. CHEMISTRY PAPER No. 5: Organic chemistry-2 (Reaction Mechanism-1) MODULE No. 19, Ambident nucleophile and regioselectivity 1968 In 1968, G. Klopman attempted to quantify Pearson's HSAB principle using frontier molecular orbital (FMO) theory. Soft [Lewis] acids bind to soft [Lewis] bases to give FMO-controlled (covalent) complexes. These interactions are dominated by the energies of the participating frontier molecular orbitals (FMO), the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). 1979 The development of hydroboration technology and the underlying concepts were recognized by the Nobel Prize in Chemistry to Herbert C. Brown. He shared the Nobel prize in chemistry with Georg Wittig in 1979 for his pioneering research on organoboranes as important synthetic intermediates. CHEMISTRY PAPER No. 5: Organic chemistry-2 (Reaction Mechanism-1) MODULE No. 19, Ambident nucleophile and regioselectivity CHEMISTRY PAPER No. 5: Organic chemistry-2 (Reaction Mechanism-1) MODULE No. 19, Ambident nucleophile and regioselectivity .
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