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Subject Chemistry Paper No and Title 5, Organic Chemistry-II, Reaction mechanism-1 Module No and Title 19, Ambident nucleophile and regioselectivity. Module Tag CHE_P5_M19 CHEMISTRY PAPER: 5, Organic Chemistry-II, Reaction mechanism 1 MODULE:19, Ambident nucleophile and regioselectivity TABLE OF CONTENTS 1. Learning Outcomes 2. Introduction 3. Hard and soft acid and base concept and factors influencing ambident reactivity 3.1 Polarizability of the nucleophile 3.2 Nature of solvent 4. Reaction mechanisms involving ambident nucleophiles 5. Regioselectivity 5.1 Alkylation of Ketones 5.2 Hydroboration 5.3 Diels Alder Reaction 5.4 Ring opening of epoxides 5. Summary CHEMISTRY PAPER: 5, Organic Chemistry-II, Reaction mechanism 1 MODULE:19, Ambident nucleophile and regioselectivity 1. Learning Outcomes After studying this module, you shall be able to Know what are ambident nucleophile Learn mechanism of ambident nucleophilic substitution Identify factors that decide point of attack by ambident nucleophile Evaluate various types of functional groups as ambident nucleophile Analyze regioselectivity because of ambident nucleophiles 2. Introduction A nucleophile is a chemical entity that donates an electron pair to an electrophile centre to form a chemical bond. The terms nucleophile was derived from nucleus and Greek word philos (meaning love) and was coined by Christopher Kelk Ingold in 1929. All molecules or ions that can donate a free pair of electrons or have at least one π bond can act as nucleophiles. A nucleophile can have more than one atoms carrying a pair of electrons or the electrons may be shared by two or more atoms, so that canonical forms can be drawn in which two atoms bear an unshared pair of electrons. If such is the case, the nucleophile may attack in two or more different ways to give different products. Such nucleophiles are called ambident nucleophiles (ambi is Latin for “both”; dent is Latin for “teeth”). According to the IUPAC, ambident nucleophiles consist of two (or more) alternative and strongly interacting distinguishable reactive centers which all can undergo the reaction. However, when the reaction occurs at either site, it generally stops or greatly retards a subsequent attack at the other sites. CHEMISTRY PAPER: 5, Organic Chemistry-II, Reaction mechanism 1 MODULE:19, Ambident nucleophile and regioselectivity A common example of ambident nucleophile is enolate ion with oxygen and carbon as two nucleophilic centers, A nucleophile with two potentially attacking sites can exhibit dual reactivity by attacking with either of the electron rich centre. There are various factors that decide which atom of an ambident nucleophile will attack a given substrate under a given set of conditions i.e., if the kinetically controlled product will predominate or the thermodynamic control will take place preferentially. Therefore, depending on reaction conditions ambident nucleophiles lead to regioselective reactions (reactions where formation of two or more constitutional isomers (e.g., ROCN or RNCO), can take place but actually only one product is formed). Structure of some of the important ambident nucleophiles can be shown to be a resonance hybrid as follows; CHEMISTRY PAPER: 5, Organic Chemistry-II, Reaction mechanism 1 MODULE:19, Ambident nucleophile and regioselectivity S C N S C N CH3 CH3 H2C H2C S S N CH3 N CH3 H3C H3C O O CH2 CH2 CH2 O O O O O O CH2 CH2 CH2 O N O N O N O O O O O O O O O R R R CHEMISTRY PAPER: 5, Organic Chemistry-II, Reaction mechanism 1 MODULE:19, Ambident nucleophile and regioselectivity 3. Hard and Soft Acid and Base Concept and Factors Influencing Ambident Reactivity Which end of an ambident nucleophile should react predominantly under a particular set of conditions? To answer this question two most important factors are: a) Polarizability of the nucleophile and b) Nature of solvent 3.1. Polarizability of the Nucleophile To explain the ambident character and polarizability of the ligands the principle of hard and soft acids and bases (HSAB) was used. According to the principle hard acids prefer hard bases and soft acids prefer soft bases (based on experimental calculation of enthalpy). For an ambident nucleophile the more electronegative atom is a harder base than the less electronegative atom. Thus in enolate anion, the more electronegative oxygen atom with negative charge is harder base than carbon atom. The following table classifies common organic ligands as hard and soft acids and base. Table1: Classification of ligands as hard and soft acids and bases. Hard base Soft base Borderline base - - - (Donor atom has high electro- (Donor atom has low electro- For e.g. ArNH2, N3 , Br , NO2 negativity and low negativity and high polarizability). For e.g. H2O, polarizability). For e.g. R2S, CHEMISTRY PAPER: 5, Organic Chemistry-II, Reaction mechanism 1 MODULE:19, Ambident nucleophile and regioselectivity - - - 2- - OH , F , AcO , SO4 , RO , RSH, I-, CN-, RCN, C2H4, RNH2, NH3, ROH C6H6 Hard acid Soft acid Borderline acid (Possess small acceptor atoms (Possess large acceptor atoms For e.g. Fe2+, Co2+, Cu2+, Zn2+, 3+ + + + and have high positive charge, and have low positive charge, Bi , R3C , NO , C6H5 without an unshared pair of and contain unshared pair of electrons). For e.g. H+, Li+, electrons). For e.g. Cu2+, Ag+, + + 2+ 2+ 2+ 2+ 2+ Na , K , Mg , Ca , BF3, Pd , Pt , Hg , BH3, I2, Br2 + AlCl3, AlH3, SO3, RCO For SN1 mechanism a carbocation intermediate is a hard acid that would prefer attack by a hard base i.e. attack by oxygen of enolate. On the other hand for SN2 mechanism the carbon at reaction centre is softer acid and would be preferably attacked by carbon atom of enolate ion. This was generalized as, as the character of a given reaction changes from SN1 to SN2 like, an ambident nucleophile would more likely attack with its less electronegative end. 3.2 Nature of Solvent The nature of solvent has been shown to guide the position of attack in a number of reactions for ambident nucleophiles. In protic solvents, through hydrogen bonding the more electronegative atom is better solvated than the less electronegative atom. Thus less electro negative atom is more prone to attack. In contrast, polar aprotic solvents offer no stabilization to either end of the nucleophile however, they stabilize the substrate cation by solvation. As a result of this the more electronegative end of the nucleophile is freer so the extent of attack by the more electronegative atom increases. CHEMISTRY PAPER: 5, Organic Chemistry-II, Reaction mechanism 1 MODULE:19, Ambident nucleophile and regioselectivity As an e.g. for the following reaction under different solvent conditions (Table 1) the ratio of N- alkylation to C-alkylation changed dramatically as shown by the proportion of products formed in the reaction. Br + N N N K H (A) (B) Solvent %A %B Tetra hydro furan 81 19 Dioxane 71 29 Di isopropyl ether 21 79 Benzene 15 85 Toluene 14 86 n-heptane 14 86 The leaving group and counter positive ion of the nucleophile also influences mechanism of reaction and therefore influences the major product formed. For the alkylation of 2-hydroxyl + pyridine (enolate ion) when the reaction was carried out with K as the counter ion, a SN2 mechanism was favored and the soft reaction centre N atom participated in the reaction (73% product). CHEMISTRY PAPER: 5, Organic Chemistry-II, Reaction mechanism 1 MODULE:19, Ambident nucleophile and regioselectivity CH3CH2I S 2 K+ N N N O O N O CH2CH3 CH CH I 3 2 SN1 Ag+ N O N O N OCH2CH3 On the other hand, if instead of K+ the counter ion is Ag+ then due to the ability of Ag+ to coordinate with the leaving group iodide ion, the reaction was found to favor SN1 mechanism and therefore O-acylation product (80%) was obtained as the major product. Steric hindrance at the nucleophilic centre may become predominating factor to decide point of attachment where applicable. For e.g. the alkylation of phenol with RI gives O-alkylated product. R R O O OH O O RI RI + Phenoxide ion 2,6-Dimethyl (Major) phenoxide ion R However, when the reaction centre is crowded as in 2,6-dimethyl phenol then, the para alkyl substitution leading to C-alkylation is favored over O-alkylation. CHEMISTRY PAPER: 5, Organic Chemistry-II, Reaction mechanism 1 MODULE:19, Ambident nucleophile and regioselectivity 4. Reaction Mechanism Involving Ambident Nucleophiles Nef Reaction involves conversion of a nitro compound to carbonyl compound through a nitronate anion. The nitronate anion is an ambident nucleophile which can either attack from the carbon end acting as a soft base or from oxygen end acting as a strong base. The reaction goes by attack from the carbon end predominantly as follows: However, in the reaction O-alkylation is possible if a hard alkylating agent such as MeOSO2F is used. Thus the reaction would proceed like: O O MeOSO2F R N R N O O Me Kolbe nitrile Synthesis: The reaction of primary aliphatic halides with alkali metal cyanides gives nitriles in good yield. + R Cl KCN DMSO R CN Mechanism: CHEMISTRY PAPER: 5, Organic Chemistry-II, Reaction mechanism 1 MODULE:19, Ambident nucleophile and regioselectivity K N: C + -KX R Cl R CN However, since cyanide is an ambident nucleophile, in the reaction, attack from nitrogen end of the nucleophile results in isocyanide product as follows: M -KX C :N + R Cl R N C The nature of cation here plays an important role as isocyanides are formed with Ag+ and Cu+ metal cations. Compounds with active methylene group such as acetyl acetone can be alkylated at either the carbon (C-alkylation) or at the oxygen (O-alkylation) atom through an enolate anion.
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