CHM 234|Conjugated Pi Systems and Pericyclic Reactions
CHM 234|Conjugated Pi Systems and Pericyclic Reactions CHM 234
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This 8 page Class Notes was uploaded by Alvin Notetaker on Tuesday March 8, 2016. The Class Notes belongs to CHM 234 at Arizona State University taught by Pillai in Fall 2015. Since its upload, it has received 11 views. For similar materials see General Organic Chemistry II in Chemistry at Arizona State University.
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Date Created: 03/08/16
Conjugated Pi Systems and Pericyclic Reactions Classes of Dienes Dienes are compounds with two C=C bond. Cumulated Conjugated Isolated Cumulated dienes (allenes): ● pi bonds are adjacent ● pi orbitals are perpendicular Conjugated dienes: ● pi bonds are separated by exactly one single bond ● pi bond overlap extends over the entire system Isolated dienes: ● pi bonds are separated by any distance greater than one single bond ● pi bonds are separated by too great a distance to experience extra overlap Conjugated Dienes Preparation SN can be avoided with tertbutoxide (tBuOK) 1,3butadiene Bond Length 2 In a conjugated diene, the single bond length is formed by theoverlapping sp orbital, while in ethane, the single bond is from theoverlapping sp3 orbital. This overlapping orbital in the conjugated diene will produced a shorter length than in the ethane. Stability Conjugated dienes are more stable than isolated dienes. Reason: The electron densities are connected. (Resonance) Conformations of 1,3Butadiene Single bond can rotate at room temperature With a strans diene, the half of the overall orbital can rotate to a 90° angle and C2 and C3 does not overlap. Due to steric hinderance, the strans is more stable. Molecular Orbital Theory ● a)sp −s 2 2 2 ● b)σ (sp −sp ) ● c)π (p−p) AMO have a node (Higher in energy) BMO no node (Lower in energy) 4π electrons(Butadiene) 6π electrons(Hexatriene) Highest occupied molecular orbital (HOMO) Lowest unoccupied molecular orbital (LUMO) Electrophilic Addition Addition of HX across 1,3butadiene Protonation creates the more stable allylic carbocation. When it react with a hydrohalide, like HBr, it will produce two product. 1,2adduct 1,4adduct 1,2adduct: the addition of hydrogen and halide will be added on C1 and C2. 1,4adduct: the addition of hydrogen and halide will be added on C1 and C4. Addition of Br 2across 1,3Butadiene Thermodynamic Control vs. Kinetic Control 1,2Adduct 1,4Adduct Low temperature 0°C 71% 29% High temperature 40°C 15% 85% The 1,2 product forms faste atlow temperature because it predominate at low temperature. Forming faster is callinetic product. The 1,4 product forms atequilibrium, thus itmore stable. The formation at equilibrium is calledthermodynamic product . Example: Introduction to Pericyclic Reactions A concerted process and do not involve either ionic or radical intermediates Cycloaddition reaction 2π bonds forms 2σ bonds, a ring, and changed πbond location. Electrocyclic reactio 1π bond converts into a 1σ bond and forms a ring. Sigmatropic rearrangements: 1σ bond breaks to from another 1σ. Pericyclic reactions’ 4 general features 1. The reaction mechanism is concerted . 2. Involves a ring of electrons moving around a closed loop. 3. The transition state iscyclic. 4. The polarity of the solvent generally has no effect on the reaction rate. Diels Alder Reaction In a DielsAlder reaction, a conjugated diene reacts with a molecule with a carboncarbon double bond. This reaction is called[4+2] cycloaddition because of the interaction of the 4π electrons in the conjugated diene and the 2π electrons in the alkene or alkyne. Mechanism for the DielsAlder Reaction The reaction is initialed byheat. pi bond breaks and two new CC sigma bonds and one new CC pi bond form. This is aconcerted reaction. Electron withdrawing substituents in the dienophile increases the reaction rate. Electron withdrawing substituents consists of electronegative atom such as oxygen and nitrogen. (C=O) ; (CN) Thermodynamic Considerations Retro DielsAlder reaction : The reverse of a DielsAlder reaction This can be achieved at high temperature where the ring decomposes into diene and dienophile. ● Ring (DielsAlder reaction) predominate at low and moderate temperature. ● Diene and dienophile (Retro DielsAlder reaction) predominate at high temperature (200°C) Conformation of the Diene strans conformation is more stable because scis causes more steric strain due to the hydrogen atoms. Reactivity (most to least) 1. scis (locked) 2. scis 3. strans 4. strans (locked) Common dienophiles in DielsAlder reaction Z=electronwithdrawal group Example: A carboncarbon triple bond can be used as a dienophile. The Stereochemistry of the DielsAlder Reaction: Syn addition If the substituents is cis in the dienophile , then the product will produce a cis product . If the substituents is trnas in the dienophile , then the product will produce a trans product . (Remember: When a substituents is bonded to a C=C, it is on the same plane as the ring.) Endo Rule When the dienophile has a substituent with pi electrons, more of theendo product is formed Endo has a interaction between the developing π bond and the electronwithdrawal group Exo has no interaction because it placement is too far away.
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