S20-Week04-Lecture15-19

S20-Week04-Lecture15-19 - D i e n es Dienes compounds with...

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GP15-01 Dienes Dienes: compounds with 2 C-C double bonds H 2 C C CH 2 allene (or propadiene) - a cumulene Structure of allenes H 2 C C CH 2 sp 2 C sp C C C H H H H ! bond #1 C H H C H H ! bond #2 sp sp 2 sp 2 p orbital overlap is maximized if the sp 2 carbons are at 90 o One consequence is that some allenes are chiral (even though they don't possess an sp 3 stereocenter!! C C H 3 C H C CH 3 H C C CH 3 H C H 3 C H mirror plane 1,2-DIENES 1 2
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GP15-02 1,3-butadiene - a conjugated diene Adjacent ! bonds can be involved in resonance (they are also described as being conjugated , or "in conjugation "). The ! Molecular Orbitals There are 4 p orbitals (one from each sp 2 carbon)... ...so there are 4 ! Molecular Orbitals – " 1 " 2 " 3 " 4 all in phase 0 extra node lowest energy 1 extra node next lowest energy 2 extra nodes next lowest energy all out of phase 3 extra nodes highest energy Energy " 1 " 2 " 3 " 4 ...add the 4 p electrons (one from each sp 2 carbon)... " 1 " 2 " 3 " 4 HOMO LUMO antibonding ( ! * MO's) bonding ( ! MO's) 1.34 Å 1.46 Å slightly longer than a "regular" C=C somewhat shorter than a "regular" C–C 1,3-DIENES 1 2 3 • • • • • • • • C C C C C C C C C C C C C C C C represented as: " 1 " 2 " 3 " 4 represented as: represented as: represented as: Relative ! Molecular Orbital Energy Levels
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GP15-03 1,3-DIENES "Ionic" additions Observation: addition of HCl to butadiene... H Cl + H Cl H Cl 1 2 4 1 (less stable product) 75% (more stable product) 25% 1,2 addition 1,4 addition Mechanism: formation of more stable (resonance stabilized) carbocation... H Cl H H Cl + H Cl 1 2 2 H H Cl 4 1 4 • In general 1,3 dienes undergo addition reactions just like simple alkenes • BUT the 'adjacent' C=C complicates things, as in the example below: more stable carbocation H CH 2 H resonance form A resonance form B Resonance form A is lower energy than form B (secondary carbocations are much more stable than primary carbocations) – so the ' actual ' structure of the carbocation intermediate has more positive charge on C2 than on C4. The nucleophile is negatively charged and so will react FASTER at C2 than C4 – so more of the less stable 1,2 addition product is formed: 2 4 2 4 Cl Cl FASTER SLOWER
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GP15-04 1,3-DIENES Diels–Alder Reaction A reaction characteristic of 1,3-dienes, and a very important organic reaction Z diene dienophile (Z = electron withdrawing group heat Z + Z it's an addition reaction, which makes a cycle (or ring) - it's a cycloaddition The reaction has STRICT STEREOELECTRONIC REQUIREMENTS (more of this later when we deal with "pericyclic reactions") So there are stereochemistry issues we must address... Z Z Z • • ! 2 Only " electrons move - so " Molecular Orbitals are very important Z is electron withdrawing - so we will use the dienophile's LUMO ... ...and the diene's HOMO • • Z NOTE: This is a concerted reaction - it does not matter which direction we draw the 3 curved arrows, providing they 'follow' each other: Z Z As long as the ends of the diene can interact with the ends of the dienophile, the reaction can proceed: Z Z Z " *
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