Conjugated Systems & Pericyclic Reactions

Transcription

1 onjugated Systems & Pericyclic Reactions 1 onjugated Dienes from heats of hydrogenation-relative stabilities of conjugated vs unconjugated dienes can be studied: Name 1-Butene 1-Pentene Structural Formula ΔH 0 kj (kcal)/mol -17 (-30.3) -16 (-30.1) cis--butene -10 (-8.6) trans--butene -115 (-7.6) conjugated 1,3-Butadien e trans-1,3-pentad iene -37 (-56.5) -6 (-54.1) 1,4-Pentadien e -54 (-60.8)

2 onjugated Dienes conjugation of the double bonds in 1,3-butadiene gives an extra stability of approximately 17 kj (4.1 kcal)/mol H catalyst ΔH 0 = (-17 kj/mol) = -54 kj/mol) H catalyst ΔH 0 = -37 kj/mol 3 onjugated Dienes The π systems of the olefins overlap H H H H S-cis conformer 4

3 onjugated Dienes H H H H S-Trans conformer 5 Summary of Bond Reactions Nucleophilic displacement of a leaving group by R - Gilman & Grignard additions Alkyne and - N anions Alkylation of enolates Alkylation of enamines 6

4 Nucleophilic addition To carbonyl & carboxyl groups Gilman, Grignard & organolithium Alkyne and N anions Aldol reactions laisen & Dieckmann Enamine acylations Wittig & variations 7 Michael reaction onjugate addition to α,β- unsaturated carbonyls arbene / carbenoid additions Simmons-Smith cyclopropanations Electrophilic Aromatic Substitutions Friedel rafts alkylation & acylations Diazonium with N 8

5 onjugated Systems systems containing conjugated double bonds, not just those of dienes, are more stable than those containing unconjugated double bonds -yclohexenone (more s table) 3-yclohexenone (less stable) Most significant are α,β-unsaturated carbonyl 9 1 1, vs 1,4-Addition H Nu 1, Addition KineticProduct RLi and RMgBr 3 4 1,4 Addition Thermodynamic Product R uli and RMgBr/u Nu 10

6 Simple Rxn oordinate Diagram Δ H is (-) e.g. an Sn Mechanism 11 A Two Step Mechanism: Δ H is (-) e.g. an Sn1 Mechanism 1

7 1, vs 1,4-Addition Addition of 1 mol of HBr to butadiene at -78 gives a mixture of two constitutional isomers: H = H- H= H HBr -78 1,3-Butadiene Br H Br H H = H- H- H H - H= H- H 3-Bromo-1-butene 90% (1,-addition) 1-Bromo--butene 10% (1,4-addition) these products can be explained by the following two-step mechanism 13 1,- and 1,4-Addition the key intermediate is a resonance-stabilized allylic carbocation: 14

8 90% 10% 15 Kinetic vs Thermodynamic ontrol 90% 10% Δ G o = Δ H o T ΔS o Gibbs Free energy For transition states: Δ G = Δ H T ΔS 16

9 Diels-Alder Reaction Diene Dienophile ycloadduct Dienophile is an alkene which is a reactant in the Diels-Alder reaction. 17 Diels-Alder Reaction Diels-Alder reaction: a cycloaddition reaction of a conjugated diene and certain types of double and triple bonds dienophile: diene-loving Diels-Alder adduct: the product of a Diels- Alder reaction 1,3-Butadiene (a diene) 3-Buten--one (a dienophile) Diels-Alder adduct 18

10 alkynes also function as dienophiles 1,3-butadiene (a diene) Et Et Diethyl -butynedioate (a dienophile) Et Et Diels-Alder adduct cycloaddition reaction: a reaction in which two reactants add together in a single step to form a cyclic product 19 the Diels-Alder reaction is shown in the following way: yclohexene products Diene Dienophile Adduct the special value of D-A D A reactions are that they (1) form six-membered rings () form new - bonds at the same time (3) are stereospecific and regioselective 0

11 Mechanism no evidence for the participation of either radical or ionic intermediates the Diels-Alder reaction is a pericyclic reaction Pericyclic reaction: a reaction that takes place in a single step, without intermediates, and involves a cyclic redistribution of bonding electrons 1 the conformation of the diene must be s-cis s-trans conformation (lower in energy) s-cis con formation (higher in en ergy)

12 (Z,4Z)-,4-hexadiene is unreactive in Diels- Alder reactions because nonbonded interactions prevent it from assuming the planar s-cis conformation methyl grou ps forced closer than allowed by van der Waals radii s-trans conformation s-cis conformation (lower energy) (higher energy) (Z,4Z)-,4-Hexadiene 3 reaction is facilitated by a combination of electronwithdrawing substituents on the dienophile and electron-releasing substituents on the diene 1,3-Butadiene Ethylene 00 pressure 140 yclohexene 1,3-Butadiene 3-Buten--one 30,3-Dimethyl- 1,3-butadiene 3-Buten--one 4

13 Electron-Releasing Groups -H 3, alkyl groups -R (ether) -R (ester) Electron-Withdrawing Groups -H (aldehyde, ketone) -H (carboxyl) -R (ester) -N (nitro) - N (cyano) Use these to activate the diene ERG Use these to activate the dienophile EWG 5 Stereochemistry of the Dienophile the configuration of the dienophile is retained H 3 H 3 H 3 A cis dienophile) H 3 Dimethyl cis-4-cyclohexene- 1,-dicarb oxylate H 3 H 3 H 3 A trans dienophile) H 3 Dimethyl trans-4-cyclohexene- 1,-dicarboxylate cis-alkene gives cis products 6

14 Stereochemistry of the Diene the configuration of the diene is retained: H 3 H 3 H H 3 H 3 H H 3 H 3 H H 3 H 3 H 7 Stereochemistry of the Diene The diene s substitutents are turned inside out 8

15 B A B D A D B rotates counterclockwise rotates clockwise This is called: Disrotation B π bond A D σ bonds 9 Summary 30

16 Summary 31