Dienes and Aromatic Compounds

•Molecular orbitals

•Reactivity Dienes (McM 14.2)

n: 1,2,3... non conjugated diene C C (CH2)n C C n=1: Skipped diene n>1: Isolated diene

C C C C conjugated diene

C C C sp cumulated diene (allene)

• Molecular orbitals of H2 (cf fig 1.20) - σ-bond Molecular orbitals of π-bond (cf fig. 1.21)

Energy π Antibonding MO Energy

H-H Antibinding MO (unfilled)

Node

H 1s orbital H 1s orbital

2 p orbitals H-H Binding MO (filled)

π Bonding MO

MO 1,3-butadiene π Molecular orbitals cf fig 14.2 Antibonding MO - 3 node Energy

Antibonding MO - 2 nodLUMOes Energy absorb UV spectra hν 1 e- 4 p orbitals Bonding MO - 1 node HOMO

delvis dobbeltbind. karakter 4 π electron Bonding MO - 0 nodes in 2 bonding orbitals (ground state) Electrophilic addition to conjugated dienes

H sec, allylic cation H H Br Br 1,2-add. prod. H-Br kinetic prod.

+ H

prim. cat. Br

1,2-add. prod. termodyn. prod.

Termodyn prod: High activation barrier, most stable prod. Vigorous cond. (high temp), reversible Kinetic prod: Low activation barrier, less stable prod. Mils cond, (low temp,) irreversible Polymerization of dienes

Natural and synthetic rubber •Radical polymerization Acid cat polymerization of 1,3-butadiene •Cationic polymerization

H H H H H

1,4-add Allylic cation Resonans stab. and so on

Hevea brasiliensis, * * tropical Americas. n Nattural rubber Cautchuc

Isoprene

* Gutta-percha n resin from the Isonandra Gutta tree (South east Asia)

Less elastic than nat. rubber Isolation under-water cables Little use today Nucleophilic 1,2- or 1.4 add.

O O O

Nu

Nu

O Nu O

H H

Nu Type of Nu - selectivity

O Nu HO 1,2-addition 1,4-addition

Dienes and cycloadd. (Diels Alder) - Chapt. 30 MO orbitals benzene - aromaticity (McM 15.4 - 15.5)

antibonding Energy

non-bonding

6 p orbitals

Degenerate orbitals node 2 at same energy level bonding

1,3,5-hexatriene Benzene only bonding MO shown Criteria for Aromaticity (Hückel) •(Monocyclic) ring •Planar •No of π-electrons in conjugation 4n+2 (n: 0, 1, 2,....)

Benzene: The 3 bonding MOs are filled Filled shell of MOs (cf. filled shell of atomic orbitals nobel gasses)

Less than 6 π-electrons: Half-filled orbital(s) - radical character

More than 6: Electron in antibonding orbitals Unstable, high-enegy species Ex. of an anti-aromatic compound - Cyclobutadiene

Energy antibonding

non-bonding

4 p orbitals

bonding

antibonding

non-bonding

Aromatic heterocyclic compds., see chapt. 28 bonding Frost circles / Frost devices (not in McM)

How to find rel. energies of MO for planar, cyclic, fully conjug. compds. (aromatic compds?) No math. involved

Benzene Energy

3 antibonding MO (2 degenerate)

3 bonding MO (2 degenerate)

Hexagon inside circle Vertex (corner) down

Molecular orbitals

Level of a non-bionding MO

Energy 6 π−electrons in 3 bonding MO Energy Benzene Cyclobutadiene 4 electrons 6 π electrons π

Diradical

Cyclopentadienyl anion Cyclopropenium cation 6 π−electrons 2 π−electrons

All π electrons in the bonding MO All π electrons in the bonding MO Tropylium cation 6 π−electrons Cyclooctatetraene 8 π−electrons

All π electrons in the bonding MO Diradical Electrophilic Aromatic Substitution and Substituent Effects (McM. 16.5 - 16.6)

E E E E Base E H H H

Aromaticity broken Resonanse satb. cationic intermediate End product aromatic

1. step ≈ 1. step in E-fil add. to alkene

•Halogenation (bromination) •Nitration •Sulfonation •Alkylation (Friedel Craft) •Acylation (Friedel Craft)

R R R influence: E •Reactivity E •Regiochemistry R R E E R: Electron donating

By resonance

ex. RO-, HO-, O-, R N-, RHN, H N-. RCOHN- 2 2 R O Y δ - R R R R δ - O O O O

δ -

o- and p-pos. Electron rich / nucleophilic

The ring is activated for E-fil ar subst More reactive than benzene R R O O OR OR OR E E E E Base E H H H

R O E Extra resonanse form H Better cation stabilisation / delocalisation E E E E Base E H H H O RO O RO RO R R

R O E Extra resonanse form H Better cation stabilisation / delocalisation

E E E E Base E H H H

OR OR OR OR OR

No extra stabilisation Reaction in o or p pos.

Y Activating, o/p directing R R R: Electron donating E E R The ring is activated for E-fil ar subst More reactive than benzene By inductive effect

ex. alkyls CH3 CH3 CH3 CH3 CH3 E E E E Base E H H H

Extra inductive stabilisation

E E E E Base E H H H

H3C H3C H3C H3C H3C

Extra inductive stabilisation

E E E E Base E H H H

CH3 CH3 CH3 CH3 CH3

No extra stabilisation

R Activating, o/p directing R R R: Electron withdrawing E E

By resonance effect:

-CN, -COR, COH, CO2H, CO2R, -NO2, -SO2R etc

Y X

δ + δ + R O R O R O R O

δ + o- and p-pos. Electron poor / less nucleophilic Especially electron poor in o/p pos

The ring is deactivated for E-fil ar subst Less reactive than benzene Least stable form

δ- R O R δ+ O R O R O R O E E E E Base E H H H

R O E H

Least stable form

E E E E Base E H H H R R R δ+ R R

O O O O O δ-

E H R O Deactivating, m directing

O O O O O E E E E E R R R R Base R H H H R: Electron withdrawing

R R E E

By inductive effect:

Alternative A:

-NR3 Deactivating, m directing

Alternative B: -Halogens Deactivating, but o/p directing!!

Inductively strong Electron donor by resonance withdrawing effect (lone pairs) weak effect Regiochemistry in E-fil aromatic subst of disubst. benzene derivs. (McM 16.7) OCH3 o/p directing OCH3 * * * * E

CN m-directing CN

OCH3 o/p directing OCH3 * * E * *

CH3 o/p-directing CH3 Resonance effects more powerfull than inductive effects

OCH3 o/p directing OCH3 OCH3 E * * * * +

OCH3 OCH * * OCH3 3 E o/p-directing

Sterical hindrance

Synth of trisubst. benzene deriv. - Planning of a good reaction sequence (McM 16.12) Nucleophilic Aromatic Substitution (McM 16.8 - 16.9)

E-fil Ar subst

R R

+ E E + H

Nucleophile

Nu-fil Ar subst

X Nu

+ Nu + X

Electrophile

X = Leaving group

More common on π-difficient heterocyles, see chapt 28 Not like SN2 (or SN1)

Nu-fil Ar subst

X Nu

+ Nu + X

Electrophile

X = Leaving group

Nu X X 2 2 ≈ SN1, not on sp C ≈ SN2, not on sp C

(but NB! diazotation, chapt. 24.8) Mechanisms:

X Nu X X X Nu •SNAr Nu Nu Nu

•SN1: Via diazonium salts (McM 24.8)

•Benzyne

•(SRN1: Involves radicals) •(VNS: Vicarious nucl. Subst.) •SNAr

X X X X Nu Nu Nu Nu Nu EWG EWG EWG EWG EWG

Only on electron defficient arenes (EWG o/p to X, Anion stabilizing effect)

(Aromatic heterocycles, McM 28.6)

1st step rate limiting (Aromaticity broken)

X=F>Cl>Br>I •Benzyne

H H Br NH 2 NH3 NH3 NH2

c.f. E2 Br ±H

“Triple bond” between sp2 C Br p-p overlap 2 2 H sp -sp overlap - weak bond Benzynes unstable / reactive intermed.

Br NH2 * NH H NH2 * 2 * * NH3 + c.f. E2 Br

14 Reactivity of benzynes: * C •Adds nucleophiles •Dienophile in Diels Alder react. Reactivity of aromatic side chains •Ox. of alkyl side chains •Halogenation benzylic position (c.f. Lab ex. 1)

•No ox. cleavage of “double bonds” in benzene •Reduction of benzene (hydrogenation) difficult