Conjugated Unsaturated Systems 46 Chapter 13 allylic substitution & allylic radicals allylic bromination sabitility of allylic radicals allylic cations.

Conjugated Unsaturated SystemsConjugated Unsaturated Systems

46

Chapter 13Chapter 13

allylic substitution & allylic radicalsallylic brominationsabitility of allylic radicalsallylic cationsresonance theory - detailed (recall chapter 1 info)alkadienes, polyunsaturated hydrocarbons1,3-butadiene, resonance delocalizationstability of conjugated dieneselectronic attack on conjugated dienes, 1,4-additiondiels-alder rx, 1,4-cycloaddition

Modified from sides of William Tam & Phillis Chang

IntroductionIntroduction

conjugated system at least one p orbital adjacent to one (or more) π bond

e.g.

Allylic Substitution vs Allyl Radical

vinylic carbons

(sp2)

allylic carbon (sp3)

mechanisms?

Radical chain reactionChain propagation (r.d.s.)

Addition rx

Allylic Bromination Allylic Bromination with with NN--BBromo-romo-SSuccinimide uccinimide ( (NBSNBS))

NBS (a solid insoluble in CCl4)

low concentration of Br•

Examples

Resonance of Allyl RadicalsResonance of Allyl Radicals

Allyl Cation (recall SN1 intermediate)

Relative order of carbocation stability

Rules for Writing Resonance StructuresRules for Writing Resonance StructuresResonance structures don’t exist But structures allow predictive description of

molecules, radicals, & ions for which a single Lewis structure is inadequate

Connect resonance structures by ↔

The hybrid (combined “weighted” avg.) of all resonance structures represents the real substance

Resonance theory

writing resonance structures move only electrons

resonance structures

not resonance structures

All structures must be proper Lewis structures

10 electrons!Xnot a proper

Lewis structure

All resonance structures must have the same number of unpaired electrons

X

All delocalized atoms of the π-electron system must lie roughly in a plane

A system described by equivalent resonance structures has a large resonance stabilization

The energy of the hybrid is lower than the energy estimated for any contributing structure

The more stable a contributing structure the greater its contribution to the hybrid

Estimating Relative Stability of Resonance Estimating Relative Stability of Resonance StructuresStructures

The more covalent bonds a structure has, the more stable it is

Structures in which all of the atoms have a complete valence shell of electrons (“octets”) make larger contributions to the hybrid

this carbon has6 electrons

this carbon has 8 electrons

Charge separation decreases stability

Alkadienes and Polyunsaturated Hydrocarbons

Alkadienes (“Dienes”)

Alkatrienes (“Trienes”)

Alkadiynes (“Diynes”)

Alkenynes (“Enynes”)

Cumulenes

Conjugated dienes

Isolated double bonds

1,3-Butadiene: Electron Delocalization

Bond Lengths of 1,3-Butadiene Bond Lengths of 1,3-Butadiene

1.34 Å

1.47 Å

1.54 Å 1.50 Å 1.46 Å

sp3 sp3spsp3sp2

Conformations of 1,3-ButadieneConformations of 1,3-Butadiene

cis

transsinglebond

singlebond

The Stability of Conjugated Dienes

Conjugated dienes are thermodynamically more stable than isomeric isolated alkadienes

Electrophilic Attack on Conjugated Dienes: 1,4 Addition

Mechanism

(a)

(b)

Kinetic versus Thermodynamic Control Kinetic versus Thermodynamic Control of a of a Chemical ReactionChemical Reaction

Diels–Alder Reaction: 1,4 Cycloaddition Reaction of Dienes

e.g.

diene dieophile adduct

Factors Favoring the Diels–Alder RXFactors Favoring the Diels–Alder RX

Types A and B are normal Diels-Alder rxs

Types C and D are Inverse Demand Diels-Alder rxs

Relative rate

Steric effects

Stereochemistry of the Diels–Alder RXStereochemistry of the Diels–Alder RX

Stereospecific: syn addition andthe dienophile configuration is retained in the product

The diene reacts in the s-cis conformation (s-trans can’t cycloadd)

X

e.g.

(diene locked s-cis)

Cyclic dienes with the double bonds s-cis are usually highly reactive in the Diels–Alder rx, e.g.

The Diels–Alder rx occurs primarily in an endo fashion

longest bridge R is exo

R is endo

DA rx can form bridged structures

Alder-Endo RuleFor dienophiles with activating groups having π bonds, the ENDO orientation in the t.s. is preferred

endo

exo

e.g.

= =

Stereospecific reaction

Stereospecific reaction

Sterics

Diene A reacts 103 times faster than diene B

Examples

Rate of Diene C > Diene D (27 times) tBu group electron donating group and favors s-cis diene end

End

Examples - steric effects

Two tBu groups cannot adopt s-cis conformation