conjugated unsaturated systems 46 chapter 13 allylic substitution & allylic radicals allylic...
TRANSCRIPT
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