Conjugated Dienes and U.V. Spectroscopy
Some Dienes
Some Conjugated Molecules
Prevents DNA from Unraveling
Y1 and Y2 for Ethylene
Orbital Depiction of Ethylene
Orbital Depiction
DHhydrogenation
Carbocation is Resonance Stabilized
Carbocation Stability
Conjugate Addition of HBr
1,2- vs. 1,4-Addition
Kinetic and Thermodynamic Products
Kinetic vs. Thermodynamic Control
2 Different 1,4- Products Form
Thermodynamic Products
Addition to 1,3,5-Hexatriene
At 0oC 1,2-addition is favored
At 40oC, 1,4-addition is preferred
Form more stable carbocation
Allylic Bromination With NBS
Free Radical Mechanism
Mechanism involves the gradual generation of Br2
Resonance stabilization leads to possibly several products
Diels-Alder Reactiona Pericyclic Reaction
Dienophiles Must be Electron Deficient
Stereochemistry
Bicyclic Adductsendo and exo
Endo Addition
Endo Rule
Identify the Diene and Dienophile that gives the Adducts
Two Phosphatase Inhibitors
A Radiosensitizer
Synthesis of LS-5
1,3-Butadiene and EthyleneMolecular Orbitals
Proper Orbital Overlap
Overlap Must be Constructive
Unsymmetrical Diels-Alder Reactions
Lewis Acid catalysis in a Diels Alder Reaction
2o Intermediate is Preferred
Stereochemistry in endo Addition
Electrocyclic Reaction in Anti-Tumor Agent
Diradical Removes H atoms from Deoxyribose in Cancer Cell
[2+2] Cycloaddion is Forbidden
Photochemical [2+2] CycloadditionNeed to Irradiate
Suprafacial Allowed
Antarafacial - Forbidden
[4+2] is Suprafacial, hence Allowed
Cycloaddition Reactions can be Planned Based on the Number of Pairs of e-s that Move
Formation of Cyclobutanes
A [4+6] Cycloaddition(odd # pairs of e-s move)Thermally Allowed
Retro Diels-Alder, followed by Diels-Alder
Intramolecular photochemical[2+2] cycloaddion
Ultraviolet Spectroscopy
Conjugated Dienes Absorb Energy in UV Region
UV Spectrum of Isoprenelmax = 222 nm
Conjugation lowers the energy differences between the HOMO and LUMO energy levels, and so conjugated dienes absorb at longer wavelengths than isolated dienes, and trienes absorb at longer wavelengths than dienes, etc.
Alkyl group substitution on double bonds also causes absorption to occur at longer wavelengths.
HOMO-LUMO Gap lessens with increased conjugation
Woodward-Fieser Rulesfor Predicting lmax
Predict lmax in U.V. Spectrum
Determination of lmax
Determination of lmax
Determination of lmax