Conjugated Dienes and U.V. Spectroscopy

Some Dienes

Some Conjugated Molecules

Prevents DNA from Unraveling

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 Reaction a Pericyclic Reaction

Dienophiles Must be Electron Deficient

Stereochemistry

Bicyclic Adducts endo and exo

Endo Addition

Identify the Diene and Dienophile that gives the Adducts

Two Phosphatase Inhibitors

A Radiosensitizer

Synthesis of LS-5

1,3-Butadiene and Ethylene Molecular 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

Photochemical [2+2] Cycloaddition Need 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

Retro Diels-Alder, followed by Diels-Alder

Intramolecular photochemical [2+2] cycloaddion

Adjacent Thymines (T’s) in DNA Susceptible to [2+2] cycloaddition in sunlight (UV-B, 290-320 nm)

Ultraviolet Spectroscopy The absorption of a 171-nm photon excites an electron from the p bonding MO of ethylene to the p* antibonding MO.

Conjugated Dienes Absorb Energy in UV Region

UV Spectrum of Isoprene lmax = 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 Rules for Predicting lmax Double bond extending conjugation 30 nm Exocyclic double bond 5 nm Alkyl group Cl, Br OH, OR 6 nm SH, SR NH2, NHR, NR2 60 nm  

Predict lmax in U.V. Spectrum

Determination of lmax

Determination of lmax

Determination of lmax