Post on 18-Jul-2015
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Elimination ReactionsElimination ReactionsE1 Reaction E2 Reaction
Weak nucleophile Strong nucleophile
General Mechanism General Mechanism
Most commonly used bases:
E1 Reaction: Stereochemistry
Substitution predominates largelyAndElimination is a minor process!
Zaitsev’s Rule is followed here, strictly!
E2 Reaction: Stereochemistry
Syn Elimination: Uncommon Anti Elimination: Very common
Example:Example:
The requirement of trans relationship
E2 Reaction: Direction of Elimination
Zaitsev’s rule: More substituted olefin is favored! Hofmann’s rule: Least substituted olefin is favored!
Reasons: Acidity and Sterics!
Steric and Electronic Factors in Eliminations:
More conjugatedand not More substituted!
Elimination Reactions: Synthetic Applications
Dehydrohalogenations:
Dehydrations:Oxidations: Loss of H2
Competition Between Elimination and Substitutions
SN2 versus E2N
• A good base/nucleophile (avoids SN1) favors SN2/E2• Sterically bulky base can favor E2 (pentacoordinated TS not possible)!
All SN2 or E2!
Tertiary, yet E2 with a strong base/nucleophile!
All SN2 or E2!
• Strong base/nucleophile always promotes elimination anda weaker base/nucleophile always yields substitution products prediminantly!
Competition Between Elimination and Substitutions
SN1 versus E1N
CH3L: No -carbon, exellent substrate for SN2, no SN1 possible, elimination cannot occur!
RCH2L: Less sterics, exellent substrate for SN2, but predominantly E2 with a strong base!2 , , p y g
R2CHL: SN2 with CH3COO-, CN-, RS-, etc.)E2 with OH-, OR-, DBU, DBN, etc.In a polar solvent: SN1 and E1 in the absence of a good nucleophile or base
R3CL: No SN2, no SN1 if substittuion is needed (no base!)E cellent Elimination ith a strong base b E2!Excellent Elimination with a strong base by E2!
Addition of Oxygen to Olefins: Epoxidation
Dih d l ti f Ol fiDihydroxylation of Olefins:
Trans‐dihydroxylation
Cis‐dihydroxylation
Reduction of Alkynes
Lindlar Catalyst: Pd/CaCO3 or BaSO4 deactivated with Lead Tetraacetate and QuinolineLindlar Catalyst: Pd/CaCO3 or BaSO4 deactivated with Lead Tetraacetate and Quinoline
Trans addition of H2Trans addition of H2
Industrial Synthesis of Vitamin A by Hoffmann-LaRoche Pharmaceutical Company
Involves reduction (Lindlar) followed by thermal isomerization!