Using curved arrows, propose a mechanism for this transformation:

Unsaturated Hydrocarbons (Part II)

1. Electrophilic addition reaction 1.1 Halogenation 1.2 Hydrohalogenation 1.3 Hydration of double or triple bonds Acid catalyzed Oxymercuration/demercuration Hydroboration/oxidation 1.4 Halohydrin formation 1.5 Addition of Carbenes to Alkenes 2. Reduction of double or triple bonds 2.1 Hydrogenation of alkene 2.2 Reduction of alkyne 3. Oxidation of Alkenes: 3.1 Hydroxylation 3.2 Ozonlysis Homework: McMurry 7.49, 7.54, 7.57

1.3 Hydration of double or triple bonds Acid catalyzed Hydration of an alkene is the addition of H-OH to to give an alcohol. Acid catalysts are used in high temperature industrial processes: ethylene is converted to ethanol Markovnikov Rule!

1.3.2 Oxymercuration/demercuration For laboratory-scale hydration of an alkene: use mercuric acetate in THF followed by sodium borohydride  Electrophile: Hg(OAc) 2  Orientation of ring opening  Reduction of C-Hg bond  Markovnikov hydration product

When the reaction was carried in alcoholic solvents:

When the substrate is triple bond:  Inert to acid catalyzed hydration  Triple bond is activated by Hg 2+  Tautomerism occurs  No reduction step  Markovnikov hydration product How does mercury ion catalyze the reaction? Detail mechanism see: Page 252

1.3.3 Hydroboration/oxidation Structure of borane: Lewis AcidLewis Base  Six electrons in outer shell  Coordinates to oxygen electron pairs in ethers

Hydroboration reaction:  Herbert Brown (HB) invented hydroboration (HB)  Borane (BH 3 ) is electron deficient, is a Lewis acid  Borane adds to an alkene to give an organoborane

 Addition of H-BH 2 (from BH 3 -THF complex) to three alkenes gives a trialkylborane  Oxidation with alkaline hydrogen peroxide in water produces the alcohol derived from the alkene

Add to double bond (orientation): Oxidation trialkylborane under basic condition (mechanism):

Regioselective reaction: Stereoselective reaction: Why?  Hydration of alkene  Anti-Markovnikov Hydration product  Syn-addition product

Why regioselective?  In a single step, concerted reaction  Lower potential energy of transition state is favored  Syn-addition in this step  Anti-Markovnikov hydration product

Why stereoselective?  H 2 O 2, OH - inserts OH in place of B  R migration, retains syn orientation

When the substrate is symmetrical triple bond: When the substrate is terminal triple bond:

Comparison of oxymercuration/demercuration and hydroboration/oxidation: To alkene: To alkyne:

1.4 Halohydrin formation  This is formally the addition of HO-X to an alkene to give a 1,2- halo alcohol, called a halohydrin  The actual reagent is the halogen (Br 2 or Cl 2 in water in an organic solvent)

Reaction: Mechanism: Transition state:

An Alternative to Bromine Bromine is a difficult reagent to use for this reaction N-Bromosuccinimide (NBS) produces bromine in organic solvents and is a safer source

1.5 Addition of Carbenes to Alkenes The carbene functional group is “half of an alkene” Carbenes are electrically neutral with six electrons in the outer shell They symmetrically across double bonds to form cyclopropanes

Formation of Dichlorocarbene Base removes proton from chloroform Stabilized carbanion remains Unimolecular Elimination of Cl - gives electron deficient species, dichlorocarbene

Reaction of Dichlorocarbene Addition of dichlorocarbene is stereospecific cis

Simmons-Smith Reaction Equivalent of addition of CH 2 : Reaction of diiodomethane with zinc-copper alloy produces a carbenoid species Forms cyclopropanes by cycloaddition

2. Reduction of double or triple bonds 2.1 Hydrogenation of alkene  Catalyst: Pt, Pd, Ni  Pd/C: Palladium on carbon  Raney Ni: Al-Ni reacts with NaOH  Heat of hydrogenation: higher ∆H, higher potential energy of alkene  Syn addition Why syn addition?

Mechanism of hydrogenation:

2.2 Reduction of alkyne  Alkene is more reactive than alkyne;  Lindlar catalyst: Pd/CaCO 3  Syn addition product: cis-alkene How to get trans-alkene?

Conversion of Alkynes to trans-Alkenes Anhydrous ammonia (NH 3 ) is a liquid below -33 ºC –Alkali metals dissolve in liquid ammonia and function as reducing agents Alkynes are reduced to trans-alkenes with sodium or lithium in liquid ammonia The reaction involves a radical anion intermediate

Mechanism of dissolved metal reduction:

3. Oxidation of Alkenes: 3.1Hydroxylation Hydroxylation adds OH to each end of C=C Catalyzed by osmium tetraoxide, then sodium bisulfate Stereochemistry: syn-addition Product is a 1,2-dialcohol or diol (also called a glycol) Via cyclic osmate di-ester

With cold, dilute KMnO 4 :  Hydroxylation reaction,  Product is diol;  Syn addition;  Via cyclic intermediate

With concentrated KMnO 4, or under acid condition:  Oxidative cleavage reaction

When the substrate is alkyne, either in dilute or concentrated solution:  Oxidative cleavage reaction

3.2 Ozonlysis

Ozone, O 3, adds to alkenes to form molozonide Reduce molozonide to obtain ketones and/or aldehydes Reductent: Zn, dimethyl sulfide Mechanism:  Used in determination of structure of an unknown alkene

Cleavage products reveal an alkene’s structure

Industrial oxidation of alkene produces epoxy….. When the substrate is alkyne:

1. Electrophilic addition reaction 1.1 Halogenation 1.2 Hydrohalogenation 1.3 Hydration of double or triple bonds Acid catalyzed Oxymercuration/demercuration Hydroboration/oxidation 1.4 Halohydrin formation 1.5 Addition of Carbenes to Alkenes 2. Reduction of double or triple bonds 2.1 Hydrogenation of alkene 2.2 Reduction of alkyne 3. Oxidation of Alkenes: 3.1 Hydroxylation 3.2 Ozonlysis Summary