Reactions of Alkenes

Reactions of Alkenes Acid Catalyzed Hydration of Alkenes

alkene reacts with water in the presence of a strong acid catalyst

Markovniknov additionRearrangements possible

Conditions:acid catalyst (dilute H2SO4 or H3PO4)excess water

RR

Reactions of Alkenes Acid catalyzed hydration to form alcohols

suffers from several disadvantages: equilibrium process

some reactions favor the alkene, not the alcohol

many alkenes are insoluble in aqueous acid

side reactions possible:polymerizationrearrangement (carbonium ion intermediate)

Reactions of AlkenesExample: Predict the major product formed during acid catalyzed hydrolysis (hydration) of 3,3-dimethyl-1-butene.

Reactions of Alkenes Hydration by Oxymercuration-Demercuration

milder, more universal method for formation of alcohols from alkenes

two step process:(1) Hg(OAc)2 (aq) mercuric acetate(2) NaBH4 sodium borohydride

HHgOAc

Reactions of Alkenes Mechanism of Oxymercuration:

Step 1: Electrophilic attack to form mercurinium ion

Reactions of Alkenes Step 2: Water attacks from the back side to open the ring.

Step 3: Water removes a proton forming the organomercurial alcohol

Reactions of Alkenes Step 4: Reduction of the organomercurial alcohol by sodium borohydride

Reactions of Alkenes Oxymercuration-demercuration gives the

Markovnikov product.

Reactions of Alkenes Ring opening of the mercurinium ion

occurs from the back side: anti addition of nucleophile

Hg(OAc)+

+

H2O - H+

HH Hg

H H

OAc

H

HO H

Hg(OAc)

Hg(OAc)+

+

H2O

- H+

HH Hg

H H

OAc

H

HO H

Hg(OAc)

Hg(OAc)+

+

H2O

- H+

HH Hg

H H

OAc

H

HO H

H

(1) H2O

(2) - H+

NaBH4

H replaces the Hg(OAc) from the same side.

Reactions of Alkenes Alkoxymercuration-Demercuration

mercuration takes place in an alcohol solvent (instead of in water)alcohol serves as nucleophile, attacking the mercurinium ion

ether forms as product

Reactions of Alkenes Example: Predict the product formed in each of the following reactions:

D

1) Hg(OAc)2 (aq)2) NaBH4

D

1) Hg(OAc)2 CH3OH2) NaBH4

Reactions of Alkenes Hydroboration-oxidation

produces anti-Markovnikov alcohols.

Two step process(1) BH3

.THF(2) H2O2, OH-

syn addition

Reactions of Alkenes Hydroboration-oxidation occurs via syn

addition of BH3 to the double bond.

Oxidation of the borane to the alcohol occurs with retention of configuration giving the overall syn addition H and OH to the double bond.

BCH3HHH

H

CH3HBH2H H

CH3HOH

BCH3HHH

H

CH3HBH2H H

CH3HOHB

CH3HHH

H

CH3HBH2H H

CH3HOH

H2O2

OH-

Reactions of Alkenes Example: Predict the product formed in each of the following reactions.

B

(1) BH3-THF(2) H2O2, OH-

CH3HHH

H

CH3HBH2H H

CH3HOH

CH3

B

(1) Hg(OAc)2 (aq)(2) NaBH4

CH3HHH

H

CH3HBH2H H

CH3HOH

CH3

CH3H

OHH

Reactions of Alkenes Example: How would you accomplish the following transformation?

CH3

BrCH3

OH

This reaction must involve more than one step…i.e. there will be an intermediate product. You can work forward or backwards to determine its identity.

Notice: Product is anti-Markovnikov

Reactions of Alkenes Working forward:

The only reactions of alkyl bromides so far are substitution or elimination.

Substitution with OH- (E2 conditions) cannot occur at a 3o carbon and would give the alcohol on the wrong carbon anyway.

1st order conditions would give mixture.

Try elimination to form the Sayzteff product NaOCH2CH3

CH3CH2OH

CH3

Br

CH3

OH

CH3

Reactions of Alkenes Use hydroboration-oxidation to form anti-

Markovnikov alcohol.

NaOCH2CH3

CH3CH2OH

(1) BH3-THF

(2) H2O2, OH-

CH3

Br

CH3

OH

CH3

CH3

NaOCH2CH3

CH3CH2OH

CH3

Br

CH3

OH

CH3(1) BH3-THF(2) H2O2, OH-

CH3

OH

(1) BH3-THF(2) H2O2, OH-

CH3

OH

Overall synthesis:

Reactions of Alkenes Halogenation

X2 adds to C=C to form vicinal dihalides

Conditions:Cl2, Br2 or I2Solvent = CH2Cl2, CHCl3, or CCl4

Note: Iodine is not used often because diiodides decompose easily

Reactions of Alkenes Step 1: Formation of a halonium ion

Step 2: Opening of halonium ion

Anti addition

Reactions of Alkenes Example: Predict the product formed in the following reaction.

Br2CH2Cl2

Note: Decolorization of a Br2/CCl4 solution is a common test for the presence of an alkene.

Reactions of Alkenes Formation of Halohydrins

formation of an alcohol with a halogen on the adjacent carbon atom

Conditions: Cl2, Br2, or I2 H2O as solvent

Br2CH2Cl2

CX2H2O

CC C

OH

X

Reactions of Alkenes Formation of halohydrins:

Anti stereochemistry

Markovnikov productOH on the more substituted carbon

Reactions of Alkenes Example: Predict the product of the following reaction.

Br2H2O

CH3

Reactions of Alkenes Catalytic Hydrogenation of Alkenes

formation of an alkane by addition of H2 across the double bond in the presence of a catalyst

Common catalysts:Pt, Pd, Ni (heterogeneous)(Ph3P)3RhCl (Wilkinson’s catalyst-

homogeneous)

Reactions of Alkenes Hydrogenation occurs at the surface of

the metal catalyst via a syn addition of H2. H2 adsorbs onto the catalyst surface Catalyst weakens the H-H bond bond of alkene adsorbs onto catalyst H2 inserts into bond

Reactions of AlkenesExample: How would you accomplish the following transformation?

CH3

OH

CH3

CH3

CH3

Reactions of Alkenes Formation of Cyclopropanes

addition of a carbene to a C=C bond

Possible Reagents:CH2N2/h or CH2N2/heatCH2I2/Zn/CuCl (Simmons-Smith Reaction)CHX3/NaOH (or KOH)/H2O

gives dihalocyclopropane

Reactions of Alkenes Examples:

CH2N2

h

CH2I2

Zn/CuCl

CH2I2

Zn/CuCl

CHBr3

KOH/H2OBr

Br

Simmons-Smith Reaction

Reactions of Alkenes Epoxidation of Alkenes

formation of a three membered cyclic ether by the reaction of an alkene with a peroxyacid

Concerted reaction Retains stereochemistry present in the

alkene

Reactions of Alkenes Common peroxyacids:

m-chloroperoxybenzoic acid (MCPBA)

peroxybenzoic acid

peroxyacetic acid

COOHO

Cl

COOHO

CH3COOH

COOHO

O


H3CCH3 MCPBA

CH2Cl2

O

H3C CH3

H3C CH3 CH2Cl2MCPBA

O

H3C CH3

Draw the epoxide ring IN THE PLANE

unless it’s fused to a

larger ring.

Reactions of Alkenes Hydrolysis of Epoxides

formation of “anti” 1,2-diols (glycols) by acid catalyzed addition of water to the epoxide

Reactions of Alkenes Depending on the conditions chosen for

epoxidation, the reaction may produce either the epoxide or the glycol. Epoxide formation

MCPBA in CCl4 (or CH2Cl2 or CHCl3)CH3CO3H

Glycol formationperoxyacetic acid/H2OMCPBA/CH2Cl2 then dilute H+ (aq)


MCPBACH2Cl2

O

CH3CO3HH3O

+

OH

OH

transanti addition

H2O

O

Reactions of Alkenes Syn Hydroxylation of Alkenes

formation of 1,2-diol (glycol) syn stereochemistry

Common Reagents:OsO4/H2O2 cold, dilute KMnO4/OH- (aq)o

r


C OsO4

H2O2

KMnO4, OH-

(cold, dil.)

C

H

H

CH3

CH3

HH

OHOH

CH3

CH3

OH

OH

C OsO4

H2O2C

H

H

CH3

CH3

HH

OHOH

CH3

CH3

Note: Decolorization of dilute KMnO4 is a common test for the presence of alkenes.

H3C CH3

HO OH

H H

Reactions of Alkenes Oxidative Cleavage of Alkenes

formation of a mixture of ketones and/or carboxylic acids by cleavage of the C=C of an alkene with warm, concentrated KMnO4

Mixture of ketone and carboxylic acid is formed in this example.

Intermediate aldehyde is oxidized further.

Ketone cannot be oxidized further.


Reactions of Alkenes Ozonolysis of Alkenes

formation of a mixture of aldehydes and ketones by oxidative cleavage of the C=C of an alkene using ozone (O3) and (CH3)2S.

Milder conditions prevent further oxidation of the aldehyde.

Reactions of Alkenes Example: Give the product of the following reaction.

(1) O3

(2) Me2S

Reactions of Alkenes Example: Show how you would accomplish the following syntheses.

(1) O3

(2) Me2S

Br

BrCH3

(1) O3

(2) Me2S

Br

BrCH3

CH3

Br

BrCH3

Br

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Reactions of Alkenes