Electrophilic Addition to Alkenes

Addition of H-X to the Carbon-Carbon Double Bond:Markovnikov’s Rule

H3C

H3C CH3

H

H-ClH3C

H3C CH3

H

Cl H

H3C

H3C CH3

H

H Cl

NOT FORMED

or

In its original form, Markovnikov’s rule states that, during the addition of HX to a C=C, the hydrogen atom goes to the side of the alkene which already possesses the most hydrogens.

H3C

H3C CH3

H

H-ClH3C

H3C CH3

H

Cl H

H3C

H3C CH3

H

H Cl

NOT FORMED

or

This is referred to as the ‘regiochemistry’ of the addition reaction (i.e. which ‘region’ of the double bond does the H and the Cl add to).

Mechanistic Explanation for Markovnikov’s Rule

H3C

H3C CH3

H

H+

H3C

H3C

CH3

H

H

FORMED(MORE stable tertiary carbocation)

:Cl-

H3C

H3C CH3

H

HCl

H3C

H3C CH3

H

H+

H3C

H3CCH3

H

NOT FORMED(LESS stable secondary carbocation)

:Cl-

H3C

H3C CH3

H

ClH

NOT FORMED

FORMED

Caveat: Under certain conditions, addition of H-Br (but not H-Cl or H-I) gives the opposite regiochemistry… Why?

HBrBr

Bror

HBrBr

Bror

HBrBr

Bror

HBrBr

Bror

Benzoyl Peroxide Ascaridole

Morris S. Kharasch

LINK

Mechanistic Reason for Effect of Peroxides on the Regiochemistry of Addition of H-Br to the alkene

RO OR 2 RO

Half headed arrow = movement of one electron (homolysis)

RO H Br ROH + Br

BrH

Br

FORMED(more stable secondary radical)

HH

NOT FORMED(less stable primary radical)

H

Br

H

BrH Br

BrH H

Br+

Likewise, in the presence of very strong acid (non-nucleophilic anions), water, alcohols, and carboxylic acids can add across double bonds. The reaction is often used to form tert-butyl esters of carboxylic acids as shown.

H3C

H3C CH3

H

H2SO4H3C

H3C CH3

H

RO HROH

RO

O H CH3H3C+

(Isobutylene)

H2SO4

RO

OCH3

CH3CH3

Oxymercuration-Demercuration: A milder method for hydration of an alkene

R2

R3

R4

R1Hg(OAc)2

H2OR2

R1 R3

HO HgOAc

R4 NaBH4R2

R1 R3

HO H

R4

H3C

CH3

H

H3CHg(OAc)2

H2O H3C

CH3

H

H3C

HgOAc

AcO-

OHH

H3C

CH3

HH3C

HgOAc

AcOH

HO

+

H3C

CH3

HH3C

HgH

HONaBH4 Reductive Elimination

H3C

CH3

HH3C

H

HOHgo+

Mechanism of Oxymercuration-Demercuration

Notice that Markovnikov’s Rule is followed

Use of Oxymercuration-Demercuration

Hydroboration-Oxidation:Anti-Markovnikov Addition of Water to an Alkene

H3C

CH3

H

H3C

H

CH3

HH3C

BR2

H3CHBR2 H2O2

NaOHH

CH3

HH3C

OH

H3C

HBR2 often equals complexes of borane (BH3) with THF, or with dimethylsulfide (Me2S).

Also, HBR2 may equal dialkylboranes, which tend to give higher regioselectivity.

One commonly used dialkylborane is 9-borabicyclononane (9-BBN), which is readily available from the hydroboration of 1,5-cyclooctadiene.

In the following slides, notice that the H and the OH group are added to the double bond from the same face.

Me R2BHMe

H

BR2syn addition ofB-H bond to alkene

H

NaOH

H2O2

MeH

OHH

(oxidation of C-B bondwith retention of configuration)

R BR

RO O

H

RB

RO

RH2O

ROH

+ HO-

R2

R3

R4

R1

Br Br+ R2

R1 R3

Br Br

R4

Br2 reacts rapidly with most alkenes, leading to vicinal dibromides

R2

R3

R4

R1

Br+ Br-

R2

R3

R4

R1 Br

Br-

R2

R1 Br

Br R4

R3

To a first approximation, the reaction be mechanistically regarded as an electrophilic attack of a highly polarized bromine molecule on the double bond to produce an intermediate carbocation as shown

However, the observed trans geometry of addition to cyclic species (see following slides) suggests that the intermediate carbocation is actually a bridged species.

Br-

Br+ Br-

Br+

H

H

Br

Br

trans-1,2-dibromocyclohexane

inversion

This intermediate bridged bromonium ion can also be intercepted by water and alcohols to form bromohydrin derivatives as shown.

ROH

Br+ Br-

Br+

H

H

Br

OR

inversion

-HBr

Br-

Hydrogenation of Alkenes

R2

R1 R3

R4 R2

R1R3

R4

H2

catalyst

H H

The most commonly used catalysts are heterogeneous (do not dissolve) and include Pd, Ni, and Pt. Often the metals are deposited on a support, like carbon.

Epoxidation of Alkenes

R2

R1 R3

R4R O

O

OH

+

R2

R1 R3

R4

O

R2

R1 R3

R4

O

+

One of the most commonly employed epoxidizing agents is m-chloroperoxybenzoic acid (mCPBA, shown at right)

Chiral Epoxidation

Allylic Alcohol

Chiral Intermediate for Sharpless Epoxidation

Dihydroxylation of Alkenes

R2

R1 R3

R4

cat. OsO4

(stoichiometric oxidant)R2

R1 R3

R4

OH OH

R2

R1 R3

R4

OH OH

+

A commonly utilized oxidant is N-methylmorpholine-N-oxide (NMO), shown to right.

Mechanism of Dihydroxylation with Osmium Tetroxide

Sharpless Asymmetric Dihydroxylation

Dihydroxylation of Alkenes is also possible with potassium permanganate (KMnO4).

R2

R1 R3

R4

KMnO4

H2OR2

R1 R3

R4

OH OH

R2

R1 R3

R4

OH OH

+

R2

R1 R3

R4

1) O3

2) Me2SO

R2

R1

O

R3

R4

+

Treatment of an alkene with ozone (O3), followed by a reducing agent (dimethyl sulfide), cleaves the double bond down the center, as shown below.

Structure of Ozone

Ozonolysis Mechanism

R2

R1 R3

R4

O

O

O

R2

R1 R3

R4

OO

O

R2

R1

R3

R4

O

O

O

O

OO R4

R3

R2

R1

SMe

Me

R2R1

O

R3

R4

O

Me2S

O

+ R3

R4

O

SMe

O

Me

+

Cationic Polymerization

H

H CH3

CH3

H+

H

H CH3

CH3H

H

H CH3

CH3

H

H CH3

CH3H H

H CH3

CH3

H

H CH3

CH3

and so on(polyisobutylene)