Alkenes and Alkynes IIAddition Reactions

Part 2

8.6 Alcohols From Alkenes through Oxymercuration-Demeruration: Markovnikov AdditionTwo steps mechanism that is useful to avoid

rearrangement Both reaction take place very rapidly at RT or

below+ H2O + Hg

OCCH3

O

HgOH OCCH3

O + CH3COH

O

+ OH + NaBH4

HOH

+ Hg +CH3CO

O

HgOH OCCH3

O

Step 1: Oxymercuration

Step 2: Demercuration

Regioselectivity of Oxymercuration-DemercurationThe orientation of the addition of the

elements of water, H– and –OH, is in accordance with Markovnikov’s rule

H

R

H

H

1) Hg(OAc)2/THF H2O

2) NaBH4, -OHR H

HH

OH H

+HO H

1-pentene

THF H2O

Hg(OAc)2

OH

HgOAc NaBH4, -OHCH2

OH

H

1-methylcyclopentene

THF H2O

Hg(OAc)2

OH

HgOAc

H

NaBH4, -OH

OH

H

H

(15 sec)(1h)

(20 sec)6 min

1-methylcyclohenxanol

2-pentanol

Hg(OAc)2 HgOAc + OHAc

+ HgOAcHgOAc

3,3-Dimethyl-1-butene Mercury-bridge carbocation

In this carbocation, the positive charge is shared between the 2o (more substituted) carbon atom and mercury atom.

Mercuric acetate dissociate to form a HgOAc cation and an acetate anion

Step 1

HgOAc

O

H

H

HgOAc

O

HH

A water molecule attacks carbon of the bridge mercuration ion that is better able to bear the partial positive charge

Step 2

HgOAc

O

HH O

H

H

HgOAc

OH

An acid-base reaction transfers a proton to another water molecule

(Hydroxylalkyl)mercury compound

Step 3

exampleStarting with an appropriate alkene, show all

steps in the synthesis of 2-methyl-2-propanol

ExampleConsider the following reaction

Outline a likely mechanism for the solvomercuration step of this ether synthesis

Show how you would use solvomercuration-demercuration to prepare tert-butyl methyl ether

Why would one use Hg(OCCF3)2 instead of Hg(Oac)2Hg(O2CCF3)/THF ROH

solvomercuration

OR

HgO2CCF3

OH

H

NaBH4, -OH

dermercuation

Answer for cThe electron-withdrawing fluorine atoms in

mercurc trifluoroacetate enhance the electrophilicity of the cation. Experiments have demonstrated that for the preparation of tertiary alcohols in satisfactory yields, the trifluoroacetate must be used rather than the acetate

8.7 Alcohols from Alkenes through Hydrocarboration-Oxidation:Anti-Markovnikov Syn HydrationAddition of water is indirect and two reactions are involved

1. addition of a boron atom and hydrogen to C= CHydrolysis of the alkylborane intermediate to an alcohol

and boric acid

BH3:THF

hydroboration B3

H2O2/-OH

oxidationOH

1-propanolpropene

CH3

H

1) BH3:THF

H2O2/-OH2)

CH3

H

OH

H

8.8 Hydroboration: Synthesis of AlkylboranesPreparation

+ H B

alkene Boron HydrideBH

alkyl borane

MechanismIn each addition step, the boron atom

becomes attached to the less subsituted carbon atom of the double bondA hydrogen atom is transferred from the boron

atom to the other carbon atom of the double bond

moresubstitutedcarbon

less substitutedcarbon

H BH2

+

B

H

H

H2nd equiv

B

HH

H

H

Con’tAs this transition state is approached, electrons

shift in the direction of the boron and away from the more substituted carbon atom of the double bondThe more substituted carbon bears an electron-

releasing alkyl group, it is better able to accommodate this positive charge

1% 99% 98% 2%

These percentage, indicating where boron becomes attached in reactions using these starting materials,illustrate the tendency for boron to bond at teh less substituted carbon of the double bond

H

H3C H

H+

H B

H

H

H

H3C H

H

H B

H

H

H

H3C H

H

H B

H

H

H3C H

H B

Four-atomconcerted transition state

H3C H

H B H

H

Syn addition of H and B

T.S

complex

the reaction takes place in complex, which changes into a cylicfour carbon transition state with the boron less hindered carbon atomthe dash line results in syn addition of hydrogen and boron group

ExampleStarting with an appropriate alken, show the

synthesis of tributyl borane

8.9 Oxidation and Hydrolysis of AlkylboranesThese reactions are occurred in the same

vessel by the addition of hydrogen peroxide in an aqueous base

R3BH2O2, aq. NaOH 25o

oxidation and hydrolysis3 R OH + B(ONa)3

B

R

R

R + O O H

R

BR O

R

HO

unstable intermediateTrialkyl borane

hydroperoxideion

R

B

R

O R

+ O H

Repeated

sequence twice

R O

B

OR

O R

Borate ester

Oxidation of Trialkylboranes

hydrolysis of the Borate ester

R O

B

OR

O R

Boron ester+ O H

O

B

O O

OR

R H

R

an alkoxide aniondeparts fromt eh borate anionreducing the formal charge onboron to zero

R

B

R

O H

O R

R

B

R

O O R

H

alcohol

ExampleStarting with the appropriate alkene, show

how you could use hydroboration-oxidation to prepare each of the alcohol1-pentanol2-methyl-1-pentenol