Download - Chapter 8
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