electrophilic addition to alkenes. addition of h-x to the carbon- carbon double bond:...
TRANSCRIPT
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
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)
H
H
NOT FORMED(less stable primary radical)
H
Br
H
Br
H 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
CH3
CH3
Oxymercuration-Demercuration: A milder method for hydration of an alkene
R2
R3
R4
R1
Hg(OAc)2
H2OR2
R1 R3
HO HgOAc
R4NaBH4
R2
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
HO
Hgo+
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
Me
H
OHH
(oxidation of C-B bondwith retention of configuration)
R B
R
R
O O
H
R
B
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
R4
R 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
O
O
O
R2
R1
R3
R4
O
O
O
O
O
O R4
R3
R2
R1
S
Me
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)