ADDITION
Many reagents add to the alkene bond. The reactions are called electrophilic
additions and thus a carbocation is formed on a carbon atom. The reaction is regioselective
as controled by the formation of the more stable carbocation. The reactions follow
somewhat complex kinetic expressions showing that a simple mechanism is not being
followed. For example, below possibly two molecules of the reagent are reacting with the
substrate in the rate determining step.
X-Y
C=C
X
X-C-C-Y
C-C Y
rate = k [alkene][XY] + k[alkene][XY][Y-] + k[alkene][XY] 2
C=C
X-Y
X-Y
Additions of HCl, HBr
The ionic addition of HX produces the more stable carbocation which then reacts
with the bromide ion. The result is that the bromide is attached to the more substituted part
of the alkene. This stereoselective reaction is called Markovnikov addition.
H
HBr
-
Br
or HBr
HBr
HOAc
Br
OAc
Br
H
H
H
HBr
H
CH3
CH3
H
CH3
C=C
Ph
Br
Br
HBr
H
Ph
H
H
CH3
H
The addition of HBr in the presence of a radical iniatiator such as a peroxide follows
a regioselective results which is opposite to the ionic addition. The two reactions are
referred to as Markovnikov and anti-Markovnikov additions but result because of the
operation of two entirely different mechanisms.
The ionic mechanism (Markovnikov) produces the more stable carbocation which
reacts with bromide. The radical mechanism (anti-Markovnikov) adds the Br radical first to
give the more stable radical which reacts with HBr to abstract a hydrogen atom.
Br
HBr
Markovnikov
ROOR
peroxide
HBr +
2
OR
OR
Br + HOR
H
HBr
Br
Br
H
Br
anti-Markovnikov
Additions of Halogens and pseudohalogens.
Halogenation of organic compounds occurs readily with halogens (X2), mixed
halogens (X-Y) and pseudohalogens (X-function). The reaction kinetics contain higher
level terms just as addition of HX and thus may have more than one molecule of halogen in
the rate controlling step. The possible stereochemical result of halogenation is shown in the
mechanism below. Most electrophilic halogenations occur through the path involving a
halonium ion to give anti addition, but syn addition is also observed as well as non
stereoselective addition.
+
X2
+
X
X
X
syn-addition
Sigma complex
X
+
+
X
Halonium ion
X
X
carbocation
X
X
X
anti-addition
X
non-stereoselective
In the bromination of an adamantyl alkene a stable bromonium ion has been
isolated.
Br
+
The reactivity of alkyl substituted alkenes is shown below. The order reflects the
nucleophility of the alkene for the electrophilic halogen.
CH3
1
CH3
61
CH3
CH3
2600
CH3
CH3
CH3
CH3
CH3
CH3
10,000
2 x 106
The amount of syn addition increases with phenyl subsituted alkenes. This is a
result of the increased stability of the intermediate benzylic cations..
Ph
CH3
H
Br
Br2
Br
H
H
H
Ph
Ph
Br
anti:syn ratios
CH3
CH3
syn
anti
H3 C
Br
CH3
Ph
Ph
Ph
H3 C
1.9:1
100:1
CH3
100:1
2:1
Fluorinated aliphatic alkenes also show a preference for anti addition as shown
below, but fluorinated styrenes show high levels of syn addition.
F
CH3
H
Br
Br2
Br
CH3
Br
H
H
H
Ph
Br
Ph
CH3
syn
anti
3%
97 %
Fluorinated styrenes, as well as other halogenated styrenes, give an increased
amount of syn addition. Sometimes the syn addition product is the major product. In these
cases, less halonium ion is formed because the fluorine atom and the phenyl ring stabilize
the cation.
F
Cl
Ph
F
Br
Ph
F
Br
Br2
Br
Ph
F
F
Cl
Br
anti
64 %
Cl
F
syn
35 %
A few representative examples of halogen addition `are shown below.
COOH
COO-
I2
O
NaHCO3
O
I
I
OH
OH
Cl2 , NaOH
CH3
CH3
CH3
( Cl - OH)
Cl
Cl
Pseudo halogens are composed of compounds such as I-NCO and I-N3 . These
reagents add with the halogen positive followed by the negative functional part. The
function allows for further organic transformations.
NCO
INCO
I
Mixed halogens can be added to alkenes. While addition of F2 is usually a
hazardous proposition, the use of BrF or IF proceed smoothly. IF can be produced in situ
from the reaction of xenon difluoride and iodine. In the example shown the F anion adds at
the site of the more stable cation.
H3C
H3C
CH3
XeF2 / I2
H
H3C
F
CH3
H3C
H
I
Addition of Boranes (Hydroboration)
Alkenes react with BH3 ( a commerical reagent is a comples with Me2S) followed
by oxidation with peroxide and base to give alcohols. The reaction is very selective and
subject to steric effects. It is not an ionic addition , but is nearly a four-center addition with
the boron atom going to the less substituted carbon atom.
H
H
H
B H
B H
H
H
BH2
R
R
H
B
R
3
R
-
OOH
H
OH
R
The regio and stereoselectivity are show in the examples below.
1) BH3
2) HOO- , -OH
H
OH
1) BH3
2) HOO- , -OH
OH
The highly sensitive regioselective nature of hydroboration may be expanded to
larger borane reagents that add almost exclusively to the least substituted carbon atom.
Three reagents are compard below.
A
6
94
A 43 57
B
1
99
B
99.9
C
C
B2 H6
= H-B-H
A
3
97
99.8
H3 C
CH3
H
2
H3 C
B
H
B
BH
C
Epoxidation and Hydroxylation
Alkenes undergo addition of peroxy reagents to produce alkenes as shown below.
The reaction occurs through a concerted addition of an oxygen atom to the alkene.
O
O
PhC-OOH +
+
O
PhC-OH
O
H
Ph-C
O
O
A procedure known as Sharpless epoxidation allows the preparation of chiral
epoxides by using chiral oxidation agents.
O
O
HO
CO2 Et
+
OH
HO
t-BuOOH
CO2 Et
Ti(OiPr)4
mol sieves
O
O
O
OH
(+)-diethyltartrate
Alkenes also add hydroxyl groups to produce glyclols on reaction with cold
potassium permanganage.
KmnO4
OH
OH
A single hydroxyl function is added in a process called oxy-mercurationdemercuration. In this reation, mercuric acetate reacts with the alkene to produce an
intermediate three membered ring similar to that seen in halogenation, called a
mercurinimium ion. Nucleophilic solvent present such as water, alcohol, or acetate can open
the ring to give an oxygenated mericury compound. Reaction of the intermediate with
sodium borohydride reduced the C-Hg bond to a C-H bond.
Hg(OAc)2
OH
H2 O
H2 O
HgOAc
Hg(OAc)2
OH
OH
NaBH4
HgOAc
The nucleophilic solvent, water in this case, attacks the carbocation at the more stable
tertiary position. After reduction the tertiary alcohol is formed. This is the same
regioselectivity observed in acid-catalyzed hydration but the mercuration reaction proceeds
much cleaner except for the presence of mercury compounds.
Hydroboration-oxidation gives the the reverse selectivity shown below. Thus for
synthetic purposes the processes compliment each other.
1) BH3
2) -OOH
OH
Alkenes also undergo addition of carbon units. The addition of divalent carbon,
carbenes or carbenoids, produce three membered rings. A very useful method for making
difluorocylopropanes comes from treatment of the trimethylsilyl ester below with catalytic
fluoride. Several products occur including the difluorocarbene which reacts immediately
with any alkene present.
F SO2 CF2 -CO2 SiMe3
+ F-
Me3SiF + CO2 + SO2 +
F SO2 CF2 -CO2 SiMe3
CF2
+ FF2
C
CF2
79 %
Dichlorocarbene can be easily generated and captured by an alkene as shown.
HCCl3
CCl2
CCl2 carbene
KOH
The addition of dienes in the Diels-Alder reaction provides six membered rings.
O
+
O
O
O
Diels-Alder
O
O
Carbene addition and the Diels-Alder reaction are coverend in more detail in the
chapter on orbital symmetry.
Addition of Fluorine to Unsaturated compounds
From Hudlicky book
Purrington
Some reagents ArIF2, CsOso4F XeF2 HF, Cf3of
Feiring
Addition of HF to alkenes and to acetylenes, Halogen Fluoride addition, OF
reagents FCLO3,. Sf, NOF,