Download PREPARATION OF ORGANOLITHIUM COMPOUNDS - GCG-42

INTRODUCTION
Organometallic compounds are the
Compounds containing a carbon
–metal bond .
Since lithium is a metal , so it forms
an organo-metallic compound.
The compounds formed by
carbon– lithium bonds are called
organolithium compounds
e.g. CH3-Li
‘CH3’ is an organic group
‘Li’ is the metallic group
Dr. HENRY GILMAN
(Father of organometallic
chemistry)
DISCOVERY
Organolithiumcompounds,
discovered by Wilhelm Schlenk
in 1917.
But only since the 1950th, based
on the pioneering work of Georg
Wittig and Henry Gilman,
organometallic reagents became
a routinely used tool in the
synthetic organic laboratory.
Dr.WILHELM SCHLENK
(German chemist,
(1879-1943)
STRUCTURE AND BONDING
d-
C
Li
d+
57% covalent
-
C:
43% ionic
Li+
strong base
good nucleophile
significant (-) charge
at the carbon
Carbon-metal bonds are polar bonds that
can be represented by a resonance hybrid of
covalent and ionic structures.
C-Li are strong bases and react with acids,
even weak acids like water and alcohols.
STRUCTURE AND BONDING
bond
C-K
C-Na
C-Li
C-Mg
C-Zn
C-Cd
C-Cu
percent ionic
51
47
43
35
18
15
9
most reactive
strongest base
Li is the
Metal used
most often
best nucleophile
least reactive
PREPARATION OF ORGANOLITHIUM
COMPOUNDS
The organometallic compounds can be prepared
by following methods:By Halogen Metal Exchange
From Terminal Alkynes
By Trans Metalation/ Metal-Metal Exchange
By Directed Metalation/ Ortho Metalation
PREPARATION OF ORGANOLITHIUM
COMPOUNDS
By Halogen Metal Exchange
Normally prepared by reaction of alkyl halides
with lithium
R
X +
2Li
R
Li + LiX
same for Ar—X
It is an oxidation-reduction reaction: carbon is
reduced
Examples
(CH3)3CCl + 2Li
diethyl
ether
–10°C
(CH3)3CLi + LiCl
(75%)
diethyl
ether
Br + 2Li
Li + LiBr
35°C
(95-99%)
It is usually chosen for aryl derivatives
PREPARATION OF ORGANOLITHIUM
COMPOUNDS
From Terminal Alkynes
PREPARATION OF ORGANOLITHIUM
COMPOUNDS
By Trans Metalation/ Metal-Metal Exchange
2Li + R2Hg
2Li + Hg
In the above equation ‘Li’ metal is replaced by ‘Hg ’
This reaction is preferred to form organolithium
compounds from other organometallic compound.
PREPARATION OF ORGANOLITHIUM
COMPOUNDS
By Directed Metalation/ Ortho Metalation
DMG
DMG
+ RLi
Li
+ R-H
Metallation of an aromatic ring near a substituent, which
acts as a “Directed Metallation Group”, is called “OrthoMetallation”.
They have in common the ability to coordinate the
approaching cation
(= lithium-ion) and/or to increase the acidity of the orthohydrogen
PROPERTIES OF ORGANOLITHIUM
COMPOUNDS
Physical properties
Highly polar due to most electro
positivity of lithium group .
Volatile in nature, easily absorbs
Polarity of Bonds
moisture.
Solubility in Hydrocarbons .
More complex bonding.
Forms covalent compounds due
to vacant ‘p’ orbital
SOLVENTS THAT ARE FREQUENTLY USED
..
CH3CH2
..
..
..
O CH2CH3
O
CH3CH2CH2CH2CH3
..
..
..
..
diethyl ether
THF
tetrahydrofuran
These solvents do not
pentane react with the metals
themselves or with
the organometallic
benzene
compounds formed.
O
1,4-dioxane
O
PROPERTIES OF ORGANOLITHIUM
COMPOUNDS
Chemical properties
R-CO-CH=CHR
HO-CH2CH2R
R1-X
9
8
R
5
6
7 R1-CO-R2
R1-C=CH
R-CO-R1
R1-CO-R
4
R-Li
R1-CHO
R1-R
CO2
epoxide
3
2
1
10
R-COOH
R1-C=C-Li
R1-C-OH
R2
R-OH
REACTIONS OF ORGANOLITHIUM
COMPOUNDS
Reaction with water
Water or traces of moisture (water vapor)
destroy the reagent !
R-Li +
strong
base
-
R
H 2O
+
R-H
H O
+
R H +
LiOH
OH
-
H
The glassware must be dry, and
we must not expose the reagent to humid air.
REACTIONS OF ORGANOLITHIUM
COMPOUNDS
Reaction with oxygen
Oxygen reacts with alkyllithium compounds to form
hydroperoxides.
CH3CH2CH2CH2
Li + O2
CH3CH2CH2CH2
- +
OO Li
H2O
butyllithium
CH3CH2CH2CH2
a hydroperoxide
This reaction usually proceeds in low yield,
meaning it is not a good synthetic method .
However, it can be a annoying problem when
trying to prepare alkyllithium compounds.
OOH
REACTIONS OF ORGANOLITHIUM
COMPOUNDS
Removing a Halogen
The reaction with water is a way of reducing ( or
dehalogenating )an alkyl halide ( converting C-X to C-H ) .
H
Li
Br
ether
H2O
Li
CH3
C Br
CH3
CH3
C
-
Li
+
C H
H O
H
Removal
of Br
REACTIONS WITH THE CARBONYL (C=O) GROUP
..
O:
C
.. - Li+
: O:
d
C
d+
-
Li
+
:C
C
new C-C
bond
NUCLEOPHILIC ADDITION TO CARBONYL
Since oxygen is more electronegative than carbon, the
carbonyl group is electron-deficient at carbon, that is, it
is an electrophile.
The organometallic compound (R-Li) behaves as a
nucleophile with an unshared pair on the carbon.
SYNTHESIS OF ALCOHOLS
Reaction of RLi with aldeydes and ketones yields alcohols.
H
CH3CH2
Li
+
O
H
formaldehyde
CH3CH2
Li
+
CH3
O
H
other aldehydes
CH3CH2
Li
+
CH
OH
secondary alcohol
CH3
CH3
CH3CH2
OH
primary alcohol
CH3
CH3CH2
CH2
O
CH3
CH3CH2
C
OH
CH3
ketones
tertiary alcohol
The type of alcohol depends on whether we use formaldehyde,
another aldehyde, or a ketone.
SYNTHESIS OF A CARBOXYLIC ACID
CH3
CH Br
CH3
Li
ether
..
:O
CH3
CH Li
CH3
Cd
+
dry-ice
:O
.. d..
O:
O
CH3
H3O+ CH3
CH C
CH C
Slow step
.. - +
CH
CH3
O : Li
3
OH
..
R-Br
Li
R-Li
CO2
R-COO- Li+
H3O+
R-COOH
usually dil. HCl
REACTION OF ,-UNSATURATED CARBONYL
COMPOUNDS WITH ORGANOMETALLIC REAGENTS
C6H5CH=CHCOC6H5
C6H5Li
C6H5CH=CH-CHO
When an organolithium compound react with an ,unsaturated carbonyl compounds it gives 1,2 addition
product
RECENT DEVELOPMENTS
Lithium hexamethyldisilazide (LHMDS) lithium amide,
30 % solution in hexane
Mol. Formula: C6H18LiNSi2
Application: Selective low nucleophilic base for e.g. enolisations
Appearance: yellowish solution
Physical Properties:
Molecular weight: 167.33
Density: 0.71 g/ccm (at 20°C)
Boiling point: 60 – 80 °C for Hexane
LIMITATIONS OF ORGANOLITHIUM COMPOUNDS
Highly volatile , so not used in mild reaction .
It easily absorbs moisture , because of which it is not
kept in open envoirnment.
These compounds are harmful for skin. so, we
should use gloves before using it