2. Defination & Importance Of OMC :-
Organometallic chemistry is the study of chemical compounds containing at
least one bond between a carbon atom of an organic compound and a metal
Organometallic compounds provide a source of nucleophilic carbon atoms
which can react with electrophilic carbon to form a new carbon-carbon bond.
Nomenclature:-
Organometallic compounds are normally named as substituted metals, e.g. alkyl
metal or alkyl metal halide.
Organomagnesium compounds are generally referred to as Grignard reagents.
Examples: CH3Li = methyllithium, CH3MgBr = methylmagnesium bromide.
Physical Properties:-
Organometallic are usually kept in solution in organic solvents due to their very
high reactivity (especially with H2O, O2 etc.)
Structure:-
Organosodium and organopotassium compounds are essentially ionic
compounds.Organolithiums and organomagnesiums have a σ bond between a C
atom and the metal: C-M
These are very polar, covalent bonds due to the electropositive character of the
metals.
3. Look at the electronegativities
of the metals Li, Na, K and Mg
compared to C and the other
atoms we have seen so far,
The images show the electrostatic potentials for methyl chloride, methyl lithium and
methyl magnesium bromide.
The more red an area is, the higher the electron density and the more blue an area
is, the lower the electron density.
In the alkyl halide, the methyl group has lower electron density (blue), and is
an electrophile.
In methyl lithium, the methyl group has higher electron density (red) and is
a nucleophile.
In methyl magnesium bromide, the methyl group is less electron rich that methyl
lithium.
4. Organo Lithium
Organolithiurn reagents react with a wide variety of organic substrates to form
carbon-carbon bonds and serve as precursors for the preparation of other
organometallic reagents.
Important points to consider when preparing and using organolithiums are,
Atmosphere :- inert atmosphere (Ar and He are best)
Nature of the halide :- Bromides generally are best; iodides have a tendency to
undergo the Wurtz reaction
Purity and physical state of the metal:- The rnetal surface should be clean and
have a large surface area.
Solvent:- Most R-Li reagents are prepared in hydrocarbon solvents. However,
phenyllithium, methyllithium
Reactivity:- The basicity of organolithium reagents decreases with increasing
stability of the carbanion moiety (e.g., t-BuLi > s-BuLi > n-BuLi).
5. Organolithium reagents exhibit reactivities similar to those of Grignard reagents
Organolithiums Via Iithium-Halogen Exchange
Aryllithium Reagents
Metal-halogen exchange provides an efficient route to aryllithiums and
heteroaromatic lithium reagents that are inaccessible by metal-hydrogen
exchange
7. Metalation Alkynes
This imparts sufficiently greater acidity to acetylene and 1-alkynes (pKa 24-26) so that
bases such as alkyllithiums, lithium dialkylamides, sodium amide in liquid ammonia,
and ethylmagnesium bromide may be used to generate the alkynyl anions.
Applications of Organo lithium:-
a) Addition to Carbonyl Compounds
b) Adddition to Acids
c) Addition to Nitriles & CO2
d) Displacement of Halo Groups
e) Opens Epoxide
8. Organo Magnesium
The Grignard reaction, reported in 1900 by Victor Grignard (Nobel Prize, 1912)
provides the synthetic chemist with one of the most powerful tools for connecting
carbon moieties.
Although 100 years have passed since Grignard published the preparation of
ethereal solutions of organornagnesiurn halides, the actual mechanism for the
formation of the reagents and their structures are still not completely understood.
The overall reaction for the formation of Grignard reagents involves an insertion
of magnesium into the carbon-halogen bond via an oxidative addition, thereby
changing its oxidation state from Mg(0) to Mg(II).
Schlenk equilibrium
9. Alkynyl Grignard reagents are obtained by deprotonation of 1-alkynes with
ethylinagnesium bromide in THF. For the preparation of ethynylrnagnesium
bromide
(HC _CMgBr), a solution of ethylmagnesium bromide in THF is slowly added to
a cooled solution of THF containing the acetylene.
Limitations Of Grignard Reagents :-
a) For thte preparation of Allyl & Benzyl Grignard reagennt always used Anthracene,d
High stable species
b) For the praparation of RMgF &RMgI Always use EtBr
c) For the preparation of Alkenyl Grignard use Already synthesized RMgX.
d) If any acidic groups present along with halo compounds that also decomposed RM
12. Cram’s rule
In 1952, Donald I. Cram (1919-2001, Nobel Prize in Chemistry, 1987) and his cowork-
er Fathy A. Abd Elhafez presented an experimental rule for the diastereoface-
differentiating reaction of ketones having a chiral center at the vicinal carbon with
metal hydride complexes or with organometallic reagents
Cram's rule states that ketones (A; L, M, and S are large, medium, and small groups,
respectively) with a chiral center The nucleophile of the reagent Y--M' then attacks
the carbonyl carbon at the less hindered diastereoface.
13.
14. In contrast to the polar nature of C-Li and C-MgX bonds,
the C-Zn bond is highly covalent and hence less
reactive,allowing the preparation of functionalized derivatives.
Utilization of organozinc reagents in organic synthesis has
mainly in preparation and utilization of compounds in organic
syntheses (Reformatsky reaction), cyclopropanation
(Simmons-Smith reaction),and transmetalations with
transition metals
Can not opens epoxide
Organo Zinc
15. The Reformatsky reaction involves condensation of a-haloester with aldehydes
or ketones in presence of zinc enolates to give the corsesponding beta hydroxy
esters.
The zinc enolates are generated by addition of an a-haloester in THF,DME,
Et-O in benzene or toluene to an activated zinc, such as a Zn-Cu couple or zinc
obtained by reduction of zinc halides with potassium .
An example of a Reformatsky condensation using Rieke zinc is shown below.
Principal
Examples
16. Mechanism
l. Generation Of Zinc Enolate Using Alpha Halo Ester With Zn
ll. Attack of Enolate on the Carbonyl Carbon(Aldehyde/Ketone))
Enantioselective addition of organozinc reagents to carbonyl
cornpounds furnishes chiral alcohols
17. In 1958, Simmons and Smith reported that treatment of a zinc-copper couple
with diiodomethane (CH2I2) in ether produces a reagent that adds to alkenes
to form cyclopropanes.
Simmons-Smith cyclopropanation Or Simmons & Smith Reaction
The cyclopropanation reaction of simple alkenes appears to proceed via
stereospecific syn-addition of a Zn-carbenoid (carbene-like species) to the double
bond without the involvement of a free carbene
18. The synthetic utility of the reaction stems from the following characteristics
1) Stereospecificity
2) Tolerance of a variety of f~lnctional groups, such as Cl, Br, OH, OR, CO,R,
C=O, and CN
3) The syn-directing effect of hydroxyl and ether functions
4)Chemoselectivity-zinc carbenoids are electrophilic and react chemoselectively
with the more nucleophilic double bond in dienes and polyenes
If any Chelating group is present i.e.OH,OR,NH2 then Cycloproponation prefer on the
same side of that group
19. Organo Copper
Organocopper complexes (RCu)
Lower-order cuprates (R2CuLi, also known as Gilmanreagents
Lower-order cyanocuprates (RCu(CN)Li)
Higher-order cyanocuprates (R2Cu(CN)Li2)
Mechanism and Stereochemistry
Substitution Reactions
