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              Selectivity of functional groups
in Organometallics

Introduction

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Organometallic
compounds are those containing M-C bond in which metal is directly linked to
carbon atom. Like other organic compounds organometallic compounds also show
chemeo , regieo and stereoselectivity. All these properties depend on the temperature
reactivity geometry and concentration of organomettalic reagent. Here we
defined the terms region,chemeo and stereoselectivity.

Regioselectivity

Regioselectivity
is a phenomenon in which addition of non_symmetrical reagents occurs in such a
way where A and B are not equal to a non_symmetrical alkene that formed two
constitutional isomers which are isomeric products.

Example

By
the reaction of HCL with propene it gives 1_chloropropane and 2_chloropropane
where 2_chloropropane is majour product which favoured over the second one so
this reaction known as regioselective.

If  2_chloropropane were the one and only product
that formed then this reaction known as regionspecifi.

 

Stereoselectivity

Stereoselectivity
is a process in which unequal mixture of stereoisomers is formed and one of the
stereoisomers predominate over other stereoisomer.

Example

Crbocupration of acetylenic
Carbonyl compounds:

•      Syn
carbocupration provides the cis product when temperature is low.

•      Cis/Trans product will formed at higher temperature by
non stereoselective process.

              REGIO & STEREOSELECTIVITY OF ORGANOCOPPER
REAGENT

Among
all of the organometallic reagents the organocopper are highly important with
respect to their selectivity in C-C bond formation. As Copper is less
electropositive than Mg and Li so copper carbon bond is less polarized this is
the reason it show higher selectivity than alkyl magnesium halide and RLi. It
easily reacts with aryl, vinyl and alkyl halide.

Order of selectivity is

Organocopper 3,4 and 3,5
Selectivity

                   FUNCTIONALIZED ORGANOMETALIC
REAGENTS

The
usefulness of Grignard and other organlithium reagent in the construction of C-C
bond and in the synthesis of organic compound is known from many centuries. As
these reagents are highly reactive so their reactivity limits the functional
groups tolerated in their use and preparation . So the presence of carbonyl  ,cyano, nitriles asters amino and hydroxyl
functional group in Grignard or lithium reagent causes compatibility issues.

So
to overcome this problem metal halogen exchange should take place at low
temperature but this solution is not still accepted as usefull.

An
alternative method  it is to first
prepare those organometalic reagent can accommodates all these functional  group carbonyl ,cyano, nitriles asters amino
and hydroxyl  and then metal metal
exchange to synthesise a reactive analogue in the presence of an electrophile .

To
achieve this purpose zinc compound have proven usefull and develop by p.knoche.

METHOD

The
following chemical reaction illustrates these method.

In
the first step metal_halogen exchange occurs between alkyl  iodide and diethylzinc by the removal of
ethyl iodide under vaccume. The resulting dialkylzinc reagent accommodates some
functional group for example esters and nitriles and  also undergo carbonyl addition reaction with
ketone and aldehyde.

 

 

 

 

Conclusion

Organometallic
reagents have been developed since 1940’s. Their structures and species have
been synthesized which clearly illustrated the proposed mechanism and provide
information regarding to reactive species. Chemical composition , concentration
and temperature control the region chemeo and steroselectivty of the process. Reactions
are effectd by geometry and reactivity of organometallic reagent. Insertion of
carbonyl, hydroxyl, amine and nitrile functional group are also depends on
reactivity of these organometallic reagents.

 

 

 

 

 

 

 

 

 

 

 

 

References

Yamanaka, M, Nakamura, E. Organometallics 2001,
20, 5675.

Mori, S.; Nakamura, E.; Morokuma, K. Organometallics
2004, 23, 1081.

Krause, N.; Gerold, A. Angewandte
Chemie International Edition in English 1997, 36 (3),
186–204.

 

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