HECk Reaction green synthesis vrs conventional, Mechanism, Pre-activation of Palladium Catalyst, Oxidative addition, Olefin Addition to the Palladium Complex, Migratory Insertion , β-Hydride Elimination, Regeneration of palladium catalyst, Merits of green synthesis

1. Assignment #3
Heck Reaction Classical vs
Green method
Hajira Mahmood
Ph.D. CHEMISTRY

2. Heck Reaction
The Heck reaction is the chemical reaction of an unsaturated halide
with an alkene in the presence of a base and a palladium catalyst to
form a substituted alkene.
American chemist
Awarded Nobel Prize in Chemistry in 2010
Richard F. Heck.

4. •Pre-activation of Palladium
Catalyst
It is noteworthy that the first step of the catalytic cycle is actually not the
oxidative addition of the substrate, as the palladium catalyst must be activated
before the reaction.
The catalytic precursor Pd(II)(OAc)2, associated with monodentate phosphine
ligands such as PPh3 is normally used to catalyze the reaction, but this Pd(II)
complex must be reduced to Pd(0) to enter the catalytic cycle.

5. Oxidative addition
An oxidative addition where the palladium is inserted into the aryl-bromide bond,
forming a palladium pi complex with the alkene.
The alkene is inserted in the carbon-palladium bond on the same side of the
double/triple bond which is known as syn addition.
It is the most difficult step of the entire catalytic cycle.
The presence of electron-donating groups on the phosphine ligands can activate the
Pd(0) catalyst such that the R-X bond can be easily broken along with the formation of
Pd-R and Pd-X bond.
The rate of oxidative addition also depends on the chemical property of halides.

6. Olefin Addition to the Palladium
Complex
Before migratory insertion of the olefin to the palladium-R bond, the olefin must first
associate onto the palladium complex, which requires the dissociation of the existing
ligands

7. Migratory Insertion
It is a type of reaction wherein two ligands on a metal complex combine.
It is a subset of reactions that very closely resembles the insertion reactions, and both are
differentiated by the mechanism that leads to the resulting stereochemistry of the products.
However, often the two are used interchangeably because the mechanism is sometimes
unknown.
Therefore, migratory insertion reactions or insertion reactions are defined not by the
mechanism but by the overall regio chemistry where in one chemical entity interposes itself
into an existing bond of typically a second chemical entity

8. β-Hydride Elimination
A reaction in which an alkyl group bonded to a metal center is converted into
the corresponding metal-bonded hydride and an alkene.
The alkyl must have hydrogens on the β-carbon.
For instance, butyl groups can undergo this reaction, but methyl groups cannot.

9. Regeneration of palladium catalyst
The addition of base is necessary to reduce the L2PdHX complex back
to the starting L2Pd(0).
Some common bases used are trialkylamines such as Et3N or
inorganic salts such as AcONa.
A proton sponge or Tl(I) or Ag(I) salts may also be employed to close
the cycle.

10. Merits of green synthesis
Green synthesis” of nanoparticles makes use
of environmentally friendly, non-toxic and safe
reagents.
Nanoparticles synthesized using biological techniques
or green technology have diverse natures, with greater
stability and appropriate dimensions since they are
synthesized using a one-step procedure.