2. • Coupling reaction involves the joining of two chemical species with the help of a
metal catalyst.
• An important type of coupling reaction is the reaction of an organic halide with an
organometallic compound which facilitates the formation of a new carbon-carbon
bond.
R1 and R3 denotes alkyl, alkene or alkylene groups and R2 denotes an H group or an
alkyl group. X denotes the halide group.
Coupling Reaction ?
4. • It is the Pd-catalyzed C-C bond formation between aryl, alklenyl, alkyl halides or pseudohalides
and organosilanes.
• This reaction is comparable to Suzuki coupling but it requires an activating agent such as fluoride
ions or a base.
• The polarization of the Si -C bond is important for the success of Hiyama coupling.
• The first necessary step is the activation of the silane with a base or fluoride ion that leads to a
pentavalent silicon compound.
• The reaction rate is increased by using silanes with fluro or alkoxy group instead of alkyl.
Hiyama Coupling
6. • The reaction proceeds through
oxidative addition,
transmetallation,
cis-trans isomerization and
reductive elimination.
• The purpose of the fluoride ion is to activate the conversion of silicon compound RSiR3 to
a pentacoordinate [RSiR3F]- intermediate which is more amiable to transmetallation.
• Reactions in which the fluoride ion is replaced by a strong base have also been reported.
7. Advantages
Hiyama coupling possesses advantages such as
• low environmental impact,
• high atom efficiency,
• safe handling compared with the coupling reactions of organoboron, organozinc or
organotin compounds.
Disadvantages
• Use of highly nucleophilic fluoride or hydroxide as an activator makes the transformation
less chemoselective, especially for substrates with a base sensitive functionality.
8. • Kumada coupling or Kumada – Corriu Coupling is the Ni or Pd catalyzed cross coupling reaction
between an alkyl, aryl, vinyl halide or triflate and aryl, alkenyl or alkyl Grignard reagents
Kumada Coupling Reaction
10. • The reactivity of the halides follows the order I> Br> Cl when Pd is used as the catalyst,
whereas with certain Ni catalysts, the order is Cl> I> Br.
• The reaction is important since it gives access to styrene compounds.
• (Z) –Alkenyl – Grignard reagents couple non- stereospecifically with Ni catalysts, but the
reaction is stereospecific with Pd catalysts.
• Ni is also used to catalyse the reaction of alkyl and aryl Grignard reagents with an aryl
and alkenyl halides
11. • It involves the Pd catalyzed substitution of the vinylic hydrogen with a vinyl, aryl or
benzyl group.
Heck Coupling
• The coupling can be intra or intermolecular.
• A base is necessary to remove the liberated acid.
13. • The reaction time and temperature depend on the nature of the organic halide to be activated.
• Heck reactions are useful on the inter and intra molecular level, although intermolecular
reactions are more common.
• Organic halides containing beta hydrogens cannot be used as they tend to form olefins at the
Pd centre.
• Electron rich, disubstituted or cyclic olefins are less reactive than electron deficient,
monosubstituted olefins.
14. • Many functional groups are compatible with Heck conditions which enables the
synthesis of carbo- and heterocyclic compounds and c-c bonded isomerized products
including natural products.
• The active form of the catalyst is a 14e Pd(0) complex with two PPh3 ligands.
• 1,2 migratory insertion step is known as carbometallation in palladium based catalytic
cycles.
15. • Discrimination in reactivity is possible in Heck reactions by controlling the
reaction conditions.
• In the given reaction when PPh3 is not added, only iodide is replaced by
acrylate whereas upon the addition of PPh3, the bromides also react giving the
tri-alkenyl substituted toluene
