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Pd pre-catalyst
L
n
Pd(0)
R
R'
R
X
Reductive
elimination
Oxidative
addtion
M = B(R'')
2
: Suzuki-Miyaura
= ZnX' : Negishi
= MgX' : Kumada-Tamao-Corriu
=Sn(R'')
3
:Sti le
=Si(R'')
3
or Si(OR'')
3
:Hiyama
R
R
L
n
Pd(II)
R'
L
n
Pd(II)
X
T ransmetallation
MX
R'M
Scheme 4.3 General accepted mechanism for cross-coupling reactions using trans-
metallating agents.
delivered on to the palladium centre. This is also the step that differentiates
the various types of cross-coupling reactions. Generally, the nature of the
transmetallating agents is directly linked to the names of the chemists who
discovered or developed their use. Thus, the Suzuki-Miyaura reaction in-
volves boronic acids or esters and the Negishi coupling utilizes organozinc
derivatives. The Kumada-Tamao-Corriu reaction uses organomagnesium
(Grignard) reagents, the Stille reaction employs organotin derivatives and
Hiyama coupling makes use of organosilanes or siloxanes.
29
The last step of
the catalytic cycle is reductive elimination, which releases the desired cross-
coupling product and regenerates the Pd(0) active species.
In the following sections, we describe the main advances in cross-coupling
chemistry, involving transmetallation-type mechanisms and Pd-NHC com-
plexes, divided into subsections tackling each type of the above-mentioned
cross-couplings separately.
4.3.1.1 Suzuki-Miyaura Coupling
Suzuki-Miyaura coupling is probably the most commonly employed and
also the most convenient protocol for the creation of C-C bonds. This is
notably because organoboron reagents offer a wide range of convenient
properties. They are generally thermally stable, inert to water and oxygen,
highly reactive as transmetallating agents and relatively non-toxic.
30
Suzuki-
Miyaura coupling involves the reaction between an organic halide or pseu-
dohalide and an organoboron reagent, generally a boronic acid or ester.
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