Chemistry Reference
In-Depth Information
The possibilities for control via stable ancillaries in Heck processes are
limited, and this is well seen particularly in comparison of the Heck process
with the cross-coupling process; the latter is chosen not only because of its
fundamental role in Pd catalysis, but also because it is an example of a stable
ancillary ligand-controlled pathway. The cross-coupling processes involve lig-
and exchange of halide or other leaving group for strong nucleophiles or
reactive organometallic compound. The chemistry of cross-coupling is
highly determinate as the reagents - the electrophile and the nucleophile -
can be bonded to the Pd centre only in the right order and the processes
involved are commonly irreversible. In this process, a single coordination
site of a formally neutral Pd complex is involved without the need for pre-
dissociation of existing ligands, which opens up ample possibilities via
controlling the reaction by stable mono- or bidentate ancillaries
(Scheme 9.29). It is not surprising that the ligand design became a highly
successful strategy in cross-coupling and the implementation of this strategy
led to exceptional progress in the 2000s. On the other hand, the Heck pro-
cesses are much more challenging in implementing a similar level of control
by the ancillaries. One of the reagents, the olefin, can be and is bonded
prematurely and this extra equilibrium is an unavoidable factor, decreasing
the concentration of active Pd species and thus the activity of the catalytic
systems.
117
Further, the Heck pathway involves the ligand exchange of an
anionic ligand (actually either s-bonded halides or labile solvent ligand in
the case of an ionic bond with nucleofugic anions such as OTf
and BF
4
)
for neutral olefin. This exchange can be realized only via predissociation
with two possibilities existing - through the neutral pathway via exchange
with a neutral ancillary or through the polar pathway via exchange with an
anionic ligand or labile ancillary in a cationic complex.
Three distinct scenarios for the roles of stable ancillaries in Heck pro-
cesses are outlined in Scheme 9.30. With stable ancillaries, the neutral
Ar
He ck r eacti on
cross-coupling
L
Pd
L
X
RM
X
-
L
Ar
Ar
Ar
L
Pd
L
L
P
d
L
L
Pd
R
X
R
Ar
Ar
L
Pd
L
Pd
L
X
R
R
po lar path
neutr al p ath
Scheme 9.29 Comparison of Heck and cross-coupling processes with respect
to the ways of binding of the second reagent (olefin in the Heck
reaction, organometallic compound or other nucleophiles in cross-
coupling).
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