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As the use of arenediazonium salts forces the reactions to follow a polar
pathway, it is particularly justified when the polar mechanism is known to
confer beneficial effects. Arylation of allylic acetates and related vinyl de-
rivatives of pentenolides with retention of the double-bond position was
realized by Correia and co-workers using diazonium salts in a simple cata-
lytic system taking advantage of PhCN as solvent, which is very likely to
provide labile ligands for stabilization of Pd species in the absence of
phosphines or other stable ancillaries. This useful protocol was successfully
used for the syntheses of a number of interesting plant metabolites
(Scheme 9.74).
264,265
Arylation by arenediazonium salts can be applied for
challenging multisubstituted olefins.
266
The same protocol using PhCN as solvent, providing a labile ligand sup-
port for transient Pd intermediates, works well for the arylation of styrenes.
These reactions give better yields if carried out at room temperature to re-
duce side reactions, and this simple trick enables the precatalyst loading to
be decreased to 1 mol% and even lower. Favourable results were also
achieved when the reactions were carried out under a CO atmosphere, which
is likely to assist the reduction of Pd(II) in the system where the generation of
Pd(0) species may be a limiting factor. The protocol was successfully used in
the synthesis of resveratrol precursor (Scheme 9.75).
267
The failure to produce amidoacrylates, the classical precursors of chiral
amino acids, by enantioselective hydrogenation is a drawback of this
N
2
BF
4
OMe
Pd
2
(dba)
3
(4 mol%)
NaOAc, PhCN
80°C (
+
O
O
µ
w), 0 .5 h
OMe
OMe
O
O
85%
OMe
Scheme 9.74 Example of arylation of 4-vinylpentenolides by arenediazonium salts.
MeO
N
2
BF
4
Pd(OA c)
2
(4 mol% )
NaOAc, PhCN
r.t, 12 h
+
OAc
OMe
OMe
MeO
95%
OAc
Scheme 9.75 Example of arylation of styrenes by arenediazonium salts.
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