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products.
276
As the reactions are run under very mild conditions at room
temperature, Pd(0) apparently cannot be dissolved. The very low loading of
precatalyst, contrasting with the 5-10% usually needed for other protocols
using arenediazonium salts, draws attention to this interesting protocol
(Scheme 9.84).
In a separate study, Felpin et al. investigated the possibilities of lowering
the precatalyst loading using soluble palladium precatalysts and found that
methanol is by far the best solvent for such reactions, if the temperature is
brought down to ambient.
277
Apparently, at higher temperatures the rate of
decomposition of diazonium salt is too high and simultaneously Pd(II) is
reduced to Pd(0) and aggregates of inactive black particles. At lower tem-
peratures, the diazonium salt is stable enough to enter the catalytic cycle
almost quantitatively. However, at low temperatures the choice of solvent
becomes the other critical factor, as too strongly coordinating solvents such
as nitriles apparently block the coordination shell - the lability of the re-
spective ligands is likely to be insucient for ligand exchange with olefin to
take place at a reasonable rate. As little as 0.05-0.1 mol% of Pd(OAc)
2
is
sucient for arylation not only of standard olefins, but also of such chal-
lenging olefins as various a-aryl acrylates. With the latter, very high dia-
stereoselectivity is observed to result in the formation of the E-isomer
(Scheme 9.85).
277
Najera and co-workers recently discovered that if neat water is used as
solvent, the protocol can be substantially simplified by exclusion of every-
thing except arenediazonium salt, olefin and 1 mol% Pd(OAc)
2
and the re-
action is performed at room temperature.
278
Interestingly, such a protocol
works not only with acrylates and similar polar olefins that are fairly soluble
in water, but also with styrenes and other hydrophobic alkenes, the solubility
CO
2
Me
Pd(II)/C (0.1 mol%)
MeOH, r.t., 12 h
N
2
BF
4
CO
2
Me
93 %
NO
2
NO
2
Scheme 9.84 Base-free arylation of acrylates using a low loading of supported
palladium precatalyst.
CO
2
Me
N
2
BF
4
NO
2
Pd(OA c)
2
(0.1 mol%)
MeOH, r.t., 12 h
+
O
2
N
CO
2
Me
NO
2
O
2
N
93% (E:Z = 98:2)
Scheme 9.85 Base-free low-loading arylation using a soluble palladium precatalyst.
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