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approach. However, racemic amino acids can be obtained by one-pot pro-
cedure of arylation in the presence of a silane reductant under very mild
conditions, the latter being compatible with all components of the catalytic
system, although the yields were only modest (Scheme 9.76).
263
Compatibility of arenediazonium salts with silanes was demonstrated in
yet another protocol. A special case of base-free Heck arylation is the aryla-
tion of olefins, e.g. norbornene, in which PdH elimination is blocked for
steric reasons and therefore base is not required as no acid is liberated, but
some hydride-transfer reducing agent is needed to terminate the cascade.
Arenediazonium salts were shown by Cacchi and co-workers to be compat-
ible with silanes and hydroarylation was realized under very mild con-
ditions.
268
The arylation is exo-stereospecific (Scheme 9.77).
High diastereoselectivity of carbopalladation and full retention of con-
figuration of the existing chiral centre, manifesting a short lifetime of the
PdH intermediate, was observed by Correia and co-workers in the arylation
of a chiral dihydrofuran precursor derived from L-glutamic acid and de-
veloped as a flexible approach to styryllactone metabolites (Scheme 9.78).
269
Arylation of methyl vinyl ketone is a special task, as this olefin is very
reactive towards common bases and nucleophiles, hence common
Mizoroki-Heck protocols often give poor results. The use of aryldiazonium
salts, described by Brunner and co-workers, allows the development of a very
N
2
BF
4
CO
2
Me
COOMe
Pd(OAc)
2
DTBMP, Et
3
SiH
MeOH, 0°C
+
NHAc
NH Ac
Me O
OMe
Scheme 9.76 Example of hydroarylation of amidoacrylates.
N
2
BF
4
Ac
Pd(OAc)
2
(5 mol%)
i
-Pr
3
SiH, THF,
0°C to r.t., 6 h
+
94%
Ac
Scheme 9.77 Hydroarylation of norbornene by arenediazonium salts.
N
2
BF
4
Pd
2
(dba)
3
(2-5 mol% )
NaOAc, MeCN, r.t.
+
O
O
OR'
OR'
R
R
R = H, OMe, NHCO
2
Me
R' = H,TBS
Scheme 9.78 Example of diastereospecific arylation of a chiral dihydrofuran
derivative.
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