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Pd( 0)
-CO
COCl
Pd Cl
Pd
Cl
O
HPdCl
R
R
R
R
R
R
H
H
PdCl
Z
Z
Scheme 9.13 Alkenylation initiated and terminated by PdH species: outline of the
mechanism.
OMe
O
O
B
O
B
O
Pd
2
(dba)
3,
(t-Bu)
3
P
iP rCOCl
EtOAc, 50°C
Me O
88%
OMe
OMe
Scheme 9.14 Example of alkenylation according to Skrydstrup and co-workers.
72
reductive elimination from R-Pd-R
0
complexes. There are cases, however,
where the difference is not definite and, at least until the mechanisms of
each particular case become established unambiguously, the margin for
interpretations will remain. Such borderline cases are excellent manifest-
ations of the diversity of Heck reactions, as in these cases the specific be-
haviour
of
common intermediates
is of
critical
importance
for
differentiation between the mechanistic paradigms.
Thus, the reactions of alkenyl- and arylpalladium intermediates with un-
saturated aromatic substrates leading to arylation and alkenylation can
formally be viewed as carbopalladation of double bonds involved in aromatic
systems. Although the detailed mechanisms of such reactions are unknown,
it seems unlikely that such transformations can really involve carbopalla-
dation. More likely is an electrophilic pathway, which is closer to the cross-
coupling paradigm. Nevertheless, true carbopalladation can take place in
annelated, particularly five-membered systems, where the double bonds are
substantially localized and indeed can behave as regular double bonds.
Particularly relevant, this formal analogy appears in alkenylation re-
actions, well known in heterocyclic chemistry (reviewed by Rossi et al.
78
), and
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