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O
O
Pd(db a-OMe)
2
(2 mol%)
PPh
3,
Cs
2
CO
3,
THF, 70°C
O
O
I
O
74%
O
Pd(0)
pl ausib le Heck mechan ism
PdI
P
dI
O
O
O
O
O
O
Pd I
H
H
O
O
O
IPd
O
O
H
O
Scheme 9.17 Heck reaction or cross-coupling? The intramolecular arylation of
a-pyrone.
Pd[(t-Bu)
3
P]
2
(5 mol%)
cyclohexene, dioxane
110°C, 14 h
NEt
NEt
Ph
+PhCl,+NaOH
110°C, 24 h
NEt
82%
Ph
Scheme 9.18 Arylation of N-alkyl-2H-isoindole.
restriction of the unavailability of a proton for syn elimination of PdH in the
hypothetical primary carbopalladation adduct can be lifted via rearrange-
ment involving a configurationally fluxional Z
3
-complex (the possibility of
such transformations via similar Z
3
-benzylic complexes in Heck reactions
with conformationally rigid systems was noted earlier
87
).
Another interesting example of such chemistry showing how subtle the
borderline between Heck reaction and arylation via C-H activation some-
times is involves the arylation of N-protected isoindoles, highly reactive
heterocyclic compounds, the structure of which is closer to that of non-
aromatic o-quinodimethanes with localized double bonds than to regular
aromatic heterocycles. Ohmura et al. showed that isoindoles are generated
in situ by dehydrogenation of readily available isoindolines and are then
arylated by aryl chlorides (Scheme 9.18).
88
Owing to the high reactivity of the
double bonds in this system, bis-arylation predominates.
There is at least one another major type of unsaturated substrates where
the Heck reaction/cross-coupling dichotomy is not readily resolved, leading
to interesting questions. The carbonyl group and its equivalents in alde-
hydes, ketones, imines, nitriles, etc., can participate in both carbopallada-
tion and cross-coupling reactions. These substrates can take part either in
direct carbopalladation of a carbon-heteroatom multiple bond, followed by
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