Chemistry Reference
In-Depth Information
O
t
-Bu
O
t
-Bu
Br
Br
Herrmann's cat.
5.58
n
-Bu
4
NOAc
+
Br
H
H
5.195
5
1
9
5
1
7
O
t
-Bu
H
H
H
5.198
Scheme 5.54
5.1.11 Heck-Like Reactions of Organometallics
Early versions of the Heck reaction employed organomercury compounds rather than organic halides.
57
This
process results in net reduction of palladium, so a stoichiometric amount was used. Other organometallic
species may be used in place of organomercury compounds, such as boronic acids
5.199
. In the presence of
an oxidant, a catalytic process is possible, delivering Heck-like products (Scheme 5.55).
58
While this reaction offers an alternative to the standard Heck reaction, under other conditions, the pathway
diverges giving a different product. The rhodium-catalysed addition of boron derivatives to enones involves
protonation of the final intermediate, rather than
-hydride elimination, leading to saturated products
5.202
,
5.205
(Schemes 5.56 and 5.57).
59,60
The reaction, therefore, is more like a Michael addition in terms of the
products formed, than a Heck reaction.
As the reaction of a
-substituted enone generates a new stereogenic centre, considerable efforts have
been invested into developing the asymmetric version. Unsurprisingly, chiral phosphines have been used
successfully (Scheme 5.58).
61
Surprisingly, chiral dienes
5.209
have also proved highly effective as ligands
(Scheme 5.59).
62
CO
2
n
-Bu
B(OH)
2
CO
2
n
-Bu
O
O
Pd(OAc)
2
, Cu(OAc)
2
, LiOAc
O
O
5.199
5.200
Scheme 5.55
O
O
B(OH)
2
acacRh(CO)
2
, dppb, H
2
O
5.201
5.202
Scheme 5.56
