Chemistry Reference
In-Depth Information
O
O
O
Pd(OAc)
2
(0.5 mol %)
Na
2
CO
3
H
2
O/PEG (3:3)
60 °C, 2 h
(HO)
2
B
+
O
Scheme 14.64
O
MeO
MeO
Ph
OH
Pd(O
2
CCF
3
)
2
(20 mol %)
Ag
2
CO
3
, 5% DMSO/DMF
120 °C, 1 h
91%
+
Ph
MeO
OMe
MeO
OMe
Scheme 14.65
OMe
70%
Boc
MeO
PG = Boc
O
OMe
Pd(OAc)
2
(10 mol %)
PPh
3
(50 mol %)
Ag
2
CO
3
, TMSO
propionic acid
DMF, 80 °C, 24 h
+
HO
N
Pg
MeO
MeO
OMe
5 equiv
OMe
65%
PG = SO
2
Ph
SO
2
Ph
MeO
Scheme 14.66
anhydride. Benzoic anhydrides are coupled with arylboronic acids in water-
PEG-2000 to give benzophenone derivatives using PdCl
2
as the precatalyst
(Scheme 14.64).
194
In this case, transmetallation and reductive elimination
occur prior to decarbonylation. The catalyst system lost activity after three
reaction cycles. By changing to a water-[bmim][PF
6
] solvent system, up to
eight cycles could be achieved. Activated 4-nitrophenyl esters can also be
used as substrates under these conditions.
195
Direct decarboxylation of benzoic acid derivatives with palladium can be
achieved at high temperature. This approach was first applied to cross-
coupling by Myers and co-workers as part of a Heck-type coupling of benzoic
acids and alkenes catalyzed by Pd(O
2
CCF
3
)
2
(20 mol%) at 120 1C using
Ag
2
CO
3
as an oxidant (Scheme 14.65).
196
At least one ortho substituent on the
benzoic acid was required to prevent ortho-palladation. The decarboxylation
reaction was proposed to involve coordination of the benzoate to palladium
followed by b-aryl elimination to release CO
2
.
197
Cyclic enamines and
enamides can be mono- or diarylated via decarboxylative coupling with the
selectivity determined by the protecting group (Scheme 14.66).
198
With a Boc
protecting group, selective monoarylation occurs; whereas selective diaryla-
tion occurs with sulfonamide, amide and benzyl protecting groups.
Decarboxylation of a-oxocarboxylic acids provides palladium-acyl species
that can be coupled with aryltrifluoroborates to give benzophenone
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