Chemistry Reference
In-Depth Information
water. The beneficial effect of quaternary ammonium salts was first noted in
1994 by Jeffery in studies of Heck couplings on water with hydrophobic
catalyst systems.
90
Jefferey found that a range of tetrabutylammonium salts
were effective promoters, whereas metal halide salts had no promoting
effect.
TBAB is an effective promoter of ligand-free cross-couplings in water. The
tetraalkylammonium ion can act as a phase-transfer catalyst (PTC), whereas
the halide counterion is thought to stabilize palladium nanoparticles. By
avoiding the use of a ligand, the cost of the process is decreased and waste is
reduced. Ligand-free Pd(OAc)
2
-catalyzed Suzuki coupling of aryl bromides
occurs more effectively in water-TBAB than in organic solvents in which
the reaction is homogeneous.
91
By preforming the trihydroxyborate, a
stoichiometric base is not required, further improving the eciency of the
reaction.
92
The Pd(OAc)
2
catalyst in water-TBAB has been applied to the
synthesis of tropolone-catechol natural product precursors (32,
Scheme 14.29).
93
This method provided more consistent reactions and
easier purification than the traditional Pd/PPh
3
organic solvent protocol.
Adding a long alkyl chain to the ammonium salt appears to improve the
performance of the PTC. Cetyltrimethylammonium bromide (CTAB) pro-
vides higher yields in the Pd/C-catalyzed Suzuki coupling of aryl iodides in
water than does TBAB.
94
Similar results were seen in the Heck coupling of
aryl iodides catalyzed by PdCl
2
.
95
N,N-Didodecyl-N
0
,N
0
,N
00
N
00
-tetra-
methylguanidinium bromide affords higher yields for the Suzuki coupling of
aryl bromides than TBAB or guanidinium ions with shorter alkyl chains.
96
The aqueous-phase catalyst could be used for five reaction runs in the Suzuki
coupling of 4-bromoacetophenone at 60 1C for 2 h with no decrease in
reaction yield.
TBAB also promotes cross-coupling on water using catalysts supported
by hydrophobic ligands, such as triphenylphosphine.
97
Water-TBAB was
used as the solvent for Sonogashira couplings of aryl iodides and bromides
catalyzed by palladacycle 33 under microwave heating (Scheme 14.30).
98
Suzuki
coupling of heteroaryl
chlorides was performed using the
MeO
OTBS
OTBS
MeO
Pd(OAc)
2
(20 mol %)
K
2
CO
3
, TBAB
H
2
O, 70 °C, 30 min
O
(HO)
2
B
O
+
95%
O
O
Br
O
O
32
Scheme 14.29
Cl
HO
N
Pd
(HO)
2
B
2
33
(0.05 mol %)
K
2
CO
3
, TBAB, H
2
O
140 °C MW, 3.5 h
Cl
+
92%
C
8
F
17
O
33
O
C
8
F
17
Scheme 14.30
Search WWH ::
Custom Search