Chemistry Reference
In-Depth Information
Ar
N
Ph
34
(0.5 mol %)
NaHCO
3
, TBAB
H
2
O, rt, 3 h
Pd
Cl
Cl
N
Cl
(HO)
2
B
N
N
+
Ar
90%
34
Ar = 2,6-diisopropylphenyl
Scheme 14.31
Pd(PPh
3
)
2
Cl
2
(2.5 mol %)
dppb (5 mol %)
DBU, STAB, H
2
O
100 °C, 12 h
Br
85%
+
2
CO
2
H
Scheme 14.32
O
O
O
O
R
m
n
n = 7, R = H: PTS
n = 1, R = Me: TPGS-750-M
O
O
Scheme 14.33
palladium-NHC complex 34 in refluxing water with TBAB as promoter
(Scheme 14.31).
99
Stearyltrimethylammonium bromide (STAB) promotes the decarboxylative
double Sonogashira coupling of aryl bromides and propiolic acid catalyzed
by Pd(PPh
3
)
2
Cl
2
/dppb on water (Scheme 14.32).
100
The hydrophobic catalyst
gave better results than when the hydrophilic TPPTS ligand was used in
place of dppb.
Anionic surfactants have also been applied to cross-coupling reactions on
water, although less commonly than cationic surfactants. Sodium dodecyl
sulfate (SDS) promotes the Hiyama coupling of aryl bromides and chlorides
catalyzed by nanoparticles derived in situ from Na
2
PdCl
2
.
101
The same sys-
tem was applied to the Suzuki coupling of aryl bromides
102
and the con-
densation of 2-iodophenols and alkynes to give benzofuran derivatives.
103
Lipshutz and co-workers developed the nonionic surfactant PEG-a-toco-
pheryl sebacate (PTS, Scheme 14.33), which is a highly ecient surfactant for
aqueous-phase palladium-catalyzed cross-coupling reactions. Suzuki,
104
Sonogashira,
105
Heck
106
and Miyaura borylation
107
couplings were all dem-
onstrated in water-PTS (2 wt%) at room temperature using palladium/
hydrophobic phosphine catalyst systems. Improved results for all reactions
were achieved with the less expensive PEG-a-tocopheryl succinate (TPGS-
750-M) surfactant.
108
The PTS and TPGS-750-M surfactants eciently
support the palladium complex and substrate in micelles to provide high
reaction rates. The catalyst is constrained to the water-micelle phase, so
catalyst separation and recycling are possible.
The micelle environment provides a hydrophobic reaction environment
that allows reactions that normally are not favorable in water to occur
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