Chemistry Reference
In-Depth Information
CH
3
Pd
2
(dba)
3
(1 mol %)
X-phos (2 mol %)
Cs
2
CO
3
, scCO
2
1800 psi, 100 °C
CH
3
N
Br
N
Me
3
Si
+
MeO
CO
2
Me
MeO
CO
2
Me
Scheme 14.56
Cl
66
(2.5 X 10
-5
mol %
)
K
2
CO
3
, MeOH (1 mL)
CO
2
(120 bar), 120 °C
Br
(HO)
2
B
O
NPd
Cl
N
O
N
+
N
MeO
MeO
66
Scheme 14.57
N-silylamines at 100 1C and 1800 psi. Palladium-oxazoline complex 66 was
highly active for the Suzuki coupling of aryl bromides in compressed CO
2
when methanol was used as a co-solvent (Scheme 14.57).
172
High yields were
obtained with 2.5
10
5
mol% palladium at 120 1C and 120 bar. Triphe-
nylphosphine-modified mesoporous SBA-15 silica complexed to palladium is
effective for the Suzuki coupling of aryl bromides at 90 1C and 20 MPa.
173
The heterogeneous catalyst could be used for seven reaction cycles with no
decrease in product yield.
14.3.4 Ionic Liquids
Ionic liquids (ILs) attracted significant attention as potential replacements
for volatile organic solvents for organic reactions, including palladium-
catalyzed cross-coupling, beginning in the early 2000s.
174
ILs are low-
volatility materials, so there is no concern with their evaporation into the
atmosphere. With proper choice of ions, they can be made to be immiscible
with both water and non-polar organic solvents. As a result, catalyst and
inorganic by-products can be removed through phase separation. Although
ILs have been touted as green solvents, such a blanket assessment cannot be
universally accepted. ILs cover a wide range of materials, some of which may
be environmentally benign, whereas others are probably fairly harmful. If
suitably designed, ILs could serve as safer and more sustainable replace-
ments for volatile organic solvents.
Early examples of palladium-catalyzed coupling reactions were typically
performed in 1-methyl-3-butylimidazolium tetrafluoroborate ([bmim][BF
4
]).
Using Pd(OAc)
2
as precatalyst, Pd-NHC complexes were observed in reaction
mixtures during Heck coupling reactions.
175
Like water, IL phases can be
used to constrain palladium catalysts, but it is not necessary to modify the
catalyst species specially. Palladium in combination with P(o-tol)
3
catalyzed
the Heck coupling of aryl bromides in [bmim][PF
6
] at 180 1C under micro-
wave heating.
176
The catalyst-containing IL solution could be used for five
reaction cycles with no decrease in product yield. Similar results have been
obtained with other systems.
177
Search WWH ::
Custom Search