Chemistry Reference
In-Depth Information
3.7.3 LPd(palladacycle)X
The third, and final, group of LPd(0) precursors in this chapter are the
palladacycles, which have shown very promising results in a number of
cross-coupling reactions.
The use of P-based palladacycles as precatalysts was demonstrated by
Bedford et al. in 2003,
114
and corresponding NHC-based palladacycles were
concurrently reported by Nolan and co-workers as ecient precatalysts for
cross-coupling reactions.
115,116
More recent examples of P-based palladacycles can be readily prepared
from an aliphatic amine, o-chlorophenethylamine (Scheme 3.37).
117
When
reacted with a base, such as NaOt-Bu or K
2
CO
3
, the catalytically active LPd(0)
species is proposed to be generated. These palladacycles have been used in
Suzuki-Miyaura coupling and amination reactions with excellent results.
Generally, the catalyst generated in situ from Pd
2
(dba)
3
and XPhos pro-
vided significantly lower conversion to the product than the XPhos-based
palladacycle 7 in Scheme 3.37 (Table 3.6).
117
Clearly, the active catalytic
species LPd(0) is formed much more e
ciently from the precatalyst, thereby
influencing the subsequent cross-coupling reaction considerably.
A number of palladacycles bearing various different dialkylarylphosphine
ligands have been prepared. Notably, the BrettPhos- and RuPhos-containing
precatalysts were demonstrated to provide very reliable and general catalysts
for amination reactions (Figure 3.21).
118
Buchwald and co-workers also reported the second-generation pallada-
cycle precatalyst 10, which can be prepared by a one-pot reaction starting
from 2-aminobiphenyl and Pd(OAc)
2
(Scheme 3.38). Its activation pathway is
identical with that of the first-generation palladacycles (see Scheme 3.37), in
Me
2
N
NH
2
XR
Cl
PdMe
2
L
Cl
Pd
+
LPd(0)
N
H
2
N
H
N
Me
2
ligand, MTBE
RX.HCl
L = XPhos
L = SPhos
L=RuPhos
7
8
9
Scheme 3.37 Palladacycle prepared from a Pd-based aliphatic amine complex and
proposed precatalyst activation.
Table 3.6 Preformed versus in situ catalysis using XPhos.
H
N
Cl
NH
2
[Pd]
+
80 °C
MeO
Me
Entry
[Pd]
Time (min)
Conversion (%)
25
a
1
0.05 mol% Pd
2
(dba)
3
/XPhos
35
2
0.1 mol% 7
35
100
a
No further conversion was observed.
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