Chemistry Reference
In-Depth Information
3.8 Effects of the Nature of the Precise Catalyst
Throughout this chapter, it has been highlighted by several examples that
the choice of ligand is crucial to the success of a cross-coupling reaction. The
second factor that can have a significant impact is the precise nature of the
catalyst, more specifically, whether it is generated in situ or from a pre-
formed catalyst. In turn, the nature of the preformed catalyst may affect the
outcome of the reaction, even if the same ligand is employed.
The dtbnp ligand may be used as an illustrative example, where cross-
coupling reactions also can be affected by the above-described behavior. The
a-arylation of acetophenone with 4-chlorotoluene was investigated using a
number of preformed and in situ-generated dtbnp-containing catalysts
(Table 3.7).
113
Overall, in this reaction, it was noted that catalysts with a
Pd-to-ligand ratio of 1 : 2 performed better than catalysts with a 1 : 1 ratio.
(dtbnp)
2
Pd and (dtbnp)
2
PdCl
2
gave quantitative conversion (Entries 3 and 4),
whereas (dtbnp)Pd(allyl)Cl and in situ-generated LPd(0) provided the product
with considerably lower conversion (Entries 1 and 2).
Another example can be extracted from the amination investigations using
Q-Phos as ligand. Notably, preformed Q-PhosPd(crotyl)Cl performed better
than in situ-generated catalyst using Pd(dba)
x
(x
¼
1.5 or 2) in conjunction
with free Q-Phos ligand (Table 3.8, Entries 1, 2 and 3).
112
Also, use of
Table 3.7 Preformed versus in situ catalysis in a-arylation reactions using dtbnp
ligand.
Me
O
O
Cl
+
[Pd] 1 mol %
NaO
t
-Bu, dioxane
100 °C
Me
Me
Me
Conversion (%)
a
Entry
[Pd]
1
Pd
2
(dba)
3
/dtbnp (1 : 1)
42
2
(dtbnp)Pd(allyl)Cl
79
3
(dtbnp)
2
Pd
99
4
(dtbnp)
2
PdCl
2
99
a
Conversion determined by GC analysis.
Table 3.8 Application of Q-Phos-based catalysts in amination reactions.
Me
N
Br
NHMe
catalyst, NaO
t
-Bu
+
MeO
toluene, rt, 40 min
MeO
Conversion (%)
a
Entry
Catalyst
1
0.5 mol% (Q-Phos)Pd(crotyl)Cl
100
2
0.25 mol% Pd
2
(dba)
3
/0.5 mol% Q-Phos
35
3
0.5 mol% Pd(dba)
2
/0.5 mol% Q-Phos
80
4
0.5 mol% (Q-Phos)
2
Pd
9
a
Conversion determined by GC analysis. Average of at least two runs.
Search WWH ::
Custom Search