Chemistry Reference
In-Depth Information
NHC = IPr
13a
:P(OR)
3
=P(OMe)
3
13b
:P(OR)
3
=P(OEt)
3
13c
:P(OR)
3
=P(O
i
Pr)
3
13d
:P(OR)
3
=P(OPh)
3
13e
:P(OR)
3
=P(OC
6
H
3
-2,4-
t
Bu
2
)
3
13f
:P(OR)
3
=P(OCH
2
CF
3
)
3
NHC = SIPr
13h
:P(OR)
3
=P(OMe)
3
13i
:P(OR)
3
= P (OCH
2
CF
3
)
3
NN
Cl
Pd
Cl
P
O
RO
OR
OR
O
xg:
P(OR)
3
=
P
Et
O
Figure 4.9
Structure of the mixed phosphite-NHC precatalysts.
R
1
R
1
NN
NN
NN
R
1
R
1
R
2
R
2
Cl
Pd
Cl
Cl
Pd
Cl
Cl
Pd
Cl
N
N
N
N
Cl
[Pd(NHC)(Im)Cl
2
]
16a
:NHC=IPr
16b
:NHC=IMes
[Pd(IPr)(Et
3
N)Cl
2
]
14
[Pd(IPr)(PE PPSI)]
15
Figure 4.10
Structures of precatalysts active in Suzuki-Miyaura coupling.
phosphite-NHC palladium complexes (13, Figure 4.9).
45
Interestingly, in
these structures, the NHC was found to modulate its bulkiness depending
on the bulkiness of the phosphite co-ligand. The reactivity of the com-
plexes was compared in Suzuki coupling. Compounds 13a and 13c ex-
hibited the highest catalytic activities. Alkoxide and hydroxide bases and/
or alcohols as solvents were found to be necessary to obtain good catalytic
activities. In a similar way, Cazin and co-workers also described the prep-
aration of a range of mixed phosphine-NHC palladium complexes.
46
These complexes were active for Suzuki coupling in a mixture of water and
iPrOH (9:1) with typically a very low catalyst loading of 0.03 mol%. As pre-
viously observed with the phosphites, the NHC was found to adapt its
bulkiness according to that of the phosphine co-ligand.
In 2011, Navarro and co-workers utilized Et
3
N as a cheap and readily
available throwaway ligand.
47
The reactivity of the new [Pd(IPr)(Et
3
N)Cl
2
]
complex (14, Figure 4.10) was directly compared with that of the [Pd(IPr)
(PEPPSI)] precatalysts (15, Figure 4.10) developed by Organ and co-workers
48
(Table 4.3). Complex 14 showed higher catalytic activity, at lower tempera-
ture. This was explained by an easier departure of the labile ligand, and/or a
higher propensity for the Et
3
N to recoordinate to the NHC-Pd(0) active
species.
Another significant example reported in the literature is the introduction
of methylimidazole as a throwaway ligand by Shao and co-workers.
49
Two
new complexes of formula [Pd(NHC)(Im)Cl
2
] were prepared (16, Figure 4.10).
Search WWH ::
Custom Search