Chemistry Reference
In-Depth Information
i
Pr
i
Pr
NN
Pr
Pr
i
i
0.1 - 0.5 mol%
O
Pd
O
d
n
4
r
4
n
g
|
2
O
R
1
R
1
O
O
H
H
O
OH
R
2
R
2
2 mol% AcOH, toluene,
1 atm O
2
or air, 5 - 20 h
60 °C, MS3Å
Figure 4.16
Sigman's (IPr-NHC)Pd(OAc)
2
(H
2
O) catalyst.
corresponding hydroxy ketones with good selectivity with catalyst loadings
as low as 1 mol%.
Sigman's group have made significant contributions to the area of
homogeneous Pd(
II
) alcohol oxidation. They developed the first Pd/NHC
(NHC
¼
N-heterocyclic carbene) catalyst system for aerobic alcohol oxi-
dations using the 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) lig-
and (Figure 4.16).
160
Other NHC systems have since been reported,
161
but the
original system exhibits some of the best performances reported to-date.
With this catalyst, acetate acts as an internal base, conducting an intra-
molecular deprotonation of the alcohol and simultaneously leaving a vacant
coordination site for b-hydride elimination. The crystal structure of the
catalyst involves a water molecule coordinated to the Pd in the vacant site
and hydrogen bonding to the neighbouring acetate ligands. The (IPr-
NHC)Pd(OAc)
2
(H
2
O) catalyst is capable of performing at a catalyst loading of
0.5 mol% for a variety of alcohols, with 4-methoxybenzyl alcohol being oxi-
dized in 20 h at loadings as low as 0.1 mol%. Catalytic quantities of acetic
acid were found to be beneficial for the system and they examined the in-
fluence of acid-base chemistry in detail.
162
The use of acid enables ambient
air to be used as the terminal oxidant source as opposed to pure O
2
, although
needing slightly extended reaction times in order to achieve comparable
yields. The use of open air makes this an accessible method to synthetic
chemists working on a small laboratory scale.
Sigman and co-workers also developed a second-generation NHC system,
the pivalate derivative (IPr-NHC)Pd(OPiv)
2
, which was found to function well
under the mild conditions of room temperature and atmospheric air, albeit
with a higher loading of 1 mol%. The group had also developed a system that
used triethylamine as a ligand and operated at room temperature,
163
and
they compared the performance of these three catalyst systems.
164
Table 4.3
gives some examples of the catalyst performance for these three systems for
a range of substrates. It can be seen that the catalysts are generally suitable
for a broad range of alcohols. It is worth noting that the NHC systems
with the larger pivalate counterions (as opposed to acetate) struggled with
more sterically demanding substrates. Also worth mentioning is that for
some substrates, when the oxidation with (IPr-NHC)Pd(OAc)
2
(H
2
O) was
.
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