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synthesize 4-8 g (9.4-18.7 mmol) of APE, as needed. These are substantial
amounts, as each palladium analysis requires 2.5-10 nmol of APE in 0.2 mL
of solution.
A potential pitfall was that different palladium species in complex samples
could react with APE at different rates, not meeting criterion 3 in section
16.3. To overcome this potential problem, excess NaBH
4
and Ph
3
P were
added to the deallylation reaction solutions to ensure that all the palladium
species with various oxidation states would be converted to Pd(PPh
3
)
4
in situ.
Under these reaction conditions (NaBH
4
,Ph
3
P, DMSO, pH 7 phosphate
buffer), the fluorescence intensity, as a measure of the conversion of APE to
Pittsburgh Green, was linearly correlated with palladium concentration. As
shown in Figure 16.16(a), palladium was the most ecient metal for cata-
lyzing the deallylation reaction. Platinum, with slightly under half the e-
ciency, was the second most reactive metal for this transformation. A better
selectivity for palladium over platinum could be achieved by carrying out the
assays in a pH 4 buffer.
62
Although a slight signal could be observed with
CuCl
2
, it was found later that the particular batch of CuCl
2
was contamin-
ated with traces of palladium.
63
As expected, 2 mM PdCl
2
and 2 mM Pd(PPh
3
)
4
produced the same fluorescence intensity due to the presence of excess
PPh
3
. This fluorimetric method was used by Semagina and co-workers to
elucidate the mechanism for the deallylation of APE catalyzed by palladium
nanoparticles and their soluble derivatives.
64
When the deallylation was carried out in THF, the reaction mixture turned
red in a palladium concentration-dependent manner [Figure 16.16(b)]. This
result indicated that the conversion of APE to Pittsburgh Green could be
used as a colorimetric method to detect palladium in the micromolar range.
The origin of the red color is currently being investigated.
Figure 16.16
(a) Metal selectivity for the deallylation of APE in the presence of Ph
3
P.
(b) Palladium-catalyzed deallylation of APE in the presence of NaBH
4
in THF. [PdCl
2
]=10mM (left) and 1.6 mM (right).
Reproduced with permission from Ref. 59. Copyright 2007 American
Chemical Society.
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