Chemistry Reference
In-Depth Information
a fluorescence plate reader costs $6000-15 000 and the plate reader in the
Koide laboratory has not required repairs during the past 5 years. Therefore,
as far as palladium analysis is concerned, a fluorescence method is more
consistently available at a lower cost. ICP-MS can analyze most metals; a
fluorescence plate reader can be used for a limited number of metals and
many enzyme assays. An ICP-MS analysis of one sample takes only a few
minutes; the APE method takes 15-60 min. However, the APE method can
analyze dozens to hundreds of samples in parallel in the 15-60 min span -
fractions of a minute are necessary per sample when many samples are
analyzed. The APE method can be employed by a first-year undergraduate
student within a few hours of training, whereas many ICP-MS operators are
PhD graduates. Although not ideal, users must prepare a solution devoid of
NaBH
4
and add a freshly made NaBH
4
solution in aqueous NaOH as needed
because an NaBH
4
solution in DMSO-pH 7 buffer cannot be stored for more
than half a day. Commercially available solutions of NaBH
4
in aqueous
NaOH were found to be ineffective in the Koide laboratory (unpublished
results). Nonetheless, this method may be the most practical to date, as it
has been implemented at Merck.
If given a specific concentration of palladium ions as a threshold, the
method developed by Baker and Phillips
75
may prove useful (Figure 16.23);
the Alloc-protected aniline derivative 43 underwent a palladium-catalyzed
deallylation-decarboxylation reaction to form 44 in situ. This compound was
then hydrolyzed to generate 2 equiv. of fluoride ions. Each fluoride removed
a TBS group from 45 to form 4-aminobenzaldehyde and 2 equiv. of fluoride
ions. Thus, the whole system was designed to propagate the color intensity
originating from 4-aminobenzaldehyde.
75
The same group developed
another method in which the palladium-catalyzed deallylation and well-
established Fmoc chemistry were combined to amplify absorption signals
autocatalytically.
76
The deallylation process was applied to another chromophore by Liu's
group (Figure 16.24);
77
the Alloc derivative 47 was subjected to palladium-
catalyzed deallylation conditions similar to those in previous work
59
to form
amine 48. The starting material was blue fluorescent and the product was
green fluorescent, allowing for a ratiometric detection of palladium.
77
The
metal selectivity favored palladium. They also screened anions to discover
that this deallylation chemistry was not subject to interference by F
,Cl
,
Br
,I
, CNS
,HSO
4
,H
2
PO
4
,NO
3
,ClO
4
and AcO
.
77
Qian's group developed a colorimetric method based on the palladium-
catalyzed conversion of the blue-fluorescent allyl ether 49 to the yellow-
fluorescent product 50 [Figure 16.25(a)].
78
To demonstrate its utility, they
formed biaryl 53 [Figure 16.25(b)], subjected it to three rounds of purifi-
cation and the palladium content was monitored after each purification step
using paper strips. The color change diminished after each purification step
due to the removal of palladium from the material. Their successful dem-
onstration indicated that it may be possible to develop paper strips for
palladium detection in the API production process. Their method has not yet
Search WWH ::
Custom Search