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Figure 16.26 Propargylic ether 54 was converted to Pittsburgh Green by palladium
catalysis.
Adapted from Ref. 79.
Figure 16.27 Compound 55 undergoes Pd-catalyzed depropargylation to form fluor-
escent resorufin eight times faster than 56.
Adapted from Ref. 83.
Kim and co-workers indicated that the depropargylation of 56 catalyzed
by palladium was too slow, but could be accelerated by installing an
aminomethylthiophene unit, as shown in 55 (Figure 16.27). The palladium-
catalyzed depropargylation to form the fluorescent product resorufin was
eight
times faster
for 55 because the thiophene moiety bound to
palladium(II).
83
Using chemodosimeters POF and AOF for palladium,
81
Bai's group de-
veloped a method for palladium speciation (Figure 16.28). They were able to
distinguish among Pd(dppf)
2
Cl
4
,K
2
PdCl
6
, Pd(PPh
3
)
4
and Pd(PPh
3
)
2
Cl
2
as the
species designated HF may or may not be generated from POF and AOF and
the resulting complexation with palladium showed different emission
spectra.
84
As Figure 28 indicates, each palladium species showed a unique
combination of the fluorescence colors, although PdCl
2
and K
2
PdCl
6
could
not be distinguished from each other.
Liu's group combined depropargylation chemistry and spirocycle-
opening chemistry (Figure 16.29); the non-fluorescent propargylic amine
58 was converted to the fluorescent rhodamine derivative 59 by palladium
catalysis.
85
They attempted to elucidate the mechanism for the depro-
pargylation, but the NMR spectrum proved to be too ambiguous to draw a
conclusion.
As demonstrated by the above studies, palladium-catalyzed depropargy-
lation provides a useful platform for palladium-selective fluorogenic re-
actions. Intramolecular acceleration of this transformation, shown by Kim
and co-workers,
83
may lead to an even faster depropargylation method in the
future. However, rational design of faster depropargylation will require
better mechanistic insights.
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