Chemistry Reference
In-Depth Information
Table 8.1 Comparison of the rate of reduction of polymer supported catalyst to non-
supported catalyst. Reprinted (adapted) with permission from ref. 29.
Copyright 1971 American Chemical Society.
RhCl(Ph
3
P)
3
on polymer
bead. Relative rate
Olefin
Free RhCl(Ph
3
P)
3
in solution
Cyclohexene
1
1
1-Hexene
2.55
1.4
D
2
-Cholestane
1/32
1/1.4
Octadecene
1/2.06
1/1.4
Cyclooctene
1/2.54
1
Cyclododecene
1/4.45
1/1.5
8.3 Mechanochemical Activation of Polymer
Resins
Given the diculty of polymer supported resins in solution (i.e., finding the
right swelling solvent, long reaction times, and differences in reactivity
depending on substrate) the use of polymer supported reagents in organic
synthesis has dwindled, along with the ability to purify and isolate substrates
without chromatography. It appears the very weakness of mechanochemical
reactions (i.e., the use of solvents to isolate products) is the very strength of
polymer supported reactions. Further, the very weakness of polymer sup-
ported reagents (i.e., the need for solvent to activate the polymer support) is
the very strength of mechanochemical reactions. It is known that mechan-
ical energy can have an effect on the size of polymer beads. Depending of the
polymer bead, sometimes simply stirring the bead can have a noticeable
impact. Therefore, instead of swelling the bead to allow the interaction
of the bead with reagents the thought is to significantly reduce the particle
size of the bead, whereby exposure of the surface will take place through
mechanical activation. There are several benefits of using mechano-
chemistry to activate polymer beads over the traditional swelling mech-
anism. Using mechanochemistry would alleviate the use of harmful organic
solvents in the process of swelling the polymer bead. The rate of the reaction
would be significantly increased because the action of mechanochemical
grinding forces reagents together, which leads to a significant increase in
the reaction rate. The forcing of reagents together under mechanical action
would also subdue the difference of reactivity of different substrates
due to size.
There are many reports in the literature of the Wittig reaction using
polymer supported resins with differing results. Simply to generate the
phosphonium salt the reaction takes 48 h at 70 1C to complete. Once that
has been satisfied there is an additional 4-24 h to conduct the deprotonation
and addition steps needed to complete the reaction, depending on the
amount of crosslinking used for the polymer support. The reaction does not
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