Chemistry Reference
In-Depth Information
K
2
CO
3
ball milling
5 mm ball
1 mL EtOH
R'
O
+
+
R
Br
P
R'
H
R
Scheme 8.4 Solvent-free Wittig reaction using polymer supported triphenylphos-
phine and liquid assisted grinding.
amount of ethanol to the milling conditions the reaction went to completion
in less than 2 h (Scheme 8.4).
It was hypothesized that instead of swelling the resin particles the
particles undergo conformational changes that depend on the polarity of
the surrounding environment. The first step in the Wittig reaction is the
attack of the triphenylphosphine on the halide, resulting in the formation
of the phosphonium salt. The phosphonium salt needs to be accessible to
the base and aldehyde for the subsequent Wittig reaction to take place. In
a polar environment, the polar triphenylphosphonium bromide functions
on the resin particles are exposed, whereby the non-polar polystyrene
backbone is shielded (Figure 8.3). This presentation allows for greater
interaction between the resin particles, base, and aldehyde, resulting
in greater reactivity. Conversely, in a non-polar environment, the poly-
styrene backbone is exposed, shielding the polar triphenylphosphonium
bromide functions. In this conformation, the base and aldehyde have
decreased interaction with the triphenylphosphonium bromide, resulting
in longer reaction times and reduced yields. Along with alleviating many
of the problems with conducting the polymer supported reaction the
desired products were still able to be isolated without
the use of
chromatography.
Another report demonstrating the power of ball milling and polymer
supported reagents was given by Chen and co-workers.
35
The reaction
of phenylacetylene with tetrakis(triphenylphosphine)palladium gave
1,4-diphenylbuta-1,3-diyne in high yields. When Bis(triphenylphos-
phine)palladium(
II
) dichloride was used as the catalyst, they observed
1,4-diphenylbut-1-en-3-yne as the primary product instead of the 1,4-
diphenylbuta-1,3-diyne. These two catalysts showed similar reactivity
under ball milling as in solution but higher yields and higher diaster-
eoselectivity were achieved under ball milling. In an effort to develop
chromatography-free conditions the authors used polymer supported
palladium catalyst. Surprisingly, the polymer supported tetra-
kis(triphenylphosphine)palladium gave 1,4-diphenylbut-1-en-3-yne product
as the major product. Furthermore, only the trans stereochemistry was
observed where the cis product as well as the head to tail product is
observed when the free catalyst was used in solution. To determine
whether the polymer supported catalysts can give 1,4-diphenylbuta-1,3-
diyne free triphenylphosphine was added to the polymer supported
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