Chemistry Reference
In-Depth Information
dextrin hydroxyl groups. Therefore, in the selective reaction either para or ortho sub-
stitution can be catalyzed if the geometry of the complex is suitable.
One important control was the study of the diazo coupling of phenol with
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. Here
para coupling formed
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, but the reaction was inhibited by
-cyclodextrin. The diazo-
nium group of
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cannot be delivered by binding to a cyclodextrin hydroxyl group, in
contrast to a chlorine atom. This supports the idea that cyclodextrin catalysis of chlor-
inations involves binding of both the chlorine atom and substrate, holding them to-
gether for rapid selective reaction.
In a further study,
a
-cyclodextrin that was partially methylated, blocking the OH
groups at C-2 and C-6 and leaving the C-3 OH group free [144], was found to catalyze
the para chlorination of anisole. Therefore, at least the cyclodextrin C-3 OH can be the
catalytic group. Furthermore, the binding of anisole was stronger than that in
a
-cy-
clodextrin, probably reflecting the flexible capping of the cavity described earlier
[84]. Thus it was possible to achieve 99% para chlorination of anisole (without the
flexible capping, there was always some free anisole in solution that led to some ortho
chlorination as well). We also synthesized a polymer, consisting of linked
a
-cyclodex-
trins, that could be used in a flow mode to produce anisole that was over 99% para
chlorinated, because there was essentially no free anisole in equilibrium with the poly-
meric
a
a
-cyclodextrin and the reaction within the complex was completely selective
[144].
1.4.2
Selective Photochemical Reactions
Our principal target in P-450 mimics was the selective oxidation of saturated carbons
directed by geometric control, not by intrinsic reactivity. In our first study, we exam-
ined selective geometrically controlled attack on aliphatic C-H bonds by photo-excited
benzophenones [145]. In a process we labeled “remote oxidation“, photolysis of a long-
chain ester
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of benzophenone-4-carboxylic acid afforded insertion into CH
2
groups
far into the chain.
In such an insertion the benzophenone is excited to its triplet state, resembling a
C-O diradical. The oxygen atom of the ketone then removes a hydrogen atom from the
methylene group, leading to a carbinyl radical and the hydroxy diphenylmethyl radical
formed by hydrogen atom addition to the benzophenone. This pair of radicals couples
