Chemistry Reference
In-Depth Information
this rearrangement asymmetrically [
30
], normally a very challenging task for
electrocyclizations.
The most recent and powerful example of supramolecular catalysis comes from
an elegant combination of the principles delineated above. The Nazarov cyclization
can be used to prepare Cp*(pentamethylcyclopentadiene) from a mixture of
pentanedienols, as in Fig.
20
. This reaction requires formation of a carbocation
by dehydration of the protonated alcohol, and then electrocyclization of the
corresponding bis-allylic carbocation.
The rate accelerations observed in the presence of the M
4
L
6
assembly are
spectacular - the encapsulated substrate cyclizes over two million times faster
than the unencapsulated alcohol [
31
]. This very high level of catalytic activity
and the observed kinetics emulate the remarkable activity of enzymes. The
observed rate acceleration is too large to be explained by an equilibrium perturba-
tion such as the increased basicity of the substrate, leading to a 1,000-fold rate
acceleration in the hydrolysis of orthoformates [
21
], the idea is that the act of
binding the guest in the cavity can only be responsible for four to five of the orders
of magnitude of rate acceleration. Thus, the million-fold rate enhancement in this
system is due at least in part to binding of the transition state.
5 Closing Remarks on Inner and Outer Beauty
The clusters whose chemistry our research group has explored in recent years have
been remarkable in continuing to show new and unusual properties. Like enzymes,
in which the inner space is controlled by protein folding and hence micro-
environments are created that differ dramatically from the surrounding bulk sol-
vent, these clusters are highly water-soluble and yet carry out chemistry normally
seen only in nonaqueous solvents. Also like enzymes, stabilization of the transition
state upon guest binding can dramatically alter the reactivity, and even change the
product distribution, for the reactions of guest molecules. Although this beauty may
be particularly appreciated in the eyes of these authors, we offer these examples -
and related chemical examples for which we express both aesthetic and scientific
admiration - to our readers, with the hope that they will also appreciate them.
Having started with a quote from the famous naturalist John Muir we will
end with a quote from an artist who had the same appreciation for the natural
beauty that Muir described. As Ansel Adams expressed in a letter to his friend
Cedric Wright [
32
]:
Art is both love and friendship and understanding: the desire to give. It is not charity, which
is the giving of things. It is more than kindness, which is the giving of self. It is both the
taking and giving of beauty, the turning out to the light of the inner folds of the awareness of
the spirit. It is a recreation on another plane of the realities of the world; the tragic and
wonderful realities of earth and men, and of all the interrelations of these.
