Chemistry Reference
In-Depth Information
tailored microenvironment. An early example of the application of this general
principle in synthetic chemistry was the encapsulating vessel produced by Cram
and coworkers (Fig.
9
) that encapsulated cyclobutadiene and stabilized this other-
wise highly unstable molecule [
16
]. Encapsulation blocks the contact of one guest
with another and prevents reaction, just as encapsulation of the iron cluster blocks
the contact with other iron clusters that would lead to a larger particle and eventu-
ally precipitation. This general principle is extremely powerful, but is limited by the
synthetic complexity of large, covalent structures like the one shown in Fig.
9
.
A powerful approach for circumventing the difficulty of the synthesis of
large hosts through conventional organic chemistry is the use of spontaneous self-
assembly. This can generate large, symmetrical structures with a defined inner and
outer space. Jean-Marie Lehn provided early examples of spontaneous assembly of
small subunits into larger ones with long-range order [
17
] and coined the term
“supramolecular assemblies” to describe these compounds.
R
R
R
R
H
H
H
H
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
H
H
H
H
R
R
R
R
Fig. 9 Encapsulation within a hemicarcerand allows cyclobutadiene, an anti-aromatic, highly
strained and reactive molecule, to be isolated
4
e
M
O
O
+
NH
4
O
O
e
e
+
M
Mg
2
,
< =
M
M
=
4
M
EtO
2
CCO
2
Et
EtO
OEt
1
Fig. 10 Ligand L and tetrahedral M
4
L
6
assembly first isolated by Saalfrank and coworkers and
reported in 1988. Taken from the table of contents illustration in [
18
]. Copyright Wiley-VCH.
Reproduced with permission
