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H
CO
2
Et
CO
2
Et
H
2
NCH
2
CO
2
Et
.
HCl
NaNO
2
, 30 min.
Ball milling
N
N
N
N
H
45
46
Scheme 1.54 Mechanochemical [2
þ
3] cycloaddition reaction of fullerene with
in situ generated diazo compound.
1.3.6 Macrocycle Formation
The use of mechanochemistry in the formation of metallomacrocycles and
cages is not very common in structural supramolecular chemistry. However,
ball milling provides a solvent-free route for the synthesis of macrocycles
with better yield than any classical solution based method.
To synthesize complex molecular architectures based on boronic acids,
Severin and co-workers reported a one-pot condensation of 4-formylphenyl-
boronic acid 47, pentaerythritol (48) and tris(2-aminoethyl)amine (tren) to
afford a macrobicyclic structure.
51
Solid state structure determination of this
macrobicyclic compound revealed a ''collapsed'' geometry with no accessible
space inside the cage. To make larger and more expanded cage structures,
the same condensation reaction was performed by exchanging the
flexible tren building block for more rigid 1,3,5-tris(aminomethyl)-2,4,6-
triethylbenzene (49). Mechanochemical condensation reaction between the
boronic acid 47, tetraol 48 and triamine 49 produced cage 50 quantitatively
within 1 h at milling frequency 20 Hz. Furthermore, if a more elongated
boronic acid, i.e. 4-(4-formylphenyl)phenylboronic acid (51) was used, it
provided the corresponding cage 52 in good yield within 1 h (Scheme 1.55).
52
In comparison with solution phase, the yields of these reactions were always
low (
o
40%) and the products also were always contaminated with significant
amounts of an incomplete condensation product. The overall geometry and
characterization of cages 50 and 52 were obtained from multinuclear NMR
spectroscopy, mass spectrometry and single-crystal X-ray diffraction data.
In continuation with macrocycle preparation in a ball mill, a multi-
component polycondensation reaction of 4-formylbenzene boronic acid (53),
t-Bu
2
Si(OH)
2
(54) and diamines (4,4
0
-bis(aminomethyl)biphenyl (55) and
(1R,2R)-1,2-diaminocyclohexane (56)) was performed under ball milling to
produce the corresponding borasiloxane-based macrocycles 57 and 58 with
an isolated yields of 85% and 65% respectively (Scheme 1.56).
53
Here, first
the condensation of silane diols with boronic acids gave the cyclic bor-
osiloxanes, and subsequent combination with the diamines produced the
corresponding borosiloxane-based macrocycles.
Another class of macrocyclic compound, rotaxanes, has shown potential
applicability as molecular actuators and switches within mesoscale mo-
lecular electronic devices.
In recent
times Chiu and co-workers have
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