ChemComm
Communication
In conclusion, for the first time, we have designed and
executed a thermal topochemical reaction in a coassembly of
two low molecular weight organogelators decorated with com-
plementary reacting motifs, viz. azides and alkynes. We have
designed two 4,6-O-benzylidene-D-galactopyranoside organo-
gelators with b-azide and a-propargyloxy motifs at the anomeric
position which coassemble in a 1 : 1 molar ratio in non-polar
solvents such that the azide and alkyne functionalities are
close enough to undergo thermal 1,3-dipolar cycloaddition. The
coassembly of the two gelators was proved by different NMR
techniques, Thermogravimetry, Powder-XRD of the xerogels and
SEM imaging of the xerogels. The xerogel of this co-assembled gel
underwent topochemical cycloaddition reaction upon mild heat-
ing, following sigmoidal kinetics as expected of a topochemical
reaction. This is not only the first report on the topochemical
reaction between two non-identical reactants in their supramole-
cular coassembly but also the first example of application of
organogelators for thermally activated topochemical reactions.
Fig. 4 (A) Possible arrangement of 3 and 4 in the coassembled state and their
topochemical reaction in the xerogel state. SEM image of the xerogel is shown in
the background. (B) and (C) Kinetics of the topochemical reaction at 55 1C.
kept at a constant temperature of 55 1C and the reaction was
monitored by TLC.13 The reaction was almost complete in one week
with the formation of both the possible products 5 and 6 in a 1 : 1
ratio.14 We have followed the kinetics of this reaction by using NMR
spectroscopy (Fig. 4B and C and ESI‡). The reaction followed a
sigmoidal kinetics (ESI‡) typical of a topochemical reaction. Impor-
tantly, the solid was physically intact throughout the course of the
reaction suggesting that the reaction occurred in the solid coassem-
bled state. Furthermore, the PXRD spectra of the xerogel during
and after the reaction revealed that the crystalline nature is
maintained throughout the course of the reaction (ESI‡). Also, a
physical mixture obtained by simple mixing of individually prepared
xerogels of 3 and 4 did not react under this condition suggesting that
the reaction is topochemically controlled by the coassembly of the
reactants in the xerogel. A DFT simulation study also supported the
coassembly of 3 and 4 favorable for topochemical reaction (ESI‡).
When the xerogel of the equimolar mixture was kept at 120 1C,
the xerogel melted and reacted to give 5 and 6 in a 2.5 : 1 ratio.
Thus, when the reaction was not under topochemical control (in the
melt), a clear selectivity for the sterically less crowded product 5
was observed as expected. A xerogel of an equimolar mixture of 3
and 4-b did not react topochemically as anticipated (ESI‡).
Notes and references
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c
This journal is The Royal Society of Chemistry 2012
Chem. Commun.