Table 1 Tandem deacetalization–Knoevenagel condensation reactions
monitored by NMR
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Entry
Conversion (%) 1
Yielda (%) 2
Yielda (%) 3
1st round
B100
B100
99.6
99.9
97.3
Trace
3.37
8.06
3.29
81.3
B100
96.6
91.9
96.7
16.0
2nd recycle
3rd recycle
4th recycle
Acidic cond.b
Basic cond.c
B0
Trace
Trace
a
The yields were calculated by integration of benzylic protons at the
b
end of the reaction. Control reaction in the presence of p-toluene-
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c
sulfonic acid. Control reaction in the presence of ethylenediamine to
block the open Cu sites.
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All reactions were performed with 2 mmol substrates in
DMSO-d6 using 0.5 mol% (0.01 mmol) PCN-124 for 12 h.
1H NMR spectroscopy was used to monitor the progress of the
reaction and to determine the reaction yield. The starting material
(dimethoxymethylbenzene), the intermediate (benzaldehyde), and
the final product (benzylidene malononitrile) have distinct
1H NMR resonances for their benzylic protons (as denoted
by different colors in Scheme 1), making it straightforward to
monitor the reaction.
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Table 1 summarizes the reaction results. PCN-124 was
found to be a highly active cooperative catalyst for this
tandem one-pot reaction with a turnover number of more
than 190. Control experiments proved that both open Cu2+
sites and amide groups are essential for the reaction, and they
work cooperatively. In the presence of p-toluenesulfonic acid,
the deacetalization reaction worked well but the Knoevenagel
condensation was inhibited. In the presence of ethylenediamine,
which alone could catalyze the second step Knoevenagel
condensation, the open Cu2+ sites in PCN-124 were blocked
and neither 2 nor 3 could be formed. Moreover, PCN-124 could be
easily recovered from the reaction medium by centrifugal separa-
tion and reused at least for three cycles without significant loss of
activity. The PXRD patterns of the fourth recycled PCN-124 were
still consistent with the as-synthesized one, indicating its excellent
chemical stability (Fig. S4, ESIz).
In conclusion, we have presented a multi-functional MOF
PCN-124 with embedded chemically antagonistic functional
groups. On one hand, the micro-porosity, open Cu2+ sites,
and amide groups in this MOF enable selective adsorption of
CO2 over CH4 as well as a cooperative catalyst for a tandem
deacetalization–Knoevenagel reaction. Our work demonstrates
that through judicious design various functional groups could
be integrated into one MOF to realize distinct functions.
This work was supported by the U.S. Department of Energy
(DOE DE-SC0001015 and DE-FC36-07GO17033), the National
Science Foundation (NSF CBET-0930079), and the Welch
Foundation (A-1725).
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c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 9995–9997 9997