Communication
RSC Advances
mogravimetric/differential thermal analysis (TG/DTA) was per-
formed on a thermal analyzer with a heating rate of 20 ꢂC minꢁ1
over a temperature range of 25–1100 ꢂC, under owing
compressed N2.
Preparation of Fe3O4–proline MNPs
5 mmol FeCl3$6H2O and 2.5 mmol FeCl2$4H2O salts were dis-
solved in 100 mL deionized water under vigorous stirring, then
2 mmol of proline and NH4OH solution (25%, w/w, 30 mL) were
added to the above mixture until the pH was raised to 11, at
which a black suspension was formed. This suspension was
ꢂ
then reuxed at 100 C for 6 h, with vigorous stirring. Fe3O4–
Scheme 4 A plausible reaction pathway for the condensation reaction
in the presence of Fe3O4–proline MNPs.
proline nanoparticles were separated from the aqueous solution
by magnetic decantation, washed with methanol, water and
ethanol several times before being dried in an oven overnight
(Scheme 1).
reaction, therefore we sonicated and heated the Fe3O4–proline
MNP suspension without reactants. Aer 4 h, we removed the
catalyst using an external magnet. If ions are released into the
solvent, this will be shown in the ICP analysis. However, the ICP
analysis didn't show any leaching of ions in the solvent.
Catalyst yield aer recycling is shown in Fig. 7. The catalyst
was recycled four consecutive runs without signicant loss in
yield.
General procedure for the direct synthesis of chromene
derivatives using Fe3O4–proline MNPs
A mixture of aryl aldehyde (1 mmol) and malononitrile (1 mmol)
in ethanol (2 mL) was stirred at room temperature for 30 min in
the presence of a catalytic amount of the Fe3O4–proline MNPs
(30 mg, containing 0.016–0.018 mmol proline). Then, 1 mmol of
2-hydroxynaphthalene-1,4-dione or 4-hydroxycoumarin was
added to the reaction mixture (Scheme 3). Aer completion of
the reaction (aer 24 h, monitored by TLC), the product was
dissolved in CH2Cl2 to remove the catalyst by an external
magnet. Aer drying, the precipitate was re-crystallized in
ethanol. The catalyst was washed with methanol and dried to
reuse. The catalyst could be recycled 4 times without any
measurable loss of activity. The desired pure products were
characterized by comparison of their physical data with those of
known compounds.9b,12
Experimental
All purchased solvents and chemicals were of analytical grade
and used without further purication. FT-IR spectra were
obtained over the region 400–4000 cmꢁ1 with a NICOLET IR100
FT-IR spectrometer with spectroscopic grade KBr. The 1H NMR
spectra were obtained with a BRUKER DRX-500 AVANCE
instrument using CDCl3 as the applied solvent and TMS as the
internal standard at 500.1. The powder X-ray spectrum was
recorded at room temperature by a Philips X'pert 1710 diffrac-
2-Amino-5-oxo-4-(4-methoxyphenyl)-4,5-dihydropyrano[3,2-
c]chromene-3-carbonitrile (2d). 1H NMR (DMSO-d6, 300 MHz): d
¼ 3.63 (s, 3H), 4.73 (s, 1H), 6.90–7.20 (m, 4H), 7.34 (s, 2H), 7.41
(d, J ¼ 8.3 Hz, 1H), 7.46 (t, J ¼ 7.6 Hz, 1H), 7.66–7.73 (m, 1H),
7.89 (d, J ¼ 7.8 Hz, 1H) ppm; 13C NMR (DMSO-d6, 75 MHz): d ¼
55.2, 57.9, 104.5, 122.8, 116.4, 119.1, 122.3, 124.6, 126.6, 123.6,
126.7, 127.8, 130.0, 132.7, 135.2, 142.2, 152.0, 153.4, 157.7, 159.5
ppm; IR (KBr, cmꢁ1): 3400, 3283, 3179, 2202, 1709, 1675, 1637,
1603, 1490, 1457, 1377, 1171, 1059, 957, 753; MS (EI, 70 eV) m/z
(%) ¼ 346 (M+, 18), 249 (24), 240 (17), 239 (100), 121 (21).
˚
tometer using Cu Ka (a ¼ 1.54056 A) in Bragg–Brentano
geometry (q–2q). The morphology of the catalyst was studied by
scanning electron microscopy using a SEM (Philips XL 30 and
S-4160) with gold coating equipped with energy dispersive X-ray
spectroscopy. The magnetic properties of the Fe3O4–proline
MNPs were measured with a homemade vibrating sample
magnetometer/Alternating Gradient Force Magnetometer
Conclusions
In summary, we have developed a nanoparticle-supported
organocatalyst analogue as an active and reusable heteroge-
neous base catalyst. Proline has emerged as a viable alternative
to magnetic nanoparticles for use as a robust, readily available,
high surface area heterogeneous support in catalytic trans-
formations. It possesses the advantage of being magnetically
recoverable, so eliminating the requirement of catalyst ltration
aer the reaction.
Fig. 7 Activity loss as a function of the number of reuses of the Fe3O4–
proline MNPs for the synthesis of 2a.
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 6508–6512 | 6511