solvent in these reactions,¶ can also be recovered on workup (in
!95% yield) and we have shown that this can be reused without
any detriment to the product yields. For example, dihydrofuran
2c was isolated in similar yields (ca. 50%) when using freshly
prepared ionic liquid or ionic liquid recovered from other
reactions. This work offers the potential for carrying out a
variety of metal-mediated radical reactions in ionic liquids and
this is currently under investigation.
Scheme 3
We thank BP Amoco Chemicals, the EU (under the
ERASMUS scheme) and The University of York for funding.
solvent system produced the dihydrofuroquinolone 4 in 42%
yield (Scheme 3).§ This can be compared to the corresponding
manganese(III) acetate/acetic reaction at 60 °C, which required
ultrasonic irradiation (because of the poor solubility of 3 in
acetic acid) to give quinolone 4 in 50% yield.8 A similar
manganese(III)-mediated reaction using quinoline-2,4-diol (1
equiv.) and a-methylstyrene (5 equiv.) in [bmim][BF4]–
dichloromethane gave a 1:1 mixture of the angular and linear
tricycles 5 and 6, respectively, in a combined 78% yield.
Notes and references
† To a solution of [bmim][BF4]10 (2 cm3) and dichloromethane (8 cm3)
containing cyclohexane-1,3-dione (0.10 g, 0.89 mmol) and a-methylstyrene
(0.53 g, 2.63 mmol, 5 equiv.) was added manganese(III) acetate dihydrate
(0.50 g, 1.88 mmol, 2.1 equiv.) under an atmosphere of nitrogen. The
reaction mixture was then heated overnight until the solution changed from
brown to yellow. Dichloromethane (40 cm3) was then added and
manganese(II) acetate was removed by filtration. The filtrate was then
washed with water (4 3 10 cm3) to remove the ionic liquid from the organic
phase. (The ionic liquid can be recovered by extracting the aqueous phase
with dichloromethane). The organic phase was then dried (MgSO4),
evaporated and purified using column chromatography (silica) to give
2-methyl-2-phenyl-3,5,6,7-tetrahydro-2H-benzofuran-4-one
1 (0.10 g,
50%) as an oil.
‡ All spectral data were in accord with the structures assigned. Selected data
for 1: dH(300 MHz, CDCl3) 7.30–7.15 (5H, m, aromatics), 2.98 and 2.89
(2H, dt, J 14.5, 1.9, CH2CPh), 2.45–2.40 (2H, m, CH2), 2.29–2.23 (2H, m,
CH2), 2.02–1.91 (2H, m, CH2), 1.62 (3H, s, CH3); dC(75 MHz, CDCl3)
196.0 (CNO), 176.4 (O–CNC), 146.0 (PhC1), 128.9, 127.9, 124.6 (PhC2–6),
113.0 (CNC–O), 93.0 (OCPh), 41.0 (PhCCH2), 36.8 (CH2CNO), 30.2 (CH3),
24.5, 22.1 (CH2CH2CH2).
§ When the reaction was carried out in dichloromethane in the absence of
the ionic liquid, quinolone 4 was not formed even after heating in an
ultrasonic bath (300 W, 30–40 kHz) for 24 h.
¶ Addition of the ionic liquid presumably increases the polarity of the
medium and similar reactions can be carried out using dichloromethane in
the presence of Et4N+2OTs. Reaction of cyclohexane-1,3-dione with a-
methylstyrene in a 0.1 M solution of Et4N+2OTs in dichloromethane also
gave tetrahydrofuranone 1 in 50% yield.
This work has shown, for the first time, that manganese(III)-
mediated radical reactions can be carried out in alternative
solvents to acetic acid in the presence of the ionic liquid
[bmim][BF4]. The ability to carry out manganese(III) reactions
in a variety of solvents, under much milder reaction conditions
than when using acetic acid, should extend the range of
precursors which can be employed in these types of reactions.
For example, we have shown that the tert-butyl ester 7 can be
reacted with a-methylstyrene in [bmim][BF4]–dichlorome-
thane to give 8 in 43% yield (Scheme 4). The same reaction at
50 °C in acetic acid afforded 8 in a much lower yield of 25%,
presumably because of the harsher (acidic) reaction condi-
tions.
At the end of the reaction, the manganese acetate can be
easily recovered (on precipitation) by addition of further
organic solvent to the reaction mixture. After filtration, the
manganese acetate was reacted with potassium permanganate to
re-oxidise the manganese(II) back to manganese(III).9 The
manganese(III) acetate can therefore easily be recycled although
this does depend on the nature of ionic liquid. For example,
when the related ionic liquid [bmim][PF6] was used, complete
removal of the manganese byproducts was problematic. The
ionic liquid [bmim][BF4], which is believed to act as a co-
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Scheme 4
Chem. Commun., 2001, 1350–1351
1351