Green photochemistry: Photo-Friedel-Crafts acylations of 1,4-naphthoquinone in room temperature ionic liquids

Article abstract of DOI:10.1039/b913252j

The photo-Friedel-Crafts acylation of 1,4-naphthoquinone with various aldehydes was investigated in a series of room temperature ionic liquids. High conversions and selectivities were achieved in [C₂mim] ⁺-based ionic liquids with the highest isolated yields found in [C₂mim][NTf₂]. The developed procedure allowed for a replacement of hazardous solvents such as benzene and acetonitrile which are commonly used for this transformation. The Royal Society of Chemistry 2009.

Full text of DOI:10.1039/b913252j

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www.rsc.org/greenchem | Green Chemistry
Green photochemistry: photo-Friedel–Crafts acylations
of 1,4-naphthoquinone in room temperature ionic liquids
Brian Murphy,a Peter Goodrich,b Christopher Hardacreb and Michael Oelgemo¨ller*a,c
Received 6th July 2009, Accepted 4th August 2009
First published as an Advance Article on the web 7th September 2009
DOI: 10.1039/b913252j
The photo-Friedel–Crafts acylation of 1,4-naphthoquinone with various aldehydes was
investigated in a series of room temperature ionic liquids. High conversions and selectivities were
achieved in [C2mim]+-based ionic liquids with the highest isolated yields found in [C2mim][NTf2].
The developed procedure allowed for a replacement of hazardous solvents such as benzene and
acetonitrile which are commonly used for this transformation.
acylation of 1,4-naphthoquinone in benzene and various room
temperature ionic liquids, the latter either commercially available
or readily synthesised by standard procedures.1
Introduction
Ionic liquids (ILs) have been intensively studied over the
last decade as non-flammable and replacement solvents for
molecular systems.1 The ease of product removal allows an
efficient reuse of the ILs and this property, coupled with the
lack of vapour pressure, has the potential to result in materials
which are much less environmentally intrusive than conventional
organic solvents. However, recent studies on the toxicity of
ionic liquids have raised some concerns regarding their broader
use.2 In addition to their physical properties, ionic liquids can
stabilise a wide range of highly reactive species and through
the ionic solvent–solute interactions can direct the chemistry.3
For example, ionic liquids have been shown to stabilize radicals
and radical ions in solution more efficiently than many con-
ventional organic solvents.4,5 This coupled with their weak or
missing absorption of light above 300 nm makes them an ideal
solvent for photochemical reactions.6 Despite these advantages,
photochemical transformations in ionic liquids are rare.7 This
is surprising since photochemistry itself is regarded as a ‘green
technology’,8 especially if conducted with sunlight.9 However,
photochemical transformations conventionally use solvents that
are toxic and/or flammable.
The photoacylation of 1,4-naphthoquinone (1) with bu-
tyraldehyde (2c) was initially chosen as a model reaction
to determine the optimum cation/anion combination for the
ionic liquid (Scheme 1). A mixture of 1,4-naphthoquinone and
excess of butyraldehyde dissolved in the IL was irradiated
for 16 h while stirring. After work-up the crude product
1
was analyzed by H-NMR spectroscopy.14 The crude mixture
generally consisted of unreacted 1,4-naphthoquinone (1), the
desired photoacylation product (3c) and the reduction product
1,4-dihydroxynaphthalene (4). The ratio of these components
critically depended on the structure of the ionic liquid employed
(Table 1).
Over the last years we have intensively studied the photo-
chemical acylation of 1,4-quinones with aldehydes as a mild
and atom efficient alternative to Friedel–Crafts acylations or
(photo-)Fries-rearrangements.10,11 The original procedure devel-
oped by Kraus and Kirihara involved the use of hazardous ben-
zene or acetonitrile as solvents.12 Supercritical carbon dioxide
(SC-CO2) has recently been reported as an alternative medium,13
but this technique suffers from technical limitations, e.g. high
pressure or small reactor volume, especially for a large-scale
application. We have, therefore, investigated the photochemical
Scheme 1 Photoacylation of 1,4-naphthoquinone with butyraldehyde.
In many of the chosen ionic liquids the photoacylation prod-
uct 3c was formed in larger amounts than in the conventional
solvent benzene. The best result was achieved in [C2mim][OTf]
and 3c was obtained in 91% yield without any photoreduction
product (4) present. The photoreaction in [C2mim][NTf2] also
resulted in a high yield of 3c at 81%; however, in this case,
1% of the photoreduction product (4) was formed as well.
In both cases, the desired product 3c was isolated by column
chromatography.
For [C2mim][OTf], the isolated yield of 3c was low with
only 10% obtained following work-up. This low yield may be
associated with the strong hydrogen bonding interaction of the
substrate with the ionic liquid anion. On changing the anion
from [OTf]- to [NTf2]- the isolated yield of 3c improved to 40%
(49% based on the amount of 1 consumed). This is still only
modest and reflects the strong affinity of aromatics and carbonyl
aSchool of Chemical Sciences and NCSR, Dublin City University,
Glasnevin, Dublin 9, Ireland
bSchool of Chemistry and Chemical Engineering/QUILL, Queen’s
University, Belfast, Northern Ireland, UK BT9 5AG
cSchool of Pharmacy and Molecular Sciences, James Cook University,
Townsville, Queensland, 4811, Australia.
E-mail: oelgemoeller@jcu.edu.au; Fax: +61-7-4781-6078;
Tel: +61-7-4781-4535
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Table 1 Product compositions for photoacylations of 1 with 2c
Table 2 Product compositions for photoacylations of 1 with various
aldehydes 2
Composition [%]b
Composition [%]a
Solventa
1
3c
4
[C2mim][OTf]
[C2mim][NTf2]
Benzene
49
9
44 (23c/45d)
7
Entry
R
1
3
4
1
3
4
[C2mim][OTf]
[C2mim][NTf2]
[C2mim][BF4]
[C4mim][NTf2]
[C6mim][PF6]
[C8mim][PF6]
[C4Py][NTf2]
[C4mPyrr][NTf2]
91 (10c/11d)
—
1
—
46
43
51
1
18
58
29
44
39
24
12
81 (40c/49d)
b
b
a
b
c
d
e
f
g
h
i
CH3
—
55
9
25
26
18
37
26
35
20
69
36
40
38
46
70
74
81
48
66
47
70
23
51
41
52
43
5
—
1
15
8
18
10
8
13
19
10
11
42
25
13
10
75
83
C2H5
C3H7
C4H9
C5H11
C6H13
C7H15
C8H17
C10H21
C11H23
Ph
45
91
73
—
—
—
27
—
30
50
39
62
—
50
38
70
50
56
32
—
—
5
5
a [Cnmim]+ = 1-alkyl-3-methylimidazolium, [C4Py]+ = 1-butyl-pyri-
dinium, [C4mPyrr]+ = 1-butyl-1-methylpyrrolidinium, [NTf2]- = bis-
{(trifluoromethyl)sulfonyl}imide, [OTf]- = trifluoromethane sulfonate.
b As determined by integration of characteristic peaks in the 1H-NMR of
the crude product. c Isolated yield. d Isolated yield based on conversion.
6
b
j
k
l
50
52
—
10
=
MeCH CH
a As determined by integration of characteristic peaks in the 1H-NMR
spectrum of the crude product. b Not studied.
species in ionic liquids. Similarly poor isolation was found for
other Friedel–Crafts processes.15 On extending the alkyl length
of the imidazolium side-chain, photoreduction to 4 increased
with the largest yield of 4 (51%) found in [C8mim][PF6]. The
influence of the anion on the photoacylation was studied using
[C2mim]+-derived ionic liquids. The chemoselectivity remained
high in all three cases studied; however, the conversion was
significantly reduced in [C2mim][BF4]. This finding may be
associated with the higher viscosity of the [BF4]- based ionic
liquid.1c,16 The higher viscosity, and thus the reduced diffusion,
hinders the necessary approach of the two reactants during the
irradiation. It was also noticed that the [PF6]- based ionic liquid
was less favourable for the photoreaction and large amounts of
the reduction product 4 were produced.
The photoacylation protocol was further applied to a range
of aldehydes (Scheme 2). Due to the high selectivity obtained
for the 1/2c model pair in combination with the good isolated
yield of 3c, [C2mim][NTf2] was selected as the ionic liquid for
this study. For comparison, selected transformations were also
studied in [C2mim][OTf].
on the solubility of the starting material in an ionic liquid has
been reported for the attempted triplet-sensitised photolysis of
myrcene.17 Likewise, benzaldehyde and crotonaldehyde gave the
desired acylated hydroquinones 3k and l in moderate yields and
selectivities.
For recycling purposes, the purity of the ILs was investigated
by NMR spectroscopy after irradiation and work-up.‡ Although
1
the H-NMR spectra of the recovered ILs did not reveal any
significant changes, a strong yellow or brown colour often
remained. Decolourisation was easily achieved by diluting the
ionic liquid with methanol and stirring with activated charcoal.
Subsequent filtration and removal of methanol gave almost
entirely colourless ionic liquids. Experiments performed with
recycled solvent showed no discernible loss in efficiency even
after 3 cycles.
The mechanism of the photoacylation has been extensively
studied by Bruce and Maruyama and both in-cage and out-of-
cage scenarios have been described. Both mechanisms operate
essentially simultaneously depending on the specific reaction
conditions of the irradiation experiment, i.e. temperature, sol-
vent, quinone/aldehyde applied.10a The photoaddition is initi-
ated by hydrogen-abstraction from the aldehyde function to the
triplet excited quinone (Scheme 3, path A). The higher viscosity
of ionic liquids compared with conventional solvents may
prevent separation of the intermediary radical pair by diffusion,
thus favouring the in-cage mechanism. A similar assumption
was also made for the photoreduction of benzophenones in
ionic liquids.18 Addition of the acyl radical to the delocalised
semiquinone radical and subsequent tautomerisation yields
the acylated hydroquinones 3. For long alkyl-chain containing
ionic liquids, which are highly viscous, hydrogen abstraction
from the ionic liquid can operate the alternative pathway
(path B) thus yielding the photoreduction product 4. It is known
that the triplet excited states of some molecules are capable
Scheme
2
Photoreaction of 1,4-naphthoquinone with various
aldehydes.
The conversion rate was lower for long chain aldehydes
(Table 2). With increasing chain-length of aliphatic aldehydes,
a general increase in the photoreduction product 4 was also
noticed.† This tendency may be best explained by the reduced
solubility of long-chained aldehydes in the selected ionic liquids
hence favouring hydrogen abstraction from the ionic liquid
instead. A similar dependency of the outcome of a photoreaction
‡ For some common organic solvents, solvent recovery can be envi-
ronmentally and economically harmful because of the large cumulative
energy demand (CED) for the process.22 However, the relatively high
price for common ionic liquids may justify recovery and reuse.
† In contrast, photoacylations in benzene give similar yields independent
from the aldehyde.11
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10 For a recent review on the photoacylation of quinones, see: (a) M.
Oelgemo¨ller and J. Mattay, in CRC Handbook of Organic Photochem-
istry and Photobiology, 2nd edn, W. M. Horspool and F. Lenci, (eds.),
CRC Press, Boca Raton, 2004, Ch. 88, p. 1; for a general review on the
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Scheme 3 Mechanistic scenario for the photoacylation and -reduction.
of abstracting hydrogen atoms from the side-chains of ionic
liquids.19 The improved efficiency, i.e. higher conversion rates,
of the photoacylation process in most ionic liquids may be best
explained by an enhanced lifetime of the triplet exited state of
the quinone as, for example, known for xanthone.4
In conclusion, the photoacylation of 1,4-naphthoquinone
with a series of aldehydes has been carried out in ionic
liquids. High conversions and chemoselectivities have been
achieved in [C2mim][OTf] and [C2mim][NTf2], respectively. The
isolated yield of model compound 3c was, however, better in
[C2mim][NTf2]. The ionic liquids replaced previously used toxic
solvents (benzene and acetonitrile) and, in comparison, showed
increased activity and selectivity. In addition, the ionic liquid
could be recovered and reused. Hence, this transformation can
be regarded as a model reaction for ‘Green Photochemistry’.20
The simple protocol is currently being transferred to ‘micro-
photochemistry’, i.e. photochemical transformations in micro-
structured devices.21
12 (a) G. A. Kraus and M. Kirihara, J. Org. Chem., 1992, 57, 3256;
(b) G. A. Kraus and P. Liu, Tetrahedron Lett., 1994, 35, 7723.
13 R. Pacut, M. L. Grimm, G. A. Kraus and J. M. Tanko, Tetrahedron
Lett., 2001, 42, 1415.
14 General procedure for irradiation: In a typical photochemical
experiment 63 mg (0.4 mmol) of 1,4-naphthoquinone and 2 mmol
of aldehyde were dissolved by mild heating and vigorous stirring in
5 ml of ionic liquid in a Pyrex test tube (transmission ≥ 300 nm).
The solution was degassed with argon and irradiated for 16 h
using a Rayonet photochemical reactor (RMR-600; Southern New
Acknowledgements
This research project was financially supported by Dublin
City University (Research Career Start Award 2006), Science
Foundation Ireland (SFI, 07/RFP/CHEF817 and 06/RFP/
CHO028), QUILL and a Portfolio Partnership grant from
the EPSRC. BM thanks the EU Marie Curie Early Stage
Training Site Fellowship programme for support, contract
number HPMT-GH-00-00147-03.
˚
England Ultraviolet Company) equipped with 8 RPR-3000 A lamps
(lmax = 300 25 nm). The ionic liquid was then extracted with 7 ¥
10 ml portions of diethyl ether. The combined organic fractions were
washed with saturated sodium bisulfite solution (3-times) and brine,
dried over MgSO4 and evaporated. The dried crude product was
analysed by 1H-NMR at this stage. Purified acylation products 3
were isolated by column chromatography (silica gel, ethyl acetate/
n-hexane mixtures).
15 (a) C. Hardacre, P. Nancarrow, D. W. Rooney and J. M. Thompson,
Org. Process Res. Dev., 2008, 12, 1156; (b) C. Hardacre, S. P. Katdare,
D. Milroy, P. Nancarrow, D. W. Rooney and J. M. Thompson,
J. Catal., 2004, 227, 44.
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