Titanium silicalite 1 (TS-1) catalyzed oxidative transformations of furan derivatives with hydrogen peroxide

Article abstract of DOI:10.1002/adsc.200303185

The oxidation of furan derivatives with titanium silicalite 1 (TS-1) and hydrogen peroxide is described. Oxidation products are identified and possible reaction pathways are discussed. It is shown that the oxidation of these compounds occurs via epoxidation of one of the furan double bonds. The initially formed epoxides immediately undergo rearrangement, furans yielding unsaturated 1,4-dicarbonyl compounds and furfuryl alcohols yielding 6-hydroxy-2H-pyran-3(6H) -ones. The latter compounds originate from cyclization of intermediate enedione alcohols. The presented method is particularly useful for the oxidation of 2,5-dimethylfuran to 3-hexene-2,5-dione and the conversion of furfuryl alcohol to 6-hydroxy-2H-pyran-3(6H)-one, a versatile synthon in organic synthesis.

Full text of DOI:10.1002/adsc.200303185

FULL PAPERS
Titanium Silicalite 1 (TS-1) Catalyzed Oxidative Transformations
of Furan Derivatives with Hydrogen Peroxide
Joos Wahlen,a Bart Moens,a Dirk E. De Vos,a Paul L. Alsters,b Pierre A. Jacobsa,*
a
Centre for Surface Chemistryand Catalysis, KU Leuven, Kasteelpark Arenberg 23, 3001 Leuven, Belgium
Fax: ( ‡ 32)-16-32-1998, e-mail: pierre.jacobs@agr.kuleuven.ac.be
b
DSM Life Sciences, Advanced Synthesis and Catalysis, P. O. Box 18, 6160 MD Geleen, The Netherlands
Fax: ( ‡ 31)-46-476-7604, e-mail: paul.alsters@dsm.com
Received: October 21, 2003; Accepted: February2, 2004
Abstract: The oxidation of furan derivatives with originate from cyclization of intermediate enedione
titanium silicalite 1 (TS-1) and hydrogen peroxide is alcohols. The presented method is particularlyuseful
described. Oxidation products are identified and for the oxidation of 2,5-dimethylfuran to 3-hexene-
possible reaction pathways are discussed. It is shown 2,5-dione and the conversion of furfuryl alcohol to 6-
that the oxidation of these compounds occurs via hydroxy-2H-pyran-3(6H)-one, a versatile synthon in
epoxidation of one of the furan double bonds. The organic synthesis.
initiallyformed epoxides immediatelyundergo rear-
rangement, furans yielding unsaturated 1,4-dicarbon-
yl compounds and furfuryl alcohols yielding 6-hy- Keywords: epoxidation; furan; furfuryl alcohol; hy-
droxy-2H-pyran-3(6H)-ones. The latter compounds drogen peroxide; titanium; zeolites
Introduction
arene[6] hydroxylation, and alcohol oxidation.[7] Its
remarkable activityin these reactions is due to site
Carbohydrates and their unsaturated derivatives, i.e., isolation of Ti(IV) centres in the hydrophobic pores of
furan, pyran and dihydropyran compounds, are current- silicalite which allows simultaneous adsorption of the
lyof interest as alternative, renewable raw materials for
hydrophobic substrate and the oxidant. Although TS-1
the synthesis of fine chemicals.[1] Since furan compounds has been applied to most oxidisable functional groups,
are readilyavailable and can be employed in a wide
range of transformations, theyare useful for the
there are several unsaturated ring systems for which its
efficacy has yet to be explored. One such system is furan
synthesis of many structurally diverse molecules. How- and its derivatives. Since furan is a cyclic, dienic ether
ever, due to their high reactivity, these oxygen hetero- stabilised byresonance, different reaction pathways are
cycles are susceptible to side reactions such as cleavage possible in its oxidation with TS-1/H2O2. Hydroxylation
and polymerisation. Therefore considerable care has to of the heteroaromatic furan ring is a reasonable reaction
be taken in the selection of reagents, catalysts and pathway, as TS-1 is known as an efficient catalyst for ring
reaction conditions in order to obtain desired products hydroxylation of, e.g., phenol.[6] The primaryproduct of
in reasonable yields. As already proven for hydro- furan hydroxylation would be 2-hydroxyfuran. On the
carbon-derived molecules, liquid-phase catalytic oxida- other hand, epoxidation of one of the furan double
tion might be a useful tool for the selective conversion of bonds, similar to monoepoxidation of a conjugated
furan derivatives. On the other hand, performing the diene such as 1,3-butadiene,[8] would yield an epoxide
reaction using a heterogeneous catalyst and an inex- as the primaryproduct. Furfurly alcohol, another
pensive and environmentallybenign oxidant such as
important furanic compound, mayalso undergo ring
hydrogen peroxide (H2O2) could lead to an even more oxidation. On the other hand, oxidation of the hydroxy-
attractive process. In this context, the confinement of methyl group and consecutive oxidation of the formed
redox-active metal centres onto the surface of an furfural to furoic acid is another possible route. In this
inorganic support is a recognised route to heteroge- respect it is interesting to note that benzyl alcohol, a
neous catalysis for liquid-phase oxidations.[2] A prom- molecule which can undergo both alcohol oxidation
inent example is the discoveryof titanium(IV) silicalite
1 (TS-1).[3] TS-1 catalyses a variety of synthetically benzaldehyde on TS-1.[7a] To distinguish between the
useful oxidation reactions under verymild conditions different possible reaction modes, a detailed studywas
and ring hydroxylation, is selectively converted to
using aqueous H2O2 as the oxidant. Well-known appli- undertaken on the oxidation of furan derivatives with
cations include olefin epoxidation,[4] alkane[5] and TS-1 and H2O2.
Adv. Synth. Catal. 2004, 346, 333 338
DOI: 10.1002/adsc.200303185
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Joos Wahlen et al.
FULL PAPERS
The behaviour of alkyl-substituted furans in the
oxidation with TS-1/H2O2 confirms the proposed mech-
anism for furan. Thus, 2-methylfuran and 2,5-dimethyl-
furan were oxidised to the corresponding unsaturated
1,4-dicarbonyl compounds (Table 1). Standard reac-
Results and Discussion
Oxidation of Furan and Alkyl-Substituted Furans
Oxidation of furan (10 mmol) with TS-1 (0.1 g) and tions with m-chloroperbenzoic acid (m-CPBA) in
H2O2 (15 mmol) in acetonitrile (10 mL) at 208C gave CH2Cl2 showed the formation of identical oxidation
quantitative conversion after 5 h reaction. Gas chroma- products from these furans.
tographyshowed the formation of mainlytwo products
2-Methylfuran furnished the unstable unsaturated
with 80% combined selectivity. According to GC-MS ketoaldehyde, 4-oxo-2-pentenal (cis/trans ratio ˆ 97/3),
1
and H NMR, these compounds are cis-2-butene-1,4- while 2,5-dimethylfuran yielded a mixture of cis- and
dial (maleic dialdehyde) and the corresponding trans- trans-3-hexene-2,5-diones (cis/trans ratio ˆ 68/32).
isomer (fumaric dialdehyde). The cis/trans ratio was 95/5 These 3-hexene-2,5-diones are more stable under the
(Scheme 1, R1 ˆ R2 ˆ H). Although gas chromatogra- reaction conditions, which is in line with the known
physhowed that the main products are the dialdehydes,
lower tendencyof ketonesto form hydrates ascompared
NMR detected rather small amounts of these com- to aldehydes. Because of their intrinsic instability, the
1
pounds. On the other hand, H and 13C NMR showed dialdehydes or ketoaldehydes derived from furans
several signals which are characteristic for the hydrates cannot be isolated in a pure state. However, theycan
of 2-butene-1,4-dial. The existence of cis-2-butene-1,4- be used in further transformations byreaction with
dial as its cyclic hydrate (2,5-dihydrofuran-2,5-diol) in suitable trapping agents.[12b]
aqueous conditions has been well documented.[9] Pre-
sumably, during GC analysis the equilibrium between
the dialdehydes and the corresponding hydrates is Oxidation of Furfuryl Alcohol and Alkyl-Substituted
shifted towards the dialdehydes due to the high temper- Furfuryl Alcohols
ature in the GC injectionport. Since NMR analysis more
closelyreflects the situation in the liquid phase, it is most
Oxidation of furfuryl alcohol (10 mmol) with TS-1
likely that in solution, the dialdehyde hydrates are the (0.1 g) and H2O2 (12.5 mmol) in acetonitrile (10 mL)
predominant products. Moreover, polymers of these at 208C gave quantitative conversion after 24 h reaction.
hydrates may be formed by intermolecular condensa- GC-MS and NMR both showed the formation of 6-
tion.
hydroxy-2H-pyran-3(6H)-one in 90% yield (Scheme 2,
Mechanistically, it is proposed that the oxidation of R1 ˆ R2 ˆ H). Selectivityto furfural or furoic acid was
furan with TS-1/H2O2 proceeds via direct epoxidation of lower than 5%. The formation of 6-hydroxy-2H-pyran-
one double bond of the furan ring (Scheme 1).[10] An 3(6H)-one can be explained byassuming a similar
unstable epoxide is formed, which immediatelyrear-
mechanism as that proposed for the oxidation of furan.
ranges to cis-2-butene-1,4-dial. The epoxide intermedi- Thus, rearrangement of the initiallyformed epoxide
ate itself was not observed. Isomerisation of cis-2- yields an enedione alcohol. This compound undergoes
butene-1,4-dial yields the corresponding trans-isomer. cyclization by intramolecular attack of the hydroxy
A similar mechanism has been proposed for the group on the aldehyde (Scheme 2). A stable cyclic
oxidation of furan and alkyl-substituted furans with hemiacetal, 6-hydroxy-2H-pyran-3(6H)-one, is formed.
other oxidising agents such as peracids[11] and dioxir-
In acetonitrile, no acyclic enedione alcohol was
anes.[12] Since water is present in the TS-1/H2O2 system, detected. Increasing the reaction temperature to 408C
the initiallyformed dialdehydes are converted to the
corresponding hydrates.
was possible without a decrease in selectivity(Figure 1).
Scheme 1. Oxidation of furans with TS-1/H2O2.
Scheme 2. Oxidation of furfuryl alcohols with TS-1/H2O2.
334
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Titanium Silicalite 1 (TS-1) Catalyzed Oxidative Transformations
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Table 1. TS-1 catalyzed oxidation of furans with H2O2.[a]
Substrate
Time [h]
3
Conv.[b] [%]
94
Product
Select.[c] [%]
cis/trans
77
83
96/4
3
3
93
94
97/3
85
68/32
[a]
Reaction conditions: substrate (10 mmol), TS-1 (0.1 g), H2O2 (35 wt %, 12.5 mmol), CH3CN (10 mL), 258C.
GC analysis on the crude reaction mixture.
Selectivityto unsaturated 1,4-dicarbonyl compounds.
[b]
[c]
hexene-2,5-dione, was also present. This is probably
due to the higher stabilityof the diketone intermediate
towards intramolecular attack of the hydroxy group on
the ketone. Further oxidation of the open-chain ene-
dione lowers the selectivity. In contrast, in the oxidation
of furfuryl alcohol and 1-(2-furyl)ethanol, the products
are smoothly cyclised to hydroxypyranones.
The rate of oxidation of the three studied furfuryl
alcohols decreased in the order furfuryl alcohol > 1-(2-
furyl)ethanol > 5-methylfurfuryl alcohol. This order
(furfuryl alcohol > 5-methylfurfuryl alcohol) disagrees
with the one that would be expected for peracids and
other electrophilic epoxidising agents; for these, induc-
tive effects of the substituent groups predominate over
steric constraints. Therefore, it is apparent that steric
effects due to methyl-substitution at the double bond
playa major role in the observed trend. Similar
behaviour has been observed for the oxidation of
butenes with TS-1/H2O2.[4d] Although both restricted
Figure 1. Conversion of furfuryl alcohol as a function of time
in acetonitrile and in methanol. Reaction conditions: furfuryl
alcohol (10 mmol), TS-1 (0.1 g), H2O2 (12.5 mmol), CH3CN
or CH3OH (10 mL), 408C.
In methanol as the solvent, the oxidation of furfuryl transition state shape selectivityand hindered diffusion
alcohol was slower and the selectivity to 6-hydroxy-2H- of reagents and products within the zeolite pores may
pyran-3(6H)-one was lower due to the addition of influence the rate of oxidation, the much lower reac-
methanol to the intermediate 5-hydroxy-4-oxo-2-pen- tivityof 5-methylfurfuryl alcohol compared to that of 1-
tenal (Figure 1, Scheme 3). At full conversion (7 h), the (2-furyl)ethanol shows that even for molecules with
selectivity towards the acyclic dimethyl acetal was 20%, equal access to the zeolite pores, the degree of sub-
while the selectivity to 6-hydroxy-2H-pyran-3(6H)-one stitution at the double bond influences the reactivity.
was 65%. Therefore, further reactions were carried out Compared to the oxidation of furans, the oxidation of
in CH3CN.
furfuryl alcohols occurred at a much lower rate. This
Next, oxidation of 1-(2-furyl)ethanol and 5-methyl- means that the electron-withdrawing effect of the
furfuryl alcohol was attempted (Table 2). Oxidation of hydroxy moiety is dominant over any other factors in
1-(2-furyl)ethanol afforded 6-hydroxy-2-methyl-2H- determining the reactivityof the furan double bonds. No
pyran-3(6H)-one as a diastereomeric mixture (cis/trans rate-enhancing effect of the alcohol in the allylic
ratio ˆ 60/40). Oxidation of 5-methylfurfuryl alcohol position, e.g., bycoordination to framework Ti species
proceeded more sluggishly. Selectivity to the cyclic as an alcoholate, was observed. Based on both electronic
hemiketal
6-hydroxy-6-methyl-2H-pyran-3(6H)-one and steric factors, it is proposed that the epoxidation
was only 20%; the acyclic enedione, 1-hydroxy-3- occurs at the double bond which is not substituted with
the hydroxymethyl group. The importance of the
electron densityat the double bond in determining the
reactivityof furan compounds is further illustrated by
the oxidation of furfural and furoic acid. In the reaction
of furfural with TS-1/H2O2 at 408C in CH3CN, small
amounts of various oxidation products were observed
after 24 h. Total conversion was 40%. Next to ring
Scheme 3. Formation of the dimethyl acetal of 5-hydroxy-4-
oxo-2-pentenal.
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Joos Wahlen et al.
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Table 2. TS-1 catalyzed oxidation of furfuryl alcohols with H2O2.[a]
Substrate
Time [h]
3.5
Conv.[b] [%]
99
Product
Select.[c] [%]
93
6.5
9.5
93
79
80[d]
20
[a]
Reaction conditions: substrate (10 mmol), TS-1 (0.1 g), H2O2 (35 wt %, 12.5 mmol), CH3CN (10 mL), 408C.
GC analysis on the crude reaction mixture.
Selectivity to cyclic 6-hydroxy-2H-pyran-3(6H)-ones.
Cis/trans ratio ˆ 60/40.
[b]
[c]
[d]
oxidation, oxidation of the aldehyde group was an
Using a reduced amount of TS-1, the oxidation of
important reaction pathway. Both furoic acid and furfuryl alcohol was carried out on a larger scale. The
products derived from a Baeyer Villiger type oxidation amount of H2O2 was lowered to 1.1 equivalents relative
of furfural were observed. Also at higher temperatures to furfuryl alcohol. Furfuryl alcohol (60 mmol), TS-1
or in other solvents, no reasonable yields to one of the (0.15 g) and H2O2 (66 mmol) were stirred at 408C in
products could be obtained. In accordance with the low CH3CN (60 mL). After 24 h, GC analysis showed 99%
reactivityof furfural, furoic acid was virtuallyinert
under these conditions. Thus, the reactivityof furan
conversion and a selectivity to 6-hydroxy-2H-pyran-
3(6H)-one of 90%. Thus, 400 mmol furfuryl alcohol
compounds towards TS-1/H2O2 decreases in the order (40 g) were converted per g TS-1 within 24 h. This
furan > furfuryl alcohol > furfural > furoic acid. experiment illustrates the high productivityand high
The TS-1 catalyst was reused three times in the oxidant efficiencyof the TS-1 catalyst for this reaction.
oxidation of furfuryl alcohol (Figure 2). The activity In addition it shows that the deactivation of TS-1
significantlydecreased upon consecutive reuse. This
deactivation maybe due to the drying treatment of TS-1
observed during the recycle experiment (Figure 2)
mainlyoccurs during the intermediate drying treatment
which was carried out at 608C between each run. and onlyto a minor extent during the reaction itself.
Decomposition of the adsorbed products mayblock the The formed 6-hydroxy-2H-pyran-3(6H)-ones are in-
pores and prevent diffusion of reactants and products. teresting compounds as theycontain an a,b-unsaturated
However, calcination of the used TS-1 fullyrestored its
catalytic activity.
ketone, a hemiacetal, and an allylic alcohol function-
ality. Owing to their multifunctional nature and the
various possibilities for further elaboration with select-
ed nucleophiles and electrophiles, 6-hydroxy-2H-pyran-
3(6H)-ones and the corresponding alkoxy- and acyloxy-
derivatives are attractive synthons for the preparation of
sugar analogues and numerous compounds showing
[13]
biological activity.
Not surprisingly, a variety of
procedures has been reported for their preparation
starting from furfuryl alcohols. The three most widely
used stoichiometric methods include oxidation with N-
bromosuccinimide (NBS),[14] peroxyacids [15] and Br2 in
methanol.[16] Onlyfew catalytic procedures are known.
For example, VO(acac)2 with tert-butyl hydroperox-
ide[17] and methyltrioxorhenium with ureum hydrogen
peroxide[18] as the oxidant have been reported.
However, most of these methods suffer from various
drawbacks including tedious work-up, use of expensive
reagents or high energycost, non-scaleability, formation
of salt waste or difficult catalyst recyclability. For
example, although the NBS and peroxyacid mediated
reactions appear tooffer simple procedures, it was found
Figure 2. Influence of TS-1 reuse on the catalytic activity in
the oxidation of furfuryl alcohol. Reaction conditions:
furfuryl alcohol (20 mmol), TS-1 (0.2 g), H2O2 (25 mmol),
CH3CN (20 mL), 408C, 6 h. The catalyst was centrifuged,
washed with CH3CN (10 mL) and dried at 608C overnight.
After 3 runs the catalyst was calcined at 5508C for 10 h.
that these methods are unreliable and unsuitable for all
[19]
but verysmall-scale work.
On the other hand, the
336
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Titanium Silicalite 1 (TS-1) Catalyzed Oxidative Transformations
FULL PAPERS
bromomethoxylation procedure is highly sensitive to Preparation of TS-1
temperature and produces stoichiometric amounts of
TS-1 (Si:Ti ratio ˆ 35:1) was prepared according to a liter-
ature procedure with a molar chemical composition of 1.0
SiO2 :0.028 TiO2 :0.33 TPAOH:35 H2O.[3] Chemicals used for
the synthesis were tetraethyl orthosilicate (Acros, 98%),
tetrabutyl orthotitanate (Merck, 98%), and tetrapropylammo-
nium hydroxide (TPAOH, Alfa Aesar, 40 wt %). Crystalliza-
tion of the gel was conducted in stainless-steel autoclaves at
1758C for three days. The resulting material was washed, dried
and finallycalcined at 550 8C for 10 h. The powder XRD
patterns (Siemens D5000 matic diffractometer) of the calcined
TS-1 samples matched those of published MFI data.[3]
NaBr. In contrast to the above-mentioned methods,
using TS-1 as a heterogeneous catalyst and aqueous
H2O2 as an inexpensive oxygen source, small-sized
furfuryl alcohols are readily converted into 6-hydroxy-
2H-pyran-3(6H)-ones with high yield and high oxidant
efficiency. Moreover, the use of other Ti-substituted
molecular sieves possessing larger pores such as zeolite
Ti-Beta or mesoporous Ti-MCM-41 mayallow the
[20]
oxidation of more bulkyfurfuryl alcohols.
General Procedure for the Oxidation of Furfuryl
Alcohol
Conclusion
A 20-mL flask was charged with acetonitrile (10 mL), furfuryl
alcohol (10 mmol), TS-1 (0.1 g), and H2O2 (35 wt %,
12.5 mmol). The mixture was stirred at 208C and the reaction
was followed byGC analysis of the crude reaction mixture.
TS-1 catalyzed oxidation of furans and furfuryl alcohols
with H2O2 occurs via epoxidation of one of the furan
double bonds. The initiallyformed epoxide is unstable
and immediatelyrearranges to unsaturated 1,4-dicar-
bonyl compounds. Furan and 2-methylfuran are con-
verted to cis-2-butene-1,4-dial and cis-4-oxo-2-pentenal,
respectively. These products undergo further transfor-
mations such as isomerisation, hydration and condensa-
tion. In contrast, 2,5-dimethylfuran yields an isolable
mixture of cis- and trans-3-hexen-2,5-diones. Furfuryl
alcohols undergo a similar oxidation-rearrangement
Spectral Data of Products Obtained by TS-1/H2O2
Oxidation of Furan Compounds
Oxidation of furan: cis-2-butene-1,4-dial: 1H NMR (300 MHz,
CD3CN): d ˆ 10.45 (dd, J1,2 ˆ 4.4 Hz, J1,3 ˆ 2.5 Hz, 2H, CHO),
ˆ
6.66 (dd, J2,1 ˆ 4.4 Hz, J2,4 ˆ 2.5 Hz, 2H, CH CH); MS (EI): m/
‡
z ˆ 84 (M ), 56, 55, 29. trans-2-butene-1,4-dial: 1H NMR
sequence and produce stable 6-hydroxy-2H-pyran- (300 MHz, CD3CN): d 9 ˆ 0.86 (dd, J1,2 ˆ 4.4 Hz, J1,3 ˆ 2.9 Hz,
ˆ
2H, CHO), 6.89 (dd, J2,1 ˆ 4.4 Hz, J2,4 ˆ 2.9 Hz, 2H, CH CH);
3(6H)-ones which originate from intramolecular cycli-
zation of enedione alcohol intermediates.
‡
MS (EI): m/z ˆ 84 (M ), 56, 55, 29.
Oxidation of 2-methylfuran: cis-4-oxo-2-pentenal: 1H NMR
(300 MHz, CD3CN): d ˆ 10.03 (d, J1,2 ˆ 6.9 Hz, 1H, CHO), 7.10
(d, J3,2 ˆ 11.7 Hz, 1H, H3), 6.15 (dd, J2,3 ˆ 11.7 Hz, J2,1 ˆ 6.9 Hz,
‡
1H, H2), 2.33 (s, 3H, H5); MS (EI): m/z ˆ 98 (M ), 83, 70, 69,
55, 43, 29.
Experimental Section
Oxidation of furfuryl alcohol: 6-hydroxy-2H-pyran-3(6H)-
one: 1H NMR (300 MHz, CD3CN): d ˆ 6.98 (dd, J5,4 ˆ 10.3 Hz,
J5,6 ˆ 2.9 Hz, 1H, H5), 6.17 (d, J4,5 ˆ 10.3 Hz, 1H, H4), 5.64 (s,
General
1H, H6), 4.57 (d, J2a,2e ˆ 16.8 Hz, 1H, H2), 4.14 (d, J2a,2e
ˆ
All materials were obtained from commerciallyavailable
sources and were used without further purification. 5-Methyl-
furfuryl alcohol was prepared by reduction of 5-methylfurfural
with NaBH4 in methanol.[13a] For GC analysis, a Hewlett
Packard 5890 gas chromatograph equipped with a 0.32 mm i.d.
by30 m WCOT fused silica column coated with a Chrompack
CP-Sil 5 CB stationaryphase (1.0 mm df) was used. The
instrument was equipped with a flame ionisation detector
(FID) and coupled to a HP 3396 integrator. Products were
identified bycomparison of their GC retention times with
authentic samples prepared by m-CPBA oxidation.[11,15] Addi-
tionally, GC-MS and NMR were used to confirm the identity of
the products. A Fisons GC 8000 Series gas chromatograph
equipped with a 0.32 mm i.d. by60 m WCOT fused silica
column coated with a Varian CP-Sil 5 CB Low bleed/MS
stationaryphase (0.25 mm df) was coupled to a Fisons MD 800
mass spectrometer. 1H and 13C NMR spectra were recorded on
a Bruker Avance 300 spectrometer at 300 MHz and 75 MHz,
16.8 Hz, 1H, H2), 4.06 (br s, 1H, OH); MS (EI): m/z ˆ 114
‡
(M ), 97, 84, 69, 56, 55, 42, 39, 29.
Oxidation of 1-(2-furyl)ethanol: 6-hydroxy-2-methyl-2H-
1
pyran-3(6H)-one: H NMR (300 MHz, CD3CN): cis isomer:
d ˆ 6.97 (d, J5,4 ˆ 10.3 Hz, 1H, H5), 6.00 (d, J4,5 ˆ 10.3 Hz, 1H,
H4), 5.50 (s, 1H, H6), 4.64 (q, J2,7 ˆ 6.6 Hz, 1H, H2), 2.8 (br,
OH), 1.28 (d, J7,2 ˆ 6.6 Hz, 3H, H7), trans isomer: d ˆ 6.96 (d,
J5,4 ˆ 10.3 Hz, 1H, H5), 6.06 (dd, J4,5 ˆ 10.3 Hz, J4,2 ˆ 1.5 Hz,
1H, H4), 5.59 (s, 1H, H6), 4.22 (qd, J2,7 ˆ 6.6 Hz, J2,4 ˆ 1.5 Hz,
1H, H2), 2.8 (br, OH), 1.32 (d, J7,2 ˆ 6.6 Hz, 3H, H7); MS (EI):
‡
m/z ˆ 128 (M ), 111, 99, 84, 56, 55, 43, 29.
Preparation of Reference Compounds
Oxidation of furan compounds with m-CPBA[11,15]: Furan and
its derivatives (1 mmol) were added to a solution of m-CPBA
(0.5 or 1 mmol) in CH2Cl2 (1 mL) at 08C. After reaction,
respectively. Reactions for product identification by NMR precipitated m-chlorobenzoic acid was removed and the
were run in CD3CN as the solvent.
sample was subjected to GC and GC-MS analysis.
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Joos Wahlen et al.
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Photooxygenation of furfural[21]: Practical-grade furfural
(10 mmol) and a few milligrams of methylene blue as sensitiser
were dissolved in methanol (5 mL). The reaction vessel was
cooled to 08C with a cryostatic bath and was externally
irradiated with two 150 W lamps while a gentle stream of
oxygen was passed through the reaction mixture. The reaction
was followed byGC until complete consumption of furfural.
After evaporation of the solvent under reduced pressure at
308C, 5-hydroxy-2(5H)-furanone was obtained. 1H NMR
(300 MHz, CD3CN): d ˆ 7.33 (dd, J4,3 ˆ 5.9 Hz, J4,5 ˆ 1.1 Hz,
1H, H4), 6.19 (d, J3,4 ˆ 5.9 Hz, 1H, H3), 6.15 (s, 1H, H5), 5.50 (br
s, 1H, OH); 13C NMR (75 MHz, CD3CN): d ˆ 171.8, 153.6,
124.4, 99.3; MS (EI): m/z ˆ 100, 99, 83, 82, 72, 71, 55, 54, 45, 44,
37, 29.
1995, 8, 903 906; d) L. A. Peterson, K. C. Naruko, D. P.
Predecki, Chem. Res. Toxicol. 2000, 13, 531 534.
[10] Recently, oxidation of furan with TS-1/H2O2 was re-
ported to yield 5-hydroxy-2(5H)-furanone. Singlet mo-
lecular oxygen (1O2) was proposed as the actual oxidizing
species: a) P. Kumar, R. Kumar, B. Pandey, Synlett 1995,
289 298; b) P. Kumar, R. K. Pandey, Green Chem. 2000,
29 31. However, the generation of significant amounts
of 1O2 from TS-1 catalysed disproportionation of H2O2 is
doubtful. More likely, 5-hydroxy-2(5H)-furanone maybe
formed byoxidation of one aldehdye group of the
initiallyformed butene-1,4-dial to the corresponding
acid. However, also at higher H2O2 to furan ratios, we
observed cis-butene-1,4-dial as the major product.
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