A new entry to sonochemical/efficient agitation switching: Alkene formation through epoxide deoxygenation

Article abstract of DOI:10.1055/s-0033-1340109

Sonochemical switching whereby the pathway of a reaction is changed under sonication conditions was first reported by Ando, and in fact could just be the consequence of efficient agitation. We herein report a new addition to this switching phenomenon in which epoxides are converted to cyclic carbonates by addition of CO₂ under standard heating/electrolysis conditions but are 'switched' to alkenes under deoxygenating sonochemical conditions. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-0033-1340109

LETTER
▌197
lAetteNr ew Entry to Sonochemical/Efficient Agitation Switching: Alkene Formati-
on through Epoxide Deoxygenation
A New Entry to Sonochemical/Efficient Agitation Switching
Benjamin R. Buckley,* Anish P. Patel, K. G. Upul Wijayantha*
Department of Chemistry, Loughborough University, Ashby Road, Loughborough, Leicestershire, LE11 3TU, UK
Fax +44(1509)223925; E-mail: b.r.buckley@lboro.ac.uk; E-mail: u.wijayantha@lboro.ac.uk
Received: 16.09.2013; Accepted after revision: 08.10.2013
many years and has been described as a useful tool for en-
Abstract: ‘Sonochemical switching’ whereby the pathway of a re-
hancing reaction rates in certain organic reactions.3 The
action is changed under sonication conditions was first reported by
phenomenon of ‘sonochemical switching’ whereby the
Ando, and in fact could just be the consequence of efficient agita-
tion. We herein report a new addition to this switching phenomenon
pathway of reaction is changed under sonication condi-
in which epoxides are converted to cyclic carbonates by addition of tions was first reported by Ando (Scheme 2).4 Debate re-
CO2 under standard heating/electrolysis conditions but are
‘switched’ to alkenes under deoxygenating sonochemical condi-
tions.
mains as to whether the ‘sonochemical effect’ actually
exists and is in fact just the effect of very efficient mixing.
Reisse and co-workers have reported an extensive study in
this area and found that through the use of efficient stir-
Key words: sonochemical, alkene, epoxide, cyclic carbonate,
carbon dioxide
ring (using a homogeniser) the pathway of the originally
proposed sonochemical switch reaction reported by Ando
can be switched using this technique thus pointing to-
wards the fact that sonochemistry has no ‘special effect’.5
Carbon dioxide utilisation (CDU) technologies have re-
cently gained increased interest as they offer an attractive
alternative to the well-established carbon dioxide storage
(CCS) processes.1 This is due to the fact that carbon diox-
ide is abundant, cheap and nontoxic and waste carbon di-
oxide under CDU protocols can be converted into useful
chemicals or fuels. Carbon dioxide has been used in the
manufacture of salicylic acid, urea and cyclic carbonates
for 50–100 years; however, due to carbon dioxide’s rela-
tive inertness these processes are significantly energy de-
manding with reactions taking place at high temperatures
and pressures. We have recently embarked on a pro-
gramme of research directed at CDU and have recently re-
ported our initial efforts in this area where we employed
an electrosynthetic system which was easy to set up,
cheap, reliable, required no expensive catalysts, ran under
atmospheric CO2 pressures and at ambient temperatures
(see Scheme 1, this type of process could be designed to
be cost neutral in terms of energy consumption if com-
bined with a suitable renewable energy source e.g. solar).2
Me
Ph
mechanical
agitation
toluene
83% yield
o/p substituted
Br
KCN, Al2O3
50 °C
ultrasound
irradiation
CN
71% yield
Scheme 2 Ando’s original sonochemical switching reactions
During the course of our optimisation studies for the elec-
trochemical incorporation of carbon dioxide into epoxides
to afford cyclic carbonates we chose to carry out the reac-
tion using sonication with a standard cleaning bath. The
reactions were run in a single compartment cell, with ace-
tonitrile as solvent, tetrabutylammonium bromide as elec-
trolyte, with a copper cathode, magnesium anode, and 60
mA current in a 50–60 Hz sonic bath. We were surprised
to find that for our test substrate styrene oxide, on analysis
of the crude reaction mixture, none of the desired cyclic
carbonate was observed, instead we obtained a >99% con-
version (90% isolated yield) to the parent alkene with no
trace of the cyclic carbonate (Scheme 3).
Sonochemistry, a technique, whereby chemistry is carried
out in an acoustically cavitating field has been known for
O
Mg
Cu
CO2 (1 atm)
O
O
O
R
Bu4NBr (1.0 equiv), MeCN
60 mA, 6 h, 50 °C
R
11 examples, 65–96%
R = alkyl, aryl, haloalkyl, alkenyl
Mg
Cu
CO2 (1 atm)
O
Scheme 1 Electrosynthetic CO2 insertion to form cyclic carbonates
Ph
Ph
Bu4NBr (1.0 equiv), MeCN
60 mA, 6 h, 50 °C + )))
1
2
SYNLETT 2014, 25, 0197–0200
90%
Advanced online publication: 13.11.2013
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
DOI: 10.1055/s-0033-1340109; Art ID: ST-2013-D0882-L
© Georg Thieme Verlag Stuttgart · New York
Scheme 3 Sonochemical deoxygenation using electrosynthesis

198
B. R. Buckley et al.
LETTER
Table 1 A Comparison of Conventional versus Ultrasonic Reaction Conditions11,12
O
Mg
Cu
O
CO2 (1 atm)
O
O
+
R
R
Bu4NBr (1.0 equiv), MeCN
60 mA, 6 h, 50 °C
or
R
B
A
Bu4NBr (1.0 equiv), MeCN
60 mA, 6 h, 50 °C + )))
Entry
Epoxide
Ratio of products under
Ratio of products under
sonochemical reaction conditions
(A:B)b,c
Conversion
(%)d
standard heating conditions
(A:B)a,b,
2
O
1
>99:1
1:>99
>99 (90)
R
R = H
R = Br
2
3
4
5
6
>99:1
>99:1
>99:1
>99:1
>99:1
62:38
10:90
93:7
67
72
R = Cl
R = F
95
R = Me
>99:1
42:58
>99
>99
R = OMe
O
7
8
>99:1
>99:1
>99:1
>99:1
>99
>99
O
HO
O
9
>99:1
>99:1
>99:1
>99:1
>99
>99
Cl
10
O
11
12
>99:1
>99:1
>99:1
>99:1
>99
>99
O
O
O
O
O
13
–
–
<5
14
15
–
–
–
–
<5
<5
O
O
a General conditions for the standard reaction: CO2 (1 atm), Cu cathode, Mg anode, Bu4NBr (1.0 equiv), MeCN, single compartment cell, 60
mA, 6 h, 50 °C.
b Ratio evaluated from the 1H NMR spectrum by integration of alkene vs. cyclic carbonate peaks.
c General conditions for the sonochemical reaction: CO2 (1 atm), Cu cathode, Mg anode, Bu4NBr (1.0 equiv), MeCN, single compartment cell,
60 mA, 50–60 Hz sonication, 6 h, 50 °C.
d Conversion evaluated from the 1H NMR spectrum by integration of epoxide vs. cyclic carbonate and alkene peaks, numbers in parenthesis
refer to isolated yield after chromatography.
Synlett 2014, 25, 197–200
© Georg Thieme Verlag Stuttgart · New York

LETTER
A New Entry to Sonochemical/Efficient Agitation Switching
199
The deoxygenation of epoxides is a challenging area of re- utilising ultrasonic irradiation promotes a single-electron-
search, since the successful reduction presents the epoxide transfer (SET) process that could be similar to that report-
unit as a viable protecting group for alkenes.6,7 Several re- ed by Huang and co-workers, presumably aromatic sys-
search groups have developed processes for epoxide de- tems tend to undergo the SET process due to stabilisation
oxygenation, often requiring harsh reaction conditions or of the benzyl radical, which cannot be achieved when em-
expensive gold catalysts.8,9 Recently Huang and co-work- ploying aliphatic epoxides. However, under our condi-
ers have reported the electrochemically supported deoxy- tions we do not observe alkene formation in the absence
genation of epoxides into alkenes in aqueous solution and of carbon dioxide, and we also do not observe conversion
this prompted us to investigate our system further.10
of cyclic carbonate to alkenes under the reaction condi-
tions.15
Intrigued by our initial findings we performed a series of
control reactions in which we found that it was essential We are also investigating the factors that influence the ra-
to have a carbon dioxide atmosphere (under argon no re- tio of products in our laboratory and full results will be
action was observed), in the absence of current no reaction published in due course.
occurred and without heating we still observed alkene for-
mation (67% conversion by 1H NMR).
Acknowledgment
With our optimised conditions in hand we then chose to
screen a range of substrates to see what effect substituents
would make to the deoxygenation reaction, with mixed re-
sults (Table 1). As a general rule under our conditions al-
iphatic epoxides produced the cyclic carbonate in
excellent conversion but di-, and trisubstituted epoxides
remained untouched. Styrene oxide remains the only sub-
strate to fully convert to the alkene with no trace of the
corresponding cyclic carbonate and disappointingly there
appears to be no correlation with the electron-withdraw-
ing or electron-donating ability of the substituents present
on the aromatic substrates. However, this work does rep-
resent the first report of sonochemical switching for this
type of process. We also found that aziridine substrates
were compatible with our carbon dioxide incorporation
process but they too did not undergo formation of the par-
ent alkene under sonochemical conditions (Scheme 4).13
B.R.B. and K.G.U.W. would like to thank Research Councils UK
for RCUK fellowships and Loughborough University for funding a
PhD studentship to A.P.P.
References and Notes
(1) Styring P., Jansen D., de Coninck H., Armstrong K.; Carbon
Capture and Utilisation in the Green Economy, Report No.
501; The Centre For Low Carbon Futures: Birmingham, UK,
2011; ISBN 978-0-9572588-0-8.
(2) Buckley, B. R.; Patel, A. P.; Wijayantha, K. G. U. Chem.
Commun. 2011, 47, 11888.
(3) (a) Cravotto, G.; Cintas, P. Angew. Chem. Int. Ed. 2007, 46,
5476; Angew. Chem. 2007, 119, 5573. (b) Ando, T.; Sumi,
S.; Kawate, T.; Ichihara, J.; Hanafusa, T. J. Chem. Soc.,
Chem. Commun. 1984, 439.
(4) Kegelaers, Y.; Eulaerts, O.; Reisse, J.; Segebarth, N. Eur. J.
Org. Chem. 2001, 3683.
(5) (a) Corey, E. J.; Su, W. G. J. Am. Chem. Soc. 1987, 109,
7534. (b) Kraus, G. A.; Thomas, P. J. J. Org. Chem. 1988,
53, 1395. (c) Johnson, W. S.; Plummer, M. S.; Reddy, S. P.;
Bartlett, W. R. J. Am. Chem. Soc. 1993, 115, 515.
(6) Corey, E. J.; Su, W. G. J. Am. Chem. Soc. 1987, 109, 7534.
(7) See for some recent examples: (a) Mahesh, M.; Murphy, J.
A.; Wessel, H. P. J. Org. Chem. 2005, 70, 4118. (b) Choi, K.
H.; Choi, K. I.; Kim, J. H.; Yoon, C. M.; Yoo, B. W. Bull.
Korean Chem. Soc. 2005, 26, 1495. (c) Firouzabadi, H.;
Iranpoor, N.; Jafarpour, M. Tetrahedron Lett. 2005, 46,
4107. (d) Oh, K.; Knabe, W. E. Tetrahedron 2009, 65, 2966.
(e) Iranpoor, N.; Firouzabadi, H.; Jamalian, A. Tetrahedron
2006, 62, 1823. (f) Justicia, J.; Jimenez, T.; Morcillo, S. P.;
Cuerva, J. M.; Oltra, J. E. Tetrahedron 2009, 65, 10837.
(g) Noujima, A.; Mitsudome, T.; Mizugaki, T.; Jitsukawa,
K.; Kaneda, K. Angew. Chem. Int. Ed. 2011, 50, 2986.
(8) (a) Mitsudome, T.; Noujima, A.; Mikami, Y.; Mizugaki, T.;
Jitsukawa, K.; Kaneda, K. Chem. Eur. J. 2010, 16, 11818.
(b) Mitsudome, T.; Noujima, A.; Mikami, Y.; Mizugaki, T.;
Jitsukawa, K.; Kaneda, K. Angew. Chem. Int. Ed. 2010, 49,
5545. (c) Noujima, A.; Mitsudome, T.; Mizugaki, T.;
Jitsukawa, K.; Kaneda, K. Angew. Chem. Int. Ed. 2011, 50,
2986. (d) Ni, J.; He, L.; Liu, Y.-M.; Cao, Y.; He, H.-Y.; Fan,
K.-N. Chem. Commun. 2011, 47, 812.
O
N
O
Mg
Cu
R
O
A
CO2 (1 atm)
+
N
Bu4NBr (1.0 equiv), MeCN
60 mA, 6 h, 50 °C + )))
R
N
R = Me 3: 95:5 ratio A:B, 87% yield
R = Ph 4: 0:100 ratio A:B, 32% yield
O
R
B
Scheme 4 Formation of cyclic carbamates; no denitrogenation is ob-
served under sonochemical conditions
In conclusion we have identified a new entry into the so-
nochemical switching process that works well for styrene
oxide but further investigations are required in order to
develop this into a practical approach to alkene group pro-
tection as epoxides. We are currently investigating the
proposed mechanism, under standard heating and stirring
conditions. We believe that an ionic mechanism similar to
that reported by North and co-workers14 is in operation as
retention of stereochemical information in the final cyclic
carbonate product is observed when employing enantio-
merically enriched epoxides. We believe that the reaction
(9) Huang, J.-M.; Lin, Z.-Q.; Chen, D.-S. Org. Lett. 2012, 14,
22.
(10) Representative Procedure for the Standard Synthesis of
Phenyl Ethylene Carbonate (Ref. 2): Styrene oxide (1;
0.12 g, 1.0 mmol) and CO2 (balloon) in MeCN (150 mL)
were electrolysed (constant current: 60 mA) for 6 h in a
© Georg Thieme Verlag Stuttgart · New York
Synlett 2014, 25, 197–200

200
B. R. Buckley et al.
LETTER
(12) General Procedure for Cyclic Carbamate Formation:
single compartment cell [Mg anode and copper(0) cathode]
containing Bu4NBr (0.32 g, 1.0 mmol) as supporting
electrolyte at 50 °C. On completion the reaction mixture was
washed with aq 0.1 M HCl (50 mL) followed by extraction
with Et2O (3 × 35 mL). The combined organic extracts were
then dried over MgSO4 and evaporated under reduced
pressure to afford an amber oil, which was suspended in
EtOAc (100 mL). After 1 h the precipitated Bu4NBr (0.30 g,
95%) was removed by filtration and the solvent evaporated
under reduced pressure to afford an amber oil. This crude
material was purified by column chromatography on silica
gel eluting with EtOAc–light petroleum to yield a colourless
solid (conversion: 99%; yield: 0.12 g, 96%); mp 49–51 °C.
IR (CH2Cl2): 1167 (C–O), 1789 (C=O) cm–1. 1H NMR (400
MHz, CDCl3/Me4Si): δ = 4.35 (dd, J = 7.9, 8.4 Hz, 1 H), 4.80
(dd, J = 8.5 Hz, 1 H), 5.68 (dd, J = 7.99 Hz, 1 H), 7.35–7.45
(m, 5 H). 13C NMR (100 MHz, CDCl3/Me4Si): δ = 71.2,
78.0, 125.9, 129.3, 129.8, 135.8, 154.8.
The aziridine (2.0 mmol) in MeCN (20 mL) was added to a
solution of supporting electrolyte Bu4NBr (0.64 g, 2.0
mmol) in MeCN (130 mL), and the resulting solution was
flushed with CO2 for 1 h, followed by heated electrolysis at
50 ºC with constant stirring at a constant current of 60 mA
for 6 h under a CO2 balloon, in a single compartment cell
containing a magnesium anode and copper cathode. On
completion the reaction mixture was filtered to remove the
precipitated MgCO3 and evaporated to dryness followed by
addition of EtOAc (50 mL). After 1 h the precipitated
Bu4NBr (ca. 70% recovered) was removed by filtration and
the solvent evaporated under reduced pressure. The crude
carbamate was then analysed by 1H NMR spectroscopy to
determine the regioselectivity and then purified by column
chromatography on silica gel eluting with EtOAc–light
petroleum.
(13) 3-Benzyl-4-methyloxazolidin-2-one (3): colourless oil,
mixture of regioisomers observed in a 95:5 ratio in favour of
the title compound (conversion: 98%; yield: 0.34 g, 87%).
IR (CH2Cl2): 1181 (C–N), 1255 (C–O), 1416 (C=C), 1721
(C=O) cm–1. 1H NMR (400 MHz, CDCl3/Me4Si): δ = 1.21
(d, J = 6.0 Hz, 3 H), 2.97 (dd, J = 6.8, 8.6 Hz, 1 H), 3.50 (dd,
J = 8.4 Hz, 1 H), 4.37–4.46 (m, 2 H), 4.57–4.67 (m, 1 H),
7.26–7.33 (m, 5 H). 13C NMR (100 MHz, CDCl3/Me4Si): δ
= 18.3, 46.7, 50.8, 64.7, 127.8, 127.9, 128.3, 128.8, 158.3.
3-Benzyl-5-phenyloxazolidin-2-one (4): colourless solid,
single regioisomer observed (conversion: 51%; yield: 0.162
g, 32%); mp 61–63 °C. IR (CH2Cl2): 1157 (C–N), 1250 (C–
O), 1454 (C=C), 1752 (C=O) cm–1. 1H NMR (400 MHz,
CDCl3/Me4Si): δ = 3.30 (dd, J = 7.5, 8.8 Hz, 1 H), 3.75 (dd,
J = 8.8 Hz, 1 H), 4.41–4.56 (m, 2 H), 5.45 (dd, J = 8.3 Hz, 1
H), 7.21–7.40 (m, 10 H). 13C NMR (100 MHz,
CDCl3/Me4Si): δ = 45.9, 48.5, 74.6, 125.5, 126.3, 128.1,
128.2, 128.8, 128.9, 135.7, 138.6, 158.0.
(14) Clegg, W.; Harrington, R. W.; North, M.; Pasquale, R.
Chem. Eur. J. 2010, 16, 6828.
(15) This was confirmed by subjecting phenyl ethylene carbonate
instead of styrene oxide to the reaction conditions reported
in reference 11. Only phenyl ethylene carbonate and
supporting electrolyte was observed in the crude reaction 1H
NMR spectrum.
(11) Representative Procedure for the Sonochemical
Synthesis of Styrene (2): Styrene oxide (1; 0.12 g, 1.0
mmol) and CO2 (balloon) in MeCN (150 mL) were
electrolysed (constant current: 60mA) for 6 h in a single
compartment cell [Mg anode and copper(0) cathode]
containing Bu4NBr (0.32 g, 1.0 mmol) as supporting
electrolyte. The reaction vessel was submerged in a
sonication bath at 50–60 Hz. On completion the reaction
mixture was washed with aq 0.1 M HCl (50 mL) followed by
extraction with Et2O (3 × 35 mL). The combined organic
extracts were then dried over MgSO4 and evaporated under
reduced pressure to afford an amber oil, which was
suspended in EtOAc (100 mL). After 1 h the precipitated
Bu4NBr (0.30 g, 95%) was removed by filtration and the
solvent evaporated under reduced pressure to afford an
amber oil. This crude material was purified by column
chromatography on silica gel eluting with EtOAc–light
petroleum to yield a colourless liquid (0.094 g, 90%). IR
(CH2Cl2): 1442 (C=C), 1610 (C=C) cm–1. 1H NMR (400
MHz, CDCl3/Me4Si): δ = 5.25 (dd, J = 0.8, 11.0 Hz, 1 H),
5.75 (dd, J = 1.2, 17.5 Hz, 1 H), 6.72 (dd, J = 11.0, 17.5 Hz,
1 H), 7.24–7.41 (m, 5 H). 13C NMR (100 MHz,
CDCl3/Me4Si): δ = 113.7, 126.2, 127.8, 128.5, 137.0, 137.7.
Synlett 2014, 25, 197–200
© Georg Thieme Verlag Stuttgart · New York