Synthesis and thermolysis of a spiro-fused oxadiazoline - Evidence for sequential formation of carbene and oxirane intermediates, and for oxirane dimerization

Article abstract of DOI:10.1139/v04-158

The spiro-fused oxadiazoline, 3,4-diaza-2,2-dimethyl-1,6,10-trioxaspiro[4. 5]dec-3-ene, when thermolysed in a sealed tube in benzene-d₆ at 110 °C, afforded acetone and an apparent oxirane intermediate (2,2-dimethyl-1,4,8-trioxaspiro[2.5]octane) that could not be isolated. Attempts to isolate the oxirane gave a dimer (8,8,11,11-tetramethyl-1,5,7,10,12,16- hexaoxadispiro[5.2.5.2]hexadecane) as the major product. The oxirane is thermally stable at 110 °C but it is very sensitive to water, as indicated by its gradual disappearance after the tube was opened. The dimer of the oxirane is believed to form from a cation arising from the ring-opening of the oxirane when it reacts with moisture. This cation then reacts with the oxirane itself to regenerate water, which is effectively a catalyst for conversion of the oxirane to the dimer.

Full text of DOI:10.1139/v04-158

1769
Synthesis and thermolysis of a spiro-fused
oxadiazoline — Evidence for sequential formation
of carbene and oxirane intermediates, and for
oxirane dimerization
Arkadiusz Klys, Wojciech Czardybon, John Warkentin, and Nick Henry Werstiuk
Abstract: The spiro-fused oxadiazoline, 3,4-diaza-2,2-dimethyl-1,6,10-trioxaspiro[4.5]dec-3-ene, when thermolysed in
a sealed tube in benzene-d6 at 110 °C, afforded acetone and an apparent oxirane intermediate (2,2-dimethyl-1,4,8-
trioxaspiro[2.5]octane) that could not be isolated. Attempts to isolate the oxirane gave a dimer (8,8,11,11-tetramethyl-
1,5,7,10,12,16-hexaoxadispiro[5.2.5.2]hexadecane) as the major product. The oxirane is thermally stable at 110 °C but
it is very sensitive to water, as indicated by its gradual disappearance after the tube was opened. The dimer of the
oxirane is believed to form from a cation arising from the ring-opening of the oxirane when it reacts with moisture.
This cation then reacts with the oxirane itself to regenerate water, which is effectively a catalyst for conversion of the
oxirane to the dimer.
Key words: carbene, dioxacarbene, dialkoxyoxirane, oxadiazoline, 2,5-dihydrooxadiazole.
Résumé : Opérant dans un tube scellé, en solution dans du benzène-d6 et à une température de 110 °C, on a réalisé la
thermolyse de l’oxadiazoline à fusion spiro, 3,4-diaza-2,2-diméthyl-1,6,10-trioxaspiro[4.5]déc-3-ène, qui conduit à la
formation d’acétone et d’un intermédiaire qui n’a pas été isolé et qui serait le 2,2-diméthyl-1,4,8-trioxaspiro[2.5]octane.
Des essais en vue d’isoler l’oxirane ont conduit à la formation, comme produit majeur, du dimère 8,8,11,11-
tétraméthyl-1,5,7,10,12,16-hexaoxadispiro[5.2.5.2]hexadécane. L’oxirane est thermiquement stable à 110 °C, mais il est
très sensible à l’eau, comme on peut en déduire de sa disparition graduelle une fois que le tube est ouvert. On croit
que le dimère de l’oxirane se forme à partir d’un cation qui se formerait à partir d’une ouverture de cycle de l’oxirane
par réaction avec de l’eau. Ce cation pourrait alors réagir avec l’oxirane lui-même pour régénérer l’eau qui ne serait de
fait qu’un catalyseur dans la conversion de l’oxirane en dimère.
Mots clés : carbène, dioxacarbène, dialkoxyoxirane, oxadiazoline, 2,5-dihydrooxadiazole.
[Traduit par la Rédaction] Klys et al. 1773
Introduction
dialkoxycarbene (3). An ylide intermediate, if real, frag-
ments rapidly to the latter two products, Scheme 1. The
carbene, a nucleophile, could then add to the ketone car-
bonyl group, either in a stepwise process via intermediate 4
or in a concerted reaction leading directly to oxirane 5.
The existence of a ylide intermediate in oxadiazoline
thermolysis has been questioned, on the basis of results of
computation at a low level of theory. Smith (3) proposed a
concerted reaction to afford a dialkoxycarbene without the
intermediacy of a carbonyl ylide. If that were the case, prod-
ucts from apparent ylide intermediates could still arise via 4
or 5, from attack of the carbene on the ketone, if 5 were to
open to 2.
Many 2,2-dialkoxy-5,5-dialkyl-∆3-1,3,4-oxadiazolines (1),
also known as 2,5-dihydro-2,2-dialkoxy-5,5-dialkyl-1,3,4-
oxadiazoles, have been prepared and thermolyzed in benzene
solution at temperatures near 110 °C (1). Their thermolysis
is generally formulated as involving a 1,3-dipolar cyclo-
reversion, by loss of N2, to form a carbonyl ylide intermedi-
ate (2). Experimental evidence for a possible carbonyl ylide
intermediate has been presented in only a few instances (2),
suggesting that they are short-lived, if they bear two alkoxy
groups, or that they are not real. Recent computational re-
sults2 suggest that whether or not an ylide is a real interme-
diate in thermolysis of some oxadiazolines depends on the
level of theory employed. In any case the ultimate products
of oxadiazoline fragmentation are N2, a ketone, and a
We now present evidence for the formation of carbene 8
and oxirane 9 intermediates in the thermolysis of the spiro-
fused oxadiazoline 6, Scheme 2. Although a carbonyl ylide
Received 7 May 2004. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on 3 February 2005.
A. Klys, W. Czardybon, J. Warkentin,1 and N.H. Werstiuk. Department of Chemistry, McMaster University, Hamilton,
ON L8S 4M1, Canada.
1Corresponding author (e-mail: warkent@mcmaster.ca).
2W. Czardybon, J. Warkentin, and N.H. Werstiuk. Unpublished computations.
Can. J. Chem. 82: 1769–1773 (2004)
doi: 10.1139/V04-158
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1770
Can. J. Chem. Vol. 82, 2004
Scheme 1.
Scheme 2.
intermediate 7 is possible from a mechanistic point of view,
there is not any experimental evidence for it in this case.
tified. The fact that formation of 9 was suppressed at the
higher concentration of trifluoroacetone must mean that
most of the 9 observed when trifluoroacetone was absent, or
present at a low concentration, arose from addition of the
free carbene (8) to acetone. Thus, fragmentation of pre-
sumed ylide 7, although slowed because of the conformation
imposed on carbene 8, must still be fast relative to electro-
cyclic ring closure of 7 to 9. Oxirane 9 must be formed
primarily by reaction of carbene 8 with acetone. Trifluoro-
acetone, when present at a relatively high concentration, is
expected to trap essentially all of the free carbene to form
14. How 14 reacts to afford 15 is not known.
Results and discussion
Oxadiazoline 6 was prepared by hydrazinolysis of carbon-
ate 10, conversion of the product (11) to the isopropylidene
hydrazone 12, oxidative cyclization of 12 to 13 with
Pb(OAc)4 in dichloromethane (2b), and acid catalysed
cyclization of 13 to 6, Scheme 3. Heating of a benzene solu-
tion of 6 (72 h) at 110 °C in a sealed tube gave a major
product (ca. 60% by GC) and several minor products, one of
which had a longer GC retention time. The initial major
product was transformed, at room temperature, into the latter
minor product after the tube was opened. Sometimes this
process took a few hours and sometimes about one day. It is
probably related to the rate at which the solution absorbed
water from the atmosphere after the reaction tube was
opened. Chromatography of the initial reaction mixture
caused the same transformation.
We suggest the following interpretation of these results,
Scheme 4. Thermolysis of 6 leads to loss of N2 to form car-
bonyl ylide 7. Fragmentation of 7 to 8 is probably slowed,
relative to the rate of fragmentation of analogous ylides that
lead to an acyclic dialkoxycarbene, because carbene 8 is
U-shaped. The U-conformation of allyloxymethoxycarbene
lies about 11 kcal mol–1 higher in energy than the W-
conformation (4). That energy difference could mean that
the transition state for formation of a U-shaped dialkoxy-
carbene from a carbonyl ylide is higher in energy than an
analogous transition state for fragmentation to a W-shaped
dialkoxycarbene. Nevertheless, a large fraction of the ylide
fragments to the carbene (8) and acetone, as discussed be-
low.
The longer lifetime of 7 could lead to some closure to
oxirane 9, which was a major initial product, but 9 must also
come from attack of the carbene on acetone, as indicated by
the results of inclusion of 1,1,1-trifluoroacetone, at two dif-
ferent concentrations, during the thermolysis. With a 5-fold
excess of trifluoroacetone (0.5 mol/L), oxiranes 9 (3 parts)
and 14 (7 parts) as well as diol 18 (4 parts) were the major
products obtained before workup. With a 10-fold excess of
trifluoroacetone (1.0 mol/L), oxirane 9 was not detected but
14 and 18 were observed in approximately a 4:3 ratio.
Workup led to the conversion of 14 to a single diastereomer
of 15 and to the loss of 18. Products from 18 were not iden-
We did not find any 17, which should have been present if
9 were in equilibrium with 16, nor was any 20 detected.
Compound 20 would be an expected product from the alter-
native ring opening of 9 to ylide 7.
The finding of 9 and 14 is supported by precedent. One
cyclic dialkoxyoxirane model (24) for 9 is known (5), and
one 2,2-dimethoxyoxirane 25 has been reported (1g). In the
absence of water, 9 must be relatively stable, since it can
survive heating of 6 at 110 °C in benzene for 72 h. Oxirane
9 cannot undergo reversible ring opening rapidly, either to
the carbonyl ylide 7 or to the isomeric zwitterion 16, be-
cause those intermediates would react with trifluoroacetone.
In the presence of moisture, which enters slowly from the
atmosphere once the sealed tube has been opened, oxirane 9
undergoes ring opening to the dihydroxy compound 18,
which must itself be in equilibrium with oxirane 9, via
cationic intermediate 19, Scheme 4. Cation 19 reacts with
oxirane 9 to afford 8,8,11,11-tetramethyl-1,5,7,10,12,16-
hexaoxadispiro[5.2.5.2]hexadecane (22) the structure of
which was secured by means of single crystal X-ray diffrac-
tion. Presumably the mechanism of formation of 22 involves
intermediate 21. Such a reaction would be unlikely in the
presence of excess water, because there would not be any
oxirane available, but it is likely when the available water is
that absorbed from the atmosphere. Water is probably cata-
lytic at very low concentrations, promoting the conversion of
9 to 22 without being consumed significantly. Lactone 23 is
presumably the result of partial hydrolysis of 22 during the
workup.
The present results may be surprising in view of the fact
that oxadiazoline 26, upon thermolysis in benzene, was re-
ported (2b) to afford the carbene dimer 27 (ca. 20%),
Scheme 5. An oxirane dimer, analogous to 22, was not re-
© 2004 NRC Canada

Klys et al.
1771
Scheme 3.
Scheme 4.
ported and in the present case, a carbene dimer was not iso-
lated from thermolysis of 6. Checking the NMR spectra and
mass spectra again confirmed that the compound isolated
from 26 was indeed 27. However, 27 was obtained in only
about 20% yield and only one additional, minor product (30)
was identified. Compound 30, analogous to 23, is presum-
ably derived from dimerization of oxirane 28 to 29 and par-
tial hydrolysis of the latter, Scheme 6 (2b). The presumed
intermediates, 28 and 29, were not seen. Similarly, the
thermolysis of 6 in the absence of trifluoroacetone gave
many products that could have included the dimer of 8.
lysis by analogous mechanisms, affording products from the
appropriate carbenes and from reaction of those carbenes
with acetone. Although not all of the products were isolated
and identified by different workers, there is no conflict be-
tween the results from thermolysis of 6 and 26, which are
expected to be qualitatively the same. However, it appears
that the products are very sensitive to the humidity and to
the adsorbent used in chromatographic separations.
Catalysis of organic reactions by water is rare but not new.
It is believed to occur, for example, during chlorination of
olefins (6) and in the ketonization of enols (7). In the present
case, water protonates and opens a highly reactive dialk-
In conclusion, it is likely that 6 and 26 undergo thermo-
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1772
Can. J. Chem. Vol. 82, 2004
Scheme 5.
Synthesis of hydrazinecarboxylic acid, 3-hydroxypropyl
ester (11)
To compound 10 (2.0 g, 20 mmol) in 10 mL of ethanol, in
a round-bottom flask with an efficient condenser, was added
hydrazine hydrate (98%, 20 mmol) during 20 min. After
refluxing for 18 h the volatiles were evaporated leaving a
colourless oil (2.7 g, 99%) that solidified in the freezer (9).
3
1H NMR (200 MHz, CDCl3) δ: 1.75 (quint, J = 6.3 Hz,
2H), 3.57 (t, 3J = 6.3 Hz, 2H), and 4.08 (t, 3J = 6.3 Hz, 2H).
Synthesis of 12
After acetone (15 mL) and 11 (1.0 g, 7.5 mmol) had
formed a homogeneous solution (30–45 min), it was treated
overnight with anhydrous Na2SO4. Decanting and evapora-
tion of the acetone left 12 as a colourless oil (1.29 g, 99%).
1H NMR (200 MHz, CDCl3) δ: 1.83 and 1.91 (s and m, 5H),
3
3
2.05 (s, 3H), 3.72 (t, J = 5.8 Hz, 2H), and 4.40 (t, J =
5.8 Hz, 2H).
Synthesis of 6
To an ice-cooled heterogeneous mixture of Pb(OAc)4
(LTA) (7.4 g, 16.5 mmol) in dry CH2Cl2 (15 mL) was added
slowly, during 1h, a solution of 12 (2.5 g, 14.4 mmol) in
10 mL of CH2Cl2. Glacial acetic acid (two drops) was added
and the yellow mixture was stirred for 4 h at 0 °C and for
16 h at room temperature. Filtration through Celite, washing
with a NaHCO3 solution (5%, 2 × 50 mL), drying over
Na2SO4, and concentration gave a crude material 13 that was
dissolved in CH2Cl2 (20 mL). Trifluoroacetic acid (3 drops)
was added and, after stirring for 90 min, thin layer chroma-
tography (TLC) analyses showed that conversion of 13 to 6
was complete. Concentration and column chromatography
on silica gel (hexane/EtOAc = 4:1) gave 6 that could be
crystallized from pentane at –55 °C to give 0.55 g (22%) of
Scheme 6.
oxyoxirane and the cationic intermediate attacks an oxirane
molecule to afford an oxirane dimer.
1
pure 6, mp 63–64 °C. H NMR (200 MHz, CDCl3) δ: 1.53
(s, 6H), 1.88 (m, 1H), 2.17 (m, 1H), 4.20 (m, 2H), and 4.60
(m, 2H). The methyl groups of the oxadiazoline ring of a
close analog of 6 absorb at δ = 1.52 (1b).
Experimental
1H NMR spectra run in benzene-d6 are referenced to the
signal for residual H in the solvent, set at 7.15 δ for benzene.
13C NMR spectra were run in CDCl3 and are referenced to
the solvent signal at 77.16 δ for the center line.
Thermolysis of 6 in benzene
A solution of 6 (344 mg, 2.0 mmol) in dry benzene was
sealed into a glass tube and the tube was immersed in an oil
bath at 110 °C for 72 h. One major product was detected by
GC along with two minor products having longer retention
times. A GC-MS gave a mass of 144 for the major product,
which is consistent with oxirane 9. The major product was
transformed into one of the minor products as the feedstock
aged. Isolation of the growing minor product and single
crystal X-ray diffraction showed it to be 22. The other mate-
rial with a long retention time was identified as 23 on the
basis of spectroscopy and analogy. A compound similar to
23, except for a phenyl substituent, has been reported (2b).
Chromatography (Chromatotron, SiO2) afforded primarily
the material with the longer GC retention time.
1,3-Dioxan-2-one
1,3-Propanediol (15.6 g, 0.20 mol) and diethyl carbonate
(30 g, 0.25 mol) in a 100 mL round-bottom flask fitted with
a distillation column, were treated with freshly prepared
NaOMe made from Na (0.18 g) and MeOH (3 mL). The so-
lution was slowly heated to 100 °C and alcohols (methanol
and ethanol, 70–85 °C at the top of the column) were col-
lected. The temperature in the flask was slowly increased to
170 °C over a 5 h period. During this time about 80% of the
product distilled at 130–150 °C. After cooling, the column
was removed, excess diethyl carbonate and a trace amount
of ethanol were distilled out (oil pump) and the product was
distilled at 130–160 °C. Some fractions crystallized sponta-
neously; those that did not were dissolved in ether and the
solutions were cooled to –55 °C to cause crystallization. The
total yield of white crystalline product (8) was 7.65 g (30%),
Compound 9
1
Compound 9 could not be isolated. In the H NMR spec-
trum of the crude (200 MHz, C6D6) there was a strong sin-
glet signal at 0.42 δ that disappeared slowly when the tube
was opened. GC-MS m/z: 144 (M+), 143 (M-1)+,129 (M-
1
mp 45–46 °C. H NMR (200 MHz, CDCl3) δ: 2.13 (quint,
Me)+, 114 (M-CH2O)+, and 87 (C4H7O2 , 100%).
3
+
3J = 5.6 Hz, 2H) and 4.44 (t, J = 5.6 Hz, 4H).
© 2004 NRC Canada

Klys et al.
1773
GC as 15 and, tentatively, as the trifluoromethyl analogue of
18.
Compound 22
1H NMR (500 MHz, C6D6) δ: 0.72 (d, J = 12 Hz, 2H),
1.68 (s, 12H), 1.73 (m, 2H), 3.44 (m, 4H), and 4.10 (t, J =
12 Hz, 4H). 13C NMR (75 MHz, CDCl3) δ: 24.76, 24.86,
30.21, 59.47, 79.13, and 108.43. MS (CI, NH3) m/z: 289
(M + H)+ (100), 288 (M+, 14), 145 (M/2 + H)+ (37), and 128
(33). The molecular structure of 22 was secured by means of
single crystal X-ray diffraction.
Acknowledgements
The authors are grateful for continued financial support
from the Natural Sciences and Engineering Research Coun-
cil of Canada (NSERC). We thank Dr. Jim Britten for the X-
ray structure of 22.
Compound 23
References
1H NMR (200 MHz, C6D6) δ: 1.35 (m (br), 2H), 1.44 (s,
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(50.3 MHz, CDCl3) δ: 23.8, 24.3, 28.6, 29.8, 60.3, 84.9,
108.0, and 172.5. MS (EI) m/z: 231 (M + 1), 215, 200, 172,
164, 129, 103, and 70 (100%); MS (CI, NH3) m/z: 231 (M +
H)+ (100), 173 (3), 145 (13), and 128 (34).
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heated as described above. Three products, with GC reten-
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should have provided (M + H)+ and (M + NH4)+ signals in
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1.52 (m, 2H), 3.34 (m, 2H), 3.74 (m, 2H). 19F NMR
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© 2004 NRC Canada