Complete ozonolysis of alkyl substituted ethenes at -60°C: Distributions of ozonide and oligomeric products

Article abstract of DOI:10.1039/b419174a

The distribution of ozonidc and oligomeric structures formed on complete ozonolysis of alkenes in a non-participating solvent at -60°C is governed by the alkyl substitution around the carbon-carbon double bond. The ozonolysis of a 1,1-alkyl substituted ethene generally favours the formation of an ozonide (a 1,2,4-trioxolane). Whereas the ozonolysis of a 1,1,2-alkyl substituted ethene also produces ozonide, a considerable amount of the ozonised products are oligomeric in nature. For example, the ozonolysis of 3-methylpent-2-ene in solution to high conversion in pentane yields oligomers with structural units derived from the fragmentation products of the primary ozonide (a 1,2,3-trioxolane) which are namely butanone carbonyl oxide and acetaldehyde; these can be characterised by electrospray ionisation mass spectroscopy (ESI-MS) under soft ionisation conditions. The predominant oligomers formed are rich in carbonyl oxide units (80 + mol%) and are cyclic in nature. A small proportion of the oligomers formed are open chain compounds with end groups that suggest that chain termination is brought about either by water or by hydrogen peroxide. Residual water in the solvent will react with the carbonyl oxides to produce 2-methoxybut-2-yl hydroperoxide, which we propose readily decomposes generating hydrogen peroxide. A significant yield of oligomers also is obtained from the ozonolysis of a 1,2-alkyl substituted ethene. The ozonolysis of trans-hex-2-ene in pentane yields oligomers containing up to four structural units and are predicted to be mainly cyclic. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b419174a

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A R T I C L E
◦
Complete ozonolysis of alkyl substituted ethenes at −60 C:
distributions of ozonide and oligomeric products†
Matthew Barton, John R. Ebdon,* Andrew B. Foster‡ and Steve Rimmer*
The Polymer Centre, Department of Chemistry, University of Sheffield, Sheffield, UK S3 7HF.
E-mail: s.rimmer@sheffield.ac.uk; Fax: +44 0114 222 9346; Tel: +44 0114 222 9565
Received 23rd December 2004, Accepted 11th February 2005
First published as an Advance Article on the web 7th March 2005
The distribution of ozonide and oligomeric structures formed on complete ozonolysis of alkenes in a
non-participating solvent at −60 ◦C is governed by the alkyl substitution around the carbon–carbon double bond.
The ozonolysis of a 1,1-alkyl substituted ethene generally favours the formation of an ozonide (a 1,2,4-trioxolane).
Whereas the ozonolysis of a 1,1,2-alkyl substituted ethene also produces ozonide, a considerable amount of the
ozonised products are oligomeric in nature.
For example, the ozonolysis of 3-methylpent-2-ene in solution to high conversion in pentane yields oligomers with
structural units derived from the fragmentation products of the primary ozonide (a 1,2,3-trioxolane) which are
namely butanone carbonyl oxide and acetaldehyde; these can be characterised by electrospray ionisation mass
spectroscopy (ESI-MS) under soft ionisation conditions. The predominant oligomers formed are rich in carbonyl
oxide units (80 + mol%) and are cyclic in nature. A small proportion of the oligomers formed are open chain
compounds with end groups that suggest that chain termination is brought about either by water or by hydrogen
peroxide. Residual water in the solvent will react with the carbonyl oxides to produce 2-methoxybut-2-yl
hydroperoxide, which we propose readily decomposes generating hydrogen peroxide. A significant yield of oligomers
also is obtained from the ozonolysis of a 1,2-alkyl substituted ethene. The ozonolysis of trans-hex-2-ene in pentane
yields oligomers containing up to four structural units and are predicted to be mainly cyclic.
Introduction
The products obtained from the ozonolysis of alkenes, here-
inafter termed ‘ozonates’, have been found by us to be usable as
radical initiators of both solution and emulsion polymerizations
of vinyl and acrylic monomers.1–4 Consequently, the character-
ization of the ozonates formed in such reactions carried out to
high conversion has been of interest.5 We have recently reported
that the complete ozonolysis of 2,3-dimethylbut-2-ene, 1, in pen-
tane at −60 ◦C yields mainly oligomeric ozonates: the individual
structures formed being readily identified by electrospray mass
spectrometry (ESI-MS) run under soft ionisation conditions.6
The structure formed in the highest yield is the cyclic hexamer
of acetone carbonyl oxide. The carbonyl compound (acetone)
produced on decomposition of the primary ozonide (a 1,2,3-
trioxolane) is not sufficiently polar to react with the carbonyl
oxide to form a 1,2,4-trioxolane and is indeed shown not to
be incorporated in these oligomeric structures. Here we explore
further the effect the substitution around the double bond of the
alkene has on the types of ozonate structures formed, oligomeric
or otherwise, by ozonolysing 2,4,4-trimethylpent-1-ene, 2, 2-
methylpent-1-ene, 3, a mixture of cis and trans-3-methylpent-
2-ene, 4 and trans-hex-2-ene, 5, the structures of which are pre-
sented in Scheme 1. The compounds were chosen because they
Scheme 1 Mechanism and predominant products of the ozonolysis of
allow us to examine in particular the effects of the alkyl substitu-
tion pattern on the structures of oligoperoxidic products (Fig. 1).
3-methylpent-2-ene, 4, in pentane at −60 ◦C.
The reaction in solution of O3 and alkenes has been ex-
tensively studied.7–9 The primary ozonide (1,2,3-trioxolane)
initially formed upon ozonolysis is unstable and cleaves to a
carbonyl oxide and a carbonyl compound. These two entities
react in different ways to produce ozonates in the form of
† Electronic supplementary information (ESI) available: Figs. 1 and
2: proton NMR spectra from reactions A and B, DEPT carbon
NMR spectra from reactions D and E; Tables 1–3: main oligomeric
ozonate structures observed from ESI mass spectrometry. See
http://www.rsc.org/suppdata/ob/b4/b419174a/
Fig. 1 Alkyl substituted ethenes ozonised at −60 ◦C in pentane.
ozonides (1,2,4-trioxolanes), diperoxides (1,2,4,5-tetroxolanes)
and oligoperoxides. If the ozonolysis reaction is carried out in
‡ Current address: School of Materials, The University of Manchester,
Grosvenor Street, Manchester, UK M1 7HS.
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 1 3 2 3 – 1 3 2 9
1 3 2 3
©

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the presence of a proton donor, alkoxyalkyl hydroperoxides are
also produced. Generally ozonides are perceived to be the major
ozonate product for reactions of mono-, di-, or trisubstituted
ethenes in non-participating solvents. Diperoxides are formed
in ozonolysis reactions of tetrasubstituted ethenes which do not
generally yield ozonides.
could be formed if the dipolar intermediate decomposed to form
dimethyldioxirane, which then broke up to form the peroxides.
The findings implied that the inability of 1 to form any ozonide is
not due solely to the lack of mutual reactivity of the malozonide
fragments (acetone carbonyl oxide and acetone). Cyclic perox-
ides can also be prepared by methods other than ozonolysis and
the main methods were recently reviewed.19,20 Also, the reactions
of these compounds have been reviewed by Cafferata.21
Oligomers have also been reported as significant products
of the ozonolysis of alkenes in solution, although, prior to
our work,6 identifying the precise structures has proven dif-
ficult. Early work proposed peroxidic and ether linkages in
oligomers formed from the ozonolysis of but-2-ene.10 Criegee11
thought that oligomers from tetrasubstituted ethenes resulted
from the polymerization of the dipolar carbonyl oxide. The
stereochemistry of the alkene has been reported to have an
effect on the distribution of products from ozonolysis: trans
isomers gave mostly oligomers, whereas cis isomers yielded
mainly ozonides.12 The reduction of the oligomers with lithium
aluminium hydride yielded alcohols, including a small amount
of a-diol, proving that there were no carbon–carbon bonds
in the backbones of the oligomers. It was also suggested that
there were a variety of different oxygen linkages present in
the oligomers, including linkages of three contiguous oxygen
atoms. Other studies of the ozonolysis of a range of mono and
di-substituted alkenes proposed oligomers containing between
three and twelve monomer units, derived from either the
aldehyde or the carbonyl oxide.13 The oligomers were defined
as cyclic structures on the basis of the absence of a carbonyl
absorption in IR spectra. Fliszar et al.14 studied oligomers
from the ozonisation of phenyl-substituted ethenes and found
that carbonyl oxides, which do not react to form an ozonide,
form oligomers instead. These workers also found that only
oligomers formed from tetra-substituted olefins were stable at
room temperature and postulated that they were open chain
polymeric peroxides. Oligomers derived from the ozonolysis
of 1 in non-participating solvents were believed to contain
peroxidic structures derived from acetone carbonyl oxide and/or
structures with hydroperoxide end groups.15 This suggested that
water and hydrogen peroxide were also participating in the
reaction, findings borne out in our studies of the alkene.6
Murray and Ramachandran16 identified oligomers as signif-
icant products from the ozonolysis of trans-di-tert-butylethene
at −90 ◦C in different solvents. They concluded from elemental
analysis and molecular weight data that the oligomers obtained
in hexane were composed of 7 or 8 carbonyl oxide units
in a cyclic arrangement. Further work on the ozonolysis of
hex-3-ene derivatives17 revealed that the solvent affects the
molecular weight of the ozonised products. Alkenes ozonised
in hexane produced higher molecular weight oligomers than did
reactions in n-butyraldehyde. However, the use of aldehyde as
solvent led to the formation, overall, of more oligomer. The
oligomers were characterized from cryoscopic molecular weight
and elemental analysis data. Other work,18 which considered
the ozonolysis of some partially brominated tetrasubstituted
ethenes in acetone, indicated that ozonide formation could
occur via a non-concerted reaction of a carbonyl oxide and
a carbonyl compound. The products of these reactions also
included acetone diperoxide and acetone triperoxide, which
Despite the previous reports, much of the work on the
oligomeric products of ozonolysis has involved a degree of
speculation since it was impossible to directly observe the
oligomers. However, we showed recently that ESI-MS could be
used to directly identify the oligomers formed from ozonolysis of
1. The products produced are mixtures of the cyclic diperoxide,
triperoxide and higher oligomers. In the absence of water, the
major products are cyclic oligomers but open chain oligomers
are also found if significant amounts of water are present. In
this work we extend this study to a series of alkenes in which the
substitution pattern around the double bond is changed.
Results and discussion
In this work we wished to study the production of oligoperoxides
during the ozonolysis of several alkenes in which the carbon–
carbon double bond was located in various places within the car-
bon skeleton. Thus, we first examined alkenes with terminal dou-
ble bonds: 2 and 3. As expected, for the ozonolysis of 2 neither
size exclusion chromatography (SEC) nor ESI-MS produced
evidencefor oligomeric structures inthe ozonates. Also1H NMR
spectra of the reaction mixture were composed of sharp lines,
characteristic of low molecular weight products, rather than the
broader peaks observed in our previous study of the ozonates
derived from1.6 On the other hand, some evidencefor oligomeric
peroxides was obtained from SEC, following the ozonolysis of 3,
but these compounds were produced in very low concentrations
and we were unable to obtain mass spectra of these materials.
The SEC chromatogram of the reaction mixture produced from
3 is shown in Fig. 2 and the molecular weight averages calculated
using polystyrene standards are given in Table 1.
Fig. 2 SEC chromatograms of the products of the ozonolysis of
2-methylpent-1-ene, 2 (reaction B), 3-methylpent-2-ene, 4 (reactions C
and D) and an equimolar mixture of 2,3-dimethylbut-2-ene, 1, and
3-methylpent-2-ene, 4 (reaction E) in pentane at −60 ◦C (low molecular
weight column, psty standards). Note: the bottom trace was obtained
for the ozonates of 1 in previous work.6
Table 1 Results from ozonolysis of alkenes in pentane at −60 ◦
C
Alkene conc./mol l−1
Alkene(s)
Yield/g (%b)
SECc (Mw, Mn)
ESI MS max m/z
A
B
C
D
E
F
0.412
0.411
0.396
0.223
0.411
0.411
2
2.75(83)
1.76(65)
1.28(82)
—
1.75(83)
1.80(64)
—
—
—
3
616, 508
438, 123
515, 379
448, 259
—
4
528 (+18)
528 (+18)
472 (+18)
398 (+18)
4
1 + 4a
5
a Equimolar amounts. b Percentage yield recovered related to the theoretical amount the fragments of the primary ozonide (1,2,3-trioxolane) can
contribute to the ozonates. Reaction D yield was not determined. c Oligomeric fraction molecular weights quoted relative to polystyrene standards.
1 3 2 4
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 1 3 2 3 – 1 3 2 9

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Thus, as expected, the terminal alkenes studied here do not
generate significant amounts of oligomeric peroxides probably
because the formaldehyde generated on breakdown of the
primary ozonide, reacts fast enough with the carbonyl oxide
to prevent oligomerization of the latter. Next we examined the
ozonolysis of some internal alkenes.
relative to carbonyl oxide units, in the ozonate structures is 20–25
mol%.
ESI-MS of the ozonates of 4 made in pentane, obtained under
soft ionisation conditions, provides the most direct evidence for
the oligomeric peroxides. The spectra are shown in Fig. 4.
A distribution of ions derived from the oligomers is readily
obtained for the products recovered from both reactions C
and D (Table 1). The spacing between each of the ions in the
main series is m/z 44. We expect the primary ozonide (a 1,2,3-
trioxolane), 4a, to decompose to yield butanone carbonyl oxide,
4b, m/z 88 and acetaldehyde, 4c, m/z 44. Therefore the spacing
of m/z 44 confirms, in combination with the NMR data, that
both carbonyl oxide and aldehyde units are included in the
oligomeric ozonates. The mass totals of the ions correspond
exactly to the ammonium adducts (+18) of oligomers, 4g,
produced from random copolymerization of butanone carbonyl
oxide and acetaldehyde. The ions can be fully rationalized to
these structures, which do not include any end groups. Thus,
the oligomers are cyclic, 4g1. For example, the ions at m/z
458 and 546 correspond to the cyclic pentamer and hexamer
of the carbonyl oxide, 4b. There is a second minor distribution
of ions that are also separated by m/z 44. These peaks appear
to correspond to open chain oligomers of x butanone carbonyl
oxide and y acetaldehyde repeat units, with end groups the mass
of which total m/z 34, i.e. m/z = 88x + 44y + 34 overall. We
suggest that these oligomers are predominantly terminated at
both ends with –OOH groups, 4g2. Such end groups may arise by
reaction of oligocarbonyl oxides with hydrogen peroxide.6 There
is also a third, minor, series of ions, also spaced at intervals of
m/z 44. This group of ions has end group masses of m/z 18. We
suggest that these ions correspond to open chain oligomers of the
two structural units terminated directly by reaction with water,
leaving structures with a hydroperoxy and a hydroxy group on
the a and x chain ends, 4g3.
Complete ozonolysis of 4 in pentane at −60 ◦C gave colour-
less, viscous residues upon removal of the solvent (reactions C
and D). A theoretical yield for the reaction can be calculated
if we consider that the products from the decomposition of the
unstable primary ozonide (a 1,2,3-trioxolane), 4a, react with
each other and as such are incorporated in the ozonate structures
ultimately formed (Scheme 1). The percentage yield recovered
on this basis in reaction C is thus estimated to be 82%. The
SEC chromatograms obtained for ozonates formed in reactions
C and D are shown in Fig. 2. Both chromatograms show that a
large proportion of the ozonates are oligomeric, with a number
average molecular weight of 379 g mol−1 (against polystyrene
standards), comparable to that obtained from the ozonates of 1
(bottom trace6).
The proton NMR spectrum of the ozonates obtained in
reaction C is shown in Fig. 3(a). The overall broadness of the
resonances between 0.5 and 2.5 ppm suggests that oligomeric
ozonates are the predominant products of this ozonolysis
reaction. The resonance at 0.90 ppm is readily attributed to
the terminal methyl group, i, but lacks the triplet fine structure
resulting from coupling to the neighbouring methylene protons,
ii. The resonance patterns between 5.0 and 6.0 ppm best
distinguish the relative amounts of ozonide and oligomer formed
on ozonolysis. The methine proton, iv, of the ozonide, 4f, is
represented by two partly overlapping quartets (J = 4.9 Hz) at
5.30 ppm from coupling to the neighbouring methyl group, v.
We attribute the broad band centred at 5.50 ppm to the methine
proton, iv, of acetaldehyde moieties (y) in oligomeric structures,
4g. The resonances found between 9.0 and 10.0 ppm are assigned
to hydroperoxy protons on the chain ends of open chain
oligomers terminated with hydrogen peroxide, 4g2. Previous
work with other alkenes has generally reported aldehyde groups
to be under-represented in the oligomeric ozonate structures17
and in these reactions the percentage of acetaldehyde units,
In some synthetic procedures or industrial processes, ozonol-
ysis may be conducted on mixtures of alkenes. This situation
should generate mixtures of peroxy oligomers that are also
amenable to analysis by joint application of ESI-MS and
NMR. Therefore, we further extended our studies to a mixture
of alkenes 1 and 4.
The complete ozonolysis in pentane at −60 ◦C of an equimolar
mixture of 2,3-dimethylbut-2-ene, 1, and 3-methylpent-2-ene,
4, yielded an inhomogeneous mixture of a white solid and
colourless viscous residue upon removal of the solvent (reaction
E). A theoretical yield for the reaction can be calculated if
we assume that there are only three reactive species generated
on ozonolysis capable of forming ozonate structures: acetone
carbonyl oxide (from 1), butanone carbonyl oxide, 4b, and
acetaldehyde, 4c (both from 4). The percentage yield recovered
on this basis is 83%.
The SEC chromatogram obtained from these ozonates formed
in pentane at −60 ◦C (reaction E) is shown in Fig. 2. The
chromatogram shows that a large proportion of the ozonates
are oligomeric, with a number average molecular weight of 259 g
mol−1 (against polystyrene standards).
The proton NMR spectrum of the ozonates is shown in
Fig. 3(b). The peak at 0.95 ppm is attributed to the methyl
protons, ii, of butanone carbonyl oxide, 4b, in all ozonate
structures shown in Fig. 5. All resonances between 5.0 and
6.0 ppm are assigned to methine protons, v, in the different
ozonate structures. The most intense band centred at 1.43 ppm
is in turn attributed to the other four sets of methyl protons,
designated i, iv and vi in the ozonate structures (1 + 4)g.
Comparing the relative integrals of these three features allows an
estimation of the proportions of the three main constituents to
be found in the ozonate structures: 37% acetone carbonyl oxide,
46% butanone carbonyl oxide and 17% acetaldehyde units. The
resonances found between 9.0 and 10.0 ppm are assigned to
hydroperoxy protons on the chain ends of open chain oligomers
terminated with hydrogen peroxide (1 + 4)g2.
Fig. 3 Proton NMR spectra of the ozonates recovered from the
complete ozonolysis of (a) 3-methylpent-2-ene, 4 (reaction C) and (b) an
equimolar mixture of 2,3-dimethylbut-2-ene, 1, and 3-methylpent-2-ene,
4 (reaction E).
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 1 3 2 3 – 1 3 2 9
1 3 2 5

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Fig. 4 ESI mass spectra of the products of ozonolysis of 3-methylpent-2-ene, 4, in pentane at −60 ◦C, part (a) reaction C and (b) reaction D: cyclic
mixed oligomers of butanone carbonyl oxide and acetaldehyde denoted with ꢀ, with linear mixed oligomers terminated by hydrogen peroxide denoted
with ꢀ and those terminated by water denoted with ᭹.
ESI-MS of the ozonates of the alkene mixture made in
pentane, obtained under soft ionization conditions, is shown
in Fig. 6. A complex distribution of oligomers is obtained for
the products recovered from reaction E (Table 1). We expect
the oligomeric structures to be composed of x acetone carbonyl
oxide, y butanone carbonyl oxide and z acetaldehyde units, with
masses m/z of 74, 88 and 44, respectively. The predominant
structures captured in the mass distribution correspond to
ammonium adducts of mixed cyclic oligomers, (1 + 4)g1, with
m/z = 74x + 88y + 44z + 18. The larger oligomeric structures
present, with masses of more than 600, correspond exclusively
to open chain oligomers terminated with hydrogen peroxide,
(1 + 4)g2, with m/z = 74x + 88y + 44z + 34 + 18. This concurs
with the findings for the ozonates obtained in the individual
reactions of 1 and 4 in which the proportion of oligomers with
open chain structures increased as the chain length increased.
This latter feature of the mass spectra is a consequence of
the cyclization process, which is an intramolecular termination
reaction and occurs at lower degrees of polymerization than
the intermolecular termination processes that lead to the open
chain oligomers. That is, if the chain does not terminate by
cyclization it grows by further addition of monomer until it is
terminated in an intermolecular reaction. Thus, termination of
the cyclic oligomers occurs earlier in the chain growth process
than termination of the open chain oligomers and this leads to
open chain oligomers with higher degrees of polymerization.
Fig. 5 Predominant products of ozonolysis of an equimolar mixture
The complete ozonolysis in pentane at −60 ◦C of 5
of 2,3-dimethylbut-2-ene, 1, and 3-methylpent-2-ene, 4, in pentane at
−60 ◦C.
yielded colourless viscous residues upon removal of the solvent
1 3 2 6
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 1 3 2 3 – 1 3 2 9

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Fig. 6 ESI mass spectra of the products of ozonolysis of an equimolar mixture of 2,3-dimethylbut-2-ene, 1, and 3-methylpent-2-ene, 4, at −60 ◦
C
in pentane (reaction E).
Fig. 7 Proton NMR spectrum of the ozonates recovered upon the
complete ozonolysis of trans-hex-2-ene, 5, in pentane at −60 ◦C (reaction
F).
neighbouring residues in the oligomers. The other broad band
hidden under some fine structure at 5.26 ppm is assigned to the
methine proton, iv, from the carbonyl oxide unit, w, (and/or a
butyraldehyde unit, x) formed from the larger side of the alkene
in oligomeric structures, 5g. Although the structures attributed
to these broad peaks make up the majority of the ozonates, the
fine structure is evidence for the formation of ozonide, 5f; two
overlapping quartets centred at 5.3 ppm are characteristic of
the methine proton, v. The other methine proton, iv, present
in the ozonide should be represented by a set of triplets from
coupling to the neighbouring methylene protons, iii: unresolved
fine structure is evident marginally upfield from the group of
quartets. The proportions of each side of the ozonised alkene
incorporated in the ozonate structures can be calculated by
comparing the relative integral intensities of the broad peaks
at 1.45 and 1.75 ppm. The peak at 1.45 ppm is attributable both
to the methylene protons, ii, and the methyl protons, vi, in all
ozonate structures, whilst the peak downfield at 1.75 ppm is due
solely to the methylene protons, iii. Entities derived from the
bulkier side of the double bond of the alkene contribute 57% to
the ozonate structures. We again attribute multiple resonances
between 9.0 and 10.0 ppm to hydroperoxy protons on oligomer
chain ends. The low relative intensity suggests that a lower
fraction of the open chain oligomers is derived from this alkene
than is derived with either 1 or 4. This may be connected with
the stability of the predominant carbonyl oxide formed in each
of the alkenes; both 1 and 4 form di-alkyl substituted carbonyl
oxides on ozonolysis.
Scheme 2 Mechanism and predominant products of ozonolysis of
trans-hex-2-ene, 5, in pentane at −60 ◦C.
(reaction E). We can calculate a theoretical yield if we predict
that both sides of the alkene form carbonyl oxide and carbonyl
compounds 5b,c,d and e on cleavage of the primary ozonide,
5a, that are capable of contributing to ozonate structures
(Scheme 2). A percentage yield of 64% was calculated on this
basis.
The proton NMR spectrum of the ozonates is shown in
Fig. 7. The overall broadness of the resonances again suggests
the formation of predominantly oligomeric ozonates. Similar
electronic substitution on each side of the double bond in the
starting alkenemakes itharder to speculate which carbonyl oxide
and carbonyl compounds are expected to form upon ozonolysis
from subsequent cleavage of the primary ozonide, 5a.
The pattern of broad resonances between 5.0 and 6.0 ppm
would suggest that methine protons exist in oligomeric struc-
tures in at least two forms. The split broad band centred
at 5.50 ppm is assigned to the methine proton, v, of an
acetaldehyde unit, z, (and/or the equivalent carbonyl oxide, y,
derived from that side of the alkene) in oligomeric structures, 5g.
The resonance band is split in two by potential chiral centres on
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 1 3 2 3 – 1 3 2 9
1 3 2 7

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Fig. 8 ESI mass spectra of the products of ozonolysis of trans-hex-2-ene, 5, in pentane at −60 ◦C (reaction F).
An ESI mass spectrum of the ozonates of 5 made in
pentane, obtained under soft ionisation conditions, is shown
in Fig. 8. A distribution of ions derived from the oligomers
is readily obtained. If the oligomers were composed only of
structural units derived from carbonyl oxide, 5b, (m/z 88) and
acetaldehyde, 5c, (m/z 44) we would expect a mass distribution
similar to that obtained for the ozonates of 4, shown in Fig. 4.
However the array of ions observed suggests that all four possible
structural units, w, x, y and z, from the ozonolysis of 5 are
present in the oligomeric structures. The masses of the w, x,
y and z residues are m/z 88, 72, 60 and 44 respectively. The
spacings between the most prominent peaks, notably m/z 12, 16,
28 and 32, in the distribution, correspond to the mass differences
between these predicted residues. However, the ions captured
do not, in almost all cases, directly conform to the ammonium
adducts (+18) of the numerous cyclic oligomers that are possible.
Instead the ions can be assigned to combinations of the four
structural units existing as either fragmented cyclic oligomers or
open chain oligomers terminated with hydrogen peroxide. All the
combinations that make up the suggested oligomeric structures
are included in the electronic supporting information.†
acetaldehyde carbonyl oxide (y, m/z = 60) and acetaldehyde
(z, m/z = 44), which are included in the oligomeric ozonates.
The stability of both ozonides and oligomers is influenced
by the R substitution in the –O–C(R2)–O– linkages. Mono-
alkyl substituted oligomers have been reported to decompose
at room temperature whilst di-alkyl substituted oligomers, i.e.
those from tetrasubstituted alkenes, were found to be stable
at room temperature.14 The ESI-MS analysis of these ozonates
captures the initial fragmentation products that are composed
solely of mono-alkyl substituted carbonyl oxide and carbonyl
structural units.
In summary, we present here for the first time unequivocal
evidence for cyclic and open chain oligomeric ozonates contain-
ing structural units derived from both fragments, carbonyl oxide
and aldehyde, generated upon the decomposition of the primary
ozonide in ozonolysis reactions of 1,1,2-trialkyl and 1,2-dialkyl
substituted alkenes. Cyclic structures are prevalent in reactions
carried out in pentane, with the open chain oligomeric structures
(best observed in the ozonates generated from 3-methylpent-2-
ene) terminated with either water or hydrogen peroxide. The
volatility of some of the ozonates generated may hinder their
recovery from the solvent, particularly in the case of 5, and we
therefore avoid quoting absolute values for the relative quantities
of ozonide and oligomers formed in each case. The yields
recovered in each case would suggest that whereas the complete
ozonolysis of 1,1-alkyl ethenes yields predominantly ozonides,
1,1,2-alkyl and 1,2-alkyl substituted ethenes yield considerably
more oligomeric ozonates.
Conclusions
The complete ozonolysis of both 2 and 3 in pentane at −60 ◦C
yielded predominantly their respective conventional ozonides
(1,2,4-trioxolanes). Any oligomeric structures that form do so in
yields that are too low to be detected by ESI-MS although some
evidence for their presence in product mixtures was obtained by
SEC analysis.
Experimental
The viscous ozonates recovered from the complete ozonolysis
of 4 in pentane at −60 ◦C (reactions C and D) are observed in the
ESI-MS under soft ionisation conditions and are predominantly
cyclic oligomers composed of butanone carbonyl oxide and
acetaldehyde; proton NMR analysis shows that the ozonates are
rich in repeat units derived from the carbonyl oxide (80 + mol%).
The predominant cyclic structure captured has m/z 546, which
we can speculate contains either six butanone carbonyl ox-
ide units or five butanone carbonyl oxide units and two acetalde-
hyde units. Distributions of open chain oligomers containing the
two structural units terminated with hydrogen peroxide are also
readily observed in the ESI-MS. As before,6 we propose that
hydrogen peroxide is generated from the reaction of carbonyl
oxide with residual water in the solvent.
Materials
2,3-Dimethylbut-2-ene (98%), 2,4,4-trimethylpent-1-ene (99%),
2-methylpent-1-ene (99 + %), a mixture of cis and trans-3-methyl
pent-2-ene (99%), trans-hex-2-ene (98 + %) and pentane (99 +
%, HPLC grade) were obtained from the Aldrich Chemical Co.
and were used as supplied. Sodium thiosulfate and potassium
iodide were purchased from BDH Laboratory Supplies and also
used as supplied.
Instrumentation
Proton and carbon NMR spectra were recorded on 250 MHz
and 400 MHz spectrometers respectively, using deuterated
chloroform as solvent. The chemical shifts quoted are relative
to tetramethylsilane. SEC measurements were carried out with
RI and UV (260 nm) detectors at ambient temperature using
THF as the solvent and toluene as a flow marker. Polymer
The ozonolysis of an equimolar alkene mixture of 1 and 4 in
◦
pentane at −60 C (reaction E) generated oligomeric ozonates
containing three structural units: acetone carbonyl oxide, bu-
tanone carbonyl oxide and acetaldehyde. The predominant ions
in the ESI-MS are again attributed to cyclic structures, whilst
most of the heavier ions (longer chain oligomers) are attributed
to open chain structures terminated by hydrogen peroxide.
The complete ozonolysis of 5 in pentane at −60 ◦C (reaction
F) revealed a complex array of oligomeric ozonate structures in
the ESI-MS. Both proton NMR and ESI-MS analysis suggest
that the primary ozonide cleaves in two ways to give four entities,
butanal carbonyl oxide (w, m/z = 88), butanal (x, m/z = 72),
˚
Laboratories PLGel, 5 lm, 500 A, and 2 × 60 cm low molecular
weight columns were used with a solvent flow rate of 1 ml min−1.
The chromatograms were calibrated against low molecular
weight polystyrene standards (Polymer Laboratories). Sample
concentrations were 5 mg/2 ml THF. The ESI-MS experiments
were carried out on a Micromass “Platform LCZ” quadrupole
mass spectrometer equipped with an atmospheric pressure
ionisation (a.p.i.) source operated in the nebulizer-assisted
1 3 2 8
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 1 3 2 3 – 1 3 2 9

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electrospray mode. The potential on the electrospray capillary
was set to 2.0 kV and the extraction cone voltage was set to
a value of 25 V, wit◦h the extractor at 4.0 V and a desolvation
temperature of 150 C. Samples were dissolved in methanol at
1 mg ml−1. Aliquots of 10 lL were injected into a flowing solvent
mixture (1 ml min−1) composed of 10% aqueous ammonium
acetate solution (10 mM) and 90% methanol by volume. A flow
rate of 0.2 ml min−1 of this mixture was diverted into the mass
spectrometer.
(CH3)C(OO)OCH2], 1.47 [3H, s, ((CH3)3CCH2)(CH3)C(OO)-
OCH2]], 1.01 [9H, s, (CH3)3CC−].
Ozonolysis of 3 in pentane at −60 ◦C (reaction B):
1
ozonates mainly in the form of ozonide and ketone, H NMR
(250 MHz, CDCl3, 25 ◦C, TMS) d 5.12 and 5.03 [2H, d,
(CH3CH2CH2)(CH3)C(OO)OCH2], 2.40 [2H, t, (CH3CH2CH2)-
(CH3)CO], 2.20 [3H, s, (CH3CH2CH2)(CH3)CO], 1.80 [2H, br,
(CH3CH2CH2)(CH3)C(OO)OCH2], 1.40 [2H + 3H, br, (CH3-
CH2CH2)(CH3)C(OO)OCH2], 0.90 [3H, t, (CH3CH2CH2−].
The other ozonolysis reactions C–F in pentane at −60 ◦C
produced mainly oligomeric ozonates, the 1H NMR (250 MHz,
CDCl3) spectra of which are discussed elsewhere in the paper.
Ozonolysis procedure
A flask containing a s◦olution of alkene was placed in a dry ice–
acetone bath at −60 C for 20 min before starting ozonolysis.
Ozone was generated by passing an oxygen stream through an
electrical discharge-type ozone generator. The O2 stream was set
to 20 ml min−1 and the electrical discharge was set at 150 V. The
O3 flow rate at these settings was measured by bubbling the
O2–O3 mixture through an alkaline boric acid-buffered aqueous
solution (50 ml) of potassium iodide (3.47 × 10−2 M) for 2 min.
The liberated iodine solution was then acidified before being
titrated with sodium thiosulfate (6.94 × 10−2 M) using starch
indicator solution. The rate of evolution of O3 was found to be
0.88 g hr−1.
Acknowledgements
We gratefully acknowledge financial support from ICI Paints
(SRF award, R3AB2352) and thank Dr P. Palasz and Dr
S. Emmett for their continued interest in the project. We also
thank Mr J. McAuley (Queen’s University, Belfast) and Prof.
B. Mann (University of Sheffield) for helpful discussions.
References
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with the outlet gas from this being passed directly into an ozone
analyser (BMT Ozone Analyser 963, Berlin). Initially, almost all
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ozonolysis reaches completion, the increasingly positive reading
on the analyser stabilised at about 40 g m−3. The completion of
alkene ozonolysis can also be observed visually, as the solution
turns blue from saturation with O3. Typically the time required
for ozonolysis was proportional to the concentration of alkene
in the solution. For example, the ozonolysis reactions of 2,4,4-
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solution by us and were never rigorously dried.
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