Facile rearrangement of 3-oxoalkyl radicals is evident in low-temperature gas-phase oxidation of ketones

Article abstract of DOI:10.1021/ja405892y

The pulsed photolytic chlorine-initiated oxidation of methyl-tert-butyl ketone (MTbuK), di-tert-butyl ketone (DTbuK), and a series of partially deuterated diethyl ketones (DEK) is studied in the gas phase at 8 Torr and 550-650 K. Products are monitored as a function of reaction time, mass, and photoionization energy using multiplexed photoionization mass spectrometry with tunable synchrotron ionizing radiation. The results establish that the primary 3-oxoalkyl radicals of those ketones, formed by abstraction of a hydrogen atom from the carbon atom in γ-position relative to the carbonyl oxygen, undergo a rapid rearrangement resulting in an effective 1,2-acyl group migration, similar to that in a Dowd-Beckwith ring expansion. Without this rearrangement, peroxy radicals derived from MTbuK and DTbuK cannot undergo HO₂ elimination to yield a closed-shell unsaturated hydrocarbon coproduct. However, not only are these coproducts observed, but they represent the dominant oxidation channels of these ketones under the conditions of this study. For MTbuK and DTbuK, the rearrangement yields a more stable tertiary radical, which provides the thermodynamic driving force for this reaction. Even in the absence of such a driving force in the oxidation of partially deuterated DEK, the 1,2-acyl group migration is observed. Quantum chemical (CBS-QB3) calculations show the barrier for gas-phase rearrangement to be on the order of 10 kcal mol^-1. The MTbuK oxidation experiments also show several minor channels, including β-scission of the initial radicals and cyclic ether formation.

Full text of DOI:10.1021/ja405892y

Article
pubs.acs.org/JACS
Facile Rearrangement of 3‑Oxoalkyl Radicals is Evident in
Low-Temperature Gas-Phase Oxidation of Ketones
Adam M. Scheer,* Oliver Welz, Darryl Y. Sasaki, David L. Osborn, and Craig A. Taatjes*
,†
†
‡
†
,†
†
Combustion Research Facility, Sandia National Laboratories, MS 9055, Livermore, California 94551, United States
‡
Biological and Materials Science, Sandia National Laboratories, MS 9292, Livermore, California 94551, United States
*
S Supporting Information
ABSTRACT: The pulsed photolytic chlorine-initiated oxida-
tion of methyl-tert-butyl ketone (MTbuK), di-tert-butyl ketone
(
(
DTbuK), and a series of partially deuterated diethyl ketones
DEK) is studied in the gas phase at 8 Torr and 550−650 K.
Products are monitored as a function of reaction time, mass,
and photoionization energy using multiplexed photoionization
mass spectrometry with tunable synchrotron ionizing radiation.
The results establish that the primary 3-oxoalkyl radicals of those ketones, formed by abstraction of a hydrogen atom from the
carbon atom in γ-position relative to the carbonyl oxygen, undergo a rapid rearrangement resulting in an effective 1,2-acyl group
migration, similar to that in a Dowd−Beckwith ring expansion. Without this rearrangement, peroxy radicals derived from MTbuK
and DTbuK cannot undergo HO elimination to yield a closed-shell unsaturated hydrocarbon coproduct. However, not only are
2
these coproducts observed, but they represent the dominant oxidation channels of these ketones under the conditions of this
study. For MTbuK and DTbuK, the rearrangement yields a more stable tertiary radical, which provides the thermodynamic
driving force for this reaction. Even in the absence of such a driving force in the oxidation of partially deuterated DEK, the 1,2-
acyl group migration is observed. Quantum chemical (CBS-QB3) calculations show the barrier for gas-phase rearrangement to be
−1
on the order of 10 kcal mol . The MTbuK oxidation experiments also show several minor channels, including β-scission of the
initial radicals and cyclic ether formation.
1
. INTRODUCTION
site initially at the γ position relative to the carbonyl oxygen,
attacks the carbonyl carbon atom (C ), which leads to a cyclic
α
Radical rearrangement isomerizations play vital roles in a wide
range of important chemical schemes including pyrolysis,
oxidation,
1
−3
transition-state structure. Fission of the C
the C and C atoms. Depending on the nature of the β-carbon
substituent groups, this rearrangement can yield a primary
radical (R1,2 = H) or a more stable secondary (R = alkyl group,
α
−C bond exchanges
β
4
,5
6,7
β
γ
and synthesis.
A large variety of radical
6
−9
rearrangements have been observed in both the liquid
and
4
,5
1
gas phases. A thorough understanding of these reactions is
necessary to build accurate kinetic models to describe complex
oxidation and ignition schemes. Furthermore, radical rearrange-
ment reactions, both unanticipated and by design, play
important roles in many synthetic schemes. In this study, we
report that one such radical rearrangement, a 1,2-acyl group
migration well-known in solution, is prominent in the gas-
phase oxidation of branched ketones. We have investigated the
low-temperature oxidation of ketones because they are among
the organic compounds produced from conversion of ligno-
cellulose by endophytic fungi and have potential application as
R2 = H) or tertiary (R1,2 = alkyl group) radical. A
thermodynamic driving force for the isomerization exists if
the final radical site has a higher degree of substitution than the
initial radical site.
9
6
Such a rearrangement was observed in solution as early as
14
15
1
1
961, and a general description was given by Karl et al. in
972. In that work, the authors studied a series of perlevulinate
10
esters. Upon thermal decomposition, the esters were designed
to give a primary radical such as the reactant in Scheme 1.
Hydrogen-donating solvents were used to terminate the reac-
tion, and products were identified using gas−liquid chromatog-
raphy. The rearrangement was found to occur rapidly, as only
trace amounts of products expected from the initial primary
radical were observed. A significant driving force was apparent
for the forward reaction if the R and R substituents stabi-
1
1−13
biofuels.
However, the fundamental chemistry related to
their autoignition is not well studied.
Scheme 1 depicts the 1,2-acyl group migration investigated in
this work. A primary carbon-centered radical, with the radical
1
2
lized the final product of the rearrangement reaction. The
authors deduced that the cyclopropoxy radical is not a discrete
intermediate of the reaction and rather suggested that a cyclic
Scheme 1
Received: June 12, 2013
Published: August 23, 2013
©
2013 American Chemical Society
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Article
Scheme 2
Scheme 3
transition state exists with radical character at both carbon atom
radical centers. Such a rearrangement was also later exploited in
the development of the Dowd−Beckwith ring expansion for
abstracts an H-atom from the ketone molecule RH to generate
an initial ketonyl (i.e., oxoalkyl) radical R. The radical R can
23
then either undergo unimolecular reactions or add O in a
2
6
,16,17
5,24
synthesis of cyclic ketones.
Calculations of the mechanism
barrierless or near barrierless reaction
to form a peroxy
of this rearrangement support a concerted rearrangement via a
radical RO , which then reacts further to yield products.
2
1
8,19
cyclic transition-state structure.
Moreover, kinetics experi-
•
2
0−22
ments
show that the rearrangement is rapid, with A-factors
Cl + R−H → R + HCl
(1)
2)
in excess of 101
0
s
−1
and activation energies (in solution) on
the order of 10 kcal mol or less. Calculations for the rearrange-
ments predict a relatively small change in the Gibbs free
activation energy upon solution (ΔΔG), suggesting rapid gas-
phase 1,2-acyl group shifts as well. However, some calcula-
−1
R + O ⇌ RO → Products
(
2
2
Reaction of Cl· with MTbuK (see Scheme 3) yields two
distinct initial radicals: a primary radical RTbu (2,2-dimethyl-3-
oxobutyl) resulting from abstraction of an H-atom from the
tert-butyl group and a primary radical RMe (3,3-dimethyl-2-
oxobutyl) resulting from H-abstraction from the methyl group
bound to C . R can rearrange via an effective 1,2 acyl group
19
1
8
tions of acyl-shift rearrangement show substantially higher
−
1
barriers, in excess of 20 kcal mol , large enough to limit the role
of such isomerizations in low-temperature gas-phase oxidation.
In this work, we demonstrate that such rearrangements are
indeed facile in the gas phase and that they play an important
role in the gas-phase low-temperature oxidation of ketones,
specifically methyl-tert-butyl ketone (3,3-dimethyl-2-butanone,
MTbuK) and di-tert-butyl ketone (2,2,4,4-tetramethylpentan-3-
one, DTbuK). Using partially deuterated diethyl ketones
α
Tbu
shift as shown in Scheme 1, which would yield a more stable
tertiary radical RR (2-methyl-4-oxopent-2-yl). The radical
abbreviations introduced in Scheme 3 will be used throughout
the text.
25
Kaiser et al. report rate constants for the reaction of Cl·
−11
−1
3 −1
with butanone of 2.3 × 10 exp(−1.8 kcal mol /RT) cm s to
(
pentan-3-one, DEK), we show that this rearrangement is
−11
−1
form HCl + 2-oxobutyl and 1.9 × 10 exp(−0.47 kcal mol /RT)
also active in DEK, even without any thermodynamic driving
force. The ketones studied are shown in Scheme 2. These com-
pounds serve as prototypes to establish chemical oxidation
mechanisms important for families of larger ketones. CBS-QB3
quantum chemical calculations of barrier heights for the 1,2-acyl
group migration support the mechanism. In addition, stationary
points on the potential energy surfaces for the reactions of the
3 −1
cm s to form HCl + 3-oxobutyl. These rate constants allow an
estimate for the ratio of RTbu to RMe (Scheme 3) expected in
this work. The additional methyl groups in MTbuK are not
expected to have major impact on the per-hydrogen rates of H-
abstraction from the 1 or 4 positions. Using the above rate
constants and normalizing to account for the number of
chemically equivalent hydrogen atoms, the ratio of RTbu/RMe is
predicted to be 8.4 at 550 K and 6.9 at 650 K.
MTbuK radicals with O are calculated to elucidate product
2
formation channels relevant to the overall gas-phase oxidation.
Low-temperature oxidation of these ketones was investigated
by using pulsed photolytic chlorine atom initiation at 8 Torr
and 550 and 650 K in the presence of a large excess of O2.
Reactions 1 and 2 show the general Cl-initiated oxidation
reactions. A Cl· atom produced by 351 nm photolysis of Cl2
Among the important typical unimolecular reactions of RO2
is the elimination of HO , which yields an unsaturated, closed-
2
5
,26−29
shell hydrocarbon byproduct.
An example of such a
reaction is shown in reaction 3 for the addition of O to the
2
n-propyl radical followed by the elimination of HO to yield
2
30
propene. The HO elimination requires that at least one of
2
1
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3
. RESULTS AND DISCUSSION
.1. HO -Elimination Channels at 550 K. 3.1.1. MTbuK.
3
2
Figure 1a shows the product mass spectrum for the Cl-initiated
the carbon atoms in the β position to the peroxy group is also
bound to at least one H-atom.
Because neither the HO2 radical nor the unsaturated
byproduct is very reactive in low-temperature autoignition
schemes, channels such as reaction 3 are effectively ‘chain-
5
terminating’ for autoignition. In the case of MTbuK and
DTbuK, every carbon atom in the alkyl branches is either
quaternary or associated with a terminating methyl group.
Therefore, in the absence of radical rearrangement reactions,
no HO -elimination pathways analogous to reaction 3 are
2
available for oxidation of these ketones. If, however, the
radical rearrangement reaction of Scheme 1 is competitive
with O addition to the initial radical, distinct coproducts following
2
HO elimination would be expected from oxidation of R .
2
R
2. EXPERIMENT
The fundamental oxidation chemistry of the ketones was studied using
multiplexed photoionization mass spectrometry (MPIMS) with
tunable vacuum ultraviolet (VUV) synchrotron ionizing radiation at
the Advanced Light Source (ALS) of Lawrence Berkeley National
Laboratory. The experiment has been described in detail previ-
Figure 1. Difference mass spectra of Cl-initiated oxidation of MTbuK
at (a) 550 and (b) 650 K from integrating the ion signal for the 20 ms
time frame immediately following photolysis and over ionizing photon
energies from 8.0 to 10.5 eV. Averaged background signal before
photolysis has been subtracted, and negative signal arising from
consumption of MTbuK is omitted for clarity.
3
1,32
ously,
and only a brief synopsis will be given here. The apparatus
consists of a heated 1.05 cm i.d. quartz flow tube with a ∼650 μm
aperture in the sidewall through which gas escapes. The effluent gas is
collimated into a molecular beam by passing through a 1.5 mm
diameter skimmer into a differentially pumped ionization chamber.
Quasi-continuous, tunable ionizing VUV radiation provided by the
ALS synchrotron intersects the molecular beam orthogonally, and the
ions that are formed are collimated and accelerated into an orthogonal
acceleration linear time-of-flight mass spectrometer. Products are
detected as a function of mass, time, and photon energy yielding a
oxidation of MTbuK (C H O) at 550 K and 8 Torr in the
6
12
16
−3
presence of 2.5 × 10 cm (∼18 mol %) O . The negative
2
signal arising from reactive removal of MTbuK (m/z = 100)
and its fragment ions (m/z = 86, m/z = 82 and m/z = 57) is
not shown in this spectrum. At 550 K, the dominant product
peaks observed are at m/z = 98 and 83. The m/z = 98
3
-dimensional data set that can be sliced and integrated to obtain
32
photoionization spectra and temporal profiles for individual species.
(C H10O) peak corresponds to the stable coproducts of HO
6
2
Comparison of the photoionization spectrum of a product mass
channel with the calibration spectra of pure compounds at a particular
mass allows the identification and quantification of products at an
loss from RO , whereas m/z = 83 is assigned as a fragment ion
2
that arises from dissociative ionization of C H O neutrals.
6
10
Reaction 4 shows the 1,2-acyl group migration of RTbu followed
3
1,32
isomer-resolved level.
The ketone is introduced either via helium flow through a bubbler
held at 20 °C or via a prepared mixture of the sample in He. Ketone
1
3
−3
densities of (2−7) × 10 cm are used. Photolysis of Cl by a pulsed
2
excimer laser operating at 351 nm and 4 Hz yields an initial Cl atom
concentration between 2 × 10 and 9 × 10 cm . The ratio of
12
12
−3
[
Cl] /[RH] is maintained at ∼0.1. The reaction proceeds in a large
0
0
16
−3
excess of O (2.5 × 10 cm ). The averaged prephotolysis signal is
2
subtracted, resulting in difference mass spectra that show positive
signals for products generated by the photolytically initiated reactions.
Signals are normalized to the ionizing photon flux. Calibrated mass
flow controllers are used to introduce reactants and carrier gas into the
flow tube. The pressure in the tube is maintained at 8 Torr by
feedback control of a butterfly valve in the exhaust line.
MTbuK, DTbuK, and diisopropyl ketone were obtained commer-
cially at stated purities of 98%. The nondeuterated diethyl ketone was
obtained commercially at a stated purity of >99%. The d - and
4
d6-diethyl ketone isotopologs were obtained commercially and were
assayed at 99.8% and 99.6% chemical purity, respectively. Mesityl
oxide was obtained commercially at a purity of >95%. The major
impurity is the isomesityl oxide isomer. A modified literature
3
3,34
by O2 addition to RR and the expected products from
procedure
oxide. Details of the synthesis are given in the Supporting Information.
,5-dimethylcyclopentanone and 2,2,5,5-tetramethylcyclopentanone
was followed to synthesize a pure sample of isomesityl
subsequent HO elimination, mesityl oxide (4-methyl-pent-3-
2
en-2-one) and isomesityl oxide (4-methylpent-4-en-2-one),
along with their calculated adiabatic ionization energies (AIEs)
2
were obtained commercially at stated purities of 98%. All samples
35,36
were freeze−pump−thawed before use.
at the CBS-QB3 level.
1
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Figure 2a compares the photoionization spectra at 550 K
associated with the m/z = 98 and 83 peaks with the
photoionization spectra of the parent ions of mesityl oxide
and isomesityl oxide at m/z = 98 and their respective m/z = 83
fragment ions. The mesityl oxide spectra match the
corresponding product spectra in MTbuK oxidation at 550 K
nearly exactly. Similarly, one can see the isomesityl oxide and
fragment ion curves poorly match the 550 K product spectra.
An upper bound of 20% for the isomesityl oxide contribution
at 550 K is estimated from a least-squares fitting procedure
in which the mesityl oxide and isomesityl oxide absolute
photoionization spectra are taken as basis functions for the
37
m/z = 98 product photoionization spectrum. These results, in
combination with the product branching ratios discussed below,
establish HO elimination as the dominant channel in MTbuK
2
oxidation at 550 K. Therefore the gas-phase 1,2-acyl group
migration is fast relative to O addition (which occurs with a
2
5
−1
pseudo-first order rate coefficient on the order of 10 s at the
conditions of this work). Figure 1b shows the product mass
spectrum for MTbuK oxidation at 650 K. At this temperature,
β-scission channels of the initial radicals become important
relative to oxidation channels. This change is reflected by the
growth of the isobutene peak (m/z = 56). Other products
observed in MTbuK oxidation will be discussed below.
Figure 3 shows stationary points on the potential energy surface
calculated for O addition to the unrearranged (RTbu) and rear-
2
ranged (R ) radicals from MTbuK. The calculations were perfor-
R
38
35,36
med with Gaussian 09 using the CBS-QB3 composite method.
The barrier for the 1,2-acyl group migration (R → R ) is
Tbu
R
Figure 2. Comparison of the photoionization spectra for the m/z = 98
peak observed in Cl-initiated oxidation of MTbuK at (a) 550 and (b)
−1
calculated to be 9.1 kcal mol . This value is far smaller than the
−1
isomerization barrier (∼20 kcal mol ) calculated with MP2
650 K with the mesityl oxide and isomesityl oxide calibration spectra.
or MP3 methods for rearrangement of the unsubstituted
The indicated fragment ion spectra resulting from dissociative
photoionization of mesityl oxide and isomesityl oxide are also
shown. The mesityl oxide and isomesityl oxide calibration spectra
were taken at 550 K.
18
(2-oxocyclopentyl)methyl radical but is similar to the barrier
−
1
19
(9.5 kcal mol ) calculated in a Dowd−Beckwith expansion of
the methyl 1-methyl-2-oxocyclopentane-1-carboxylate. A single-point
Figure 3. Stationary points on the potential energy surface for reaction of O with the MTbuK radicals, RTbu and R . Energies at 0 K are calculated at
2
R
the CBS-QB3 level. Several β-scission channels are also possible but for clarity are not included.
1
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Figure 4. Left: Geometry at the saddle point associated with the 1,2-acyl group migration reaction for the initial RTBu radical of MTbuK (2,2-
dimethyl-3-oxobutyl) calculated at the B3LYP/6-311G(d,p) level of theory. Middle: HOMO calculated at the saddle point (isovalue = 0.08). Right:
Spin density calculated at the saddle point (isovalue = 0.01) resulting from the same calculation.
energy calculation at the CCSD/6-31G(d) level was carried out
to investigate the extent of multireference character at the (RTbu
R ) saddle point. A value of 0.0197 was obtained for the T1
are consistent with the stabilization of the cyclic structure
expected from substituents at the β carbon.
→
3.1.2. DTbuK. Observation of an analogous 1,2-acyl group
migration would be expected in the Cl-initiated oxidation of
DTbuK. In fact, this rearrangement was studied by Ingold and
R
39
diagnostic formulated by Lee et al., within the range in which
the use of single-reference methods is appropriate. The
geometry at this saddle point is given in Table S1. The
resulting R radical is calculated to be more stable than R by
2
1
co-workers, who measured a rate coefficient in CCl solution
4
of 8.7 × 1010 exp(−7.77 kcal mol /RT) s . Even if the
activation energy in the gas phase is several kcal mol larger,
−1
−1
R
Tbu
−1
−1
3
.8 kcal mol (with similar entropy), sufficient to shift the
equilibrium to favor R by >30:1 at 550 K. After rearrangement
the rearrangement should compete with O addition under the
R
2
and O2 addition, the lowest-energy unimolecular reaction
pathway for the resulting R O is HO elimination to form
present conditions. The 550 K product mass spectrum for
Cl-initiated oxidation of DTbuK (C H O, m/z = 142) indeed
R
2
2
9
18
mesityl oxide, in accord with the experimental observations.
displays a strong signal from the product of HO elimination at
2
Both the barrier and product energies are calculated to be
m/z = 140 (C H O) as well as a corresponding fragment ion
9
16
3 kcal mol− higher for isomesityl oxide formation. These
1
∼
at m/z = 83, analogous to that from mesityl oxide ionization
(see reactions S2 and S3). The respective photoioniza-
tion spectra are shown in Figure 5a along with the time traces in
HO -elimination channels are shown in bold. Other channels in
2
Figure 3 are discussed below.
The geometry of the saddle point for the 1,2-acyl group
migration is shown in Figure 4, along with the orbital diagram
for the highest (singly) occupied molecular orbital (HOMO)
and the spin density plot (the molecular orientation in all
diagrams is equivalent). The HOMO at the saddle point
contains significant bonding character between both the α and
β carbons and the α and γ carbons. Similarly, these regions
contain a high degree of spin density, suggesting the unpaired
electron resides both at the initial and final radical sites. Efforts
to locate a distinct cyclopropoxy radical intermediate failed,
tending to confirm that a discrete cyclic intermediate is not
involved in the reaction when stabilizing substituents are
present at C and that the rearrangement rather occurs over a
β
1
5
single barrier, as suggested by Karl et al., and noted in other
1
8
calculations of similar rearrangements. Because the config-
uration at the saddle point involves radical character at both the
β and γ carbons, a larger degree of alkyl substitution at the
initial β site is indeed expected to facilitate isomerization.
The CBS-QB3 calculations for the simplest case of this
rearrangement, the reversible ring-opening of the cyclopropoxy
radical, provide a useful comparison. The distribution of elec-
tronic spin density at the saddle point for cyclopropoxy ring-
opening is completely analogous to that shown in Figure 4.
Ring closure of 3-oxopropyl to cyclopropoxy has a barrier of
Figure 5. (a) Photoionization spectra for the m/z = 140 peak observed
in Cl-initiated oxidation of DTbuK at 550 K and for a 2,2,5,5-tetra-
methylcyclopentanone standard. (b) Time traces of the m/z = 140 product
of HO -elimination and its m/z = 83 fragment ion in DTbuK oxidation.
2
−1
1
1.2 kcal mol . The electronic energy of the cyclopropoxy
−1
radical is <0.25 kcal mol below the electronic energy of the
saddle point leading to ring-opening. When including zero1-
point corrections, the energy of cyclopropoxy is 0.5 kcal mol−
above the saddle point energy, which implies the cyclopropoxy
radical is essentially unbound. The RTbu → RR reaction
Figure 5b. Scheme 4 shows the expected products of HO2
elimination for DTbuK oxidation after the 1,2-acyl group
migration in analogy to MTbuK (reaction 4). These products
were not commercially available nor did we attempt to
synthesize them. However, the calculated AIEs of both species
are in good agreement with the observed ionization onset of 8.7 eV
in the m/z = 140 product spectrum.
−1
associated with MTbuK has a lower barrier of 9.1 kcal mol ,
and no stable cyclic intermediate is found. These observations
1
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Scheme 4
An alternative pathway, as shown in reaction 5, involves a
ring closure upon ejection of HO via the peroxy radical formed
2
rent masses depending on whether HO elimination originates
2
from the original or rearranged radical. These pathways are
depicted in Scheme 5 for both molecules, with the rearranged
radical pathway in the right column.
by O addition to the initial DTbuK radical without acyl migration.
2
Chlorine atom reaction with undeuterated DEK is expected
to yield the primary DEK radical and the secondary DEK rad-
Such a reaction would yield 2,2,5,5-tetramethylcyclopentanone.
Similarly, a ring closure to form a three-membered ring could
also be possible but would be high in energy and therefore not
expected.
40
ical (3-oxopent-2-yl) in a ratio of approximately 1:1 at 550 K.
In the case of the initial secondary radical, HO /DO
2
2
elimination will only yield one isotopolog for d - and d -DEK
4
6
The measured photoionization spectrum of 2,2,5,5-tetra-
methylcyclopentanone (m/z = 140) is given in Figure 5a.
Though its ionization onset matches the observed onset of the
m/z = 140 product spectrum of DTbuK oxidation, the shapes
of the curves clearly disagree. However, it is possible that two
or more isomers may contribute to the m/z = 140 product
photoionization spectrum, and the onset of the 2,2,5,5-
tetramethylcyclopentanone photoionization spectrum does
not exclude it from consideration. The 1,2-acyl group migration
would also be expected in diisopropyl ketone and an analogous
reaction to eq 5 would yield 2,5-dimethylcyclopentanone.
Figure S1 shows the ionization onset of 2,5-dimethylcyclo-
pentanone occurs below the photon energy of the m/z = 112
oxidation, which, as shown in reactions 7a and 7b, will yield VEK
at the same value of m/z as would be observed for reaction of the
initial (nonrearranged) primary radical (Scheme 5).
product of HO elimination in diisopropyl ketone, providing
2
strong evidence that such ring-closing channels are unimportant.
Figure 6a shows the product mass spectra resulting from the
3
.1.3. DEK. Unlike MTbuK and DTbuK, the initial primary
5
50 K Cl-initiated oxidation of DEK and d - and d -DEK in the
4 6
3
-oxopentyl radical derived from H-abstraction at the methyl
range of the coproducts of HO /DO elimination. Figure 6b
2
2
group of DEK does not have a thermodynamic driving force
favoring the 1,2-acyl group migration, as the rearrangement of
this radical regenerates the same species (reaction 6a). The
compares the photoionization spectrum for VEK (C H C(O)-
2
5
CHCH ; m/z = 84) plotted next to m/z = 84 from DEK
2
oxidation, m/z = 87 and 88 from d -DEK, and m/z = 89 from
4
d6-DEK. These spectra are consistent with VEK (ionization
onset 9.4 eV) as the principal HO -elimination product. The
2
Figure 6b inset reveals a minor onset near 8.4 eV for m/z = 84
in DEK oxidation. This signal may be due to formation of a
small amount of 3-hydroxy-penta-1,3-diene (AIE = 8.24 eV).
The presence of this minor m/z = 84 product in the
photoionization spectrum likely causes the slight vertical shift
relative to the VEK standard for much of the spectrum. In
addition to producing signal at its parent mass (m/z = 84), the
VEK calibration spectrum also shows a small fragment ion
signal at m/z = 83 and by analogy explains at least a portion of
CBS-QB3 calculations on reaction 6a do show a stable cyclic
intermediate as well as two (symmetric) saddle points for ring1
closure and ring opening, with a barrier height of 11.2 kcal mol−
above the initial 3-oxopentyl radical. Selectively deuterating
either the β (d -DEK) or γ (d -DEK) sites yields a molecule that
can give distinct products depending on rearrangement (see
reactions 6b and 6c).
4
6
the m/z = 86 and 88 signal in d - and d -DEK oxidation,
4 6
respectively.
The signal at m/z = 88 in d -DEK oxidation is too large to
4
13
arise solely from the C isotopolog of the m/z = 87 signal from
d3-VEK (Figure 6a) and is in very good agreement with the
authentic VEK photoionization spectrum (Figure 6b). This
excess signal is attributed to d -VEK from the rearrangement
4
channel of Scheme 5. The relative amount of VEK resulting
from rearrangement (observed at m/z = 88 for both initial
isotopologs) is clearly less in d -DEK than in d -DEK (Figure 6a),
From reactions 6b and 6c, it can be seen that HO2
elimination from the primary radical of d - and d -DEK
would yield vinyl ethyl ketone (1-penten-3-one, VEK) at diffe-
4
6
6
4
because of kinetic deuterium isotope effects in the initial radical
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Scheme 5
formation and in the HO elimination. In d -DEK oxidation a
channels, including β-scission reactions, which yield small
molecule products, and the ‘chain-propagating’ formation of
2
6
preference of H- over D-atom abstraction will result in more
initial production of the secondary radical, a species expected to
only produce d -VEK (m/z = 89) via the associated HO -
5
,24
OH + cyclic ethers
(discussed below) are minor (but
nonzero) at this temperature.
5
2
elimination channel. Furthermore, the RO resulting from the
As discussed above, RMe is expected to be a minor product of
H-abstraction of MTbuK by Cl· (Scheme 3). Figure 7 shows
the potential energy surface calculated for important channels
anticipated for O addition to R . This radical shows vinoxy-
2
41
rearranged primary radical, allowing other channels such as
5
cyclic ether formation to become more competitive. In
contrast, in d -DEK oxidation these kinetic isotope effects
4
2
Me
will favor VEK from the rearranged radical.
type resonance stabilization. As a result, the associated RO well
2
−1
3.2. Other Product Channels in MTbuK Oxidation. To
is shallow (22 kcal mol ) relative to the entrance channel, so
that the barriers to bimolecular products lie above the R + O2
reactants, and the equilibrium of the RMe + O ↔ R O asso-
further support the prominence of HO elimination from the
2
reaction of the rearranged R radical with O in MTBuK, we
R
2
2
Me
2
compare the yields of mesityl and isomesityl oxide to the yields
of the other stable products observed. Table 1 gives branching
ratios determined for the Cl-initiated oxidation experiments of
this work at 550 and 650 K. The mesityl oxide product of HO2
elimination (following O addition to R ) dominates at 550 K,
ciation reaction favors reactants even at a relatively low tem-
5
perature. In contrast, O addition to either R or R results
2
Tbu
R
−1
in a much deeper RO well (35−40 kcal mol ). With no likely
2
rearrangement reactions and no neighboring C−H bonds, HO2
elimination originating from the oxidation channels of R is
2
R
Me
accounting for ∼70% of the product distribution. Other
expected to be unimportant.
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Table 1. Branching Ratios Determined for the Products of
a
Cl-Initiated Oxidation of MTbuK at 550 and 650 K
standard
cross
section
(Mb)
energy
(eV)
branching
e
m/z
assignment
ketene
ratio
b
5
50 K
42
9.95
9.375
10.675
9.5
18.4
3.93
0.02
5
5
9
6
8
8
isobutene
acetone
0.11
0.12
0.71
0.03
14.61
8.39
mesityl oxide
c
d
1
14 cyclic ether
10.35
18
6
50 K
42
ketene
9.95
9.375
10.675
9.5
18.4
3.93
0.04
0.74
5
6
8
8
isobutene
acetone
5
9
14.61
8.39
<0.01
0.22
isomesityl oxide
(
0.75) + mesityl
f
oxide (0.25)
d
1
14 cyclic ether
10.35
18
<0.01
a
Values result from integrating product signals over the 20 ms
immediately following photolysis. Energies listed are the point at
which the signal was compared with a standard. These energies vary to
ensure fragment ions of the same m/z value are not incorporated into
the measurement for a particular species. The photoionization cross
sections from calibration spectra of the pure compounds (“standard
b
cross sections”) given are at the corresponding energies. Due to
c
uncertainty in the assignment, value is an upper bound. Expected
products are 4,4-dimethyl-tetrahydrofuran-3(2H)-one and 5,5-dimeth-
d
yl-tetrahydrofuran-3(2H)-one. Cross section is estimated. Error is
e
expected to be ±50%. Includes empirical correction for time-of-flight
f
mass discrimination factor. Branching ratio of 3:1 for isomesityl oxide:
mesityl oxide is taken as an estimate based on uncertainty limits given
from a fit of the photoionization spectra.
Several other significant peaks at 550 K are observed in the
mass spectrum of Figure 1a. Those at m/z = 99 and 43 display
rapid temporal onsets and do not decay precipitously as would
be expected of a reactive radical. This kinetic behavior is in
contrast to that of the stable products, which display more
gradual temporal onsets. Alkylperoxy radicals do not typically
have stable parent cations and are thus usually only detected as
Figure 6. (a) Bottom: Product mass spectrum of Cl-initiated oxidation
of DEK at 550 K normalized to photocurrent resulting from
integrating the ion signal for the 45 ms time frame immediately
following photolysis and over ionizing photon energies from 8.3 to
1
1.0 eV. Middle: Same for d -DEK integrated over 9.0−11.0 eV. Top:
4
Same for d -DEK integrated over 9.0−11.0 eV. (b) Photoionization
6
spectra for the indicated peaks observed in Cl-initiated oxidation of
DEK and deuterated DEKs at 550 K along with that for a VEK
standard.
42
13
fragment ions. The m/z = 99 peak in excess of the
C
isotopologs from mesityl and isomesityl oxide might arise from
dissociative ionization of one or more of the alkylperoxy
+
+
Aside from HO elimination, a typical RO species can undergo
radicals formed ([RO ] → [R] + O ), and the m/z = 43 peak
2
2
2 2
reversible isomerization to a hydroperoxyalkyl radical (QOOH)
via internal H-abstraction. These QOOH radicals can eject an OH
radical upon ring closure to form cyclic ethers. Figures 3 and 7
is also likely a fragment ion. The β-scission channels of RTbu
,
R , R , and QOOH species can yield several small molecule
R
Me
5
products, including acetone (m/z = 58), isobutene (m/z = 56),
show that QOOH → OH + dimethyldihydrofuran-3(2H)-one
ketene (m/z = 42), CO, and CH . A more detailed discussion,
including figures, for these bond homolysis channels is given in
the Supporting Information.
3
(
m/z = 114) channels are available for the reaction of O + R ,
2
Me
RTbu, and R . The feature at m/z = 114 in Figure 1a might arise
R
from these products, i.e., 4,4-dimethyl-dihydrofuran-3(2H)-one
and 5,5-dimethyldihydrofuran-3(2H)-one. Several three- and
four-membered cyclic ether products are also possible but are
less likely because they must follow higher energy pathways; we
therefore did not include them in Figures 3 and 7. Figure 8
shows the m/z = 114 photoionization spectrum from MTbuK
oxidation at 550 K. The adiabatic ionization energies calculated
for the expected dimethyldihydrofuran-3(2H)-one products agree
well with the observed ionization onset. Formation of 5,5-
3.3. MTbuK Oxidation at 650 K. At 650 K, several
changes from the 550 K case are observed in the oxidation
spectra of MTbuK (Figure 1; Table 1). First, the peak at m/z =
56 (isobutene) becomes the dominant feature in the mass
spectrum reflecting the greater degree of β-scission at higher
temperature. Second, almost no acetone (m/z = 58) or cyclic
ether product (m/z = 114) is seen. Finally, the shape of the
photoionization spectrum of the m/z = 98 product attributed to
HO -elimination changes, coming into better agreement with
2
dimethyldihydrofuran-3(2H)-one from RMe + O requires a 1,2-
that of isomesityl oxide (Figure 2b). The assignment of
isomesityl oxide is corroborated further by the agreement of the
m/z = 43 and 83 photoionization spectra with the fragment
ions of isomesityl oxide (see Supporting Information for further
discussion). The emergence of isomesityl oxide at 650 K is
2
acyl group migration in the resulting QOOH (Figure 7). This
primary QOOH → tertiary QOOH isomerization occurs with a
−1
calculated barrier of 6.1 kcal mol relative to the primary QOOH
−1
radical, 3.0 kcal mol less than that calculated for RTbu ↔ RR.
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Journal of the American Chemical Society
Article
Figure 7. Stationary points on the potential energy surface for the reaction of the primary MTbuK radical RMe with O . Energies at 0 K are at the
2
CBS-QB3 level. Several β-scission channels are also possible but for clarity are not included.
4
. CONCLUSIONS
The 1,2-acyl migration in 3-oxoalkyl radicals, known to be facile
in solution, is also rapid in the gas phase and will play an
important role in developing accurate mechanisms and kinetics
for the low-temperature oxidation of branched ketones. As
demonstrated by a study of deuterated diethyl ketones, the
rearrangement may take place even without a thermodynamic
driving force provided by β substituents. Quantum chemical
characterization of the saddle point for the 1,2-acyl group shift
15
supports the reasoning of Karl et al. and other calculations on
1
8,19
Dowd−Beckwith rearrangements
that the configuration at
the saddle point shows spin density at both the β and γ carbons
relative to the O atom of the carbonyl group. No discrete
cyclopropoxy radical intermediate is located when substituents
at C provide a thermodynamic driving force for the reaction.
β
If the product of a 1,2-acyl rearrangement is more stable than
the initial radical, such rearrangements are expected to be
important even for parent molecules such as isopropyl ketones, in
Figure 8. Photoionization spectrum of the m/z = 114 peak observed in
Cl-initiated oxidation of MTbuK at 550 K. The inset shows ionization
onset and calculated (CBS-QB3) AIEs for 4,4-dimethyl-tetrahydrofuran-
which bonding motifs allow traditional HO -elimination pathways
2
3(2H)-one and 5,5-dimethyl-tetrahydrofuran-3(2H)-one.
without radical rearrangement. Characterization of fundamental
oxidation chemistry is a key step toward predicting ignition
behavior and identifying particular compounds as potentially
useful liquid fuels. Future experimental and modeling efforts to
describe the pyrolysis, oxidation, combustion, and ignition of
branched ketones should address the gas phase 1,2-acyl group
migration reactions described here. A detailed theoretical kinetics
somewhat surprising considering that at 550 K the photo-
ionization spectra are consistent with >80% of the HO2-
elimination channel yielding the conjugated mesityl oxide
isomer. At higher temperature, barriers for RO → R (-H) +
2
HO are more readily overcome for production of both mesityl
2
and isomesityl oxide. It is possible that at 650 K the entropic
favorability (3:1 ratio of primary to secondary H-atoms) of the
isomesityl oxide channel dominates energetic effects that would
favor mesityl oxide.
study both on the 1,2-acyl rearrangement and the O addition
2
reactions to the original and rearranged ketone radicals would be
valuable, giving accurate gas-phase rate coefficients for these
competing processes to be used in combustion models.
1
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Journal of the American Chemical Society
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(
18) Ryu, I.; Fukushima, H.; Okuda, T.; Matsu, K.; Kambe, N.;
ASSOCIATED CONTENT
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oxide, 2,5-dimethylcyclopentanone, and 2,2,5,5-tetramethylcy-
clopentanone, absolute energies and Gaussian archive entries
for all stationary points from Figures 3 and 7, Tables S1−S6,
Figure S1−S5. This material is available free of charge via the
Internet at http://pubs.acs.org.
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Corresponding Authors
amschee@sandia.gov
cataatj@sandia.gov
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The authors thank Prof. Barry Carpenter, Dr. Judit Zad
■
(
́
or, and
Phys. Chem. A 2003, 107, 4415.
Dr. John D. Savee for useful discussions and Mr. Howard
Johnsen for technical support of these experiments. We thank
an anonymous reviewer for alerting us to the important work
on the Dowd−Beckwith ring expansion. This work is supported
by the Laboratory Directed Research and Development
program at Sandia National Laboratories, a multiprogram
laboratory operated by Sandia Corporation, a Lockheed Martin
Company, for the United States Department of Energy
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1
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(
USDOE)’s National Nuclear Security Administration under
contract DEAC04-94AL85000. The Advanced Light Source is
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Energy Sciences, of the USDOE, under contract DE-AC02-
5CH11231 between Lawrence Berkeley National Laboratory
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