FTIR study of thermally induced transformations of trimethylsilylmethyl acetate in the temperature range 623-813 K

Article abstract of DOI:10.1016/S0022-328X(99)00762-7

The chemistry of trimethylsilylmethyl acetate in the gas phase in the temperature range 623-813 K has been investigated under static conditions using Fourier transform IR spectroscopy. Little change occurs at temperatures lower than 623 K, at which temperature a thermally induced isomerization to form ethyldimethylsilyl acetate occurs. Some ethanoic acid is also produced at this temperature. The ethyldimethylsilyl acetate produced undergoes thermolysis at temperatures > 723 K giving methane, ethene, carbon monoxide, carbon dioxide, ethanoic acid, 1,3-diethyl-1,1,3,3-tetramethyldisiloxane, cyclohexamethyltrisiloxane, and cyclooctamethyltetrasiloxane as products. Loss of ethyldimethylsilyl acetate is first order over the whole temperature range, and first-order rate constants vary from 4.87 × 10-⁵ s-¹ at 723 K to 33.5 × 10-⁵ s-¹ at 813 K, respectively, leading to an activation energy, E_a,, of 110(4) kJ mol-¹. An intramolecular five-centre process is proposed for the isomerization reaction. The thermolysis is interpreted in terms of principally radical reactions involving initial homolytic dissociation of the EtMe₂SiO-C(O)CH₃ bond.

Full text of DOI:10.1016/S0022-328X(99)00762-7

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 599 (2000) 185–194
FTIR study of thermally induced transformations of
trimethylsilylmethyl acetate in the temperature range 623–813 K
Ian K. Ball, Philip G. Harrison *, Ashley Torr
School of Chemistry, Uni6ersity of Nottingham, Uni6ersity Park, Nottingham NG7 2RD, UK
Received 7 October 1999; received in revised form 9 December 1999
Abstract
The chemistry of trimethylsilylmethyl acetate in the gas phase in the temperature range 623–813 K has been investigated under
static conditions using Fourier transform IR spectroscopy. Little change occurs at temperatures lower than 623 K, at which
temperature a thermally induced isomerization to form ethyldimethylsilyl acetate occurs. Some ethanoic acid is also produced at
this temperature. The ethyldimethylsilyl acetate produced undergoes thermolysis at temperatures \723 K giving methane, ethene,
carbon monoxide, carbon dioxide, ethanoic acid, 1,3-diethyl-1,1,3,3-tetramethyldisiloxane, cyclohexamethyltrisiloxane, and cyclo-
octamethyltetrasiloxane as products. Loss of ethyldimethylsilyl acetate is first order over the whole temperature range, and
first-order rate constants vary from 4.87×10−5 s−1 at 723 K to 33.5×10−5 s−1 at 813 K, respectively, leading to an activation
energy, Ea, of 110(4) kJ mol−1. An intramolecular five-centre process is proposed for the isomerization reaction. The thermolysis
is interpreted in terms of principally radical reactions involving initial homolytic dissociation of the EtMe2SiOꢀC(O)CH3 bond.
© 2000 Elsevier Science S.A. All rights reserved.
Keywords: Trimethylsilylacetate; Thermal decomposition; FTIR
a-effect [6,7]. No chemistry is known either in the gas
phase or at high temperature for this reagent, and
hence we report here the results of a detailed investiga-
tion of the thermolysis of neat, gaseous trimethylsilyl-
methyl acetate.
1. Introduction
The increasing use of high- temperature gas-phase
conditions for new material formation has led to a need
for information concerning the behaviour of organosili-
con compounds under these conditions. We have previ-
ously [1] shown that trimethylsilyl acetate decomposes
by a radical process in the temperature range 723–818
K giving as principal gaseous products methane, hexa-
methyldisiloxane, cyclohexamethyltrisiloxane, carbon
dioxide, and carbon monoxide, the relative amounts of
which vary with the temperature. Small amounts of
water and ethanoic acid are also formed. Trimethyl-
silylmethyl acetate is another potential precursor for the
formation of siloxane films by MOCVD, although in
this molecule the silicon is insulated from the oxygen of
the acetate group by a methylene group. Previous stud-
ies of this molecule have all been in the solution phase,
and have concentrated on reduction/substitution reac-
tions [2–5] of the acetyl group and the nature of the
2. Experimental
The general experimental technique has been de-
scribed in detail previously [8]. Prior to any qualitative
or quantitative studies, the reaction cell was condi-
tioned by allowing trimethylsilylmethyl acetate
(Aldrich) (ca. 5 Torr) to decompose at 743 K several
times.
3. Results
3.1. IR spectrum and Beer–Lambert characteristics of
trimethylsilylmethyl acetate at ambient temperature
The gas-phase IR spectrum of trimethylsilylmethyl
acetate at a pressure of 10.8 Torr is shown in Fig. 1,
* Corresponding author. Fax: +44-115-9513563.
E-mail address: p.g.harrison@nottingham.ac.uk (P.G. Harrison)
0022-328X/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(99)00762-7

186
I.K. Ball et al. / Journal of Organometallic Chemistry 599 (2000) 185–194
Fig. 1. Ambient-temperature infrared spectrum of trimethylsilymethyl acetate in the gas phase (10.8 Torr).
with band positions and assignments [7] summarised in
Table 1. Quantitative measurements of the abundance
of trimethylsilylmethyl acetate were made using the in-
tense carbonyl stretching band at 1769 cm−1. A Beer–
Lambert plot of the integrated peak envelope area of the
w(CꢁO) band in the range 1702–1826 cm−1 versus gas-
phase abundance was constructed in the pressure range
0.0–6.1 Torr (Fig. 2), all measurements being made at
ambient temperature. The least-squares fit (correlation
coefficient=0.965) of the linear plot was of the form:
position has a maximum at 1740 cm−1 (cf. 1769 cm−1
w(CꢁO) at ambient temperature). Also present in this re-
gion is a new band centred at 1788 cm−1 (due to
ethanoic acid w(CꢁO) [9]). After 600 s at 623 K, the reac-
tion cell was cooled to ambient temperature when the
characteristic bands of trimethylsilylmethyl acetate at
1425, 1373, 1292, 1259, 1226, 1068, 1034, 977, 863, 848,
795, 765, 703 and 614 cm−1 reduce in intensity and
eventually disappear. After 600 s at 623 K and cooling
to ambient temperature, the spectrum exhibits new
bands at 1426, 1372, 1290, 1260, 1182, 1069, 1012, 964,
A= −0.8867+1.2614×105 C
935, 888, 838, 829, and 802 cm−1
.
where A is the integrated peak envelope area for the
w(CꢁO) band (absorbance units cm−1) and C is the mo-
lar abundance (mol l−1). From the Beer–Lambert law
the gradient, ml, was determined to be 1.26×105 (ab-
sorbance units cm−1) mol−1 l, giving a value of 9.34
(0.33)×105 (absorbance units cm−1) mol−1 l m−1 for
the magnitude of the extinction coefficient, m.
Table 1
Infrared assignment data for trimethylsilyl methylacetate in the gas
phase at ambient temperature
Band position (cm−1
)
Assignment a
3052 vvw
2966 m
2916 w
2859 vvw
1769 vs
1425 w
1373 m
1292 ms
1259 s
1226 vvs
1068 w
1034 mw
977 vvw
863 vvs
848 vs
w(CꢀH)
w(CꢀH)
w(CꢀH)
w(CꢀH)
3.2. Analysis of the gas-phase reaction products
w(CꢁO)
la(CꢀH) (SiꢀCH3)
ls(CꢀH) (SiꢀCH3)
k(CH2)
ls(CH3)
was(OꢀC)OCH3
z(CH3)
w(ꢀCH2ꢀOꢀ)
z(CH3)
w(SiꢀCH2)
?
3.2.1. Effects of thermal treatment prior to
decomposition
As the temperature is increased the IR bands charac-
teristic of trimethylsilylmethyl acetate begin to broaden
and decrease in intensity. At 373 K, very little change
occurs in the IR. However, when a temperature of ca.
623 K is attained, these bands exhibit subtle but distinct
differences from the bands recorded at ambient temper-
ature, and are illustrated in the series of IR spectra
shown in Fig. 3. The carbonyl stretching band of
trimethylsilylmethyl acetate is observed to broaden from
1826 to 1702 cm−1 at ambient temperature to 1831–
1695 cm−1 when heated to 373 K. On heating to 623 K,
the carbonyl band shifts down wavenumber. The new
795 mw
765 w
703 w
?
was(SiꢀCH2ꢀO)
was(SiꢀCH3)
ws(SiꢀCH3)
614 vw
a By comparison with data from Refs. [6,7], and A.L. Smith,
Spectrochim. Acta 16 (1960) 87.

I.K. Ball et al. / Journal of Organometallic Chemistry 599 (2000) 185–194
187
new bands appear at 838 (w(SiꢀO)), 964 (w(CꢀC)), 1260
(w(CꢀCOꢀO)+ls(CH3)), and 1740cm−1 (w(CꢁO)). The
similarity of the IR spectra of the product of heating
trimethylsilylmethyl acetate and trimethylsilyl acetate is
illustrated in Table 2. Hence, both have similar struc-
tures with the former containing an additional car-
bonꢀcarbon bond (w(CꢀC) at 964 cm−1
)
[10].
Isomerisation is possible for molecules containing
SiꢀCH2ꢀO linkages [3], and it would appear that such a
process is occurring producing dimethylethylsilyl
acetate.
Traces of ethanoic acid are also observed at 623 K
(w(CꢁO) 1788 cm−1, w(CꢀO) 1182 cm−1) [11], and its
formation probably arises from homolytic cleavage of
the skeletal SiꢀCH2(CO)OMe bond, followed by hydro-
’
gen abstraction by the CH2(CO)OMe radical so
formed.
3.2.2. Thermolysis in the temperature range 723–813 K
The decomposition of neat trimethylsilylmethyl ace-
tate was investigated quantitatively in the temperature
range 723–813 K. A typical series of IR spectra for the
decomposition at 773 K is illustrated in Fig. 5. When
isothermal conditions are attained (773 K), the
trimethylsilylmethyl acetate has isomerised completely,
and it is the decomposition of dimethylethylsilyl acetate
(with characteristic IR bands at 1740 cm−1 w(CꢁO),
1372 cm−1 ls(CꢀH), 1260 cm−1 [skeletal CꢀCOꢀO+
ls(SiꢀO)], 1069 cm−1 w(CH3), 1012 cm−1 [skeletal
CꢀCOꢀO+z(CH3)], 964 cm−1 w(CꢀC), 935 cm−1
[skeletal CꢀCOꢀO+z(CH3)], 838 cm−1 w(SiꢀO)) which
is observed. Interpretation of the spectra shown in Fig.
5 demonstrates that the products of decomposition are
Fig. 2. Beer–Lambert plot of integrated area of the w(CꢁO) peak vs.
gas-phase molar abundance for trimethylsilylmethyl acetate.
It is interesting to note that the position of the new
acetate carbonyl band at 1740 cm−1 is almost identical
to that of trimethylsilyl acetate (1741 cm−1), suggesting
a structural similarity. This is further highlighted in Fig.
4, where the spectrum of the product of heating
trimethylsilylmethyl acetate to 623 K for 600 s is com-
pared with the ambient-temperature IR spectra of
trimethylsilyl acetate and trimethylsilymethyl acetate.
In particular, characteristic bands in the IR spectrum of
trimethylsilylmethyl acetate at 1034 (w(ꢀH2CꢀO), 1226
(was(OꢀC)OMe), and 1769cm−1 (w(CꢁO)) are lost, while
Fig. 3. Infrared spectra of trimethylsilylmethyl acetate in the gas phase: (a) at ambient temperature; (b) after heating to 375 K; (c) after heating
to 623 K; (d) after heating to 623 K for 600 s, and (e) as for (d), followed by cooling to ambient temperature.

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I.K. Ball et al. / Journal of Organometallic Chemistry 599 (2000) 185–194
Fig. 4. Comparison of the gas-phase infrared spectra of trimethylsilyl acetate at ambient temperature (a) and trimethylsilylmethyl acetate at
ambient temperature (c) and after heating to 623 K for 600 s, followed by cooling to ambient temperature (b).
Table 2
Comparison of the infrared spectra (1800–800 cm−1) of trimethylsilyl acetate (TSA) and the product obtained by the thermal rearrangement of
trimethylsilylmethyl acetate
Trimethylsilyl acetate (cm−1
)
Assignment
Rearranged product (cm−1
)
Tentative assignment
1741 s
1428 vw
1375 m
–
1245 vvs
–
1073 w
1019 mw
w(CꢁO)
la(CꢀH)
ls(CꢀH)
–
CꢀCOꢀO+ls(CH3)
–
z(CH3) (CꢀCH3)
CꢀCOꢀO+z(CH3)
–
CꢀCOꢀO+z(CH3)
–
w(SiꢀO)
–
–
1740 s
w(CꢁO)
la(CꢀH)
ls(CꢀH)
1426 vw
1372 mw
1290 m
1260 vs
1182 vw
1069 vw
1012 mw
964 w
935 w
888 mw
838 s
829 ms
802 m
a
?
CꢀCOꢀO+ls(CH3)
w(CꢀO) b
z(CH3) (CꢀCH3)
CꢀCOꢀO+z(CH3)
w(CꢀC) (SiCꢀC)
CꢀCOꢀO+l(CH3)
–
937 ms
–
858 vs
–
–
a
?
w(SiꢀO)
a
?
?
a
a Unassigned bands.
b Ethanoic acid.
methane (3019 cm−1 (w(CꢀH)), 1306 cm−1
(l(H3CꢀH)), ethanoic acid (3580 cm−1 (w(OꢀH), 1788
cm−1 w(CꢁO), 1182 cm−1 w(CꢀO)), ethene (949 cm−1
(CH2 wag)), carbon dioxide (2349 cm−1 (was(CO2), 667
cm−1 (l(CO2))), cyclohexamethyltrisiloxane (1025
cm−1 (was(SiꢀOꢀSi))) and carbon monoxide (2144 cm−1
(w(CO))). A band at 1073 cm−1 is also observed, prob-
ably due to the was(SiꢀOꢀSi) vibration of another silox-
ane product. A typical ambient-temperature ratio (after
thermolysis) of methane:ethene:carbon dioxide is ca.
8:5:3.
trisiloxane, and diethyltetramethyldisiloxane is obtained
from the mass spectrum (70 eV) of the gaseous product
mixture after thermolysis at 773 K (12 000 s, \75%
reaction), which exhibits principal peaks at m/e values
of 207, 175, 161, 147, 133, 119, 117, 89, 75, 73, 66, 61
and 59. Assignments and probable origins for these
fragments are shown in Table 3. The highest m/e
fragment occurs at 281 and can be attributed to
(Me7Si4O4)+, which is derived from cyclooctamethyl-
tetrasiloxane. The m/e fragment at 207 corresponds to
(Me5Si3O3)+ due to cyclohexamethyltrisiloxane. The
series of five m/e fragments with maxima at 175, 161,
147, 133 and 119 are attributed to (Me3Et2Si2O)+,
Corroboration of the formation of the siloxane spe-
cies cyclooctamethyltetrasiloxane, cyclohexamethyl-

I.K. Ball et al. / Journal of Organometallic Chemistry 599 (2000) 185–194
189
(Me4EtSi2O)+, (Me3Et(H)Si2O)+, (Me3Et(H)2Si2O)+,
and (Me3(H)2Si2O+, respectively, derived from di-
ethyltetramethyldisiloxane, (Me2EtSi)2O, which is most
probably responsible for the band observed at 1073
cm−1 in the IR.
Table 3
Summary of the mass spectrum fragmentation (70 eV) of the gaseous
products from the thermolysis of trimethylsilylmethyl acetate at 773
K for 12 000 s
m/e
% Base
Fragment a
Molecule of origin
Plots of the integrated peak envelope areas with time
showing the production of carbon monoxide and car-
bon dioxide as the thermolysis temperature increases are
281
207
175
161
147
133
119
89
1.3
7.8
(Me7Si4O4)+
(Me5Si3O3)+
(Me3Et2Si2O)+
(Me4EtSi2O)+
(Me3Et(H)Si2O)+
(Me3Et(H)2Si2O)+
(Me3(H)2Si2O+
(Me3SiO)+
(Me2SiO)4
(Me2SiO)3
(EtMe2Si)2O
(EtMe2Si)2O
(EtMe2Si)2O
(EtMe2Si)2O
(EtMe2Si)2O
–
10.7
80.9
26.4
100.0
24.2
12.8
39.5
21.0
75
73
(Me3SiH2)−
–
–
(Me3Si)+
a Based on 28Si.
Fig. 6. Plot of integrated area of the w(CꢁO) peak vs. time for carbon
monoxide production at 733 K (ꢀ), 753 K (ꢁ), 773 K ( ), 803 K
(X) and 813 K (").
illustrated in Figs. 6 and 7, respectively. The profiles
observed for each are quite different. At each tempera-
ture, the production of carbon monoxide initially in-
creases to a maximum value before declining, whereas
the production of carbon dioxide increases steadily with
time. At low thermolysis temperatures no carbon diox-
ide is observed until the thermolysis has been underway
for some considerable time. The maximum observed in
the profiles for the production of carbon monoxide is
very broad at low thermolysis temperatures, but steadily
sharpens as the temperature is increased. At the highest
temperature studied, the amount of carbon monoxide
rapidly decreases to zero. From these obser-vations we
conclude that it is carbon monoxide that is produced
initially in the reaction, and is subsequently converted to
carbon dioxide as the thermolysis processes.
Fig. 5. Time-resolved infrared spectra of the thermolysis of
trimethylsilylmethyl acetate at 773 K and (a) 0 s; (b) 2900 s; (c) 6700
s, and (d) 7200 s.

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I.K. Ball et al. / Journal of Organometallic Chemistry 599 (2000) 185–194
Fig. 7. Plot of integrated area of the w(CꢁO) peak vs. time for carbon
dioxide production at 733 K (ꢀ), 753 K (ꢁ), 773 K ( ), 803 K (X)
and 813 K (").
Fig. 9. First-order plots for the decomposition of trimethylsilylmethyl
acetate at 723 K (ꢂ), 733 K (ꢀ), 743 K (ꢃ), 753 K (ꢁ), 763 K (ꢄ),
773 K ( ), 783 K (+), 803 K (×), and 813 K ("). Solid lines
represent least-squares best fits.
3.3. Determination of rate data and acti6ation energy
of decomposition
The integrated peak envelope area of the carboyl
stretching vibration characteristic of ethyldimethylsilyl
acetate, derived by the isomerization of trimethylsilyl-
methyl acetate after molecular rearrangement, was par-
tially masked on the higher wavenumber side by the
carbonyl stretching band w(CꢁO) of ethanoic acid.
Hence, the integrated peak envelope area was calcu-
lated using the range 1740–1673 cm−1 (i.e. the lower
wavenumber half band area) and then doubled. Using
the extinction coefficient derived from the Beer–Lam-
bert plot (Fig. 2), the time dependence of the loss of the
rearranged acetate was determined for each tempera-
ture in the range 723–813K. Plots of molar abundance
versus time are illustrated in Fig. 8, and show exponen-
tial loss (correlation coefficients \0.98) of the isomer-
ized product. Plots of ln C versus time are linear
(correlation coefficients \0.99) (Fig. 9) showing that
the loss of dimethylethylsilyl acetate is first-order over
the whole temperature range studied. Rate constants
vary from 4.87×10−5 to 33.5×10−5 s−1 in the tem-
perature range 723–813 K. All kinetic runs were taken
to ca. 75% completion. Rate data are summarised in
Table 4.
Fig. 10 shows the corresponding Arrhenius plot from
which the relationship
ln C=8.23−110.3/RT
was derived by least-squares analysis (R2=0.961) lead-
ing to values for the activation energy, Ea, and the
pre-exponential factor, A, of 110(4) kJ mol−1 and
3.8×103 s−1, respectively.
Fig. 8. Plot of gas-phase molar abundance (mol l−1) of trimethylsilyl-
methyl acetate vs. time at 723 K (ꢂ), 733 K (ꢀ), 743 K (ꢃ), 753 K
(ꢁ), 763 K (ꢄ), 773 K ( ), 783 K (+), 803 K (×), and 813 K (").
Solid lines represent least-squares best fits.

I.K. Ball et al. / Journal of Organometallic Chemistry 599 (2000) 185–194
191
Table 4
First-order rate constant data
Temperature (K) First-order rate constant (k1) (s−1 (×105))
Temperature (K)
First-order rate constant (k1) (s−1 (×105))
723
733
743
753
763
4.87
4.90
5.57
7.11
11.0
773
783
803
813
11.6
16.2
22.5
33.5
followed by hydrogen abstraction by the resulting ace-
toxyl radical.
A similar rearrangement to that proposed here for
trimethylsilylmethyl acetate has also been observed for
benzyltriphenylsilanes [13] in which there is an ex-
change of groups between the silicon atom and the
adjacent carbon atom (Eq. (2)):
(2)
The ease of migration of the group (X) from carbon
to silicon decreases in the order X=F\OTs\OAc\
Cl\BrꢀOMe, which coincides with the expected or-
der of stabilisation of a penta-coordinated silicon atom
based on bond energies and the electronegativities of X.
It was observed that a-chlorobenzyltriphenylsilane rear-
ranged faster than the analogous germanium com-
pound, probably reflecting the greater bond energy of
SiꢀX compounds and the lower energy of the silicon d
orbitals.
Fig. 10. Arrhenius plot for the loss of trimethylsilylmethyl acetate in
the temperature range 723–813 K.
Thermal rearrangements involving [1,2]-shifts have
also been observed for tin-containing compounds.
Seyferth and Armbrecht [14] reported that
dimethyldichlorostannane was one of the products of
the thermolysis of trimethyl(a,a-dichlorobenzyl)tin at
150°C in cyclohexene. The reaction course proposed
was initial Sn···Cl attraction, followed by a four-centre
transition state involving the tin atom, a methyl group
associated with tin, the carbon a to tin and a chloride
atom (Eq. (3)). The exchange of methyl and the chlo-
ride was followed by a further step and yielded
dimethyldichlorostannane.
4. Discussion
When trimethylsilylmethyl acetate is heated to 623 K
it undergoes an isomerisation to give dimethylethylsilyl
acetate, most probably via a concerted, five-centred,
transition state (Eq. (1)):
(1)
No substantial isomerisation occurs below this tem-
perature. The driving force for the isomerisation is the
strength of the incipient SiꢀO bond, the polarity of the
bond Me3Sil+ l−CH2ꢀ allowing nucleophilic attack
–
by the carbonyl oxygen at silicon. Some ethanoic acid is
also produced at this temperature. This may arise from
the homolytic fission of the Me3SiCH2ꢀOCOMe bond,
(3)

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I.K. Ball et al. / Journal of Organometallic Chemistry 599 (2000) 185–194
K. The neat reaction was shown to occur by two
parallel reaction pathways: a concerted dyotropic rear-
rangement and a free radical chain, which appeared to
be wall initiated. Davidson and Ijadi-Maghsoodi [16]
also studied the rearrangement of this molecule in a
stirred flow apparatus with nitrogen as carrier gas
between 467 and 550°C. The rearrangement followed
first-order kinetics and was insensitive to added radical
traps. The assumption was made that the unimolecular
rearrangement followed a three-centre transition state
with hydrogen migration rather than methyl migration
(Eq. (6)).
Scheme 1. Proposed reaction mechanism for the thermolysis of
dimethylethylsilylmethyl acetate in the temperature range 723–813 K
(final products are in italicised bold font).
(6)
Trimethylsilylmethyl acetate undergoes a molecular
rearrangement prior to decomposition and the major
products of this rearrangement are dimethylethylsilyl
acetate and ethanoic acid. After the thermally induced
isomerisation, the isomerization product, dimethyle-
thylsilyl acetate, undergoes decomposition in the tem-
perature range 723–813 K. Gas-phase products include
methane, ethene, carbon dioxide, cyclohexamethyl-
trisiloxane, diethyltetramethyldisiloxane, ethanoic acid,
carbon monoxide and cyclooctamethyltetrasiloxane
(Eq. (7)):
Two elimination pathways are possible in the pyroly-
sis of 2-trimethylsilylethyl acetate in the temperature
range 600–800 K [12]. The products of thermolysis at
the lower end of the temperature region studied were
due solely to the elimination of ethanoic acid and
vinyltrimethylsilane (Eq. (4)):
(4)
Me2EtSiOAc“CH4+C2H4+CO2+(Me2SiO)3
+(EtMe2Si)2O+MeCO2H+CO
At higher temperature (\750 K) however, another
type of elimination occurs, the extrusion of ethene
(Eq. (5)), but which contributed only one-third of the
overall elimination. This process has a higher activation
energy and only occurs at the higher temperatures
studied. Both eliminations from 2-trimethylsilylethyl ac-
etate were proposed to involve normal six-centre reac-
tions.
+(Me2SiO)4
(7)
Loss of dimethylethylsilyl acetate follows first-order
kinetics over the whole temperature range studied, and
rate constants vary from 4.87×10−5 s−1 at 723 K to
33.5×10−5 s−1 at 813 K, respectively, leading to an
activation energy, Ea, of 110(4) kJ mol−1. This be-
haviour is very similar to that of trimethylsilyl acetate
[1], which also exhibited first-order kinetics with rate
constants varying from 1.66×10−5 s−1 at 723 K to
15.7×10−5 s−1 at 818 K, respectively, leading to an
activation energy, Ea, of 104(5) kJ mol−1. In that case,
the behaviour in the presence of radical initiators indi-
cated strongly the operation of a free radical mecha-
nism involving the initial generation of the
(5)
Another reported rearrangement is the gas-phase iso-
merisation of (chloromethyl)dimethylsilane to trimethyl-
chlorosilane [15] in a static reactor between 636 and 690
’
trimethylsilyloxyl radical, Me3SiO , and a similar pro-
cess is also thought to occur in the present case.

I.K. Ball et al. / Journal of Organometallic Chemistry 599 (2000) 185–194
193
A reaction scheme that accounts for the formation of
all products is shown in Scheme 1. Due to the strength
of the SiꢀO band, the initial process would be ho-
molytic cleavage of the skeletal CꢀO bond generating
trimethylsilyloxyl and acetyl radicals (step 1). Two reac-
tion pathways are possible for the further reaction of
the dimethyethylsilyloxyl radical:
1. abstraction of hydrogen to form trimethylsilanol
(step 2),
2. elimination of an ethyl radical with the formation of
dimethylsilanone (step 3).
processes are fast relative to this, then loss of
trimethylsilyl acetate in the neat reaction will be first-
order as observed.
As carbon monoxide is present at the start tempera-
ture and rises to a maximum, the acetyl radical formed
in step 1 must thermally decompose to give carbon
monoxide and a methyl radical (step 7). After reaching
a maximum value, the carbon monoxide abundance
then decreases with time. Oxidation, presumably at the
wall surface where there is a large presence of oxide,
provides carbon dioxide (step 8).
Ethene is not present at the start, but grows in
continuously with time later in the thermolysis. Ethyl
radicals, produced into the reaction sphere, can propa-
gate to form ethene (step 9). Methane forms later than
ethene in the reaction and is probably produced from a
methyl radical hydrogen abstraction (step 10).
A third possible reaction pathway, silyloxyl radical
attack at a silicon centre, as far as we are aware, has
not as yet been observed. Hydrogen abstraction by
trimethylsilyloxyl radicals is known to occur with a
variety of substrates [18–21]. In particular, hydrogen
abstraction from bistrimethylsilyl peroxide giving the
’
Me3SiOOSiMe2CH2 radical has been observed. The
product of hydrogen abstraction, trimethylsilanol, read-
ily undergoes condensation to hexamethyldisiloxane
and water [17], and an analogous process (step 6)
accounts for the formation of diethyltetramethyldisilox-
ane in the present system. The hexamethylcyclotrisilox-
ane and octamethyltetrasiloxane observed almost
certainly result from the oligomerization of dimethyl-
silanone [22,23] (steps 4 and 5), formed by ethyl radical
elimination from the dimethylethylsilyloxyl radical (step
3).
The acetyl radical formed in step 1 is known to
eliminate carbon monoxide [24–27] (step 7). Methyl
radicals are also formed in this step, leading to methane
formation by hydrogen abstraction (step 10). Ethanoic
acid may be formed by the formation of an acetoxyl
radical, followed by hydrogen abstraction as described
above. An alternative pathway for the formation of
ethanoic acid is via internal hydrogen abstraction from
dimethylethylsilyl acetate (Eq. (8)):
Acknowledgements
We thank the EPSRC for a research studentship (to
A.T.)
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