Substituent effects on silene reactivity - reactive silenes from photolysis of phenylated tri- and tetrasilanes

Article abstract of DOI:10.1139/V08-165

Laser flash photolysis of 2-phenylheptamethyltrisilane (5d), 2,2-diphenylhexamethyltrisilane (5e), and phenyl tris(trimethylsilyl)silane (5f) in hexane and acetonitrile solution affords strong, long-lived transient absorptions centered in the 440-470 nm range, which are assigned to the transient silenes formed via [l,3]-trimethylsilyl migration into the ortho-position of a phenyl ring on the basis of their UV-vis spectra and kinetic data for their reactions with methanol (MeOH), acetic acid (AcOH), acetone, 2,3-dimethyl-1,3-butadiene (DMB), carbon tetrachloride (CCl₄), and oxygen. The silene derivatives are formed along with the corresponding silylenes (SiMePh, SiPh₂, and Si(SiMe₃)Ph, respectively) upon photolysis of these compounds in solution, and indeed, weak, short-lived transient absorptions assignable to the silylenes can also be detected in laser photolysis experiments with the three compounds in hexane, superimposed on the much more prominent absorptions due to the silenes. The silylene absorptions are quenched by MeOH and triethylsilane (Et₃SiH) with absolute rate constants varying over the narrow ranges of (1.1-1.8) x 10¹⁰ (mol/L)^-1 s^-1 and (2.5-3.6) × 10⁹ (mol/L)^-1 s^-l, respectively, in excellent agreement with previously reported values for SiPh₂ and SiMe₂ under the same conditions. The kinetic data obtained for the silenes are compared to previously reported data for the reactions of the same substrates with the related silenes (6a-6c) formed by photolysis of pentamethylphenyl-, 1,1,1,2-tetramethy 1-2,2-dipheny 1-, and 1,1,1 -trimethyl-2,2,2- triphenyldisilane (5a-5c, respectively) under similar conditions. The comparison provides the first systematic, quantitative assessment of the stabilizing effects of trialkylsilyl substitution at the silenic silicon atom in silene derivatives.

Full text of DOI:10.1139/V08-165

1105
Substituent effects on silene reactivity — Reactive
silenes from photolysis of phenylated tri- and
tetrasilanes
William J. Leigh, Andrey G. Moiseev, Eugenie Coulais, Farahnaz Lollmahomed,
and Mohammad S. Askari
Abstract: Laser flash photolysis of 2-phenylheptamethyltrisilane (5d), 2,2-diphenylhexamethyltrisilane (5e), and
phenyltris(trimethylsilyl)silane (5f) in hexane and acetonitrile solution affords strong, long-lived transient absorptions
centered in the 440–470 nm range, which are assigned to the transient silenes formed via [1,3]-trimethylsilyl migration
into the ortho-position of a phenyl ring on the basis of their UV–vis spectra and kinetic data for their reactions with
methanol (MeOH), acetic acid (AcOH), acetone, 2,3-dimethyl-1,3-butadiene (DMB), carbon tetrachloride (CCl4), and
oxygen. The silene derivatives are formed along with the corresponding silylenes (SiMePh, SiPh2, and Si(SiMe3)Ph,
respectively) upon photolysis of these compounds in solution, and indeed, weak, short-lived transient absorptions
assignable to the silylenes can also be detected in laser photolysis experiments with the three compounds in hexane,
superimposed on the much more prominent absorptions due to the silenes. The silylene absorptions are quenched by
MeOH and triethylsilane (Et3SiH) with absolute rate constants varying over the narrow ranges of (1.1–1.8) ×
1010 (mol/L)–1 s–1 and (2.5–3.6) × 109 (mol/L)–1 s–1, respectively, in excellent agreement with previously reported val-
ues for SiPh2 and SiMe2 under the same conditions. The kinetic data obtained for the silenes are compared to previ-
ously reported data for the reactions of the same substrates with the related silenes (6a–6c) formed by photolysis of
pentamethylphenyl-, 1,1,1,2-tetramethyl-2,2-diphenyl-, and 1,1,1-trimethyl-2,2,2-triphenyldisilane (5a–5c, respectively)
under similar conditions. The comparison provides the first systematic, quantitative assessment of the stabilizing effects
of trialkylsilyl substitution at the silenic silicon atom in silene derivatives.
Key words: silene, silylene, trisilane, tetrasilane, kinetics.
Résumé : La photolyse éclair au laser du 2-phénylheptaméthyltrisilane (5d), du 2,2-diphénylhexaméthyltrisilane (5e) et
du phényltris(triméthylsilyl)silane (5f), en solution dans l’hexane et l’acétonitrile, permet d’observer de fortes
absorptions transitoires de longue durée, centrées autour de 440 à 470 nm et qui, sur la base de leurs spectres UV/vis
et des données cinétiques de leurs réactions avec le méthanol (MeOH), l’acide acétique (AcOH), l’acétone, le 2,3-
diméthyl-1,3-butadiène (DMB), le tétrachlorure de carbone (CC4) et l’oxygène, ont été attribuées aux silènes
transitoires formés par le biais d’une migration [1,3]-triméthylsilyle vers la postion ortho d’un noyau phényle. Lors de
la photolyse des ces composés en solutions, les dérivés silènes se forment aux côtés des silylènes correspondants
[respectivement (SiMePh, SiPh2 et Si(SiMe3)Ph] et, dans les expériences de photolyse laser des trois composés en
solution dans l’hexane, on peut de fait détecter des absorptions transitoires de courtes durées attribuables aux silylènes
qui sont superposées sur les absorptions beaucoup plus prééminentes dues aux silènes. Les absorptions des silylènes
peuvent être piégées par le méthanol et le triéthylsilane (Et3SiH), avec des constantes de vitesse absolues qui s’étalent
respectivement sur une plage étroite allant de (1,1–1,8) x 1010 (mol/L)–1 s–1 à (2,5–3,6) x 1010 (mol/L)–1 s–1, en
excellent accord avec les valeurs rapportées antérieurement pour le SiPh2 et le SiMe2, dans les mêmes conditions. On
compare les données cinétiques obtenues pour les silènes avec celles obtenues antérieurement pour les réactions des
mêmes substrats avec les silènes apparentés, 6a–6c, formés par photolyse des pentaméthylphényl-, 1,1,1,2-tétraméthyl-
2,2-diphényl- et 1,1,1-triméthyl-2,2,2-triphényldisilanes (5a–5c, respectivement), dans des conditions similaires. La
comparaison fournit la première évaluation systématique et quantitative des effets stabilisants de la substitution par un
groupement trialkylsilyle au niveau de l’atome de silicium silénique dans les dérivés silènes.
Mots-clés : silène, silylène, trisilane, tétrasilane, cinétique.
[Traduit par la Rédaction] Leigh et al. 1117
Received 1 September 2008. Accepted 14 October 2008. Published on the NRC Research Press Web site at canjchem.nrc.ca on
19 November 2008.
W.J. Leigh,1 A.G. Moiseev, E. Coulais, F. Lollmahomed, and M. Askari. Department of Chemistry, McMaster University,
1280 Main Street West, Hamilton, ON L8S 4M1, Canada.
1Corresponding author (e-mail: leigh@mcmaster.ca).
Can. J. Chem. 86: 1105–1117 (2008)
doi:10.1139/V08-165
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Chart 1.
Introduction
The Si=C double bond is an intrinsically reactive bonding
arrangement that has received a great deal of theoretical and
experimental attention over the past few decades (1–4). Most
silenes are transients in solution or the gas phase, reacting
rapidly with nucleophiles or oxygen, or by dimerization
when reactive substrates are absent. Successful attempts to
synthesize isolable silene derivatives have invariably relied
on the presence of sterically bulky substituents to suppress
dimerization, which in most cases proceeds via formal
head-to-tail [2+2] cycloaddition. This purely kinetic stabiliz-
ing effect has also been supplemented with thermodynamic
stabilization, most notably with the introduction of
trialkylsilyl substituents at the silenic silicon atom and alkyl-
and (or) alkoxy-substituents at carbon, such as in the well-
known silenes of Brook (1 (5)) and Apeloig (2 (6)) and
their co-workers. It is well established that such substitution
patterns lead to marked reductions in the natural (δ+Si=Cδ–)
polarity associated with the Si=C double bond, slowing the
normally diffusion-controlled dimerization reaction (7, 8)
and reversing its normal (head-to-tail) regiochemistry (6, 9)
(Chart 1).
While an abundance of evidence suggests that, by and
large, weakly polar silenes such as 1 and 2 display similar
reactivity as do more polar derivatives toward most sub-
strates in terms of reaction products, relatively few studies
have been carried out with the aim of quantifying the effects
of reduced Si=C bond polarity on reaction kinetics or mech-
anisms. Our group has studied the kinetics of the reaction of
2 and a sterically stabilized derivative with methanol
(MeOH) in hexane solution, and found that not only does
this silene derivative react several orders of magnitude more
slowly with the alcohol than do other transient silenes that
have been studied, it reacts via a different mechanism (8).
The results (both experimental and computational) are con-
sistent with a concerted mechanism involving reaction of the
hydrogen-bonded dimer of the alcohol in hexane solution, in
contrast to the two-step mechanism, initiated by nucleophilic
attack at silicon followed by proton transfer to carbon,
which is typical of more electrophilic, naturally polarized
transient silenes such as 1,1-diphenylsilene (3) (3, 10, 11).
Milnes and Baines (12) have recently reported a mechanistic
probe study of the effects of Si=C bond polarity on the
mechanism of the reaction of two kinetically stabilized
silenes with aldehydes, and have shown it to change from
one involving a zwitterionic intermediate in the case of the
naturally polarized silene 4 to a biradical pathway in the
case of the very weakly polar Brook silene 1 (R = t-butyl or
adamantyl) (Chart 2).
Chart 2.
Scheme 1.
silene reactivity, in addition to nucleophilic addition reac-
tions, in as systematic a manner as possible.
It occurred to us that just such an opportunity presents it-
self with the series of 1-silahexatriene derivatives, 6a–6f,
which are derived from the corresponding oligosilanes, 5a–
5f, by photochemical [1,3]-trimethylsilyl migration
(Scheme 1). The first three members of the series (6a–6c)
have been studied by us previously by laser photolysis meth-
ods (15–17), and so a considerable body of kinetic data al-
ready exists for these compounds. The last three members of
the series (6d–6f) are known to be formed as coproducts in
the photolysis of the tri- and tetrasilanes, 5d–5f, which are
better known as photochemical precursors of the transient
silylene derivatives SiPh2, SiMePh, and Si(Ph)SiMe3, respec-
tively (18–22). Photolysis of these compounds in solution is
known to afford the corresponding silylenes in chemical
yields of 40%–60%, from the results of trapping studies in
which the silylenes of primary interest were scavenged with
substrates such as alkenes, dienes, ketones, trialkylsilanes,
and alcohols. Some of these studies also reported significant
yields of products derived from trapping of the transient 1-
silahexatriene derivatives, 6d–6f, in chemical yields as high
as ca. 40% (18, 23–30). Gaspar and co-workers employed
5d and 5e as precursors to SiMePh (31, 32) and SiPh2 (33),
respectively, in some of the earliest reported attempts to
study reactive silylenes in solution by laser flash photolysis
methods. They described strongly absorbing, long-lived tran-
sient species that they assigned (with some reservation) to
the corresponding silylenes (31–33); however, as we have
recently shown in the case of 5e (34), it is more likely that
the species they characterized in their experiments are in fact
the corresponding 1-silahexatriene derivatives and not the
silylenes of interest.
Some time ago, we reported two experimental studies di-
rected at quantifying the effects of substituents at silicon and
carbon on the kinetics of nucleophilic additions to transient
silenes (13, 14). Though systematic, these studies were lim-
ited to alcohol additions, and furthermore, covered only
silenes of the general structure Me(R)Si=CH2 and
Me2Si=CHR. We have continued to search for opportunities
to extend the scope of these studies to include both mono-
and bis-SiMe3 substitution at the silenic silicon atom and to
examine the effects of these substituents on other aspects of
The course of the reactions of silenes of this type with
typical “silenophiles” is often different than that found with
most other silenes, owing to the fact that the silenic carbon
is part of a carbocyclic ring system containing both an al-
lylic hydrogen and a dienyl moiety in conjugation with the
Si=C bond. For example, aliphatic dienes and alkenes react
with these silenes via ene-addition rather than by the usual
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Leigh et al.
1107
Fig. 1. Transient absorption spectra recorded 128–256 ns (-᭺-)
and 4.86–5.09 µs (-ٗ-) after the laser pulse, from the laser flash
photolysis of 0.11 mmol/L solutions of 2-phenylheptamethyl-
trisilane (5d) in (a) hexane and (b) hexane containing
3.0 mmol/L Et3SiH. The insets show transient decays recorded at
440 and 510 nm. The spectrum obtained from a photolysed solu-
tion of 5d in 3-methylpentane at 78 K (—; scaled to match the
absorption intensities below 310 nm) is also shown in (a).
[4+2] or [2+2] cycloaddition pathways that are typical of
most other silene derivatives (1, 2, 18, 35); these reactions
are diverted from their normal course by the labile allylic
hydrogen, whose transfer to a carbon in the substrate results
in aromatization of the carbocyclic ring. Reaction with ace-
tone proceeds via competing [2+2] and ene-addition (30,
36–38), the latter again involving transfer of the allylic hy-
drogen in the silene to the substrate rather than via the usual
pathway (as exemplified by 3 (39)) to produce a silyl enol
ether. Finally, the presence of the cyclohexadienyl-
substituent in silenes of this type results in unusually high
reactivity toward molecular oxygen and CCl4, which are
thought to react via mechanisms involving radical or
biradical intermediates (16).
In the present paper, we report the results of a laser flash
photolysis study of 5d–5f in various solvents. As suggested
above, the main goal of the work was to establish that the 1-
silahexatriene derivatives are the dominant transient products
detectable in experiments of this type with these compounds,
to characterize their reactivities with a variety of substrates
that show characteristically high reactivity toward silenes of
this type, and to compare the results to previously published
data for the related silenes 6a–6c in the interest of elaborat-
ing on our understanding of substituent effects on various
aspects of silene reactivity. A secondary goal was to attempt
to detect the reactive silylenes SiMePh and Si(Ph)SiMe3 in
solution by laser photolysis of 5d and 5f, to provide a bench-
mark for future studies with potentially more appropriate
photochemical precursors to these silylene derivatives; to our
knowledge, 5f has not been studied previously by laser
photolysis methods.
Results
Laser flash photolysis of rapidly flowed solutions of 2-
phenylheptamethyltrisilane (5d; ~10–4 mol/L) in dry,
deoxygenated hexane, with the pulses from a KrF excimer
laser (248 nm; ~20 ns; ~100 mJ) afforded strong transient
absorptions centered at λmax = 440 nm, which decay over
several hundred microseconds with multiexponential kinet-
ics, in good agreement with the results of earlier flash
photolysis studies of this compound (31, 32). We assign this
absorption to silene 6d based on the similarity of its spec-
trum to those of 6e (λmax = 460 nm (34)) and 6a–6c (λmax
=
425–490 nm (16, 17)), and the kinetic evidence described
later in the paper. However, we also detected a second, much
shorter-lived transient absorption that was superimposed on
those due to the long-lived species and exhibited a lifetime τ
~ 3 µs. The species exhibited a distinct absorption band
centered at λmax ~ 270 nm and weaker absorptions through-
out the 400–600 nm range. Figure 1 shows representative
transient spectra, recorded 128–256 ns and 4.86–5.09 µs af-
ter the laser pulse, along with decay traces recorded at
440 nm and 510 nm. As in our previous study of 5e (34), ad-
dition of 3 mmol/L Et3SiH reduced the lifetime of the short-
lived species to ca. 100 ns without affecting the lifetime of
the strong 440 nm absorption, thus cleaning up the 128–256
ns spectrum considerably (see Fig. 1b). We also recorded a
UV–vis spectrum of a photolysed solution of 5d in 3-
methylpentane (MP) at 78 K, which is shown as the solid
line in Fig. 1. The low temperature matrix spectrum (λmax
=
268, 493 nm) agrees well with that reported by West and co-
workers (40), who assigned it to methylphenylsilylene
(SiMePh).
The low temperature spectrum is reproduced again in
Fig. 2, along with solution phase difference spectra calcu-
lated from the spectra of Fig. 1 in two ways. The first spec-
trum (-ٗ-) was calculated as the difference between the end-
of-pulse (128–256 ns) spectra in pure hexane (Fig. 1a) and
in hexane containing 3 mmol/L Et3SiH (Fig. 1b), and thus
isolates that portion of the end-of-pulse spectrum that is
quenched by a primary reaction with the silane. Clearly, the
difference spectrum (λmax = 270, 510 nm) agrees with the
matrix spectrum of SiMePh quite closely; it is also quite
similar to the reported solution phase and low temperature
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Can. J. Chem. Vol. 86, 2008
Fig. 2. Transient difference spectra, calculated as the difference
between the 128–256 ns spectrum of Fig. 1a minus the 128–
256 ns spectrum of Fig. 1b (-ٗ-), and as the difference between
the 128–256 ns and 4.86–5.09 µs spectra of Fig. 1a (-᭺-). The
dashed line is the spectrum recorded of a photolysed solution of
5d in 3-methylpentane at 78 K, reproduced from Fig. 1a.
Fig. 3. (a) Transient absorption spectra recorded 16–80 ns (-᭺-)
and 432–544 ns (-ٗ-) after the laser pulse, from the laser
photolysis of a 0.07 mmol/L solution of phenyltris(trimethyl-
silyl)silane (5f) in deoxygenated hexane, and the scaled spectrum
of a photolysed solution of 5f in 3-methylpentane at 78 K (—);
the dotted line shows the latter spectrum magnified by a factor
of 5. The inset shows transient decays recorded at 290 and
450 nm. (b) Plots of kdecay vs [Q] for quenching of the short-
lived transient (λmax ~ 285 nm) from 5f by MeOH (᭺) and
Et3SiH (ٗ) in deoxygenated hexane at 25 °C; the solid lines are
the linear least-squares fits of the data to eq. [1].
matrix spectra of SiPh2 (34, 40). We thus assign the short-
lived absorptions centered at 270 and 510 nm to the silylene,
SiMePh. Further support for the assignment is provided by
the second order rate constants for quenching of the species
by MeOH (kMeOH = (1.8 0.3) × 1010 (mol/L)–1 s–1), Et3SiH
(kEt SiH = (3.4 0.3) × 109 (mol/L)–1 s–1), and molecular
oxy3gen (kO = (1.8 0.2) × 108 (mol/L)–1 s–1), which were
determined2from (linear) plots of the pseudo first-order rate
constant for decay of the transient signal at 520 nm versus
substrate (Q) concentration according to eq. [1]. The rate
constants are quite similar to those for the reaction of SiMe2
and SiPh2 with these three substrates in hexane under similar
conditions (41); furthermore, the ratio of the absolute rate
constants for quenching of the species by MeOH and Et3SiH
(kMeOH/kEt SiH = 5.3 1.5) are quite close to the rate ratio
3
determined by Hawari et al. (42) from the competitive trap-
ping of SiMePh by ethanol and Et3SiH in cyclohexane
(kEtOH/kEt SiH = 4.8). The lifetime of the 440 nm absorption
3
in the presence of millimolar concentrations of MeOH and
Et3SiH was roughly the same as in pure hexane solution,
consistent with its assignment to silene 6d.
[1]
kdecay = k0 + kQ[Q]
The second spectrum of Fig. 2 (-᭺-) was calculated as the
difference between the end-of-pulse and 4.9–5.1 µs spectra
of Fig. 1a, and thus isolates that portion of the 128–256 ns
spectrum of Fig. 1a that decays in the first 5 µs after excita-
tion in pure hexane solution (i.e., in the absence of a silylene
scavenger). This difference spectrum clearly contains contri-
butions from both SiMePh and silene 6d, consistent with the
presence of a fast initial decay process for the silene that ex-
ists only in the absence of a silylene scavenger. We tenta-
tively identify this process as a bimolecular reaction of the
silene and silylene; comparison of the maximum absorbance
values of the early and late spectra of Figs. 1a and 1b indi-
cate that roughly 20% of the initially produced amount of 6d
undergoes this rapid initial decay process. No evidence for
the formation of the silylene dimerization product, 1,2-
dimethyl-1,2-diphenyldisilene (Si2Me2Ph2; λmax = 420 nm
(43)), could be obtained in any of these experiments, consis-
tent with its formation being quenched as a result of the
competing reaction of SiMePh with silene 6d.
Similarly, laser flash photolysis of a rapidly flowed 7 ×
10–5 mol/L solution of phenyltris(trimethylsilyl)silane (5f) in
hexane afforded a transient spectrum consisting of two main
components, one very long-lived one centered at λmax
=
450 nm, which decayed over several milliseconds with
mixed order kinetics, and one very short-lived one centered
at λmax ~ 285 nm that decayed with first order kinetics and
© 2008 NRC Canada

Leigh et al.
1109
Table 1. Absolute rate constants (units of 106 (mol/L)–1s–1, unless otherwise noted) for the quenching of silenes 6a–6f by various sub-
strates in hydrocarbon solvents and MeCN at 25 °C.
Substrate
6a
6b
6c
6d
6e
6f
λmax
425 nm
460 nm
490 nm
440 nm
460 nm
450 nm
MeOHa
k1 = 230 70b
k1 = 60 25b
k1 = 17 10b
k2 = 2.47 0.03c k2 = 0.80 0.01c τMeOH ~ 38 µs
k2 = 4900 1000b,c k2 = 2800 500b,c k2 = 1100 400b,c
AcOHa
Acetone
DMB
290 10b
4500 60d
128 3d
141 5b
1580 60d
68 1d
70 2b
11.7 0.3
4.3 0.1
0.11 0.006e
2.7 0.1
1.9 0.1
0.083 0.003e
0.025 0.001
0.09 0.01
0.011 0.006
4.9 0.2
542 5d
30 1d
CCl4
440 10d
160 10d
110 10d
78
2
23
1
O2
690 40d
350 10d
190 20d
120
5
39
2
11.4 0.2
Note: In hexane solution unless otherwise noted.
aIn MeCN solution, see eq. [2].
bData from ref. 17.
cIn units of 106 (mol/L)–2 s–1.
dIn isooctane solution, data from ref. 16.
eIn cyclohexane solution, data from refs. 31 and 33.
lifetime τ ~ 300 ns. Figure 3a shows transient spectra re-
corded 16–80 and 432–544 ns after the laser pulse, along
with the UV–vis spectrum of a photolysed sample of 5f in a
3-MP matrix at 78 K. The matrix spectrum (λmax = 280,
680 nm) agrees well with the one reported by Conlin and
co-workers, who assigned it to phenyltrimethylsilylsilylene
(Si(Ph)SiMe3) (44). The lifetime of the transient signal at
290 nm was shortened upon addition of MeOH or Et3SiH to
the solution. Plots of kdecay versus [Q] were linear, analysis
of which, according to eq. [1], afforded the second order rate
constants kMeOH = (1.1 0.1) × 1010 (mol/L)–1 s–1 and
Addition of acetone or carbon tetrachloride (CCl4) to the
hexane solutions of 5d–5f caused the decay of the silene ab-
sorptions to accelerate in proportion to substrate concentra-
tion and follow clean pseudo first kinetics. Plots of kdecay
versus [Q] were linear, and were analyzed according to
eq. [1] to obtain the second order rate constants listed in
Table 1. Figure 4 shows representative plots of kdecay versus
[Q] for quenching of silenes 6d–6f by acetone and CCl4.
The lifetimes of the silenes were also observed to decrease
upon addition of oxygen to the solutions; second-order
quenching rate constants were estimated from three-point
plots of kdecay values recorded in argon-, air-, and O2-
saturated solutions of 5d–5f in hexane versus [O2], which
exhibited good linearity in all three cases. Addition of up to
10 mmol/L 2,3-dimethyl-1,3-butadiene (DMB) to hexane so-
lutions of 5f had little effect on the intensity or lifetime of
the 450 nm signal due to silene 6f, as expected considering
the reported rate constants for reaction of the long-lived
transients from 5d and 5e (i.e., silenes 6d and 6e) with this
diene, which are in the range of ca. 105 (mol/L)–1 s–1 in cyc-
lohexane (31, 33). Addition of larger quantities of the diene
(30–40 mmol/L) led to corresponding reductions in the
intensity of the silene absorption due to screening of the
excitation light by the added substrate, and very modest in-
creases in the pseudo first order decay rate constant. A plot
of kdecay versus [DMB] was linear over the 30–40 mmol/L
kEt SiH = (2.6 0.4) × 109 (mol/L)–1 s–1 for reaction of the
3
species with MeOH and Et3SiH, respectively. These results
are consistent with the assignment of the short-lived 285 nm
absorption band to Si(Ph)SiMe3. Unfortunately, the presence
of the weak long wavelength band, which is observable in
the matrix spectrum, could not be detected in solution, due
undoubtedly to the much lower extinction coefficient of this
absorption relative to that at 285 nm, which is itself rela-
tively weak. The lifetime of the 450 nm transient was unaf-
fected in the presence of the millimolar quantities of MeOH
and Et3SiH that were required to effect a change in the life-
time of the 285 nm absorption, allowing preliminary assign-
ment of the 450 nm absorption to silene 6f.
Laser photolysis of solutions of 5e (~10–4 mol/L) in
deoxygenated hexane afforded similar results to those re-
ported in our preliminary study of this compound (45): two
range in added diene, and afforded an estimate of kDMB
=
(1.1 0.1) × 104 (mol/L)–1 s–1 for the second order rate con-
distinct transient species are formed, one that exhibits λmax
=
stant for reaction of DMB with 6f.
460 nm and decays over several milliseconds, and one that
exhibits a discrete absorption band at λmax = 290 nm, weaker
absorptions in the 490–530 nm range and decays over 1–
2 µs. The latter is quenched efficiently by added MeOH
(kMeOH = (1.32 0.04) × 1010 (mol/L)–1 s–1) and Et3SiH
Quenching of the three silenes by methanol (MeOH) and
acetic acid (AcOH) was studied in acetonitrile (MeCN) solu-
tion, as preliminary experiments indicated that the effects of
these substrates on the silene lifetimes were too small for
meaningful rate constants to be determined in hexane, given
the solubility limits of the two substrates in the hydrocarbon
solvent. Laser photolysis of 5d–5f in deoxygenated MeCN
led in each case to the formation of two long-lived transient
(kEt SiH = (3.5 0.2) × 109 (mol/L)–1 s–1), consistent with a
3
silylene (SiPh2) assignment (34). Again, the lifetime of the
long-lived species is unaffected by these reagents in
millimolar concentrations (34).
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Can. J. Chem. Vol. 86, 2008
Fig. 4. Plots of kdecay vs [Q] for the quenching of silenes 6d
(ٗ), 6e (᭺), and 6f (∆) by (a) acetone and (b) CCl4 in
deoxygenated hexane at 25 °C. The solid lines are the linear
least-squares fits of the data to eq. [1].
Fig. 5. (a) Transient absorption spectra recorded 32–96 ns (-᭺-),
13.1–13.8 µs (-ٗ-), and 139.5–140.6 µs (∆) after the laser pulse,
from the laser photolysis of a 0.07 mmol/L solution of 5d in
deoxygenated MeCN; the inset shows transient decays recorded
at 300 and 440 nm. (b) Transient absorption spectra recorded 0–
1.2 µs (-᭺-), 15.4–16.6 µs (-ٗ-), and 99.8–101.1 µs (∆) after the
laser pulse, from the laser photolysis of a 0.07 mmol/L solution
of 5e in deoxygenated MeCN; the inset shows transient decays
recorded at 300 and 460 nm.
species, one exhibiting an absorption maximum in the 440–
460 nm range and a lifetime similar to that of the corre-
sponding transient in hexane and hence assignable to the
corresponding silenes, and the second exhibiting λmax
~
310 nm. The latter absorptions decayed on a distinctly faster
time scale with both 5d and 5e, but on a similar time scale
to the 450 nm absorption in the case of 5f. Transient absorp-
tion spectra recorded at various times after the laser pulse
for 5d and 5e in MeCN are shown in Fig. 5, while the analo-
gous spectra obtained with 5f are shown in Fig. 6a. Addition
of MeOH to the MeCN solutions of 5d and 5e caused the
lifetimes at 450 nm to shorten and the decays to follow clean
pseudo first order kinetics. As we anticipated based on our
earlier studies of silenes 6a–6c (16, 17), plots of kdecay
versus [MeOH] exhibited positive curvature; polynomial
least-squares fitting of the data to eq. [2] revealed a good
correlation with the square of the MeOH concentration, as is
illustrated in Fig. 7a. In contrast, the lifetime of silene 6f in
MeCN was barely affected upon addition of up to ca.
2 mol/L MeOH, so no attempt was made to determine a rate
constant in this case. Silene 6f could actually be detected
quite easily in neat MeOH solution, where it exhibited a life-
time of τ ~ 38 µs and the same absorption maximum as in
hexane and MeCN; the spectrum is shown in Fig. 6b. The
effect of added MeOH on the short wavelength signals ob-
served in experiments in MeCN solution was not investi-
gated.
[2]
kdecay = k0 + k1[MeOH] + k2[MeOH]2
In contrast to the behavior observed with MeOH, addition
of AcOH to MeCN solutions of 5d–5f resulted in increases
in kdecay that varied linearly with concentration in all three
© 2008 NRC Canada

Leigh et al.
1111
Fig. 7. Plots of (a) kdecay vs [MeOH]2 and (b) kdecay vs [AcOH]
for the quenching of silenes 6d (ٗ) and 6e (᭺) by MeOH and
AcOH in deoxygenated MeCN at 25 °C; the inset in (b) shows
the quenching plot for silene 6f by AcOH.
Fig. 6. (a) Transient absorption spectra recorded 0–3.2 µs (-᭺-),
57.6–60.8 µs (-ٗ-), and 540–542 µs (∆) after the laser pulse,
from the laser photolysis of a 0.07 mmol/L solution of 5f in
deoxygenated MeCN; the inset shows transient decays recorded
at 290 and 450 nm. (b) Transient absorption spectra recorded 0–
0.6 µs (-᭺-), 23.0–24.3 µs (-ٗ-), and 135–136 µs (∆) after the
laser pulse, from the laser photolysis of a 0.07 mmol/L solution
of 5f in deoxygenated MeOH; the inset shows transient decays
recorded at 320 and 450 nm.
solution produces the corresponding silene derivatives in
yields comparable to those of the corresponding silylenes
and their co-product, Si2Me6, in all three cases. Thus, both
reactive intermediates should in principle be detectable by
laser flash photolysis methods, given an adequate apprecia-
tion of what to expect in the way of relative absorption in-
tensities and reactivity patterns for the two species. The
lowest energy (π,π*) absorption band in the UV–vis spectra
of silenes of this general structure is relatively intense
(εmax ≥ 2 × 104 (mol/L)–1 cm–1) (15), in contrast to the rela-
tively weak (εmax ~ 103 (mol/L)–1 cm–1 (46)) lowest energy
(n,p) absorption band in the UV–vis spectra of arylsilylenes.
Given that the two types of products are formed in similar
yields in all three cases, detection of the silene will clearly
be a much easier task than detection of the silylene.
cases. The corresponding quenching plots are shown in
Fig. 7b.
The kinetic data obtained in these experiments are sum-
marized in Table 1, along with the corresponding, previously
reported data for 6a–6c.
Discussion
While early product studies of the photochemistry of 5d–
5f focused largely on the reactions of the corresponding
silylenes (SiMePh, SiPh2, and Si(Ph)SiMe3, respectively)
with various substrates (18, 19, 21, 22), several of them also
reported the isolation of products derived from trapping of
silenes 6d–6f; specifically, those studies that employed
aliphatic dienes (23, 26, 37), terminal alkenes (29, 30), ace-
tone (37), or alcohols (24, 25, 27) as trapping agents. It is
clear from these studies that the photolysis of 5d–5f in fluid
Our flash photolysis results for 5d–5f are indeed consis-
tent with the formation of two distinct transient products in
all three cases. The dominant transient absorptions, which
we assign to the silenes 6d–6f based on comparisons of their
absorption spectra and reactivities to those of the related an-
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1112
Can. J. Chem. Vol. 86, 2008
Chart 3.
Scheme 2.
alogues 6a–6c (vide supra), are centered in the 440–460 nm
spectral range and exhibit lifetimes of several hundred mi-
croseconds or greater; these are the species that were charac-
terized in the earlier laser photolysis studies of 5d (31, 32)
and 5e (33) in cyclohexane solution. Superimposed on these
absorptions in hexane are those assignable to the corre-
sponding silylenes, which are considerably shorter-lived and
are quenched several orders of magnitude more rapidly than
the 440–460 nm species by MeOH and Et3SiH; in all three
cases, the rate constants are close to the diffusion-controlled
limit and are in good agreement with the values reported by
us earlier for the reactions of SiMe2 and SiPh2 with the same
substrates under similar conditions (34, 45). The presence of
SiPh2 in the end-of-pulse spectrum from 5e is revealed by a
relatively strong absorption band centered at λmax = 290 nm,
which is due to the S0 → S2 transition of the silylene, and
much weaker absorptions in the 480–530 nm range due to
the S0 → S1 transition, which we have shown is centered at
λmax = 515 nm in hexane at 25 °C (34). Similar short-lived
absorptions (λmax ~ 270, 510 nm) are observed in the end-of-
pulse spectrum from laser photolysis of 5d, superimposed on
the much stronger absorptions of the long-lived transient
coproduct (i.e., silene 6d). The spectrum of this short-lived
species is again quite similar to the low temperature matrix
spectrum from 5d (40), consistent with it being due to the
corresponding silylene, SiMePh. With 5f, on the other hand,
the only short-lived absorptions detectable in the end-of-
pulse spectrum are centered at λmax ~ 290 nm, and the long
wavelength absorption band expected for Si(Ph)SiMe3 on
the basis of its low temperature spectrum (44) could not be
detected. This is very likely due simply to the lower sensi-
tivity of our instrument at wavelengths above 600 nm cou-
pled with the much lower extinction coefficient of the
absorption compared to that at 290 nm (see Fig. 3a). Assign-
ment of the latter to Si(Ph)SiMe3 is supported by the obser-
vation that it is quenched by MeOH and Et3SiH with rate
constants similar to those measured for the other two
silylenes.
Similarly, the relative reactivities of 6d and 6e toward
MeOH, AcOH, acetone, CCl4, and O2, with 6d exhibiting
modestly higher reactivity than 6e in every case, are quite
similar to the differences between the rate constants for re-
action of 6a and 6b with the same six substrates, as well as
to the corresponding differences between 6b and 6c (16, 17)
(see Table 1). Substitution of a methyl substituent with
trimethylsilyl at the silenic Si-atom is known to cause a
somewhat smaller red-shift in the UV–vis spectra of silenes
compared to phenyl-for-methyl substitution (cf. 7 (λmax
=
255 nm) (48), 8 (λmax = 285 nm) (13), and 9 (λmax = 315 nm)
(49)), but results in substantially larger reductions in reactiv-
ity toward MeOH addition (cf. 7 (k1 = 4.9 × 109 (mol/L)–1 s–1)
(48), 8 (k1 = 1.8 × 108 (mol/L)–1 s–1) (13), and 9 (k1 = 3.2 ×
109 (mol/L)–1 s–1) (49)). These trends lead to the prediction
that compared to the series 6a–6c, there should be a smaller
red-shift in λmax as one proceeds through the series 6a, 6d,
and 6f, accompanied by a much larger reduction in reactivity
towards MeOH. Clearly, the results are in excellent agree-
ment with this prediction. It can be concluded that the UV–
vis spectra and reactivity exhibited by 6d–6f follow trends
that are not only internally consistent, but are consistent as
well with those exhibited by 6a–6c given the known effects
of methyl-, phenyl-, and trimethylsilyl-substitution at silicon
on the spectra and reactivity of other transient silene deriva-
tives.
Comparison of the kinetic data for the two groups of com-
pounds 6a/6d/6f and 6c/6e/6f allows a more detailed assess-
ment of the effects of sequential trimethylsilyl substitution
on the kinetics of the six silene-substrate reactions that have
been studied in this work. In general, increasing TMS substi-
tution in both series of compounds leads to a reduction in
the rate constant for all six of these reactions, which is con-
sistent with previously established trends for other silene de-
rivatives (8, 50). However, the effect varies tremendously
with the specific reaction type. For example, the sequential
decrease in the rate constants for quenching by CCl4 and O2
with increasing TMS substitution in the two series of com-
pounds is relatively small and more or less additive, while
for quenching by acetone and DMB the introduction of the
first TMS group results in a substantially larger reduction in
reactivity than the introduction of the second TMS group.
Interestingly, the opposite is true for quenching by MeOH
and AcOH.
The assignment of the dominant, long-lived absorptions to
the corresponding silenes 6d–6f is based on comparisons of
their spectra and reactivities to those of the related analogues
6a–6c (16, 17), coupled with a knowledge of the effects of
methyl-, phenyl- and trimethylsilyl-substitution on the UV–
vis spectra and reactivity of the “simple” silene derivatives
7–9 (Chart 3) and 3 (3, 13, 47–49). The difference in the ab-
sorption maxima of silenes 6d (λmax = 440 nm) and 6e
(λmax = 460 nm) is consistent with the trend expected from
consideration of the spectra of 6a–6c, which exhibit a regu-
lar red-shift in λmax as the methyl groups in 6a (λmax
=
425 nm) are replaced with first one phenyl substituent (i.e.,
6b; λmax = 460 nm) and then a second (i.e., 6c; λmax
=
490 nm) (16). As has also been observed for 6a–6c (16),
there is no change in the positions of the absorption maxima
of 6d–6f upon changing the solvent from hexane to MeCN.
Of the six reactions studied, that with MeOH exhibits the
largest substituent effect on the reaction kinetics. The mech-
© 2008 NRC Canada

Leigh et al.
1113
Scheme 3.
Scheme 4.
anism has been shown to involve initial, reversible nucleo-
philic attack at silicon to form a zwitterionic complex that
proceeds to product via rate-controlling proton transfer from
oxygen to the silenic carbon, the latter by competing uni-
molecular and catalytic pathways (Scheme 2) (10, 15, 17,
47). While this results in overall 1,2-addition in simple (non-
conjugated) silenes, three regioisomers corresponding to
1,2-, 1,4-, and 1,6-addition are possible in the reaction of
silenes of the type considered here, where the Si=C bond
forms the terminus of a 1-silahexatrienyl system. The 1,4-
and 1,6-addition products normally dominate the product
mixtures from silenes of this type (51); this is exemplified
by the results of Kira and co-workers (24, 25) for the
photolysis of 5e in hexane containing 1.0 mol/L ethanol,
which yields ethoxydiphenylsilane (10; 50%; from trapping
of SiPh2) and the 1-silahexatriene-derived products 11a and
11b in a combined yield of 37% at 25 °C (Scheme 3). The
corresponding 1,2-addition product has been reported in
only one instance that we are aware of, from reaction of
silene 6a (generated by photolysis of 5a) with MeOH (17).
The product distribution obtained from reaction of silene 6a
with MeOH (Scheme 4) depends on the alcohol concentra-
tion, with reasonable yields of the 1,2-addition product (12a)
being obtained only at very low alcohol concentrations (17).
The 1,4- and 1,6-addition products (12b and 12c, respec-
tively) dominate the reaction mixture at higher alcohol con-
centrations, conditions that are typical of most of the
published studies of the chemistry of silenes of this type. It
can thus be concluded that these two regioisomers arise
from the catalytic H-transfer pathway for decomposition of
the initially formed silene-alcohol complex, while the 1,2-
addition product results from the unimolecular H-transfer
pathway (17).
The competition between unimolecular and catalytic
intracomplex proton transfer is revealed in kinetic experi-
ments by a quadratic dependence of the silene decay rate
constant on alcohol concentration, since the catalyst for the
latter pathway is a second molecule of the alcohol (see
Scheme 2) (10, 15, 17). A variety of mechanisms are possi-
ble for the catalysis. With silenes such as 6a–6c in polar
solvents, the evidence suggests that it proceeds via a
deprotonation-protonation sequence (17), favoring the for-
mation of 1,4- and 1,6-adducts, as mentioned above. In
principle, the process could alternatively proceed via a
protonation-deprotonation sequence leading to predominant
trans-addition (10) or via a concerted “proton-shuttling”
mechanism in which the second molecule of alcohol acts si-
multaneously as acid and base. The transition state for this
latter process is envisioned as being similar to that involved
in the concerted reaction of the Si=C bond with the
hydrogen-bonded dimer of the alcohol, as was proposed re-
cently for the addition of MeOH to silene 2 in hexane solu-
tion (8) and should result in overall syn-addition. In any
event, whether or not the predicted quadratic dependence of
the silene decay rate constant (kdecay) on alcohol concentra-
tion can actually be observed with a given silene depends on
the magnitude of the overall second-order rate constant and
the maximum time resolution of the kinetic method.
Strongly electrophilic silenes such as 3 and 9 (for which k1 ~
109 (mol/L)–1 s–1 in MeCN) show only the first-order (in
alcohol) kinetic component, because it is so fast that the life-
time of the silene becomes too short to be detected on the
nanosecond time scale at the higher alcohol concentrations
necessary for the second-order component (k2) to become
kinetically significant (47, 49). Moderately reactive silenes
(such as 6a–6c, for which k1 ≤ 3 × 108 (mol/L)–1 s–1 in
MeCN) typically show both the first- and second-order com-
ponents (3). Only the second-order component is observed
with 6d and 6e (see Fig. 7a), indicating that the overall
second-order reaction pathway is slowed so dramatically rel-
ative to that with 6a–6c that it cannot be detected at all; the
only kinetically significant mechanism for reaction of
MeOH with these silenes involves two molecules of the al-
cohol in the transition state for the rate controlling step, and
overall third order reaction kinetics. The rate constant for
this process is more than 500 times slower than in 6a–6c, re-
flecting a substantial reduction in electrophilicity due to the
introduction of the TMS substituent at the silenic silicon
atom. Silene 6f, which bears two TMS-substituents at sili-
con, is so weakly electrophilic that its lifetime in MeCN is
unaffected by addition of up to 2 mol/L MeOH, and it re-
mains remarkably long-lived even in neat MeOH solution.
There is not even a shift in the absorption maximum in the
alcohol solvent relative to that in hexane or MeCN, which
provides a further indication of the extremely low degree of
electrophilicity of this silene derivative. The absorption
spectra of all three derivatives are identical in THF and
MeCN solution, as is also true of silenes 6b and 6c; of this
series of silenes only 6a exhibits a blue-shift of its absorp-
tion maximum in THF relative to its value in MeCN, due to
complexation with the ether solvent (16). Silene 6f is esti-
mated to be roughly two orders of magnitude more reactive
toward MeOH addition than the Apeloig silene 2, from com-
parison of the lifetimes of the two species in MeOH (τ6f
=
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1114
Can. J. Chem. Vol. 86, 2008
Scheme 5.
Scheme 6.
38 µs) or 9:1 v/v MeOH:THF (τ2 ~ 14 ms (8)) at 25 °C. This
could be the simple result of the pronounced differences in
steric bulk of the adamantylidene and cyclohexadienylidene
substituents at the silenic carbon in the two silenes.
Scheme 7.
More moderate variations in rate constants are observed
for quenching of the six silenes by acetone, AcOH, and
DMB, which proceeds with clean overall second order kinet-
ics in all cases. The major product of the reaction of silene
6e with acetone is the silyl enol ether derived from formal
ene-addition of the ketone C=O bond across the 1-silaallyl
moiety of the silene (13; Scheme 5) (37); the analogous
compounds are also the major products of the reactions of
6a–6c with this ketone, accompanied by minor amounts of
the siloxetanes derived from formal [2+2] cycloaddition
(16). The formation of the latter products from silenes of
this type is most likely general, but as they are rather sensi-
tive to thermal and (or) hydrolytic decomposition, they have
been detected in only a few instances (16, 38). The reaction
is thought to proceed via a two-step mechanism initiated by
formation of a biradical or zwitterionic intermediate that col-
lapses to products via competing ring-closure and
intramolecular H-migration (16, 38). The course of the reac-
tions of silenes of this type with AcOH has not been fully
established; the product mixture resulting from reaction of
AcOH with silene 6a proved to be intractable in earlier stud-
ies, leading to a complex mixture of at least eight products
in MeCN solution (17). However, with simpler silenes such
as 3 the reaction is known to proceed via formal 1,2-addition
of AcOH across the Si=C bond (47). The mechanism is pre-
sumably analogous to an ene-addition, involving bonding of
the carbonyl oxygen to silicon and transfer of the hydroxyl
proton to carbon. The reactions of 1,3-butadiene with 6d and
DMB with 6e and 6f afford the corresponding ene-addition
products (14–16; Scheme 6) (23, 26, 37), in identical fashion
to the reaction of aliphatic dienes with other (phenyldisilane-
derived) 1-silahexatrienes, such as 6a–6c (16, 52). It is inter-
esting to note that the rate constants for reaction of 6a/6d/6f
and 6c/6e/6f with DMB follow similar trends as those for re-
action with acetone (see Table 1), suggesting that for these
silenes, Si=C bond polarity provides a similar driving force
in both reactions.
The reaction with O2 is thought to proceed via rate-
determining attack of the substrate at the silenic Si atom to
yield the corresponding triplet 1,4-biradical, which in the
present series of compounds enjoys significant resonance
stabilization owing to the fact that the C-terminus is part of a
cyclohexadienyl system (16). In the cases of 6a and 6c, the
final products of the reaction are phenyltrimethylsilane and
the corresponding silanone oligomers, (Me2SiO)n from 6a
and (Ph2SiO)n from 6c, which have been proposed to be
formed from carbene 17 and dioxasilirane 18, respectively,
produced by cleavage of the 1,4-biradical (Scheme 7) (16).
The product mixtures resulting from the reaction of
silenes of this type with CCl4 have not been fully character-
ized; we reported previously that photolysis of 5a and 5c in
cyclohexane-d12 containing 0.03 mol/L CCl4 leads to com-
plex mixtures of products that include the chlorosilanes de-
rived formally from Si–Si bond homolysis, and
hexachloroethane (16). A reinvestigation of the photolysis of
5a in the presence of CCl4 under similar conditions to those
used in our earlier study (0.03 mol/L 5a; 0.03 mol/L CCl4),
and monitoring the course of the photolysis at low (ca. 3%–
1
12%) conversions by (600 MHz) H NMR spectroscopy, al-
lowed quantification of the chemical yields of PhMe2SiCl
(50%) and Me3SiCl (26%) relative to consumed 5a; chloro-
form (as CHCl3; 14%) was also identified in the mixture.
The spectra also showed evidence for the formation of at
least one additional aromatic product, and a series of reso-
nances in the δ 2.7–4 and δ 5.7–6.4 regions that are charac-
teristic of 1,3- and 1,4-cyclohexadiene derivatives (16, 17),
in significant yields. On the basis of these results, we sug-
gest that the reaction of 6a with CCl4 most likely proceeds
via Cl atom abstraction to yield cyclohexadienyl radical 19
and the trichloromethyl radical (Scheme 8). The formation
of PhMe2SiCl is then accounted for by β-scission of radical
19, while CHCl3 and the unidentified aromatic and
cyclohexadiene products are the products expected from
coupling and disproportionation processes involving 19 and
The reactions of silenes of this type with O2 and CCl4 are
unique in that they proceed much faster than is the case with
simpler silene derivatives (3). This suggests that the substitu-
tion pattern at the silenic carbon is the main factor control-
ling the rates of these reactions, with substitution at the
silenic Si atom playing only a secondary role. Accordingly,
the span in reactivity throughout the present series of com-
pounds toward these two substrates is relatively small, vary-
ing by a factor of less than 100 throughout the series (see
Table 1); with 6d–6f, these are the fastest reactions that
these silenes undergo.
© 2008 NRC Canada

Leigh et al.
1115
Scheme 8.
nol (MeOH), acetic acid (AcOH), acetone, 2,3-dimethyl-1,3-
butadiene (DMB), molecular oxygen, and carbon tetrachlo-
ride in hexane or MeCN solution. The results allow the first
systematic, quantitative study of the well-known stabilizing
effects of trimethylsilyl substitution at the silicon atom in
silenes, applied to a selection of different reaction types.
These include the nucleophilic addition of alcohols, the for-
mal ene-addition reactions of carboxylic acids, ketones, and
dienes, and the radical-like reactions with oxygen and CCl4.
All are slowed significantly by trimethylsilyl substitition at
the silenic silicon atom, underlining the importance of Si=C
bond polarity as a common, general driving force in the re-
actions of silenes.
trichloromethyl and trimethylsilyl radicals, the latter being
produced as the coproduct of the β-scission process in 19.
It has been suggested that the reaction is likely initiated
by electron transfer, based on the facts that quenching of 6a–
6c by CCl4 is significantly faster in MeCN than in hydrocar-
bon solvents and more than 100 times faster than quenching
by chloroform (16). The very modest variation in rate con-
stant throughout the present series of compounds is probably
also consistent with this notion, to the extent that varying
combinations of methyl-, phenyl-, and trimethylsilyl-
substitution at the silenic silicon would be expected to have
relatively small effects on the oxidation potential of the 1-
silahexatrienyl moiety that is common to all six of these
silenes.
Finally, it is interesting to note that the UV–vis spectra re-
corded after photolysis of 5d–5f in 3-MP matrixes at 78 K
differ significantly from the time-resolved spectra obtained
by flash photolysis in fluid solution at room temperature
(see Figs. 1–3 and ref. (33)). This is consistent with a
marked difference in the temperature and (or) medium
dependences of the quantum yields for formation of the cor-
responding silylenes and silenes from photolysis of these
compounds, with silene formation being suppressed almost
entirely when the three compounds are photolysed in solid
matrixes at low temperature.
Experimental
Compounds 5a and 5d–5f were prepared by the published
methods (26, 53–55) and purified by vacuum distillation fol-
lowed by column chromatography on silica gel using hex-
anes as eluant. Acetone (Caledon Reagent) was used as
received from the supplier. Methanol (Baker Photrex) and
2,3-dimethyl-1,3-butadiene (Sigma-Aldrich) were distilled.
Carbon tetrachloride (Sigma-Aldrich) was purified by distil-
lation over P2O5 and passed through a short silica gel col-
umn. Glacial acetic acid (Sigma-Aldrich), acetonitrile
(Caledon HPLC), and 3-methylpentane (Sigma-Aldrich
spectrophotometric grade) were used as received from the
suppliers. Triethylsilane (Sigma-Aldrich) was distilled from
lithium aluminum hydride. Hexanes (EMD OmniSolv) and
tetrahydrofuran (Caledon Reagent) were dried by passage
through activated alumina under nitrogen using a Solv-Tek
solvent purification system (Solv-Tek, Inc). Deuterated
solvents were used as received from Cambridge Isotope Lab-
oratories.
Laser flash photolysis experiments employed the pulses
from a Lambda Physik Compex 120 excimer laser filled
with F2/Kr/Ne mixtures (248 nm; 25 ns; 90–120 mJ/pulse),
and a Luzchem Research mLFP-111 laser flash photolysis
system, modified as described previously (56). Solutions
were prepared at concentrations (ca. 10–4 mol/L) such that
the absorbance at the excitation wavelength (248 nm) was
ca. 0.7, and were flowed through a 7 × 7 mm Suprasil flow
cell from calibrated 100 or 250 mL reservoirs, fitted with a
glass frit to allow bubbling of argon gas through the solution
for at least 30 min prior to and then throughout the duration
of each experiment, using a Masterflex™ 77390 peristaltic
pump fitted with Teflon tubing (Cole-Parmer Instrument
Co.). The sample cell and transfer lines were dried before
use in a vacuum oven at 65–85 °C, while the reservoir was
flame-dried and allowed to cool under an argon atmosphere.
Reagents were added directly to the reservoir by microliter
syringe as aliquots of standard solutions. Rate constants
were calculated by linear least-squares analysis of decay
rate-concentration data consisting of at least 10 points for all
substrates except oxygen. For the latter, they were calculated
from kdecay values in Ar-, air- and O2-saturated solution, with
use of neutral density filters to achieve reasonably clean
pseudo first order kinetics in Ar-saturated solution. The
errors in the absolute rate constants are quoted as twice the
standard error obtained from the least-squares analyses.
Summary and conclusions
Laser flash photolysis of the well-known silylene precur-
sors 5d–5f in hexane solution leads to strong, long-lived
transient absorptions due to the corresponding silene deriva-
tives 6d–6f, superimposed on the very weak, much shorter-
lived absorptions due to the silylenes SiMePh (from 5d),
SiPh2 (from 5e), and Si(Ph)SiMe3 (from 5f). The UV–vis
spectra of the silylenes in hexane solution can be extracted
from the raw transient spectra by selective quenching with
Et3SiH, which reacts several orders of magnitude more rap-
idly with the silylenes than with the corresponding silenes.
The resulting spectra are quite similar to those recorded in a
3-methylpentane matrix at 78 K, where photolysis of 5d–5f
affords the silylenes with high selectivity. In solution at
room temperature, photolysis of the three compounds af-
fords the corresponding silenes and silylenes in nearly equal
yields.
Along with the related, phenyldisilane-derived silene de-
rivatives 6a–6c, the three silenes studied in the present work
form a contiguous series of silenes bearing the six possible
combinations of methyl-, phenyl-, and trimethylsilyl-
substituents at the silenic silicon atom. The UV–vis spectra
of the six silenes vary in a rational way with substitution, as
do the absolute rate constants for their reactions with metha-
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1116
Can. J. Chem. Vol. 86, 2008
14. W.J. Leigh, C. Kerst, R. Boukherroub, T.L. Morkin, S.
Jenkins, K. Sung, and T.T. Tidwell. J. Am. Chem. Soc. 121,
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Low temperature UV–vis spectroscopic experiments em-
ployed 2 cm × 1 cm × 1 cm cuvettes constructed from
Suprasil quartz tubing (Vitro Dynamics, Inc.) and an Oxford
Optistat™ liquid nitrogen cryostat equipped with an Oxford
ITC601 temperature controller. Samples were irradiated in a
Rayonet Photochemical Reactor (Southern New England Ul-
traviolet Company) fitted with 12 RPR-2537 lamps.
Steady state photolysis of a 0.031 mol/L solution of 5a in
C6D12 containing CCl4 (0.03 mol/L) and CH2Cl2
(0.005 mol/L; internal standard) was carried out in a quartz
NMR tube, after deoxygenating the solution with a fine
stream of argon and sealing the tube with a rubber septum.
The photolysis was monitored at periodic time intervals by
600 MHz 1H NMR spectroscopy (Bruker AV600), and
chemical yields were determined from the slopes of concen-
tration versus time plots, constructed by integration of the
peaks due to 5a and those products that could be specifically
identified in the photolysate (PhMe2SiCl, Me3SiCl, and
CHCl3) by spiking the mixture with authentic samples. Rep-
resentative NMR spectra and concentration versus time plots
are given in the Supplementary Data.2
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Acknowledgement
25. T. Miyazawa, S.Y. Koshihara, C. Liu, H. Sakurai, and M. Kira.
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We thank the Natural Sciences and Engineering Research
Council of Canada for financial support and for an Under-
graduate Student Research Award for MSA.
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