Silanones and silanethiones from the reactions of transient silylenes with oxiranes and thiiranes in solution. The direct detection of diphenylsilanethione

Article abstract of DOI:10.1021/ja107881p

The transient silylenes SiMe₂ and SiPh₂ react with cyclohexene oxide (CHO), propylene oxide (PrO), and propylene sulfide (PrS) in hydrocarbon solvents to form products consistent with the formation of the corresponding transient silanones and silanethiones, respectively. Laser flash photolysis studies show that these reactions proceed via multistep sequences involving the intermediacy of the corresponding silylene-oxirane or -thiirane complexes, which are formed with rate constants close to the diffusion limit in all cases and exhibit UV absorption spectra similar to those of the corresponding complexes with the nonreactive O- and S-donors, tetrahydrofuran and tetrahydrothiophene. The SiMe₂-PrO and SiPh₂-PrO complexes both exhibit lifetimes of ca. 300 ns, and are longer-lived than the corresponding complexes with CHO, which are both in the range of 230-240 ns. On the other hand, the silylene-PrS complexes are considerably shorter-lived and vary with silyl substituent; the SiMe₂-PrS complex decays with the excitation laser pulse (i.e., τ ≥ 25 ns), while the SiPh₂-PrS complex exhibits τ = 48 ± 3 ns. The decay of the SiPh₂-PrS complex affords a long-lived transient product exhibiting λ _max ≈ 275 nm, which has been assigned to diphenylsilanethione (Ph₂Si=S) on the basis of its second order decay kinetics and absolute rate constants for reaction with methanol, tert-butanol, acetic acid, and n-butyl amine, for which values in the range of 1.4 × 10⁸ to 3.2 × 10⁹ M^-1 s^-1 are reported. The experimental rate constants for decay of the SiMe₂-epoxide and -PrS complexes indicate free energy barriers (ΔG^?) of ca. 8.5 and ≥7.1 kcal mol^-1 for the rate-determining steps leading to dimethylsilanone and -silanethione, respectively, which are compared to the results of DFT (B3LYP/6-311+G(d,p)) calculations of the reactions of SiH ₂ and SiMe₂ with oxirane and thiirane. The calculations predict a stepwise C-O cleavage mechanism involving singlet biradical intermediates for the silylene-oxirane complexes, and a concerted mechanism for silanethione formation from the silylene-thiirane complexes, in agreement with earlier ab initio studies of the SiH₂-oxirane and -thiirane systems.

Full text of DOI:10.1021/ja107881p

ARTICLE
pubs.acs.org/JACS
Silanones and Silanethiones from the Reactions of Transient
Silylenes with Oxiranes and Thiiranes in Solution. The Direct
Detection of Diphenylsilanethione.
Svetlana S. Kostina and William J. Leigh*
Department of Chemistry and Chemical Biology, McMaster University, 1280 Main Street West, Hamilton, ON Canada, L8S 4M1
S Supporting Information
b
ABSTRACT: The transient silylenes SiMe2 and SiPh2 react with
cyclohexene oxide (CHO), propylene oxide (PrO), and propylene
sulfide (PrS) in hydrocarbon solvents to form products consistent
with the formation of the corresponding transient silanones and
silanethiones, respectively. Laser flash photolysis studies show
that these reactions proceed via multistep sequences involving the
intermediacy of the corresponding silylene-oxirane or -thiirane
complexes, which are formed with rate constants close to the
diffusion limit in all cases and exhibit UV absorption spectra
similar to those of the corresponding complexes with the non-
reactive O- and S-donors, tetrahydrofuran and tetrahydrothiophene. The SiMe2-PrO and SiPh2-PrO complexes both exhibit
lifetimes of ca. 300 ns, and are longer-lived than the corresponding complexes with CHO, which are both in the range of
230-240 ns. On the other hand, the silylene-PrS complexes are considerably shorter-lived and vary with silyl substituent;
the SiMe2-PrS complex decays with the excitation laser pulse (i.e., τ e 25 ns), while the SiPh2-PrS complex exhibits τ = 48 (
3 ns. The decay of the SiPh2-PrS complex affords a long-lived transient product exhibiting λmax ≈ 275 nm, which has
been assigned to diphenylsilanethione (Ph2SidS) on the basis of its second order decay kinetics and absolute rate constants
for reaction with methanol, tert-butanol, acetic acid, and n-butyl amine, for which values in the range of 1.4 ꢀ 108 to 3.2 ꢀ 109
M-1 s-1 are reported. The experimental rate constants for decay of the SiMe2-epoxide and -PrS complexes indicate
free energy barriers (ΔGq) of ca. 8.5 and e7.1 kcal mol-1 for the rate-determining steps leading to dimethylsilanone
and -silanethione, respectively, which are compared to the results of DFT (B3LYP/6-311þG(d,p)) calculations of the
reactions of SiH2 and SiMe2 with oxirane and thiirane. The calculations predict a stepwise C-O cleavage mechanism
involving singlet biradical intermediates for the silylene-oxirane complexes, and a concerted mechanism for silanethione
formation from the silylene-thiirane complexes, in agreement with earlier ab initio studies of the SiH2-oxirane and -
thiirane systems.
’ INTRODUCTION
of oxygen from oxiranes, which yields the corresponding alkene
and silanone or carbonyl compound, respectively (eq 1).10-13
Early product- and kinetic studies of the reactions of SiMe2 with
the parent oxirane and various substituted derivatives were
interpreted in terms of a multistep mechanism involving the
initial formation of a silylene-oxirane complex.10-12,14,15 Tzeng
and Weber proposed that the collapse of the complex proceeds
via a stepwise mechanism involving a zwitterionic intermediate,
in order to account for the formation of siloxacyclohexenes in the
reactions of SiMe2 with vinyl epoxides, in addition to dimethyl-
silanone and the corresponding diene (eq 2).11 Various details of
the mechanism have received support from subsequent theore-
tical calculations,16-18 and by recent kinetic studies of the
reaction of SiH2 with oxirane in the gas phase.19 The analogous
reaction with singlet carbenes is also thought to proceed with the
It is well established that the chemistry of divalent silicon
compounds, or silylenes, shares many common characteristics
with that of singlet carbenes.1-3 Closer scrutiny, however,
usually reveals fundamental mechanistic differences between
the two homologous species. For example, both species
typically react rapidly with water and alcohols via net O-H
insertion, yet singlet carbenes most commonly do so via a two-
step mechanism involving initial protonation followed by ion-
pair combination,4 while silylenes react via initial nucleophilic
attack followed by (catalytic) proton transfer.5 The latter
mechanism evidently occurs much less commonly with singlet
carbenes than that involving initial protonation at the divalent
center.4,6,7 Conversely, there are few recognized examples
known of silylene reactions initiated by protonation at
silicon.8,9
A second reaction with oxygen-containing substrates that is
common to both silylenes and singlet carbenes is the abstraction
Received: September 1, 2010
Published: March 08, 2011
r
2011 American Chemical Society
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initial formation of a polar carbene-oxirane complex, or
transient silanethiones have been observed directly, either in low-
temperature matrixes,60 the gas phase,61-63 or in solution.40
In the present paper, we report the results of kinetic and
product studies of the reactions of the transient silylenes SiMe2
and SiPh2 with cyclohexene oxide (CHO), propylene oxide
(PrO), and propylene sulfide (PrS) in hexanes solution. The
goals of the study were to investigate and compare the reactions
of these silylenes with simple oxiranes and thiiranes and deter-
mine the kinetics and mechanisms as completely as possible, to
see whether the intermediate complexes proposed in earlier
studies could be detected and characterized kinetically, and to
attempt to detect the resulting (diaryl) silanones and sila-
nethiones directly and study their reactivities in a quantitative
way. As in our previous studies of these two transient silylenes in
solution,5,64,65 they were generated and detected by laser photo-
lysis of the corresponding oligosilane derivatives 1 and 2,
respectively. The well-known acyclic SiPh2-precursor, trisilane
3,66-70 was also employed in some of the product studies. We
have also carried out a density functional theory (DFT) study of
the reactions of SiH2 and SiMe2 with oxirane and thiirane, and
compare the results to the experimental data for SiMe2.
ylide.20-22
Singlet carbenes are also known to abstract sulfur from
thiiranes,22,23 doing so substantially more rapidly than O-atom
abstraction from oxiranes according to the kinetic studies of
Pezacki et al for several carbene systems in solution.22 The
corresponding reaction with silylenes has not yet been studied
experimentally, though theory predicts the reaction should
proceed with similar facility as that with oxiranes, and also via a
mechanism initiated by the spontaneous formation of a Lewis
acid-base complex between the silylene and the heteroatom in
the substrate.16-18
According to the ab initio (MP2/6-31G(d,p) and QCISD/6-
31G(d,p)) calculations of Apeloig and co-workers, the main
mechanistic difference between the reactions of SiH2 with
oxirane and thiirane lies in the mode of collapse of the inter-
mediate complexes, with the SiH2-oxirane complex undergoing
sequential C-O bond cleavage to yield a biradical intermediate
in the initial step, and the SiH2-thiirane complex undergoing
concerted fragmentation.16 The calculations predict that disso-
ciation of the complex is the rate-controlling step in both cases,
and proceeds with similar free energy barriers. Two subsequent
DFT studies17,18 afforded results that were in reasonable general
agreement with those of the earlier ab initio study, but failed to
locate the biradical intermediate in the decomposition of the
SiH2-oxirane complex that was found by Apeloig and Sklenak.16
As a result, concerted asynchronous cleavage mechanisms were
predicted for both the oxirane- and thiirane-complexes, the latter
via a lower enthalpic barrier than the former.17,18 Similar ener-
getics and mechanisms were predicted for the corresponding
reactions of SiMe2 with oxirane and thiirane.18
These reactions represent a potentially useful method for the
photochemical synthesis of transient silanone and silanethione
derivatives, for study of their reactivities in solution by time-
resolved spectroscopic methods. Remarkably little quantitative
information on the reactivity of these species exists, in spite of
considerable interest over the past few decades in the chemistry
of silicon-chalcogen double bonds24-27 and the synthesis of
stable compounds containing these intrinsically highly reactive
moieties.28-44 Several stable silacarbonyl compounds have been
reported,30-36 all but one of them31 containing chelating ligands
to stabilize the highly reactive silicon-oxygen double bond.
However, a stable dialkyl- or diarylsilanone (with or without
chelating ligands) has yet to be reported, and thus, most of our
knowledge regarding the spectroscopic properties and structures
of these compounds has been gained through low-temperature
matrix isolation studies13,24,25,45-50 and theoretical calcula-
tions.26,48,51-58 Silanethiones are thought to be more thermo-
dynamically stable than silanones due to a lower polarity of the
SidS bond,26,51,59 and indeed two sterically stabilized, base-free
silanethiones have been reported41-44 in addition to a few
donor-coordinated derivatives.36-38 However, remarkably few
’ RESULTS
Laser Flash Photolysis Experiments. Laser flash photolysis
experiments were carried out with rapidly flowed, deoxygenated
solutions of 1 (ca. 4 ꢀ 10-4 M) and 2 (ca. 6 ꢀ 10-5 M) in
anhydrous hexanes at 25 ( 1 ꢀC using the pulses from a KrF
excimer laser (248 nm, 90-105 mJ, ca. 20 ns) for excitation. The
silylenes are observed as promptly formed transients with UV-
vis absorption bands centered at λmax = 465 nm (SiMe2; τ ≈ 500
ns) and λmax = 300 and 515 nm (SiPh2; τ ≈ 1.7 μs), and in the
absence of added substrates decay with the concomitant forma-
tion of longer-lived UV-vis absorptions due to the correspond-
ing disilenes, Si2Me4 (λmax = 360 nm, τ ≈ 20 μs) and Si2Ph4
(λmax = 290 and 460 nm, τ ≈ 100 μs), respectively.5,64,65 Silylene
formation from 2 is accompanied by the formation of minor
amounts (ca. 3%) of silene 4, which gives rise to long-lived
residual absorptions centered at 460 nm on which the spectra due
to SiPh2 and Si2Ph4 are superimposed.64,70 This species exhibits
much lower reactivity than SiPh2 toward most substrates, so its
presence in the photolysis mixture does not compromise the
determination of rate constants or the characterization of tran-
sient products from the reactions of the silylene with added
substrates.
Addition of CHO (0.1-1.5 mM) to hexanes solutions of 1
resulted in shortening of the lifetime of the silylene, suppression
of the formation of Si2Me4, and the appearance of a new transient
species giving rise to absorptions centered at λmax = 310 nm. The
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Figure 1. (a) Plot of the pseudo-first-order decay coefficient (kdecay) of the SiMe2 absorption at 470 nm vs [CHO]. (b) Transient absorption spectra
from laser flash photolysis of a hexanes solution of 1 containing 1 mM CHO, recorded 26-38 ns (O), 128-154 ns (9), and 1.49-1.51 μs (Δ) after the
laser pulse (the portion of the 26-38 ns spectrum below 340 nm is distorted due to sample fluorescence and is not shown); the inset shows transient
absorbance vs time profiles recorded at 320, 370, and 470 nm.
latter grew in on a similar time scale as the silylene decay at low
CHO concentrations, both its growth rate and maximum in-
tensity increasing with increasing CHO concentration over the
0.2-1.5 mM range. The bimolecular rate constant for its
formation from free SiMe2 and CHO was determined from the
slope of a plot of the pseudo-first-order decay rate coefficients for
decay of the silylene absorption (kdecay) versus CHO concentra-
tion according to the expression of eq 3, where Q denotes the
substrate, kQ is the bimolecular rate constant for its reaction with
the free silylene, and k0 is the apparent pseudo-first-order rate
coefficient for silylene decay in the absence of added substrate.
The plot (see Figure 1a) showed excellent linearity, and exhibited
a slope of kQ = (1.9 ( 0.2) ꢀ 1010 M-1 s-1. The value is the same
within experimental error to that for reaction of free SiMe2 with
THF under similar conditions (k = (1.7 ( 0.2) ꢀ 1010 M-1 s-1
in hexanes at 25 ꢀC).65
residual absorptions remained after the complexes decayed,
indicating that the products of decomposition of the complexes
do not absorb significantly in the 270-600 nm range of the
spectrum.
Efficient reductions in the lifetime of SiMe2 and suppression of
the formation of Si2Me4 were also observed upon addition of PrS
to hexanes solutions of 1; a bimolecular rate constant of kQ = (2.1
( 0.2) ꢀ 1010 M-1 s-1 for the primary reaction with the silylene
was determined from the slope of a linear plot of kdecay versus
[PrS] according to eq 3 (see Figure S3a). A transient spectrum
recorded in the presence of 5 mM PrS (Figure S3b) showed
evidence of new, very short-lived transient absorptions centered
at λmax ≈ 320 nm (τ e 25 ns), though they were difficult to
detect reproducibly due to interference from strong sample
fluorescence in the 280-330 nm range of the spectrum. The
species is assigned to the SiMe2-PrS complex, the expected
spectrum of which was established using tetrahydrothiophene
(THT) as a representative S-donor; this substrate reacts with
SiMe2 with the same rate constant as that for PrS (see Figure
S4a), and results in the formation of a relatively long-lived
kdecay ¼ k0 þ kQ ½Q ꢁ
ð3Þ
The transient product decayed with first-order kinetics, ex-
hibiting a lifetime (τ = 230 ( 12 ns) that was independent of
CHO concentration up to at least 0.17 M (see Figure S1).
Figure 1b shows a series of transient absorption spectra recorded
for 1 in the presence of 1 mM CHO, where the lifetime of SiMe2
is reduced to ca. 60 ns and the spectrum of the 310 nm species
grows in to achieve its maximum intensity ca. 75 ns after the laser
pulse. The spectrum of the latter species is typical of SiMe2-O-
donor complexes such as those with THF and aliphatic alcohols
under similar conditions,5,65,71 and is thus assigned to the
SiMe2-CHO complex.
transient with UV-absorption maximum centered at λmax
=
325 nm (τ > 10 μs; Figure S4b). The spectrum agrees well with
that reported by Shizuka and co-workers in cyclohexane,72 who
assigned it to the SiMe2-THT complex. The decay of the
SiMe2-THT complex is accompanied by the growth of the
characteristic absorptions due to Si2Me4 (λmax = 370 nm), which
were quite prominent at low (0.2-2.0 mM) THT concentra-
tions but then decreased modestly as the concentration was
increased to higher levels, probably due to the presence of low
concentrations of reactive impurities in the solvent and/or
substrate. In contrast, the formation of Si2Me4 is completely
quenched in the presence of PrS, indicating that irreversible
reaction is responsible for the rapid decay of the SiMe2-PrS
complex. Again, the products of this reaction do not absorb
sufficiently strongly in the 270-600 nm wavelength range for us
to detect them by time-resolved UV-vis spectroscopy.
Addition of CHO, PrO, or PrS to hexanes solutions of 2
caused similar behavior as was observed in the corresponding
experiments with 1; the decay of the SiPh2 absorption (monito-
red at 530 nm) was accelerated in the presence of added
substrate, the signal decayed completely to baseline with clean
first-order kinetics, and formation of the silylene dimer (Si2Ph4;
λmax = 460 nm) was suppressed, all to an increasing extent with
Lifetime quenching of SiMe2 by propylene oxide (PrO) was
equally efficient (kQ = (1.6 ( 0.2) ꢀ 1010 M-1 s-1) and also led
to the formation of a single product absorption centered at λmax
=
310 nm (Figure S2), which can be assigned to the SiMe2-PrO
complex. The species decayed with clean first-order kinetics and
lifetime τ = 325 ( 20 ns, the average of several measurements
over the 0.005-0.093 M concentration range in added PrO
(Figure S1).
The formation of Si2Me4 is completely quenched in the
presence of CHO or PrO at concentrations of ca. 2 mM or
higher, indicating that irreversible reaction is responsible for the
decay of the SiMe2-oxirane complexes, and that it proceeds with
a rate constant considerably greater than reversion of the
complex back to the free silylene and substrate. Only very weak
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Figure 2. Transient absorption spectra recorded 65-77 ns (O), 0.27-
0.29 μs (0), and 3.50-3.57 μs (Δ) after the laser pulse, by laser flash
photolysis of a deoxygenated hexanes solution of 2 containing 17.0 mM
CHO; the inset shows transient decay/growth profiles recorded at 300
and 370 nm. The weak residual absorption centered at ca. 460 nm is due
to the long-lived photolysis coproduct, silene 4.
Figure 3. Transient absorption spectra recorded 48-54 ns (O) and
0.70-0.72 μs (0) after the laser pulse, by laser flash photolysis of a
deoxygenated hexanes solution of 2 containing 4.7 mM PrS; the dashed
line shows the difference spectrum, constructed by subtracting the 0.70
μs spectrum from the 48 ns spectrum. The inset shows transient decay/
growth profiles recorded at 290 and 370 nm.
increasing substrate concentration. Plots of kdecay for the silylene
absorption versus [Q] were linear in all three cases (Figures S5-
S7), and afforded bimolecular rate constants in the range of
(1.1-1.2) ꢀ 1010 M-1 s-1. With all three substrates, the decay of
the silylene was accompanied by the growth of a transient
product with absorption bands centered at λmax ≈ 290 and
370 nm, which (at higher substrate concentrations) decayed with
a first-order rate constant that was independent of substrate
concentration (Figure S8).
Table 1. Absolute Rate Constants(kQ, in Units of 109 M-1 s-1
for Quenching of SiMe2 and SiPh2 by Cyclohexene Oxide
(CHO), Propylene Oxide (PrO), Propylene Sulfide (PrS),
and Tetrahydrothiophene (THT). and First-Order Rate
Constants for Decay of the Resulting Complexes (kcplx, in
Units of 106 s-1) in hexanes at 25 ( 1 ꢀCa
)
SiMe2
SiPh2
substrate kQ/109 M-1 s-1 kcplx/106 s-1 kQ/109 M-1 s-1 kcplx/106 s-1
Figure 2 shows transient absorption spectra and representative
absorbance-time profiles obtained with hexanes solutions of 2
containing 17 mM CHO; the corresponding spectra with PrO as
substrate (Figure S9) were virtually identical. The spectra are in
both cases indistinguishable from those of the SiPh2-THF
complex under similar conditions,65 allowing their assignment
to the corresponding SiPh2-oxirane complexes. The lifetime of
the SiPh2-CHO complex (τ = 240 ( 10 ns) was modestly
shorter than that of the SiPh2-PrO complex (τ = 295 ( 15 ns),
as found for the corresponding SiMe2-oxirane complexes (vide
supra). Again, no new products absorbing in the 270-600 nm
range arising from the decay of the complexes could be detected
in either case.
Laser photolysis of optically matched solutions of 2 containing
ca. 10 mM CHO and ca. 10 mM THF, under conditions of equal
laser intensity, revealed the maximum absorbance at 365 nm to
be ca. 25% lower for the SiPh2-CHO complex than for the
SiPh2-THF complex. Kinetic simulations showed the value to
be within the range expected given the difference in the decay
kinetics of the two species, assuming equal extinction coefficients
at the monitoring wavelength. Provided the assumption of equal
extinction coefficients is valid, this shows that the lower peak
intensity observed for the SiPh2-CHO complex (τ = 240 (
10 ns) is due entirely to its much more rapid decay compared to
that of the SiPh2-THF complex (τ > 20 μs), and not to the
presence of a second, undetectable reaction channel that com-
petes with the formation of the complex from the free silylene
and the oxirane. Thus, complexation of the free silylene proceeds
quantitatively in the presence of both O-donor substrates at
concentrations of 10 mM.
CHO
PrO
19 ( 2
16 ( 2
21 ( 2
21 ( 2
4.3 ( 0.3
3.1 ( 0.2
g40 ( 4
<0.5b
10.0 ( 0.4
12 ( 2
4.2 ( 0.3
3.4 ( 0.2
18 ( 3
PrS
11.2 ( 0.4
15 ( 2
THT
<0.5b
a
Errors in kQ-values are reported as twice the standard error from linear
least-squares analysis of plots of silylene kdeccay values vs [Q] according
to eq 1. The kcplx-values and their uncertainties are reported as the
average and standard deviation of 9-13 independent determinations.
b. Decay is second order.
clear evidence for the formation of two transient products of the
reaction of SiPh2 with the thiirane: a short-lived one exhibiting
absorption bands centered at ca. 300 and 370 nm and lifetime τ =
48 ( 8 ns, and a second, much longer-lived one that is detected as
a tail absorption at the blue edge of the spectral window, in the
range which is normally dominated by the bleaching signal due to
the precursor (e.g., see Figure 2). The shorter-lived species is
assigned to the SiPh2-PrS complex, based on comparisons of its
spectrum to that recorded with a solution of 2 in hexanes
containing 5.3 mM THT (Figure S10). It should be noted that
the long-lived species present at the end of the decay of the
SiPh2-PrS complex is different from the one observed in the
presence of THT, which complexes strongly with SiPh2 but does
not inhibit the formation of the dimer (Si2Ph4; λmax = 460 nm)
and higher oligomerization products that form over a more
extended time scale. The latter also give rise to strong long-lived
absorptions at the blue edge of our spectral window, and are also
formed in the presence of THF.5,65
Table 1 lists the absolute rate constants determined in this
work for quenching of SiMe2 and SiPh2 by CHO, PrO, PrS, and
THT in hexanes at 25 ( 1 ꢀC, along with the first-order decay
Transient UV-vis spectra recorded by laser photolysis of a
hexanes solution of 2 containing 4.7 mM PrS (Figure 3) showed
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Figure 4. (a) Plot of the pseudo-first-order decay coefficient (kdecay) of the 290 nm absorption observed from 2 in the presence of PrS (20 mM) and
MeOH (0.05-0.5 mM) vs [MeOH]; the inset shows transient decays recorded in the presence of 0 and 0.05 mM MeOH. (b) Transient absorption
spectra recorded 0.18-0.24 (O) and 8.56-8.72 μs (0) after the laser pulse, by laser flash photolysis of deoxygenated hexanes solution of 2 containing
20 mM PrS and 0.5 mM MeOH; the inset shows transient decays recorded at 290 and 460 nm. The third spectrum (b) was calculated by subtraction of
the 8.56-8.72 μs spectrum from the 0.18-0.24 μs spectrum.
reaction with PrS remains the dominant mode of decay of the
free silylene.
Table 2. Absolute second-Order Rate Constants (kQ, in
Units of 109 M-1 s-1) for Quenching of the Long-Lived
Transient Product of the Reaction of SiPh2 with PrS by
MeOH, t-BuOH, AcOH, and n-BuNH2 in Hexanes
Containing 0.02 M PrS at 25 ( 1 ꢀCa
As indicated above, the raw UV/vis spectra shown in Figure 3
contain contributions from those of silene 4 and the precursor
bleaching signal, superimposed on the spectrum of the long-lived
product of the reaction of SiPh2 and PrS. Figure 4b shows tran-
sient spectra recorded over a longer time scale with a solution of 2
containing 0.02 M PrS and 0.5 mM MeOH, and the difference
spectrum obtained by subtraction of the spectrum remaining at
the end of the decay of the long-lived product from that at the
beginning of the decay. The difference spectrum shows that the
long-lived product of the reaction of SiPh2 with PrS exhibits a
strong absorption band centered at λmax ≈ 275 nmand(perhaps) a
secondary long wavelength band centered at λmax ≈ 460 nm,
though the latter is very weak and should be accepted with caution.
We assign the species to diphenylsilanethione (Ph2SidS) on the
basis of its UV-vis spectrum and kinetic behavior.
substrate
MeOH
t-BuOH
AcOH
n-BuNH2
kQ/109 M-1 s-1
0.85 ( 0.09 b
0.14 ( 0.01
3.2 ( 0.7
2.1 ( 0.1
a
Unless otherwise noted, errors are reported as twice the standard error
from linear least-squares analysis of plots of kdeccay values vs [Q]
according to eq 3. b The average and standard deviation of two
independent determinations.
rate constants of the resulting Lewis acid-base complexes. The
latter are reported as the average of 9-13 independent determi-
nations in each case.
Experiments with 2 and PrS carried out on longer time scales
revealed that the long-lived absorptions formed in the reaction of
SiPh2 with PrS are also transient, decaying over roughly 100 μs
with approximate second-order kinetics (2k/ε290-nm = (2.3 (
0.2) ꢀ 106 cm s-1; see Figure S11). Addition of submillimolar
amounts of MeOH (to hexanes solutions of 2 containing 20 mM
PrS) resolved the absorption into two components, consisting of
a first-order decay superimposed on a weak, nondecaying resi-
dual absorption. The decay rate of the main component in-
creased with increasing methanol concentration, and a plot of
kdecay versus [MeOH] was linear with slope kMeOH = (8.5 ( 0.9)
ꢀ 108 M-1 s-1, the average of two independent determinations.
Figure 4a shows the plot of kdecay versus [MeOH] that was
obtained in one experiment. Similar results were obtained upon
addition of tert-butanol (t-BuOH; 0.2-1.5 mM), acetic acid
(AcOH; 0.05-0.5 mM), or n-butyl amine (n-BuNH2; 0.1-
1.5 mM). The quenching plots are shown in Figure S12, while
the corresponding second-order rate constants are listed in
Table 2. The weak residual absorption that was observed
in these experiments may be due to direct photolysis of PrS,
Product Studies. A series of steady-state photolysis experi-
ments were carried out in order to determine the ultimate pro-
ducts of the reactions of SiMe2 and SiPh2 with CHO, PrO, and
PrS, and their chemical yields relative to consumed silylene
precursor and/or substrate. The experiments were carried out
in quartz NMR tubes containing deoxygenated cyclohexane-d12
solutions of 1 or 2 (0.05 M), the substrate (0.1-0.2 M), and
internal standard (dioxane and/or Si2Me6; ca. 0.01 M), both in
the absence and presence of added D3 (0.2 M) as a scavenger for
the expected silanones and silanethiones. The solutions were
irradiated with 254 nm light, following the course of the
photolyses by 1H NMR spectroscopy and/or GC/MS over the
0 to ca. 10% conversion range in silylene precursor. Products
were identified in the crude photolysis mixtures by comparison of
their (1H and 29Si) NMR and mass spectral characteristics to
literature data or to those of independently prepared authentic
samples. Product yields were determined relative to consumed
precursor and/or substrate from the relative slopes of concentra-
tion versus time plots, constructed from the 1H NMR integrals
for as many of the components of the mixtures as was practically
possible. This was confined to the alkene relative to consumed
substrate in the experiments with 1 because the overlap of the 1H
NMR resonances due to 1 and its photoproducts make it impos-
sible to follow the consumption of the precursor by this analytical
method; more complete quantitation of the photolyses was possible
which absorbs weakly at the excitation wavelength (ε248-nm
=
23 M-1 cm-1); laser photolysis of a 10 mM hexanes solution of
PrS gives rise to similarly weak, nondecaying absorptions in the
270-290 nm range of the spectrum, which is consistent with this
interpretation. It should be noted that the relative concentrations
of PrS and secondary substrate in these experiments are such that
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in the experiments with 2. The results of the experiments with 1
and CHO were analogous to those reported previously by Goure
and Barton in their early study of the photolysis of 1 in the
presence of cyclooctene oxide,10 and are described in the
Supporting Information. The results of the other experiments
are summarized below, while representative NMR spectra and
concentration versus time plots are shown in the Supporting
Information.
the anticipated product of reaction of Ph2SidS with D3,78
could be obtained.
Photolysis of 1 in the presence of propylene sulfide (PrS)
and D3 afforded as the major products pentamethylcyclopen-
tasilane (5), propene (6), and compound 7, the product of
formal insertion of Me2SidS into D3,73,74 along with smaller
amounts of compound 8 from competing reaction of SiMe2
with D375 (eq 4 and Figures S15, S16). A variety of other minor
products containing SiMe2 moieties were also formed in the
photolysis (see Figure S16), but none of them could be
identified.
Finally, a solution of 2 (0.05 M), PrS (0.08 M), and MeOH
(0.005 M) in C6D12 was photolyzed in an attempt to trap the
1
silanethione as the methanol adduct. Indeed, the H NMR
spectra of the photolyzed mixture showed evidence for the
formation of two methoxyl-containing products (eq 7; Figure
S22, S23). The minor of these was identified as methoxydiphe-
nylsilane (14) by spiking the mixture with an authentic sample,
while the major one was identified as methoxydiphenylsilanethiol
(15) on the basis of NMR and mass spectral data. The char-
acteristic aromatic resonances of the silanethione dimer (12)
could not be detected in this experiment.
Photolysis of 2 in the presence of PrO and D3 afforded 6,
disilacyclopentane 9,64 and cyclotetrasiloxane 1076 as the major
products (eq 5; Figures S17, S18). Compound 11, arising
presumably from insertion of SiPh2 into a Si-O bond of D3,
was also detected in trace amounts by GC/MS and was tenta-
tively assigned on the basis of its mass spectrum. The aromatic
1
region of the H NMR spectrum of the photolysate exhibited
broad underlying absorptions, suggesting the formation of oligo-
or polymeric material. Support for the structural assignment of
11 was obtained by photolyzing a deoxygenated 0.05 M hexanes
solution of 2 containing only D3 (0.2 M), which showed the same
compound to be formed as the major SiPh2-containing product.
Photolysis of 2 and PrO in the absence of D3 led to the formation
of 6 (51 ( 2%), 9 (92 ( 3%), and significantly greater amounts
of oligo- or polymeric material than were obtained in the
presence of D3 (Figure S19).
Computational Studies. The thermochemical parameters
associated with the formation and first step in the decomposition
of the complexes of SiH2 and SiMe2 with oxirane and thiirane
were modeled using density functional theory, employing the
B3LYP exchange-correlation functional79,80 in conjunction with
the standard 6-311þG(d,p) basis set. Starting structures for
geometry-optimization of the complexes and the transition states
for their decomposition were constructed using key geometric
parameters taken from the earlier ab initio study of the SiH2-
oxirane and -thiirane systems by Apeloig and Sklenak.16 The
transition states were initially optimized using the restricted
procedure, and then reoptimized using the unrestricted proce-
dure with HOMO-LUMO mixing in the initial guess leading to
unrestricted wave functions. This had, at most, only very minor
effects on the energies and geometries of the transition structures
compared to the results obtained using the restricted procedure
without HOMO-LUMO mixing. Intrinsic Reaction Coordinate
(IRC) calculations starting at the saddle points were carried out
in the reverse direction to verify the connections between the
transition structures and the complexes, and in the forward
direction to establish starting structures for the final geometry-
optimizations of the primary decomposition products; these
calculations were carried out using the unrestricted procedure
with HOMO-LUMO mixing in all cases, as were the final
geometry optimizations and frequencies calculations for the
biradicals. All minima and transition states were characterized
by their vibrational frequencies. The reported thermodynamic
Photolysis of 2 in the presence of PrS afforded 6, 9 and
tetraphenylcyclodisilthiane (12) as the major products (eq 6
and Figure S20a). Compound 12 was identified by spiking the
photolysis mixture with an authentic sample.77 A similar
mixture of products was obtained upon photolysis of 2 and
PrS in the presence of D3 (Figures S20b, S21); while trace
amounts of compound 11 were also detected in the photolysate
by GC/MS, no evidence for the formation of compound 13,
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Figure 5. Calculated (B3LYP/6-311þG(d,p)) structures and selected bond distances (in Å) and angles (in degrees) of the anti and gauche conformers
of the (A) SiMe2-oxirane and (B) SiMe2-thiirane complexes (“CX”), the transition states for C-X bond cleavage of the gauche conformers (“TSX”),
and the first-formed products of the reaction; “β” denotes the C3-Si-X-C2 dihedral angle. The methyl-hydrogens have been omitted from the
structures for clarity.
Table 3. Calculated (B3LYP/6-311þG(d,p)//B3LYP/6-311þG(d,p)) Standard Enthalpies (ΔH298) and Free Energies (ΔG298
)
of Stationary Points on the Potential Energy Surfaces for Reaction of SiH2 and SiMe2 with Oxirane and Thiirane, in kcal mol-1
Relative to the Energies of the Reactive Gauche Conformers of the Corresponding Silylene-Substrate Complexes
SiH2 þ oxirane
SiMe2 þ oxirane
SiH2 þ thiirane
SiMe2 þ thiirane
ΔH298
ΔG298
ΔH298
ΔG298
ΔH298
ΔG298
ΔH298
ΔG298
Free Reactants
anti-Complex
Transition State
Biradical
þ14.8
þ0.6
þ4.4
þ0.1
þ7.2
-1.6
-4.0
-1.6
þ18.0
-1.2
þ6.5
-
þ7.8
-1.2
þ5.9
-
þ8.0
-2.0
þ3.2
-
-3.4
-2.2
þ3.5
-
þ8.8
þ9.2
þ5.6
-18.4
-57.2
þ6.6
-19.5
-67.8
-11.1
-42.7
-13.0
-54.2
R2Si=X þ C2H4
-30.2
-41.4
-44.7
-57.0
data (vide infra) are given at 298.15 K from unscaled vibrational
frequencies in the harmonic approximation.
SiMe2-complexes with oxirane and thiirane, respectively, at the
B3LYP/6-311þG(d,p) level of theory; the gauche and anti
conformers of the SiH2-oxirane complex are roughly equal in
energy. In any event, in each case the two conformers can be
expected to be in rapid equilibrium at 25 ꢀC, with only small
barriers for their interconversion. Table 3 lists the B3LYP/6-
311þG(d,p) standard enthalpies and free energies of the various
species investigated, in each case relative to that of the reactive
gauche conformer of the silylene-substrate complex.
Geometry optimization of the H2SiOCH2CH2 biradical at the
UMP2(full)/6-31G(d,p) and UQCISD(full)/6-31G(d,p) levels
of theory with HOMO-LUMO mixing afforded significantly
different structures and energies for the species than those
obtained using the restricted procedure without orbital mixing.16
Both levels of theory afforded a transoid biradical structure with a
Si-O-C-C torsional angle of ca. 164ꢀ, with total electronic
energies of ΔE = -5.2 kcal mol-1 (UMP2) and -6.5 kcal mol-1
(UQCISD) relative to the gauche SiH2-oxirane complex. These
values, along with the zpe and thermal corrections obtained from
Figure 5 shows the calculated (B3LYP/6-311þG(d,p)) struc-
tures of the Lewis acid-base complexes of SiMe2 with oxirane
and thiirane, the transition states for C-X bond cleavage from
the reactive gauche conformers of the complexes, and the
primary products of these reactions.
Dissociation of the gauche SiR2-oxirane complexes led to the
corresponding R2SiOCH2CH2 biradicals, while dissociation of
the SiR2-thiirane complexes led directly to ethylene and the
corresponding silanethione, in general agreement with Apeloig’s
and Sklenak’s ab initio results for the SiH2-oxirane and -
thiirane systems.16 As expected, two low-energy conformers were
found for each of the complexes, one with the silylene substit-
uents oriented anti to the oxirane or thiirane ring and the other
with the substituents in a gauche arrangement. In all cases except
for the SiH2-oxirane complex, the anti conformer is lower in
free energy, by an amount ranging from 1.2 kcal mol-1 for
the SiH2-thiirane complex to 1.6 and 2.2 kcal mol-1 for the
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a frequencies calculation at the UMP2/6-31G(d,p) level, af-
forded standard free energy values of ΔG298 = -8.6 and -9.9
kcal mol-1, respectively. Full details of these calculations are
reported in the Supporting Information. In contrast, optimiza-
tion using the restricted procedure affords a gauche geometry in
both cases (—Si-O-C-C ≈ 120ꢀ (MP2) and 94ꢀ (QCISD)) and
corresponding energies of ΔE = -0.6 and þ9.1 kcal mol-1 at the
MP2 and QCISD levels, as reported earlier.16 As was found with
the B3LYP calculations, use of the unrestricted procedure had no
effect on the structure or energy of the transition state linking the
complex and the biradical, compared to those obtained using the
restricted procedure. Further exploration of the model chemistry
of the silylene-oxirane derived biradicals is beyond the scope of
the present work.
DFT calculations were also carried out for the SiH2- and
SiMe2-oxirane systems at the (U)B3LYP/6-311G(d) level of
theory (Table S1); the geometries and ΔEo-values of the gauche
complexes, transition states, and final products (R2SidO þ
ethylene) relative to the free reactants are nearly identical in
every case to those reported previously by Su.18 These calcula-
tions afforded more negative complexation free energies (by ca.
2 kcal mol-1) and more positive free energies for the transition
states (by ca. 0.5 kcal mol-1), biradicals (by 1 to 2 kcal mol-1),
and final products (by 0.5-2.4 kcal mol-1) compared to the
corresponding calculations employing the 6-311þG(d,p) basis
set. Geometry optimizations and frequencies calculations were
also carried out at the B3LYP/6-311G(d) level of theory for
SiPh2, Ph2SidO, Me2SidS, and Ph2SidS (Table S2), affording
overall enthalpies of reaction of -61.8 kcal mol-1 for SiPh2 þ
oxirane, -51.5 kcal mol-1 for SiMe2 þ thiirane, and -49.4 kcal
mol-1 for SiPh2 þ thiirane.
presence of additional reaction pathways involving the silylenes
and the oxiranes that we have been unable to characterize
completely.
The study also provides the first experimental demonstration
of the homologous reactions of the two silylenes with thiiranes,
which afford products consistent with the formation of the
corresponding transient silanethiones (Me2SidS and Ph2SidS)
along with the corresponding alkene, in chemical yields of 33-
65%. Various aspects of the reactivity of Me2SidS have been
reported previously under high-temperature thermolytic
conditions,73,74,78 where the species dimerizes to the correspond-
ing cyclodisilthiane (16) or can be trapped by added D3 as the
insertion product (7). Our experiments indicate that D3 is also a
reasonably effective trapping agent for Me2SidS at ambient
temperatures in solution, though the yield of 7 in this experiment
was only half that of the propene produced; we could find no
evidence for the formation of the silanethione dimer (16) by 1H
NMR spectroscopy78 or GC/MS. In contrast, the oligosiloxane is
a completely ineffective trapping agent for the diphenyl analog
(Ph2SidS), whose main fate is dimerization to the correspond-
ing cyclodisilthiane (12) both in the absence and presence of 0.2
M D3. As expected, however,42,43 the silanethione can be trapped
efficiently by MeOH to afford the corresponding methoxysila-
nethiol (15), even at quite low concentrations (∼5 mM) of
added alcohol. As with the photolyses in the presence of PrO and
CHO, the chemical yields of alkene and silanethione-derived
products obtained in the experiments with 1 and 2 in the
presence of PrS are significantly less than 100%, and we were
similarly unsuccessful in detecting the bulk of the missing
material.
Time-dependent calculations, again at the B3LYP/6-311G(d)
level of theory, afforded predicted UV-vis absorption maxima of
530 nm (f 0.026; n f p) and 309 nm (f 0.244; π f π*) for SiPh2,
317 nm (f 0.001; n f π*) and 254 nm (f 0.070; π f π*) for
Ph2SidO, and 421 nm (f 0.002; n f π*), 305 (f 0.160; π f π*),
275 nm (f 0.160; π f π*), and 269 nm (f 0.086; π f π*) for
Ph2SidS. The three short-wavelength (π f π*) transitions in
the calculated spectrum of Ph2SidS overlap to form a single,
slightly asymmetric band centered at λmax ≈ 280 nm in the
simulated spectrum, in excellent agreement with that assigned to
the molecule in our laser photolysis experiments (vide supra).
The calculated spectrum of SiPh2 is also in outstanding agree-
ment with the experimental (solution phase) spectrum.64
Goure and Barton also reported (isolated) material balances in
the range of 40-50% in their study of the reaction of thermally-
and photochemically generated SiMe2 with cyclooctene oxide.10
In the present cases, several minor molecular products were
evident in each case in the GC/MS chromatograms of the crude
photolyzates, but in no case did any of them stand out as being
particularly prominent. We suspect that polymerization pro-
cesses account for at least some of the deficiency in the material
balance, though definitive signs of this were observed (by NMR)
in only two instances: the photolysis of 2 in the presence of PrO
with and without added D3. Interestingly, comparison of the
NMR spectra of these photolyzates to those of an authentic
sample of the anticipated product of Ph2SidO oligomerization,
hexaphenylcyclotrisiloxane (17), demonstrated that it was not
formed in appreciable yield under the conditions of these
experiments.
The laser photolysis experiments show conclusively that the
reactions of both SiMe2 and SiPh2 with CHO, PrO, and PrS
proceed via multistep mechanisms involving the initial formation
of the corresponding Lewis acid-base complexes, which have
been detected as discrete intermediates that are formed on the
same time scale as the decay of the free silylenes and subse-
quently decay with first-order kinetics. The complexes have been
identified on the basis of their UV absorption spectra, which are
indistinguishable from those of the corresponding silylene-
THF65,71,72 and silylene-THT complexes (Figures S4, S10
’ DISCUSSION
The product studies demonstrate that SiPh2 reacts with simple
oxirane derivatives in solution to afford the corresponding alkene
and products consistent with the formation of diphenylsilanone
(Ph2SidO), in an analogous fashion to SiMe2 under similar
conditions (vide supra and ref 10). The relative yields of alkene
and D3-trapping products in the photolyses of 1 and 2 in the
presence of the oxiranes and D3 are close to 1:1 in all cases,
indicating that both transient silanones are trapped by the
oligosiloxane with close to 100% efficiency under the conditions
of our experiments. It is also apparent that the reactions of SiMe2
and SiPh2 with CHO and PrO all proceed with similar efficien-
cies under comparable experimental conditions, in terms of the
chemical yields of alkene and silanone-derived products relative
to consumed silylene precursor and/or oxirane derivative. These
efficiencies are considerably less than 100%, indicating the
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and ref 72) in each case. The absolute rate constants for
formation of the complexes, as given by the rate constants for
quenching of the free silylenes by the substrates, all lie within a
factor of 2 of the diffusional rate constant in hexanes at 25 ꢀC;
they are consistently close to a factor of 2 larger for SiMe2 than
for SiPh2, as usual,65 but do not vary with the identity of the O- or
S-donor. This indicates that complexation is an essentially bar-
rierless process in all cases. The comparison of the optical yields
of the SiPh2-CHO and SiPh2-THF complexes under equiva-
lent conditions (vide supra) allows the conclusion that at subs-
trate concentrations of 10 mM, complexation is the exclusive
reaction of the free silylene with CHO. This is an important
point, as it indicates that the deficiency in material balance that
characterizes the steady state photolysis experiments must be due
to competing decomposition channels of either the complex or a
second reactive intermediate that is formed from the complex,
and not to a second primary reactive process involving SiPh2 and
CHO that competes with complexation and which leads to
products other than the alkene and silanone. It seems reasonable
to assume that this is also true at the higher substrate concentra-
tions employed in the product studies, and that it applies as well
to the reactions of the other substrates with both SiPh2 and
SiMe2; lifetime quenching of these silylenes by CHO, PrO and
PrS proceeds with essentially the same rate constants as those for
quenching by the corresponding “non-reactive” ether and sulfide
substrates, THF and THT, respectively, whose interactions with
SiMe2 and SiPh2 are confined to Lewis acid-base complexation
and no further reaction involving the substrate ensues.
The absorbance versus time behavior observed for the two
silylenes in the presence of submillimolar concentrations of each
of these substrates is consistent with equilibrium constants for
complexation in excess of Kcplx > 30 000 M-1 in all cases, which
corresponds to a standard free energy value of ΔGꢀ e -4.2 kcal
mol-1, where the standard state is the gas phase at 1 atm pressure
and 298.15 K. Calculated free energies of complexation of SiH2
with oxirane and THF19 lead to the expectation of somewhat
larger equilibrium constants for complexation with THF and
THT than with the oxiranes and PrS, respectively, but they are all
larger than we can measure by our methods so the prediction
cannot be verified experimentally, at least at room temperature.
The B3LYP calculations predict a slightly more favorable free
energy of complexation for SiMe2 with thiirane than with
oxirane, though they underestimate the stabilities of the com-
plexes by at least 5-7 kcal mol-1 relative to our experimentally
estimated upper limit of ΔGꢀ e -4.2 kcal mol-1. The experi-
mental data afford no indication of the relative Lewis acidities of
SiMe2 and SiPh2 toward the O- or S-donors; we note, however,
that the equilibrium constants for complexation of the corre-
sponding germylene derivatives (GeMe2 and GePh2) with THF
and alcohols indicate that the phenylated derivative is signifi-
cantly more acidic than the methylated one, the values being
consistent with differences in free energy of complexation on the
order of ca. 0.5 kcal mol-1 in hexanes at 25 ꢀC.81
concomitant formation of the corresponding disilenes, indicating
that they each possess one or more low energy reaction channels
that are absent in the complexes with THF and THT. The first-
order rate coefficients for decay of the complexes are in each case
independent of the concentration of the corresponding oxirane
or thiirane, from which it can be concluded that they reflect the
rates of unimolecular C-X bond cleavage, leading ultimately to
the formation of alkene and silanone or silanethione in addition
to other (unknown) products. As expected, there is no catalysis
or reaction of the complexes that involves a second molecule of
the substrate. The kinetic data, which are summarized in Table 1,
reveal several distinct trends.
First, the kdecay-values for the oxirane complexes do not vary
significantly as a function of substitution at silicon, indicating that
differences in the steric and electronic properties of the methyl
and phenyl substituents have a minimal impact on the rate cons-
tant for reaction of the silylene-oxirane complexes; on the other
hand, dissociation of the two silylene-CHO complexes is
roughly 35% faster than that of the silylene-PrO complexes.
The rate constants correspond to effective free energies of activa-
tion (ΔG‡) of 8.4-8.5 kcal mol-1 for the reaction(s) responsible
for their decay. The calculated free energy of activation for C-O
bond cleavage in the SiMe2-oxirane complex, relative to the
lower energy (anti-) conformer, is 8.1 kcal mol-1 (see Table 3).
Methyl-substitution in the ring may lead to a slight lowering of
the activation energy for C-O bond cleavage, but nevertheless,
the calculated value for the parent oxirane-SiMe2 complex is in
quite reasonable agreement with the experimental values for the
complexes with PrO and CHO in hexanes solution. Extrapola-
tion of these considerations to the parent silylene suggests that
the SiH2-oxirane complex should possess a lifetime on the order
of a few microseconds in the gas phase under conditions of
complete vibrational relaxation.
In agreement with the ab initio calculations on the SiH2-
oxirane system (vide supra and ref 16), the present DFT
calculations indicate that cleavage of the silylene-oxirane com-
plexes proceeds in stepwise fashion from the gauche-conformer,
yielding the corresponding (singlet) R2SiOCH2CH2 biradical in
the first step. The C-O bond that cleaves in this initial step is the
one that bisects the silylene substituents in the gauche-complex,
and the approach to the transition state involves only slight
twisting about the developing Si-O bond; the main changes in
geometry are a shortening of the Si-O bond distance, a
substantial lengthening of the O-C1 distance, and slight short-
ening of the O-C2 and C1-C2 bond distances (C1 is defined as
the carbon of the C-O bond that is cleaved; see Figure 5).
Formation of the H2SiOCH2CH2 biradical is predicted to be
exergonic by ca. 13 kcal mol-1 at the UB3LYP/6-311þG(d,p)
level of theory, in quite reasonable agreement with the value of ca.
10 kcal mol-1 that is predicted at the UQCISD/6-31G(d,p)
level. An even greater exergonicity is predicted for the formation
of the Me2SiOCH2CH2 biradical—ca. 18 kcal mol-1 relative to
the anti conformer of the complex—and a lower free energy of
activation for its formation by ca. 2.5 kcal mol-1. The theoretical
prediction of exergonic biradical formation is consistent with the
experimental rate constants for decomposition of the SiMe2-
and SiPh2-PrO and CHO complexes. On the basis of silyl
radical bond dissociation energies, one would expect phenyl
substitution to lead to slightly greater stabilization of the SiPh2-
derived biradicals compared to those derived from SiMe2;82 the
effect is small, but should nonetheless result in a faster rate of
decomposition in the SiPh2-complexes if the process is endergonic
In keeping with their role as the first-formed intermediates in
the O- and S-transfer reactions, the kinetic behavior exhibited by
the silylene-PrO, -CHO, and -PrS complexes is distinctly
different than that of the corresponding nonreactive complexes
with THF and THT. The latter are quite long-lived, decaying
over several tens of microseconds with second-order kinetics and
the concomitant formation of the corresponding silylene dimers,
Si2Me4 and Si2Ph4. On the other hand, the oxirane- and thiirane-
complexes decay with clean first-order kinetics, without the
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Scheme 1
well with that of the SiMe2-THT complex (Figure S4b). A
better spectrum has been obtained for the longer-lived SiPh2-
PrS complex (dashed line, Figure 3), which compares very well
with that of the THT-derived species (Figure S10). The SiPh2-
PrS complex decays with a first-order rate constant of kdecay
≈
2 ꢀ 107 s-1, corresponding to a free energy of activation of ΔG‡
≈ 7.4 kcal mol-1 for the reaction(s) responsible for its decay. A
similar treatment of the data for the SiMe2-PrS complex leads to
a corresponding estimated upper limit of ΔG‡ e 7.1 kcal mol-1
.
The calculated value of ΔG‡ = 5.7 kcal mol-1 for reaction of the
SiMe2-thiirane complex is consistent with this upper limit.
The variation in the decay rate constants for decay of the
SiMe2- and SiPh2-PrS complexes contrasts the behavior ob-
served for the silylene-oxirane complexes and suggests a
different mechanism for their reaction, as indeed the calculations
predict. The consistently faster rates of decay of the PrS com-
plexes compared to the PrO- and CHO-complexes are also
consistent with the results of the DFT calculations, which predict
lower free energy barriers for reaction of both the SiH2- and
SiMe2-thiirane complexes compared to those of the corre-
sponding complexes with oxirane. The difference in the free
energies of activation for decay of the SiMe2- and SiPh2-PrS
complexes is consistent with a greater degree of exergonicity in
the dissociation of the SiMe2-complex compared to the pheny-
lated system. Indeed, the calculated (B3LYP/6-311G(d)) values
of the overall free energies for formation of ethylene and
Me2SidS and Ph2SidS from thiirane and the corresponding
silylenes are -52.4 and -49.5 kcal mol-1, respectively (see
Table S2). Steric factors may also contribute to some extent, as
the calculations indicate that compared to the situation with the
silylene-oxirane systems, a greater degree of twisting about the
developing Si-S bond is required to achieve the transition state
from the gauche conformer of the SiR2-thiirane complex. This
results in a transition state structure in which one of the silylene
substituents is in a near-eclipsed arrangement with one of the C-
S bonds in the thiirane ring.
and the Hammond Postulate applies. The fact that essentially no
difference in rate constant is observed suggests that biradical
character is not substantially developed in the transition state for
C-O bond cleavage, which is consistent with an exergonic
process.
The somewhat faster rates of cleavage of the silylene-CHO
complexes compared to those of the complexes with PrO is also
consistent with the stepwise cleavage mechanism, assuming that
the O-C bond to the more substituted carbon in PrO is the one
preferentially cleaved in the initial step; all else being equal, one
would expect cleavage of the complexes with the disubstituted
oxirane derivative (CHO) to proceed with twice the rate con-
stant exhibited by the PrO-complexes, based simply on statistical
factors. The fact that the CHO-complexes react less than twice
faster than the PrO-complexes could indicate that the enthalpic
barrier for O-C bond cleavage is slightly higher for the disubs-
tituted system, perhaps due to steric or strain-related factors.
Biradical involvement could also explain the low material
balances observed in the steady-state photolysis experiments,
since there are in principle at least two other modes of reaction
possible for these species in addition to (the observed) β-
cleavage to yield silanone and alkene: cyclization and 1,3-H
migration to form the corresponding siloxetane and silyl enol
ether, respectively (Scheme 1). Formation of these other pro-
ducts would in principle be possible if the dynamics of the
biradical are appropriate, or if there is communication with the
triplet state, which could lengthen the lifetime of the biradical to
the point where conformational interconversion might be pos-
sible. Of the two additional potential products mentioned above,
the silyl enol ethers should be most easily identifiable in the NMR
spectra of the photolyzates, and we saw no evidence that they
were formed. The possible competing formation of the corre-
sponding siloxetanes is more difficult to rule out, though again we
observed nothing that definitively suggested their presence in the
NMR spectra. Neither product type would be expected to be
detectable by GC/MS, though both should be sufficiently stable
at room temperature in solution to be detected by NMR, if they
were formed. While the present calculations suggest that the
biradical is formed in a conformation that should favor β-clea-
vage over the other possible reaction pathways, higher levels of
theory will be required in order to model its reactivity in a
reasonably reliable way.
Finally, the present study has allowed the transient sila-
nethione Ph2SidS to be detected directly and some aspects of
its reactivity in solution to be studied for the first time. The
species exhibits an intense (π f π*) absorption band and
(apparently) a second very weak band centered at λmax ≈ 275
and 460 nm, respectively, in reasonable agreement with the
theoretically predicted spectrum. If the 460 nm band is indeed
due to the n f π* absorption in Ph2SidS, it is red-shifted by ca.
65 nm relative to that reported by Tokitoh and co-workers for the
sterically stabilized diarylsilanethione 18 (λmax = 396 nm,
ε = 100).41 The parent diarylsilanethione undergoes rapid (2k/
ε290-nm = (2.3 ( 0.2) ꢀ 106 cm s-1) head-to-tail dimerization as
its primary mode of decay in the absence of reactive substrates;
the rate coefficient corresponds to a value of kdim ≈ 1 ꢀ 1010
M-1 -1 assuming a value of ca. 10 000 dm mol1 cm-1 for the
s
extinction coefficient at 290 nm. As expected, based on the
behavior of other silanethiones that have been reported,37,42,43
the species reacts rapidly with nucleophilic substrates such as
aliphatic alcohols, acetic acid, and n-butyl amine, with rate
constants that vary in more or less the expected manner with
variations in substrate acidity, nucleophilicity, and steric bulk
throughout the limited series of substrates that have been
studied. One interesting point of comparison that can be made
is with 1,1-diphenylsilene (Ph2Si=CH2), which reacts with the
same substrates in hydrocarbon solvents with rate constants that
are of similar magnitudes to those obtained for the silanethione
Dissociation of the silylene-PrS complexes proceeds signifi-
cantly more rapidly than in the corresponding silylene-PrO
complexes, and there is a distinct effect of silyl substituent on the
rate constant. The decay of the SiMe2-PrS complex is too fast
to be resolved from our laser pulse, so only a lower limit of kdecay
g 4 ꢀ 107 s-1 could be obtained in this case and the quality of
the spectrum obtained for the species (Figure S3b) is consequen-
tly rather poor; nevertheless, the spectrum compares reasonably
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Journal of the American Chemical Society
ARTICLE
under similar conditions, and which vary in a similar way as a
function of substrate structure.83,84 Both species also undergo
head-to-tail dimerization at close to the diffusion-controlled rates
in the absence of added substrates.85,86
’ ACKNOWLEDGMENT
We thank the Natural Sciences and Engineering Research
Council of Canada for financial support for this work, and Drs. N.
H. Werstiuk and P. Ayers for helpful advice and discussion. Part
of this work was made possible by the facilities of the Shared
Hierarchical Academic Research Computing Network
(SHARCNET:www.sharcnet.ca) and Compute/Calcul Canada.
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Details pertaining to the synthesis and characterization of the silylene
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’ ASSOCIATED CONTENT
S
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b
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leigh@mcmaster.ca
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