Comparison of the reactivities of dimethylsilylene (SiMe2) and diphenylsilylene (SiPh2) in solution by laser flash photolysis methods

Article abstract of DOI:10.1021/om700659d

Laser flash photolysis studies have been carried out with the goal of comparing the reactivities of dimethyl- and diphenylsilylene (SiMe₂ and SiPh₂, respectively) toward a comprehensive series of representative substrates in hexane solution under common conditions. The silylenes are generated using dodecamethylcyclohexasilane (1) and 1,1,3,3-tetramethyl-2,2-diphenyl-1,2,3-trisilacyclohexane (9), respectively, as photochemical precursors. The reactions examined include O-H, O-Si, and M-H (M = Si, Ge, Sn) bond insertions, Lewis acid-base complexation with tetrahydrofuran, (1+2) cycioaddition to C=C and C=C multiple bonds, chlorine atom abstraction from CCl₄, and reaction with 'molecular oxygen; the kinetic data for SiPh₂ are accompanied by product studies in most cases. Further insight into the mechanisms of these reactions is provided by the identification of reaction intermediates and/or transient products in some cases; notable examples include the reactions of SiPh₂ with methoxytrimethylsilane, carbon tetrachloride, and molecular oxygen. With several of the substrates that were studied, comparison of the kinetic data for SiPh₂ to previously reported rate constants for reaction of dimesitylsilylene in cyclohexane allows an assessment of the role of steric effects in affecting silylene reactivity. Rate constants could also be determined for quenching of tetramethyldisilene (Si₂Me₄) with molecular oxygen, CCl₄, and several other reagents that were studied.

Full text of DOI:10.1021/om700659d

Organometallics 2007, 26, 6277-6289
6277
Comparison of the Reactivities of Dimethylsilylene (SiMe2) and
Diphenylsilylene (SiPh2) in Solution by Laser Flash Photolysis
Methods
Andrey G. Moiseev and William J. Leigh*
Department of Chemistry, McMaster UniVersity, 1280 Main Street West,
Hamilton, Ontario, Canada L8S 4M1
ReceiVed July 1, 2007
Laser flash photolysis studies have been carried out with the goal of comparing the reactivities of
dimethyl- and diphenylsilylene (SiMe2 and SiPh2, respectively) toward a comprehensive series of
representative substrates in hexane solution under common conditions. The silylenes are generated using
dodecamethylcyclohexasilane (1) and 1,1,3,3-tetramethyl-2,2-diphenyl-1,2,3-trisilacyclohexane (9), re-
spectively, as photochemical precursors. The reactions examined include O-H, O-Si, and M-H (M )
Si, Ge, Sn) bond insertions, Lewis acid-base complexation with tetrahydrofuran, (1+2) cycloaddition
to CdC and CtC multiple bonds, chlorine atom abstraction from CCl4, and reaction with molecular
oxygen; the kinetic data for SiPh2 are accompanied by product studies in most cases. Further insight into
the mechanisms of these reactions is provided by the identification of reaction intermediates and/or transient
products in some cases; notable examples include the reactions of SiPh2 with methoxytrimethylsilane,
carbon tetrachloride, and molecular oxygen. With several of the substrates that were studied, comparison
of the kinetic data for SiPh2 to previously reported rate constants for reaction of dimesitylsilylene in
cyclohexane allows an assessment of the role of steric effects in affecting silylene reactivity. Rate constants
could also be determined for quenching of tetramethyldisilene (Si2Me4) with molecular oxygen, CCl4,
and several other reagents that were studied.
precursors to the silylenes of interest. The first attempts to study
reactive silylenes by laser flash photolysis methods in solution
employed the acyclic trisilanes 312,13 and 414 as precursors to
the phenylated silylenes SiMePh and SiPh2, respectively, as it
was well known that photolysis of both compounds (as well as
1 and 2 and a number of other aryltrisilane derivatives) in low-
temperature glasses afforded the corresponding silylenes cleanly,
allowing their UV/vis absorption spectra to be characterized.15
These compounds were also well known to afford the corre-
sponding silylenes in high yields upon photolysis in solution at
room temperature. Unfortunately, they also undergo competing
photorearrangements under the latter conditions, producing
transient silene derivatives (e.g., 5; eq 1) whose strong UV/vis
absorptions occur in a similar spectral range as the more weakly
absorbing silylenes, obscuring the transient spectra to the point
where the silylenes of primary interest were missed altogether.
Introduction
Despite great experimental and theoretical interest over the
past several decades in the chemistry of divalent organosilicon
compounds (silylenes),1 relatively little is known of their reaction
kinetics in solution. The parent molecule (SiH2) and several
simple substituted derivatives SiXY (X,Y ) H, Me, or halogen)
have received extensive attention in gas-phase kinetic studies,2-5
and the results of this work have served to provide a useful
link between theoretical and experimental studies of silylene
reactivity in the gas phase. Until very recently however,6 only
dimethyl- and dimesitylsilylene (SiMe2 and SiMes2, respec-
tively) had been successfully detected and studied in solution
by time-resolved spectroscopic methods,7-11 using the well-
known oligosilane derivatives 1 and 2 as photochemical
* Corresponding author. E-mail: leigh@mcmaster.ca.
(1) (a) Gaspar, P. P. In ReactiVe Intermediates, Vol. 3; Jones, M., Jr.,
Moss, R. A., Eds.; John Wiley & Sons: New York, 1985; pp 333-427;
Vol. 2, 1981; pp 335-385; Vol. 1, 1978; pp 229-277. (b) Gaspar, P. P.;
West, R. In The Chemistry of Organic Silicon Compounds, Vol. 2;
Rappoport, Z., Apeloig, Y., Eds.; John Wiley and Sons: New York, 1998;
pp 2463-2568. (c) Tokitoh, N.; Ando, W. In ReactiVe Intermediate
Chemistry; Moss, R. A., Platz, M. S., Jones, M., Jr., Eds.; John Wiley &
Sons: New York, 2004; pp 651-715.
(2) Safarik, I.; Sandhu, V.; Lown, E. M.; Strausz, O. P.; Bell, T. N. Res.
Chem. Intermed. 1990, 14, 105.
(3) Jasinski, J. M.; Becerra, R.; Walsh, R. Chem. ReV. 1995, 95, 1203.
(4) Becerra, R.; Walsh, R. Res. Chem. Kinet. 1995, 3, 263.
(5) Becerra, R.; Walsh, R. Phys. Chem. Chem. Phys. 2007, 9, 2817.
(6) Moiseev, A. G.; Leigh, W. J. J. Am. Chem. Soc. 2006, 128, 14442.
(7) Levin, G.; Das, P. K.; Lee, C. L. Organometallics 1988, 7, 1231.
(8) Levin, G.; Das, P. K.; Bilgrien, C.; Lee, C. L. Organometallics 1989,
8, 1206.
(9) Shizuka, H.; Tanaka, H.; Tonokura, K.; Murata, K.; Hiratsuka, H.;
Ohshita, J.; Ishikawa, M. Chem. Phys. Lett. 1988, 143, 225.
(10) Yamaji, M.; Hamanishi, K.; Takahashi, T.; Shizuka, H. J. Photo-
chem. Photobiol. A: Chem. 1994, 81, 1.
(11) Conlin, R. T.; Netto-Ferreira, J. C.; Zhang, S.; Scaiano, J. C.
Organometallics 1990, 9, 1332.
(12) Gaspar, P. P.; Boo, B. H.; Chari, S.; Ghosh, A. K.; Holten, D.;
Kirmaier, C.; Konieczny, S. Chem. Phys. Lett. 1984, 105, 153.
(13) Gaspar, P. P.; Holten, D.; Konieczny, S.; Corey, J. Y. Acc. Chem.
Res. 1987, 20, 329.
(14) Konieczny, S.; Jacobs, S. J.; Braddock Wilking, J. K.; Gaspar, P.
P. J. Organomet. Chem. 1988, 341, C17.
(15) Michalczyk, M. J.; Fink, M. J.; De Young, D. J.; Carlson, C. W.;
Welsh, K. M.; West, R.; Michl, J. Silicon, Germanium, Tin Lead Compd.
1986, 9, 75.
10.1021/om700659d CCC: $37.00 © 2007 American Chemical Society
Publication on Web 11/08/2007

6278 Organometallics, Vol. 26, No. 25, 2007
MoiseeV and Leigh
complexation with THF, O-H, O-Si, and group 14 M-H (M
) Si, Ge, Sn) σ-bond insertions, reaction with molecular oxygen,
(2+1) cycloaddition to alkenes, dienes, and alkynes, and chlorine
atom abstraction from CCl4. Advantage is taken of the fact that
the presence of the phenyl chromophore in SiPh2 results in a
red-shift of the absorption spectra of the silylene and its reaction
products relative to those of SiMe2, enhancing our ability to
detect and characterize intermediates that may be involved in a
given silylene-substrate reaction. In several cases, this provides
important new insight into the mechanisms of these reactions.
As outlined in the preceding paper,16 we have recently
developed two photochemical precursors to SiPh2 with the idea
of exploiting ring-strain effects to suppress the undesired
photorearrangements characteristic of 2-phenyl-substituted trisi-
lanes, which occur at the expense of silylene extrusion. Indeed,
preliminary product and laser photolysis studies demonstrated
the approach to be successful. The first compound studied,
trisilacycloheptane 6, affords SiPh2 and the accompanying
disilacyclohexane derivative (7) in significantly higher yields
than 4 (eq 2), allowing the UV/vis spectrum of SiPh2 to be
recorded in fluid solution and its reaction kinetics studied in
spite of still-significant interference from transient absorptions
assignable to the silene coproduct, 8.6 Even better results were
obtained with the trisilacyclohexane derivative 9, which affords
SiPh2 and disilacyclopentane 10 in close to quantitative yields
under the same conditions (eq 3).16 This was shown by the
results of steady-state photolysis experiments with methanol
(MeOH) and acetone, which trap the silylene as the formal O-H
insertion and ene-addition products 12 and 13, respectively.16
Though the isomeric silene (11) still appears to be formed, its
yield is so low that it presents minimal interference in laser
flash photolysis studies with the molecule. The UV/vis spectrum
of SiPh2 thus emerges in almost identical form to that recorded
in earlier matrix experiments with 4.15 In hexane solution in
the absence of scavengers, SiPh2 dimerizes to afford tetraphe-
nyldisilene (Si2Ph4), whose spectrum was also recorded. Inter-
estingly, it coincides almost exactly with that of the minor silene
coproduct; the situation thus remains complicated, but is
nevertheless tractable. It is now possible to study the chemistry
of this benchmark transient silylene in solution directly and at
an unprecedented level of detail.
Results and Discussion
Laser flash photolysis experiments were carried out as
described in the preceding paper,16 using rapidly flowed
solutions of 9 (∼10-4 M) and 1 (ca. 5 × 10-4 M) in dry,
deoxygenated hexane and the pulses from a KrF excimer laser
(248 nm, ca. 25 ns, ca. 100 mJ) for excitation. Silylene decays
were monitored at 530 nm for SiPh2 and 470 nm for SiMe2;
they generally fit acceptably to first-order decay kinetics in the
absence of added scavenger, with lifetimes that varied over the
ranges 0.6-1.5 µs and 250-600 ns, respectively. As detailed
in the preceding paper,16 the decay of the silylenes was
accompanied by the growth of absorptions at shorter wave-
lengths, due to the formation of the corresponding disilenes
Si2Ph4 (λmax ) 290, 370, 460 nm) and Si2Me4 (λmax ) 360 nm)
by silylene dimerization. The variation in the lifetimes of the
silylenes is due to an acute sensitivity to the presence of moisture
in the solvent and the glassware, which was difficult to control
(particularly on humid days) despite the routine of flame-drying
the apparatus prior to each experiment. As expected, the initial
intensities of the disilene signals also varied from experiment
to experiment, correlating inversely with the silylene lifetimes.
Addition of the substrates shown in Chart 1 caused the
pseudo-first-order decay of the silylenes to accelerate in
proportion to the substrate concentration; in all cases but THF,
this was accompanied by increases in the growth rates of the
disilene absorptions and reductions in their maximum intensities.
Rate constants were determined by linear least-squares analysis
of plots of the pseudo-first-order decay rate constants for decay
(kdecay) of SiPh2 and SiMe2 according to eq 6, where k0 is the
pseudo-first-order rate constant for decay of the species in the
absence of the substrate (Q) and kQ is the second-order rate
constant for the reaction with Q; these plots were linear in all
but one case (Vide infra). The data are summarized in Table 1,
along with reported rate constants for reaction of seven of the
substrates with SiMes2 in cyclohexane or hexane solution.11,16
Each experiment was completed by recording transient spectra
in the presence of the scavenger at a concentration where
the silylene lifetime was reduced to ca. 300 ns or less, and is
hence determined mainly by reaction with the substrate. This
was done in an effort to probe for the formation of the primary
product(s) of the silylene-trapping reaction. These additional
details will be presented later in the paper.
kdecay ) k0 + kQ[Q]
(6)
The first such study is the subject of the present paper: a
preliminary survey of the kinetics of a broad selection of well-
known silylene reactions in solution, focusing on SiPh2 and its
methylated counterpart, SiMe2, with data recorded under a
common set of conditions so as to enable direct comparisons
between the two silylene derivatives. The specific reactions
studied are shown in Chart 1 and include Lewis acid-base
The outcomes of these reactions are all well documented for
SiMe2 and/or other transient silylenes,1,17 but only those with
Et3SiH, dienes, alkenes, and acetone have been studied previ-
ously for SiPh2.6,16,18-20 We therefore carried out NMR-scale
photolyses of ca. 0.05 M solutions of 9 in C6D12 containing ca.
(17) Kira, M.; Iwamoto, T.; Ishida, S. Bull. Chem. Soc. Jpn. 2007, 80,
258.
(16) Moiseev, A. G.; Leigh, W. J. Organometallics 2007, 26, 6268-
6276.
(18) Bobbitt, K. L.; Gaspar, P. P. J. Organomet. Chem. 1995, 499, 17.

Comparison of the ReactiVities of SiMe2 and SiPh2 in Solution
Organometallics, Vol. 26, No. 25, 2007 6279
Chart 1
Table 1. Diphenylsilylene Trapping Products and Absolute Rate Constants for Reaction of Diphenyl-, Dimethyl-, and
Dimesitylsilylene with Various Silylene Scavengers in Hexane Solution at 25 °C
SiPh2
SiMe2
SiMes2
trapping agent
product(s)
kQ
/109 M-1 s-1
kQ
/109 M-1 s-1
kQ
/109 M-1 s-1
MeOHa
acetonea
THF
MeOSiMe3
AcOH
12 (93%)
13 (86%)
Ph2Si-THF complex
14 (54%)
13.4 ( 1.0
18.1 ( 1.5
0.82 ( 0.03
14.4 ( 2.0
15.2 ( 1.3
4.2 ( 0.7
10.1 ( 1.0
0.12 ( 0.02
3.3 ( 0.2a
3.1 ( 0.2
5.3 ( 0.6
7.9 ( 0.7
7.9 ( 1.6e
14.5 ( 1.3
13.6 ( 1.4
7.6 ( 0.6
8.3 ( 0.7e
1.37 ( 0.09
14.1 ( 1.2
17.3 ( 1.5
6.2 ( 0.6b
15.9 ( 1.0
0.47 ( 0.04
3.6 ( 0.3
8.3 ( 0.4
< 0.001d
16 (84%)
O2
17 (R ) Ph)c
0.032 ( 0.004d
0.079 ( 0.004d
Et3SiH
Et3GeH
n-Bu3SnH
cyclohexene
DMP
19
20 (90%)
2.5 ( 0.2
18.9 ( 1.4
7.8 ( 1.0
11.7 ( 0.5
15.9 ( 0.7
19.8 ( 0.7
13.8 ( 0.5
17.8 ( 0.6
3.4 ( 0.2
21 (87%)
2218
0.0028 ( 0.0003d
0.0088 ( 0.0007d
c
DMB
23 (87%) + 24 (<18%)
c
26 (67%)
c
27 (78%)
isoprene
BTMSE
TBE
CCl4
b
c
a Data from ref 16. Rate constant obtained from linear least-squares analysis of kdecay data at low concentrations (0.1-0.5 mM). Product not determined.
d In cyclohexane at 25 °C; data from ref 11. Data from ref 6.
e
0.20 M AcOH, MeOTMS, Et3GeH, Bu3SnH, BTMSE, DMB,
and CCl4, following the course of the photolyses by H NMR
and laser flash photolysis experiments with each of the substrates
studied are presented below.
1
spectroscopy and GC/MS, the latter being recorded before and
after photolysis to 50-60% conversion of 9. The products
formed in these experiments (see Chart 1) were in most cases
identified by comparison of the NMR and GC/MS data for the
crude reaction mixtures to the corresponding data for authentic
samples or to literature data; only two of them (vinylsilirane
23 and silirene 26) were tentatively identified by comparison
of the spectral data to reported data for closely related
compounds. As in the MeOH- and acetone-trapping experiments
described in the preceding paper,16 yields were determined
relative to consumed 9 from the slopes of concentration versus
time plots constructed from the NMR data over the 0-25%
conversion range. The photolysis mixtures were generally quite
clean, with the yield of disilacyclopentane 10 matching that of
the SiPh2-derived product(s) reasonably closely in most cases.
The chemical yields of the SiPh2-derived products identified in
these experiments are shown in Table 1 along with the kinetic
data. Additional specific details pertaining to the steady state
Finally, for several of the reagents studied with the SiMe2
precursor 1 (AcOH, CCl4, TBE, acetone, DMP, isoprene, O2,
and BTMSE), the reduction in the maximum transient absor-
bance at 360 nm (due to Si2Me4) with increasing substrate
concentration was accompanied by a distinct shortening of the
lifetime of the absorption over the substrate concentration ranges
studied, with the decays fitting well to first-order kinetics. Rate
constants for quenching of Si2Me4 were determined from plots
of kdecay versus [Q] according to eq 6 and are listed in Table 2.
Reactions of SiPh2 and SiMe2 with Oxygenated Substrates.
The Lewis acid-base complexation of silylenes with ethers and
other heteroatom-containing substrates is well known and has
been studied extensively in low-temperature matrixes with
SiMe2 and a number of other reactive silylene derivatives.21-25
(21) Gillette, G. R.; Noren, G. H.; West, R. Organometallics 1987, 6,
2617.
(22) Ando, W.; Sekiguchi, A.; Hagiwara, K.; Sakakibara, A.; Yoshida,
H. Organometallics 1988, 7, 558.
(23) Gillette, G. R.; Noren, G. H.; West, R. Organometallics 1989, 8,
487.
(24) Belzner, J.; Ihmels, H. AdV. Organomet. Chem. 1999, 43, 1.
(25) Boganov, S. E.; Faustov, V. I.; Egorov, M. P.; Nefedov, O. M. Russ.
Chem. Bull., Int. Ed. 2004, 53, 960.
(19) Sakurai, H.; Kobayashi, Y.; Nakadaira, Y. J. Organomet. Chem.
1976, 120, C1.
(20) Tortorelli, V. J.; Jones, M., Jr.; Wu, S.; Li, Z. Organometallics 1983,
2, 759.

6280 Organometallics, Vol. 26, No. 25, 2007
MoiseeV and Leigh
Table 2. Rate Constants for Quenching of Si2Me4 by
Various Substrates in Hexane Solution at 25 °C, Determined
from Plots of kdecay vs [Q] According to Eq 6
this wavelength, and not to residual free SiPh2 in equilibrium
with the complex. This conclusion is supported by the fact that
the long-lived absorption remained of similar relative intensity
with solutions of 9 containing up to 0.05 M THF, where the
lifetime of the free silylene was too short to be detected, and is
consistent with an equilibrium constant in excess of 105 M-1
for formation of the silylene-ether complex under these
conditions. On the other hand, the temporal profiles of the
absorptions at 350 and 460 nm varied only modestly over this
concentration range. This indicates that disilene formation occurs
mainly via a mechanism involving dimerization of the complex
under these conditions, even in the presence of as little as 0.3
mM THF. The lengthening of both the growth and decay times
of the disilene absorption in the presence of the ether compared
to those in its absence indicates that this process occurs
significantly more slowly than that involving the free silylene,
which is close to diffusion-controlled.16 Analysis of the 350 nm
signal as a second-order decay afforded a rate coefficient of
reagent
kQ/109 M-1 s-1
AcOH
CCl4
O2
DMP
isoprene
TBE
2.7 ( 0.2
2.3 ( 0.2
0.29 ( 0.04b
0.5 ( 0.2b
0.4 ( 0.2
0.81 ( 0.04
0.59 ( 0.05
BTMSE
a In most cases, the plots were constructed using data recorded at 5-9
b
substrate concentrations; errors are reported as ( 2σ. From kdecay values
recorded at only two concentrations.
Kinetic studies have been limited to SiH2 and SiMe2 in the gas
phase26,27 and SiMe2 in solution.8,10 Though the complexation
process is ultimately nonproductive in the cases of unstrained
dialkyl ethers,28 it has the effect of moderating the reactivity of
silylenes toward other substrates in ether compared to hydro-
carbon solvents8,24,29,30 and forms the basis for our current
understanding the mechanisms of the reactions of silylenes with
alcohols, alkoxysilanes, ketones, and strained cyclic ethers such
as oxetanes and oxiranes.1
2k/ꢀ ≈ 106 cm s-1
.
Addition of dioxane to hexane solutions of 9 led to similar
effects; the ether quenched the lifetime of SiPh2 at the diffusion-
controlled rate (kQ ) (1.8 ( 0.1) × 1010 M-1 s-1) and led to
the formation of a new species exhibiting λmax ) 370 nm. The
species decayed with a lifetime τ ≈ 5 µs in the presence of 11
mM dioxane, with the concomitant growth of the disilene
absorption at 460 nm. These results are relevant to the
interpretation of the various steady-state photolysis experiments
reported throughout this paper, in which dioxane was employed
as an internal integration standard. It clearly also serves as a
moderator in the silylene-trapping reactions, though it appears
to have no effect on their ultimate outcomes.
Lewis acid-base complexation of SiPh2 with THF proceeds
at close to the diffusion limit in hexane solution and leads to
the formation of new, longer-lived absorptions at shorter
wavelengths (λmax ≈ 350 nm), which appear to grow in over a
similar time scale as the silylene decay over the 0.1-0.5 mM
concentration range in THF (see Figure 1a). The spectrum of
the species is similar to that reported for the 2-methyltetrahy-
drofuran complex of SiMes2 (λmax ≈ 350 nm) at 77 K21 and is
red-shifted compared to that of the complex of SiMe2 (λmax
≈
310 nm) with THF in cyclohexane;8,10 it can thus be assigned
with confidence to the SiPh2-THF Lewis acid-base complex.
A similar trend in λmax is known for the complexes of the
germanium analogues of the three silylenes with THF.31 Disilene
formation is slowed quite substantially over this concentration
range, but appears to proceed in similar yield to that in pure
hexane solution. The behavior is illustrated in Figure 1a, which
shows transient absorption spectra recorded with a solution of
9 in hexane containing 0.3 mM THF at four time intervals after
the laser pulse and transient growth/decay profiles at 530, 460,
and 350 nm. A dotted line has been placed at the 500 ns point
of the growth/decay profiles to emphasize the equivalence of
the silylene decay rate and the growth rate of the complex, which
are both just discernible on the time scale employed for this
particular experiment. The absorption intensity at 460 nm grows
to a maximum over the first 5 µs after the pulse, 4-5 times
slower than in the absence of THF, and then decays at the same
rate as the absorption due to the complex.
Laser photolysis of hexane solutions of 1 in the presence of
THF afforded quite similar results to those described previously
in cyclohexane solution8,9 and allowed the determination of kTHF
) 1.73 × 1010 M-1 s-1 for formation of the SiMe2-THF
complex (λmax ) 310 nm) in hexane at 25 °C. The rate constant
is the same within experimental error as that for formation of
the SiPh2-THF complex under the same conditions.
The behavior observed in the presence of the reactive ether
methoxytrimethylsilane (MeOTMS) was markedly different
from that observed with THF. As with THF, addition of the
alkoxysilane to hexane solutions of 9 led to reductions in the
lifetime of SiPh2 over the 0-1 mM concentration range,
affording the second-order rate constant kMeOTMS ) 4.2 × 109
M-1 s-1 from the resulting linear plot of kdecay versus [MeOT-
MS]. Growth/decay profiles recorded at 460 nm showed the
formation of Si2Ph4 to be strongly quenched over the 0.1-4.0
mM concentration range in added MeOTMS, which is charac-
teristic of essentially irreversible scavenging of SiPh2, the
immediate precursor to the disilene. At 350 nm, the characteristic
growth/decay associated with the S0 f S2 transition of Si2Ph4
in pure hexane16 was replaced with a stronger, more rapidly
decaying signal whose maximum intensity increased with
increasing alkoxysilane concentration, superimposed on a
longer-lived decay which decreased in overall intensity with
increasing concentration. No further changes in either compo-
nent of the 350 nm signal were observed upon increasing the
alkoxysilane concentration above 4 mM. The lifetime of the
rapidly decaying component of the 350 nm signal was deter-
mined by two-exponential decay analysis of traces recorded in
the presence of 1, 2, and 4 mM MeOTMS and was found to be
independent of concentration within this range (τ ) 730 ( 5
ns). The only unusual feature noted in these experiments was
that the initial intensity of the silylene signal (at 530 nm)
The long-lived residual absorption present in the 530 nm
decay profile is due to silene 11, which absorbs very weakly at
(26) (a) Baggott, J. E.; Blitz, M. A.; Frey, H. M.; Lightfoot, P. D.; Walsh,
R. Int. J. Chem. Kinet. 1992, 24, 127. (b) Becerra, R.; Cannady, J. P.; Walsh,
R. J. Phys. Chem. A 1999, 103, 4457. (c) Becerra, R.; Cannady, J. P.; Walsh,
R. Phys. Chem. Chem. Phys. 2001, 3, 2343. (d) Alexander, U. N.; King, K.
D.; Lawrance, W. D. Phys. Chem. Chem. Phys. 2001, 3, 3085. (e) Alexander,
U. N.; King, K. D.; Lawrance, W. D. J. Phys. Chem. A 2002, 106, 973. (f)
Becerra, R.; Cannady, J. P.; Walsh, R. J. Phys. Chem. A 2003, 107, 11049.
(g) Becerra, R.; Bowes, S. J.; Ogden, J. S.; Cannady, J. P.; Adamovic, I.;
Gordon, M. S.; Almond, M. J.; Walsh, R. Phys. Chem. Chem. Phys. 2005,
7, 2900.
(27) Alexander, U. N.; King, K. D.; Lawrance, W. D. Phys. Chem. Chem.
Phys. 2001, 3, 3085.
(28) Gu, T.-Y. G.; Weber, W. P. J. Am. Chem. Soc. 1980, 102, 1641.
(29) Steele, K. P.; Weber, W. P. J. Am. Chem. Soc. 1980, 102, 6095.
(30) Steele, K. P.; Weber, W. P. Inorg. Chem. 1981, 20, 1302.
(31) Leigh, W. J.; Lollmahomed, F.; Harrington, C. R.; McDonald, J.
M. Organometallics 2006, 25, 5424.

Comparison of the ReactiVities of SiMe2 and SiPh2 in Solution
Organometallics, Vol. 26, No. 25, 2007 6281
Figure 1. (a) Transient absorption spectra recorded 64-128 ns (-0-), 0.64-0.70 µs (-O-), 3.52-3.58 µs (-4-), and 17.76-17.92 µs (---)
after the laser pulse, from laser flash photolysis of a 0.09 mM solution of 9 in deoxygenated hexane containing 0.3 mM THF. The inset
shows transient growth/decay profiles recorded at 350, 460, and 530 nm; a dotted line has been drawn at ∼0.6 µs to illustrate the coincidence
in the decay of the signal at 530 nm and the growth at 350 nm. (b) Transient absorption spectra recorded 0-64 ns (-0-), 1.01-1.07 µs
(-O-), and 8.5-8.6 µs (-4-) after the pulse, from laser photolysis of 9 in hexane containing 4.0 mM MeOTMS; the inset shows transient
decay profiles recorded at 350 and 480 nm.
appeared to be reduced in the presence of MeOTMS, to an extent
that increased systematically with increasing concentration. We
do not yet understand the reasons for this behavior, but can at
least rule out excited-state quenching on the basis of the product
studies described below.
The quenching of SiMe2 by MeOTMS was found to be
somewhat different than the behavior observed with SiPh2.
Addition of the alkoxysilane to hexane solutions of 1 resulted
in the expected increase in the silylene decay rate constant, but
the resulting plot of kdecay versus [MeOTMS] exhibited down-
ward curvature over the 0.1-5.0 mM concentration range. An
estimate of the limiting second-order rate constant (i.e., as
[MeOTMS] f 0) of kQ ) 6.2 × 109 M-1 s-1 was therefore
determined by linear least-squares analysis of data recorded over
the 0.1-0.4 mM concentration range. The value is ca. 3 times
lower than the rate constant for reaction of SiMe2 with MeOH
under the same conditions, in reasonable agreement with the
rate constant ratio determined by Steele et al. for reaction of
SiMe2 with MeOTMS and ethanol in dilute hydrocarbon
solution, which was based on competition experiments.34
Quenching of the silylene was accompanied by the appearance
of a new transient absorption centered at λmax ≈ 310 nm, which
can be assigned to the SiMe2-MeOTMS complex on the basis
of its similarity to the spectrum of the SiMe2-THF complex.8
The absorption decayed with clean first-order kinetics and a
lifetime τ ) 150 ns in hexane containing 5.3 mM MeOTMS,
nearly the same as that exhibited by the much weaker signal at
470 nm due to free SiMe2 at this concentration (τ ≈ 110 ns).
The formation of Si2Me4, which is barely detectable in the
presence of 1.4 mM MeOTMS (see Supporting Information),
could not be detected at the higher alkoxysilane concentration,
and so we interpret the similar lifetimes at 470 and 350 nm as
being due to the free silylene and the complex in mobile
equilibrium and decaying predominantly via unimolecular
rearrangement of the complex to yield the formal O-Si insertion
product. The underlying reasons for the nonlinear dependence
of the silylene decay rate on alkoxysilane concentration are not
clear at the present time, though we note that the behavior is
consistent with saturation kinetics, as would result if there were
a pathway for reaction of the complex that involves a second
molecule of alkoxysilane and regenerates SiMe2.
Figure 1b shows transient absorption spectra recorded over
various time windows after the laser pulse with a hexane solution
of 9 containing 4 mM MeOTMS, along with decay profiles
recorded at 350 and 480 nm. The decay profiles show that the
long-lived component of the 350 nm signal is in fact associated
with the minor transient coproduct formed along with SiPh2 in
the photolysis pulse, which we assign to silene 11.16 The lifetime
of the species is largely unaffected in the presence of the
alkoxysilane, which is consistent with the silene assignment.32
The short-lived component can be assigned to the Lewis acid-
base complex of SiPh2 with MeOTMS on the basis of its
similarity to the spectrum of the SiPh2-THF complex (see
Figure 1a), while its first-order decay can be associated with
the rate of SiMe3 migration to yield the final product formed in
the reaction, alkoxydisilane 14 (eq 7). The identity of the final
product was confirmed by steady-state photolysis of 9 in C6D12
containing 0.19 M MeOTMS, which afforded 10 and 14 in
yields of 74% and 46%, respectively. A third product was also
detected by GC/MS of the crude photolysate after ca. 60%
conversion of 9 and was tentatively identified as trisilane 15,
the (secondary) product of competing trapping of SiPh2 by 14.
Its formation was in fact expected, as it is known that
alkoxydisilanes are considerably more effective silylene scav-
engers than alkoxysilanes.33,34
Our results are consistent with the two-step mechanism
originally proposed by Weber and co-workers for the O-Si
insertion reaction with alkoxysilanes, involving initial Lewis
acid complexation of silylene with the substrate,34 but add
considerable new insight into the finer mechanistic details of
the reaction. As with MeOH,16 formation of the complex is the
rate-determining step for silylene decay at low substrate
concentrations; however, this stage of the process is considerably
slower in the case of MeOTMS, as might be expected consider-
(32) Leigh, W. J.; Sluggett, G. W. Organometallics 1994, 13, 269.
(33) Gu, T.-Y. G.; Weber, W. P. J. Organomet. Chem. 1980, 195, 29.
(34) Steele, K. P.; Tzeng, D.; Weber, W. P. J. Organomet. Chem. 1982,
231, 291.

6282 Organometallics, Vol. 26, No. 25, 2007
MoiseeV and Leigh
ing the lower basicity of the alkoxysilane compared to the ether
and alcohol.35 Our results also show that the intermediate
complex is not a steady-state intermediate, but rather constitutes
a bottleneck in the reaction pathway owing to its relatively slow
rate of conversion to the final product, 14. It is interesting to
note that Conlin and co-workers found the reaction of SiMes2
with MeOTMS to be too slow to be detected under the
conditions of their kinetic experiments with this silylene in fluid
solution and estimated an upper limit for the rate constant of
kMeOTMS < 106 M-1 s-1 11
. This suggests that complexation with
of theoretical calculations by several groups;37,39-41 kinetic
studies of the reaction of the parent molecule (SiH2) with O2 in
the gas phase have also been reported.41 Ando and co-workers
first studied the reaction of SiMes2 with O2 by UV/vis and IR
spectroscopy in an oxygen matrix at 16 K.37 They reported the
formation of a species exhibiting a weak UV/vis absorption
maximum at λmax ≈ 320 nm, which they assigned to the
corresponding triplet silanone oxide (17; see eq 10) on the basis
of IR spectroscopic evidence and RHF/6-31(g) calculations on
the triplet state of the parent silanone oxide, 3(H2Si-O2). Sander
the alkoxysilane is significantly more sensitive to steric effects
than is the case for the reaction with MeOH, as might be
expected on the basis of the fact that complexation is the rate-
determining step for silylene decay in both cases and considering
the differences in steric bulk between the two substrates.
While the rate constants for formation of the silylene-
methoxysilane complexes from SiPh2 and SiMe2 (see Table 1)
are quite similar, larger differences are observed in the first-
order rate constants for their decay. The decay process can most
reasonably be associated with the 1,2-SiMe3 migration that takes
the intermediate complex to the final product of the reaction.
The ca. 5-fold slower decay rate constant observed for the
SiPh2-MeOTMS complex (kdecay ≈ 1.4 × 106 s-1) relative to
that of the SiMe2-MeOTMS complex under the same condi-
tions (kdecay ≈ 6.6 × 106 s-1) is consistent with a somewhat
higher secondary barrier for formation of the net O-Si insertion
product in the phenylated system. The rate constants for decay
of the complexes are consistent with activation energies for the
38
and co-workers later studied the reactions of O2 with SiMe2
and SiMePh39 in 0.5% O2-doped argon matrixes by IR spec-
troscopy and showed that the two species react rapidly at ca.
40 K to form the corresponding dioxasiliranes (18), without the
intervention of detectable intermediates. It is thus clear that
following intersystem crossing to the singlet (ground) state
surface, the initially formed silanone oxide closes to the cor-
responding dioxasilirane derivative via an extremely low
activation barrier. This is supported by theoretical calculations,
which place the height of the barrier at ca. 5 kcal mol-1 for the
parent silanone oxide41 and lower still in substituted systems
such as that derived from SiMePh.39
second step in the reaction on the order of Ea ≈ 7.5 kcal mol-1
,
.
assuming pre-exponential factors on the order of 1012 s-1
However, it is clear from the unusual concentration dependence
observed with SiMe2 that the reaction is significantly more
complicated than this analysis might suggest, and further work
will be necessary in order to understand it completely.
The reactions of SiPh2 with acetone and acetic acid afford
the formal ene-type addition products 1316,18 and 16 (see eq 8),
respectively, and both proceed with rate constants close to the
diffusion-controlled limit. As might therefore be expected, the
rate constants are also quite similar to those measured for
reaction of SiMe2 with these substrates. While it is reasonable
to expect that these reactions also proceed via initial complex-
ation, and indeed spectroscopic evidence for this has been
reported for the reaction of a nonenolizable ketone with SiMes2
at cryogenic temperatures,36 we have been unable to detect any
signs of intermediate complexes in the reactions of these three
silylenes with acetone16 or AcOH in fluid solution (see
Supporting Information). We thus conclude that if such species
are indeed generally involved in the reactions with these two
substrates, they must be true steady-state intermediates, rear-
ranging to the final products with rate constants large enough
that the intermediates do not build up in high enough concentra-
tions to enable detection (eq 9). This is consistent with the results
of theoretical calculations for the reaction of SiH2 with
acetone.26b
We have measured absolute rate constants for the reactions
of SiPh2 and SiMe2 with oxygen in hexane in the present study,
obtaining values of kO2 ) 1.2 × 108 and 4.7 × 108 M-1 s-1
,
respectively, from the slopes of three-point plots of kdecay in
argon-, air-, and O2-saturated hexane versus [O2]. The value
for SiPh2 is only ca. 4 times higher than that reported by Conlin
and co-workers for SiMes2 in cyclohexane solution,11 showing
that steric effects on the rate constant for the process are quite
small, as might be expected. Interestingly, the reaction with O2
is the slowest that we have examined with SiMe2 and SiPh2,
which most likely reflects the fact that the reaction yields in
the first step a relatively high-energy triplet 1,3-biradical product.
The present value for SiMe2 of kO2 ) 4.7 × 108 M-1 s-1 revises
the ca. 5-fold higher value reported by Levin et al., which was
based on a single lifetime determination in air-saturated cyclo-
hexane.8
Transient absorption spectra were recorded by laser photolysis
of an O2-saturated hexane solution of 9, where the lifetime of
SiPh2 is reduced to τ ≈ 330 ns; the results of the experiment
are shown in Figure 2. Transient decays recorded at 460 nm
suggest that disilene formation is largely quenched under these
conditions; the transient absorption observed at 460 nm appears
The reaction of transient silylenes with molecular oxygen has
been studied in low-temperature matrixes in the cases of
SiMes2,37 SiMe2,38 and SiMePh39 and has also been the subject
(35) Pitt, C. G.; Bursey, M. M.; Chatfield, D. A. J. Chem. Soc., Perkin
Trans. 2 1976, 434.
(36) Ando, W.; Hagiwara, K.; Sekiguchi, A. Organometallics 1987, 6,
2270.
(37) Akasaka, T.; Nagase, S.; Yabe, A.; Ando, W. J. Am. Chem. Soc.
1988, 110, 6270.
(38) Patyk, A.; Sander, W.; Gauss, J.; Cremer, D. Angew. Chem., Int.
Ed. Engl. 1989, 28, 898.
(39) Bornemann, H.; Sander, W. J. Am. Chem. Soc. 2000, 122, 6727.
(40) Nagase, S.; Kudo, T.; Akasaka, T.; Ando, W. Chem. Phys. Lett.
1989, 163, 23.
(41) Becerra, R.; Bowes, S.-J.; Ogden, J. S.; Cannady, J. P.; Adamovic,
I.; Gordon, M. S.; Almond, M. J.; Walsh, R. Phys. Chem. Chem. Phys.
2005, 7, 2900.

Comparison of the ReactiVities of SiMe2 and SiPh2 in Solution
Organometallics, Vol. 26, No. 25, 2007 6283
provide an estimate of kO2 ) (2.9 ( 0.4) × 108 M-1 s-1 for the
rate constant for reaction of Si2Me4 with O2 under these
conditions, based on the lifetimes of the species in air- and O2-
saturated solution. The value compares favorably with those
that have been measured for other transient disilenes in solution43
and is a factor of about 5 larger than that for the corresponding
reaction of the germanium analogue, Ge2Me4.44
Further studies of the reactions of arylsilylenes with molecular
oxygen in hydrocarbon solvents are in progress.
Reactions of SiPh2 and SiMe2 with Group 14 Hydrides
R3MH (M ) Si, Ge, Sn). The Si-H insertion reaction of
silylenes with hydridosilanes is among the best known of
silylene reactions and has been particularly extensively studied
in the gas phase with several simple silylene derivatives such
as SiH2, SiMeH, SiPhH, SiClH, and SiMe2 as a function of
methyl substitution in the silane (MenSiH4-n; n ) 0-3).4,5 The
accepted mechanism involves the initial formation of a weakly
bound H-bonded silylene(acceptor)-hydridosilane(donor) com-
plex, which rearranges to yield the final insertion product (eq
11); the latter is thought to be the rate-determining step in the
reactions of the substituted silylenes that have been studied.4,45
The effects of substituents on the high-pressure-limiting rate
constants for these reactions in the gas phase vary somewhat
depending on the degree of methyl substitution in the silane,
but with trimethylsilane (Me3SiH) vary in the order PhSiH ∼
MeSiH ∼ SiH2 . SiMe2 > ClSiH.5 The effect has been
rationalized as the result of destabilization of both the intermedi-
ate complex and the barrier for the second step, relative to the
situation in the corresponding reaction of the parent molecule
(SiH2), with the effect being much greater for SiMe2 than for
MeSiH and PhSiH.45 Ge-H insertions have also received
attention in gas-phase studies, though much less extensively than
is the case for Si-H insertions. Surprisingly, SiMe2 has recently
been found to react close to 10 times more slowly with Me2-
GeH2 than with Me2SiH2, which runs counter to what might be
expected on the basis of the relative strengths of the M-H bonds
in the two substrates.5
Figure 2. Transient absorption spectra recorded 0-48 ns (-0-),
0.54-0.62 µs (-O-), and 2.14-2.22 µs (-4-) after the laser pulse,
by laser flash photolysis of a 0.09 mM solution of 9 in oxygen-
saturated hexane at 25 °C; the inset shows transient growth/decay
profiles recorded at 520 (O), 460 (4), and 350 nm (0).
to consist of only a single component, which is formed with
the laser pulse and decays to the prepulse level with clean first-
order kinetics and a lifetime τ ≈ 1.6 µs. The much reduced
lifetime of the transient absorption at this wavelength in O2-
compared to Ar-saturated hexane provides additional support
for its assignment to silene 11, as the estimated second-order
rate constant for its reaction with O2 (k ≈ 4 × 107 M-1 s-1
;
calculated from the 1.6 µs lifetime and a value of [O2] ) 15
mM42) is of the magnitude expected on the basis of reported
kinetic data for other silenes of this general structure.32 It is
difficult to rule out the possibility that the 460 nm signal in
O2-saturated solution contains minor contributions from the
silylene dimer, Si2Ph4. However, if this is the case, then the
fact that the signal decays with clean first-order kinetics suggests
that Si2Ph4 and 11 have similar lifetimes and hence similar rate
constants for reaction with O2. This is, in fact, consistent with
reported data for other phenyl-substituted disilenes.43
In addition to the prompt absorptions due to SiPh2 (τ ≈ 330
ns) and silene 11 (τ ≈ 1.6 µs), transient spectra recorded with
the O2-saturated solution (see Figure 2) showed clear evidence
for the formation of a new transient product (λmax ≈ 370 nm),
which grows in over a similar time scale as the silylene decay
and is then consumed on a time scale of several microseconds
(τ ≈ 3.3 µs). While we clearly have insufficient data for a
reliable assignment for the species to be made at the present
time, the lifetime seems rather short and the absorption
maximum too far to the red for it to be due to 1,1-diphenyl-
dioxasilirane (18; R ) Ph); the similarity in the spectrum to
those of the complexes of SiPh2 with the other O-donors studied
herein suggests the triplet silanone oxide may be the most likely
candidate for the species. It should be noted that the spectrum
of the species is strikingly similar to the matrix spectrum of
the SiMes2-O2 adduct reported by Ando and co-workers.37 It
should also be noted that the fact that the species is significantly
longer-lived than free SiPh2 under the same conditions indicates
that it is formed irreversibly.
The rate constant determined in the present work for reaction
of SiMe2 with Et3SiH in hexane (kEt3SiH ) (3.6 ( 0.3) × 109
M-1 s-1) is the same as the values reported previously in
cyclohexane within experimental error,8,9 as should be expected
given that it is well below the diffusion-controlled limit, and is
also in good agreement with the value reported for reaction of
the same silylene with Me3SiH in the gas phase (k ) 2.7 × 109
M-1 s-1).46 The value is also quite similar to the value found
for SiPh2 + Et3SiH in hexane,6,16 indicating once again the close
similarities in the reactivities of SiMe2 and SiPh2. The two
silylenes are ca. 40 times more reactive toward Si-H insertion
than SiMes2 in cyclohexane under similar conditions,11 indicat-
ing that steric effects on the Si-H insertion reaction are
moderate and of a similar magnitude to those observed in the
reaction with MeOH.16
Nothing but SiMe2 (τ ≈ 100 ns), Si2Me4 (τ ≈ 250 ns), and
the long-lived photolysis coproduct (λmax ) 280 nm) could be
detected in experiments with 1 in O2-saturated hexane, so
unfortunately we cannot address the mechanistic details of the
reaction of SiMe2 with O2 in fluid solution. We can, however,
Silylene insertions into Ge-H and Sn-H bonds have been
much less widely studied than Si-H insertions, but at least one
(44) Leigh, W. J.; Lollmahomed, F.; Harrington, C. R. Organometallics
2006, 25, 2055.
(45) Becerra, R.; Carpenter, I. W.; Gordon, M. S.; Roskop, L.; Walsh,
R. Phys. Chem. Chem. Phys. 2007, 9, 2121.
(46) Baggott, J. E.; Blitz, M. A.; Frey, H. M.; Walsh, R. J. Am. Chem.
Soc. 1990, 112, 8337.
(42) Battino, R.; Rettich, T. R.; Tominaga, T. J. Phys. Chem. Ref. Data
1983, 12, 163.
(43) Morkin, T. L.; Owens, T. R.; Leigh, W. J. In The Chemistry of
Organic Silicon Compounds, Vol. 3; Rappoport, Z., Apeloig, Y., Eds.; John
Wiley and Sons: New York, 2001; pp 949-1026.

6284 Organometallics, Vol. 26, No. 25, 2007
MoiseeV and Leigh
Figure 3. (a) Plots of the pseudo-first-order decay rate constants (kdecay) for SiMe2 (0) and SiPh2 (O) vs [Et3GeH] in hexane solution at
25 °C. The solid lines are the linear least-squares fits of the data to eq 6. (b) Transient absorption spectra recorded 16-80 ns (-0-) and
0.86-0.98 µs (-O-) after the laser pulse, by laser flash photolysis of a 0.09 mM solution of 9 in deoxygenated hexane containing 1.7 mM
Et3GeH. The inset shows transient growth/decay profiles recorded at 460 and 530 nm.
example of each process is known for transient silylenes such
Reactions of SiPh2 and SiMe2 with Alkenes and Alkynes.
The reactions of silylenes with C-C unsaturated compounds
have had a long and fascinating history, which is well
documented in early reviews.1 The corresponding three-
membered ring compoundsssiliranes from alkenes and silirenes
from alkynessare well known to be the primary products of
these reactions, which proceeds stereospecifically in the case
of addition to alkenes.20,51-53 The stereochemistry of the addition
to alkenes was in fact first established with SiMe2 and SiPh2,
employing the photolysis of 1 and 4 to generate the two species
in the presence of high concentrations of cis- or trans-2-butene
and trapping the resulting siliranes, which are often rather
unstable, as their methanolysis products.20,51
as SiMe2 and SiMePh19 in solution. The expected M-H
47
insertion products from SiPh2 (20 and 21; eq 12) are obtained
along with 10 in excellent yields upon photolysis of 9 in C6D12
in the presence of Et3GeH and n-Bu3SnH, respectively (see eq
12). Figure 3a shows the plots of kdecay versus [Et3GeH] for
SiMe2 and SiPh2 in hexane at 25 °C, while Figure 3b shows
time-resolved absorption spectra recorded by flash photolysis
of a solution of 9 containing 1.7 mM Et3GeH, where the lifetime
of SiPh2 is reduced to τ ≈ 300 ns and the formation of Si2Ph4
is strongly quenched. The spectra illustrate the absence of
detectable intermediates in the primary reaction of SiPh2 with
Et3GeH; similar results were obtained with Bu3SnH.
The kinetics of the reactions of both SiMe2 and SiMes2 with
alkenes were examined in the earlier flash photolysis studies
of these species in cyclohexane solution, with values of 7.3 ×
109 and 5.9 × 109 M-1 s-1 being reported for SiMe2 with
1-hexene and trimethylsilylethylene, respectively,8 and k ) 2.8
× 106 M-1 s-1 for SiMes2 with cyclohexene.11 Rate constants
for the reactions of SiMe2 and SiPh2 with cyclohexene and the
terminal alkene 3,3-dimethyl-1-pentene (DMP) have been
measured in the present work and again reveal close similarities
between the reactivities of the two species. Though we have
not carried out product studies in either case, Tortorelli et al.
showed that SiMe2 and SiPh2 both react with cyclohexene by
1,2-addition, the latter yielding 22 (see Chart 1).20 Steric effects
are felt considerably more strongly in this reaction than in
MeOH or Et3SiH insertions, as shown by the ca. 2000-fold lower
rate constant for reaction of SiMes2 with cyclohexene11 com-
pared to that for SiPh2 (k ) 7.9 × 109 M-1 s-1). The difference
is of a similar magnitude to that observed for the complexation
reaction with MeOTMS; in both cases, it is impossible to avoid
some degree of nonbonded interaction between the substituents
on the silylene and the substrate in the transition state for the
rate-determining step of the reaction, and hence the sensitivity
of the rate constant to steric bulk associated with the silylene
substituents is greater.
Perhaps surprisingly given the systematic decrease in M-H
bond strengths throughout the series M ) Si > Ge > Sn,48 the
rate constants for the reactions of SiPh2 with the three metallyl
hydrides are all quite similar, with those for the Si-H and
Ge-H insertions being essentially identical and less than a factor
of 2 lower than that for the Sn-H insertion. A somewhat larger
spread in reactivity is observed for SiMe2, with the reaction
with Et3GeH proceeding slightly slower than with Et3SiH, and
that with Bu3SnH ca. 5 times faster. The difference in rate
constants for reaction of SiMe2 with the germane and silane is
small, but is in the same direction that has been observed for
the reactions of Me2GeH2 and Me2SiH2 with SiMe2 in the gas
phase,5 as mentioned above. The absence of detectable transient
products in laser photolysis experiments with SiPh2 in the
presence of these substrates verifies, for the Sn-H insertion in
particular, that the reaction does not proceed via H atom
abstraction, as the tributylstannyl radical should be readily
detectable if it was formed as a discrete intermediate.49,50
The rate constants for reaction of the three silylenes with 2,3-
dimethyl-1,3-butadiene (DMB) and isoprene are 2-3 times
larger than those for reaction with the alkenes, but exhibit a
similar trend with silylene structure, decreasing in the order
(47) Appler, H.; Neumann, W. P. J. Organomet. Chem. 1986, 314, 247.
(48) Luo, Y.-R. In Handbook of Bond Dissociation Energies in Organic
Compounds; CRC Press LLC: Boca Raton, 2003; p 287.
(49) Chatgilialoglu, C.; Ingold, K. U.; Lusztyk, J.; Nazran, A. S.; Scaiano,
J. C. Organometallics 1983, 2, 1332.
(51) Tortorelli, V. J.; Jones, M., Jr. J. Am. Chem. Soc. 1980, 102, 1425.
(52) Zhang, S.; Wagenseller, P. E.; Conlin, R. T. J. Am. Chem. Soc.
1991, 113, 4278.
(50) Shaw, W. J.; Kandandarachchi, P.; Franz, J. A.; Autrey, T.
Organometallics 2004, 23, 2080.
(53) Pae, D. H.; Xiao, M.; Chiang, M. Y.; Gaspar, P. P. J. Am. Chem.
Soc. 1991, 113, 1281.

Comparison of the ReactiVities of SiMe2 and SiPh2 in Solution
kSiMe ∼ kSiPh ∼ 1600kSiMes . The reaction is thought to proceed
Organometallics, Vol. 26, No. 25, 2007 6285
which is consistent with the results of Bobbitt and Gaspar for
the reaction of SiPh2 with 1,3-butadiene in the presence of
acetone.1,18
2
2
2
analogously to that with alkenes, forming the corresponding
2-vinyl-1-silirane derivative as the main primary product, which
undergoes rapid secondary rearrangement to the isomeric
1-silacyclopent-3-ene and (under photochemical conditions)
other (acyclic) isomers.1 The evidence for vinylsilirane formation
has come largely from trapping experiments, but was put on a
firm footing by Zhang and Conlin’s successful isolation of
several vinylsilirane derivatives from reaction of SiMes2 with
aliphatic dienes.56 The isomeric 1-silacyclopent-3-ene is appar-
ently always formed in the reactions of silylenes with dienes,
but has been suggested to arise mainly via secondary thermal
and/or photochemical rearrangement of the vinylsilirane as
opposed to a direct (4+1) cycloaddition pathway.1,18
While numerous examples illustrating the course of the
reaction of silylenes with alkynes have been reported, to our
knowledge none have specifically addressed SiPh2. We thus
photolyzed 9 in the presence of bis(trimethylsilyl)acetylene
(BTMSE) to verify the course of the reaction expected on the
58
basis of the reported studies of the reaction of SiMe2 and
SiMes259 with this alkyne. Indeed, a single major (SiPh2-derived)
product was formed in the reaction and was assigned to silirene
1
26 on the basis of the H and 29Si NMR spectra of the crude
photolysate (see eq 15). The 29Si spectrum showed, in addition
to the resonances due to 9 and the coproduct 10, resonances at
δ -11.37 and -114.46 due to the SiMe3 and ring-silicon atoms,
respectively, in 26; the latter is in close correspondence with
the analogous data reported for the silirenes derived from
reaction of the same alkyne with SiMe258 and SiMes2.59 As with
the other C-C unsaturated compounds studied, near diffusion-
controlled quenching was observed for both SiMe2 and SiPh2
in hexane solutions of 1 and 9 containing submillimolar
concentrations of BTMSE, leading to absolute rate constants
of kBTMSE ) 1.38 × 1010 and 7.6 × 109 M-1 s-1 for SiMe2 and
SiPh2, respectively.
The reaction of SiPh2 with DMB was first studied by Jones
and co-workers, who isolated silacyclopentene 24 and the formal
ene-type product 25 from photolysis of 4 in the presence of the
diene (eq 13).20 Both compounds were suggested to arise from
“rapid isomerization” of vinylsilirane 23, citing the earlier
conclusions of Ishikawa, Ohi, and Kumada regarding the origins
of the analogous products from photolysis of the SiMePh
precursor 3 in the presence of DMB under similar conditions.54,55
In light of this and a recent study of our own of the
photochemistry of 24,57 we were surprised to discover that
vinylsilirane 23 is easily detectable by NMR spectroscopy upon
photolysis of 9 in C6D12 in the presence of DMB, where it is
formed in a chemical yield of ca. 87% along with minor amounts
of silacyclopentene 24 (eq 14). The structural assignment is
1
based on comparison of the H and 29Si NMR spectra of the
Transient absorption spectra recorded with hexane solutions
of 9 containing 0.4-1.0 mM DMP, DMB, and BTMSE,
conditions under which the lifetime of SiPh2 is reduced to τ <
200 ns, showed absorptions centered at ca. 280 nm that did not
decay during the maximum time window (of 0.9 s) that can be
monitored with our system (see Supporting Information). The
spectra and extended lifetimes of these species are consistent
with their assignment to the corresponding three-membered-
ring compounds. The UV/vis spectrum of the product formed
from DMB (see Supporting Information) is identical to that
reported by us previously for the long-lived product detected
in laser photolysis experiments with silacyclopentene 24, which
was assigned to vinylsilirane 23.57 No spectroscopic evidence
for intermediate complexes could be obtained for any of these
reactions.
photolyzed mixture (see Supporting Information) to the data
reported by Zhang and Conlin for the corresponding vinylsilirane
obtained from reaction of DMB with SiMes2.56 As expected,
the compound shows marked thermal instability, rearranging
within 48 h (in the dark) to yield 24. Isomer 25 could not be
detected as a coproduct in this experiment, indicating that the
quantum yield for its formation by secondary photolysis of 23
is substantially lower than that for formation of 24 by the same
route. The concentration versus time plot for product 24 shows
good linearity over the 0-30% conversion range in 9 (see
Supporting Information), which would seem to be inconsistent
with it being formed exclusiVely via secondary rearrangement
of vinylsilirane 23, but rather also via direct (4+1) cycloaddition.
However, it should be pointed out that the lowest conversion
probed in our steady-state photolysis experiment was ca. 5%,
and the ratio of 24:23 present appeared to be significantly lower
than its value at ca. 10% conversion. It thus seems more likely
that 24 is formed mainly via secondary rearrangement of 23,
Reactions of SiPh2 and SiMe2 with CCl4. The reactions of
silylenes with halocarbons have also been of considerable
interest.17,47,60-70 The reaction varies in overall course depending
(58) Seyferth, D.; Annarelli, D. C.; Vick, S. C. J. Am. Chem. Soc. 1976,
98, 6382.
(59) Ishikawa, M.; Nishimura, K.; Sugisawa, H.; Kumada, M. J.
Organomet. Chem. 1980, 194, 147.
(60) Ishikawa, M.; Nakagawa, K.-I.; Katayama, S.; Kumada, M. J.
Organomet. Chem. 1981, 216, C48.
(61) Ishikawa, M.; Nakagawa, K.-I.; Katayama, S.; Kumada, M. J. Am.
Chem. Soc. 1981, 103, 4170.
(54) Ishikawa, M.; Ohi, F.; Kumada, M. J. Organomet. Chem. 1975,
86, C23.
(55) Ishikawa, M.; Nakagawa, K.-I.; Enokida, R.; Kumada, M. J.
Organomet. Chem. 1980, 201, 151.
(56) Zhang, S.; Conlin, R. T. J. Am. Chem. Soc. 1991, 113, 4272.
(57) Leigh, W. J.; Huck, L. A.; Held, E.; Harrington, C. R. Silicon Chem.
2005, 3, 139.

6286 Organometallics, Vol. 26, No. 25, 2007
MoiseeV and Leigh
Figure 4. (a) Plots of the pseudo-first-order decay rate constants (kdecay) for SiMe2 (-0-) and SiPh2 (-O-) vs [CCl4] in hexane solution at
25 °C. The solid lines are the linear least-squares fits of the data to eq 6. (b) Transient absorption spectra recorded 48-80 ns (-0-),
0.32-0.43 (-O-), and 0.88-0.99 µs (-4-) after the laser pulse, by laser flash photolysis of a 0.09 mM solution of 9 in deoxygenated hexane
containing 4 mM CCl4. The inset shows transient growth/decay profiles recorded at 330, 460, and 530 nm.
on the silylene and the halocarbon; for example, transient
silylenes such as SiMe2 undergo C-Cl insertion with monochlo-
roalkanes60,71 and halogen atom abstraction with CCl4, the latter
leading ultimately to the formation of the corresponding
dichlorosilane and other radical-derived products.62-65,69 The
latter is precisely the behavior exhibited by SiPh2, as shown by
the results of steady-state photolysis of a solution of 9 in C6D12
containing 0.2 M CCl4 (eq 16). The reaction affords dichlo-
rodiphenylsilane (27; 78%) as the main SiPh2-derived product,
in addition to 10 (89%), CHCl3 (25%), and hexachloroethane,
with the latter being detected (but not quantified) by GC/MS;
we did not determine whether chloroform-d was formed along
with the protiated isotopomer in the reaction. At least one
additional product was detected by 1H NMR spectroscopy,
which indicated it to be formed in ca. 25% yield relative to
consumed 9. This could not be confirmed by GC/MS, however.
The NMR spectrum suggests that the product retains the ring
structure of the starting material, and thus it seems most likely
that it is a radical-derived product resulting from reaction of 9
with trichloromethyl radicals. Chlorodiphenylsilane-d (Ph2Si-
(D)Cl) was ruled out as a possible product by spiking the crude
sample with a small quantity of an authentic sample of the
protiated isotopomer.
Taraban and co-workers have studied the photochemistry of a
benzosilanorbornadiene-type precursor to SiMe2 (28) in the
presence of CCl4 by flash photolysis and CIDNP methods and
assigned a relatively long-lived transient they observed by flash
photolysis in hexane containing 2 M CCl4 to the SiMe2-CCl4
complex.69 The free silylene cannot be detected directly in laser
photolysis experiments with this precursor,72,73 and so an indirect
method was employed to estimate a value of k ≈ 106 M-1 s-1
for the rate constant for complexation of the (singlet) silylene
with CCl4.69
Addition of CCl4 to hexane solutions of 1 and 9 led in both
cases to linear dependences of the silylene decay rate constants
on CCl4 concentration (see Figure 4a) and concomitant quench-
ing of the disilene signal intensities, as was observed with the
other scavengers. Analysis of the kdecay-concentration data
The reaction is thought to proceed in general via the
intermediacy of a silylene-halocarbon Lewis acid-base com-
plex,60,66,69,70 whose partitioning between halogen atom abstrac-
tion and other pathways depends primarily on the nature of the
halocarbon; Cl abstraction is known to be very strongly favored
in the case of CCl4.47,63,66 No absolute rate constants have yet
been reported for a silylene-halocarbon reaction, although
according to eq 6 afforded absolute rate constants of kCCl
)
4
3.4 × 109 and 1.37 × 109 M-1 s-1 for SiMe2 and SiPh2,
respectively. Transient absorption spectra recorded by laser
photolysis of 9 in the presence of 4 mM CCl4, where the lifetime
of SiPh2 was reduced to τ ≈ 225 ns, showed the characteristic
absorption bands due to SiPh2 and the long-lived residual
absorber (λmax ) 460 nm), both of which were formed promptly
with the laser pulse (see Figure 4b). The lifetime of the latter
species (τ ≈ 1.4 µs) was roughly 100-fold shorter than its value
in neat hexane; this indicates a quenching rate constant of kQ
≈ 2 × 108 M-1 s-1, which is again consistent with the silene
(11) assignment.32 Interestingly, the signals in the 330-350 nm
region of the spectrum of 9 in the presence of 4 mM CCl4
exhibited somewhat longer lifetimes compared to those at 300
and 530 nm (see inset, Figure 4b), suggesting the presence of
(62) Kira, M.; Sakamoto, K.; Sakurai, H. J. Am. Chem. Soc. 1983, 105,
7469.
(63) Nakao, R.; Oka, K.; Dohmaru, T.; Nagata, Y.; Fukumoto, T. J.
Chem. Soc., Chem. Commun. 1985, 766.
(64) Kolesnikov, S. P.; Egorov, M. P.; Galminas, A. M.; Ezhova, M.
B.; Nefedov, O. M.; Leshina, T. V.; Taraban, M. B.; Kruppa, A. I.;
Maryasova, V. I. J. Organomet. Chem. 1990, 391, C1.
(65) Lehnig, M.; Klaukien, H.; Reininghaus, F. Ber. Bunsenges. Phys.
Chem. 1990, 94, 1411.
(66) Oka, K.; Nakao, R. J. Organomet. Chem. 1990, 390, 7.
(67) Moser, D. F.; Bosse, T.; Olson, J.; Moser, J. L.; Guzei, I. A.; West,
R. J. Am. Chem. Soc. 2002, 124, 4186.
(68) Su, M.-D. J. Am. Chem. Soc. 2003, 125, 1714.
(69) Taraban, M. B.; Volkova, O. S.; Plyusnin, V. F.; Kruppa, A. I.;
Leshina, T. V.; Egorov, M. P.; Nefedov, O. M. J. Phys. Chem. A 2003,
107, 40962.
(70) Becerra, R.; Cannady, J. P.; Walsh, R. J. Phys. Chem. A 2006, 110,
6680.
(71) Ishida, S.; Iwamoto, T.; Kabuto, C.; Kira, M. Chem. Lett. 2001,
2001, 1102.
(72) Hawari, J. A.; Griller, D. Organometallics 1984, 3, 1123.
(73) Taraban, M. B.; Volkova, O. S.; Kruppa, A. I.; Plyusnin, V. F.;
Grivin, V. P.; Ivanov, Y. V.; Leshina, T. V.; Egorov, M. P.; Nefedov, O.
M. J. Organomet. Chem. 1998, 566, 73.

Comparison of the ReactiVities of SiMe2 and SiPh2 in Solution
Organometallics, Vol. 26, No. 25, 2007 6287
a transient product that absorbs in this wavelength range. The
identity of this species is uncertain, but we consider the most
likely possibility to be the chlorodiphenylsilyl radical (Ph2SiCl),
which the product studies suggest is formed as the primary
product of the reaction of the silylene with the halocarbon. The
lifetime of the species shows a similar (but slightly lower)
sensitivity to CCl4 concentration than SiPh2; a plot of kdecay
values at 330 nm versus [CCl4] was linear, affording a rate
constant of kQ ) (8 ( 1) × 108 M-1 s-1. The value is similar
to those reported for the (Cl abstraction) reactions of other
arylsilyl radicals with CCl4.74
Transient spectra recorded with a solution of 1 containing
0.6 mM CCl4 showed only the characteristic absorptions due
to SiMe2 (τ ≈ 180 ns), Si2Me4 (peak intensity ca. 25% of that
in pure hexane; τ ≈ 680 ns), and the long-lived photoproduct
that is characteristic of all the experiments carried out with this
precursor; no new transient products or intermediates could be
detected in the experiment (see Supporting Information). The
lifetime of Si2Me4 under these conditions provides an estimate
of k ≈ 2.4 × 109 M-1 s-1 for the rate constant for reaction of
the disilene with CCl4, which is thought to proceed via initial
Cl atom abstraction.75,76 This is 10 orders of magnitude higher
than the value reported by Kira and co-workers for reaction of
CCl4 with a stable tetrasilyldisilene in solution under similar
conditions.75 The value for Si2Me4 can also be compared to that
for reaction of the germanium analogue, Ge2Me4, with CCl4 in
Figure 5. Representations of the transition states for the rate-
determining steps in the reactions of silylenes with various
substrates, illustrating the variation in silylene-substrate steric
interactions with reaction type. Also shown are the rate constant
ratios kSiMes /kSiPh for each of the systems pictured, which provides
2
2
a quantitative description of the effect.
between SiMe2- and SiPh2-derived reaction intermediates, as
only a few of the reactions studied in the present work proceed
via intermediates that build up in sufficient concentrations for
us to detect them. Nevertheless, the few examples for which
this is the case indicate that the differences here are also small.
Those derived from SiPh2 tend to be longer-lived, absorb at
longer wavelengths, and exhibit more intense UV/vis spectra
than their SiMe2-derived counterparts, all distinct advantages
in studies of this type. Though it has not been specifically
quantified, dimerization appears to be somewhat slower in the
case of SiPh2, which imparts scavenging reactions with intrinsi-
cally greater efficiency; this makes kinetic analyses more reliable
at low substrate concentrations and provides better sensitivity
in experiments directed at the detection and characterization of
reaction intermediates that are involved in some cases. The
phenylated system has the additional advantage of being readily
amenable to the introduction of polar ring substituents, which
will undoubtedly prove to be very useful in future, more detailed
mechanistic studies of silylene reactivity.
Wherever possible, we have made a point of comparing our
kinetic data for SiPh2 to previously reported rate constants for
the corresponding reactions of the sterically hindered diarylsi-
lylene, SiMes2,11 in the interest of developing kinetic bench-
marks describing the sensitivity of various silylene reaction types
to steric effects. A convenient measure of this sensitivity is the
relative rate constants for reaction of SiMes2 and SiPh2 with a
given substrate, subject to the proviso that these rate ratios may
also contain contributions to the rate constants from the
electronic effects of the mesityl methyl groups; there are six of
them, so this effect may not be insignificant. The list of reactions
for which direct comparisons can be made is somewhat limited
at present, but it is nevertheless already clear that trends of
valuable predictive power will emerge from further comparisons
of this type. The seven substrates for which we have data divide
into three groups: those with little apparent sensitivity to steric
effects (acetone, O2; kSiMes2/kSiPh2 > 0.1), those with moderate
sensitivity (MeOH, Et3SiH; 0.01 < kSiMes2/kSiPh2 < 0.1), and
those with high sensitivity (cyclohexene, DMB, MeOTMS;
kSiMes2/kSiPh2 e 10-3). These trends make good sense when
considered against the expected structures of the transition states
for the rate-determining steps in the various reactions and the
degree of nonbonded interactions that must develop between
the substrate and the silylene substituents. This is illustrated
pictorially in Figure 5 for the reactions with acetone, O2, MeOH,
hexane at 25 °C, k ) 2.3 × 107 M-1 s-1 44
.
The difference in
rates for the two tetramethyldimetallenes is in order-of-magni-
tude agreement with the recent calculations of Su on the ener-
getics of Cl atom abstraction from CCl4 by these compounds.76
Summary and Conclusions
Absolute rate constants have been determined for the reactions
of the transient silylenes SiMe2 and SiPh2 with a wide variety
of representative reagents, under a common set of conditions.
The reactions that have been studied are all well known, and
many of the associated mechanistic details have already been
reasonably well established with the aid of product studies,
competition experiments, and low-temperature (matrix) spec-
troscopic studies. Gas-phase kinetics and theoretical studies of
the parent molecule and the few other small molecules that lend
themselves readily to work of this type have also played a
particularly significant role in developing our understanding of
silylene reactivity. In particular, this work provides a quantitative
link between the behavior of the parent molecule, for which
kinetic studies are limited to the gas phase, and simple
substituted derivatives such as SiMe2, which can be studied in
both the gas phase and solution.
The present work has been directed at further developing the
links between these various efforts and exploring some of the
finer mechanistic details that are associated with silylene
chemistry. We have shown that substitution of methyl with
phenyl groups generally leads to almost no change in intrinsic
reactivity toward a wide variety of representative substrates;
where differences do exist, they are invariably small, with SiPh2
being only slightly less reactive than SiMe2. Similar trends are
evident in the relative reactivities of Si(H)Ph and Si(H)Me
toward the methylsilanes (MenSiH4-n; n ) 0-3) in the gas
phase, as mentioned earlier in the paper.5 We have fewer data
available at present to address the differences in reactivity
(74) Chatgilialoglu, C. Chem. ReV. 1995, 95, 1229.
(75) Kira, M.; Ishima, T.; Iwamoto, T.; Ichinohe, M. J. Am. Chem. Soc.
2001, 123, 1676.
(76) Su, M.-D. J. Phys. Chem. A 2004, 108, 823.

6288 Organometallics, Vol. 26, No. 25, 2007
MoiseeV and Leigh
-78 °C with a dry ice/acetone bath. A 2 M solution of n-
butyllithium (0.82 mL, 1.65 mmol) in pentane was added dropwise
via syringe over 20 min, and the mixture was warmed to ca. 0 °C
in an ice bath and then kept at this temperature for 15 min. The
mixture was recooled to -78 °C, and a solution of triethylgermane
(0.27 g, 1.69 mmol) in THF (1.5 mL) was added dropwise over 1
h. The bath was removed, and the contents were taken up in a
syringe and then added dropwise over 1 h to a solution of
chlorodiphenylsilane (0.31 g, 1.42 mmol) in THF (2 mL) under
argon at -40 °C (acetonitrile-dry ice bath) in an oven-dried 25
mL two-necked round-bottom flask fitted with condenser, argon
inlet, and magnetic stir bar. The reaction mixture was left to stir
overnight, with the temperature rising slowly to room temperature.
The mixture was diluted with diethyl ether (5 mL) and filtered
through a short silica gel column to remove white precipitate and
polar impurities, and the solvent was removed under vacuum to
yield a colorless oil (0.27 g). Column chromatography on silica
gel with hexanes as eluant afforded triethyl(diphenylsilyl)germane
(20; 0.077 g, 16%), which was identified on the basis of the
following spectroscopic data: 1H NMR (600 MHz, CDCl3) δ 0.96
(q, J ) 7.8 Hz, 6H), 1.05 (t, J ) 7.8 Hz, 9H), 5.08 (s, 1H), 7.37-
(m,6H), 7.58 (dd, J ) 1.8, 7.8 Hz, 4H); 13C NMR (150 MHz,
CDCl3) δ 9.0, 13.8, 27.5, 30.1, 128.2, 129.1, 135.0, 135.8; 29Si
NMR (119 MHz, CDCl3) δ -29.25 (Ph2SiH, JSi-H ) 186.0 Hz);
IR (neat) ν (cm-1) ) 3068 (m), 3052 (m), 2950 (s), 2928 (s), 2909
(s), 2874 (s), 2099 (s), 1486 (m), 1459 (m), 1430 (s), 1104 (m),
1014 (m), 791 (s), 754 (s), 722 (s); GC/MS (EI) m/z ) 344(8; M+),
317 (17), 316 (18), 315 (65), 314 (32), 313 (61), 311 (47), 289
(23), 288 (22), 287 (100), 286 (40), 285 (80), 284 (17), 283 (56),
261 (14), 260 (9). 259 (49), 258 (19), 257 (49), 256 (13), 255 (39),
253 (9), 212 (16), 184 (16), 183 (83), 181 (26), 151 (31), 149 (25),
147 (15), 135 (15), 133 (12), 131 (11), 107 (18), 105 (65), 103
(27), 101 (13), 75 (10), 53 (17); exact mass calculated for
C18H26Si74Ge 344.1016, found 344.1024.
Et3SiH, DMB, and cyclohexene. The reader is directed to the
extensive body of theoretical work that has been done on most
of these reactions in the case of the parent molecule, for more
accurate depictions of the transition-state structures.40,41,45,77-80
A few of the systems studied in this and the preceding paper
have afforded particularly rich mechanistic information as a
result of the successful detection of intermediates formed as
the primary products of the silylene-substrate reactions. These
include the reactions of SiPh2 with methanol,16 methoxytrim-
ethylsilane, CCl4, and oxygen. In all four cases, intriguing
preliminary results have been obtained, underlining the fact that
much remains to be learned of the mechanistic details of many
silylene reactions. Future work from our laboratory will address
these reactions, and others, in greater detail.
Experimental Section
1H and 13C NMR spectra were recorded on Bruker AV200 or
AV600 spectrometers in deuterated chloroform and were referenced
to the solvent residual proton and 13C signals, respectively, while
29Si spectra were recorded on the AV600 using the HMBC pulse
sequence and referenced to an external solution of tetramethylsilane.
GC/MS analyses were determined on a Varian Saturn 2200 GC/
MS/MS system equipped with a VF-5ms capillary column (30 m
× 0.25 mm; 0.25 µm; Varian, Inc.). High-resolution electron mass
spectra and exact masses were determined on a Micromass TofSpec
2E mass spectrometer using electron impact ionization (70 eV).
Infrared spectra were recorded as thin films on potassium bromide
plates using a Bio-Rad FTS-40 FTIR spectrometer. Column
chromatography was carried out using a 3 × 60 cm column using
silica gel 60 (230-400 mesh; Silicycle).
1,1,3,3-Tetramethyl-2,2-diphenyl-1,2,3-trisilacyclohexane (9) was
synthesized according to the method described in the preceding
paper,16 while dodecamethylcyclohexasilane (1) was used as
received from Sigma-Aldrich. Methoxytrimethylsilane was syn-
thesized by reaction of methanol with excess hexamethyldisila-
zane and purified by distillation (bp 56-58 °C).81 All the other
scavengers investigated in this work were obtained from commercial
sources in the highest purity available. Triethylsilane (Et3SiH),
triethylgermane (Et3GeH), and tri-n-butylstannane (Bu3SnH) were
stirred at room temperature for 18 h over lithium aluminum hydride
and then either distilled at atmospheric pressure (Et3SiH) or under
mild vacuum (Bu3SnH) or passed through a short silica gel column
in the case of Et3GeH. CCl4 was refluxed over phosphorus pentoxide
and distilled. 4,4-Dimethyl-1-pentene (DMP), isoprene, and 2,3-
dimethyl-1,3-butadiene (DMB) were purified by passage of the neat
liquids through a silica gel column. 3,3-Dimethyl-1-butyne (TBE)
and cyclohexene were distilled. Bis(trimethylsilyl)acetylene (Gelest),
acetone (Caledon Reagent), and glacial acetic acid (Sigma-Aldrich)
were used as received from the suppliers. Hexanes (EMD Omni-
Solv), diethyl ether (Caledon Reagent), and tetrahydrofuran (Cale-
don 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 Laboratories.
Tri-n-butyl(diphenylsilyl)stannane (21). The compound was
prepared in identical fashion to 20, from tri-n-butyltin hydride
(0.37 g, 1.27 mmol) and chlorodiphenylsilane (0.26 g, 1.19 mmol).
Column chromatography of the crude reaction mixture on silica
gel with hexanes as eluant afforded tri-n-butyl(diphenylsilyl)-
stannane (21; 0.055 g, 10%), which was identified on the basis of
the following spectroscopic data: 1H NMR (600 MHz, CDCl3) δ
0.84 (t, J ) 7.8 Hz, 9H), 1.00 (t, J ) 7.8 Hz, 6H), 1.26 (sextet, J
) 7.8 Hz, 6H), 1.46 (quint, J ) 7.8 Hz, 6H), 7.36 (m,6H), 5.28 (s,
1H), 7.57 (m, 4H); 13C NMR (150 MHz, CDCl3) δ 9.0, 13.8, 27.5,
30.1, 128.2, 129.1, 135.0, 135.8; 29Si NMR (119 MHz, CDCl3) δ
-28.23 (Ph2SiH, JSi-H ) 187.4 Hz); IR (neat) ν (cm-1) ) 3068
(m), 2958 (s), 2871 (s), 2853 (s), 2099 (s), 1464 (m), 1428 (s),
1376 (m), 1102 (s), 787 (s), 714 (s), 698 (s); GC/MS (EI) m/z )
417 (10; M+ - n-Bu), 416 (7), 415 (9), 414 (6), 361 (31), 360
(17), 359 (31), 358 (16), 357 (16), 309 (16), 307 (21), 306 (20),
305 (100), 304 (39), 303 (87), 301 (46), 197 (12), 195 (10), 184
(11), 183 (50), 181 (14), 177 (9), 121 (8), 105 (17), 79 (3), 53 (5);
exact mass calculated for C24H38SiSn 474.1765, found 474.1794;
exact mass calculated for C20H29SnSi (M+ - n-Bu) 417.1060, found
417.1043.
Triethyl(diphenylsilyl)germane (20). In an oven-dried 15 mL
two-necked round-bottom flask fitted with condenser, argon inlet,
and magnetic stir bar was placed diisopropylamine (0.17 g, 1.7
mmol) in THF (2 mL) under argon, and the solution was cooled to
Steady-State Photolysis Experiments. In a typical procedure,
a solution of 9 (0.039-0.051 M) in cyclohexane-d12 (0.7 mL) was
placed in a quartz NMR tube and the tube was capped with a rubber
septum. The solution was deoxygenated with dry argon for 15 min,
and then the appropriate volumes of trapping agent and dioxane
(as internal integration standard) were added with a microlitre
syringe to result in concentrations of ca. 0.20 and 0.01 M,
respectively. The solution was then photolyzed for 18-24 min (ca.
44-60% conversion of 9) in a Rayonet photochemical reactor
(Southern New England Ultraviolet Co.) equipped with a merry-
(77) Heaven, M. W.; Metha, G. F.; Buntine, M. A. J. Phys. Chem. A
2001, 105, 1185.
(78) Heaven, M. W.; Metha, G. F.; Buntine, M. A. Aust. J. Chem. 2001,
54, 185.
(79) Sakai, S. Int. J. Quantum Chem. 1998, 70, 291.
(80) Skancke, P. N.; Hrovat, D. A.; Borden, W. T. J. Am. Chem. Soc.
1997, 119, 8012.
(81) Schrock, R. R.; Sancho, J.; Pederson, S. F. Inorg. Synth. 1989, 26,
44.
1
go-round and two RPR-2537 lamps, monitoring by 600 MHz H
NMR spectroscopy in 2 min time intervals throughout the experi-

Comparison of the ReactiVities of SiMe2 and SiPh2 in Solution
Organometallics, Vol. 26, No. 25, 2007 6289
ment. Each mixture was also analyzed by GC/MS before and after
photolysis.
versus time plots for the product(s) and 9 in each case. Concentra-
tions were calculated from the 1H NMR integrals, using the SiMe
proton resonances to characterize 9 and 10 and one of the peaks
for the remaining product(s) to characterize it; C-H resonances
were used wherever possible. Details are provided in the Supporting
Information.
With most of the trapping agents studied, photolysis led to the
formation of two major products. The first was common to all and
was identified as 1,1,2,2-tetramethyl-1,2-disilacyclopentane (10) by
1
comparison of its H NMR and mass spectra to reported data:82
1H NMR (600 MHz, C6D12) δ 0.07 (s, 12H; SiMe2), 0.67 (t, 4H, J
) 6.6 Hz; C3,5H2), 1.67 (quint, 2H, J ) 6.6 Hz; C4H2); GC/MS
(EI) m/z ) 158 (42; M+), 143 (43), 130 (41), 117 (33), 116 (44),
115 (100), 99 (8), 85 (10), 73 (41), 59 (19), 45 (13), 43 (28). The
second major product was unique to each of the trapping agents
studied and could be assigned to the product of reaction of the trap
with SiPh2. Additional minor product(s) were evident in a few cases.
In the majority of cases, the silylene-trapping products were identi-
Laser Flash Photolysis Experiments. Laser flash photolysis
experiments were carried out as described in the preceding paper.16
Rate constants were calculated by linear least-squares analysis of
decay rate-concentration data. Errors in absolute second-order rate
constants are quoted as twice the standard deviation obtained from
the least-squares analyses.
Acknowledgment. We thank the Natural Sciences and
Engineering Research Council of Canada for financial support.
1
fied by comparison of the H NMR and mass spectra to literature
data or to the corresponding spectra of authentic, independently
prepared samples. Two of the products (23 and 26) were tentatively
identified from the 1H NMR spectra of the crude photolysis
mixtures, on the basis of comparisons to the reported spectra of
closely related analogues. Chemical yields of the major products
formed were determined from the relative slopes of concentration
1
Supporting Information Available: Representative H NMR
spectra and concentration versus time plots from steady-state
photolysis experiments, plots of kdecay versus [Q] for quenching of
SiMe2 and SiPh2, and transient absorption spectra from laser
photolysis of 9 and 1 in the presence of scavengers. This material
is available free of charge via the Internet at http://pubs.acs.org.
(82) Gusel’nikov, L. E.; Polyakov, Yu. P.; Volnina, E. A.; Nametkin,
N. S. J. Organomet. Chem. 1985, 292, 189.
OM700659D