Mechanism of di-tert-butylsilylene transfer from a silacyclopropane to an alkene

Article abstract of DOI:10.1021/ja0301370

Kinetic and thermodynamic studies of the reactions of cyclohexene silacyclopropane 1 and monosubstituted alkenes suggested a possible mechanism for di-tert-butylsilylene transfer. The kinetic order in cyclohexene silacyclopropane 1 and cyclohexene were de

Full text of DOI:10.1021/ja0301370

Published on Web 08/06/2003
Mechanism of Di-tert-Butylsilylene Transfer from a
Silacyclopropane to an Alkene
Tom G. Driver and K. A. Woerpel*
Contribution from the Department of Chemistry, UniVersity of California,
IrVine, California 92697-2025
Received February 25, 2003; Revised Manuscript Received May 16, 2003; E-mail: kwoerpel@uci.edu
Abstract: Kinetic and thermodynamic studies of the reactions of cyclohexene silacyclopropane 1 and
monosubstituted alkenes suggested a possible mechanism for di-tert-butylsilylene transfer. The kinetic
order in cyclohexene silacyclopropane 1 and cyclohexene were determined to be 1 and -1, respectively.
Saturation kinetic behavior in monosubstituted alkene concentration was observed. Competition experiments
between substituted styrenes and a deficient amount of di-tert-butylsilylene from 1 correlated well with the
Hammett equation and provided a F value of -0.666 ( 0.008, using σp constants. These data supported
a two-step mechanism involving reversible di-tert-butylsilylene extrusion from 1, followed by irreversible
concerted electrophilic attack of the silylene on the monosubstituted alkene. Eyring activation parameters
were found to be ∆Hq ) 22.1 ( 0.9 kcal‚mol-1 and ∆Sq ) -15 ( 2 eu. Competition experiments between
cycloalkenes and allylbenzene determined cycloalkenes to be more efficient silylene traps (krel )1.3, ∆∆Gq
) 0.200 kcal‚mol-1). A summary of the data resulted in a postulated reaction coordinate diagram. The
mechanistic studies enabled rational modification of reaction conditions that improved the synthetic utility
of silylene transfer. Removal of the volatile cyclohexene from the reaction mixture into an evacuated
headspace led to the formation of previously inaccessible cyclohexene-derived silacyclopropanes.
pane 1,3,16 have been employed for silylene transfer. Our
laboratory has shown that cyclohexene silacyclopropane 1 is
an efficient source of silylene, providing silacyclopropanes from
chiral, functionalized alkenes (eq 1).17,18
Through qualitative observations of silacyclopropane stability,
the mechanism for thermal silylene transfer was postulated to
involve the reversible extrusion of free silylene from a cyclic
silane, followed by trapping of the intermediate silylene with
an alkene.2,4,8 Kinetic evidence that di-tert-butylsilylene forma-
tion is reversible, however, has not been obtained. If free di-
tert-butylsilylene is formed from extrusion, its electronic nature
has remained unknown. Additionally, little is known about the
activation parameters of silylene transfer.4b
Introduction
Insight into formation of silacyclopropanes is of particular
interest because of the emerging synthetic utility of these
strained-ring silanes.1 Silacyclopropanes can be formed by
thermolysis of a cyclic silane in the presence of an alkene,2-8
a process that involves silylene intermediates.9-13 Since the
initial discovery by Seyferth and co-workers that hexamethyl-
silirane can generate silylene thermally,6 other cyclic silanes,
including cyclotrisilanes2,5,14,15 and cyclohexene silacyclopro-
(1) Franz, A. K.; Woerpel, K. A. Acc. Chem. Res. 2000, 33, 813-820.
(2) Belzner, J.; Ihmels, H.; Kneisel, B. O.; Gould, R. O.; Herbst-Irmer, R.
Organometallics 1995, 14, 305-311.
(3) Boudjouk, P.; Black, E.; Kumarathasan, R. Organometallics 1991, 10,
2095-2096.
(4) (a) Pae, D. H.; Xiao, M.; Chiang, M. Y.; Gaspar, P. P. J. Am. Chem. Soc.
1991, 113, 1281-1288. (b) Recently, the activation parameters of (t-
Bu3Si)(i-Pr3Si)Si formation were reported: Jiang, P.; Trieber, D., II; Gaspar,
P. P. Organometallics 2003, 22, 2233-2239.
(5) Scha¨fer, A.; Weidenbruch, M.; Peters, K.; von Schnering, H. G. Angew.
Chem., Int. Ed. Engl. 1984, 23, 302-303.
(6) Seyferth, D.; Annarelli, D. C. J. Am. Chem. Soc. 1975, 97, 7162-7163.
(7) Seyferth, D.; Annarelli, D. C.; Vick, S. C.; Duncan, D. P. J. Organomet.
Chem. 1980, 201, 179-195.
To understand the fundamental reactivity of silacyclopropanes
and their intermediates, we conducted a quantitative analysis
of the thermal behavior of cyclohexene silacyclopropane 1 in
the presence of an alkene. We believed that a mechanistic
understanding of this reaction would guide improvements to
silacyclopropane synthesis. Analysis of the kinetic behavior and
(8) Seyferth, D.; Annarelli, D. C.; Duncan, D. P. Organometallics 1982, 1,
1288-1294.
(9) Gaspar, P. P.; West, R. In The Chemistry of Organosilicon Compounds;
Rappoport, Z., Apeloig, Y., Eds.; Vol. 2; Wiley: Chichester, 1998; pp
2463-2468.
(10) Haaf, M.; Schmedake, T. A.; West, R. Acc. Chem. Res. 2000, 33, 704-
714.
(11) Scha¨fer, A.; Weidenbruch, M. J. Organomet. Chem. 1985, 282, 305-313.
(12) Weidenbruch, M. Coord. Chem. ReV. 1994, 130, 275-300.
(13) Silylenes can also be produced using photochemical methods. For recent
examples, refer to: (a) Miyazawa, T.; Koshihara, S.; Liu, C.; Sakurai, H.;
Kira, M. J. Am. Chem. Soc. 1999, 121, 3651-3656. (b) Gaspar, P. P.;
Beatty, A. M.; Chen, T.; Haile, T.; Lei, D.; Winchester, W. R.; Braddock-
Wilking, J.; Rath, N. P.; Klooster, W. T.; Koetzle, T. F.; Mason, S. A.;
Albinati, A. Organometallics 1999, 18, 3921-3932. (c) Weidenbruch, M.;
Meiners, F.; Saak, W. Can. J. Chem. 2000, 78, 1469-1473.
(14) Belzner, J.; Dehnert, U.; Ihmels, H. Tetrahedron 2001, 57, 511-517.
(15) Weidenbruch, M. Chem. ReV. 1995, 95, 1479-1493.
(16) Boudjouk, P.; Samaraweera, U.; Sooriyakumaran, R.; Chrusciel, J.;
Anderson, K. R. Angew. Chem., Int. Ed. Engl. 1988, 27, 1355-1356.
(17) Driver, T. G.; Franz, A. K.; Woerpel, K. A. J. Am. Chem. Soc. 2002, 124,
6524-6525.
(18) CÄ irakovic´, J.; Driver, T. G.; Woerpel, K. A. J. Am. Chem. Soc. 2002, 124,
9370-9371.
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A R T I C L E S
Driver and Woerpel
Figure 2. Saturation kinetic behavior observed for allylbenzene, 60 °C.
Scheme 1. Mechanism of Di-tert-Butylsilylene Transfer
Figure 1. Determination of kinetic order for cyclohexene silacyclopropane
1, 60 °C.
Eyring activation parameters suggested the formation of di-tert-
butylsilylene as the reactive intermediate. The electrophilicity
of the di-tert-butylsilylene intermediate was established by the
identification of a Hammett F value. These results and data from
competition experiments permit construction of a reaction
coordinate diagram for thermal silylene transfer from 1. Finally,
the mechanistic study culminated in the rational modification
of reaction parameters to allow synthesis of previously inac-
cessible cyclohexene-derived silacyclopropanes.
allylbenzene. Saturation dependence in allylbenzene concentra-
tion was observed (Figure 2), confirming the formation of an
intermediate.
Results and Discussion
Inhibition by cyclohexene and saturation with allylbenzene
established that 1 is reversibly converted to an intermediate that
can be intercepted by allylbenzene. Because the data is not
consistent with a bimolecular step, this reactive intermediate
must not contain either cyclohexene or allylbenzene, and the
intermediate is postulated to be di-tert-butylsilylene (Scheme
1). Because di-tert-butylsilylene was never observed using NMR
spectroscopy, it was treated as a steady-state intermediate for
derivation of the experimentally determined rate law (eq 3). The
saturation kinetic behavior observed indicates the silylene
intermediate reacts at similar rates with cyclohexene (k-1) and
allylbenzene (k2). At 30 equiv of allylbenzene, k-1[cyclohexene]
no longer impacted the rate, simplifying the rate law to d[2]/dt
) k1[1], in which the extrusion of silylene was rate-limiting. If
k-1 was significantly larger than k2, a pre-equilibrium would
be established and the kinetic order in allylbenzene would have
been one throughout all allylbenzene concentrations. This
mechanistic analysis quantitatively confirms the aforementioned
conclusions of Belzner,2 Gaspar,4b and Seyferth8 that extrusion
of a silylene species from a cyclic silane is reversible.
Initial experiments between cyclohexene silacyclopropane 1
and an alkene focused on optimizing the conditions for
measuring kinetic parameters (eq 2). Allylbenzene was chosen
as the di-tert-butylsilylene acceptor to facilitate analysis using
1H NMR spectroscopy. Several signals, including diagnostic
t-Bu and Bn signals of both the product and starting material,
were distinct. In addition, the similar dielectric constant of
allylbenzene (ꢀ ) 2.63)19 as compared to toluene (ꢀ ) 2.38)19
would ensure that the polarity of the solvent would remain
unchanged at high concentrations of alkene. Because of its high
1
boiling point, low polarity, and distinct H NMR spectrum,
phenyltrimethylsilane was chosen as the internal standard. For
all experiments, the components were heated in a valved NMR
tube under a headspace of nitrogen at ambient pressure.
k1k2[1][allylbenzene]
d[2]
dt
Insight into the mechanism of thermal silylene transfer was
obtained through an examination of the initial rates under
pseudo-first-order reaction conditions. The kinetic order was
determined to be one in cyclohexene silacyclopropane 1
concentration (Figure 1) and negative one in the concentration
of cyclohexene.20 Inhibition of silylene transfer by cyclohexene
prohibits a bimolecular reaction mechanism between 1 and
)
(3)
k-1[cyclohexene] + k2[allylbenzene]
A series of competition experiments between various para-
substituted styrenes (p-OMe, p-Me, p-F, p-Cl, and p-CF3) and
a deficient amount of 1 provided insight into the electronic
nature of the transition state for cycloaddition (eq 4). Higher
reactivity was observed with more electron-rich styrenes. The
rate difference in formation of substituted styrene silacyclopro-
panes correlated linearly with the Hammett equation using σp
(19) Wohlfarth, C. In CRC Handbook of Chemistry and Physics, 83rd ed.; Lide,
D. R., Ed.; CRC Press: Boca Raton, FL; 2003, pp 6.153-6.173.
(20) Refer to the Supporting Information for tabular and graphical presentation
of the kinetic data.
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Mechanism of Di-tert-Butylsilylene Transfer
A R T I C L E S
Figure 3. Linear correlation of log(krel) with σpara constants using the
Hammett equation.
Figure 4. Plot of ln(kobs/T) vs 1/T for the reaction of cyclohexene
silacyclopropane 1 and allylbenzene.
constants21 to result in a F value of -0.666 ( 0.008 at 87 °C
(Figure 3). The magnitude and sign of the F value were validated
through examination of temperatures within (30 °C that did
not reveal an isokinetic point. The F values at 55 and 120 °C
were found to be -0.57 and -0.79, respectively.22 The linear
relationship observed between temperature and F value22 allowed
estimation of the isokinetic temperature to be -110 °C. Because
the reaction was carried out at temperatures substantially above
the isokinetic point (and therefore entropy controlled), the
apparent F value should be evaluated cautiously. The reaction
was affected, however, in a linear fashion by the electronic
nature of the para-substituent on styrene: the rate of silylene
addition was accelerated by electron-donating groups and
retarded by electron-withdrawing substituents.
proceeded by an initial electrophilic phase resulting from the
interaction of the silylene LUMO with the π-electrons of the
alkene. A similar small negative F value (-0.619 obtained using
σ+ constants) was observed in the concerted, electrophilic
cyclopropanation of substituted styrenes with phenyl(bromo-
dichloromethyl)mercury-derived dichlorocarbene.28
The activation parameters of di-tert-butylsilylene extrusion
from 1 were identified.4b A correlation between the observed
rate and temperature was obtained between 55 and 95 °C (Figure
4). Saturation by allylbenzene was chosen to simplify the rate
law (d[2]/dt ) k1[1]). Under saturation conditions, the extrusion
of di-tert-butylsilylene from cyclohexene silacyclopropane 1
could be described using Eyring activation parameters (∆Hq )
22.1 ( 0.9 kcal‚mol-1, ∆Sq ) -15 ( 2 eu). The negative ∆Sq
value was unexpected for a unimolecular reaction in which one
particle becomes two.29 Since the synchronous fission of two
bonds in a three-membered ring is forbidden,30 the loss of
entropy in the transition state could result from an asynchronous
cheletropic elimination of di-tert-butylsilylene which would
require extensive organization by the silacyclopropane 1.31,32
Because the reaction involved a polarized transition state (vide
supra) between a silylene and an alkene, an additional entropic
penalty would result from solvent reorganization. The displace-
ment of several solvent molecules by the disassociation of the
sterically large di-tert-butylsilylene could also account for the
negative entropy. Alternatively, a di-tert-butylsilylene-Lewis
base complex33 (with either cyclohexene or toluene34 serving
as the Lewis base) could be involved.
The small negative F value provided insight into the electronic
nature of the cycloaddition. The negative value revealed the
nucleophile to be the alkene and established the electrophilicity
of di-tert-butylsilylene.23 Stepwise reactions that involved
charged intermediates, such as the hydration and bromination
of styrene, were typified by large negative F values (hydration,24
F ) -4; bromination,25 F ) -4.3). The small negative F value
suggested that a concerted electrophilic attack of free di-tert-
butylsilylene on the acceptor alkene involves the formation of
a slightly polar transition state. This reaction mechanism agreed
with gas-phase studies26 and ab initio calculations27 describing
the reaction of silylene with ethylene in which the reaction
The ∆∆Gq between cycloaddition onto cyclohexene and
allylbenzene was estimated by comparison of the relative rates
of di-tert-butylsilylene trapping by cyclopentene and cyclooctene
(26) Baggott, J. E.; Blitz, M. A.; Frey, H. M.; Lightfoot, P. D.; Walsh, R. J.
Chem. Soc., Faraday Trans. 2 1988, 84, 515-526.
(27) Anwari, F.; Gordon, M. S. Isr. J. Chem. 1983, 23, 129-132.
(28) Seyferth, D.; Mui, J. Y.-P.; Damrauer, R. J. Am. Chem. Soc. 1968, 90,
6182-6186.
(29) For two examples of positive ∆Sq parameters resulting from one molecule
becoming two, refer to: (a) ∆Sq ) 3.5 eu (retro-Diels-Alder); Beno, B.
R.; Wilsey, S.; Houk, K. N. J. Am. Chem. Soc. 1999, 121, 4816-4826. (b)
∆Sq ) 11.9 eu (SN1); Hinkle, R. J.; McNeil, A. J.; Thomas, Q. A.; Andrews,
M. N. J. Am. Chem. Soc. 1999, 121, 7437-7438.
(21) Hansch, C.; Leo, A.; Taft, R. W. Chem. ReV. 1991, 91, 165-195.
(22) Please refer to the Supporting Information.
(23) The apparent electrophilicity of t-Bu2Si contrasts with the observed
nucleophilicity (F ) +0.85 ( 0.2) of Ar2Si (Ar ) 2-(Me2NCH2)C6H4)
observed by Belzner and co-workers in ref 14.
(24) Schubert, W. M.; Lamm, B.; Keefe, J. R. J. Am. Chem. Soc. 1964, 86,
4727-4729.
(25) Dubois, J. E.; Schwarcz, A. Tetrahedron Lett. 1964, 5, 2167-2173.
(30) Hoffmann, R. W. J. Am. Chem. Soc. 1968, 90, 1475-1485.
(31) For a general discussion of cheletropic reactions, refer to Woodward, R.
B.; Hoffmann, R. W. The ConserVation of Orbital Symmetry; VCH:
Weinheim, Germany, 1970; pp 152-163.
(32) Recently, a ∆Sq term of -7.6 eu was observed for the cheletropic
decomposition of a nitrosoamine in heptane by Shustov, G. V.; Rauk, A.
J. Org. Chem. 2000, 65, 3612-3619.
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A R T I C L E S
Driver and Woerpel
Figure 6. Reaction coordinate diagram describing silylene transfer from 1
to allylbenzene.
Figure 5. Correlation of cycloalkene trapping efficiency to estimate the
cyclohexene silacyclopropane silylene extrusion thermodynamic profile.
>99% completion. Since cyclic alkenes are more reactive than
acyclic alkenes (krel ) 1.3), the transition state for extrusion is
marginally lower in energy than cycloaddition of silylene onto
allylbenzene by 200 cal‚mol-1. The kinetic data did not provide
insight into the relative energies of the silylene-alkene com-
plexes and free silylene.
versus allylbenzene (eq 5). Competition experiments were
performed using 15 equiv of each alkene. Both cyclopentene
and cyclooctene were equally reactive (within experimental
error) and more efficient than allylbenzene in trapping silylene
(cyclopentene, krel ) 1.37 ( 0.11; cyclooctene krel ) 1.34 (
0.08). Because cyclopentene- and cyclooctene-derived silacy-
clopropanes differ in thermodynamic stability (eq 6), the similar
krel values indicate that the transition state for extrusion must
be product-like (t-Bu2Si:), and the observed krel of 1.3 should
extend to describe cycloaddition of di-tert-butylsilylene with
cyclohexene (Figure 5).
The mechanistic insight gained in this study was applied
toward the development of methodology to improve the
construction of silacyclopropanes. Because the thermodynamic
driving force to transfer di-tert-butylsilylene from 1 to substi-
tuted cyclohexenes was missing, statistical mixture of products
were obtained by using the previously reported methods.17 The
reversibility of the extrusion from 1 could be exploited to access
these desirable silacyclopropanes. If cyclohexene could be
removed from the reaction mixture, the formation of function-
alized cyclohexene-derived silacyclopropanes could be achieved.
Cyclohexene could be effectively removed from the reaction
mixture using an evacuated headspace and toluene solvent
because the lowest-boiling component of the reaction is cyclo-
hexene. Rational modification of reaction conditions provided
functionalized silacyclopropanes 8 and 10 (eqs 8 and 9).
Conclusion
A quantitative analysis of the thermal behavior of cyclohexene
silacyclopropane 1 in the presence of an alkene improved our
understanding of the fundamental reactivity of silacyclopro-
panes. The kinetic behavior confirmed that the formation of di-
(33) For leading references to the coordination of Lewis bases with silylenes,
refer to: (a) Steele, K. P.; Weber, W. P. J. Am. Chem. Soc. 1980, 102, 2,
6095-6097. (b) Weidenbruch, M.; Piel, H.; Lesch, A.; Peters, K.; von
Schnering, H. G. J. Organomet. Chem. 1993, 454, 35-43. (c) Bott, S. G.;
Marshall, P.; Wagenseller, P. E.; Wang, Y.; Conlin, R. T. J. Organomet.
Chem. 1995, 499, 11-16. (d) Takeda, N.; Suzuki, H.; Tokitoh, N.; Okazaki,
R.; Nagase, S. J. Am. Chem. Soc. 1997, 119, 1456-1457. (e) Belzner, J.;
Ihmels, H. AdV. Organomet. Chem. 1999, 43, 1-42. (f) Bharatam, P. V.;
Moudgil, R.; Kaur, D. Organometallics 2002, 21, 3683-3690.
The thermodynamic and kinetic data can be represented as a
reaction coordinate diagram to describe di-tert-butylsilylene
transfer from cyclohexene silacyclopropane 1 to allylbenzene
(eq 7; Figure 6). The difference in ground-state energies (∆G°)
can be estimated to be g4.8 kcal, since the reaction goes to
(34) The reviewers are gratefully acknowledged for this insight.
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Mechanism of Di-tert-Butylsilylene Transfer
A R T I C L E S
The general procedure is illustrated for the reaction of cyclohexene
silacyclopropane 1 with excess allylbenzene. To a solution of cyclo-
hexene silacyclopropane 1 (0.200 mL of a 0.4455 M solution in toluene-
d8, 0.0891 mmol) in 0.600 mL of a 0.02346 M solution of PhSiMe3 in
toluene-d8 (contained in a 2 mL volumetric flask) was added allylben-
zene (0.275 mL, 2.07 mmol). The resulting solution was diluted to 2
mL with additional 0.02346 M solution of PhSiMe3 in toluene-d8. The
reaction mixture was then divided among three valved NMR tubes
1
(0.600 mL in each). Initial H NMR spectra were obtained, and the
temperature of the reaction mixture was regulated to 80 ( 1 °C.
Periodically, the reaction mixture was rapidly cooled to 0 °C, and the
reaction progress was measured relative to the internal standard of
1
PhSiMe3 at ambient temperature using H NMR spectroscopy.
General Procedure for the Construction of the Cyclohexene-
Derived Silacyclopropanes 8 and 10. The general procedure is
illustrated for the reaction of cyclohexene silacyclopropane 1 with 1,
4-dihydronaphthalene. To a solution of 1,4-dihydronaphthalene (25.0
mg, 0.192 mmol) in 0.300 mL of a 0.02346 M solution of PhSiMe3 in
toluene-d8 (contained in a medium wall NMR tube) was added 0.200
mL of a 0.4455 M solution of cyclohexene silacyclopropane 1 in
toluene-d8. The reaction mixture was degassed and sealed under
vacuum. After an initial 1H NMR spectrum was obtained, the reaction
mixture was warmed to 100 °C for 16 h and monitored periodically
tert-silylene is reversible and established that the rate of
cycloaddition with cyclohexene and allylbenzene is similar. The
observed small, negative Hammett F value suggested that
cycloaddition occurs through a concerted, electrophilic attack
of di-tert-butylsilylene on the alkene. Determination of the
Eyring activation parameter ∆Sq to be -15 eu suggested that
the silylene transition state may consist of an alkene-silylene
complex. Mechanistic insight guided the rational modification
of reaction conditions to permit construction of previously
inaccessible cyclohexene-derived silacyclopropanes.
1
using H and 29Si NMR spectroscopy. The 29Si NMR spectrum was
obtained to verify the formation of product silacyclopropane (δ -44.15
ppm). The yield of the reaction was determined by comparison of peaks
associated with the product silacyclopropane 8 (diagnostic t-Bu peaks
at δ 1.144 and 0.943 ppm) with the internal standard PhSiMe3 (t-Bu
peak at δ 0.19 ppm, area set to be one). The same procedure was
followed with cyclohexene 9 to provide silacyclopropane 10 in 85%
yield. Diagnostic peaks: t-Bu peaks at δ 1.16 and 0.99 ppm; 29Si NMR
peak at δ -54.7 ppm.
Experimental Section
General Comments Concerning the Procedure. Unless otherwise
noted, all reagents were commercially obtained and, where appropriate,
purified prior to use. Toluene-d8 was distilled from CaH2 and degassed
prior to use. All reagents and cyclohexene di-tert-butylsilacyclopropane
1 were stored in an Innovative Technologies nitrogen atmosphere
drybox.
Acknowledgment. This research was supported by the
National Institute of General Medical Sciences of the National
Institutes of Health (GM54909). K.A.W. thanks the Camille and
Henry Dreyfus Foundation, Merck Research Laboratories,
Johnson & Johnson, and the Sloan Foundation for awards to
support research. T.G.D. thanks Novartis Pharmaceuticals
through the ACS Division of Organic Chemistry and Eli Lilly
and Company for predoctoral fellowships. We thank Dr. Phil
Dennison for assistance with NMR spectrometry and Professors
K. J. Shea and F. Freeman (UCI) for insightful discussions.
The apparatuses (volumetric flasks and valved NMR tubes) were
washed with a solution of 2:1 HCl (12 N):HNO3 (16 N), rinsed with
water and acetone, washed with ammonium hydroxide, and rinsed with
water and acetone. The valved NMR tubes were dried at 130 °C. Each
experiment was set up in the nitrogen atmosphere drybox and was
carried out in triplicate (three at a time). The reaction mixture was
heated to the desired temperature either in the NMR probe (kinetic
order in allylbenzene and 1) or using a 1000 mL oil bath. The reaction
progress over the first half-life (or, when appropriate, five half-lives)
was monitored periodically using 1H NMR spectroscopy. The concen-
trations of the reactants and products were obtained through comparison
of the area of the standard (PhSiMe3 peak at 0.19 ppm, area ) 1.0000)
and the area of the t-Bu peaks of the silacyclopropanes 1 and 2 (1,
1.00 and 1.19 ppm; 2, 1.06 and 0.98 ppm). The data obtained were fit
to the best straight line using a least-squares program. For tabular and
graphical representation of the data, refer to the Supporting Information.
Supporting Information Available: Experimental procedures,
tabular, graphical, and mathematical derivations of kinetic and
thermodynamic parameters, and spectroscopic data for the
products (PDF). This material is available free of charge via
the Internet at http://pubs.acs.org.
JA0301370
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