Stereoelectronic requirement and effect of ring size on silanol elimination: A kinetic study

Article abstract of DOI:10.1039/b108237j

Acid catalyzed elimination of silanol from the cyclic cis-α,β-dihydroxysilanes 3-8 to cycloalkanones was studied at three different temperatures (298, 313 and 328 K). Silyldiols 3-7 hydrolyzed smoothly, while 8 was resistant to hydrolysis. The observed ease of elimination (reflected in the rate constants) was found to be related to the conformational flexibility of the ring and intramolecular hydrogen bonding. The results have confirmed the requirement of perfect antiperiplanar geometry of the eliminating groups, SiMe₂ and β-OH. The quantitation of the hydrolysis products, the cycloalkanones, was done using the gas chromatographic peak area normalization method using an internal standard. The rate data obtained at three different temperatures were employed to calculate ΔH?, ΔS?, and ΔG?. The rate constants and thermodynamic parameters increase with the ring size. There is an interesting demarcation between these two values for common ring silyldiols and the larger ring silyldiols.

Full text of DOI:10.1039/b108237j

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Stereoelectronic requirement and effect of ring size on silanol
elimination: a kinetic study
M. Narendra Mallya and Gopalpur Nagendrappa*
2
Department of Chemistry, Bangalore University, Central College Campus,
Bangalore-560 001, India
Received (in Cambridge, UK) 11th September 2001, Accepted 17th October 2001
First published as an Advance Article on the web 15th November 2001
Acid catalyzed elimination of silanol from the cyclic cis-α,β-dihydroxysilanes 3–8 to cycloalkanones was studied at
three different temperatures (298, 313 and 328 K). Silyldiols 3–7 hydrolyzed smoothly, while 8 was resistant to
hydrolysis. The observed ease of elimination (reflected in the rate constants) was found to be related to the
conformational flexibility of the ring and intramolecular hydrogen bonding. The results have confirmed the
requirement of perfect antiperiplanar geometry of the eliminating groups, SiMe3 and β-OH. The quantitation of the
hydrolysis products, the cycloalkanones, was done using the gas chromatographic peak area normalization method
using an internal standard. The rate data obtained at three different temperatures were employed to calculate ∆H‡,
∆S‡, and ∆G‡. The rate constants and thermodynamic parameters increase with the ring size. There is an interesting
demarcation between these two values for common ring silyldiols and the larger ring silyldiols.
the β-carbon (called vertical stabilization 1) or by the formation
of silacyclopropylium cation (non vertical stabilization 2)
(Scheme 2). Also, the amount of stabilization depends on R3Si–
Cα–Cβ–Lg dihedral angle θ. The synperiplanar (θ = 0Њ) β-effect
Introduction
Acid catalyzed silanol elimination of α,β-dihydroxysilanes to
give carbonyl compounds is well documented.1–3 The obser-
vations by Stork, Hudrlik, and Whitham have clearly
demonstrated that the elimination proceeds via path (a) of
is proven to be much smaller than the antiperiplanar (θ = 180Њ)
β-effect.6–9
Scheme 1. However, in a rare observation by Cunico,4 it was
revealed that the TFA-catalyzed reaction of α,β-dihydroxy-
silanes containing bulky groups on the silyl moiety (R = t-Bu,
Scheme 1) yielded α-silylketones by silyl group migration, called
silapinacol rearrangement. These α-silylcarbonyl compounds
that are prone to protonic desilylation5 can also lead to carbonyl
compounds, path (b), (Scheme 1).
The process of silanol elimination begins with the proton-
ation of β-OH leading to the formation of a cation stabilized by
a β-silicon effect.6 The stereochemistry of elimination is similar
Scheme 2
to that observed in E2 reactions. Instead of the base abstracting
the proton β to the leaving group, the nucleophile (water)
attacks the silyl group resulting in simultaneous fission of the
C–ϩOH2 and C–SiMe3 bonds. The rate of elimination depends
on the magnitude of stabilization of the β-cation.7,8 The induct-
ive contribution (ϩI) from silicon is shown to be small and
independent of dihedral angle, as it occurs through bonds.6 The
magnitude of β-silicon stabilization depends on the (p–σ)-C–Si
hyperconjugation, which is maximized when the 2p orbital of
the cationic β-carbon and the C–Si σ-bonding orbital are
aligned in the same plane. Several studies7–10 have shown that,
in a system like R3Si–Cα–Cβ–Lg (Lg = leaving group), the
incipient β-cation may be stabilized in two ways, either by
hyperconjugation of the C–Si bond with the vacant p-orbital of
The β-effect providing low energy transition state and/or
intermediates causing rate enhancement of up to 1012 with
respect to model systems has been reported,7 especially in
solvolysis reactions of β-silylesters. The rate enhancement
was remarkably high (∼1012) for antiperiplanar disposition,
and moderately high (∼104) when these groups are gauche or
synperiplanar, compared with unsilylated analogues.
The existence of β-effect even in ground state has been noted
by a low temperature X-ray structure study10 of β-trimethylsilyl
esters. The observed lengthening of the C–O bond when the Si–
C–C–O dihedral angle is close to 180Њ is attributed to the
β-effect. The measured lengthening of the ester C–O bond in
Scheme 1
2248
J. Chem. Soc., Perkin Trans. 2, 2001, 2248–2252
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b108237j

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synperiplanar geometry was much smaller in comparison with
that of antiperiplanar geometry and has been ascribed to
much poorer vertical overlap between the σ-C–Si orbital and
neighbouring σ*-C–O orbital.
2 mm id). Temperature programming was as follows: initial
temperature 80 ЊC, hold time 2 min, heating rate 5 ЊC minϪ1
final temperature 180 ЊC. The injector and detector temper-
atures were 200 and 220 ЊC respectively.
,
The stereoelectronic requirement of β-silicon effect plays
a crucial role in silanol elimination and has been extensively
Chromatographic methods
utilized in stereo- and regiospecific C᎐C bond forming
᎐
1
Thin layer chromatographic method. In a 25 mL flask,
reactions under acid as well as base-catalyzed conditions.11 In
acyclic systems of any diastereomeric form, a free rotation
about the central Cα–Cβ bond can bring the β-cation into
the same plane as C–Si bond. However, in cyclic systems, only
if ring conformations can allow antiperiplanar disposition
of the silyl moiety with the β-OH group, the acid-catalyzed
silanol elimination is possible.3,12,13 Though some studies of
1 mmol of the cis-diol was taken and dissolved in 3 mL of 1,4-
dioxane. The flask was then placed in a water bath kept over a
magnetic stirrer fitted with a temperature sensor (sensitivity
0.2 ЊC). The desired temperature was set and when the bath
had reached the set temperature, 0.5 mL of 1 M H2SO4 was
added to the flask with a micro-pipette. The reaction was
followed at regular intervals by TLC. The TLC was performed
on glass plates coated with silica gel G and 10% ethyl acetate in
petroleum ether was used as eluent. The chromatogram was
developed in an iodine chamber. Initially the progress of the
reaction was checked every 30 min. As the reaction proceeded,
the rate slowed and the TLC was performed every hour. The
diols had Rf values of 0.4 and the ketones 0.8. The reaction was
taken to be complete when the diol spot could not be detected
on the TLC plate. At this stage the contents of the flask were
diluted with water (10 mL), the excess acid was neutralized
with 10% NaHCO3 solution and the ketone was extracted into
ether (3 × 10 mL). The combined ether layers were washed with
water (2 × 10 mL), saturated NaCl solution (10 mL) and dried
over anhydrous Na2SO4. The solvent was removed on a rotary
evaporator to obtain the ketone. The ketone was characterized
by its IR spectrum and by its semicarbazone and oxime
derivatives.
acid-catalyzed hydrolysis of
a few cis- and trans-1-tri-
methylsilyl-1,2-cycloalkanediols have been made, a thorough
investigation on the conformational requirements of acid
hydrolysis, and especially the related kinetic and thermo-
dynamic data are lacking. The present study addresses these
points.
The earlier studies by Lambert and co-workers7–9 for
assessing the β-silicon effect in different geometries were
carried out for solvolysis reactions that are mechanistically
quite different from silanol elimination.1,2 The developing
positive charge on β-carbon during silanol elimination, is
predominantly stabilized by vertical participation 1 (Scheme 2).
Thus, the dihedral angle of 180Њ is crucial for the silanol
elimination.
In acyclic α,β-dihydroxysilanes, whether erythro or threo,
the required antiperiplanar disposition of β-OH and SiMe3
(i.e., θ = 180Њ) is easily attainable by free rotation about Cα–Cβ
bond. However, in cyclic systems the attainability of the all
important 180Њ dihedral angle is decided by the conformational
flexibility of the ring, which means the ring size of the silyldiol.
Stork and co-workers,1 and Hudrlik and co-workers2 have
demonstrated smooth silanol elimination from both erythro
and threo silyldiols of acyclic systems. Robbins and Whitham3
2
Gas chromatographic method. Into a 25 mL flask, 1 mmol
of the cis-diol was weighed, and 3 mL of 1,4-dioxane and 150
mg of toluene were added. The flask was then placed in a water
bath kept on a magnetic stirrer fitted with a temperature sensor
(sensitivity 0.2 ЊC). When the solution reached the desired
temperature, 0.5 mL of 1 M H2SO4 was added to the flask with
a micro-pipette. An aliquot of 0.1 mL was immediately drawn
from the reaction mixture, two drops of saturated aqueous
NaHCO3 were added to neutralize the excess acid followed by
0.5 mL diethyl ether and shaken well. The ether layer was
separated, dried, and injected (1 µL) into the GC. The peak
areas of toluene and ketone were noted. The initial aliquot
showed no ketone in the reaction mixture. Aliquots taken at
regular intervals were worked up as above and injected (1 µL)
into GC. The area percentage of ketone was found to increase
with each injection. The percentage of ketone was measured
relative to that of toluene. The reaction was assumed to be
complete when the percentage of ketone relative to that of
toluene remained constant for three successive measurements.
observed
that
1-trimethylsilyl-trans-1,2-cyclohexanediol
obtained from acid hydrolysis of trimethylsilylcyclohexene
epoxide was resistant to further reaction even under drastic
conditions.
Though silanol eliminations have been studied for long, no
attempt has been made to correlate their kinetic characteristics
and thermodynamic parameters with ring structure. Most of
the silanol eliminations were observed in trans-silyldiols derived
from acyclic α,β-epoxysilanes and a few from the cyclic systems.
In an earlier study12 of the hydrolysis of cyclic α,β-epoxysilanes
derived from different ring sizes (medium to large rings), we
had noted that the medium ring (five and six membered)
epoxysilanes provided the most stable trans-α,β-dihydroxy-
silanes which did not hydrolyze further to cycloalkanone
while the 12-membered ring diol did.12 The essentially syn
stereochemistry of β-OH and SiMe3 groups in medium rings
prevented the attainment of antiperiplanar geometry, whereas
in the large ring there was no such restriction. The trimethyl-
silylcycloheptene epoxide and the trimethylsilylcyclooctene
epoxide did not give the expected silyldiols due to transannular
hydride migration followed by loss of silyl group.12,13
Quantitation of the chromatogram
Peak area normalization method was used to quantify the
chromatogram.15 To avoid the error due to variations in
injection volume and aliquot dilution, the concentration of
the cycloalkanone [x] was measured relative to an internal
standard. GC-pure toluene was used as an internal standard
because it is inert under the reaction conditions and has a
convenient retention time. The progress of hydrolysis was noted
by increase in the concentration of cycloalkanone, and the
reaction was assumed to be complete when three successive
GC measurements showed the same concentration of
cycloalkanone. Since a ketone is the only product of hydrolysis,
its final concentration is a direct measure of the initial
concentration of the diol [a]. The hydrolysis was carried out at
three different temperatures (298, 313 and 328 K). Plotting
ln [a/a Ϫ x] against time (t) produced a straight line graph in
each case, indicating that the rate of each reaction is first order
in silyldiol. A typical kinetic plot is given in Fig. 1. The rate
Experimental
Materials
The silyldiols 3–8 were prepared as described previously.14 All
chemicals and solvents were purified by standard procedures.
The identity and purity of the substrates and the products were
ascertained by their physical and spectral data, and those of
their derivatives.
GC analyses were carried out on a Varian Vista 6000
instrument using 15% FFAP on Chromosorb W column (2 m ×
J. Chem. Soc., Perkin Trans. 2, 2001, 2248–2252
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Table 1 Hydrolysis of cyclic cis-α,β-dihydroxysilanes 3–6. Rate constants and thermodynamic parameters
Rate constant of hydrolysis 105 k sϪ1
Diol
298 K
313 K
328 K
Ea/kJ molϪ1
∆H‡/kJ molϪ1
∆S‡/J KϪ1 molϪ1
∆G‡/kJ molϪ1
3
4
5
6
2.99
4.08
8.24
9.01
13.17
32.98
74.07
23.00
37.03
111.19
270.83
56.01
60.32
73.63
75.89
53.41 0.11
57.60 0.06
70.99 0.06
73.29 0.06
Ϫ152.50 3.17
Ϫ135.54 3.42
Ϫ85.39 4.20
Ϫ71.33 4.31
101.13 0.27
100.17 0.24
97.82 0.16
95.65 0.13
17.48
Fig. 1 Hydrolysis of silyldiol 3 at 298 K (᭺), 313 K (᭹), 328 K (᭛).
Scheme 3
ammonium permanganate.14 The silanol elimination was
carried out in 1,4-dioxane as the solvent and aqueous 1 M
H2SO4 as catalyst (see Experimental section for details).
As expected, the stereoelectronic requirement for silanol
elimination could be easily attained by a small rotation in 3–7,
because the eliminating groups in the substrates have the anti
stereochemistry. The diols 3–7 hydrolyzed smoothly to the
corresponding cycloalkanones, while 8 was resistant to acid
even at 75 ЊC, and decomposed on heating to 100 ЊC. A 60Њ
rotation around C2–C3 bond in the anticlinal (120Њ) position of
β-OH and SiMe3 groups to secure the required perfect anti-
periplanar geometry is prevented due to rigidity of the
bicycloheptane system 8.
Fig. 2 Arrhenius plot (Ϫln k vs 1/T ) of hydrolysis of silyldiol 3.
constants and energy of activation (Ea) were evaluated by
standard procedures.16 A representative Arrhenius plot (ln k vs.
1/T ) used to evaluate energy of activation is given in Fig. 2.
The regression coefficient (r) was found to be >0.9997. The
enthalpy of activation (∆H‡), entropy of activation (∆S‡), and
the free energy of activation (∆G‡) were calculated using
standard relations.17 The rate constants and thermodynamic
parameters are presented in Table 1.
The hydrolytic elimination of diols 5 and 6 occurred
smoothly without forming any transannular product. This is
in contrast to what was noted during the hydrolytic opening
of epoxides derived from trimethylsilylcycloheptene and
trimethylsilylcyclooctene.12
The silyldiol 7 was a mixture of diastereomers 7a and 7b,
formed from the cis hydroxylation of a 48 : 52 mixture
of cis- and trans-trimethylsilylcyclododecene. Though the
hydrolysis of the diastereomeric mixture was observed at
room temperature, heating to 50 ЊC for 2 h was required for
completion of the reaction. This may be attributed to the
difference in solubilities of the diastereomers and the rapid
conformational changes for attainment of antiperiplanar
array (Scheme 4).
Results and discussion
Complementary to our earlier observations, we have now
observed that cyclic cis-α,β-dihydroxysilanes which have
β-OH and SiMe3 groups anti to each other in rings of all sizes
hydrolyze smoothly to the corresponding cycloalkanones. A
detailed kinetic study was carried out, and the findings clearly
demonstrate the stereoelectronic requirement of β-silicon effect
in silanol elimination reactions, and its relationship with the
ring structure.
Kinetic results
For the present study, six different cyclic cis-silyldiols
comprising of five- and six-membered ring diols 3 and 4, seven-
and eight-membered ring diols 5 and 6, one large ring (twelve-
membered) diol 7, and one rigid bicyclic diol 8 were used
(Scheme 3), all of which evidently have the β-OH group anti to
the SiMe3 group. It should be noted that the two groups in 8
have the dihedral angle fixed at 120Њ, while in others it can
attain 180Њ.
To measure the rates of hydrolysis, the time required for each
reaction was qualitatively estimated by TLC. However, the
actual rate data were obtained by measuring the progress of the
reaction by GC using an internal standard. The TLC method
was useful in obtaining the approximate time required for
hydrolysis and it also gave an idea about the susceptibility of
the diol for hydrolysis. The method was applied to all the diols
(3–8) at two different temperatures (298 and 313 K).
The silyldiols were prepared by cis-hydroxylation of the
corresponding cyclic vinysilanes using cetyltrimethyl-
Kinetically, the silanol elimination is first order in silyldiol.
Hence, by measuring the rate of consumption of the silyldiol
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Scheme 4
Scheme 5
or the rate of formation of cycloalkanone it is possible to get
kinetic data. Since the hydrolysis product, the ketone, is a
liquid, it is convenient to measure its concentration by the
quantitation of gas chromatograms of aliquots taken at regular
intervals during hydrolysis. However, this method was useful in
measuring the concentration of cycloalkanones obtained in the
hydrolysis of silyldiols 3–6, but not 7 as it was not possible to
measure accurately the concentration of cyclododecanone by
GC.
The silanol elimination is taking place from a protonated
intermediate 9 (Scheme 3), which may be considered as the
activated complex.19 There would be a thermal equilibrium
between the activated complex and the substrate. The critical
configuration of 9, i.e. antiperiplanar array, leads to the
product. The rate at which 9 collapses to product would
determine the rate of hydrolysis. The difference between the
average energy of the reactant and that of the activated
complex is the enthalpy of activation. It includes the difference
in bond dissociation energy, conformational changes, strain,
and solvation energy between the substrate and the activated
complex. Since all these factors contribute to ∆H‡, it is bound
to vary from system to system. The activated complex seems to
eliminate silanol faster in the larger ring diols (5 and 6) due to
higher probability to attain the favourable conformation in
these cases. This is perhaps determined by the proportion of
different conformers present, governed by their relative energies
and the Boltzmann distribution. The observed increasing trend
in rate constants with increasing enthalpy must be ultimately
controlled by entropy cancellation. The four atoms Me3Si–Cα–
Cβ–ϩOH2, and the attacking nucleophile H2O must be in the
same plane in the transition state. In order to achieve this
specific orientation, the molecule must surrender the freedom it
normally has to assume many possible forms in space, and
adopt only the one that leads to elimination. Thus, there is a
considerable loss of entropy, and ∆S# is negative. The ∆S‡ for 3
and 4 involves reorganization of the conformationally more
stable diequatorial geometry of the eliminating groups to the
less stable diaxial geometry at some stage of formation of the
transition state. It is reflected in the greater negative values of
∆S‡ for these systems than in the case of the more agile 5 and 6
(Table 1). In the case of 8, where this geometry is unattainable,
the expected hydrolysis does not occur even at relatively high
temperatures, and decomposition of the substrate takes place.
The kinetic results were as expected. As the ring size
increased, the conformational freedom increased,18 and with
that the rate of hydrolysis increased from the five-membered
ring silyldiol to the eight membered one. The intra-
molecular hydrogen bonding is one factor that may influence
the conformational characteristics of the silyldiols. This
bonding is strongest in the five-membered ring cis-diol18 and
decreases along the series and reaches a minimum in the
eight-membered ring, as observed in their IR spectra.14 Though
we have not been able to quantify this under our reaction
conditions, the fact that each substrate has to follow a similar
mechanistic pathway, as indicated by similar ∆G‡ values, we
consider that stronger intramolecular hydrogen bonding is at
least partly responsible for the slower reaction rates in lower
member diols (3 and 4) than in larger rings (5 and 6). Another
factor that could influence the rate of elimination is the
stereoelectronic effect, especially in six-membered ring silyldiol
4. Since trans eliminations are most favoured in cyclohexane
systems, the diol 4 was expected to hydrolyze faster than 5 and
6. This turned out to be not the case. In the cyclohexane system
(4) the only conformation that fulfills the requirement of
leaving groups, i.e. SiMe3 and β-OH, to be antiperiplanar is the
one in which they are trans diaxial (4b). In 4 the preferred
position for the bulky SiMe3 group is naturally equatorial.
Thus, the β-OH and SiMe3 would be in a trans-diequatorial
position (4a), which is a considerable deviation (60Њ) from the
needed antiperiplanar geometry. For elimination to occur, the
molecule has to transform such that SiMe3 and β-OH groups
attain the trans-diaxial geometry. This process entails some
energy barrier that probably causes the hydrolysis of 4 to be
slower than that of 5 and 6 (Scheme 5).
Though the rate of hydrolysis increased with ring size, it was
noticed that the enthalpy of activation also increased along the
series. The ∆H‡ for the set of 3 and 4 are of similar value and
those for the set of 5 and 6 are of similar value. Thus there is a
significant difference between the ∆H‡ values of the two sets.
Though the reaction rates increased with temperature, which is
on the expected lines, the rate enhancement for every 15 ЊC rise
in temperature was higher for 5 and 6 (∼4 times) than for 3 and
4 (∼3 times). For an enthalpy controlled reaction, lower rate
means higher enthalpy. However, in this case, though the rate of
hydrolysis was higher for larger rings (5 and 6), the enthalpy of
activation was also higher. This is an unexpected but interesting
observation, which we believe may be explained in the following
way.
Acknowledgements
This work was partly financed by UGC-DRS and COSIST
programmes and DST, New Delhi. MNM thanks UGC, New
Delhi, for a Senior Research Fellowship. Donation of some
equipment by the Alexander-von-Humboldt Foundation,
Germany, is gratefully acknowledged.
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