Protonation of trimethylsilyl-substituted carbon-carbon multiple bonds in aliphatic systems. Conformational dependence of the β-silyl stabilizaton of carbocations

Article abstract of DOI:10.1021/ja9537686

Rates of carbon protonation to give carbocation products were measured in concentrated perchloric acid solutions for cyclohexene, propyne, 1-hexyne, and their 1-trimethylsilyl-substituted analogs. The trimethylsilyl substituent accelerated the reaction markedly and provided the following β-silyl carbocation stabilizing effects: δΔG^? = 5.7 kcal/mol^-1 for the cyclohexyl system and δΔG^? = 6.5 kcal mol^-1 for both acetylenic systems. These effects are substantially greater than δΔG^? = 2.9 and 3.4 kcal mol^-1 found previously for the protonation of ethyl vinyl ether and phenylacetylene, which suggests that the silyl effects in these previous systems were attenuated by additional carbocation stabilization provided through their ethoxy and phenyl groups. The present effects, on the other hand, fall far short of δΔG^? = 16-17 kcal mol^-1 found for conformationally optimum systems. The influence of conformation on the magnitude of β-silyl effects and how this impinges on the presently studied systems is discussed.

Full text of DOI:10.1021/ja9537686

3
838
J. Am. Chem. Soc. 1996, 118, 3838-3841
Protonation of Trimethylsilyl-Substituted Carbon-Carbon
Multiple Bonds in Aliphatic Systems. Conformational
Dependence of the â-Silyl Stabilizaton of Carbocations
V. Gabelica and A. J. Kresge*
Contribution from the Department of Chemistry, UniVersity of Toronto,
Toronto, Ontario M5S 1A1, Canada
X
ReceiVed NoVember 9, 1995
Abstract: Rates of carbon protonation to give carbocation products were measured in concentrated perchloric acid
solutions for cyclohexene, propyne, 1-hexyne, and their 1-trimethylsilyl-substituted analogs. The trimethylsilyl
q
substituent accelerated the reaction markedly and provided the following â-silyl carbocation stabilizing effects: δ∆G
-
1
q
-1
)
5.7 kcal/mol for the cyclohexyl system and δ∆G ) 6.5 kcal mol for both acetylenic systems. These effects
q
-1
are substantially greater than δ∆G ) 2.9 and 3.4 kcal mol found previously for the protonation of ethyl vinyl
ether and phenylacetylene, which suggests that the silyl effects in these previous systems were attenuated by additional
carbocation stabilization provided through their ethoxy and phenyl groups. The present effects, on the other hand,
q
-1
fall far short of δ∆G ) 16-17 kcal mol found for conformationally optimum systems. The influence of
conformation on the magnitude of â-silyl effects and how this impinges on the presently studied systems is discussed.
It is well known that â-trimethylsilyl substituents can stabilize
carbocations strongly: effects of several tens of kcal mol have
the conformational requirements for maximum stabilization,
1
-1
while satisfied in the systems producing the large â-silyl
2
3
4,5
been calculated and observed for systems in the gas phase
effects, were probably not met in the transition states of the
4
5
-1
6,7
and stabilizations of 17 and 16 kcal mol have been found
for reactions in solution. In contrast to this, effects of only 2.9
vinyl ether and acetylene protonation reactions. The develop-
ing carbocationic centers in the latter reactions, however, are
also stabilized by ethoxy and phenyl groups, and these additional
substituents could have served to attenuate the â-silyl effect.
In order to determine whether or not this was so, we have now
examined the purely aliphatic systems shown in eqs 3-5, X )
H, SiMe3. We have found that the â-silyl effects here are
-
1
and 3.4 kcal mol have been observed in the carbon protonation
6
7
of ethyl vinyl ether and phenylacetylene, eqs 1 and 2, X ) H,
SiMe3.
We have attributed this striking difference to the largely
hyperconjugative nature of the â-silyl effect2,4,8 and the fact that
X
Abstract published in AdVance ACS Abstracts, April 1, 1996.
(
1) For reviews, see: Lambert, J. B. Tetrahedron 1988, 46, 2677-2689.
Apeloig, Y. In The Chemistry of Organic Silicon Compounds; Patai, S.,
Rappoport, Z., Eds.; Wiley-Interscience: New York, 1989; pp 57-225.
Fleming, I. CHEMTRACTS Org. Chem. 1993, 6, 113-116.
substantially greater than those in the additionally stabilized
systems of eqs 1 and 2, but that they still fall considerably short
of the very large effects found in conformationally optimum
(2) Wierscke, S. G.; Chandrasekhar, J.; Jorgensen, W. L. J. Am. Chem.
Soc. 1985, 107, 1496-1500. Ibrahim, M. R.; Jorgensen, W. L. J. Am. Chem.
Soc. 1989, 111, 819-824. Apeloig, Y.; Arad, D. J. Am. Chem. Soc. 1985,
1
07, 5285-5286. Apeloig, Y.; Karmi, M.; Stanger, A.; Schwarz, H.;
4
,5
systems.
Drewello, T.; Czekay, G. J. Chem. Soc., Chem. Commun. 1987, 989-991.
Nguyen, K. A.; Gordon, M. S.; Wang, G.-t.; Lambert, J. B. Organometallics
1
991, 10, 2798-2803.
3) Hajdasz, D.; Squires, R. J. Chem. Soc., Chem. Commun. 1988, 1212-
Experimental Section
(
1
214. Li, X.; Stone, J. A. J. Am. Chem. Soc. 1989, 111, 5586-5592.
Materials. 1-(Trimethylsilyl)cyclohexene was prepared from cy-
McGibbon, G. A.; Brook, M. A.; Terlouw, J. K. J. Chem. Soc., Chem.
Commun. 1992, 360-362.
clohexanone via the tosylhydrazone, as described.4 All other materials
were best available commercial grades.
(4) Lambert, J. B.; Wang, G.-t.; Finzel, R. B.; Teramura, D. H. J. Am.
Product Analyses. Large volumes (ca. 100 mL) of reaction
solutions with substrate and acid concentrations similar to those used
in the kinetic measurements were allowed to react for 10 half-lives.
These solutions were then extracted with small (2 mL) portions of
Chem. Soc. 1987, 109, 7838-7845.
(5) Bausch, M. J.; Gong, Y. J. Am. Chem. Soc. 1994, 116, 5963-5964.
(6) Kresge, A. J.; Tobin, J. B. J. Phys. Org. Chem. 1991, 4, 587-591.
(7) Kresge, A. J.; Tobin, J. B. Angew. Chem., Int. Ed. Engl. 1993, 32,
7
21-723.
3
CDCl . This was done by shaking the reaction solutions with CDCl
3
(8) Lambert, J. B.; Emblidge, R. W.; Malany, S. J. Am. Chem. Soc. 1993,
in flasks to whose bottoms small cones had been fused; the CDCl
3
1
4
1
15, 1317-1320. White, J. M.; Robertson, G. B. J. Org. Chem. 1992, 57,
638-4644. Kuan, Y. L.; White, J. M. J. Chem. Soc., Chem. Commun.
994, 1195-1196. Green, A. J.; Kuan, Y. L.; White, J. M. J. Chem. Soc.,
extracts settled into these cones, and could then be easily withdrawn
using Pasteur pipets. The extracts were dried over molecular sieves
and were then analyzed by NMR. This method produced extracts
Chem. Commun. 1994, 2023-2024.
0
002-7863/96/1518-3838$12.00/0 © 1996 American Chemical Society

Protonation of Me3Si-Substituted C-C Multiple Bonds
J. Am. Chem. Soc., Vol. 118, No. 16, 1996 3839
whose product concentrations were substantially greater than the
solutions extracted, thus facilitating the NMR analysis.
Kinetics. Reaction rates were determined spectroscopically by
monitoring the decrease in alkene or acetylene absorbance at 200-
2
05 nm. Measurements were made with Cary 118 and 2200 spec-
trometers whose cell compartments were thermostated at 25.0 ( 0.05
C. Portions (3.0 mL) of aqueous acids contained in cuvettes were
°
first allowed to come to temperature equilibrium with the spectrometer
cell compartments, and reactions were then initated by adding 5.0-µL
aliquots of stock solutions of substrates dissolved in acetonitrile to these
acid solutions; final substrate concentrations in the reaction mixtures
-
4
were ca. 10 M. Absorbance decreases were followed for several
half-lives. The data conformed to the first-order rate law well, and
first-order rate constants were obtained by nonlinear least squares fitting
of exponential functions. Acid concentrations were determined by
titration of weighed samples.
Results
Figure 1. Cox-Yates plot for the protonation of 1-(trimethylsilyl)-
1
-propyne in aqueous perchloric acid at 25 °C.
Product analyses were conducted for the reaction of 1-(tri-
methylsilyl)cyclohexene and 1-(trimethylsilyl)-1-hexyne with
aqueous perchloric acid at acid concentrations near the midpoints
of the ranges used for kinetic measurements (13.8 and 27.4 wt
Table 1. Summary of Rate Data for the Protonation of
Cyclohexene, Propyne, 1-Hexyne, and Their Trimethylsilyl
Derivatives in Aqueous Solution at 25 °C
%
, respectively). Proton and carbon-13 NMR spectra of the
H+/M- s-1
1
ma
+/k
+
D
substrate
cyclohexene
-(trimethylsilyl)cyclohexene
propyne
-(trimethylsilyl)-1-propyne
1-hexyne
k
k
H
spent reaction mixtures showed cyclohexene to be the only
product formed from the cyclohexene substrate and 1-hexyne
to be the only product formed from the hexyne substrate.
Elimination of the trimethylsilyl moiety from the carbocations
formed in these olefin and acetylene protonation reactions thus
occurs more rapidly than capture of the cations by solvent. This
-
-
8
1.73 × 10
2.10 × 10
6.11 × 10
3.32 × 10
1.37 × 10
7.78 × 10
1.50
1.54
1.13
1.45
1.02
1.26
4
1
2.25
1.55
1.28
-10
-5
1
-
-
9
5
1
-(trimethylsilyl)-1-hexyne
is consistent with results obtained in the protonation of
a
Cox-Yates correlation slope.
7
(phenyltrimethylsilyl)acetylene and the solvolysis of 2-(tri-
4
methylsilyl)cyclohexyl derivatives, as well as with the general
propensity of â-trimethylsilyl cations to lose their trimethylsilyl
groups.
Rates of protonation of the presently studied alkenes and
alkynes were measured in moderately concentrated aqueous
perchloric acid solutions. For each substrate, duplicate or
triplicate determinations were made at each of 5-6 acid
concentrations. The data so obtained are summarized in Table
examined conformed to this relationship well; an example is
shown in Figure 1. The values of kH obtained in this way are
+
9
listed in Table 1.
Some rate constants were also determined for the silyl-
substituted substrates in solutions of DClO4 in D2O. Replicate
measurements were made for each substrate at a single acid
concentration in the concentration range used for the measure-
10
ments in H2O solution; these data are summarized in Table S2.
1
0
S1.
Kinetic isotope effects were evaluated from the results obtained
by comparison with H2O rate constants at the same acid
concentration; the latter were obtained by interpolation of the
Cox-Yates correlations. The results are listed in Table 1.
Rates of protonation of cyclohexene in aqueous perchloric
As is usual for the protonation of carbon-carbon double and
triple bonds, rates of reaction of the present substrates increased
with increasing acidity of the medium more rapidly than in direct
proportion to acid concentration, and an acidity function was
1
1
therefore used to analyze the data. The Cox-Yates method
14
acid have been measured before, and the dilute-solution
12
using the X0 function appears to be the best procedure currently
avialable for this purpose.13 The data were fitted using the
expression shown in eq 6, where kH+ is the bimolecular
hydronium ion rate constant that applies in dilute solution and
-8
-1
+
hydronium ion rate constant reported, kH ) 4.43 × 10
M
-1
+
s , though broadly consistent with our result, kH ) 1.73 ×
-8
-1 -1
1
0
M
s , is not in good numerical agreement with it. In
the previous study, however, extrapolation to dilute solution was
performed using the H0 acidity function, which is not an
appropriate acidity scale for this reaction. Fortunately, the raw
+
log (kobs/[H ]) ) log kH
+
+ mX0
(6)
data were also published in this earlier report, and reanalysis
+
8
m is a slope parameter. The data obtained for all six substrates
of that using the Cox-Yates method gives k
) 1.67 × 10
H
-1 -1
M
s , in very good agreement with the presently determined
value.
Rates of protonation of propyne and 1-hexyne were also
(9) Olah, G. A.; Berrier, A. L.; Field, L. D.; Prakash, G. K. S. J. Am.
Chem. Soc. 1982, 104, 1349-1355. Prakash, G. K. S.; Reddy, V. K.; Rasul,
G.; Casanova, J.; Olah, G. A. J. Am. Chem. Soc. 1992, 114, 3076-3078.
Siehl, H.-U.; Kaufmann, F.-P.; Apeloig, Y.; Braude, V.; Danovich, D.;
Berndt, A.; Stamatis, N. Angew. Chem., Int. Ed. Engl. 1991, 30, 1479-
15
determined before, but the previous measurements were made
in sulfuric acid rather than the perchloric acid used here.
Sulfuric acid solutions at the concentrations employed contain
appreciable quantities of undissociated bisulfate ion, which
functions as an efficient general acid proton donor and makes
rates of protonation in this medium considerably faster than
1
4
1
1
482. Siehl, H.-U.; Kaufmann, F.-P. J. Am. Chem. Soc. 1992, 114, 4937-
939. Siehl, H.-U.; Kaufmann, F.-P.; Hori, K. J. Am. Chem. Soc. 1992,
14, 9343-9349. Lew, C. S. Q.; McClelland, R. A. J. Am. Chem. Soc.
993, 115, 11516-11520. Lew, C. S. Q.; McClelland, R. A.; Johnston, L.
J.; Schepp, N. P. J. Chem. Soc., Perkin Trans. 2 1994, 395-397.
10) Supporting information; see paragraph at the end of this paper
regarding availability.
(
(
11) (a) Cox, R. A.; Yates, K. J. Am. Chem. Soc. 1978, 100, 3861-
(14) Chwang, W. K.; Nowlan, V. J.; Tidwell, T. T. J. Am. Chem. Soc.
1977, 99, 7233-7238.
(15) Cramer, P.; Tidwell, T. T. J. Org. Chem. 1981, 46, 2683-2686.
Allen, A. D.; Chiang, Y.; Kresge, A. J.; Tidwell, T. T. J. Org. Chem. 1982,
47, 775-779.
3
867. (b) Cox, R. A. Acct. Chem. Res. 1987, 20, 27-31.
(
(
12) Cox, R. A.; Yates, K. Can. J. Chem. 1981, 59, 2116-2124.
13) Kresge, A. J.; Chen, H. J.; Capen, G. L.; Powell, M F. Can. J. Chem.
1
983, 61, 249-256.

3
840 J. Am. Chem. Soc., Vol. 118, No. 16, 1996
Gabelica and Kresge
Table 2. Rate Accelerations Provided by â-Trimethylsilyl
a
Substituents in Alkene and Alkyne Protonation Reactions
the conformational requirements for maximum hyperconjugative
interaction were met in the systems for which these large effects
were observed, but were not met in the presently studied cases.
Hyperconjugative stabilization of a carbocation involves
interaction between the filled orbital of the stabilizing bond and
the vacant orbital of the cationic center. This interaction will
be maximum when these orbitals are coplanar, i.e., when the
dihedral angle between them is zero. This angle was constrained
to be zero in 1 by the conformationally biasing 4-tert-butyl group
and the trans arrangement of the trimethylsilyl substituent and
4
the leaving group in the cation precursor, and it was probably
a
In aqueous solution at 25 °C.
zero in 2 as well, for that would put its trimethylsilyl substituent
in a plane perpendicular to the aromatic framework, free of steric
interaction with peri hydrogens in the 1 and 8 positions; the
â-silyl effect in the case of 2, moreover, was on an equilibrium
constant rather than a rate constant, and it should consequently
those in a completely dissociated acid such as perchloric; this
is so even when comparison is made at the same value of an
appropriate acidity function.16 Numerical agreement of the
present and previous results should therefore not be expected,
but it is gratifying that Cox-Yates treatment of the previous
data gives rate constants that are consistently greater than those
determined here.
5
refer to 2 in its most stable equilibrium position.
In the presently studied cyclohexyl system, on the other hand,
this dihedral angle was not zero. In the carbon protonation of
an olefin such as cyclohexene, the proton attacks the olefinic
π-system in a direction perpendicular to its σ-bond framework,
forming a bond with one of the π-system’s constituent p-orbitals
and converting the other into the vacant p-orbital of the cationic
center. In the case of (trimethylsilyl)cyclohexene, attack occurs
at the carbon atom bearing the silyl substituent, as shown in eq
Discussion
Simple alkenes and alkynes such as the cyclohexene, propyne,
and 1-hexyne used in the present study undergo hydration to
the corresponding alcohols in concentrated aqueous mineral
acids; in the case of alkynes, the alcohols formed are enols,
which then tautomerize to their keto isomers. It is now well
established that the first and rate-determining step in these
processes is carbon protonation to give carbocation reaction
intermediates, as shown in eqs 3-5.14,15 The trimethylsilyl
derivatives of these substrates examined here also undergo
carbon protonation, but now the carbocations formed lose their
labile trimethylsilyl groups rather than become hydrated.
Nevertheless, the carbon protonation step is still rate-determin-
ing, as indicated by the normal isotope effects (kH/kD > 1) that
these reactions give and also by the fact that their Cox-Yates
slope parameters are in the range expected for a rate-determining
7
, and the state of hybridization of this carbon atom changes
2
3
from sp to sp . At the beginning of this process, the dihedral
angle between the carbon-silicon bond and the forming vacant
p-orbital is θ ) 90°, as shown in the Newman projection 3,
and at the end of the process it reaches the nominal value θ )
0°, as shown in 4. In the transition state of this reaction the
dihedral angle will have some value between these limits, but
proton transfer reaction.1
1b,17
6
The protonation of each of these trimethylsilyl substituted
substrates is considerably faster than that of its unsilylated
counterpart. The rate ratios, listed in Table 2, range from 15 000
to 57 000 and correspond to â-silyl effects, expressed as
differences in free energy of activation, of δ∆G ) 5.7 to 6.5
kcal mol . These effects are considerably greater than those
q
-
1
q
found before for the protonation of ethyl vinyl ether, δ∆G )
.9 kcal mol , and phenylacetylene, δ∆G ) 3.4 kcal mol .
-1 6
q
-1 7
2
This difference indicates that the â-silyl effects in the vinyl ether
and phenylacetylene systems were in fact attenuated by the
cation-stabilizing ability of their ethoxy and phenyl groups.
The presently determined â-silyl effects, however, are still
it is unlikely to be less than 60°. If, as is commonly believed,
the interaction varies as the cosine of the dihedral angle squared,
then the hyperconjugative stabilization in the protonation of
-
1
much less than the 16-17 kcal mol stabilizations found for
1
-(trimethylsilyl)cyclohexene will not be greater than (cos 60°)2
4
generation of the 2-(trimethylsilyl)cyclohexyl cation 1 and the
)
0.25 of the maximum value expressed in a conformationally
optimum system.
5
9
-((trimethylsilyl)methyl)fluorenyl cation 2 in solution. We
believe this difference to be due to the now quite well established
The situation is somewhat different in the case of the
acetylenic systems studied here, because now the substrate has
two π-systems formed from p-orbitals at right angles to one
another. Proton attack will occur on one of these π-systems,
leaving the other intact. This will create a vinyl cation in which,
as shown in eq 8, the carbon-silicon bond and the vacant
hyperconjugative nature of the â-silyl effect2 and the fact that
,4,8
(16) Kresge, A. J.; Hakka, L. E.; Mylonakis, S.; Sato, Y. Discuss.
Faraday Soc. 1965, 39, 75-83. Kresge, A. J.; Mylonakis, S.; Hakka, L. E.
J. Am. Chem. Soc. 1972, 94, 4917-4199.
(
17) Cox, R. A.; Grant, E.; Whitaker, T.; Tidwell, T. T. Can. J. Chem.
1
990, 68, 1876-1881.

Protonation of Me3Si-Substituted C-C Multiple Bonds
J. Am. Chem. Soc., Vol. 118, No. 16, 1996 3841
systems, but in those cases this angle will be much less
unfavorable. In the systems showing the very large â-silyl
4
,5
effects, for example, the silyl substituent was connected to
the cationic center by a saturated carbon atom for which the
2
nominal value of φ is 109.5° and (cos (109.5° - 90°)) ) 0.89,
not far from its maximum value of 1.00 for exactly parallel
orbitals with φ ) 90°. In the cyclohexyl case studied here,
this aspect of the conformational problem was also quite good.
The carbon atom connecting the silyl substituent to the cationic
center there changed from olefinic to saturated as the reaction
progressed, and φ therefore varied from 120° to 109.5°. The
p-orbital lie in the same plane and the dihedral angle conse-
quently has its optimum value of zero. This dihedral angle,
moreover, will be zero throughout the entire protonation process.
There is another angle, however, designated φ in side view 5,
that must be considered in acetylenic systems. At the beginning
of the proton addition process, the carbon-silicon bond is
colinear with the acetylenic carbon-carbon triple bond and φ
2
corresponding values of (cos (φ - 90°)) are 0.75 and 0.89,
again not far away from the optimum 90° value of 1.00.
These considerations show that, whereas the angle φ plays a
dominant role in controlling the magnitude of the â-silyl effect
in the protonation of acetylenes, it has much less influence on
this effect in the protonation of alkenes. In the latter reaction,
on the other hand, the magnitude of the â-silyl effect is strongly
reduced by unfavorable values of the dihedral angle θ.
)
180°. In this configuration, the carbon-silicon bond and
the developing p-orbital are perpendicular to one another and
no hyperconjugative interaction is possible. Hyperconjugation
will of course develop as the protonation process progresses,
2
for φ will change toward its nominal sp value of 120° and
approach the optimum value of 90° for exactly parallel orbitals.
In the transition state of the reaction, however, the change will
not be complete and hyperconjugative stabilization will be less
than it would be in a conformationally optimum system. If the
cosine squared relationship holds here as well, then the
Acknowledgment. We are grateful to the Natural Sciences
and Engineering Research Council of Canada for financial
support of this work.
Supporting Information Available: Tables S1 and S2 of
rate data (4 pages). This material is contained in many libraries
on microfiche, immediately follows this article in the microfilm
version of the journal, can be ordered from the ACS, and can
be downloaded from the Internet; see any current masthead page
for ordering information and Internet access instructions.
magnitude of the interaction will vary in proportion to (cos (φ
2
-
90°)) , and for a transition state in which the geometrical
2
change is half complete and φ ) 150°, (cos (150° - 90°)) )
.25, and the stabilization will be only a quarter of its maximum
value.
The value of an angle corresponding to φ will also influence
the magnitude of hyperconjugative interactions in nonacetylenic
0
JA9537686