Effect of a trimethylsilyl moiety on the nucleophilic character of the C=C bond: A comparative kinetic investigation of the epoxidation of substituted and unsubstituted cycloalkenes

Article abstract of DOI:10.1039/b101752g

The rates of epoxidation of twelve cycloalkenes (6-17) with MCPBA were determined at four temperatures (298, 303, 308 and 313 K). All of them were found to follow second-order kinetics. The silylated cycloalkenes (10-13) react faster than the corresponding unsubstituted cycloalkenes (6-9), but slower than the corresponding methyl cycloalkenes (14-17). Thus, for epoxidation, the silyl moiety is a deactivating group relative to an alkyl group, while it is activating in comparison with hydrogen. When the homologous series 6-9, 10-13 and 14-17 are considered, the order of the rates of epoxidation in each series seems to follow the order of strain energy of its members, i.e., the rate decreases in the sequence: 5-membered > 8-membered > 7-membered > 6-membered. The rate data obtained at four temperatures were employed to calculate ΔH?, ΔS? and ΔG?, based on which a common reaction mechanism is proposed.

Full text of DOI:10.1039/b101752g

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Effect of a trimethylsilyl moiety on the nucleophilic character
of the C᎐C bond: a comparative kinetic investigation of the
epoxidation of substituted and unsubstituted cycloalkenes
2
Govindagouda S. Patil and Gopalpur Nagendrappa*
Department of Chemistry, Bangalore University (Central College Campus),
Bangalore-560 001, India
Received (in Cambridge, UK) 23rd February 2001, Accepted 14th May 2001
First published as an Advance Article on the web 12th June 2001
The rates of epoxidation of twelve cycloalkenes (6–17) with MCPBA were determined at four temperatures (298, 303,
08 and 313 K). All of them were found to follow second-order kinetics. The silylated cycloalkenes (10–13) react
3
faster than the corresponding unsubstituted cycloalkenes (6–9), but slower than the corresponding methyl
cycloalkenes (14–17). Thus, for epoxidation, the silyl moiety is a deactivating group relative to an alkyl group, while
it is activating in comparison with hydrogen. When the homologous series 6–9, 10–13 and 14–17 are considered, the
order of the rates of epoxidation in each series seems to follow the order of strain energy of its members, i.e., the rate
decreases in the sequence: 5-membered > 8-membered > 7-membered > 6-membered. The rate data obtained at four
‡
‡
‡
temperatures were employed to calculate ∆H , ∆S and ∆G , based on which a common reaction mechanism is
proposed.
Introduction
The ionization energies measured by photoelectron and mass
1
2
3
4
spectroscopy, and the EPR, NMR, vibrational and electron
5
transmission spectra of various organosilicon compounds,
and also the theoretical calculations of their molecular orbital
6
energies have shown that a vinylic silyl group decreases the
nucleophilicity of the C᎐C bond due to (p–d) back bonding or
᎐
π
the interaction of the σ* orbital of the Si–C bond with vicinal π
electrons. A decrease in the nucleophilicity of the C᎐C bond
᎐
due to the presence of a vinylic silyl group, relative to an
unsilylated olefin, has been observed in some reactions of7
vinylsilanes with electrophilic reagents such as free radicals,
Scheme 2
8
9
10
carbenes, iodomethylzinc iodide and singlet oxygen. Kinetic
studies of the epoxidation of hept-1-ene (1) and vinylsilanes 2
and 3 with perbenzoic acid have shown that the vinylsilanes are
observed behaviour of silylated and unsilylated C᎐C bonds in 5:
Although the vinylsilane group is said to be mildly deactivat-
᎐
“
ing towards electrophilic attack owing to electron acceptor
properties of d orbital or antibonding orbitals, examination of
reported epoxidation rates shows that it is the replacement of
carbon by silicon which results in deactivation. The silylated
double bond in 5 differs from the unsilylated one in that hydro-
gen has been replaced by silicon to give a more substituted
double bond, possibly activated towards epoxidation . . .” The
same explanation may be extended to the epoxidation of 4.
However, no systematic investigation to correlate the character-
istics of the silylated double bond with those of unsilylated or
alkylated double bonds towards epoxidation has so far been
reported. It is important, in the chemistry of vinylsilanes, that
we have clear understanding of the role of the silyl moiety in
such reactions and resolve the controversies noted earlier. We
report here the results of a kinetic study of the epoxidation of
several cycloalkenes aimed at appreciating the comparative role
of the trimethylsilyl group.
11
less reactive than hept-1-ene (1) (Scheme 1). However, reports
Scheme 1
on the relative rates of the epoxidation of vinylsilanes and sim-
ple olefins are rather confusing. While two examples show the
epoxidation of the unsilylated double bond to be faster than that
11
of the silylated double bond (Scheme 1), a few other cases
12
indicate the opposite.
The epoxidation of 1-trimethylsilylcycloocta-1,5-diene (4)
1
2a
13
with peracetic acid, monoperphthalic acid (MPPA) and
13
m-chloroperbenzoic acid (MCPBA) has been found to give, in
each case, a mixture of monoepoxides, with the epoxide of the
silylated double bond in excess of the unsilylated epoxide. The
reaction of 2-trimethylsilylhexa-1,5-diene (5) with MCPBA has
also shown that the silylated double bond is considerably more
Experimental
Materials
Cycloheptene (8), (Z)-cyclooctene (9), 1-methylcyclopentene
(14) and 1-methylcyclohexene (15) were commercial analytical
reagent grade samples and were distilled before use. MCPBA
12b
reactive towards epoxidation than the unsilylated one
Scheme 2). Peterson gave the following explanation for the
12b
(
DOI: 10.1039/b101752g
J. Chem. Soc., Perkin Trans. 2, 2001, 1099–1102
This journal is © The Royal Society of Chemistry 2001
1099

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3
Ϫ1
Ϫ1
(
Lancaster, UK) was estimated by an iodimetric method for its
Table 1 Second-order rate constants (10k/dm mol
s ) for the
epoxidation of the cycloalkenes 6–17 with MCPBA in benzene at
peracid content. Cyclopentene (6), cyclohexene (7), 1-methyl-
cycloheptene (16) and 1-methylcyclooctene (17) were prepared
by the dehydration of their respective alcohols. 1-Trimethyl-
silylcycloalkenes (10–13) were prepared by the reported
a
various temperatures
14
T/K
15
procedure. All the solvents were distilled before use.
Cycloalkene
298
303
1.58
308
2.38
1.83
2.00
2.04
6.75
4.33
4.50
4.53
19.50
8.00
10.00
12.67
313
3.54
Kinetic measurements
6
7
8
9
1.04
0.67
0.79
0.96
3.33
1.83
2.08
2.17
12.56
4.50
6.06
7.78
1.08
1.29
1.50
4.67
2.92
3.00
3.17
16.11
6.00
8.00
10.37
2.92
3.17
3.54
9.00
6.75
6.92
Equal volumes (12.5 mL each) of the solutions of an olefin
Ϫ3
Ϫ3
(
6–17, 0.01 mol dm ) and MCPBA (0.01 mol dm ) in
benzene, taken separately in 50 mL round-bottomed flasks,
were thermostatted at the desired temperature for 30 min.
When the solutions had attained the bath temperature, they
were mixed rapidly and stirred magnetically. The progress of
reaction was monitored by withdrawing aliquots at regular time
intervals and determining the amount of unreacted MCPBA
iodimetrically. The course of reaction was studied up to 85–
1
0
11
12
1
1
1
1
1
3
4
5
6
7
7.17
b
—
10.25
12.90
16.00
9
5% conversion. The calculated second-order rate constants
a
Ϫ3
Ϫ3
b
[
MCPBA] = 0.01 mol dm , [cycloalkene] = 0.01 mol dm
.
The
were reproducible to within ±3%.
reaction was too fast to follow.
Qualitative examination of competitive epoxidation by GC
Table 2 Thermodynamic parameters for the epoxidation of the
cycloalkenes 6–17 with MCPBA calculated using the k values in Table 1
A mixture of equimolar quantities of n-dodecane (used as an
internal standard), cycloalkene (6–9), and the corresponding
‡
‡
‡
1
-trimethylsilylcycloalkene (10–13) in CH Cl was treated with
E
a
/
∆H /
∆S /
∆G /
2
2
Ϫ1
Ϫ1
Ϫ1
Ϫ1
Ϫ1
Cycloalkene
kJ mol
kJ mol
J K mol
kJ mol
MCPBA (1.5 equivalents divided into four portions, each
portion being added at 30 min intervals). A GC trace was
recorded before adding the first portion of MCPBA. Sub-
sequently, GC analysis was performed after adding each of
the remaining portions of MCPBA and stirring the mixture for
6
7
8
9
0
1
2
62.07
73.16
72.33
66.51
45.45
59.86
58.19
57.36
34.29
42.95
39.49
37.41
59.53
70.62
69.79
63.97
42.91
57.32
55.65
54.82
31.77
40.41
36.95
34.87
Ϫ64.01
Ϫ30.37
Ϫ31.97
Ϫ50.09
Ϫ109.67
Ϫ66.02
Ϫ71.22
Ϫ73.63
Ϫ136.50
Ϫ115.54
Ϫ125.25
Ϫ130.06
79.09
79.69
79.56
79.27
76.41
77.48
77.40
77.32
73.13
75.87
75.21
74.60
3
0 min.
1
1
1
Results and discussion
For the present study of the kinetics of epoxidation, cyclo-
alkenes 6–9, the corresponding 1-trimethylsilylcycloalkenes 10–
13
1
1
1
4
5
6
1
3, and 1-methylcycloalkenes 14–17 were selected (Scheme 3).
17
Scheme 3
MCPBA was the reagent of choice for epoxidation because of
its efficiency, convenience of handling, reliability of analysis
and ready availability.
Fig. 1 Epoxidation of 1-trimethylsilylcyclopentene (10) by MCPBA in
benzene at 298 (᭹), 303 (᭺), 308 (᭿) and 313 K (ᮀ).
The reaction of olefins with peracid has been established as a
second-order reaction, first order with respect to olefin and first
16
order with respect to peracid. Our experimental results show
similar trends. The epoxidation of 1-trimethylsilylcycloalkenes
by MCPBA is found to follow second-order kinetics, as
Table 3 that the reactivity of cycloalkenes towards epoxidation
is considerably influenced by the substituent at the vinylic
position and the reactivity is found to increase in the follow-
ing order: unsubstituted cycloalkenes < 1-trimethylsilylcyclo-
alkenes < 1-methylcycloalkenes.
observed from the straight line graph obtained by plotting
Ϫ1
[
MCPBA] versus time, measured from aliquots of the reaction
mixture taken at regular intervals. A typical graph is given in
Fig. 1.
The kinetic data for the epoxidation of 6–17 at four different
temperatures (298, 303, 308 and 313 K) were measured and the
values of the activation parameters for the overall reactions
were calculated from the Arrhenius plots of ln k vs. 1/T (Tables
A qualitative examination of the competitive epoxidation of
an equimolar mixture of a cycloalkene (6–9) and the corre-
sponding 1-trimethylsilylcycloalkene (10–13) by MCPBA using
a GC procedure (with internal standard) also indicated a
similar trend. That is, in each mixture of cycloalkene and
1-trimethylsilylcycloalkene a higher proportion of cyclic vinyl-
silane was found to be epoxidized than its unsilylated
analogue.
1
and 2). A representative Arrhenius plot is given in Fig. 2.
It is evident from the relative rate constants furnished in
1
100
J. Chem. Soc., Perkin Trans. 2, 2001, 1099–1102

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The observation that 1-methylcycloalkenes react faster than
the corresponding 1-trimethylsilylcycloalkenes, which in turn
react faster than the corresponding unsubstituted cycloalkenes,
The deactivation by (p–d)π or (Si–C) σ*-π interaction is
probably not sufficient to affect the slightly greater charge flow
from Si to C in 1-trimethylsilylcycloalkenes than from H to C
in simple cycloalkenes, at least in the case of epoxidation
reactions. It is also likely that the inductive effect of the three
12b
suggests that the discerning explanation given by Peterson
referred to earlier (see Introduction section) about the influence
of the trimethylsilyl group is correct. That is, the trimethylsilyl
group deactivates the double bond relative to an alkyl group (as
is seen while going from 14–17, respectively, to 10–13), but it is
actually mildly activating if it is replacing a hydrogen (which is
observed while going from 6–9, respectively, to 10–13). The
diminished reactivity of silylated cycloalkenes as compared to
CH groups on Si plays some role in this charge distribution.
3
Another notable feature emerges when we analyze the kinetic
data for the three sets of homologous series 6–9, 10–13 and
14–17. In each series it is observed that the five-membered
cycloalkene (6, 10 and 14) reacts faster than any other
member of the series, while the six-membered cyclic analogue
(7, 11 and 15) reacts slower than the others (Table 4). The order
of reactivity of cyclic olefins according to ring size is:
6-membered < 7-membered < 8-membered < 5-membered
ring.
1
-methylcycloalkenes can be explained as being due to the
strong inductive and hyperconjugative effects of the methyl
group, while the silyl group is mildly electron withdrawing by
(
p–d) overlapping. The reason that the silylated double bond
π
maintains still a certain higher degree of nucleophilic character,
compared to its unsilylated counterpart, may lie in the fact that
the electronegativity of silicon is less than that of hydrogen.
Similar trends have been noticed in the reactions of
cycloalkenes with other electrophilic reagents, e.g., iodine thio-
17
17
18
cyanate, nitrosyl chloride, iodomethylzinc iodide and
19
peracetic acid.
The rate of electrophilic addition depends on several factors,
such as the strain in the molecule, the electronic and steric
effects of the substituents, etc. When an electrophile adds to a
2
3
C᎐C bond the sp hybridized carbons change their state to sp ,
᎐
resulting in the loss of a certain amount of strain due to the
change of angle from 120 to 109.5Њ (or near to it). But in reac-
tions involving three-centered cyclic activated complexes (as
in the epoxidation of olefins) the change in the strain due to
2
the transformation of the sp hybrid state of the carbons of a
C᎐C bond to a cyclic three-centered activated complex is
᎐
20
very small. Because of this the relative rates of electrophilic
addition, which involves a cyclic three-centered activated com-
plex, essentially reflect the strain energy differences (Table 5).
The energies of activation (E ) and enthalpies of activation
a
‡
(
∆H ) for the epoxidation of the substrates 6–17 increase with
Fig. 2 Arrhenius plots of 6 (᭹), 10 (᭺) and 14 (᭿).
the decrease in rates. This indicates that the epoxidation of all
the substrates is enthalpy controlled. The negative values of
entropy of activation (∆S ) imply the formation of a rigid
Table 3 Relative rate constants (ksubstituted/kunsubstituted) for rings of the
same size
‡
activated complex from the reactants. The free energies of acti-
‡
Ϫ1
vation (∆G ) are nearly the same (73.13–79.69 kJ mol ) for all
the substrates, which shows that the epoxidation of all the cyclic
olefins, 6–17, follows a similar mechanism. The mechanism of
epoxidation of 1-trimethylsilylcycloalkenes, and of the other
cycloalkenes studied here, may be visualized as involving
the three-centered π complex that is proposed in the case of
n
1
2
3
4
H
SiMe3
1.00
3.20
1.00
2.73
6.72
1.00
2.63
7.67
1.00
2.26
8.10
CH3
12.08
16
alkenes (Scheme 4).
Table 4 Relative rate constants for the epoxidation of homologous series of cycloalkenes
n
Relative rate
n
Relative rate
n
Relative rate
6
7
8
9
1
2
3
4
1.55
1.00
1.18
1.43
10
11
12
13
1
2
3
4
1.82
1.00
1.14
1.19
14
15
16
17
1
2
3
4
2.74
1.00
1.34
1.73
Table 5 Strain energies and relative rate constants for the reactions of cycloalkenes with various electrophilic reagents, taking kcyclohexene = 1.00
Relative rate constant (k/kcyclohexene
)
Strain energy/
kcal mol
Ϫ1 a
b
c
c
d
e
Cycloalkene
MCPBA
ISCN
NOCl
ICH ZnI
CH CO H
3 3
2
6
7
8
9
6.93
2.01
7.35
8.21
1.55
1.00
1.18
1.43
2.10
1.00
1.90
1.10
88.00
1.00
29.00
27.00
1.60
1.00
1.18
—
1.51
1.00
1.36
—
a
b
c
d
e
Ref. 21. Present work. Ref. 17. Ref. 18. Ref. 19.
J. Chem. Soc., Perkin Trans. 2, 2001, 1099–1102
1101

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(
c) G. E. Evans, B. Jerome and N. H. Rees, J. Chem. Soc., Perkin
Trans. 2, 1973, 2, 2091; (d) A. L. Alfred and W. Bush, J. Am. Chem.
Soc., 1968, 90, 3352.
(a) C. S. Kraihanzel and M. L. Losee, J. Organomet. Chem., 1967,
3
1
0, 427; (b) M. P. Simonnin, Bull. Soc. Chim. Fr., 1966, 1774;
(
c) W. F. Reynolds, G. K. Hamer and A. R. Bassindale, J. Chem.
Soc., Perkin Trans. 2, 1977, 971.
4
5
6
R. West and C. S. Kraihanzel, Inorg. Chem., 1962, 1, 967.
J. C. Giordan, J. Am. Chem. Soc., 1983, 105, 6544.
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1
981, 219, 21; (b) M. Horn and J. N. Morrell, J. Organomet. Chem.,
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Scheme 4
7
8
H. Sakurai, A. Hosomi and M. Kumada, J. Org. Chem., 1969, 34,
1
764.
In conclusion, the trimethylsilyl group in cyclic vinylsilanes is
found to enhance significantly the rate of epoxidation relative
to that of unsilylated cycloalkenes, but it is less effective than a
methyl group in the vinylic position. The results, we believe, are
sufficiently convincing and should clear the prevailing con-
fusion about the role of the trimethylsilyl moiety in influencing
the rate of the epoxidation of vinylsilanes. The thermodynamic
parameters give good insight into the nature and mechanism of
the epoxidation of vinylsilanes.
(a) I. A. Dyakonov, G. Y. Goldonikov and I. B. Repinskaya,
Zh. Obshch. Khim., 1965, 35, 2181; Chem. Abstr., 1966, 64, 11238;
(b) J. Culdin and V. Chvalovsky, Collect. Czech. Chem. Commun.,
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Chem. Commun., 1963, 28, 3088; (d) D. Seyferth and H. Dertoczos,
J. Organomet. Chem., 1968, 11, 263.
9
D. Seyferth and H. M. Cohen, Inorg. Chem., 1962, 1, 913.
0 (a) W. E. Fristad, T. R. Bailey and L. A. Paquette, J. Org. Chem.,
1
1
1
980, 45, 3028; (b) W. E. Fristad, T. R. Bailey, L. A. Paquette,
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1 H. Sakurai, N. Hayashi and M. Kumada, J. Organomet. Chem.,
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Acknowledgements
12 (a) A. P. Davis, G. J. Hughes, P. R. Lowndes, C. M. Robbins,
E. J. Thomas and G. H. Whitham, J. Chem. Soc., Perkin Trans. 1,
The authors thank the DST and the UGC (under the
DRS programme), New Delhi for financial help. G. S. P.
thanks the CSIR, New Delhi for a Senior Research Fellowship.
Donation to G. N. of some equipment used in this work by the
Alexander von Humboldt Foundation, Germany, is gratefully
acknowledged.
1
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1
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