Steric parameters for substituents bound to atoms of silicon and some other elements of the third period

Article abstract of DOI:10.1080/10426501003713106

The kinetics of a tetraethoxysilane reaction with n-butylmagnesium chloride, stoichiometrically monosolvated with isopropyl ether or with methyl tert-butyl ether, was studied in toluene. The pseudo-first-order rate constants determined at a great excess of Grignard reagent were used for separation of the appropriate equilibrium and rate constants. Equilibrium constants for five alkyl ether ligands at the magnesium center are in an excellent correlation with isosteric ES(Si) parameters. It was concluded that these constants should be applicable to all elements of the third period of the periodic table. Taylor & Francis Group, LLC.

Full text of DOI:10.1080/10426501003713106

Phosphorus, Sulfur, and Silicon, 185:2503–2510, 2010
Copyright ꢀ
Taylor & Francis Group, LLC
C
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426501003713106
STERIC PARAMETERS FOR SUBSTITUENTS BOUND
TO ATOMS OF SILICON AND SOME OTHER ELEMENTS
OF THE THIRD PERIOD
Anu Ploom, Ants Tuulmets, and Jaak J a¨ rv
Institute of Chemistry, University of Tartu, Tartu, Estonia
The kinetics of a tetraethoxysilane reaction with n-butylmagnesium chloride, stoichiomet-
rically monosolvated with isopropyl ether or with methyl tert-butyl ether, was studied in
toluene. The pseudo–first-order rate constants determined at a great excess of Grignard
reagent were used for separation of the appropriate equilibrium and rate constants. Equi-
librium constants for five alkyl ether ligands at the magnesium center are in an excellent
correlation with isosteric ES(Si) parameters. It was concluded that these constants should
be applicable to all elements of the third period of the periodic table.
Keywords Grignard reaction; isosteric parameters; silanes; solvation effects
INTRODUCTION
The quantitative structure–reactivity analysis, also known as correlation analysis, has
been widely practiced in organic chemistry since the pioneering work of Hammett and
Taft. Besides the quantum chemical calculations and other complementary methods, the
correlation analysis has maintained its importance as a useful tool for investigating reaction
mechanisms and for the prediction of rate and equilibrium constants.1
–3
In traditional organic chemistry, several protocols for the quantitative description of
1
–3
polar and steric effects of substituents are available. However, these protocols have been
elaborated mostly for reactions taking place at the carbon atom or its closest neighbors in
the second period. Therefore an unequivocal approach still needs to be developed for other
reaction centers, including organometallic chemistry in general.4
–6
Recently we have carried out a revision of quantitative data regarding the reactivity
of organosilicon compounds in light of views that stem from organic reactions at the carbon
7
–10
atom.
opposed to carbon compounds, the Taft ES scale of steric parameters is not applicable,
and the inductive effect of substituents must be expressed by two terms, involving one that
It was found that in structure–reactivity analysis for organosilicon compounds, as
7
,8
10
describes electronegativity.
Cartledge11 has defined a set of ES(Si) parameters for alkyl groups from reaction
rates of silicon compounds, and these constants appeared to be in a good correlation with
Received 11 November 2009; accepted 18 February 2010.
This work was financially supported by the Estonian Ministry of Education and Research (Grant SF
0
180064s08).
Address correspondence to Anu Ploom, Institute of Chemistry, University of Tartu, Tartu 50411, Estonia.
E-mail: anu.ploom@ut.ee
2
503

2
504
A. PLOOM ET AL.
ꢁ
12
3
ν parameters derived by Charton from replacement reactions at sp -carbon. The scale
ꢁ
of ν was thereafter successfully applied in correlating reactivity data of alkyl substituted
phosphorus compounds.13 The major difference between these parameters, derived for
silicon compounds, and those derived previously for carbonyl compounds by Taft, can be
observed if the effect of substituent branching in α and β positions is compared. Charton
has concluded12 that expansion of coordination from four to five at the reaction center
will show different sensitivity to α and β branching than a reaction involving change of
coordination from three to four, as we observe in the case of carbonyl compounds.
ꢁ
We have shown that the sets of parameters ES(Si) and ν are practically equivalent
and almost equally applicable for quantitative description of steric effects exerted by alkyl
8
substituents upon the reactivity of organosilicon compounds. Further we extended the
10
ES(Si) scale for several polar substituents, the data for which were not previously available.
The novel protocol for the correlation analysis in organosilicon chemistry proved to
be applicable to all available experimental data for nucleophilic replacements and some
other reactions at the silicon center,8 including acid- and base-catalyzed hydrolyses,
bromination, Grignard reactions, etc.
,10
As far as the difference between reactivities of silicon and carbon compounds is
mostly related to differences in atomic dimensions and in the electron structure, the question
can be raised as to whether the conclusions above can be extended to other elements of the
third period of the periodic table.
In this report, we show that ES(Si) parameters also are suitable to express the steric
effect of substituents at the magnesium atom. To this end, we correlate equilibrium constants
for the replacement of ethers as donor ligands at the magnesium center in Grignard reagent
for an alkoxysilane as a donor using the isosteric parameters of the ethers. It has been
quantitatively proven14 that donor ligands at the magnesium atom in Grignard reagents
can be considered as substituents equally to other groups at the center. Kinetic data for
the reaction between n-butylmagnesium chloride and tetraethoxysilane in toluene solution
in the presence of various alkyl ethers were exploited to calculate the needed equilibrium
constants.
RESULTS AND DISCUSSION
We have elaborated an experimental method determining rate and equilibrium con-
15
stants for the Grignard reaction with alkoxysilanes. The method is based on understanding
that the reaction proceeds through complexation of Grignard reagent with silane, which
replaces the donor molecule bound at the magnesium center (Scheme 1). Subsequently, this
16,17
complex rearranges to reaction products through a four-center transition state.
As alkylmagnesium chlorides exist essentally as dimeric species, which are stable
over a wide concentration range,18 the overall reaction can be presented as follows:
K
−
→
(
GE)2 + S
G2ES + E,
−
k
(1)
(2)
←
G2ES −→ products
where G denotes the magnesium compound, S is silane, E is the ether molecule, (GE)2 is
the dimeric monosolvated Grignard reagent, and G2ES is the substitution complex. The
parameters K and k are the equilibrium and rate constants, respectively.

STERIC PARAMETERS FOR ATOMS OF THE THIRD PERIOD
2505
Et
O
+
Et
S
O
Cl
Cl
S
Cl
Cl
A.
B.
K
S
Si
Mg
Si
Mg
Mg
Et
Mg
+
S
Bu
Bu
Bu
Bu
O
S
Cl
Cl
Si
Mg
Mg
CH2
Pr
Bu
Scheme 1 (A) Schematic representation of the replacement of the donor molecule at the magnesium center (donor
bonds are not distinguished separately). (B) The transition state for the reaction of n-butylmagnesium chloride
with an alkoxysilane. In (A) and (B), S stands for the donor solvent molecule.
In the presence of an excess of Grignard reagent, the reaction follows a first-order
rate equation.17 Assuming that the complexation Equilibrium (1) is fast, according to the
principle of steady state kinetics,1 Eq. (3) can be deduced for the observed rate constant.
9,20
kK [G]
kobs =
.
(3)
1
+ K [G]
Eq. (3) can be converted into the linear form Eq. (4), suitable for data processing in
coordinates 1/kobs vs. 1/[G].
1
1
1
= k
+
.
(4)
kobs
kK [G]
The pseudo–first-order rate constants of the reaction between tetraethoxysilane and
monosolvated with alkyl ethers n-butylmagnesium chloride were measured in toluene
using a large excess of the Grignard reagent. An important advantage of such an approach
consists in preclusion of nonspecific solvation effects when the effects of donor solvents
are considered.
In all experiments, only the first replacement at the silicon center was observed,
◦
because at the temperature used (20 C), further substitutions are extremely slow. Concen-
trations of the monosolvated Grignard reagents changed from 0.15 M to 0.5 M, and the
exact values were determined by titration. As alkylmagnesium chlorides are essentially
in dimeric form,18 the actual concentration range of the reacting species was two times
smaller than the titrated concentration. The obtained rate constants were analyzed as de-
scribed above, and an example is shown in Figure 1. The results are collected in Table I
15
together with data obtained in our previous work.
According to Eq. (1), the equilibrium constants K should reflect the solvating ability
of donors, the strongest of them shifting the equilibrium most to the left (see Scheme 1A).
21
As the Brønsted basicities of the ethers used are practically similar, their solvating power
Lewis basicity) should be determined solely by their steric effects. Indeed, the equilibrium
constants aligned well with the presumable steric properties of the ethers.
(

2
506
A. PLOOM ET AL.
3
3
2
2
1
5
0
0
0
0
0
0
.06
.05
.04
.03
.02
.01
0
0
5
0
5
10
5
0
0.1 0.2 0.3 0.4 0.5 0.6
n -BuMgCl], mol/l
0
1
2
3
4
5
6
7
[
1/[n -BuMgCl]
Figure 1 (a) Pseudo-first order rate constants kobs vs. the titrated concentration of n-BuMgCl i-Pr2O complex
◦
for the reaction with tetraethoxysilane in toluene at 20 C. (b) Determination of the equilibrium constant (K) and
rate constant (k).
On the other hand, the rate constants k varied relatively little with variation of the
donor ethers. This can be explained by the remote positioning of the retained donor molecule
in the transition state of the reaction (see Scheme 1B).
In order to quantify the structure–reactivity relations for the Equilibrium (1), loga-
rithms of the equilibrium constants were contrasted with corresponding isosteric constants
in Table II. The ethers were considered as substituents at the magnesium center, and fol-
lowing the principle of isostericity, the oxygen atom of the ether was taken to be equivalent
to the CH-group in the corresponding alkyl substituents, as shown in Scheme 2.
CH2
CH2
CH2
CH2
CH3
CH3
O
CH
3
CH
CH
3
Scheme 2 Ethyl ether and the corresponding isosteric 3-pentyl group.
ꢁ
The appropriate constants ES were taken from the revised and extended analysis
by MacPhee et al.22 made by default for carbon compounds. The corresponding isosteric
a
Table I Rate and equilibrium constants calculated from Eq. (4) for the reaction of tetraethoxysilane with
◦
monosolvated n-butylmagnesium reagents in toluene at 20 C
K, L·mol−
1
−1
k, s
Grignard reagent
b,c
n-BuMgCl·THF
0.40 ± 0.10
1.22 ± 0.19
3.30 ± 0.66
3.95 ± 0.19
6.15 ± 0.31
0.12 ± 0.03
0.31 ± 0.05
0.33 ± 0.05
0.13 ± 0.01
0.10 ± 0.01
b
·
·
·
·
Et2O
b
Bu2O
d
MTBE
i-Pr2O
a
Si(OEt)4 + BuMgCl → BuSi(OEt)3 + EtOMgCl.
b
c
15
From Ref. , standard errors recalculated.
A GLC method was used, see Ref.
15
d
Methyl tert-butyl ether.

STERIC PARAMETERS FOR ATOMS OF THE THIRD PERIOD
2507
Table II Logarithms of the equilibrium constants from Table I and the corresponding isosteric constants
ꢁ
-ES
a
-ES(Si)b
Ether ligand
log K
THF
Et2O
Bu2O
MTBE
i-Pr2O
–0.40
0.086
0.52
0.60
0.79
0.41
2.00
2.08
3.21
5.01
0.66c
0.78
0.87
0.98
0.99
d
aConstants from Ref.22
b
Calculated in this work (see Results and Discussion section).
c
8
Approximated with cyclo-hexyl group (from Ref. ).
d
Methyl tert-butyl ether.
ES(Si) constants were not available in the literature. However, extensive kinetic data for
23
the hydrolysis of alkyldimethylchlorosilanes published by Shimizu et al., and their un-
24
expectedly good additive scheme for calculation of the missing rate data, enabled us to
obtain the rate constants for silanes with the required alkyl substituents. For the hydrolysis
24
reaction, empirical Eqs. (5) and (6) have been derived, where S(R) denotes the structural
effect of a group R at silicon, defined as log krel for RMeSiCl2.
S(RCH ) = 0.20 S(R) − 0.57
(5)
(6)
2
1
2
3
1
2
3
logk (R R R Si) = S(R ) + 1.15S(R ) + 1.35S(R )
rel
i
1 2 3
In Eq. (6), the structural effect of an alkyl group R in R R R Si is assumed to be the
1
3
largest with R and the smallest with R . Further, excellent correlation for compounds with
alkyl substituents from the same reaction with ES(Si) scale[ was obtained
8]
logk = ( − 0.09 ± 0.07) + (2.28 ± 0.12) E (Si), R = 0.991
(7)
rel
S
and a significant susceptibility of the reaction to steric effects permitted reliable calculation
of the values of the isosteric constants, as shown in Table III.
For the THF ligand, data for the isosteric cyclo-pentyl group are not available.
However, we consider the approximation with cyclo-hexyl group (Table II) appropriate,
because in both cyclo-hexyl and cyclo-pentyl groups, branching in α- or β-positions relative
8
,11,12
to Mg or Si center is the same (for the branching effects, see Refs.
).
Table III Calculation of isosteric constants for the ether ligands
log krela
Corresponding isosteric
Ether ligand
alkyl group
Obs.b
Calc.c
-ES(Si)d
Et2O
Et2CH-
Bu2CH-
i-Pr2CH-
t-BuMeCH-
–1.71
—
—
–1.77
–1.99
–2.26
–2.23
0.78
0.87
0.99
0.98
Bu2O
i-Pr2O
MTBEe
—
a
23
For the hydrolysis of alkyldimethylchlorosilanes.
b
c
23
From Ref.
Calculated from Eq. (6).
d
Calculated from Eq. (7).
e
Methyl tert-butyl ether.

2
508
A. PLOOM ET AL.
logK
logK
B
A
0.9
0.9
0.6
0.3
0
0.6
0.3
0
0
1
2
3
4
5
0
0.2 0.4 0.6 0.8
1
-
0.3
-0.3
-0.6
-0.6
-E `
-E (Si)
S
S
ꢁ
Figure 2 Correlation of equilibrium constants with isosteric parameters ES (A) and ES(Si) (B).
ꢁ
As seen in Figure 2, correlation of the equilibrium constants with the isosteric ES
2
parameters is hardly satisfactory (R = 0.804), while ES(Si) constants provide an excellent
2
correlation (δ = –3.34 ± 0.45; R = 0.950). Thus the steric effect of substituents on reactions
taking place at the Mg center can be reliably expressed by means of ES(Si) constants.
In conclusion, the scale ES(Si) of steric parameters derived from organosilicon chem-
istry proved also to be pertinent in the case of organomagnesium compounds.
Charton and Charton13 have extensively investigated substituent effects in
organophosphorus chemistry. Rate constants for the reaction of 22 sets of XZ(PO)Cl with
water, and rate constants for sets of some other reactions, were successfully correlated
ꢁ
with the steric parameters ν by means of the modified Taft equation. The results were
in agreement with the transition state of trigonal–bipyramidal geometry, similar to the
replacement reactions at the silicon center. As already stressed in the Introduction section,
8
ꢁ
we have shown that the scale ν is equivalent to the ES(Si) scale (R = 0.996). Therefore
it can also be concluded that the scale ES(Si) can be equally used for organophosphorus
compounds. And moreover, as far as the scale ES(Si) was proved to be applicable for
magnesium, silicon, and phosphorus compounds, it is reasonable to assume that the same
parameters can be applied also for aluminum and sulfur compounds, if these will be
available.
EXPERIMENTAL
Preparation of Monosolvated Grignard Reagents
Toluene solutions of n-butylmagnesium chloride stoichiometrically monosolvated
with isopropyl ether or MTBE (methyl tert-butyl ether) were prepared in a stainless-
steel autoclave equipped with a magnetic stirrer, a thermometer, and a manometer. The
autoclave was charged with an adequate amount of Mg powder, and iodine was used as
activator. The reaction vessel was purged thoroughly with argon, then calculated volumes
of toluene, donor ether, and n-butyl chloride were added and the autoclave was sealed. The
reaction mixture was heated until the reaction began. With isopropyl ether, the induction
◦
period lasted several hours, and the synthesis was accomplished in 2 h at 90 C. In the

STERIC PARAMETERS FOR ATOMS OF THE THIRD PERIOD
2509
case of MTBE, the induction period was short, and the preparation was performed at
◦
5
0 C.
After the reaction mixture was cooled down, the concentration of the Grignard
reagent was determined. For kinetic measurements, these solutions were diluted with the
appropriate amounts of toluene. The reagents and solutions were handled under dry argon
and transferred by use of cannulae or syringes. Yields of the syntheses of monosolvated
reagents (about 90%) were very reproducible, and the rate constants measured with solutions
from different batches were in excellent agreement between different experiments.
Kinetic Measurements
Reactions were investigated in a thermostatic flask equipped with a stirrer and a
thermistor. The thermistor was connected through a bridge circuit to a PC. The reaction
vessel was purged thoroughly with pure argon. All parts of the equipment and the reagents
were thermostatted. After the thermal equilibrium was set, 0.05 mL of tetraethoxysilane
was injected into 15 mL of the Grignard reagent (providing at least a 10-fold excess of
◦
Grignard reagent), and the temperature change of the reaction solution (up to 0.5 C) was
recorded as a plot of temperature vs. time. The pseudo–first-order rate constants were
calculated from these kinetic data (Table IV).
Table IV Pseudo-first order rate constants for the reaction of tetraethoxysilane with mono-
◦
solvated n-butylmagnesium reagents in toluene at 20 C
Titrated conc. of the
Grignard reagent
2
−1 a
Solvent
THFb,c
kobs×10 , s
0.23
0.52 ± 0.01
0.91 ± 0.02
1.39 ± 0.03
1.79 ± 0.01
4.60 ± 0.10
6.50 ± 0.11
9.26 ± 0.10
10.9 ± 0.10
13.9 ± 0.40
17.0 ± 0.60
19.8 ± 0.14
3.19 ± 0.10
4.60 ± 0.12
5.72 ± 0.19
6.35 ± 0.09
3.05 ± 0.07
4.05 ± 0.10
5.07 ± 0.02
5.80 ± 0.16
0
0
0
.41
.67
.90
Et2Oc
0.28
0
0
0
.44
.66
.89
Bu2Oc
0.44
0
0
.66
.88
MTBEd
0.17
0
0
0
.28
.42
.51
i-Pr2O
0.15
0
0
0
.24
.36
.48
a
Mean values at least three determinations.
b
c
15
A GLC method was used, see Ref.
Initial kinetic data from the work Ref.
15
d
Methyl tert-butyl ether.

2
510
A. PLOOM ET AL.
REFERENCES
1. J. Shorter, Correlation Analysis of Organic Reactivity (Wiley, Chichester, 1982).
2. O. Exner, Correlation Analysis of Chemical Data (Plenum Press, New York, 1988).
3. A. Williams, Free Energy Relationships in Organic and Bio-organic Chemistry (RSC, Cam-
bridge, 2003).
4
5
6
7
8
9
. V. I. Galkin, R. D. Sayakhov, and R. A. Tcherkasov, Usp. Khim., 60, 1617 (1991).
. D. White and N. J. Coville, Adv. Organomet. Chem., 36, 95 (1994).
. K. K a¨ ppler, U. Scheim, K. R u¨ hlmann, and A. Porzel, J. Organomet. Chem., 441, 15 (1992).
. D. Panov, A. Ploom, and A. Tuulmets, Phosphorus, Sulfur, and Silicon, 181, 2807 (2006).
. A. Ploom, D. Panov, and A. Tuulmets, Arkivoc, 37 (2006).
. O. Golubev, D. Panov, A. Ploom, A. Tuulmets, and B. T. Nguyen, J. Organomet. Chem., 692,
3
700 (2007).
1
1
1
1
1
1
1
1
1
1
2
0. A. Ploom and A. Tuulmets, J. Organomet. Chem., 694, 313 (2009).
1. F. K. Cartledge, Organometallics, 2, 425 (1983).
2. M. Charton, J. Am. Chem. Soc., 97, 3694 (1975).
3. M. Charton and B. J. Charton, J. Org. Chem., 43, 2383 (1978).
4. A. Tuulmets, Organic Reactivity (USSR), 11, 81 (1974).
5. A. Ploom, D. Panov, J. J a¨ rv, and A. Tuulmets, J. Organomet. Chem., 693, 2351 (2008).
6. A. Tuulmets, D. Panov, and M. Sassian, Tetrahedron Lett., 44, 3943 (2003).
7. A. Tuulmets, B. T. Nguyen, D. Panov, M. Sassian, and J. J a¨ rv, J. Org. Chem., 68, 9933 (2003).
8. F. W. Walker and E. C. Ashby, J. Am. Chem. Soc., 91, 3845 (1969).
9. L. P. Hammett, Physical Organic Chemistry (McGraw-Hill, New York, 1970), 2nd ed., Chap. 4.
0. E. A. Moelwyn-Hughes, The Chemical Statics and Kinetics in Solutions (Academic Press,
London, 1971), Chap. 4.
21. I. A. Koppel and V. A. Palm, In Advances in Linear Free Energy Relationships, N. B. Chapman
and J. Shorter, eds. (Plenum Press, New York, 1972), Chapter 5.
2
2
2. J. A. MacPhee, A. Panaye, and J. E. Dubois, Tetrahedron, 34, 3553 (1978).
3. N. Shimizu, N. Takesuke, A. Yamamoto, T. Tsutsumi, S. Yasuhara, and Y. Tsuno, Chem. Lett.,
1
263 (1992).
24. N. Shimizu, N. Takesuke, S. Yasuhara, and T. Inazu, Chem. Lett., 1807 (1993).

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