SYNLETT
Accounts and Rapid Communications in Synthetic Organic Chemistry
With compliments of the Author
Thieme
REPRINT
Selectivity in the Grignard Reaction with Silanes GrignardReactionwithSilanes Tuulmets,* Anu Ploom, Dmitri Panov, Jaak Järv Ants Institute of Chemistry, University of Tartu, 2 Jakobi St., Tartu 51014, Estonia Fax +372(7)3795264; E-mail:
[email protected] Received 10 September 2009
k1
Abstract: Selectivity problems in preparation of silanes by Grignard reaction were discussed. A quantitative approach in terms of LFE analysis was proposed. It appeared that the inductive effect controls the rate of replacement more considerably than steric requirements in the transition state. Rates of subsequent substitution reactions at silicon decrease stepwise. Only with methylmagnesium halides are the subsequent steps faster. Key words: Grignard reactions, selectivity, silicon, steric hindrance, substituent effects
The genesis of silane and silicone technology traces back to the Grignard reaction.1 The first practical synthesis of organosilanes was accomplished by Kipping in 1904 by the Grignard reaction.2 However, despite the versatility of Grignard chemistry for the formation of silicon–carbon bonds, its use in current silane technology has been partly supplanted by the direct process and hydrosilylation reactions.1,3 Although the production of organosilanes by nonGrignard processes is considerably greater than with Grignard process, the latter remains essential for many of the specialty silanes. Employment of the Grignard process allows the introduction of a broader range of functionalities than alternative technologies. Thus, Grignard chemistry remains a vital and versatile method for the production of organosilanes.1
RMgCl + SiX4
However, there have been indications that the sequence of rate constants above may not hold for some cases.4–6 In this report we present a quantitative analysis of the selectivity problems based on recent developments in the LFE (linear free energy) analysis for the silicon
R2SiX2 + MgXCl
RMgCl + RSiX3 k3
RMgCl + R2SiX2
R3SiX + MgXCl k4
RMgCl + R3SiX
R4Si + MgXCl
X = halide, MeO, EtO
Scheme 1 reagent
Replacement reactions at the silicon atom by a Grignard
chemistry7,8 and on the results of our kinetic investigations. A small number of kinetic studies on the Grignard reaction with silanes had been published9,10 until we launched a systematic investigation into quantitative aspects of the reaction.11–16 The main attention was focused on the structure–reactivity relationships. Kinetic investigations into structural and solvent effects confirmed the understanding that the reaction with alkoxysilanes proceeds through complexation of Grignard reagent with silane which replaces the donor molecule (commonly ether) bound at the magnesium center (Scheme 2). Subsequently, this complex is involved in formation of the reaction products.11,12,16 Chlorosilanes react differently without solvent molecule replacement.
For practical purposes, alkoxy- and chlorosilanes are the most convenient precursors to alkylsilanes. The Grignard reaction with alkoxysilanes is very general and found to be more selective than the analogous reaction with chlorosilanes.3 The selectivity problems rise from the stepwise substitution reaction at the silicon atom (Scheme 1). It is generally believed that in Scheme 1, k1 > k2 > k3 > k4. Control of substitution can be accomplished on statistical grounds (variation of ratios of the reagents) and basing on the relative reactivity. Multiple substitutions are favored when the activation energy for sequential substitution varies over a narrow range.
RSiX3 + MgXCl k2
GE + S
K
GS
GS + E products
G = a magnesium compound S = a silane E = an ether molecule
Scheme 2
Mechanism of the Grignard reaction with alkoxysilanes
Controversial results from our kinetic data obtained with traditional methods of the LFE analysis17 impelled us to undertake an extensive revision of the correlation analysis in organosilicon chemistry.7,8,15,16 It turned out that the well-known Taft equation (Equation 1) implicating terms for inductive (r*s*) and steric (dES) effects, differently of carbon compounds, cannot be applied for organosilicon compounds. log k = log k0 + ρ*σ* + δES
Equation 1
SYNLETT 2010, No. 2, pp 0291–0293xx. 201 Advanced online publication: 04.01.2010 DOI: 10.1055/s-0029-1219167; Art ID: Y02009ST © Georg Thieme Verlag Stuttgart · New York Synthesis 2000, No. X, x–xx
ISSN 0039-7881
© Thieme Stuttgart · New York
is a copy of the author's personal reprint l
291
l This
CLUSTER
292
CLUSTER
A. Tuulmets et al.
A set of steric parameters ES(Si) for substituents at the silicon center, originally defined by Cartledge for several alkyl groups,18 has been elaborated and also extended to polar groups.7,8 More surprisingly, it was found that the inductive effect of substituents, at least in nucleophilic reactions at silicon, must be expressed by two terms involving one for electronegativity.8
steric effects,3,5 it appears that the inductive effect controls the rate more substantially. In Equation 3 the contribution of the inductive effect is from 38–76 kJ mol–1 against that for the steric effect, ranging from 16–45 kJ mol–1. Similar correlations for methoxysilanes and chlorosilanes cannot be performed at present because of scarcity of the experimental data.
Subsequently, we apply the obtained protocol for correlation analysis to the reactivity data for alkoxysilanes. In Table 1 experimental activation enthalpies (DH≠exp) for reactions with ethoxysilanes in dibutyl ether are presented. Use of activation enthalpies instead of rate constants allows comparison of reactions which cannot be measured within a common temperature range. Table 1 includes a span in rate constants for more than four magnitudes.
Correlations in the form of Equation 3 are very general and such relationships, if available, permit to predict quantitatively the selectivity in the Grignard reagent coupling with silanes.
As the observed activation enthalpies involve the enthalpy for the ligand-exchange equilibrium, DHo, (see Scheme 2), the activation enthalpy must be corrected as in Equation 2. ΔHocorr = ΔH≠exp – ΔHo.
Equation 2 16
In a previous paper we have suggested that in dibutyl ether the equilibrium enthalpy DHo can be approximated with the value –48 kJ mol–1. In Table 1 the corrected values for activation enthalpies, DH≠corr, are presented. In a nucleophilic displacement at silicon only steric and inductive effects of substituents are operating and alkyl groups do not exert any polar effect.7,8 Therefore, for the correlation analysis of data the sum of steric constants for ligands in the transition state20 SES(Si) (Figure 1) and number of ethoxy groups, n(OEt), were employed for the correlation analysis of data (See Table 1 and Equation 3). ΔH≠corr = (120±23) – (31±12)ΣΕS(Si) – (19±7)n(OEt) a R = 0.915
Evidently, the last substitution at silicon, frequently not achieved practically because of the extremely low reaction rate, is not as much hindered by steric circumstances, but rather caused by the decrease in electrophilicity of the silicon center. The catalytic methods,1,3 if not switching the reaction to a different path, actually involve an enhancement of the electrophilicity. Based on their preparative results, Voronkov and Yakubovskaya6 concluded that alkyltrimethoxysilanes react with alkylmagnesium halides considerably faster than tetramethoxysilane itself. Calculation of steric effects for the reaction steps of a methyl Grignard reagent with tetramethoxysilane, performed as described above, yielded the following file: –0.48, –0.36, –0.24, and –0.12. Evidently, considering these experimental facts, the dramatic decrease in the inductive effect, caused by the loss of methoxy groups, does not overweigh the favorable decrease in steric hindrances. Et O
EtO Si
EtO
Figure 1
Mg Cl Bu Me
Ligands at the silicon center in a transition state
However, the analogous file of SES(Si) values for the reaction with an ethyl Grignard reagent is: –0.63, –0.66, –0.69, and –0.72, thus limiting the conclusion of the Russian authors to methyl Grignards only.
Equation 3
The relatively good correlation expressed by Equation 321 demonstrates quantitatively that steric effects enhance and the inductive effect lowers the activation energy, thus retarding and accelerating the reaction, respectively. Contrary to the common opinion of predominant impact of
With ethoxysilanes, methylmagnesium halides provide a sequence of SES(Si) values rather close to that with methoxysilanes (see above). Nevertheless, from Equation 3 for ethoxysilanes a slight increase in activation energies can be found. However, although being statistically a good
Table 1 Experimental and Corrected Activation Enthalpies (in kJ mol–1) for Grignard Reactions with Ethoxysilanes in Dibutyl Ether and Corresponding Parameters for the Correlation Analysis Reaction
Temp range (°C)
Method19
DH≠exp
DH≠corr
SES(Si)
n(OEt)
t-BuMgCl + i-PrSi(OEt)3
90–130
A
70 ± 11
118
–1.54
3
n-BuMgCl + Me2Si(OEt)2
20–50
B
49 ± 3
97
–0.51
2
n-BuMgCl + i- PrSi(OEt)3
30–80
A
42 ± 4
90
–1.21
3
n-BuMgCl + MeSi(OEt)3
20–40
B
42 ± 1
90
–0.65
3
n-BuMgCl + Si(OEt)4
20–40
B
20 ± 1
68
–0.79
4
Synlett 2010, No. 2, 291–293
© Thieme Stuttgart · New York
CLUSTER
correlation, Equation 3 is not yet a precise instrument for predictions. Therefore, a slightly reverse order of reaction rates cannot be excluded. For the substitution reaction of tetraethoxysilane with ethyl Grignard reagents, steric demands are almost equal for all the reaction steps. Thus, because of the decrease in the inductive effect, a decrease in reaction rates is inevitable. From Equation 3 an increase in activation energies by about 20 kJ mol–1 for each step could be predicted, however, our experimental data for ethylmagnesium chloride10 indicate even greater decrease in the reaction rates. In the preparation of alkylsilanes by Grignard reactions, selectivity problems rise from the stepwise replacement reaction at the silicon atom. Control of substitution can be accomplished by variation of the ratio of the reagents being, however, also sensitive to relative reactivity issues. The selectivity problems have been discussed and a quantitative approach in terms of LFE analysis has been proposed. It appeared that the inductive effect controls the rate more considerably than steric requirements in the transition state. Rates of subsequent substitution reactions at silicon decrease stepwise, while only for reactions of methylmagnesium halides do the subsequent steps proceed faster.
Acknowledgment This work was financially supported by the Estonian Ministry of Education and Research, Grant SF189964s08.
References and Notes (1) Arkles, B. Grignard Reagents and Silanes, In Handbook of Grignard Reagents; Silverman, G. S.; Rakita, P. E., Eds.; Marcel Dekker: New York, 1996. (2) Kipping, F. S. Proc. Chem. Soc., London 1904, 20, 15. (3) Brook, M. A. Silicon in Organic, Organometallic, and Polymer Chemistry; Wiley: New York, 2000. (4) See ref. 3, p. 387. (5) Eaborn, C. Organosilicon Compounds; Butterworths: London, 1960. (6) Voronkov, M. G.; Yakubovskaya, A. Y. Zh. Obsch. Khim. 1955, 25, 1124. (7) Ploom, A.; Panov, D.; Tuulmets, A. ARKIVOC 2006, (v), 37. (8) Ploom, A.; Tuulmets, A. J. Organomet. Chem. 2009, 694, 313.
Grignard Reaction with Silanes
293
(9) (a) Reid, A. F.; Wilkins, C. J. J. Chem. Soc. 1955, 4029. (b) Corriu, R. J. P.; Henner, B. J. Organomet. Chem. 1975, 102, 407. (10) Tuulmets, A.; Hõrak, M.; Kõopere, T.; Ruotsi, J. Org. React. (USSR) 1982, 19, 102. (11) Tuulmets, A.; Panov, D.; Sassian, M. Tetrahedron Lett. 2003, 44, 3943. (12) Tuulmets, A.; Nguyen, B. T.; Panov, D.; Sassian, M.; Järv, J. J. Org. Chem. 2003, 68, 9933. (13) Tuulmets, A.; Nguyen, B. T.; Panov, D. J. Org. Chem. 2004, 69, 5071. (14) Panov, D.; Ploom, A.; Tuulmets, A. Phosphorus, Sulfur Silicon Relat. Elem. 2006, 181, 2807. (15) Golubev, O.; Panov, D.; PloomA, ; Tuulmets, A. J. Organomet. Chem. 2007, 692, 3700. (16) Ploom, A.; Panov, D.; Järv, J.; Tuulmets, A. J. Organomet. Chem. 2008, 693, 2351. (17) (a) Exner, O. Correlation Analysis of Chemical Data; Plenum Press: New York, 1988. (b) Williams, A. Free Energy Relationships in Organic and Bioorganic Chemistry; RSC: Cambridge, 2003. (18) Cartledge, F. K. Organometallics 1983, 2, 425. (19) The rate constants were determined under pseudo-first-order conditions with a great excess of Grignard reagent. The rate measurements were carried out with 0.5 M solutions of Grignard reagents. The rate constants will be published elsewhere together with a discussion of mechanistic issues. Method A Method A is a GLC method. The reaction vessel equipped with a magnetic stirrer and containing 40 mL of the Grignard reagent was thermostatted. According to the concentration of the Grignard reagent, 0.1–1 mL of silane (providing a 9to 20-fold excess of the Grignard reagent) was added into the flask to start the reaction. At appropriate times 2 mL aliquots were taken from the reaction mixture and quenched with ice cold water. The organic layer was instantly separated, dried, and analyzed for the silane using GLC. Method B Method B is a thermographic method. Fast reactions were investigated in a thermostatic flask equipped with a stirrer and a thermistor. The thermistor was connected through a bridge circuit to a recording potentiometer. 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 silane was added to 15 mL of the Grignard reagent (providing a 20–40-fold excess of the Grignard reagent), and the temperature change of the reaction solution (less than 1 °C) was recorded as a plot of temperature vs. time. (20) Values of ES(Si) for methyl, methoxy, ethoxy, ethyl, n-butyl, and isopropyl groups are: 0, –0.12, –0.14, –0.15, –0.23, and –0.56, respectively.8 (21) Scales of parameters in Equation 3 are not normalized, therefore the intercept has a formal meaning. Nevertheless, Equation 3 can be used for calculation of relative effects.
Synlett 2010, No. 2, 291–293
© Thieme Stuttgart · New York
