Mechanism of Dehydrogenative Condensation of (o-borylphenyl

Article pubs.acs.org/Organometallics

Mechanism of Dehydrogenative Condensation of (oborylphenyl)hydrosilanes with Methanol László Könczöl,† Atsushi Kawachi,‡ and Dénes Szieberth*,† †

Department of Inorganic and Analytical Chemistry, Materials Structure and Modeling Research Group of the Hungarian Academy of Sciences, Budapest University of Technology and Economics, Szt. Gellért tér 4., H-1111 Budapest, Hungary ‡ Department of Chemistry, Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8526, Japan S Supporting Information *

ABSTRACT: The dehydrogenative condensation of hydrosilanes and alcohols is a facile way of protecting alcohols in chemical synthesis. Recently, Kawachi and co-workers combined the B(C6F5)3 catalyst and the hydrosilyl group in one molecule, enabling the coupling reaction to take place at room temperature without additional catalyst. In this paper, the mechanism of the reaction of this novel protecting group with methanol is explored using computational techniques. A stepwise reaction pathway was found to be preferred to the concerted one. The mechanism can be interpreted using the FLP framework. In the preferred pathway, the activation of the Si−H and O−H bonds happens simultaneously. While the hydride-like hydrogen on the Si atom is activated by the Lewis acidic boron site, the O−H bond is activated by the π-system of one of the mesityl substituents on the boron atom. Further calculations show that, by changing the acidity of the Lewis acidic site, the activation barrier of the reaction can be tuned.

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INTRODUCTION Silyl ethers are widely used in synthetic chemistry for the protection of alcohols, owing to their ability to be selectively removed in the presence of other protecting groups.1 For the synthesis of silyl ethers, R3SiH-type reagents are advantageous due to the fact that the only byproduct of the synthesis is H2 (see Scheme 1).

Scheme 2. Dehydrogenative Condensation of Hydrosilanes with Alcohols without Additional Catalyst

In the crystal structure of 1, no lengthening of the Si−H bond was observed, so no significant Si−H···B interaction exists in the starting material. Varying the R groups in the alcohols allowed the observation that the reaction rate decreases with the bulkiness of the alcohol. Substitution of the dimesityl-boryl group in 1 with pinacolborane resulted in the loss of reactivity toward alcohols. Reactions involving proton transfer and the elimination of H2 were studied extensively in the literature. In case of the reactions of transition-metal hydrides and proton donors, the elimination pathway contains an M−H···H−X dihydrogenbonded (DHB) intermediate, and an η2-H2 complex.8 Main group hydrides react in a different way: in an investigation of the reaction EH4− + ROH = EH3OR− + H2 (E = B, Al, Ga; R = CH3, CF3CH2, CF3), the η2-H2 complex was only found in the BH4− + CF3OH case due to the weak interaction between the main group atoms and H2.9 Although the energy and geometry of the DHB complex was found to depend mainly on the acidity of the alcohol and influenced less by the type of the

Scheme 1. Protection of Alcohol with Hydrosilanes

Traditionally, the reaction is carried out using transitionmetal catalysts; however, since 1999, the successful use of B(C6F5)3 was reported as a catalyst of the silylation of a wide variety of alcohols.2−5 The key feature of the reaction was postulated to be the hydrogen abstraction from the silyl group by the strong Lewis acid borane, and the subsequent coordination of the alcohol substrate to the silylium cation formed this way.2 B(C6F5)3 was also successfully applied in catalyzing the polyetherification of dihydrosilanes and bisphenols.6 In 2007, the synthesis of o-[(dimesitylboryl)phenyl]methoxydimethylsilane (2) was reported by Kawachi et al.7 The ortho-boryl group of o-[(dimesitylboryl)phenyl]dimethylsilane (1) activates the Si−H bond, enabling it to react with alcohols at room temperature, without additional catalyst (see Scheme 2). A push−pull mechanism was suggested for the reaction, with the alcohol attacking the silicon from the opposite side to the boryl group. © 2011 American Chemical Society

Received: July 11, 2011 Published: December 20, 2011 120

dx.doi.org/10.1021/om2006003 | Organometallics 2012, 31, 120−125

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hydrogens. The comparison of the calculated activation and reaction energies with the results obtained by the SCS-MP2 method served as a benchmark for the selection of a suitable DFT functional. From the tested functionals, the MPW1K one produced the closest match. Table 1 shows the relative energies and selected geometrical parameters obtained with this functional as well as the results of SCS-MP2 calculations. Comparing the values in Table 1, it is apparent that increasing the basis set from 6-31+G* (266 basis functions on 1′) to aug-cc-pvtz (947 basis functions) did not produce significant changes in the relative energies. The calculated geometrical parameters also did not show significant deviations between the two basis sets, with the exception of structure 3′. Because the optimized H−Si and Si−O bond lengths in 3′ have changed perceptibly (the H−Si distance increased by 0.086 Å, and the Si−O distance decreased by 0.067 Å for the increase of the basis set), while the relative energy of 3′ changed only by 0.9 kcal/mol, we can conclude that the potential surface around the intermediate 3′ is very flat, and the relative energies are not sensitive to the exact geometrical parameters. Single-point SCS-MP2 calculations on the MPW1K/6-31+G* geometry also gave very similar results to the ones obtained by full geometry optimization at the SCS-MP2 level, showing the eligibility of the MPW1K/6-31+G* geometries. The experimentally determined geometrical parameters of 17 also gave good agreement with the ones optimized at the MPW1K/6-31+G* level of theory (see Table 2s, Supporting Information). Single-point CCSD(T)/cc-pVDZ calculations have also been carried out to test the reliability of the SCS-MP2 results. The higher-level correlation method gave similar relative energies to the SCS-MP2 ones. Consequently, calculations at the SCS-MP2/TZVPP//MPW1K/6-31+G* level of theory are expected to give acceptable results for the reaction of the real reactant 1 as well. Relative energies reported further on in this paper are obtained at this level of theory, whereas geometrical and electron density data are given at the MPW1K/6-31+G* level. All the DFT and MP2 calculations were carried out using the Gaussian 03 suite of programs package.16 The RI-DFT-d and the SCSRI-MP2 single-point calculations were carried out using TURBOMOLE.17 For the determination of the bond critical points and the electron densities at the bond critical points, the AIM200018 program was used. At all the optimized structures, vibrational analysis was performed to check the nature of the stationary point (at a minimum, all the eigenvalues of the Hessian matrix are positive; at the transition states, there is exactly one negative eigenvalue). ZPE corrections and gas-phase corrections to Gibbs free energies were obtained from frequency calculations at the DFT level. Applying these corrections to the electronic energies calculated at higher levels of theory, however, did not influence either the relative ordering of the reaction paths or the conclusions drawn from the energy ordering of the structures; therefore, we included these numbers only in the Supporting Information (Table 4s). In the case of transition states, IRC (intrinsic reaction coordinate) calculations were performed to check which minima are connected by the transition state. The molecules and orbitals were visualized by the MOLDEN19 program.

hydride, the barrier of the hydrogen elimination depends on the properties of the proton acceptor as well: borane has a significantly higher TS energy with all investigated alcohols than AlH4− or GaH4−.9 The investigation of [(CF3)3BH−] [HPH3−nMen+] complexes revealed that reducing the proton donor strength decreases the stability of the dihydrogen bond.10 The H2 elimination reactions discussed above are closely related to hydrogen activation reactions, being essentially the reverse of those. A new concept emerging in the investigation of hydrogen activation reactions is the frustrated Lewis pairs (FLPs) concept, introduced by Stephan11−13 and explained on theoretical grounds by Pápai.14,15 Hydrogen activation is facilitated by a Lewis acid−base pair sterically hindered from forming a conventional Lewis acid−base adduct. It was also pointed out that the reaction barrier for the hydrogen activation depends on the frustration energy (the difference in energy between the classical and the frustrated Lewis pair).10 The aim of this work is the detailed investigation of the mechanism of the hydrosilylation reaction reported by Kawachi using ab initio calculations. Gaining access to the mechanistic details of the reactionbesides providing the explanation of the observed substituent effects in the reaction ratesalso facilitates the design of novel protecting groups. The expected similarities between the reverse of the investigated reactions and the hydrogen activation reactions allow us to analyze the reaction in the framework of the FLP concept.

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COMPUTATIONAL METHODS

Because the SCS-MP2 method has been proven to describe hydrogentransfer reactions reliably,14 we chose this method for our calculations as well. The size of the real system, however, prohibits the calculation of first and second derivatives at this level of theory, so we chose the strategy to determine the optimum and TS geometries at the DFT level, then carry out single-point SCS-MP2 calculations at these geometries. To find a DFT functional suitable for the description of the potential surface, we have calculated the dehydrogenative condensation of the simplified molecule 1′ with methanol (shown in Scheme 3) with several DFT functionals (see Table 1s in the

Scheme 3. Dehydrogenative Condensation of the Simplified Hydrosilane

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RESULTS AND DISCUSSION The conformational space of the starting molecule 1 and product 2 was explored, and three main conformations were found in both cases (denoted with the a, b, and c symbols; see

Supporting Information). In the simplified molecules denoted with a prime symbol, the mesityl groups on the boron atom are substituted with methyl groups and the methyl groups on the silicon changed to

Table 1. Calculated Activation Energies and Reaction Energies (kcal/mol) of the Reaction Shown in Scheme 3 and Some Selected Bond Distances (Å) TS1′|3′ MPW1K/631+G* MPW1K/aug-cc-pVTZ SCS-MP2/TZVPP SCS-MP2/TZVPP//MPW1K/631+g* (SP) CCSD(T)/cc-pVDZ//MPW1K/631+G* (SP)

3′

TS3′|2′

2′

E

H−Si

Si−O

E

H−Si

Si−O

E

H−Si

Si−O

E

Si−O

14.5 14.9

1.627 1.621

2.084 2.071

14.1 13.2 11.0 11.4 10.6

1.749 1.833 1.915

1.915 1.848 1.845

15.2 14.0

2.341 2.321

1.821 1.795

−19.9 −18.5 −25.5 −25.5 −21.8

1.706 1.691 1.702

13.3 11.1 121

10.9 10.6


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site. This effect can be traced on the Si−H distances in the transition states TS1a|2a and TS1b|2b. In the former, this distance is 1.832 Å, whereas in the second case, the polarizing effect of the catalyst group increases it slightly to 1.841 Å with the concomitant formation of a B−H bond that is indicated by the short B−H distance (d[B−H] = 1.346 Å) and the appearance of a new bond critical point (ρ[B−H] = 0.104). The decreased electron density at the bond critical point of the Si−H bond ( ρ[Si−H](Ia) = 0.062, ρ[Si−H](Ib) = 0.055) also shows the polarizing effect. The reaction barrier is lowered by 18.1 kcal/mol compared with that of the first pathway. The resulting barrier is 22.1 kcal/mol, significantly lower than that of Ia. This mechanism agrees with the push−pull mechanism suggested by Kawachi.7,20 In the reaction pathway Ic, the alcohol approaches from a different angle. The proton from the hydroxyl group comes near to one of the phenyl rings of a mesityl group connected to the boron center. Instead of the concerted mechanism of the previous two casessimilar to the 1′ moleculea stepwise path is obtained with a new minimum on the potential surface (denoted as 3). Both reaction barriers on this pathway are considerably lower than that of the concerted Ia and Ib (but higher than that of the reaction described by Scheme 3 due to the steric congestion). The overall reaction barrier is determined by the first transition state (16.8 kcal/mol). This barrier height agrees better with the observed reaction rate. To understand the underlying effects causing the decrease of the barriers, the geometries of both the transition states (TS1c|3 and TS3|2c) and the intermediate 3 must be investigated (see Table 2). In the first transition state (TS1c|3), the Si−H bond is less polarized (d[Si−H] = 1.736 Å, ρ[Si−H](Ic) = 0.067), whereas the H−B distance is longer (1.416 Å) compared with that of the TS1b|2b, which hints at an earlier transition state than TS1b|2b. The distance between the HO proton and the C2 atom of the mesityl group becomes short (2.029 Å), which indicates an interaction between the π-system of the phenyl ring and the proton. The O−H bond is slightly elongated (0.976 vs 0.967 Å). Such interactions were described earlier.21−23 This interaction can be observed also on the electrostatic potential map of the transition state (see blue circle in Figure 2). The topological analysis of the electron density also shows a weak interaction between C2 and HO (the electron density at the bond critical point is ρ = 0.026). This interaction provides the extra stabilization of TS1c|3 compared with TS1b|2b, where the only stabilizing effect is that of the boryl group.

Table 3s in the Supporting Information). The conformers' energies are close to each other; the maximum difference is 2.5 kcal/mol in case of 1 and 3.6 kcal/mol in case of 2. They are separated by low energy barriers; thus, the dimethylsilyl group is expected to rotate freely around the C−Si axis, confirming the absence of significant Si−H···B7 or O−B interaction. According to the orientation of the hydrogen of the dimethylsilyl group in the starting material and the approach direction of the methanol molecule, four different possible reaction pathways have been found (see Figure 1).

Figure 1. Possible reaction pathways of the dehydrogenative condensation.

The reaction pathway Ia goes through a single energy barrier. Both in the starting material and in the transition state, the hydrogen atom connected to the silyl group is oriented away from the boron center; thus, the polarization of the Si−H bond by the Lewis base cannot take place. The reaction barrier is high (40.2 kcal/mol), not consistent with the experimentally observed reaction rates at room temperature.7 Because interaction with the boryl group is absent, this reaction provides a reference point to assess the catalytic effect of the boron center in the subsequent mechanisms. The reaction pathway Ib also follows a concerted mechanism. The orientation of the silyl hydrogen, however, allows the participation of the boryl group. On the approach of the alcohol, the Si−H bond is polarized by the Lewis basic boron

Table 2. Geometries of the Transition States (TS1c|3 and TS3|2c) and the Intermediate (3)

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and the π-system of the nearby mesityl group, while the lone pair of the boron and the silicon atom act as Lewis acidic sites. An interesting insight can be gained by comparing the geometries of 2b (that has bulky substituents on the boron center) and 2′ (which is much less bulky), which are the starting geometries of the reverse reaction. The Si−O distance is 1.717 Å in the case of 2′ and 3.177 Å in the case of 2b. The increase of the Si−O distance can be contributed to the steric hindrance caused by the bulky substituents on the boron center. The steric crowding acts as a frustration on the Lewis acid−base pair formed by the boron and the oxygen atoms and decreases the barrier corresponding to the hydrogen splitting from the 36.4 kcal/mol in the case of 2′ to 25.5 kcal/mol in the case of 2. The pinacolborane derivative of 1 (1pin) was also investigated experimentally by Kawachi.7 It was found that the hydrosilylation did not take place using this starting material. The lack of reactivity was explained by the decreased acidity of the pinacolborane group. To evaluate the effect of this functional group on the reaction mechanism, we have recalculated the stepwise Ic pathway with the starting material 1pin (see Figure 3). In accordance with the decreased acidity of

Figure 2. Electrostatic potential map of the transition state (TS1c|3) (the blue color indicates the positive, whereas the red color indicates the negative electrostatic potential).

In the intermediate 3, similar interactions are present as in the previous TS1c|3. The stabilizing effect of the π-system of the phenyl ring is even more pronounced, the distance between the HO proton and the ring is decreased to 1.746 Å, and the electron density of the bond critical point increased to 0.048, althoughbecause of the decreasing O−Si distancethe closest carbon atom is C1 instead of C2. The O−HO distance is also significantly elongated (1.021 Å). Presumably, this interaction is the dominant factor that creates the intermediate minimum on the reaction pathway. The H−Si bond is already completely broken. The H−Si distance is 2.731 Å, and the H−Si bcp disappears. The B−H distance is further shortened to 1.233 Å (ρ[B−H] = 0.152). The HSi−HO distance is 2.157 Å, which is consistent with the formation of a weak DHB interaction that can also contribute to the stabilization of the intermediate, although a corresponding bcp is not found. The elimination of the hydrogen molecule is completed through TS3|2c. Both the B−HSi and the O−HO distances are elongated compared with those of the intermediate (1.265 and 1.052 Å, respectively), whereas the HO−HSi distance decreases to 1.240 Å, showing the formation of the hydrogen molecule (ρ[HO−HSi] = 0.067). The activation barrier from the intermediate is only 0.4 kcal/mol, making the intermediate kinetically unstable. The fourth possible reaction pathway is Id, where, instead of the silyl group, the alcohol attacks the other Lewis acidic site: the boron center. Although the formation of a complex between the oxygen of the attacking alcohol and the boron was expected, no such intermediate was located on this pathway; the migration of the methoxy group from the boron to the silicon and the formation of the hydrogen molecule take place simultaneously. The barrier corresponding to this transition state, however, is relatively high (37.7 kcal/mol). The absence of the intermediate and the high barrier is explained by the bulky groups on the boron atom, hindering both the approach of the alcohol and the formation of the pyramidal arrangement around the boron that is preferred for the dative bond. The reverse of reaction I (R′−O−SiR3 + H2 → R′−OH + H−SiR3) can be conceived as a hydrogen activation reaction. Although the reverse reaction is endothermic, the individual steps can be compared to the steps of known hydrogen activation reactions, for example, the reactions activated by frustrated Lewis acid−base pairs (FLPs). In our case, the role of the Lewis base is played by the oxygen of the methoxy group

Figure 3. Reaction pathway of the dehydrogenative condensation in the case of 1pin.

the boron center, the first barrier on this pathway increases by 7.2 kcal/mol. Contrary to Ic, the cleavage on the proton from the alcohol is assisted by one of the oxygens of the pinacolborane. The interaction between the lone pair of the oxygen and the proton is stronger than the π-H interaction in 3. The second barrier on this pathway is also comparably high. The elimination of the hydrogen molecule from the reaction complex is hindered by the strong O−H+ interaction (see Figure 3). The effect of the changes of Lewis acidity of the electronacceptor site of the starting material was also investigated by changing the mesityl substituents on the boron center to pentafluorphenyl groups (1pf) and substituting boron to aluminum (1Al). The stepwise reaction pathway was calculated with both 1pf and 1Al (see Figure 4). In both cases, the ratedetermining step is the first one. The first barrier is also decreased in both cases due to the increased Lewis acidity of the acidic center, in a good agreement with the results obtained for alcoholysis reactions of main group hydrides.9 In the case of the less-acidic 1Al, the first barrier decreases less than in the case of 1pf. It is also interesting to note that, whereas in the case of TS1c|3, the silyl hydrogen was activated by the proximity of the π-system of one of the mesityl rings, in the case of 123


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ASSOCIATED CONTENT

S Supporting Information *

The optimized geometry of the molecules at the MPW1K/631+G* and MP2/TZVPP levels of theory are available, as well as the energetic and geometrical data of the reaction in Scheme 3 with various DFT functionals. Relative energies of the possible conformers of the starting material 1 and product 2 are also included. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

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ACKNOWLEDGMENTS The results discussed above were supported by the grant TÁ MOP-4.2.2.B-10/1--2010-0009. Figure 4. Reaction pathway of the dehydrogenative condensation in the case of 1pf and 1Al.

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TS1pf|3pf, the silyl hydrogen interacts with one of the fluorine atoms. The relative energies of the products are also significantly lowered in the case of both 1Al and 1pf. The stabilization of the products can be contributed to the formation of a dative bond between the Lewis acidic center and the oxygen of the methoxy group. The formation of this bond is also made possible by the decrease of the steric crowding around the boron and the aluminum atoms.

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REFERENCES

CONCLUSIONS

First-principles methods were used to investigate the mechanism of the dehydrogenative condensation of o[(dimesitylboryl)phenyl]dimethylsilane with methanol. A stepwise mechanism with a zwitterionic intermediate was found to be preferred over the previously suggested concerted one. The activation of the Si−H and O−H bonds takes place simultaneously. While the hydride-like hydrogen on the Si atom is activated by the Lewis acidic boron site, the activation of the O−H bond of the incoming methanol also plays a significant role in reducing the activation energy. The Lewis basic site providing the activation of the O−H bond is the πsystem of one of the mesityl substituents on the boron atom. The elimination of H2 from the intermediate is hindered by a small barrier only; the rate-determining step is the first one. Changing the Lewis acidity of the acidic site enables us to control the height of the first barrier on the preferred reaction path: changing the mesityl groups on the boron atom to pentafluorophenyl groups or substituting B to Al (thus increasing the Lewis acidity of the site) decreases the first barrier significantly. Surprisingly, the experimentally observed lack of reactivity of the pinacol-substituted derivative is caused not only by the change in Lewis acidity but also by the hindrance of the H2 elimination. The reverse reaction of the dehydrogenative condensation can be conceived as the activation of a H2 molecule by a frustrated Lewis pair, where the boron atom acts as the acidic and the π-system of the mesityl ring and the methoxy oxygen as the basic site. 124


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Mechanism of Dehydrogenative Condensation of (o-borylphenyl

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