Organometallics 2005, 24, 6077-6084
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Articles Mechanism of the B(C6F5)3-Catalyzed Reaction of Silyl Hydrides with Alkoxysilanes. Kinetic and Spectroscopic Studies Julian Chojnowski,*,† Sławomir Rubinsztajn,‡ James A. Cella,‡ Witold Fortuniak,† Marek Cypryk,† Jan Kurjata,† and Krzysztof Kaz´mierski† Center of Molecular and Macromolecular Studies, Polish Academy of Sciences, Sienkiewicza 112, 90-363 Ło´ dz´ , Poland, and General Electric Company, Global Research Center, Niskayuna, New York 12309 Received July 6, 2005
The coupling reaction of silyl hydrides with alkoxysilanes to produce siloxanes and hydrocarbons catalyzed by tris(pentafluorophenyl)borane was studied by gas chromatography and UV spectroscopy using model reagent systems: Ph2MeSiH + Ph2MeSiOn-Oct (I) and Ph2MeSiH + Me3SiOn-Oct (II). Detailed kinetic studies performed for system I showed that the reaction is first order in both substrates and the rate is proportional to the catalyst concentration. A highly negative apparent entropy of activation points to a crowded transition state structure, leading to a significant dependence of the rate on steric effects. Studies of system II demonstrated that the exchange of the Si-H and Si-OR functionality accompanies the coupling process and in many cases is the dominating reaction in this system. Ultraviolet spectra recorded during the reaction show a distinct strong absorption band with λmax ) 303-306 nm, which is due to an allowed electronic transition in the uncomplexed B(C6F5)3 molecule. This absorption also gives rise to intense fluorescence with a maximum of the emission band at 460 nm. When the borane is complexed by oxygen nucleophiles, such as water, alcohol, or silanol and is not active as a catalyst, it does not show the absorption in the 303-306 nm region. This absorption may serve as a measure of the concentration of the active uncomplexed catalyst in the reaction system. Since complexes of B(C6F5)3 with the alkoxysilane substrates and the disiloxane products are relatively weak, the catalyst appears in the reaction system mostly as an uncomplexed species and its concentration is not significantly changed during the reaction. The mechanism proposed includes the transient formation of a complex between hydrosilane, borane, and alkoxysilane in which H- is transferred from silicon to boron and an oxonium ion moiety is generated by interaction of alkoxysilane with positive silicon. The decomposition of the complex occurs by the H- transfer to one of the three electrophilic centers of the oxonium structure, which explains the competition between the siloxane formation and the Si-H/Si-OR exchange. In the case of alkoxysilanes derived from primary alcohols, H- is preferably transferred to silicon. However, for alkoxysilanes derived from a secondary alcohol, such as isopropyl alcohol, the secondary carbon is more readily attacked than silicon by H-, which leads to a high yield of mixed disiloxane. Introduction Tris(pentafluorophenyl)borane is a powerful and selective catalyst for many reactions in organic chemistry (for reviews see refs 1-3). This compound is particularly * To whom correspondence should be sent. E-mail: jchojnow@ bilbo.cbmm.lodz.pl. † Polish Academy of Sciences. ‡ General Electric Company. (1) (a) Piers, W. E.; Chivers, T. Chem. Soc. Rev. 1997, 26, 345. (b) Piers, W. E. Adv. Organomet. Chem. 2005, 52, 1. (2) Ishihara, K.; Yamamoto, H. Eur. J. Org. Chem. 1999, 527, 7. (3) Coates, G. W.; Hustal, P. D.; Reiinartz, S. Angew. Chem., Int. Ed. 2003, 41, 2236.
active in promoting the reduction of an organic functionality with silyl hydrides. Parks and Piers4 found that B(C6F5)3 catalyzes the hydrosilylation of carbonyl compounds. Since then, many other reductive processes using the R3SiH/B(C6F5)3 system have been reported. This system is particularly useful in the transformation of aldehydes, ketones, carboxylic acids, and their derivatives into silyl ethers,4,5 but further reduction to hydrocarbons also occurs smoothly.6-8 Hydrosilylation (4) Parks, D. J.; Piers, W. E. J. Am. Chem. Soc. 1996, 118, 9440. (5) Parks, D. J.; Blackwell, J. M.; Piers, W. E. J. Org. Chem. 2000, 65, 3090.
10.1021/om050563p CCC: $30.25 © 2005 American Chemical Society Publication on Web 10/21/2005
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Organometallics, Vol. 24, No. 25, 2005
Chojnowski et al.
Table 1. Results of Preliminary Studies of the Condensation of Triorganohydrosilanes with Triorganoalkoxysilanes substratea R3SiH R′3SiOR′′ 1 2 3 4 5 6 7 8 9 a
Et3SiH Et3SiH Et3SiH PhMe2SiH PhMe2SiH PhMe2SiH Ph2MeSiH Ph2MeSiH
Me3SiOMe Me3SiOEt Me3SiOn-Oct Me3SiOMe Me3SiOEt Me3SiOn-Oct Me3SiOn-Oct Me3SiOi-Pr
Ph2MeSiH
Et3SiOi-Pr
[B(C6F5)3]0 (10-3 mol/L)
approx t1/2 of R3SiH conversion
time of quenching of reaction
2.5 2.5 1.5 4.0 4.0 3.0 0.6 11.6
∼400 s ∼1000 s
