Articles Mechanism of the B(C 6 F 5 ) 3 -Catalyzed Reaction of Silyl Hydrides with Alkoxysilanes. Kinetic and Spectroscopic Studies Julian Chojnowski,* ,† Slawomir 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 Lo ´ 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: Ph 2 MeSiH + Ph 2 MeSiOn-Oct (I) and Ph 2 MeSiH + Me 3 SiOn-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 λ max ) 303-306 nm, which is due to an allowed electronic transition in the uncomplexed B(C 6 F 5 ) 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(C 6 F 5 ) 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 se- lective catalyst for many reactions in organic chemistry (for reviews see refs 1-3). This compound is particularly active in promoting the reduction of an organic func- tionality with silyl hydrides. Parks and Piers 4 found that B(C 6 F 5 ) 3 catalyzes the hydrosilylation of carbonyl com- pounds. Since then, many other reductive processes using the R 3 SiH/B(C 6 F 5 ) 3 system have been reported. This system is particularly useful in the transformation of aldehydes, ketones, carboxylic acids, and their de- rivatives into silyl ethers, 4,5 but further reduction to hydrocarbons also occurs smoothly. 6-8 Hydrosilylation * 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. (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. 6077 Organometallics 2005, 24, 6077-6084 10.1021/om050563p CCC: $30.25 © 2005 American Chemical Society Publication on Web 10/21/2005