Host−Guest Chemistry in a Urea Matrix: Catalytic and Selective Oxidation of Triorganosilanes to the Corresponding Silanols by Methyltrioxorhenium and the Urea/Hydrogen Peroxide Adduct
journal contributionposted on 02.03.1999 by Waldemar Adam, Catherine M. Mitchell, Chantu R. Saha-Möller, Oliver Weichold
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The oxidation of silanes to silanols, catalyzed by methyltrioxorhenium (MTO), proceeds in high conversions and excellent selectivities in favor of the silanol (no disiloxane product) when the urea/hydrogen peroxide adduct (UHP) is used as oxygen source instead of 85% aqueous H2O2. It is proposed that this novel Si−H oxidation takes place in the helical urea channels, in which the urea matrix serves as host for the silane substrate, the H2O2 oxygen source, and the MTO metal catalyst as guests. In this confined environment, the metal catalyst is stabilized against decomposition, and this enhances higher conversions while condensation of the silanol to its disiloxane is avoided for steric reasons. The oxidation of the optically active silane (S)-(α-Np)PhMeSiH proceeds with retention of configuration in excellent yield. To date, no catalytic Si−H oxygen insertion has been reported for the preparation of optically active silanols. In analogy with the stereoselectivity in the dioxirane oxidation of (+)-(α-Np)PhMeSiH to (+)-(α-Np)PhMeSiOH, a concerted spiro-type transition-state structure is proposed for this novel Si−H oxidation. Herewith, a valuable synthetic method for the preparation of silanols has been made available through catalytic and selective oxidation of silanes to silanols by the MTO/UHP system.