Host-guest chemistry in a urea matrix: Catalytic and selective oxidation of triorganosilanes to the corresponding silanols by methyltrioxorhenium andthe urea/hydrogen peroxide adduct

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 (UH P) 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 whi ch 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 )-(alpha-Np)PhMeSiH proceeds with retention of configuration in excellent y ield. To date, no catalytic Si-H oxygen insertion has been reported for the preparation of optically active silanols. In analogy with the stereoselect ivity in the dioxirane oxidation of (+)-(alpha-Np)PhMeSiH to (+)-(alpha-Np) PhMeSiOH, a concerted spiro-type transition-state structure is proposed for this novel Si-H oxidation. Herewith, a valuable synthetic,method for the p reparation of silanols has been made available through catalytic and select ive oxidation of silanes to silanols by the MTO/UHP system.