Host-guest chemistry in a urea matrix: Catalytic and selective oxidation of triorganosilanes to the corresponding silanols by methyltrioxorhenium andthe urea/hydrogen peroxide adduct
W. Adam et al., Host-guest chemistry in a urea matrix: Catalytic and selective oxidation of triorganosilanes to the corresponding silanols by methyltrioxorhenium andthe urea/hydrogen peroxide adduct, J AM CHEM S, 121(10), 1999, pp. 2097-2103
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.