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

Abstract

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 H 2 O 2 . 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 H 2 O 2 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)-(R-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 (+)-(R-Np)PhMeSiH to (+)-(R-Np)PhMeSiOH, a concerted spiro-type transitionstate 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.

Zirconium phenylphosphonate-anchored methyltrioxorhenium as novel heterogeneous catalyst for epoxidation of cyclohexene

“…1). is an effective oxidant in the olefin epoxidation over MTO with [17] or without [10], [11] support materials. By using MTO/Nb 2 O 5 catalyst with UHP as oxidant, a 100% selectivity to epoxide was also obtained (entry 8 [18]).…”

“…Several efficient methods have been developed to avoid the side reaction. For example, urea hydrogen peroxide adduct (UHP) with helical channel supposed to serve as a host confined stabilizing environment for MTO was adopted as oxidant in the organic reaction medium, e.g., dichloromethane (CH 2 Cl 2 ) [10], [11].…”

Synthesis and Characterization of Methyltrioxorhenium(VII) Immobilized in Bipyridyl-Functionalized Mesoporous Silica

“…It was found that a urea/hydrogen peroxide complex is a very effective oxidant in heterogeneous olefin epoxidations and silane oxidations catalyzed by MTO. [11] With NaY-zeolite as host for these reactions also resulted in high yields and excellent product selectivities. [12] MTO has also been supported on silica functionalized with polyether tethers.…”

Catalytic Oxidation of Silanes by Carbon Nanotube–Gold Nanohybrids

“…[2] Silanes are classically converted into silanols using strong oxidizing agents such as osmium tetroxide, [3] permanganate, [4] ozone, [5] peracids, [6] or peroxides. [7] However, under these reaction conditions, significant amounts of siloxanes and toxic by-products are formed. To overcome these drawbacks, catalytic systems involving water and oxygen have been recently devised.…”