The highly selective catalytic epoxidation of terminal alkenes by the complex WVI/PV/H202/CHC13/PTC (PTC = phase transfer catalyst) system (Ishii-Venturello chemistry) has been extensively investigated by groups in several countries and recently commercialized, yet little is known with certainty about the mechanism. The substrate conversions and epoxide selectivities observed under biphasic conditions, aqueous H202/alkene in CHCI3, with 21 polyoxometalates (cetylpyridinium chloride as the phase-transfer catalyst, PTC) including the Ishii precursor complex, [PW12O40]3~, clearly indicate that only [PW^Oito]3-456and [PWnC>3C)]7-89, which both rapidly form {P04[W0(C>2)2]4}3-, 1, are effective. Simultaneous monitoring of organic oxygenated products and gaseous products (nearly all O2) with several of these polyoxometalates confirm that H2O2 disproportionation is by far the dominant side reaction with several d-electron transition metal-substituted polyoxometalate catalyst precursors. Analysis of the 2/w-p coupling satellites in the 31P NMR spectra of the polytungstophosphate products from the stoichiometric reaction of 1 with alkene substrates as a function of cation, solvent, field strength, and time indicates that both a PW4 and a PW3 specie are formed initially and one PW2 specie subsequently. Several lines of kinetic and spectroscopic evidence indicate that two processes dominate over all others during Ishii-Venturello epoxidation: a slow epoxidation, 1 (PW4) + alkene -PW4, PW3, and PW2 (henceforth called "subsequent peroxo species" or SPS) + epoxide, followed by a rapid regeneration of 1 with H2O2. First, little epoxidation is observed until 1 is in appreciable concentration. Second, the rate law for epoxidation of 1-octene by the Arquad salt of 1, Arql (Arquad = [(CigH37)75% +Third, 1 is the dominant polytungstophosphate present under steady state turnover conditions. Fourth, the ratio of the initial rates of epoxidation is vo(Aiqsps/vo(Arqi) = 0.13 ± 0.01. Fifth, the dominant inorganic product in the formation of 1, {[WCX* 1C^hdUOjhO}2-, is two orders of magnitude slower in alkene epoxidation than Arql under identical conditions at both 23 and 60 °C. Additional 31P NMR studies address both ion pairing effects and dynamic exchange in 1 and the SPS PW4. A linear correlation was found between the change in both chemical shift and 27w-p coupling constant for the SPS PW4 specie but not the SPS PW2 or the SPS PW3 species as a function of reaction time. This is consistent with the SPS PW4 specie undergoing rapid dynamic exchange on the 31P NMR time scale. Addition of 1 equiv of 1,2-epoxybutane to tetran-hexylammonium SPS (THASPS) does shift the SPS PW4 resonance to high field with a larger 2Jw-p coupling constant in accord with the correlation. Consequently, the dynamics of SPS PW4 may reflect exchange of epoxide product. Rapid catalyst inactivation despite being one of two success limiting features of Ishii-Venturello epoxidation was not addressed in any previous work. Under the typical biphasic reaction conditions, ...

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Developing methods to probe the nature and structure of nanoscale environments continues to be a challenge in nanoscience. We report a cyclic voltammetry investigation of the internal, hydrogen-bond-driven phase separation of amide-containing thiols and alkanethiols. Amide-containing thiols with a terminal ferrocene carboxylate functional group were investigated in two binary monolayers, one homogeneously mixed and the other phase separated. The electrochemical response of the ferrocene probe was used to monitor adsorbate coverage, environment, and phase separation within each of these monolayers. The results demonstrate that the behavior of ferrocene-containing monolayers can be used to probe nanoscale organization.

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