MECHANISM AND DYNAMICS IN THE H-3[PW12O40]-CATALYZED SELECTIVE EPOXIDATION OF TERMINAL OLEFINS BY H2O2 - FORMATION, REACTIVITY, AND STABILITY OF (PO4[WO(O-2)(2)](4))(3-)

The highly selective catalytic epoxidation of terminal alkenes by the complex W-VI/P-V/H2O2/CHCl3/PTC (PTC = phase transfer catalyst) system (Ishii-Venturello chemistry) has been extensively investigated by gro ups in several countries and recently commercialized, yet little is kn own with certainty about the mechanism. The substrate conversions and epoxide selectivities observed under biphasic conditions, aqueous H2O2 /alkene in CHCl3, with 21 polyoxometalates (cetylpyridinium chloride a s the phase-transfer catalyst, PTC) including the Ishii precursor comp lex, [PW12O40](3-), clearly indicate that only [PW12O40](3-) and [PW11 O39](7-), which both rapidly form {PO4[WO(O-2)(2)](4)}(3-), 1, are eff ective. Simultaneous monitoring of organic oxygenated products and gas eous products (nearly all O-2) with several of these polyoxometalates confirm that H2O2 disproportionation is by far the dominant side react ion with several d-electron transition metal-substituted polyoxometala te catalyst precursors. Analysis of the (2)J(W-P) coupling satellites in the P-31 NMR spectra of the polytungstophosphate products from the stoichiometric reaction of 1 with alkene substrates as a function of c ation, solvent, field strength, and time indicates that both a PW4 and a PW3 specie are formed initially and one PW2 specie subsequently. Se veral lines of kinetic and spectroscopic evidence indicate that two pr ocesses dominate over ah 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 observ ed until 1 is in appreciable concentration. Second, the rate law for e poxidation of 1-octene by the Arquad salt of 1, Arq1 (Arquad = [(C18H3 7)(75%) + (C16H33)(25%)](2-) [CH3]N-2), in CHCl3 at 23 degrees C is v( 0) = k[1][1-octene]. Third, 1 is the dominant polytungstophosphate pre sent under steady state turnover conditions. Fourth, the ratio of the initial rates of epoxidation is v(0(ArqSPS))/V-0(Arq1) = 0.13 +/- 0.01 . Fifth, the dominant inorganic product in the formation of 1, {[WO(O- 2)(2)(H2O)]O-2}(2-), is two orders of magnitude slower in alkene epoxi dation than Arq1 under identical conditions at both 23 and 60 degrees C. Additional P-31 NMR studies address both ion pairing effects and dy namic exchange in 1 and the SPS PW4. A Linear correlation was found be tween the change in both chemical shift and (2)J(W-P) coupling constan t 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 spec ie undergoing rapid dynamic exchange on the P-31 NMR time scale. Addit ion of 1 equiv of 1,2-epoxybutane to tetra-n-hexylammonium SPS (THASPS ) does shift the SPS PW4 resonance to high field with a larger (2)J(W- P) coupling constant in accord with the correlation. Consequently, the dynamics of SPS PW4 may reflect exchange of epoxide product. Rapid ca talyst inactivation despite being one of two success limiting features of Ishii-Venturello epoxidation was not addressed in any previous wor k. Under the typical biphasic reaction conditions, catalysis nearly st ops after 500 turnovers. The effects of alkene, H2O, and epoxide produ ct on epoxidation rates and polytungstophosphate speciation monitored by P-31 NMR establish that epoxide, but apparently neither alkene nor H2O, leads to irreversible catalyst inactivation.