Titanium Silicalite-1 (TS-1) shows an outstanding ability to catalytically epoxidize olefins with hydrogen peroxide (H2O2), leaving only water as byproduct. 1,2 Despite the industrial use of the TS-1/H2O2 system for the production of more than one million tons of propylene oxide per year, 3 the active site structure remains elusive, although it has been studied for almost 40 years by spectroscopic and computational methods. 4-10 TS-1 is a zeotype of MFI structure in which a small fraction of Si-atoms (1-2 %) are substituted by Ti, and its catalytic properties are generally attributed to isolated Ti(IV) sites. 1 Herein, we analyze a series of highly active and selective TS-1 propylene epoxidation catalysts. By UV-Vis and Raman spectroscopy, as well as electron microscopy, we show that Ti is well-dispersed in all samples, with formation of small TiOx clusters at high Ti-loadings. Most notably, irrespective of Ti-content, all samples show a characteristic solid-state 17 O NMR signature when contacted with H2 17 O2, indicating the formation of bridging peroxo species on dinuclear Tisites. Using DFT (density functional theory) calculations, we propose a mechanism of propylene epoxidation on a dinuclear site, in which the cooperativity between two titanium atoms enables a low-energy reaction pathway where the key oxygen-transfer transition state bears strong resemblance to that of olefin epoxidation by peracids.The active species in TS-1 are commonly proposed to be isolated Ti(IV) sites bearing peroxo 11 or hydroperoxo moieties, 12 although the involvement of terminal Ti-oxo and activated H2O2 on Ti(IV) has also been discussed (Fig. 1a,b). 7 In contrast, the only homogeneous Ti-based epoxidation catalysts able to efficiently utilize H2O2 as primary oxidant are dinuclear, such as the Berkessel-Katsuki epoxidation catalyst 1 (Fig. 1c). [14][15][16][17][18][19] While the structural characterization of molecular systems is well-established and has enabled the isolation of peroxo compounds, obtaining information on the structure of Ti-sites in TS-1 with molecular-level precision has proven more challenging.Recent work by some of us has shown that solid-state 17 O NMR spectroscopy is a powerful tool for understanding and assessing the reactivity of peroxo species. 20 Oxygen-17 is an NMRactive quadrupolar nucleus whose spectroscopic properties can be readily measured by solidstate NMR and computed by DFT. The NMR signature (chemical shift and quadrupolar coupling) is highly sensitive to the symmetry and electronic structure around the oxygen atoms. We thus reasoned that 17 O NMR spectroscopy would be a valuable tool to harness the signature of the active sites in TS-1 and thereby probe their structure. In this study, we investigate five TS-1 samples prepared in the BASF laboratories (Table 1). 21,22 Two of these samples have a Ti-content of 1.9 wt%, one of which was prepared on hundred-kg scale (sample 1), the other three samples have Ti-loadings of 1.5 wt%, 1.0 wt%, and 0.5 wt%. The five samples have surface areas between 4...

Based on a mechanistic study, we have discovered a Brønsted acid catalyzed formation of ketone radicals. This is believed to proceed via thermally labile alkenyl peroxides formed in situ from ketones and hydroperoxides. The discovery could be utilized to develop a multicomponent radical addition of unactivated ketones and tert-butyl hydroperoxide to olefins. The resulting γ-peroxyketones are synthetically useful intermediates that can be further transformed into 1,4-diketones, homoaldol products, and alkyl ketones. A one-pot reaction yielding a pharmaceutically active pyrrole is also described.

We report a new and readily accessible class of titanium salalen complexes derived from cis‐1,2‐diaminocyclohexane (cis‐DACH) and fluorinated salicylic aldehyde derivatives. With aqueous hydrogen peroxide as the oxidant, these complexes catalyze the epoxidation of terminal, nonconjugated olefins in high yields with high enantioselectivities. We furthermore discovered that the addition of certain acidic or basic co‐catalysts significantly accelerated the epoxidation. For example, in the presence of 1 mol % Ti catalyst and 1 mol % pentafluorobenzoic acid, 1‐octene epoxidation (95 % ee) was completed at room temperature within 8 h. The catalytic process was compatible with many functional groups (e.g., ethers, esters, halides, nitriles, and nitro groups), whereas free hydroxy groups appeared to slow down the reaction to some extent. Catalyst recycling was possible.

Titanium–salalen complexes have recently solved a long-standing problem in homogeneous epoxidation catalysis by enabling the selective catalytic epoxidation of terminal, nonconjugated olefins with hydrogen peroxide. In this work, we disclose the mechanism of this intriguing catalyst system, based on XRD analyses, kinetic studies, and NMR elucidation of intermediate structures, complemented by DFT computations. Titanium–salalen catalysts are typically prepared/stored as bis-μ-oxo or μ-oxo-μ-peroxo dimers. Under reaction conditions, while the μ-oxo bridged catalyst dimers remain intact, the epoxidation occurs through an octahedral, yet altered, coordination geometry of the homochiral monomeric subunits. This catalytically active coordination mode is accessed by a slow pre-equilibrium, involving uptake of hydrogen peroxide, and subsequent rearrangement of the coordination sphere of the dinuclear complex. This configuration allows a three-pronged electrophilic activation of hydrogen peroxide, which enables oxygen transfer by the joint action of (i) the Lewis acidic titanium center, (ii) H-bond donation by the ligand’s NH, and (iii) π-chalcogen interaction with the ligand’s pentafluorophenyl moieties. This efficient activation of H2O2 by a dinuclear site parallels recent findings on the active sites of the industrial heterogeneous titanium silicalite TS-1 catalyst.

In preceding studies the neotropical ascomycete Hypoxylon rickii turned out to be a prolific source of new secondary metabolites, considering that we had obtained terpenoids with five different scaffolds along with a series of terphenyls. From the mycelial extracts of a 70 L scale fermentation of this strain we additionally isolated nine new macrolides (1-9) by RP-HPLC. The planar structures were elucidated by NMR spectroscopy complemented by HR-ESIMS. The relative configurations were assigned by J-based configuration analyses and confirmed by Kishi's Universal Database. Subsequently, the absolute configurations were assigned by Mosher's method using the shift analysis of a tetra-MTPA derivative. For rickiol A (1) and E (5) we observed transesterification of 20-membered ring structures to 22-membered isomers rickiol A2 (6) and E2 (7), and to 24-membered isomers rickiol A3 (8) and rickiol E3 (9), respectively. Cytotoxic effects and moderate antibiotic activity against Gram-positive bacteria were observed for 1-8 and 1-6 and 8, respectively. The total synthesis of rickiol E3 (9) established easier access to these compounds.