Efficient epoxidation over dinuclear sites in titanium silicalite-1.

Titanium silicalite-1 (TS-1) is a zeolitic material with MFI framework structure, in which 1 to 2 per cent of the silicon atoms are substituted for titanium atoms. It is widely used in industry owing to its ability to catalytically epoxidize olefins with hydrogen peroxide (H2O2), leaving only water as a byproduct1,2; around one million tonnes of propylene oxide are produced each year using this process3. The catalytic properties of TS-1 are generally attributed to the presence of isolated Ti(IV) sites within the zeolite framework1. However, despite almost 40 years of experimental and computational investigation4-10, the structure of these active Ti(IV) sites is unconfirmed, owing to the challenges of fully characterizing TS-1. Here, using a combination of spectroscopy and microscopy, we characterize in detail a series of highly active and selective TS-1 propylene epoxidation catalysts with well dispersed titanium atoms. We find that, on contact with H217O2, all samples exhibit a characteristic solid-state 17O nuclear magnetic resonance signature that is indicative of the formation of bridging peroxo species on dinuclear titanium sites. Further, density functional theory calculations indicate that cooperativity between two titanium atoms enables propylene epoxidation via a low-energy reaction pathway with a key oxygen-transfer transition state similar to that of olefin epoxidation by peracids. We therefore propose that dinuclear titanium sites, rather than isolated titanium atoms in the framework, explain the high efficiency of TS-1 in propylene epoxidation with H2O2. This revised view of the active-site structure may enable further optimization of TS-1 and the industrial epoxidation process.

The Rickiols: 20-, 22-, and 24-membered Macrolides from the Ascomycete Hypoxylon rickii.

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.

Acid-catalyzed oxidative radical addition of ketones to olefins.

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.