Titaniumcis-1,2-Diaminocyclohexane Salalen Catalysts of Outstanding Activity and Enantioselectivity for the Asymmetric Epoxidation of Nonconjugated Terminal Olefins with Hydrogen Peroxide

Tracking Coordination Environment and Reaction Intermediates in Homogeneous and Heterogeneous Epoxidation Catalysts via Ti L2,3-Edge Near-Edge X-ray Absorption Fine Structures

Ti-based molecules and materials are ubiquitous and play a major role in both homogeneous and heterogeneous catalytic processes. Understanding the electronic structures of their active sites (oxidation state, local symmetry, and ligand environment) is key to developing molecular-level structure-property relationships. In that context, X-ray absorption spectroscopy (XAS) offers a unique combination of elemental selectivity and sensitivity to local symmetry. Commonly, for early transition metals such as Ti, K-edge XAS is applied for in situ characterization and subsequent structural analysis with high sensitivity toward tetrahedral species. Ti L2,3-edge spectroscopy is in principle complementary and offers specific opportunities to interrogate the electronic structure of five-and six-coordinated species. It is, however, much more rarely implemented because the use of soft X-rays implies ultrahigh vacuum conditions. Furthermore, the interpretation of the data can be challenging. Here, we show how Ti L2,3-edge spectroscopy can help to obtain unique information about both homogeneous and heterogeneous epoxidation catalysts and develop a molecular-level relationship between spectroscopic signatures and electronic structures. Toward this goal, we first establish a spectral library of molecular Ti reference compounds, comprising various coordination environments with mono- and dimeric Ti species having O, N, and Cl ligands. We next implemented a computational methodology based on multiplet ligand field theory and maximally localized Wannier orbitals benchmarked on our library to understand Ti L2,3-edge spectroscopic signatures. We finally used this approach to track and predict the spectra of catalytically relevant intermediates, focusing on Ti-based olefin epoxidation catalysts.

Titanium Salalen Catalyzed Enantioselective Benzylic Hydroxylation

The titanium complex of the cis-1,2-diaminocyclohexane (cis-DACH) derived Berkessel-salalen ligand is a highly efficient and enantioselective catalyst for the asymmetric epoxidation of terminal olefins with hydrogen peroxide ("Berkessel-Katsuki catalyst"). We herein report that this epoxidation catalyst also effects the highly enantioselective hydroxylation of benzylic C-H bonds with hydrogen peroxide. Mechanism-based ligand optimization identified a novel nitro-salalen Ti-catalyst of the highest efficiency ever reported for asymmetric catalytic benzylic hydroxylation, with enantioselectivities of up to 98 % ee, while overoxidation to ketone is marginal. The novel nitro-salalen Ti-catalyst also shows enhanced epoxidation efficiency, as evidenced by e.g. the conversion of 1-decene to its epoxide in 90 % yield with 94 % ee, at a catalyst loading of 0.1 mol-% only.

syn ‐Selective Epoxidation of Chiral Terminal Allylic Alcohols with a Titanium Salalen Catalyst and Hydrogen Peroxide

In the Sharpless asymmetric epoxidation of chiral secondary allylic alcohols, one substrate enantiomer is predominantly converted to the anti‐epoxy alcohol. We herein report the first highly syn‐selective epoxidation of terminal allylic alcohols using a titanium salalen complex as catalyst, at room temperature, and aqueous hydrogen peroxide as oxidant. With enantiopure terminal allylic alcohols as substrates, the epoxy alcohols were obtained with up to 98 % yield and up to >99 : 1 dr (syn). Catalyst loadings as low as 1 mol % can be applied without eroding the syn‐diastereoselectivity. Modification of the allylic alcohol to an ether does not affect the diastereoselectivity either [>99 : 1 dr (syn)]. Inverting the catalyst configuration leads to the anti‐product, albeit at lower dr (ca. 20 : 1). The synthetic potential is demonstrated by a short, gram‐scale preparation of a tetrahydrofuran building block with three stereocenters, involving two titanium salalen catalyzed epoxidation steps.

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 byproduct^1,2; 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 investigation^4-10, 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.

Wheel-Like Icosanuclear Peroxotitanate-A Stable Water-Soluble Catalyst for Oxygen Transfer Reactions

Water-soluble wheel-like icosanuclear peroxotitanate K₁₆[Ti₂₀(μ-O)₈(HO₂)₈(O₂)₁₂( R, R-tart)₁₂]·52H₂O (1) chelated by tartrate has been successfully isolated. As the largest peroxotitanate reported, {Ti₂₀} features 20 (hydro)peroxo groups with three kinds of coordination modes in μ-η¹:η², μ-η²:η², and η² fashions. The cluster is stable in solution and solid states. It has been tested for the catalytic oxidations of methyl phenyl sulfide and pyridine with hydrogen peroxide, respectively, which shows reversible elimination and the addition of peroxo groups. This provides a rare example of well-characterized titanium peroxide for homogeneous catalysis and mechanism research.

Electronic Structure and Reactivity Studies of a Nonsymmetric One-Electron Oxidized CuII Bis-phenoxide Complex

The tetradentate mixed imino/amino phenoxide ligand (N-(3,5-di-tert-butylsalicylidene)-N'-(2-hydroxyl-3,5-di-tert-butylbenzyl))-trans-1,2-cyclohexanediamine (salalen) was complexed with CuII, and the resulting Cu complex (2) was characterized by a number of experimental techniques and theoretical calculations. Two quasi-reversible redox processes for 2, as observed by cyclic voltammetry, demonstrated the potential stability of oxidized forms, and also the increased electron-donating ability of the salalen ligand in comparison to the salen analogue. The electronic structure of the one-electron oxidized [2]+ was then studied in detail, and Cu K-edge X-ray Absorption Spectroscopy (XAS) measurements confirmed a CuII-phenoxyl radical complex in solution. Subsequent resonance Raman (rR) and variable temperature 1H NMR studies, coupled with theoretical calculations, showed that [2• ]+ is a triplet (S = 1) CuII-phenoxyl radical species, with localization of the radical on the more electron-rich aminophenoxide. Attempted isolation of X-ray quality crystals of [2• ]+ afforded [2H]+, with a protonated phenol bonded to CuII, and an additional H-bonding interaction with the SbF6 - counterion. Stoichiometric reaction of dilute solutions of [2• ]+ with benzyl alcohol showed that the complex reacted in a similar manner as the oxidized CuII-salen analogue, and does not exhibit a substrate-binding pre-equilibrium as observed for the oxidized bisaminophenoxide CuII-salan derivative.

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.