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New polymers, properly designed for end-of-life and efficiently formed from renewable carbon, are key to the transition to a more sustainable circular plastics economy. Ring-opening polymerization (ROP) of bicyclic lactones is a promising method for the production of intrinsically recyclable polyesters, but most lactone monomers lack an efficient synthesis route from biobased starting materials, even though this is essential to sustainably account for material loss during the life cycle. Herein, we present the exceptionally rapid and controlled polymerization of a fully biobased tricyclic oxanorbornene-fused γ-butyrolactone monomer (M1). Polyester P(M1) was formed in low dispersity (D̵ = 1.2–1.3) and controllable molecular weight up to Mn= 76.8 kg mol–1and exhibits a high glass transition temperature (Tg= 120 °C). The orthogonal olefin and lactone functionalities offer access to a wide range of promising materials, as showcased by postpolymerization modification by hydrogenation of the olefin, which increased polymer thermal stability by over 100 °C. Next to rapid hydrolytic degradation and solvolysis, the poly(oxanorbornene-fused γ-butyrolactone) could be cleanly chemically recycled back to the monomer (CRM), in line with its favorable ceiling temperature (Tc) of 73 °C. The density functional theory (DFT)-computed ΔH° of ring-opening with methanol of γ-butyrolactone-based monomers provided a model to predict Tc, and the DFT-computed and X-ray crystal structure-derived structural parameters of M1, hydrogenated analogue M1-H2, and regioisomer M2offered insights into the structural descriptors that cause the high polymerizability of M1, which is key to establishing structure–property relations.

Herein, we report the use of structured hydroxyapatite composite (SHCs) as highly efficient and recyclable solid base catalysts for various condensation reactions. Catalyst performance as function of catalyst loading, reaction time and reaction temperature were studied in the solventless self-aldol condensation reaction of butyraldehyde to 2-ethylhexenal under mild reaction conditions. SHC catalysts were found to outperform benchmark solid base catalysts such as MgO, TiO2, calcium carbonate and hydroxyapatites. Characterization of the synthesized SHC catalysts by a range of surface analysis, spectroscopic and electron microscopy techniques, showed that a moderate acid/base ratio and high BET surface area to be key to their high efficiency. Furthermore, recycling experiments showed the catalyst to be stable over multiple runs. Moreover, the most active SHC catalyst was investigated in other prototypical condensation reactions such as the Knoevenagel condensation, Claisen–Schmidt condensation and Henry reaction, again showing excellent performance. These results highlight the versatility of these SHC materials and their potential for industrial employment as solid base catalysts.

A study of the influence of reaction conditions on the Pd/TOMPP-catalyzed (TOMPP = tris(2-methoxyphenyl)phosphine) telomerization of 1,3-butadiene with phenols revealed that the composition of the reaction medium strongly influences the regioselectivity of the telomer products. Mechanistic studies show this effect to be related to the stability of the key catalytic intermediate cis-[Pd((1–3η)-octa-2,7-dien-1-yl)(TOMPP)2]+(E), which mainly depends on the solvent composition. The crotyl analogue cis-[Pd((1–3η)-but-2-en-1-yl)(TOMPP)2]BF4(4) was synthesized as a model for Eand was fully characterized, including by an X-ray crystal structure determination. Comparison of the crystal structures of 4and [Pd((1–3,7,8η)-octa-2(E),7-dien-1-yl)(TOMPP)]BF4(1), an analogue of intermediate C, shows that the difference in regioselectivity for nucleophilic attack of these complexes is not reflected in the structure of the allyl moiety. Furthermore, Pd/TOMPP-catalyzed n/iso equilibration reactions of telomers show that all product formation routes in the Pd-catalyzed telomerization mechanism are reversible. Telomer product equilibration studies resulted in complete conversion to 1,3,7-octatriene, with the selectivity changing over time in favor of formation of the Zisomer of octatriene.

Bite angle and electronic effects on the ruthenium–diphosphine catalysed ether bond cleavage of the lignin β-O-4 model compound 2-phenoxy-1-phenylethanol were tested. Enhanced conversion of the substrate was observed with increasing σ-donor capacity of the ligands. Kinetic and thermodynamic data suggest oxidative addition of the dehydrogenated model compound to the diphosphine Ru(0) complex to be rate-limiting.

The synthesis and functionalization of imines and amines are key steps in the preparation of many fine chemicals and for pharmaceuticals in particular. Traditionally, metal complexes are used as homogeneous catalysts for these organic transformations. Here we report gold–palladium and ruthenium–palladium nano-alloys supported on TiO2acting as highly efficient heterogeneous catalysts for the one-pot synthesis of the imine N-benzylideneaniline and the secondary amine N-benzylaniline directly from the easily available and stable nitrobenzene and benzyl alcohol precursors using a hydrogen auto-transfer strategy. These reactions were carried out without any added external hydrogen, sacrificial hydrogen donor or a homogeneous base. The bimetallic catalysts were prepared by the recently developed modified impregnation strategy, giving efficient control of size and nano-alloy composition. Both bimetallic catalysts were found to be far more active than their monometallic analogues due to a synergistic effect. Based on the turnover numbers the catalytic activities follow the order Ru < Pd < Au Au–Pd < Ru–Pd. Aberration corrected scanning transmission electron microscopy (AC-STEM) and X-ray absorption spectroscopy (XAFS) studies of these catalysts revealed that the reason for the observed synergistic effect is the electronic modification of the metal sites in the case of the Au–Pd system and a size stabilisation effect in the case of the Ru–Pd catalyst.

The aqueous-phase reforming (APR) of a crude glycerol that originates from an industrial process and the effect of the individual components of crude glycerol on APR activity have been studied over 1 wt% Pt/Mg(Al)O, 1 wt% Pt/Al2O3, 5 wt% Pt/Al2O3and 5 wt% Pt/C catalysts at 29 bar and 225 °C. The use of a 10 wt% alkaline crude glycerol solution in water, containing 6.85 wt% glycerol, 1.62 wt% soaps, 1.55 wt% methanol, and 0.07 wt% ester, resulted in a dramatic drop in APR activity compared to the corresponding 6.85 wt% solution of pure glycerol in water. Catalytic performance in crude glycerol APR increased in the order: 1 wt% Pt/Al2O3< 5 wt% Pt/Al2O3< 1 wt% Pt/Mg(Al)O < 5 wt% Pt/C. A H2selectivity of only 1% was obtained with crude glycerol over a 1 wt% Pt/Al2O3catalyst, while the same catalyst material under identical reaction conditions gave 64% H2with pure glycerol. The cause of deactivation was investigated with synthetic mixtures that mimicked the composition of the crude glycerol and contained glycerol, methanol and varying amounts of NaCl and sodium oleate. The results showed that Na salts of fatty acids have a much more pronounced negative influence on APR activity than NaCl does and greatly inhibit H2formation. Stearic acid and long chain aliphatics and olefins were shown to be formed and to be involved in the deactivation of the catalyst. The relatively high activity/selectivity of the 1 wt% Pt/Mg(Al)O could be attributed to intercalation of oleate/stearate in the sheets of the layered double hydroxide that is formed under reaction conditions.

Adipic acid and its esters are important bulk chemicals whose principal use is in the production of the nylon-6,6 polymer. There is considerable interest in finding novel green routes from sustainable feedstocks towards these important intermediates. Herein, we describe the catalytic oxidative cleavage of catechol to muconic acids using a catalyst prepared in situfrom iron(iii) nitrate, tris(2-pyridylmethyl)amine and ammonium acetate. An investigation of catalyst loading, temperature and oxygen pressure, allowed a turnover frequency of 120 h−1to be obtained. The subsequent hydrogenation and transesterification of the obtained muconic acid products were shown to proceed well over commercially available supported catalysts. After vacuum distillation, dimethyl adipate could be isolated in 62% yield from catechol, thus demonstrating a green and sustainable route to this important bulk chemical.

The first Fe-based catalytic system for the oxidative cleavage of unsaturated fatty acids and esters to carboxylic acids is reported. The system comprises [Fe(OTf)2(6-Me-PyTACN)] (2) (6-Me-PyTACN = 1-[(6-methyl-2-pyridyl)methyl]-4,7-dimethyl-1,4,7-triazacyclononane, OTf = trifluoromethane sulfonate anion) as the catalyst (3 mol%) either with a combination of hydrogen peroxide and NaIO4or exclusively with NaIO4as the oxidant, and operates at 0 °C or ambient temperature. Under these standard conditions (method A), methyl oleate is converted in a one-pot procedure into 50–55% of both nonanoic and azelaic acid, together with some epoxide and aldehyde intermediates as byproducts. These yields can be further improved by addition of sulfuric acid (method B) to hydrolyze the epoxide byproducts, by including a pH neutralization step and addition of more catalyst (1 mol%). Under the optimized conditions, both methyl oleate and oleic acid are converted into high yields of the corresponding carboxylic acids (80–85%). Overall, this catalytic system provides an alternative to the industrial ozonolysis of oleic acid and to catalytic Ru- and Os-based systems for the oxidative cleavage of unsaturated fatty acids and esters.

The oxidative cleavage of the CC double bond in unsaturated fatty acids into aldehydes or carboxylic acids is a reaction of current interest in biomass valorization. The products of this reaction, which is currently being performed on an industrial scale by means of ozonolysis, can be applied for the production of commodity chemicals such as plasticizers and polymers. Alternative, catalytic methods for this conversion that are not based on the hazardous ozone oxidant are of high interest. In this respect, the use of transition metal catalysts would allow the application of milder reaction conditions and more benign oxidants. This review covers the various transition metal-based catalysts that have been reported for the oxidative cleavage of alkenes in general and the cleavage of unsaturated fatty acids in particular. Catalytic systems based on second- and third-row metals such as Ru, Os, and W have been widely studied and are well-known to catalyze the cleavage of various alkenes and unsaturated fatty acids. In addition to simple metal salts, metal oxides or peroxides, various metal coordination complexes have been investigated for chemo- and regioselective olefin cleavage. Heterogeneous catalyst systems based on these metals have, in addition, been studied with a focus on catalyst reuse. To circumvent the use of expensive, toxic or less abundant second- and third-row metal catalysts, the use of first-row transition metal catalysts for the cleavage reaction might be preferred. However, examples of such catalysts for oxidative olefin cleavage are much less documented. The application of Fe- and Mn-based catalysts currently is largely limited to activated alkenes, e.g.styrene derivatives. The many possibilities that are offered by careful ligand design together with the recent advances in the field of first-row transition metal oxidation catalysis may allow for the development of catalyst systems that substitute ozone or second- and third-row metal catalysts for the oxidative cleavage of simple alkenes and unsaturated fatty acids.

The Pd/TOMPP-catalysed (TOMPP = tris(2-methoxyphenyl)phosphine) telomerisation of 1,3-butadiene was studied under solvent- and base-free conditions with phenolic substrates that can be potentially derived from lignin. Large differences in catalytic activity were observed, with reactivity increasing in the order of phenol, p-cresol, guaiacol, creosol and syringol. This reactivity trend can be attributed to the substrates' relative nucleophilicities, as induced by the donating effects of the p-methyl and o-methoxy substituents. The chosen reaction conditions, i.e.temperature, ligand/metal and butadiene/substrate ratios, strongly influenced both the conversion and selectivity of the reaction. Remarkably, the composition of the reaction medium, i.e. the butadiene/substrate ratio, exerted a strong influence on the linear/branched ratio. High conversions and selectivities to the linear products are obtained when excess butadiene is used. The linear telomer products could be readily converted from O-alkylated to C-alkylated phenolics viathe thermal Claisen rearrangement. High conversions and selectivities were observed after 2 hours at 200 °C. Branched o-octadienyl phenols were obtained in all cases except for the syringol telomer which gave the linear p-octadienyl product exclusively.

The improved synthesis, characterisation and application of a microporous 4,4′-biphenyl/phosphine-based covalent organic framework (COF) for the heterogeneous Pd-catalysed telomerisation of 1,3-butadiene with phenol and glycerol are presented. The solid polyphosphine is amorphous, microporous and an excellent support for Pd(acac)2. Solid-state NMR and DRIFT analysis of materials of varying Pd-loading show that bis-phosphine complexes of palladium are preferably formed. Under solvent- and base-free conditions, high conversions and selectivities are obtained for this catalyst material with both phenol and glycerol as substrates. The product selectivity, with both butenylation and telomerisation activity observed with phenol, can be tuned by variation of the metal loading. For glycerol it is shown that the selectivity to the undesired tri telomer is low under all applied conditions and, remarkably, that the heterogeneous catalyst outperforms its homogeneous PPh3-based counterpart.

Proteins sharing the same “2‐His‐1‐carboxylate” structural motif have little amino acid sequence similarity and are able to perform many different reactions. Many factors have been cited to explain their different specificity and turnover rates, like protein environment, coordinated ligand geometry, electronic structure of the active site, etc. In this paper, we present a combined approach applying high‐resolution XANES spectroscopy and theory simulations to different model complexes that mimic the binding modes of the amino acids to the metal site. Experiments were performed on three compounds showing three metal sites: ferrous hexacoordinate, ferric pentacoordinate and ferrous pentacoordinate. The first two compounds bear an N,N,O‐tridentate 3,3‐bis(1‐alkylimidazol‐2‐yl)propionate ligand that features a monodentate carboxylate group. These complexes mimic the activity of extradiol dioxygenases but also exhibit intradiol cleavage activity. The third compound features a bidentate terphenylcarboxylate ligand and a sterically hindered bidentate N,N‐donor, thus providing a good structural mimic of the ternary enzyme‐tetrahydrobiopterin‐substrate complex in pterin‐dependent phenylalanine hydroxylase, which also contains a bidentate carboxylate. Modeling of high‐resolution XANES on well‐defined model complexes of different geometry can aid in protein structure elucidation. XANES gives the oxidation state and coordination number of the metal in the non‐crystallized protein at natural pH. The accuracy of the results is limited by the core‐hole and experimental broadenings. We found that high‐resolution XANES experiments give increased resolution at the pre‐edges, but limited improvement at the main edge. These high‐resolution pre‐edges can be accurately simulated by using crystal field multiplet theory (CFM). We show that by combining modelling and XANES simulations with FEFF8, detailed structural and chemical information can be obtained. We found that a short O2–metal distance for the carboxylate oxygen atom not bound to the metal causes a higher white line in FeII, which is similar to the results obtained for the pterin‐dependent hydroxylase, tyrosine hydroxylase (TYH). Full‐potential FDMNES simulations for each sample confirm the accuracy of the main results with muffin‐tin approximation (FEFF8).

Found at last: A simple and efficient one‐pot synthesis procedure leads to several cationic [Pd(1,2,3,7,8‐η5‐octa‐2,7‐dien‐1‐yl)(PR3)]+complexes (see structure; Pd pink, O red, P orange), which are the long sought‐after key reactive intermediates in the Pd/PR3‐catalyzed telomerization of butadiene.

Doch noch fündig geworden: Eine einfache und effiziente Eintopfsynthese liefert mehrere kationische [Pd(1,2,3,7,8‐η5‐Octa‐2,7‐dien‐1‐yl)(PR3)]+‐Komplexe (siehe Struktur; rosa Pd, rot O, orange P), bei denen es sich um die lange gesuchten Schlüsselintermediate in der Pd/PR3‐katalysierten Telomerisierung von Butadien handelt.

Sugar and alcohol—a superior combo: The telomerization activity of the Pd/TOMPP catalyst is screened using thirteen different biomass‐derived carbohydrates and sugar alcohols. High substrate conversions are achieved by using low Pd loading and without the use of an added base. In terms of butadiene conversion, a clear structure–activity relationship is found, which is in line with the series: sugar alcohols>sucrose>aldohexoses>aldopentoses>ketohexoses.

Copper(II) complexes of the potentially tripodal N,N,O ligand 3,3‐bis(1‐methylimidazol‐2‐yl)propionate (L1) and its conjugate acid HL1 have been synthesised and structurally and spectroscopically characterised. The reaction of equimolar amounts of ligand and CuIIresulted in the complexes [Cu(L1)]n(X)n(X=OTf−, PF6−; n=1,2), for which a new bridging coordination mode of L1 is inferred. Although these complexes showed moderate catecholase activity in the oxidation of 3,5‐di‐tert‐butylcatechol, surprising reactivity with the pseudo‐substrate tetrachlorocatechol was observed. A chloranilato‐bridged dinuclear CuIIcomplex was isolated from the reaction of [Cu(L1)]n(PF6)nwith tetrachlorocatechol. This stoichiometric oxidative double dehalogenation of tetrachlorocatechol to chloranilic acid by a biomimetic copper(II) complex is unprecedented. The crystal structure of the product, [Cu2(ca)Cl2(HL1)2], shows a bridging bis‐bidentate chloranilato (ca) ligand and ligand L1 coordinated as its conjugate acid (HL1) in a tridentate fashion. Magnetic susceptibility studies revealed weak antiferromagnetic coupling (J=−35 cm−1) between the two copper centres in the dinuclear complex. Dissolution of the green complex [Cu2(ca)Cl2(HL1)2] resulted in the formation of new pink‐purple mononuclear compound [Cu(ca)(HL1)(H2O)], the crystal structure of which was determined. It showed a terminal bidentate chloranilato ligand and N,N‐bidentate coordination of ligand HL1, which illustrates the flexible coordination chemistry of ligand L1.

The Rieske dioxygenases are a group of non‐heme iron enzymes, which catalyze the stereospecific cis‐dihydroxylation of its substrates. Herein, we report the iron(II) coordination chemistry of the ligands 3,3‐bis(1‐methylimidazol‐2‐yl)propionate (L1) and its neutral propyl ester analogue propyl 3,3‐bis(1‐methylimidazol‐2‐yl)propionate (PrL1). The molecular structures of two iron(II) complexes with PrL1were determined and two different coordination modes of the ligand were observed. In [FeII(PrL1)2](BPh4)2(3) the ligand is facially coordinated to the metal with an N,N,O donor set, whereas in [FeII(PrL1)2(MeOH)2](OTf)2(4) a bidentate N,N binding mode is found. In 4, the solvent molecules are in a cisarrangement with respect to each other. Complex 4is a close structural mimic of the crystallographically characterized non‐heme iron(II) enzyme apocarotenoid‐15–15′‐oxygenase (APO). The mechanistic features of APO are thought to be similar to those of the Rieske oxygenases, the original inspiration for this work. The non‐heme iron complexes [FeII(PrL1)2](OTf)2(2) and [FeII(PrL1)2](BPh4)2(3) were tested in olefin oxidation reactions with H2O2as the terminal oxidant. Whereas 2was an active catalyst and both epoxide and cis‐dihydroxylation products were observed, 3showed negligible activity under the same conditions, illustrating the importance of the anion in the reaction.

The synthesis of a topologically new type of dendrimer is presented in which a permanently positively charged core is decorated with a shell that is apolar and aliphatic in nature. The novel polyionic dendrimers were successfully applied as hosts for a predefined number of anionic guests, as is shown for the dye methyl orange. The aliphatic periphery of the dendrimers efficiently shields the ion pairs and renders the dendrimers fully soluble in organic solvents that are as apolar as hexane and, furthermore, provides them with an interesting phase behavior that appears not to be sensitive to the type of anion associated with it.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

A general synthetic route towards new tridentate, tripodal N,N,O ligands, bearing one carboxylate group and two of the biologically relevant imidazole moieties, is presented. The parent compounds of this new family of ligands, A[MIm2Pr] (1) and A[BenzMIm2Pr] (2) (A = H, K, Bu4N) were synthesized in two steps and high overall yield. The structures of HMIm2Pr (1a) and HBenzMIm2Pr (2a) were confirmed by X-ray crystal structure determination, both molecules forming infinite one-dimensional hydrogen bonded chains. The synthetic route allows for the fine-tuning of the physical properties of both ligand systems, e.g., solubility and ease of crystallization. The capability of the two ligands to coordinate facially through all donor atoms was investigated and two isostructural mononuclear copper(II) complexes of the type [CuL2] were obtained. Both [Cu(MIm2Pr)2]·2H2O (3) and [Cu(BenzMIm2Pr)2]·3.12(H2O)·1.74(C2H5OH) (4) have been characterized by X-ray crystallography, ESI-MS, elemental analysis, UV/Vis, IR, and EPR spectroscopy. In both structures the copper atoms are on an inversion center, which results in a tetragonally distorted octahedral coordination geometry. The CuN4O2 chromophores consist of four equatorially coordinated imidazole nitrogen atoms and two axially coordinated carboxylate groups. The non-coordinated water molecules of both crystal structures are involved in a one-dimensional hydrogen-bonded network. Two neighboring octahedrons are connected by two water molecules, each water molecule forming two hydrogen bonds with the non-coordinated oxygen atoms of the carboxylate group. This results in the formation of infinite one-dimensional chains. EPR spectroscopy and ESI-MS measurements provide evidence for the integrity of the complexes in solution. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

The cover picture showsthe work described in the paper: High‐resolution XANES spectra of model compounds of Fe dioxygenase show features with enhanced accuracy, which can be analyzed with simulation codes to obtain the electronic structure at the active metal site of the model. Three compounds showing three metal sites, ferrous hexacoordinate, ferric pentacoordinate, and ferrous pentacoordinate, were studied. The results show that there is subtle correlation between the structure and the absorption edge features; in particular, the influence of the orientation of the carboxylate ligand on these features is demonstrated. Details are discussed in the article by A. Mijovilovich, B. M. Weckhuysen et al. on p. 1589 ff.