Mark Mascal

Putting Anion–π Interactions Into Perspective

Supramolecular chemistry is a field of scientific exploration that probes the relationship between molecular structure and function. It is the chemistry of the noncovalent bond, which forms the basis of highly specific recognition, transport, and regulation events that actuate biological processes. The classic design principles of supramolecular chemistry include strong, directional interactions like hydrogen bonding, halogen bonding, and cation-π complexation, as well as less directional forces like ion pairing, π-π, solvophobic, and van der Waals potentials. In recent years, the anion-π interaction (an attractive force between an electron-deficient aromatic π system and an anion) has been recognized as a hitherto unexplored noncovalent bond, the nature of which has been interpreted through both experimental and theoretical investigations. The design of selective anion receptors and channels based on this interaction represent important advances in the field of supramolecular chemistry. The objectives of this Review are 1) to discuss current thinking on the nature of this interaction, 2) to survey key experimental work in which anion-π bonding is demonstrated, and 3) to provide insights into the directional nature of anion-π contact in X-ray crystal structures.

High-yield conversion of plant biomass into the key value-added feedstocks 5-(hydroxymethyl)furfural, levulinic acid, and levulinic esters via 5-(chloromethyl)furfural

5-(Hydroxymethyl)furfural, levulinic acid, ethyl levulinate and butyl levulinate are produced by the solvolysis of 5-(chloromethyl)furfural (CMF). Since CMF can be derived in high yield from sugars, cellulose, or lignocellulosic feedstocks, the process described here presents an efficient entry into the value-added manifold of biomass-derived products of relevance to the organic materials and fuel industries.

Dramatic advancements in the saccharide to 5-(chloromethyl)furfural conversion reaction.

In 2008, we reported that glucose, sucrose, and cellulose could be converted into 5-(chloromethyl)furfural (CMF) 1 in extraordinary yields in a simple biphasic reactor. The process was conducted under mild conditions and involved nothing more sophisticated than aqueous HCl as a reagent. More recently, we have shown that the reaction can be applied equally well to raw biomass (cotton, newsprint, wood, corn stover, and straw), producing not only 1 and related, minor products 2–4 in excellent overall yield, but also converting the hemicellulose present into furfural 5 (Scheme 1). CMF is of interest because it can be easily processed into drop-in biofuel candidates, such as 5-(ethoxymethyl)furfural 6 and 5-methylfurfural 7 by reacting with ethanol and hydrogen, respectively (Scheme 2). In terms of biofuel precursors, CMF can be thought of like plant oils, which are likewise not appropriate for direct use in automobiles, but can be converted into suitable fuels by reacting with alcohols. The two are complementary in the sense that they are ultimately sourced from different biological pathways in plants. While the method described above gave, to our knowledge, the highest isolated yields to date of simple organic compounds from cellulose and cellulosic biomass, we originally noted that two of the reaction parameters would be targeted for optimization, that is, reaction time and the volume of solvent required to extract the products. Herein, we report significant advancements on these fronts and in related aspects of the process. As originally described, reactions were generally run to 30 h duration in order to obtain the maximum yields of products. 2] Although still faster than glucose fermentation to alcohols, the process was slower than many other approaches to biomass deconstruction, particularly thermal conversion and reforming methods which, despite producing a range of products of varied utility and in moderate yield, are able to process raw biomass in a matter of minutes. We had observed, in the course of the reaction, a substantial outgassing of hydrogen chloride, which correlated with a decreasing rate of conversion to products, such that it became necessary after 18 h to introduce additional concentrated HCl to the reactor to drive the process to completion. We have since determined that conducting the same reaction in a closed system results in a steep reduction in reaction time, with the substrates shown in Table 1 all being converted to CMF 1 within 3 h. Additionally, no alternative cyclization isomer 2, 5-(hydroxymethyl)furfural (HMF) 3, or levulinic acid (LA) 4 is now observed in the organic extract, the crude product being essentially pure CMF 1 The mass balance consists of a small amount of humic material and LA 4 found in the aqueous phase of the reaction mixture, and the yields of the latter are also reported (Table 1). The increase in CMF yields, compared to those previously reported, 2] appear to be the result of a reduction in side products 2 and 3. The published method also involved the continuous extraction of products 1–4 into an organic medium, thus sequestering them from the aqueous acid and preventing degradation reactions that have long plagued the acidic processing of biomass. The extraction rate used was 10 mL min 1 which, over a 30 h period, involved the distillation of ca. 18 L of solvent for the production of a few grams of product. Of course, the reactor made use of a solvent loop whereby the solvent was continuously recycled, so only 650 mL of solvent was needed to charge the reactor, and a total of volume of 1.25 L of extracts [a] Prof. M. Mascal, Dr. E. B. Nikitin Department of Chemistry and Bioenergy Research Group University of California, Davis 1 Shields Avenue, Davis, CA 95616 (USA) Fax: (+ 1) 5307528995 E-mail : mascal@chem.ucdavis.edu Scheme 1. Concurrent processing of corn stover into hexoseand pentose-derived products.

Hydrodeoxygenation of the Angelica Lactone Dimer, a Cellulose-Based Feedstock: Simple, High-Yield Synthesis of Branched C7–C10 Gasoline-like Hydrocarbons†

Dehydration of biomass-derived levulinic acid under solid acid catalysis and treatment of the resulting angelica lactone with catalytic K2 CO3 produces the angelica lactone dimer in excellent yield. This dimer serves as a novel feedstock for hydrodeoxygenation, which proceeds under relatively mild conditions with a combination of oxophilic metal and noble metal catalysts to yield branched C7 -C10 hydrocarbons in the gasoline volatility range. Considering that levulinic acid is available in >80 % conversion from raw biomass, a field-to-tank yield of drop-in, cellulosic gasoline of >60 % is possible.

Towards the Efficient, Total Glycan Utilization of Biomass

From biomass to mass transportation: Waste biomass (newspaper, corn stover, straw, and wood) is converted into biofuel precursors and value-added products in excellent yield using a simple, inexpensive process involving concurrent hydrolysis, dehydration, and substitution reactions in a biphasic reactor. The hemicellulose fraction of these substrates is simultaneously converted into furfural, and together these constitute an efficient means for the total exploitation of the carbohydrate content of biomass.

Hydrogen‐Bonded Molecular Ribbons as Templates for the Synthesis of Modified Mineral Phases

The combination of two important organizational principles, that is surfactant monolayer assembly and the triazinetriamine - triazinetrione hydrogen-bonding molecular ribbon, produces a hybrid assembly which is used as a novel template to control the orientation and morphology in the crystallization of calcium carbonate.

Comparison of solution-mixed and sequentially processed P3HT:F4TCNQ films: effect of doping-induced aggregation on film morphology

Doping polymeric semiconductors often drastically reduces the solubility of the polymer, leading to difficulties in processing doped films. Here, we compare optical, electrical, and morphological properties of P3HT films doped with F4TCNQ, both from mixed solutions and using sequential solution processing with orthogonal solvents. We demonstrate that sequential doping occurs rapidly (<1 s), and that the film doping level can be precisely controlled by varying the concentration of the doping solution. Furthermore, the choice of sequential doping solvent controls whether dopant anions are included or excluded from polymer crystallites. Atomic force microscopy (AFM) reveals that sequential doping produces significantly more uniform films on the nanoscale than the mixed-solution method. In addition, we show that mixed-solution doping induces the formation of aggregates even at low doping levels, resulting in drastic changes to film morphology. Sequentially coated films show 3–15 times higher conductivities at a given doping level than solution-doped films, with sequentially doped films processed to exclude dopant anions from polymer crystallites showing the highest conductivities. We propose a mechanism for doping induced aggregation in which the shift of the polymer HOMO level upon aggregation couples ionization and solvation energies. To show that the methodology is widely applicable, we demonstrate that several different polymer:dopant systems can be prepared by sequential doping.

Synthesis of the natural herbicide δ-aminolevulinic acid from cellulose-derived 5-(chloromethyl)furfural

Cellulose-derived 5-(chloromethyl)furfural is converted into δ-aminolevulinic acid in three chemical steps involving conversion to 5-(azidomethyl)furfural, photooxidation, and catalytic hydrogenation in 68% overall yield. δ-Aminolevulinic acid is a natural product with important agrochemical and pharmaceutical applications.

S-CYLINDROPHANES : FROM METAL TWEEZERS TO METAL SANDWICHES

Inclusion of AgIand CuIions by the preorganized cylindrophane cage compound 1 provides the first examples of bis(η6) sandwich complexes of these metals. Locked in a trigonal-planar heteroatom ligand field, the guest ions are shown by NMR spectroscopy to withdraw electron density from the aromatic rings. A statistical study of η6 coordination in the solid state supports the description of arene–metal contacts between 2.5 and 3.5 A as bonds.

Efficient, metal-free production of succinic acid by oxidation of biomass-derived levulinic acid with hydrogen peroxide

A practical, scalable, metal-free synthesis of succinic acid from the biomass-derived platform chemical levulinic acid is described. Treatment of levulinic acid with the inexpensive, simple oxidant hydrogen peroxide under the catalytic action of trifluoroacetic acid gives succinic acid in high yield and enables facile product isolation by simple distillation of the volatile catalyst and byproducts.