Eduardo C. Escudero-Adán

A Powerful Aluminum Catalyst for the Synthesis of Highly Functional Organic Carbonates

An aluminum complex based on an amino triphenolate ligand scaffold shows unprecedented high activity (initial TOFs up to 36,000 h(-1)), broad substrate scope, and functional group tolerance in the formation of highly functional organic carbonates prepared from epoxides and CO(2). The developed catalytic protocol is further characterized by low catalyst loadings and relative mild reaction conditions using a cheap, abundant, and nontoxic metal.

Simple, stable, and easily accessible well-defined CuCF3 aromatic trifluoromethylating agents.

matic building blocks are made by a Swarts-type process involving exhaustive chlorination of a methyl group on the ring and subsequent Cl/F exchange on the resultant ArCCl3 with HF. This process deals with hazardous materials and generates large quantities of chlorine waste. An ecologically friendly alternative to the Swarts reaction is the direct introduction of a CF3 group into the desired position on the ring. This has been achieved by coppermediated or -catalyzed and palladium-mediated or -catalyzed trifluoromethylation of aromatic substrates. The most widely utilized are the Cu CF3 reagents, originally discovered by McLoughlin and Thrower and further developed by Kobayashi and Kumadaki for stoichiometric trifluoromethylation of haloarenes. These reagents, which have been known for over 40 years, are still poorly defined, and their structure and composition remain unknown. In their classic report on the F NMR spectroscopy studies of “CuCF3”, Wiemers and Burton [9] identified these compounds as “elusive and complex species”. Understanding mechanisms of trifluoromethylation reactions with Cu reagents is important for further progress in the area. This understanding, however, is hardly conceivable without studies on well-defined CF3 Cu I complexes that can trifluoromethylate aromatic electrophiles. Such complexes are extremely rare, being limited to a few derivatives of only one type of ligand, N-heterocyclic carbenes (NHCs). Vicic and co-workers have recently prepared a handful of air-sensitive complexes of the type [(NHC)Cu(CF3)] and demonstrated their ability to trifluoromethylate iodoarenes. Air-stable well-defined Cu trifluoromethylating reagents that are easy to make and handle are still unknown. Herein we report a highyielding simple preparation, full characterization, and aromatic trifluoromethylation reactions of [(Ph3P)3Cu(CF3)] (1), a remarkably airstable Cu trifluoromethyl complex. Complex 1 has been previously prepared by Komiya et al. by a multistep route requiring an organocopper reagent. Although microanalytical data and room-temperature NMR (H, F, P) and IR spectroscopic data for 1 have been provided, structural data, solution behavior, and reactivity studies for 1 have not been reported. We have developed the first organocopper-free, exceedingly simple and efficient synthesis of 1 (Scheme 2). It was found that [(Ph3P)3Cu(F)]·2MeOH [13] (2) smoothly reacted

Quantitative Evaluation of Anion–π Interactions in Solution

Anions included: A series of meso-tetraaryl calix[4]pyrrole receptors have been used as a model system to quantify chloride–π interactions in solution (see picture; green balls are chloride ions). The free energy values are generally indicative of a repulsive interaction; their magnitude depends on the substituent on the aromatic ring.

Fast and persistent electrocatalytic water oxidation by Co-Fe Prussian blue coordination polymers.

The lack of an efficient, robust, and inexpensive water oxidation catalyst (WOC) is arguably the biggest challenge for the technological development of artificial photosynthesis devices. Here we report the catalytic activity found in a cobalt hexacyanoferrate (CoHCF) Prussian blue-type coordination polymer. This material is competitive with state-of-the-art metal oxides and exhibits an unparalleled long-term stability at neutral pH and ambient conditions, maintaining constant catalytic rates for weeks. In addition to its remarkable catalytic activity, CoHCF adds the typical properties of molecule-based materials: transparency to visible light, porosity, flexibility, processability, and low density. Such features make CoHCF a promising WOC candidate for advancement in solar fuels production.

[(NHC)CuX] complexes: Synthesis, characterization and catalytic activities in reduction reactions and Click Chemistry. On the advantage of using well-defined catalytic systems

The preparation of three series of [(NHC)CuX] complexes (NHC = N-heterocyclic carbene, X = Cl, Br, or I) is reported. These syntheses are high yielding and only use readily available starting materials. The prepared complexes were spectroscopically and structurally characterized. Notably, two of them present a bridging NHC ligand between two copper centers in the solid state, an extremely rare coordination mode for these ligands. These complexes were then applied to two distinct organic reactions: the hydrosilylation of ketones and the 1,3-dipolar cycloaddition of azides and alkynes. In both transformations, outstanding catalytic systems were found for preparing the corresponding products in excellent yields and short reaction times. Most remarkably, the screening of well-defined systems in the hydrosilylation reaction allowed for the identification of a pre-catalyst previously overlooked since, originally, catalytic species were in situ generated. Under such conditions, major formation of [(NHC)(2)Cu](+) species, inactive in this reduction reaction, occurred instead of the expected copper hydride. These results highlight one of the most important advantages of employing well-defined complexes in catalysis: gaining an improved control of the nature of the catalytically relevant species in the reaction media.

Highly active aluminium catalysts for the formation of organic carbonates from CO2 and oxiranes

Al(III) complexes of amino-tris(phenolate) ligand scaffolds have been prepared to attain highly Lewis acidic catalysts. Combination of the aforementioned systems with ammonium halides provides highly active catalysts for the synthesis of organic carbonates through addition of carbon dioxide to oxiranes with initial turnover frequencies among the highest reported to date within the context of cyclic carbonate formation. Density functional theory (DFT) studies combined with kinetic data provides a rational for the relative high activity found for these Al(III) complexes, and the data are consistent with a monometallic mechanism. The activity and versatility of these Al(III) complexes has also been evaluated against some state-of-the-art catalysts and the combined results compare favorably in terms of catalyst construction, stability, activity, and applicability.

Multicatalytic asymmetric synthesis of complex tetrahydrocarbazoles via a Diels-Alder/benzoin reaction sequence.

Expanding upon the recently developed aminocatalytic asymmetric indole-2,3-quinodimethane strategy, a straightforward synthesis of structurally and stereochemically complex tetrahydrocarbazoles has been devised. The chemistrys complexity-generating power was further harnessed by designing a multicatalytic, one-pot Diels-Alder/benzoin reaction sequence to stereoselectively access trans-fused tetracyclic indole-based compounds having four stereogenic centers with very high fidelity.

Carbon Dioxide as a Protecting Group: Highly Efficient and Selective Catalytic Access to Cyclic cis-Diol Scaffolds†

The efficient and highly selective formation of a wide range of (hetero)cyclic cis-diol scaffolds using aminotriphenolate-based metal catalysts is reported. The key intermediates are cyclic carbonates, which are obtained in high yield and with high levels of diastereo- and chemoselectivity from the parent oxirane precursors and carbon dioxide. Deprotection of the carbonate structures affords synthetically useful cis-diol scaffolds with different ring sizes that incorporate various functional groups. This atom-efficient method allows the simple construction of diol synthons using inexpensive and accessible precursors and green metal catalysts and showcases the use of CO2 as a temporary protecting group.

Effective chirogenesis in a bis(metallosalphen) complex through host-guest binding with carboxylic acids.

Transfer of chiral information through supramolecular interactions (chirogenesis) has been observed in many natural systems including DNA and proteins, and is nowadays widely used in the development of smart artificial and biomimetic materials. The induction of chirality in bis(metalloporphyrins) for example, has been successfully applied in assigning the absolute configuration of amines, diamines and aminoamides, aminoalcohols and epoxyalcohols, and diols by using a circular dichroism (CD) protocol. Effective chirality transfer with carboxylic acids, however, has proven to be highly difficult, and has only been achieved by using potassium carboxylate salts followed by tedious extractions, or by the addition of a huge excess of substrate to a metal-free host. The low efficiency of chiral induction with these previous methods is mainly due to the relatively weak host–guest interactions with carboxylic acid groups. To overcome this problem, we have designed a bis[Zn(salphen)] complex 1 (salphen = N,N’-phenylenebis(salicylimine)), which, similar to 2,2’-biphenol units, exists in dynamic equilibrium between two chiral conformations (S and R enantiomers; see Scheme 1). We reasoned that the energy barrier of rotation increases upon binding of a ditopic ligand to the Lewis acidic Zn centers. 12] Herein, we demonstrate that 1 binds very strongly with acetic acid and that axial chirality can be effectively induced by exchange for chiral a-substituted carboxylic acids, with the practical advantage that substrate derivatization or use of excessive substrate is not required. Compound 1 was prepared in a single step by reaction of a bis(salicylaldehyde) molecule with two equivalents of a monoimine precursor and Zn(OAc)2 in the presence of pyridine. Subsequent precipitation from MeOH afforded the product in excellent yield (73 %) and purity (see the Supporting Information). Characterization by NMR spectroscopy indicated the presence of one equivalent of acetic acid, which had formed as a by-product in the synthesis. Slow evaporation of a solution of the product in toluene/ MeCN 1:1 resulted in single crystals suitable for X-ray analysis (Figure 1). The solid-state structure revealed that the two Zn centers of 1 are bridged by AcOH (through the oxygen atoms of the carboxylic acid unit) to give a complex with 1:1 stoichiometry (1 AcOH). As anticipated, both the S and the R conformer were present in a 1:1 ratio in the unit cell; each conformer has the same dihedral angle (44.98) and Zn–O(acetate) distance (2.01 ). This distance is identical to that previously found in a related acetate-bridged complex. Since the exact position of the acidic proton of AcOH could not be resolved by X-ray diffraction, the proton was placed in Scheme 1. Conformational isomerism of 1. The tBu groups are omitted for clarity in the line drawings of the conformers. P denotes righthandedness and M left-handedness.

Oxidative Diamination Promoted by Dinuclear Iodine(III) Reagents

New dinuclear iodine(III) reagents for the intermolecular diamination of alkenes are reported. These are accessible through protolytic aminolysis events, which generate defined imido-iodine(III) groups.