Expedited Synthesis of Metal Phosphides Maximizes Dispersion, Air Stability, and Catalytic Performance in Selective Hydrogenation.

Metal phosphides have been hailed as potential replacements for scarce noble metal catalysts in many aspects of the hydrogen economy from hydrogen evolution to selective hydrogenation reactions. But the need for dangerous and costly phosphorus precursors, limited support dispersion, and low stability of the metal phosphide surface toward oxidation substantially lower the appeal and performance of metal phosphides in catalysis. We show here that a 1-step procedure that relies on safe and cheap precursors can furnish an air-stable Ni2P/Al2O3 catalyst containing 3.2 nm nanoparticles. Ni2P/Al2O3 1-step is kinetically competitive with the palladium-based Lindlar catalyst in selective hydrogenation catalysis, and a loading corresponding to 4 ppm Ni was sufficient to convert 0.1 mol alkyne. The 1-step synthetic procedure alters the surface ligand speciation of Ni2P/Al2O3, which protects the nanoparticle surface from oxidation, and ensures that 85 % of the initial catalytic activity was retained after the catalyst was stored under air for 1.5 years. Preparation of Ni2P on a variety of supports (silica, TiO2, SBA-15, ZrO2, C and HAP) as well as Co2P/Al2O3, Co2P/TiO2 and bimetallic NiCoP/TiO2 demonstrates the generality with which supported metal phosphides can be accessed in a safe and straightforward fashion with small sizes and high dispersion.

A Cooperative Rhodium/Secondary Phosphine Oxide [Rh/P(O)nBu2 ] Template for Catalytic Hydrodefluorination of Perfluoroarenes.

The selective activation of C-F bonds under mild reaction conditions remains an ongoing challenge of bond activation. Here, we present a cooperative [Rh/P(O)nBu2 ] template for catalytic hydrodefluorination (HDF) of perfluoroarenes. In addition to substrates presenting electron-withdrawing functional groups, the system showed an exceedingly rare tolerance for electron-donating functionalities and heterocycles. The high chemoselectivity of the catalyst and its readiness to be deployed at a preparative scale illustrate its practicality. Empirical mechanistic studies and a density functional theory (DFT) study have identified a rhodium(I) dihydride complex as a catalytically relevant species and the determining role of phosphine oxide as a cooperative fragment. Altogether, we demonstrate that molecular templates based on these design elements can be assembled to create catalysts with increased reactivity for challenging bond activations.

Surface and Bulk Chemistry of Mechanochemically Synthesized Tohdite Nanoparticles.

Aluminum oxides, oxyhydroxides, and hydroxides are important in different fields of application due to their many attractive properties. However, among these materials, tohdite (5Al2O3·H2O) is probably the least known because of the harsh conditions required for its synthesis. Herein, we report a straightforward methodology to synthesize tohdite nanopowders (particle diameter ∼13 nm, specific surface area ∼102 m2 g-1) via the mechanochemically induced dehydration of boehmite (γ-AlOOH). High tohdite content (about 80%) is achieved upon mild ball milling (400 rpm for 48 h in a planetary ball mill) without process control agents. The addition of AlF3 can promote the crystallization of tohdite by preventing the formation of the most stable α-Al2O3, resulting in the formation of almost phase-pure tohdite. The availability of easily accessible tohdite samples allowed comprehensive characterization by powder X-ray diffraction, total scattering analysis, solid-state NMR (1H and 27Al), N2-sorption, electron microscopy, and simultaneous thermal analysis (TG-DSC). Thermal stability evaluation of the samples combined with structural characterization evidenced a low-temperature transformation sequence: 5Al2O3·H2O → κ-Al2O3 → α-Al2O3. Surface characterization via DRIFTS, ATR-FTIR, D/H exchange experiments, pyridine-FTIR, and NH3-TPD provided further insights into the material properties.

Statistical evaluation of simulated NMR data of flexible molecules.

A new probability score-named χ-probability-is introduced for evaluating the fit of mixed NMR datasets to calculate molecular model ensembles, in order to answer challenging structural questions such as the determination of stereochemical configurations. Similar to the DP4 parameter, the χ-probability is based on Bayes theorem and expresses the probability that an experimental NMR dataset fits to a given individual within a finite set of candidate structures or configurations. Here, the χ-probability is applied to single out the correct configuration in four example cases, with increasing complexity and conformational mobility. The NMR data (which include RDCs, NOE distances and 3J couplings) are calculated from MDOC (Molecular Dynamics with Orientational Constraints) trajectories and are investigated against experimentally measured data. It is demonstrated that this approach singles out the correct stereochemical configuration with probabilities more than 98%, even for highly mobile molecules. In more demanding cases, a decisive χ-probability test requires that the datasets include high-quality NOE distances in addition to RDC values.

An Adaptive Rhodium Catalyst to Control the Hydrogenation Network of Nitroarenes.

An adaptive catalytic system that provides control over the nitroarene hydrogenation network to prepare a wide range of aniline and hydroxylamine derivatives is presented. This system takes advantage of a delicate interplay between a rhodium(III) center and a Lewis acidic borane introduced in the secondary coordination sphere of the metal. The high chemoselectivity of the catalyst in the presence of various potentially vulnerable functional groups and its readiness to be deployed at a preparative scale illustrate its practicality. Mechanistic studies and density functional theory (DFT) methods were used to shed light on the mode of functioning of the catalyst and elucidate the origin of adaptivity. The competition for interaction with boron between a solvent molecule and a substrate was found crucial for adaptivity. When operating in THF, the reduction network stops at the hydroxylamine platform, whereas the reaction can be directed to the aniline platform in toluene.

A Unified Approach to Polycyclic Alkaloids of the Ingenamine Estate: Total Syntheses of Keramaphidin B, Ingenamine, and Nominal Njaoamine I.

Many polycyclic marine alkaloids are thought to derive from partly reduced macrocyclic alkylpyridine derivatives via a transannular Diels-Alder reaction that forms their common etheno-bridged diaza-decaline core ("Baldwin-Whitehead hypothesis"). Rather than trying to emulate this biosynthesis pathway, a route to these natural products following purely chemical logic was pursued. Specifically, a Michael/Michael addition cascade provided rapid access to this conspicuous tricyclic scaffold and allowed different handles to be introduced at the bridgehead quarternary center. This flexibility opened opportunities for the formation of the enveloping medium-sized and macrocyclic rings. Ring closing alkyne metathesis (RCAM) proved most reliable and became a recurrent theme en route to keramaphidin B, ingenamine, xestocyclamine A, and nominal njaoamine I (the structure of which had to be corrected in the aftermath of the synthesis). Best results were obtained with molybdenum alkylidyne catalysts endowed with (tripodal) silanolate ligands, which proved fully operative in the presence of tertiary amines, quinoline, and other Lewis basic sites. RCAM was successfully interlinked with macrolactamization, an intricate hydroboration/protonation/alkyl-Suzuki coupling sequence, or ring closing olefin metathesis (RCM) for the closure of the second lateral ring; the use of RCM for the formation of an 11-membered cycle is particularly noteworthy. Equally rare are RCM reactions that leave a pre-existing triple bond untouched, as the standard ruthenium catalysts are usually indiscriminative vis-à-vis the different π-bonds. Of arguably highest significance, however, is the use of two consecutive or even concurrent RCAM reactions en route to nominal njaoamine I as the arguably most complex of the chosen targets.

NMReDATA: Tools and applications.

The nuclear magnetic resonance extracted data (NMReDATA) format has been proposed as a way to store, exchange, and disseminate nuclear magnetic resonance (NMR) data and physical and chemical metadata of chemical compounds. In this paper, we report on analytical workflows that take advantage of the uniform and standardized NMReDATA format. We also give access to a repository of sample data, which can serve for validating software packages that encode or decode files in NMReDATA format.

Strong and Confined Acids Enable a Catalytic Asymmetric Nazarov Cyclization of Simple Divinyl Ketones.

We report a catalytic asymmetric Nazarov cyclization of simple, acylic, alkyl-substituted divinyl ketones using our recently disclosed strong and confined imidodiphosphorimidate Brønsted acids. The corresponding monocyclic cyclopentenones are formed in good yields and excellent regio-, diastereo-, and enantioselectivities. Further, the chemical utility of the obtained enantiopure cyclopentenones is demonstrated.

Conformational Investigations in Flexible Molecules Using Orientational NMR Constraints in Combination with 3J-Couplings and NOE Distances.

The downscaling of NMR tensorial interactions, such as dipolar couplings, from tens of kilohertz to a few hertz in low-order media is the result of dynamics spanning several orders of magnitudes, including vibrational modes (~ns-fs), whole-molecule reorientation (~ns) and higher barrier internal conformational exchange (

Direct Detection of the Terminal Hydride Intermediate in [FeFe] Hydrogenase by NMR Spectroscopy.

Hydride state intermediates are known to occur in various hydrogen conversion enzymes, including the highly efficient [FeFe] hydrogenases. The intermediate state involving a terminal iron-bound hydride has been recognized as crucial for the catalytic mechanism, but its occurrence has up to now eluded unequivocal proof under (near) physiological conditions. Here we show that the terminal hydride in the [FeFe] hydrogenase from Chlamydomonas reinhardtii can be directly detected using solution 1H NMR spectroscopy at room temperature, opening new avenues for detailed in situ investigations under catalytic conditions.

Structure and Reactivity of Half-Sandwich Rh(+3) and Ir(+3) Carbene Complexes. Catalytic Metathesis of Azobenzene Derivatives.

Traditional rhodium carbene chemistry relies on the controlled decomposition of diazo derivatives with [Rh2(OAc)4] or related dinuclear Rh(+2) complexes, whereas the use of other rhodium sources is much less developed. It is now shown that half-sandwich carbene species derived from [Cp*MX2]2 (M = Rh, Ir; X = Cl, Br, I, Cp* = pentamethylcyclopentadienyl) also exhibit favorable application profiles. Interestingly, the anionic ligand X proved to be a critical determinant of reactivity in the case of cyclopropanation, epoxide formation and the previously unknown catalytic metathesis of azobenzene derivatives, whereas the nature of X does not play any significant role in -OH insertion reactions. This perplexing disparity can be explained on the basis of spectral and crystallographic data of a representative set of carbene complexes of this type, which could be isolated despite their pronounced electrophilicity. Specifically, the donor/acceptor carbene 10a derived from ArC(═N2)COOMe and [Cp*RhCl2]2 undergoes spontaneous 1,2-migratory insertion of the emerging carbene unit into the Rh-Cl bond with formation of the C-metalated rhodium enolate 11. In contrast, the analogous complexes 10b,c derived from [Cp*RhX2]2 (X = Br, I) as well as the iridium species 13 and 14 derived from [Cp*IrCl2]2 are sufficiently stable and allow true carbene reactivity to be harnessed. These complexes are competent intermediates for the catalytic metathesis of azobenzene derivatives, which provides access to α-imino esters that would be difficult to make otherwise. Rather than involving metal nitrenes, the reaction proceeds via aza-ylides that evolve into diaziridines; a metastable compound of this type has been fully characterized.

1H NMR Spectroscopy of [FeFe] Hydrogenase: Insight into the Electronic Structure of the Active Site.

The [FeFe] hydrogenase HydA1 from Chlamydomonas reinhardtii has been studied using 1H NMR spectroscopy identifying the paramagnetically shifted 1H resonances associated with both the [4Fe-4S]H and the [2Fe]H subclusters of the active site "H-cluster". The signal pattern of the unmaturated HydA1 containing only [4Fe-4S]H is reminiscent of bacterial-type ferredoxins. The spectra of maturated HydA1, with a complete H-cluster in the active Hox and the CO-inhibited Hox-CO state, reveal additional upfield and downfield shifted 1H resonances originating from the four methylene protons of the azadithiolate ligand in the [2Fe]H subsite. The two axial protons are affected by positive spin density, while the two equatorial protons experience negative spin density. These protons can be used as important probes sensing the effects of ligand-binding to the catalytic site of the H-cluster.

Half-Sandwich Ruthenium Carbene Complexes Link trans-Hydrogenation and gem-Hydrogenation of Internal Alkynes.

The hydrogenation of internal alkynes with [Cp*Ru]-based catalysts is distinguished by an unorthodox stereochemical course in that E-alkenes are formed by trans-delivery of the two H atoms of H2. A combined experimental and computational study now provides a comprehensive mechanistic picture: a metallacyclopropene (η2-vinyl complex) is primarily formed, which either evolves into the E-alkene via a concerted process or reacts to give a half-sandwich ruthenium carbene; in this case, one of the C atoms of the starting alkyne is converted into a methylene group. This transformation represents a formal gem-hydrogenation of a π-bond, which has hardly any precedent. The barriers for trans-hydrogenation and gem-hydrogenation are similar: whereas DFT predicts a preference for trans-hydrogenation, CCSD(T) finds gem-hydrogenation slightly more facile. The carbene, once formed, will bind a second H2 molecule and evolve to the desired E-alkene, a positional alkene isomer or the corresponding alkane; this associative pathway explains why double bond isomerization and over-reduction compete with trans-hydrogenation. The computed scenario concurs with para-hydrogen-induced polarization transfer (PHIP) NMR data, which confirm direct trans-delivery of H2, the formation of carbene intermediates by gem-hydrogenation, and their evolution into product and side products alike. Propargylic -OR (R = H, Me) groups exert a strong directing and stabilizing effect, such that several carbene intermediates could be isolated and characterized by X-ray diffraction. The gathered information spurred significant preparative advances: specifically, highly selective trans-hydrogenations of propargylic alcohols are reported, which are compatible with many other reducible functional groups. Moreover, the ability to generate metal carbenes by gem-hydrogenation paved the way for noncanonical hydrogenative cyclopropanations, ring expansions, and cycloadditions.

Cromolyn/gelatin mixtures as aqueous alignment media and utilization of their mechanical stability for a layering technique.

In this study, aqueous blends of cromolyn and gelatin ("cromogels") are introduced as anisotropic media. The addition of gelatin enables an advantageous adjustability of the strength, the homogeneity, and the stability of the cromolyn alignment. The mechanical stability of these polymer-dispersed liquid crystals is further utilized by stacking layers of D2 O/cromolyn/gelatin with varying component ratio. The resulting distinct phases with correspondingly different degrees of alignment can be targeted by spatially resolved NMR techniques. As a case study, we investigated sucrose in a two-phase system with neat D2 O and analyte layered over the anisotropic medium. A recently presented spatially selective coupled-type HSQC experiment allows the determination of one-bond C-H splitting in both phases.

NMReDATA, a standard to report the NMR assignment and parameters of organic compounds.

Even though NMR has found countless applications in the field of small molecule characterization, there is no standard file format available for the NMR data relevant to structure characterization of small molecules. A new format is therefore introduced to associate the NMR parameters extracted from 1D and 2D spectra of organic compounds to the proposed chemical structure. These NMR parameters, which we shall call NMReDATA (for nuclear magnetic resonance extracted data), include chemical shift values, signal integrals, intensities, multiplicities, scalar coupling constants, lists of 2D correlations, relaxation times, and diffusion rates. The file format is an extension of the existing Structure Data Format, which is compatible with the commonly used MOL format. The association of an NMReDATA file with the raw and spectral data from which it originates constitutes an NMR record. This format is easily readable by humans and computers and provides a simple and efficient way for disseminating results of structural chemistry investigations, allowing automatic verification of published results, and for assisting the constitution of highly needed open-source structural databases.

Catalytic Asymmetric [4+2]-Cycloaddition of Dienes with Aldehydes.

Despite its significant potential, a general catalytic asymmetric [4+2]-cycloaddition of simple and electronically unbiased dienes with any type of aldehyde has long been unknown. Previously developed methodologies invariably require activated, electronically engineered substrates. We now provide a general solution to this problem. We show that highly acidic and confined imidodiphosphorimidates (IDPis) are extremely effective Brønsted acid catalysts of the hetero-Diels-Alder reaction of a wide variety of aldehydes and dienes to give enantiomerically enriched dihydropyrans. Excellent stereoselectivity is generally observed and a variety of scents and natural products can be easily accessed.

Enantioselective Synthesis of [6]Carbohelicenes.

The use of α-cationic phosphonites derived from TADDOL as ancillary ligands has allowed a highly regio- and enantioselective synthesis of substituted [6]carbohelicenes by sequential Au-catalyzed intramolecular hydroarylation of diynes. Key for these results is the modular structure of these new ligands, and the enhanced reactivity that they impart to Au(I)-centers after coordination.

Structures of Reactive Donor/Acceptor and Donor/Donor Rhodium Carbenes in the Solid State and Their Implications for Catalysis.

Owing to its tremendous preparative importance, rhodium carbene chemistry has been studied extensively during past decades. The invoked intermediates have, however, so far proved too reactive for direct inspection, and reliable experimental information has been extremely limited. A series of X-ray structures of pertinent intermediates of this type, together with supporting spectroscopic data, now closes this gap and provides a detailed picture of the constitution and conformation of such species. All complexes were prepared by decomposition of a diazoalkane precursor with an appropriate rhodium source; they belong to either the dirhodium(II) tetracarboxylate carbene series that enjoys widespread preparative use, or to the class of mononuclear half-sandwich carbenes of Rh(III), which show considerable potential. The experimental data correct or refine previous computational studies but corroborate the currently favored model for the prediction of the stereochemical course of rhodium catalyzed cyclopropanations, which is likely also applicable to other reactions. Emphasis is put on stereoelectronic rather than steric arguments, with the dipole of the acceptor substituent flanking the carbene center being the major selectivity determining factor. Moreover, the very subtle influence exerted by the anionic ligands on a Rh(III) center on the chemical character of the resulting carbenes species is documented by the structures of a homologous series of halide complexes. Finally, the isolation of a N-bonded Rh(II) diazoalkane complex showcases that steric hindrance represents an inherent limitation of the chosen methodology.

Atomic structure of the KEOPS complex: an ancient protein kinase-containing molecular machine.

Kae1 is a universally conserved ATPase and part of the essential gene set in bacteria. In archaea and eukaryotes, Kae1 is embedded within the protein kinase-containing KEOPS complex. Mutation of KEOPS subunits in yeast leads to striking telomere and transcription defects, but the exact biochemical function of KEOPS is not known. As a first step to elucidating its function, we solved the atomic structure of archaea-derived KEOPS complexes involving Kae1, Bud32, Pcc1, and Cgi121 subunits. Our studies suggest that Kae1 is regulated at two levels by the primordial protein kinase Bud32, which is itself regulated by Cgi121. Moreover, Pcc1 appears to function as a dimerization module, perhaps suggesting that KEOPS may be a processive molecular machine. Lastly, as Bud32 lacks the conventional substrate-recognition infrastructure of eukaryotic protein kinases including an activation segment, Bud32 may provide a glimpse of the evolutionary history of the protein kinase family.

Stabilization of a Chiral Dirhodium Carbene by Encapsulation and a Discussion of the Stereochemical Implications.

For the first time, the stereochemical course of an asymmetric cyclopropanation can be discussed on the basis of experimental structural information on a pertinent chiral dirhodium carbene intermediate. Key to success was the formation of racemic single crystals of a heterochiral [Rh2 {(S*)-PTTL}4 {=C(Ar)COOMe}][Rh2 {(R*)-PTTL}4 ] (Ar=MeOC6 H4 ; PTTL=N-phthaloyl-tert-leucinate) capsule, which has been characterized by X-ray diffraction. NMR spectroscopic data confirm that the obtained structural portrait is also relevant in solution and provide additional information about the dynamics of this species. The chiral binding pocket is primarily defined by the conformational preferences of the N-phthaloyl-protected amino acid ligands and reinforced by a network of weak interligand interactions that get stronger when chlorinated phthalimide residues are used.