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Global energy consumption is projected to increase, even in the face of substantial declines in energy intensity, at least 2-fold by midcentury relative to the present because of population and economic growth. This demand could be met, in principle, from fossil energy resources, particularly coal. However, the cumulative nature of CO2 emissions in the atmosphere demands that holding atmospheric CO2 levels to even twice their preanthropogenic values by midcentury will require invention, development, and deployment of schemes for carbon-neutral energy production on a scale commensurate with, or larger than, the entire present-day energy supply from all sources combined. Among renewable energy resources, solar energy is by far the largest exploitable resource, providing more energy in 1 hour to the earth than all of the energy consumed by humans in an entire year. In view of the intermittency of insolation, if solar energy is to be a major primary energy source, it must be stored and dispatched on demand to the end user. An especially attractive approach is to store solar-converted energy in the form of chemical bonds, i.e., in a photosynthetic process at a year-round average efficiency significantly higher than current plants or algae, to reduce land-area requirements. Scientific challenges involved with this process include schemes to capture and convert solar energy and then store the energy in the form of chemical bonds, producing oxygen from water and a reduced fuel such as hydrogen, methane, methanol, or other hydrocarbon species.

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Powering the planet: Chemical challenges in solar energy utilization

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The site-selective formation of carbon-carbon bonds through direct functionalizations of otherwise unreactive carbon-hydrogen bonds constitutes an economically attractive strategy for an overall streamlining of sustainable syntheses. In recent decades, intensive research efforts have led to the development of various reaction conditions for challenging C-H bond functionalizations, among which transition-metal-catalyzed transformations arguably constitute thus far the most valuable tool. For instance, the use of inter alia palladium, ruthenium, rhodium, copper, or iron complexes set the stage for chemo-, site-, diastereo-, and/or enantioselective C-H bond functionalizations. Key to success was generally a detailed mechanistic understanding of the elementary C-H bond metalation step, which depending on the nature of the metal fragment can proceed via several distinct reaction pathways. Traditionally, three different modes of action were primarily considered for CH bond metalations, namely, (i) oxidative addition with electronrich late transition metals, (ii) σ-bond metathesis with early transition metals, and (iii) electrophilic activation with electrondeficient late transition metals (Scheme 1). However, more recent mechanistic studies indicated the existence of a continuum of electrophilic, ambiphilic, and nucleophilic interactions. Within this continuum, detailed experimental and computational analysis provided strong evidence for novel C-H bond metalationmechanisms relying on the assistance of a bifunctional ligand bearing an additional Lewis-basic heteroatom, such as that found in (heteroatom-substituted) secondary phosphine oxides or most prominently carboxylates (Scheme 1, iv). This novel insight into the nature of stoichiometric metalations has served as stimulus for the development of novel transformations based on cocatalytic amounts of carboxylates, which significantly broadened the scope of C-H bond functionalizations in recent years, with most remarkable progress being made in palladiumor ruthenium-catalyzed direct arylations and direct alkylations. These carboxylate-assisted C-H bond transformations were mostly proposed to proceed via a mechanism in which metalation takes place via a concerted base-assisted deprotonation. To mechanistically differentiate these intramolecular metalations new acronyms have recently been introduced into the literature, such as CMD (concerted metalationdeprotonation), IES (internal electrophilic substitution), or AMLA (ambiphilic metal ligand activation), which describe related mechanisms and will be used below where appropriate. This review summarizes the development and scope of carboxylates as cocatalysts in transition-metal-catalyzed C-H functionalizations until autumn 2010. Moreover, experimental and computational studies on stoichiometric metalation reactions being of relevance to the mechanism of these catalytic processes are discussed as well. Mechanistically related C-H bond cleavage reactions with ruthenium or iridium complexes bearing monodentate ligands are, however, only covered with respect to their working mode, and transformations with stoichiometric amounts of simple acetate bases are solely included when their mechanism was suggested to proceed by acetate-assisted metalation.

Carboxylate-assisted transition-metal-catalyzed C-H bond functionalizations: mechanism and scope.

Formal addition of diazomethane's terminal nitrogen atom to the 9,10-positions of anthracene yields H2CN2A (1, A = C14H10 or anthracene). The synthesis of this hydrazone is reported from Carpino's hydrazine H2N2A through treatment with paraformaldehyde. Compound 1 has been found to be an easy-to-handle solid that does not exhibit dangerous heat or shock sensitivity. Effective umpolung of the diazomethane unit imbues 1 with electrophilicity at the methylene carbon center. Its reactivity with nucleophiles such as H2CPPh3 and N-heterocyclic carbenes is exploited for CC bond formation with elimination of dinitrogen and anthracene. Similarly, 1 is demonstrated to deliver methylene to a nucleophilic singlet d2 transition metal center, W(ODipp)4 (2), to generate the robust methylidene complex [2CH2]. This behavior is contrasted with that of the Wittig reagent H2CPPh3, a more traditional and Bronsted basic methylene source that upon exposure to 2 contrastingly forms the methylidyne salt [MePPh3][2CH].

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Diazomethane umpolung atop anthracene: an electrophilic methylene transfer reagent

Cage-opening reactions of the highly strained tri-tert-butylphosphatetrahedrane (1), shown here to function as a synthon of (tri-tert-butylcyclopropenyl)phosphinidene, are described. Treatment of 1 with a base-stabilized silylene led to the corresponding phosphasilene, which was isolated in 72% yield as a red crystalline solid. Phosphinidene transfer was also observed when 1 (2 equiv) was combined with the Wittig reagent Ph3PCH2 to form a diphosphirane (50% isolated yield). The reaction is proposed to proceed through a generated phosphaalkene intermediate, which was characterized by NMR spectroscopy. In addition, we report on nickel-catalyzed phosphinidene transfer to styrene, ethylene, neohexene, and 1,3-cyclohexadiene; the corresponding phosphiranes were isolated in 51-64% yield. Computational studies suggest the intermediacy of a nickel phosphinidene species. Treatment of the ethylene-derived phosphirane product with triflic acid delivered elimination of [tBu3C3]OTf and formation of a P-H bond, illustrating the ability of the tri-tert-butyl cyclopropenyl group to serve as a protecting group that is removable following phosphinidene transfer.

Reactions of Tri-tert-Butylphosphatetrahedrane as a Spring-Loaded Phosphinidene Synthon Featuring Nickel-Catalyzed Transfer to Unactivated Alkenes.

Three‐Coordinate Complexes of “Hard” Ligands: Advances in Synthesis, Structure and Reactivity

Phosphatetrahedranes (tBuCP)2 and (tBuC)3P were recently reported and represent the first tetrahedranes containing a mixed carbon/phosphorus core. Herein, we report that tetrahydrofuran (THF) solutions of the parent triphosphatetrahedrane HCP3 may be generated in 31% yield (NMR internal standard yield) by combining [Na(THF)3][P3Nb(ODipp)3] (Dipp = 2,6-diisopropylphenyl), INb(ODipp)3(THF), and bromodichloromethane in thawing THF. While HCP3 was found to be stable in dilute THF solutions for extended periods of time, the concentration of the solution at -40 °C led to the formation of a black precipitate, which has been tentatively assigned as a polymerized form of HCP3. HCP3 reacts readily with (dppe)Fe(Cp*)Cl (dppe = 1,2-bis(diphenylphosphino)ethane, Cp*= η5-C5Me5) in the presence of Na[BPh4] to form a purple cationic iron complex of triphosphatetrahedrane (50% yield), which was structurally characterized in a single-crystal X-ray diffraction experiment. Additionally, we present a series of homodesmotic equations analyzed via quantum chemical calculations that suggest triphosphatetrahedrane is the least strained of the mixed C/P phosphatetrahedranes.

Alleviating Strain in Organic Molecules by Incorporation of Phosphorus: Synthesis of Triphosphatetrahedrane.

Selecting an appropriate workgroup size is critical for the performance of OpenCL kernels, and requires knowledge of the underlying hardware, the data being operated on, and the implementation of the kernel. This makes portable performance of OpenCL programs a challenging goal, since simple heuristics and statically chosen values fail to exploit the available performance. To address this, we propose the use of machine learning-enabled autotuning to automatically predict workgroup sizes for stencil patterns on CPUs and multi-GPUs. 
We present three methodologies for predicting workgroup sizes. The first, using classifiers to select the optimal workgroup size. The second and third proposed methodologies employ the novel use of regressors for performing classification by predicting the runtime of kernels and the relative performance of different workgroup sizes, respectively. We evaluate the effectiveness of each technique in an empirical study of 429 combinations of architecture, kernel, and dataset, comparing an average of 629 different workgroup sizes for each. We find that autotuning provides a median 3.79x speedup over the best possible fixed workgroup size, achieving 94% of the maximum performance.

Christopher C. Cummins

Papers

Diazomethane umpolung atop anthracene: an electrophilic methylene transfer reagent

Reactions of Tri-tert-Butylphosphatetrahedrane as a Spring-Loaded Phosphinidene Synthon Featuring Nickel-Catalyzed Transfer to Unactivated Alkenes.

Three‐Coordinate Complexes of “Hard” Ligands: Advances in Synthesis, Structure and Reactivity

Alleviating Strain in Organic Molecules by Incorporation of Phosphorus: Synthesis of Triphosphatetrahedrane.

Autotuning OpenCL Workgroup Size for Stencil Patterns