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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

語用論(Pragmatics)を考える

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.

In 1958 Professor Alan Davison started his research career at an exciting time for the field of organometallic chemistry. New developments in spectroscopy, instrumentation and techniques to manipulate materials in controlled environments to avoid reaction with water or oxygen were becoming widely available. Controlling exposure of an element with highly reactive oxygen facilitated the isolation, characterization and discovery of an abundance of unknown compounds. Alan was an insightful and talented synthetic chemist and made many new and interesting organometallic compounds. He used the earliest commercial nuclear magnetic resonance instruments to characterize the then poorly understood transition metal hydrides and also to identify the earliest fluxional organometallic molecules. In 1970 he entered a collaboration with Professor Alun G. Jones, a nuclear chemist at Harvard Medical School, to characterize and develop the chemistry of technetium. They made a major discovery of technetium molecules which had the ability to selectively locate in human heart muscle, thereby vastly expanding the practice of nuclear medicine to a global community. Professor Alan Davison was also widely known for his outstanding qualities as a teacher and mentor.

https://royalsocietypublishing.org/doi/pdf/10.1098/rsbm.2017.0004

Alan Davison. 24 March 1936 — 14 November 2015

This short review describes a breakthrough embodied by the synthesis of a niobaziridine hydride complex. This reactive entity reacts directly with white phosphorus to provide a bridging diphosphorus diniobium complex that upon reduction splits to afford a terminal niobium phosphide anion, isolated as its sodium salt. Reactions of the latter with acid chlorides constitute a new synthesis of phosphaalkynes, while treatment with chlorodiorganophosphanes leads to complexed 1,1-diorganophosphanylphosphinidene systems. Additionally, reactions of the sodium salt of the niobium phosphide anion with divalent main group element salts (E = Ge, Sn, or Pb) provide complexed triatomic EP2 triangles. Dinitrogen cleavage was realized via reduction of a heterodinuclear niobium/molybdenum dinitrogen complex, and this provided an entry to a nitrogen-15 labeled terminal nitride anion of niobium as its sodium salt. In a fashion analogous to the aforementioned phosphaalkyne synthesis, acid chlorides are transformed upon reaction with the niobium nitride anion into corresponding nitrogen-15 labeled organic nitriles. Complete synthetic cycles are achieved in both the phosphaalkyne and the organic nitrile syntheses, as the oxoniobium(V) byproduct can be recycled in high yield to the title niobaziridine hydride complex.

A Niobaziridine Hydride System for White Phosphorus or Dinitrogen Activation and N- or P-Atom Transfer

Pragmatic enrichments arising from the use of modified fractions have been little studied, but offer interesting insights into the subtleties of scale structure and granularity. In this chapter I present some new experimental data on the interpretation of these expressions. I argue that these data suggest that modified fractions, like modified integers, give rise to pragmatic enrichments which are conditioned by scale granularity, but that we need to refine the notion of granularity somewhat to extend it to this domain. There is also evidence for enrichments that are not easily captured in classical quantity implicature terms, but which I suggest could be explained by appeal to typicality effects.

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Modified Fractions, Granularity and Scale Structure

Finding the right heuristics to optimize code has always been a difficult and mostly manual task for compiler engineers. Today this task is near-impossible as hardware-software complexity has scaled up exponentially. Predictive models for compilers have recently emerged which require little human effort but are far better than humans in finding near optimal heuristics. As any machine learning technique, they are only as good as the data they are trained on but there is a severe shortage of code for training compilers. Researchers have tried to remedy this with code generation but their synthetic benchmarks, although thousands, are small, repetitive and poor in features, therefore ineffective. This indicates the shortage is of feature quality more than corpus size. It is more important than ever to develop a directed program generation approach that will produce benchmarks with valuable features for training compiler heuristics. We develop BenchPress, the first ML benchmark generator for compilers that is steerable within feature space representations of source code. BenchPress synthesizes compiling functions by adding new code in any part of an empty or existing sequence by jointly observing its left and right context, achieving excellent compilation rate. BenchPress steers benchmark generation towards desired target features that has been impossible for state of the art synthesizers (or indeed humans) to reach. It performs better in targeting the features of Rodinia benchmarks in 3 different feature spaces compared with (a) CLgen - a state of the art ML synthesizer, (b) CLSmith fuzzer, (c) SRCIROR mutator or even (d) human-written code from GitHub. BenchPress is the first generator to search the feature space with active learning in order to generate benchmarks that will improve a downstream task. We show how using BenchPress, Grewe's et al. CPU vs GPU heuristic model can obtain a higher speedup when trained on BenchPress's benchmarks compared to other techniques. BenchPress is a powerful code generator: Its generated samples compile at a rate of 86%, compared to CLgen's 2.33%. Starting from an empty fixed input, BenchPress produces 10× more unique, compiling OpenCL benchmarks than CLgen, which are significantly larger and more feature diverse. We release BenchPress's source code on a publicly available repository at https://github.com/fivosts/BenchPress. This work was supported by the Engineering and Physical Sciences Research Council (grant EP/L01503X/1), EPSRC Centre for Doctoral Training in Pervasive Parallelism at the University of Edinburgh, School of Informatics. This work was supported by the Royal Academy of Engineering under the Research Fellowship scheme.

https://dl.acm.org/doi/pdf/10.1145/3559009.3569644

BenchPress: A Deep Active Benchmark Generator

Donor-stabilized adducts of the P2O5 molecule are reported. The compounds are of the general formula P2O5D2, where D = DABCO, pyridine, and DBU. The DABCO adduct was initially prepared from trimetaphosphate but could be prepared more conveniently from phosphorus pentoxide. The reactions leading to P2O5D2 (D = DABCO) also generate P3O8D3 and both adducts were structurally characterized. The pyridine adduct P2O5D2 (D = pyridine) was most easily prepared in good purity and yield and thus its diphosphorylation chemistry was developed. Its reaction with protic nucleophiles MeOH and HNEt2 yield salts of [P2O7Me2]2-, a dimethyl ester of pyrophosphate, and [P2O5(NEt2)2]2-, a phosphoramidate derivative which was structurally characterized. Diphosphorylation of adenosine monophosphate yielded a cyclic intermediate which could be opened by nucleophiles including water to provide ATP, diethylamine to provide γ-diethylamino-ATP, and uridine monophosphate to provide adenosine uridine tetraphosphate. Adenosine diphosphate was diphosphorylated to provide, following hydrolytic ring opening, adenosine tetraphosphate.

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Christopher C. Cummins

Papers

Alan Davison. 24 March 1936 — 14 November 2015

A Niobaziridine Hydride System for White Phosphorus or Dinitrogen Activation and N- or P-Atom Transfer

Modified Fractions, Granularity and Scale Structure

BenchPress: A Deep Active Benchmark Generator

N-donor Base Adducts of P2O5 as Diphosphorylation Reagents