287 research outputs found

    Asymmetric Photocatalysis by Hydrogen Atom Transfer with Chiral-at-Rhodium Complexes

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    Hydrogen atom transfer provides straightforward methods to generate open-shell radical intermediates from C-H bonds and offers unique opportunities for green and sustainable synthesis. Visible-light-induced C-H functionalization enabled by HAT is an emerging strategy for substrate activation in photocatalyzed organic synthesis. In recent years, photoinduced HAT reactions have seen substantial development of their versatility, efficiency, and selectivity. However, asymmetric photocatalysis via hydrogen atom transfer has rarely been reported. In the first section, a visible-light-induced asymmetric rearrangement of 3-(2-formylphenyl)-1-pyrazol-1-yl-propenones to benzo-[d]cyclopropa[b]pyranones with up to ] 99% ee is introduced, which is catalyzed by a bis-cyclometalated chiral-at-metal rhodium complex (RhS). Mechanistic experiments and DFT calculations support a mechanism whereby the photoexcited catalyst/substrate complex generates triplet excited-state species through photoiduced intersystem crossing, which triggers an intramolecular hydrogen atom transfer subsequent highly stereocontrolled hetero-Diels–Alder reaction. In this reaction scheme, the rhodium catalyst fulfills multiple functions by 1) enabling visible-light π→π* excitation of the catalyst-bound enone substrate, 2) facilitating the hydrogen atom transfer, and 3) providing the asymmetric induction for the hetero-Diels–Alder reaction. In the second section, a visible light driven deracemization of ketones as a new type of deprotonation and asymmetric protonation is demonstrated for the first time, which leads to the formation of chiral carbonyl compounds in an efficient way with high yield (up to 97%) and high enantioselectivity (up to 97%). This new photoinduced deprotonation process is achieved by single electron transfer and subsequent hydrogen atom transfer. A bis-cyclometalated chiral-at-metal rhodium complex (RhInd) is used as the photocatalyst to induce the redox process and is responsible for the asymmetric induction, while the amine acts as the single electron reductant, HAT reagent and proton source. This conceptually simple light-driven strategy of coupling a photoredox deprotonation with a stereocontrolled protonation serves as a blueprint for other deracemizations of ubiquitous carbonyl compounds

    Active Example Selection for In-Context Learning

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    With a handful of demonstration examples, large-scale language models show strong capability to perform various tasks by in-context learning from these examples, without any fine-tuning. We demonstrate that in-context learning performance can be highly unstable across samples of examples, indicating the idiosyncrasies of how language models acquire information. We formulate example selection for in-context learning as a sequential decision problem, and propose a reinforcement learning algorithm for identifying generalizable policies to select demonstration examples. For GPT-2, our learned policies demonstrate strong abilities of generalizing to unseen tasks in training, with a 5.8%5.8\% improvement on average. Examples selected from our learned policies can even achieve a small improvement on GPT-3 Ada. However, the improvement diminishes on larger GPT-3 models, suggesting emerging capabilities of large language models.Comment: EMNLP 2022, code is available at https://github.com/ChicagoHAI/active-example-selectio

    The Determinants of Corporate Hedging with Derivatives in UK Companies

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    Abstract: This paper attempts to examine the determinants of corporate hedging with derivatives in UK companies. Based on previous evidences, this paper demonstrates several potential determinants of corporate hedging, which are: Growth opportunities, Alternatives for hedging, Firm size, Leverage, Tax and Foreign exposure. The sample contains 400 UK companies in different industries, and the data are collected from the annual reports from financial years of 2015 to 2016. By employing mean difference tests, logistic regressions and robustness checks, this paper suggests that liquidity, firm size, interest coverage and leverage are positively related to corporate hedging, while growth opportunities, dividend, profitability, debt maturity and foreign exposures are negatively related to corporate hedging with derivatives in UK companies

    Constructions of Binary Optimal Locally Repairable Codes via Intersection Subspaces

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    Locally repairable codes (LRCs), which can recover any symbol of a codeword by reading only a small number of other symbols, have been widely used in real-world distributed storage systems, such as Microsoft Azure Storage and Ceph Storage Cluster. Since binary linear LRCs can significantly reduce coding and decoding complexity, constructions of binary LRCs are of particular interest. The aim of this paper is to construct dimensional optimal binary locally repairable codes with disjoint local repair groups. We introduce how to connect intersection subspaces with binary locally repairable codes and construct dimensional optimal binary linear LRCs with locality 2b2^b (b≥3b\geq 3) and minimum distance d≥6d\geq 6 by employing intersection subspaces deduced from the direct sum. This method will sufficiently increase the number of possible repair groups of dimensional optimal LRCs, and thus efficiently expanding the range of the construction parameters while keeping the largest code rates compared with all known binary linear LRCs with minimum distance d≥6d\geq 6 and locality 2b2^b (b≥3b\geq 3).Comment: Accepted for publication in the SCIENCE CHINA Information Science

    How the internal family factors and external environmental factors influence the innovation capabilities and willingness of family business.

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    A family business is a complex system composed of a family and a business. With the development of the economy, how family businesses can improve their competitiveness has become a problem that must be faced. Innovation as the source of the core competitiveness of an enterprise has a direct influence on the development of the family business. This paper aims to explore different impacts that influence the innovation performance and management of the family business. Referring to the current problems of the family business, this paper analyzes the factors which may affect the innovation of family business based on the related theories: the agency theory, resource-based view and social capital theory. Furthermore, based on the theory of family business innovation, an integrated framework is adopted to explore the different variables that influence the family business innovation capabilities and willingness and analyse the relationship between each other. At the end, suggestions to the innovation management of the family business are listed in terms of management models, governance structure and human resource management
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