25 research outputs found

    Low coordinate NHC-Zinc-Hydride Complexes Catalyze Alkyne C-H Borylation and Hydroboration using Pinacolborane

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    Organozinc compounds containing sp, sp<sup>2</sup>, and sp<sup>3</sup> C–Zn moieties undergo transmetalation with pinacolborane (HBPin) to produce Zn–H species and organoboronate esters (RBPin). This Zn–C/H–B metathesis step is key to enabling zinc-catalyzed borylation reactions, and it is used in this work to develop both terminal alkyne C–H borylation and internal alkyne hydroboration. These two conversions can be combined in one pot to achieve the zinc-catalyzed conversion of terminal alkynes to 1,1-diborylated alkenes without isolation of the sensitive (to protodeboronation) alkynyl boronate ester intermediates. Mechanistic studies involving the isolation of intermediates, stoichiometric experiments, and DFT calculations all support mechanisms involving organozinc species that undergo metathesis with HBPin. Furthermore, zinc-catalyzed hydroboration can proceed via a hydrozincation step, which does not require any exogenous catalyst in contrast to all previously reported alkyne hydrozincations. Bulky <i>N</i>-heterocyclic carbenes (NHCs) are key for effective catalysis as the NHC steric bulk enhances the stability of the NHC–Zn species present during catalysis and provides access to low-coordinate (NHC)­Zn–H cations that are electrophilic yet Brønsted basic. This work provides an alternative approach to access synthetically desirable pinacol–organoboronate esters using earth-abundant metal-based borylation catalysts

    Improved change detection with nearby hands

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    Recent studies have suggested altered visual processing for objects that are near the hands. We present three experiments that test whether an observer’s hands near the display facilitate change detection. While performing the task, observers placed both hands either near or away from the display. When their hands were near the display, change detection performance was more accurate and they held more items in visual short-term memory (experiment 1). Performance was equally improved for all regions across the entire display, suggesting a stronger attentional engagement over all visual stimuli regardless of their relative distances from the hands (experiment 2). Interestingly, when only one hand was placed near the display, we found no facilitation from the left hand and a weak facilitation from the right hand (experiment 3). Together, these data suggest that the right hand is the main source of facilitation, and both hands together produce a nonlinear boost in performance (superadditivity) that cannot be explained by either hand alone. In addition, the presence of the right hand biased observers to attend to the right hemifield first, resulting in a right-bias in change detection performance (experiments 2 and 3)

    fMRI Evidence for a Dual Process Account of the Speed-Accuracy Tradeoff in Decision-Making

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    Background: The speed and accuracy of decision-making have a well-known trading relationship: hasty decisions are more prone to errors while careful, accurate judgments take more time. Despite the pervasiveness of this speed-accuracy tradeoff (SAT) in decision-making, its neural basis is still unknown. Methodology/Principal Findings: Using functional magnetic resonance imaging (fMRI) we show that emphasizing the speed of a perceptual decision at the expense of its accuracy lowers the amount of evidence-related activity in lateral prefrontal cortex. Moreover, this speed-accuracy difference in lateral prefrontal cortex activity correlates with the speedaccuracy difference in the decision criterion metric of signal detection theory. We also show that the same instructions increase baseline activity in a dorso-medial cortical area involved in the internal generation of actions. Conclusions/Significance: These findings suggest that the SAT is neurally implemented by modulating not only the amount of externally-derived sensory evidence used to make a decision, but also the internal urge to make a response. We propose that these processes combine to control the temporal dynamics of the speed-accuracy trade-off in decisionmaking

    The Emergence of Emotions

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    Emotion is conscious experience. It is the affective aspect of consciousness. Emotion arises from sensory stimulation and is typically accompanied by physiological and behavioral changes in the body. Hence an emotion is a complex reaction pattern consisting of three components: a physiological component, a behavioral component, and an experiential (conscious) component. The reactions making up an emotion determine what the emotion will be recognized as. Three processes are involved in generating an emotion: (1) identification of the emotional significance of a sensory stimulus, (2) production of an affective state (emotion), and (3) regulation of the affective state. Two opposing systems in the brain (the reward and punishment systems) establish an affective value or valence (stimulus-reinforcement association) for sensory stimulation. This is process (1), the first step in the generation of an emotion. Development of stimulus-reinforcement associations (affective valence) serves as the basis for emotion expression (process 2), conditioned emotion learning acquisition and expression, memory consolidation, reinforcement-expectations, decision-making, coping responses, and social behavior. The amygdala is critical for the representation of stimulus-reinforcement associations (both reward and punishment-based) for these functions. Three distinct and separate architectural and functional areas of the prefrontal cortex (dorsolateral prefrontal cortex, orbitofrontal cortex, anterior cingulate cortex) are involved in the regulation of emotion (process 3). The regulation of emotion by the prefrontal cortex consists of a positive feedback interaction between the prefrontal cortex and the inferior parietal cortex resulting in the nonlinear emergence of emotion. This positive feedback and nonlinear emergence represents a type of working memory (focal attention) by which perception is reorganized and rerepresented, becoming explicit, functional, and conscious. The explicit emotion states arising may be involved in the production of voluntary new or novel intentional (adaptive) behavior, especially social behavior

    Iron/Tetramethylethylenediamine-catalyzed ambient-temperature coupling of alkyl grignard reagents and aryl chlorides

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    Tetramethylethylenediamine (TMEDA) acts as cheap and readily removed ligand in the iron-catalyzed coupling of alkyl Grignard reagents and activated aryl chlorides. The use of TMEDA allows for low ligand and iron catalyst loading as well as an increased reaction concentration and an ambient reaction temperature on a mole scale

    Ventral prefrontal cortex is not essential for working memory

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    It is widely held that the prefrontal cortex is important for working memory. It has been suggested that the inferior convexity (IC) may play a special role in working memory for form and color (Wilson et al., 1993). We have therefore assessed the ability of monkeys with IC lesions to perform visual pattern association tasks and color-matching tasks, both with and without delay. In experiment 1, six monkeys were trained on a visual association task with delays of up to 2 sec. Conservative IC lesions that removed lateral area 47/12 in three animals had no effect on the task. Further experiments showed that these lesions had no effect on the postoperative new learning of a It is widely held that the prefrontal cortex is involved in working memory (Goldman-Rakic, 1987, 1996; Funahashi and Kubota, 1994). In animal-based research, working memory refers to the ability to hold information “on-line ” to guide behavior in the absence o

    Landscape transformations and human responses c. 11,500-c. 4500 years ago

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    This chapter discusses the character of human activity at Niah after the transition to the Holocene, the modern climate era. The beginning of the Holocene was marked by an abrupt warming in the palaeotemperature records of the Greenland ice cap at 11,700 BP (Rasmussen et al. 2006; as in the case of Chapter 5, this is rounded here to 11,500 Br), and the end of the major climate swings which had marked the terminal stages of the Pleistocene (Rasmussen et al. 2008; Svensson et al. 2006). The geography of Southeast Asia had changed markedly in the millennia leading up to the Holocene, as the vast exposed plains of the Pleistocene Sundaland continent were progressively inundated by rising sea levels (Sathiamurthy & Voris 2006). The process continued but with complex and localized dynamics into the Holocene, relative sea levels in some parts of Southeast Asia reaching and briefly surpassing modern values by 3-5 m between about 6000 BP and 4500 BP before gradually stabilizing (Bird et al. 2006; 2010; Hanebuth et al. 2000; Horton et al. 2005; Tanabe et al. 2006; Tjia 1996; Fig. 6.1). The Mid Holocene high sea stand is taken as the approximate boundary between this chapter and the next, though it is not easily visible at Niah

    Landscape transformations and human responses, c. 11,500-c. 4500 years ago

    No full text
    This chapter discusses the character of human activity at Niah after the transition to the Holocene, the modern climate era. The beginning of the Holocene was marked by an abrupt warming in the palaeotemperature records of the Greenland ice cap at 11,700 BP (Rasmussen et al. 2006; as in the case of Chapter 5, this is rounded here to 11,500 Br), and the end of the major climate swings which had marked the terminal stages of the Pleistocene (Rasmussen et al. 2008; Svensson et al. 2006). The geography of Southeast Asia had changed markedly in the millennia leading up to the Holocene, as the vast exposed plains of the Pleistocene Sundaland continent were progressively inundated by rising sea levels (Sathiamurthy & Voris 2006). The process continued but with complex and localized dynamics into the Holocene, relative sea levels in some parts of Southeast Asia reaching and briefly surpassing modern values by 3-5 m between about 6000 BP and 4500 BP before gradually stabilizing (Bird et al. 2006; 2010; Hanebuth et al. 2000; Horton et al. 2005; Tanabe et al. 2006; Tjia 1996; Fig. 6.1). The Mid Holocene high sea stand is taken as the approximate boundary between this chapter and the next, though it is not easily visible at Niah
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