10,000 publications from this institution
Abstract Few allylic electrophiles containing two different substituents at a single allyl terminus and none in which one of the two substituents is a heteroatom, have been shown previously to react with iridium catalysts to form substitution products. We report that iridium‐catalysts are uniquely suited to form tertiary allylic fluorides enantioselectively by the addition of a diverse range of carbon‐centered nucleophiles at the fluorine‐containing terminus of 3‐fluoro‐substituted allylic esters. The products contain tertiary stereogenic centers bearing a single fluorine, which are isosteric with common tertiary stereocenters containing a single hydrogen. Computational studies reveal the principal steric interactions influencing the stability of endo and exo π‐allyl intermediates formed from 3,3‐disubstituted allylic electrophiles.
In this work, a gold-catalyzed oxidative coupling of alkenes with aryltrimethylsilanes is disclosed. Using trimethylsilanes instead of boronic acids reduces homocoupling byproducts and allows intramolecular coupling reactions. Furthermore, the scope of functional groups on the coupling partners can be expanded considerably, and substitution is tolerated in para, meta, and ortho positions.
We experimentally manipulated nutrient input to a rocky intertidal community, using nutrient-diffusing flowerpots, to determine ( i ) whether nutrients limited intertidal productivity, ( ii ) how a large-scale oceanographic disturbance (an El Niño event) affected patterns of nutrient limitation, ( iii ) the relative impacts of molluscan grazers and nutrient limitation, and ( iv ) if responses to experimental nutrient addition among trophic levels were more consistent with prey-dependent or ratio-dependent food chain models. Nutrients measurably increased the abundance of micrograzers (amphipods and chironomid larvae), but not algal biomass, during the summer of an El Niño year. Nutrients had no effects in two non-El Niño years and during the autumn of an El Niño year. Adding nutrients did not affect food chain stability as assessed by temporal variation in algal biomass and micrograzer abundance. Large molluscan grazers caused large reductions in micrograzers and smaller reductions in algae, indicating consistent consumer effects. The results demonstrate that in this intertidal community, nutrient limitation can occur under conditions of nutrient stress, that top-down grazing effects are typically stronger than bottom-up nutrient effects, and that prey-dependent models are more appropriate than ratio-dependent models.
Pristine monocrystalline graphene is claimed to be the strongest material known with remarkable mechanical and electrical properties. However, graphene made with scalable fabrication techniques is polycrystalline and contains inherent nanoscale line and point defects—grain boundaries and grain-boundary triple junctions—that lead to significant statistical fluctuations in toughness and strength. These fluctuations become particularly pronounced for nanocrystalline graphene where the density of defects is high. Here we use large-scale simulation and continuum modelling to show that the statistical variation in toughness and strength can be understood with ‘weakest-link’ statistics. We develop the first statistical theory of toughness in polycrystalline graphene, and elucidate the nanoscale origins of the grain-size dependence of its strength and toughness. Our results should lead to more reliable graphene device design, and provide a framework to interpret experimental results in a broad class of two-dimensional materials.
We ask whether epidemic exposure leads to a shift in financial technology usage within and across countries and if so who participates in this shift. We exploit a datasetcombining Gallup World Polls and Global Findex surveys for some 250,000 individuals in 140 countries, merging them with information on the incidence of epidemics and local 3G internet infrastructure. Epidemic exposure is associated with an increase in remote-access (online/mobile) banking and substitution from bank branch-based to ATM-based activity. Using a machine-learning algorithm, we show that heterogeneity in this response centers on the age, income and employment of respondents. Young, high-income earners in full-time employment have the greatest propensity to shift to online/mobile transactions in response to epidemics. These effects are larger for individuals in subnational regions with better ex ante 3G signal coverage, highlighting the role of the digital divide in adaption to new technologies necessitated by adverse external shocks.
