This article describes detailed mechanistic studies focused on elucidating the impact of pyridine ligands on the Pd-catalyzed C–H acetoxylation of benzene. Three different catalysts, Pd(OAc)2, ...Pd(OAc)2/pyridine (1:1), and Pd(OAc)2/pyridine (1:2), are compared using a combination of mechanistic tools, including rate and order studies, Hammett analysis, detailed characterization of catalyst resting states, and isotope effects. The data from these experiments implicate C–H activation as the rate-limiting step in all cases. The major difference between the three catalysts is proposed to be the resting state of Pd. Under the reaction conditions, Pd(OAc)2 rests as an acetate bridged dimer, while the Pd(OAc)2/pyridine (1:2) catalyst rests as the monomer (pyridine)2Pd(OAc)2. In contrast, a variety of experiments suggest that the highly active catalyst generated from the 1:1 combination of Pd(OAc)2 and pyridine rests as the dimeric structure (pyridine)Pd(OAc)22.
The C–H bonds of methane are generally more kinetically inert than those of other hydrocarbons, reaction solvents, and methane functionalization products. Thus, developing strategies to achieve ...selective functionalization of CH₄ remains a major challenge. Here, we report transition metal–catalyzed C–H borylation of methane with bis-pinacolborane (B₂pin₂) in cyclohexane solvent at 150°C under 2800 to 3500 kilopascals of methane pressure. Iridium, rhodium, and ruthenium complexes all catalyze the reaction. Formation of mono- versus diborylated methane is tunable as a function of catalyst, with the ruthenium complex providing the highest ratio of CH₃Bpin to CH₂(Bpin)₂. Despite the high relative concentration of cyclohexane, minimal quantities of borylated cyclohexane products are observed. Furthermore, all three metal complexes catalyze borylation of methane with >3.5:1 selectivity over ethane.
There is a well-documented relationship between academic match and bachelor’s degree completion; students who undermatch are less likely to complete a BA than those who match or overmatch, net of ...academic qualifications and demographic characteristics. Little is known, however, about whether this association has changed over time. I argue that recent trends in U.S. higher education may have altered this association. Thus, while prior research has documented gaps in outcomes between undermatched, matched, and overmatched students, it is important to understand how these gaps may be evolving. The present study uses nationally representative data from three cohorts of first-time college students—students who began college in 1995, 2003, and 2011—to examine this question. Findings show that, in some ways, the association between academic match and BA completion has remained stable over time; across all three cohorts, undermatched students are less likely to graduate than matched and overmatched students, after controlling for academic qualifications and demographic characteristics. In other ways, the association may be evolving; overall, overmatched students’ odds of graduation have increased over time, while matched and undermatched students’ have not. There are multiple possible explanations for this, including the fact that graduation rates in recent years have become increasingly stratified by college selectivity. The study concludes with recommendations for policy and practice, as well as suggestions for future research.
To determine the impact of city-level cannabis decriminalization and medical marijuana laws (MMLs) on fatal traffic crashes in US cities.
Using a census of fatal traffic crashes from the 2010 to 2017 ...Fatality Analysis Reporting System, we examined MMLs and cannabis decriminalization on fatal crashes by age and sex of driver. We used a Poisson difference-in-differences approach, exploiting temporal and geographic variation in marijuana decriminalization laws.
Cities experienced a 13% increase in fatal crashes involving 15- to 24-year-old male drivers following decriminalization (incidence rate ratio = 1.125; 95% confidence interval = 1.014, 1.249). This effect was immediate and strongest on weekend nights. We found no effect on female drivers or older males. Conversely, we found that MMLs were associated with fewer fatal crashes for both males and females, which was most pronounced in 15- to 24-year-old drivers.
Unlike MMLs, which are associated with fewer fatal crashes, cities experienced a relative increase in fatal crashes involving young male drivers following marijuana decriminalization.
MMLs stipulate consumption occurs at home, whereas decriminalization only lessens the penalty for marijuana possession. Therefore, travel incentives of such laws have heterogeneous effects on traffic safety.
Objectives: To determine the impact of fraternity and sorority membership on condom use during sex. Methods: Utilizing data from The American College Health Association (ACHA) National College Health ...Assessment from 2016-2018, we estimated a linear probability, logistic, and instrumental variable model to determine the relationship between fraternity and sorority membership and the frequency of condom use during sex. Results: Compared to non-fraternity members, fraternity members experience an increase in sex without a condom for both oral sex (13.5%, 95% ci= 0.048, 0.221) and anal sex (28.5%, 95% ci= 0.174, 0.396). Compared to non-sorority members, sorority members are more likely to engage in oral sex without a condom. This result is not robust to single sorority members. Conclusions: Fraternity membership increases the probability of risky oral and anal sex. Policy Implications: The reduced use of condoms has implications on sexual health including an increased risk of contracting or spreading sexually transmitted infections.
SuperAgers are adults over the age of 80 with superior episodic memory performance and at least average-for-age performance in non-episodic memory domains. This study further characterized the ...neuropsychological profile of SuperAgers compared to average-for-age episodic memory peers to determine potential cognitive mechanisms contributing to their superior episodic memory performance.
Retrospective analysis of neuropsychological test data from 56 SuperAgers and 23 similar-age peers with average episodic memory was conducted. Independent sample
-tests evaluated between-group differences in neuropsychological scores. Multiple linear regression determined the influence of non-episodic memory function on episodic memory scores across participants.
As a group, SuperAgers had better scores than their average memory peers on measures of attention, working memory, naming, and speeded set shifting. Scores on tests of processing speed, visuospatial function, verbal fluency, response inhibition, and abstract reasoning did not differ. On an individual level, there was variability among SuperAgers with regard to non-episodic memory performance, with some performing above average-for-age across cognitive domains while others performed in the average-for-age range on non-memory tests. Across all participants, attention and executive function scores explained 20.4% of the variance in episodic memory scores.
As a group, SuperAgers outperformed their average memory peers in multiple cognitive domains, however, there was considerable intragroup variability suggesting that SuperAgers' episodic memory strength is not simply related to globally superior cognitive functioning. Attention and executive function performance explained approximately one-fifth of the variance in episodic memory and maybe areas to target with cognitive interventions.
Abstract
We report on improved sky localizations of 13 repeating fast radio bursts (FRBs) discovered by CHIME/FRB via the use of interferometric techniques on channelized voltages from the telescope. ...These so-called “baseband localizations” improve the localization uncertainty area presented in past studies by more than three orders of magnitude. The improved localization regions are provided for the full sample of FRBs to enable follow-up studies. The localization uncertainties, together with the limits on the source distances from their dispersion measures, allow us to identify likely host galaxies for two of the FRB sources. FRB 20180814A lives in a massive passive red spiral at
z
∼ 0.068 with very little indication of star formation, while FRB 20190303A resides in a merging pair of spiral galaxies at
z
∼ 0.064 undergoing significant star formation. These galaxies show very different characteristics, further confirming the presence of FRB progenitors in a variety of environments even among the repeating subclass.
The oxidative addition of Pd to Si-H bonds is a crucial step in a variety of catalytic applications, and many aspects of this reaction are poorly understood. One important yet underexplored aspect is ...the electronic effect of silane substituents on reactivity. Herein we describe a systematic investigation of the formation of silyl palladium hydride complexes as a function of silane identity, focusing on electronic influence of the silanes. Using (μ-dcpe)Pd
2
(dcpe = dicyclohexyl(phosphino)ethane) and tertiary silanes, data show that equilibrium strongly favours products formed from electron-deficient silanes, and is fully dynamic with respect to both temperature and product distribution. A notable kinetic isotope effect (KIE) of 1.21 is observed with H/DSiPhMe
2
at 233 K, and the reaction is shown to be 0.5
th
order in (μ-dcpe)Pd
2
and 1
st
order in silane. Formed complexes exhibit temperature-dependent intramolecular H/Si ligand exchange on the NMR timescale, allowing determination of the energetic barrier to reversible oxidative addition. Taken together, these results give unique insight into the individual steps of oxidative addition and suggest the initial formation of a σ-complex intermediate to be rate-limiting. The insight gained from these mechanistic studies was applied to hydrosilylation of alkynes, which shows parallel trends in the effect of the silanes' substituents. Importantly, this work highlights the relevance of in-depth mechanistic studies of fundamental steps to catalysis.
Mechanistic studies reveal the rate law, an H/D KIE, and that the silane's electronics impact the thermodynamic and kinetic energetics of the oxidative addition reaction. These electronic effects are relevant in the hydrosilylation of alkynes.
Less is more: The rational optimization and general applicability of the catalytic system Pd(OAc)2/pyridine is described (see scheme). The catalyst shows excellent reactivity in the CH oxygenation ...of simple aromatic substrates. The Pd/pyridine ratio is critical as the use of one equivalent of pyridine per Pd center leads to dramatic enhancements in both reactivity and site selectivity in comparison to Pd(OAc)2 alone.
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