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1.
J Appl Meas ; 21(3): 347-360, 2020.
Artigo em Inglês | MEDLINE | ID: mdl-33983904

RESUMO

Research using the National Teacher and Principal Survey (NTPS) has consistently demonstrated that teachers' reported working conditions are related to both intentions to leave the profession and attrition (Tickle, Chang, and Kim, 2011). However, limited research evaluates teacher appraisals of job-related demands and resources as an antecedent to job dissatisfaction. We tested for differential item functioning (DIF) using a partial credit model approach within a Rasch modeling context to examine whether elementary and secondary teachers with similar overall stress levels respond to the NTPS Demands and Resources items in similar ways. For the Demands items, seven of the items displayed differences that were negligible, four were intermediate, and three items indicated large DIF contrasts. For the Resources items, 10 items displayed differences that were negligible, two were intermediate, and zero items indicated large DIF contrasts. These results indicate elementary and secondary teachers exhibit different appraisal patterns, suggesting implications for the development and use of survey data in public school settings in general, and for the use of the NTPS data in particular.


Assuntos
Professores Escolares , Instituições Acadêmicas , Local de Trabalho , Humanos , Psicometria , Inquéritos e Questionários
3.
J Appl Meas ; 17(3): 354-364, 2016.
Artigo em Inglês | MEDLINE | ID: mdl-28027057

RESUMO

The purpose of this study was to examine the measurement properties of the Assessing Math Concepts AMC Anywhere Hiding and Ten Frame Assessments, formative assessments of primary students' number sense skills. Each assessment has two parts, where Part 1 is intended to be foundational skills for part two. Part 1 includes manipulatives whereas Part 2 does not. Student data from 228 kindergarten through second grade teachers with a total of 3,666 students was analyzed using Rasch scaling. Data analyses indicated that when the two assessments were examined separately the intended order of item difficulty was clear. When the parts of both assessments were analyzed together, the items in Part 2 were not consistently more difficult that the items in Part 1. This suggests an alternative sequence of tasks in that students may progress from working with a specific number with manipulatives then without manipulatives rather than working with a variety of numbers with manipulatives before moving onto assessments without manipulatives.


Assuntos
Algoritmos , Avaliação Educacional/métodos , Conceitos Matemáticos , Modelos Estatísticos , Psicometria/métodos , Estudantes/estatística & dados numéricos , Interpretação Estatística de Dados , Aprendizagem Baseada em Problemas , Instituições Acadêmicas/estatística & dados numéricos , Estudantes/classificação , Inquéritos e Questionários
4.
Air Med J ; 35(2): 86-7, 2016.
Artigo em Inglês | MEDLINE | ID: mdl-27021675

RESUMO

The prognosis of pediatric patients who require prolonged resuscitation after ice water drowning and hypothermic cardiac arrest remains guarded. We report a case of successful prolonged resuscitation of a pediatric patient in hypothermic cardiac arrest who showed severe metabolic derangements and went on to make a rapid and full neurologic recovery without the use of extracoproreal rewarming or mechanical cardiac support. Many ground and air medical emergency medical service programs have policies against interfacility transfer of patients in hypothermic cardiac arrest, calling into question the need to revise current protocols.


Assuntos
Afogamento , Hipotermia/terapia , Parada Cardíaca Extra-Hospitalar/terapia , Ressuscitação/métodos , Reaquecimento , Humanos , Lactente , Masculino
5.
J Am Chem Soc ; 137(2): 940-7, 2015 Jan 21.
Artigo em Inglês | MEDLINE | ID: mdl-25531937

RESUMO

Scanning tunneling microscopy, temperature-programmed reaction, near-edge X-ray absorption fine structure spectroscopy, and density functional theory calculations were used to study the adsorption and reactions of phenylacetylene and chlorobenzene on Ag(100). In the absence of solvent molecules and additives, these molecules underwent homocoupling and Sonogashira cross-coupling in an unambiguously heterogeneous mode. Of particular interest is the use of silver, previously unexplored, and chlorobenzene-normally regarded as relatively inert in such reactions. Both molecules adopt an essentially flat-lying conformation for which the observed and calculated adsorption energies are in reasonable agreement. Their magnitudes indicate that in both cases adsorption is predominantly due to dispersion forces for which interaction nevertheless leads to chemical activation and reaction. Both adsorbates exhibited pronounced island formation, thought to limit chemical activity under the conditions used and posited to occur at island boundaries, as was indeed observed in the case of phenylacetylene. The implications of these findings for the development of practical catalytic systems are considered.

6.
Angew Chem Int Ed Engl ; 54(20): 5919-23, 2015 May 11.
Artigo em Inglês | MEDLINE | ID: mdl-25810151

RESUMO

Previous theoretical studies of C3B have suggested that boron-doped graphite is a promising H2- and Li-storage material, with large maximum capacities. These characteristics could lead to exciting applications as a lightweight H2-storage material for automotive engines and as an anode in a new generation of batteries. However, for these applications to be realized a synthetic route to bulk C3B must be developed. Here we show the thermolysis of a single-source precursor (1,3-(BBr2)2C6H4) to produce graphitic C3B, thus allowing the characteristics of this elusive material to be tested for the first time. C3B was found to be compositionally uniform but turbostratically disordered. Contrary to theoretical expectations, the H2- and Li-storage capacities are lower than anticipated, results that can partially be explained by the disordered nature of the material. This work suggests that to model the properties of graphitic materials more realistically, the possibility of disorder must be considered.

7.
Nature ; 454(7207): 981-3, 2008 Aug 21.
Artigo em Inglês | MEDLINE | ID: mdl-18719586

RESUMO

Supported gold nanoparticles have excited much interest owing to their unusual and somewhat unexpected catalytic properties, but the origin of the catalytic activity is still not fully understood. Experimental work on gold particles supported on a titanium dioxide (110) single-crystal surface has established a striking size threshold effect associated with a metal-to-insulator transition, with gold particles catalytically active only if their diameters fall below approximately 3.5 nm. However, the remarkable catalytic behaviour might also in part arise from strong electronic interaction between the gold and the titanium dioxide support. In the case of industrially important selective oxidation reactions, explanation of the effectiveness of gold nanoparticle catalysts is complicated by the need for additives to drive the reaction, and/or the presence of strong support interactions and incomplete understanding of their possible catalytic role. Here we show that very small gold entities ( approximately 1.4 nm) derived from 55-atom gold clusters and supported on inert materials are efficient and robust catalysts for the selective oxidation of styrene by dioxygen. We find a sharp size threshold in catalytic activity, in that particles with diameters of approximately 2 nm and above are completely inactive. Our observations suggest that catalytic activity arises from the altered electronic structure intrinsic to small gold nanoparticles, and that the use of 55-atom gold clusters may prove a viable route to the synthesis of robust gold catalysts suited to practical application.

8.
J Appl Meas ; 15(4): 405-21, 2014.
Artigo em Inglês | MEDLINE | ID: mdl-25232673

RESUMO

This article reports the results of an application of the Rasch rating scale model to the Teaching Strategies GOLD assessment system in a norm sample of children aged birth to 71 months. The analyses focused on the examination of dimensionality, rating scale effectiveness, the hierarchy of item difficulties, and the relationship of developmental scale scores to child age. Results show that each subscale satisfies the Rasch model for unidimensionality. Ratings were found to be less reliable at the lowest and highest ends of the scale and less distinct at 'In-between' levels. Items appear to form theoretically expected hierarchies, supporting evidence for construct validity for the measures. Moderately high correlations of developmental scale scores with child age suggest that teachers are able to make valid ratings of the developmental progress of children across the intended age range.


Assuntos
Desenvolvimento Infantil/classificação , Avaliação Educacional/métodos , Docentes/estatística & dados numéricos , Modelos Estatísticos , Competência Profissional/estatística & dados numéricos , Psicometria/métodos , Pré-Escolar , Avaliação Educacional/estatística & dados numéricos , Feminino , Humanos , Lactente , Recém-Nascido , Masculino , North Carolina , Psicometria/estatística & dados numéricos , Reprodutibilidade dos Testes , Sensibilidade e Especificidade , Ensino/estatística & dados numéricos
9.
BMJ Case Rep ; 17(8)2024 Aug 08.
Artigo em Inglês | MEDLINE | ID: mdl-39122381

RESUMO

Vitamin K is an essential dietary cofactor required for the synthesis of active forms of vitamin K-dependent procoagulant proteins. Vitamin K deficiency, particularly late-onset deficiency occurring between 1 week and 6 months of age, can cause a life-threatening bleeding disorder. An exclusively breastfed, full-term, 6-week-old infant male presented with severe haemorrhagic shock and multi-system organ failure related to caregiver refusal of intramuscular vitamin K after birth. Coagulation studies were normalised within 8 hours of intramuscular vitamin K administration. An increasing number of caregivers are refusing intramuscular vitamin K which has led to a rise in the incidence of vitamin K deficiency bleeding. Health policy organisations around the world emphasise the benefits of intramuscular vitamin K and risks of refusal, particularly in exclusively breastfed infants who are at higher risk due to low vitamin K levels in breast milk. This case highlights the multi-system severity of this life-threatening yet preventable disorder.


Assuntos
Insuficiência de Múltiplos Órgãos , Choque Hemorrágico , Deficiência de Vitamina K , Vitamina K , Humanos , Masculino , Insuficiência de Múltiplos Órgãos/etiologia , Deficiência de Vitamina K/complicações , Lactente , Choque Hemorrágico/etiologia , Vitamina K/uso terapêutico , Vitamina K/administração & dosagem , Aleitamento Materno , Sangramento por Deficiência de Vitamina K/diagnóstico , Injeções Intramusculares , Recusa do Paciente ao Tratamento
10.
Sch Psychol ; 2023 Nov 16.
Artigo em Inglês | MEDLINE | ID: mdl-37971816

RESUMO

Teacher stress is at an all-time high. The COVID-19 pandemic created unprecedented challenges for teachers, which resulted in a record number of teachers intending to leave the classroom citing stress as a contributing factor. Understanding teachers' positive and negative psychological experiences, or well-being, during the height of the COVID-19 pandemic will inform teacher wellness interventions intended to keep teachers in the classroom. Two hundred forty-six teachers from a suburban school district participated in a survey of well-being indicators, including affect, perceived stress, teaching self-efficacy, and preventive coping. A latent profile analysis identified four patterns of well-being: strained, resilient, lower teaching self-efficacy, and lower emotional well-being. While many teachers were resilient during the crisis, approximately two-thirds experienced global or isolated areas of strain. Additionally, the Classroom Appraisal of Resources and Demands-Revised, a theory-driven assessment of teachers' workplace appraisal of demands and resources, was associated with teachers' strained and resilient well-being profiles, making it a suitable screening tool for these groups. Layered screening and tailored intervention, based on teachers' well-being patterns, may help minimize teacher attrition during and postcrises. (PsycInfo Database Record (c) 2023 APA, all rights reserved).

11.
Sch Psychol ; 38(6): 370-384, 2023 Nov.
Artigo em Inglês | MEDLINE | ID: mdl-38127528

RESUMO

School psychologists have the psychological and consultative expertise necessary to support teachers who are vulnerable to stress. Transactional theory offers a lens to guide such support, as it posits that each teacher's unique appraisals of their work demands and resources determine the degree to which they are at risk for stress. This study used a multiphase sequential mixed method design with a transactional theory lens to examine the association of leadership quality and stability with teachers' ratings of workplace conditions. The four phases consisted of (a) input from an expert panel, (b) scale development and validation, (c) interviews with key informants, and (d) multilevel modeling informed by all previous phases. Through key informant interviews, district-level administrators provided ratings of the quality and stability of school leadership. The researchers examined the associations between these ratings and teacher appraisals of classroom demands, classroom resources, job satisfaction, and perceived levels of instructional support collected via a district-wide climate survey. Multilevel models with key informant ratings and school characteristics at Level 2 (Nschools = 47) and teacher characteristics and perceptions at Level 1 (Nteachers = 1,850) demonstrated that the quality and stability of school leadership were associated with teachers' appraisals of their occupational demands and resources, job satisfaction, and ratings of instructional support. Findings show that the quality and stability of school leadership play an important role in the incidence of stress vulnerability, suggesting important pathways for school psychologists seeking to promote the occupational health of teachers. (PsycInfo Database Record (c) 2023 APA, all rights reserved).


Assuntos
Pessoal de Educação , Liderança , Humanos , Instituições Acadêmicas , Inquéritos e Questionários , Professores Escolares/psicologia
12.
Langmuir ; 27(16): 9687-95, 2011 Aug 16.
Artigo em Inglês | MEDLINE | ID: mdl-21449570

RESUMO

Some aspects of metal-catalyzed heterogeneous enantioselective reactions are reviewed with specific reference to four different systems where the phenomena that control enantioselection appear to be very different. In the case of glucose electro-oxidation, it is clear that any intrinsic chirality present at the metal surface plays a vital role. With α-keto hydrogenation, achiral surfaces modified by the adsorption of chiral agents become effective enantioselective catalysts and the formation of extended arrays of chiral species appears not to be of importance: instead a 1:1 docking interaction controlled by hydrogen bonding between the adsorbed chiral modifier and the prochiral reactant determines the outcome. Hydrogen bonding also plays a central role in ß-ketoester hydrogenation, but here fundamental studies indicate that the formation of ordered arrays involving the reactant and chiral ligand is of importance. Asymmetric C═C hydrogenation, though relatively little studied, has the potential for major impact in synthetic organic chemistry both on the laboratory scale and in the manufacture of fine chemicals and pharmaceuticals. The structural attributes that determine whether a given chiral ligand is effective have been identified; the ability to form strong covalent bonds with the metal surface while also resisting hydrogenation and displacement by the strongly adsorbing reactant under reaction conditions is an essential necessary condition. Beyond this, ligand rigidity in the vicinity of the chirality center coupled with resistance to SAM formation is a critically important factor whose absence results in racemic chemistry.


Assuntos
Metais/química , Catálise , Glucose/química , Hidrogenação , Oxirredução , Estereoisomerismo
13.
Pediatr Crit Care Med ; 12(6): 655-9, 2011 Nov.
Artigo em Inglês | MEDLINE | ID: mdl-21725274

RESUMO

OBJECTIVES: The use of bedside ultrasound in critically ill adults has become standard practice. The current state of bedside ultrasound use in pediatric critical illness is unknown. The purpose of this study was to describe the use of bedside ultrasound in critically ill children with an emphasis on its use for establishing central vascular access. We also sought to describe current methods of training for bedside ultrasound use in pediatric critical care. DESIGN: We conducted a cross-sectional survey on the use of bedside ultrasound in pediatric intensive care units in the United States. MEASUREMENTS AND MAIN RESULTS: Pediatric critical care medical directors or their representatives from 128 of 230 eligible hospitals responded (56% response rate). The use of bedside ultrasound for vascular access was statistically more likely in units with >12 beds, >1,000 yearly admissions, and those with an active fellowship or pediatric cardiothoracic surgery program. Ultrasound was used at least once for vascular access in 82% (105 of 128) of responders, with 86% (90 of 105) using it on a regular basis. When using bedside ultrasound for vascular access, the preferred site is the internal jugular vein. A significant portion of responders use bedside ultrasound for nonvascular procedures such as assessing pleural or pericardial effusions. Despite the widespread use of bedside ultrasound, formal training is rare, occurring in only 20% (18 of 90) of current institutions that utilize bedside ultrasound. CONCLUSIONS: This national survey of the use of bedside ultrasound in pediatric critical care reveals widespread use of the technology. When using bedside ultrasound for vascular access, the preferred site is the internal jugular vein. Despite widespread use of bedside ultrasound, formal training that occurs routinely in other subspecialties is lacking. This survey provides meaningful demographic data that can be useful in planning future prospective studies and implementing formal training in bedside ultrasound for pediatric critical care fellows.


Assuntos
Unidades de Terapia Intensiva Pediátrica , Sistemas Automatizados de Assistência Junto ao Leito/estatística & dados numéricos , Ultrassonografia/estatística & dados numéricos , Cateterismo Venoso Central , Estudos Transversais , Humanos , Veias Jugulares/diagnóstico por imagem , Estados Unidos
14.
J Prim Prev ; 32(3-4): 147-60, 2011 Aug.
Artigo em Inglês | MEDLINE | ID: mdl-21833787

RESUMO

This study evaluated the effectiveness of the Adults and Children Together (ACT) Against Violence Parents Raising Safe Kids program, developed by the American Psychological Association in collaboration with the National Association for the Education of Young Children, as an economical primary prevention intervention for child maltreatment. Using an experimental design with random assignment to groups, program impact on participating parents' knowledge, behavior, and attitudes compared to those of a comparison group of parents receiving standard community-based support services was examined. As hypothesized, the ACT Parents Raising Safe Kids program achieved positive results in several areas related to effective parenting, including a reduction in the use of harsh verbal and physical discipline and an increase in nurturing behavior. Positive results were observable both at the conclusion of the ACT program and at three-month follow-up. Results further indicated a positive impact on parent expectations and social support for those parents with the greatest need in these areas. Qualitative data collected through focus groups demonstrated that parents themselves perceived numerous benefits to the ACT program, including assistance in controlling their anger, learning and implementing better parenting and discipline strategies, and recognizing when their child's behavior is developmentally appropriate. Overall, findings suggest that the ACT Parents Raising Safe Kids program is a promising primary prevention strategy that can be implemented across diverse community settings.


Assuntos
Maus-Tratos Infantis/prevenção & controle , Poder Familiar/psicologia , Pais/psicologia , Prevenção Primária/métodos , Desenvolvimento de Programas/métodos , Adulto , Fatores Etários , Criança , Coleta de Dados , Avaliação Educacional , Escolaridade , Feminino , Grupos Focais , Educação em Saúde/métodos , Humanos , Masculino , Relações Pais-Filho , Avaliação de Programas e Projetos de Saúde , Psicometria , Apoio Social , Estatística como Assunto , Inquéritos e Questionários , Violência/prevenção & controle
15.
Nanomaterials (Basel) ; 11(11)2021 Oct 28.
Artigo em Inglês | MEDLINE | ID: mdl-34835645

RESUMO

The production of syngas (H2 and CO)-a key building block for the manufacture of liquid energy carriers, ammonia and hydrogen-through the dry (CO2-) reforming of methane (DRM) continues to gain attention in heterogeneous catalysis, renewable energy technologies and sustainable economy. Here we report on the effects of the metal oxide support (γ-Al2O3, alumina-ceria-zirconia (ACZ) and ceria-zirconia (CZ)) on the low-temperature (ca. 500-750 ∘C) DRM activity, selectivity, resistance against carbon deposition and iridium nanoparticles sintering under oxidative thermal aging. A variety of characterization techniques were implemented to provide insight into the factors that determine iridium intrinsic DRM kinetics and stability, including metal-support interactions and physicochemical properties of materials. All Ir/γ-Al2O3, Ir/ACZ and Ir/CZ catalysts have stable DRM performance with time-on-stream, although supports with high oxygen storage capacity (ACZ and CZ) promoted CO2 conversion, yielding CO-enriched syngas. CZ-based supports endow Ir exceptional anti-sintering characteristics. The amount of carbon deposition was small in all catalysts, however decreasing as Ir/γ-Al2O3 > Ir/ACZ > Ir/CZ. The experimental findings are consistent with a bifunctional reaction mechanism involving participation of oxygen vacancies on the support's surface in CO2 activation and carbon removal, and overall suggest that CZ-supported Ir nanoparticles are promising catalysts for low-temperature dry reforming of methane (LT-DRM).

16.
J Am Chem Soc ; 132(35): 12246-8, 2010 Sep 08.
Artigo em Inglês | MEDLINE | ID: mdl-20715838

RESUMO

XPS, TEM, and reaction studies were used to examine the catalytic behavior of gold species deposited on lanthana toward the cross-coupling of phenylacetylene and iodobenzene. Atomically dispersed Au(I) and Au(III) were catalytically inert, whereas metallic Au(0) nanoparticles were both active and very selective. Thus it is metallic gold and not ionic gold that provides the catalytically active sites. Au(0) nanoparticles supported on silica, gamma-alumina, and BaO were active but relatively unselective; however, as with lanthana, ceria-supported Au(0) nanoparticles showed high selectivity. This strong promoting effect of the lanthanide oxide supports on Sonogashira selectivity cannot be accounted for in terms of acid/base, redox, or SMSI effects; it may be tentatively ascribed to metal --> support hydrogen spillover.


Assuntos
Acetileno/análogos & derivados , Ouro/química , Iodobenzenos/química , Nanopartículas Metálicas/química , Acetileno/síntese química , Acetileno/química , Catálise , Domínio Catalítico , Estrutura Molecular
17.
J Am Chem Soc ; 132(23): 8081-6, 2010 Jun 16.
Artigo em Inglês | MEDLINE | ID: mdl-20491472

RESUMO

Temperature-programmed reaction measurements supported by scanning tunneling microscopy have shown that phenylacetylene and iodobenzene react on smooth Au(111) under vacuum conditions to yield biphenyl and diphenyldiacetylene, the result of homocoupling of the reactant molecules. They also produce diphenylacetylene, the result of Sonogashira cross-coupling, prototypical of a class of reactions that are of paramount importance in synthetic organic chemistry and whose mechanism remains controversial. Roughened Au(111) is completely inert toward all three reactions, indicating that the availability of crystallographically well-defined adsorption sites is crucially important. High-resolution X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure spectroscopy show that the reactants are initially present as intact, essentially flat-lying molecules and that the temperature threshold for Sonogashira coupling coincides with that for C-I bond scission in the iodobenzene reactant. The fractional-order kinetics and low temperature associated with desorption of the Sonogashira product suggest that the reaction occurs at the boundaries of islands of adsorbed reactants and that its appearance in the gas phase is rate-limited by the surface reaction. These findings demonstrate unambiguously and for the first time that this heterogeneous cross-coupling chemistry is an intrinsic property of extended, metallic pure gold surfaces: no other species, including solvent molecules, basic or charged (ionic) species are necessary to mediate the process.

18.
Med Confl Surviv ; 26(3): 207-22, 2010.
Artigo em Inglês | MEDLINE | ID: mdl-21291168

RESUMO

The objective of this study was to determine whether perceived health status of Iraqi immigrants and refugees residing in the United States was related to pre-migration environmental stress, current unemployment, and if they had emigrated before or after the 1991 Gulf War. A random sample of Iraqis residing in Southeast Michigan, US, was interviewed using an Arab language structured survey. The main outcome measure was self-rated health (SRH). Major predictors included socioeconomics, employment status, pre-migration environmental stress, and health disorders. Path analysis was used to look at mediating effects between predictors and SRH. We found that SRH was significantly worse among participants that had left Iraq after the 1991 Gulf War. Unemployment and environmental stress exposure were inversely related to SRH. There was a direct path between Gulf War exposure and poor health. In addition, there were indirect paths mediated through psychosomatic and psychiatric disorders to SRH. Another path went from Gulf War exposure, via environmental stress and somatic health to poor health. Unemployment had a direct path, as well as indirect paths mediated through psychiatric and psychosomatic disorders, to poor self-rated health. In conclusion, these results suggest that pre- as well as post-migration factors, and period of migration, affect health.


Assuntos
Emigrantes e Imigrantes/psicologia , Nível de Saúde , Refugiados/psicologia , Adulto , Feminino , Guerra do Golfo , Humanos , Masculino , Michigan , Pessoa de Meia-Idade , Qualidade de Vida , Fatores Socioeconômicos , Estresse Psicológico , Desemprego/psicologia , Desemprego/estatística & dados numéricos
19.
J Am Chem Soc ; 131(47): 17286-90, 2009 Dec 02.
Artigo em Inglês | MEDLINE | ID: mdl-19888721

RESUMO

The adsorption and hydrogenation of acrolein on the Ag(111) surface has been investigated by high resolution synchrotron XPS, NEXAFS, and temperature programmed reaction. The molecule adsorbs intact at all coverages and its adsorption geometry is critically important in determining chemoselectivity toward the formation of allyl alcohol, the desired but thermodynamically disfavored product. In the absence of hydrogen adatoms (H(a)), acrolein lies almost parallel to the metal surface; high coverages force the C=C bond to tilt markedly, likely rendering it less vulnerable toward reaction with hydrogen adatoms. Reaction with coadsorbed H(a) yields allyl alcohol, propionaldehyde, and propanol, consistent with the behavior of practical dispersed Ag catalysts operated at atmospheric pressure: formation of all three hydrogenation products is surface reaction rate limited. Overall chemoselectivity is strongly influenced by secondary reactions of allyl alcohol. At low H(a) coverages, the C=C bond in the newly formed allyl alcohol molecule is strongly tilted with respect to the surface, rendering it immune to attack by H(a) and leading to desorption of the unsaturated alcohol. In contrast with this, at high H(a) coverages, the C=C bond in allyl alcohol lies almost parallel to the surface, undergoes hydrogenation by H(a), and the saturated alcohol (propanol) desorbs.

20.
J Am Chem Soc ; 131(40): 14584-9, 2009 Oct 14.
Artigo em Inglês | MEDLINE | ID: mdl-19754049

RESUMO

Asymmetric hydrogenation of C=C bonds is of the highest importance in organic synthesis, and such reactions are currently carried out with organometallic homogeneous catalysts. Achieving heterogeneous metal-catalyzed hydrogenation, a highly desirable goal, necessitates forcing the crucial enantiodifferentiating step to take place at the metal surface. By synthesis and application of six chiral sulfide ligands that anchor robustly to Pd nanoparticles and resist displacement, we have for the first time accomplished heterogeneous enantioselective catalytic hydrogenation of isophorone. High resolution XPS data established that ligand adsorption from solution occurred exclusively on the Pd nanoparticles and not on the carbon support. All ligands contained a pyrrolidine nitrogen to enable their interaction with the isophorone substrate while the sulfide functionality provided the required interaction with the Pd surface. Enantioselective turnover numbers of up to approximately 100 product molecules per ligand molecule were found with a very large variation in asymmetric induction between ligands: observed enantiomeric excesses increased with increasing size of the alkyl group in the sulfide. This likely reflects varying degrees of ligand dispersion on the surface: bulky substituent groups hinder close approach of ligand molecules to each other, inhibiting close-packed island formation, favoring dispersion as separate molecules, and leading to effective asymmetric induction. Conversely, small substituents favor island formation leading to very low asymmetric induction. Enantioselective reaction most likely involves initial formation of an enamine or iminium species, confirmed by use of an analogous tertiary amine, which leads to racemic product. Ligand rigidity and resistance to self-assembled monolayer formation are important attributes that should be designed into improved chiral modifiers.

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