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1.
Artigo em Inglês | MEDLINE | ID: mdl-32343867

RESUMO

The reaction of a Lewis acidic borane with an alkyne is a key step in a diverse range of main group transformations. Alkyne 1,1-carboboration, the Wrackmeyer reaction, is an archetypal transformation of this kind. 1,1-Carboboration has been proposed to proceed through a zwitterionic intermediate. We report the isolation and spectroscopic, structural and computational characterization of the zwitterionic intermediates generated by reaction of B(C6 F5 )3 with alkynes. The stepwise reactivity of the zwitterion provides new mechanistic insight for 1,1-carboboration and wider B(C6 F5 )3 catalysis. Making use of intramolecular stabilization by a ferrocene substituent, we have characterized the zwitterionic intermediate in the solid state and diverted reactivity towards alkyne cyclotrimerization.

2.
Chemistry ; 2020 Apr 18.
Artigo em Inglês | MEDLINE | ID: mdl-32304609

RESUMO

Hydroboration is an emerging method for mild and selective reduction of carbonyl compounds. Typically, transition-metal or reactive main-group hydride catalysts are used in conjunction with a mild reductant such as pinacolborane. The reactivity of the main-group catalysts is a consequence of the nucleophilicity of their hydride ligands. Silicon hydrides are significantly less reactive and are therefore not efficient hydroboration catalysts. Here, a readily prepared silyl anion is reported to be an effective initiator for the reduction of aldehydes and ketones requiring mild conditions, low catalyst loadings and with a good substrate scope. The silyl anion it is shown to activate HBpin to generate a reactive borohydride in situ which reacts with aldehydes and ketones to afford the hydroboration product.

3.
Inorg Chem ; 58(17): 11439-11448, 2019 Sep 03.
Artigo em Inglês | MEDLINE | ID: mdl-31411460

RESUMO

Aluminum hydrides, once a simple class of stoichiometric reductants, are now emerging as powerful catalysts for organic transformations such as the hydroboration or hydrogenation of unsaturated bonds. The coordination chemistry of aluminum hydrides supported by P donors is relatively underexplored. Here, we report aluminum dihydride and dimethyl complexes supported by amidophosphine ligands and study their coordination behavior in solution and in the solid state. All complexes exist as κ2-N,P complexes in the solid state. However, we find that for amidophosphine ligands bearing bulky aminophosphine donors, aluminum dihydride and dimethyl complexes undergo a "ligand-slip" rearrangement in solution to generate κ2-N,N complexes. Thus, importantly for catalytic activity, we find that the coordination behavior of the P donor can be modulated by controlling its steric bulk. We show that the reported aluminum hydrides catalyze the hydroboration of alkynes by HBPin and that the variable coordination mode exhibited by the amidophosphine ligand modulates the catalytic activity.

4.
Angew Chem Int Ed Engl ; 58(5): 1329-1333, 2019 Jan 28.
Artigo em Inglês | MEDLINE | ID: mdl-30461143

RESUMO

The equilibrium between disilenes (R2 Si=SiR2 ) and their silylsilylene (R3 Si-SiR) isomers has previously been inferred but not directly observed, except in the case of the parent system H2 Si=SiH2 . Here, we report a new method to prepare base-coordinated disilenes with hydride substituents. By varying the bulk of the coordinating base and other silicon substituents, we have been able to control the rearrangement of disilene adducts to their silylsilylene tautomers. Remarkably, 1,2 migration of a trimethylsilyl group is preferred over hydrogen migration. A DFT study of the reaction mechanism provides a rationale for the observed reactivity and detailed information on the bonding situation in base-stabilized disilenes.

5.
Chem Commun (Camb) ; 54(60): 8399-8402, 2018 Jul 24.
Artigo em Inglês | MEDLINE | ID: mdl-29998230

RESUMO

1,2,3-Trisilacyclopentadienes are obtained from the reactions of cyclotrisilene c-Si3R4 (R = iPr3C6H2) with phenyl and diphenyl acetylene, respectively. With 1,4-diethynyl benzene the cross-conjugated bridging of two of the Si3C2 cycles by a para-phenylene linker is achieved. UV/vis spectroscopy indicates a small but significant effect of cross-conjugation, which is confirmed by TD-DFT calculations. The formation mechanism of the 1,2,3-trisilacyclopentadienes is elucidated by VT NMR.

6.
Chemistry ; 24(3): 672-680, 2018 Jan 12.
Artigo em Inglês | MEDLINE | ID: mdl-29119625

RESUMO

Four-membered rings with a P2 BCh core (Ch=S, Se) have been synthesized by the reaction of phosphinidene chalcogenide (Ar*P=Ch) and phosphaborene (Mes*P=BNR2 ). The mechanistic pathways towards these rings are explained by detailed computational work that confirmed the preference for the formation of P-P, not P-B, bonded systems, which seems counterintuitive given that both phosphorus atoms contain bulky ligands. The reactivity of the newly synthesized heterocycles, as well as that of the known (RPCh)n rings (n=2, 3), was probed by the addition of N-heterocyclic carbenes, which revealed that all investigated compounds can act as sources of low-coordinate phosphorus species.

7.
Chem Commun (Camb) ; 54(4): 378-380, 2018 Jan 04.
Artigo em Inglês | MEDLINE | ID: mdl-29242890

RESUMO

Cp*Al reacts with diphenylacetylene to form a Cp*-substituted 1,4-dialuminacyclohexene. The dialuminacyclohexene reacts with four equivalents of an isonitrile to couple the terminal carbon atoms, forming 6 new carbon-carbon bonds and resulting in a zwitterionic diamide ligand which contains a carbocationic backbone.

8.
Angew Chem Int Ed Engl ; 56(33): 9953-9957, 2017 08 07.
Artigo em Inglês | MEDLINE | ID: mdl-28643472

RESUMO

Formal exchange of C=C units with isoelectronic B=N or B=P units can provide access to molecules with unique electronic or chemical properties. Herein, we report the simple solution-phase generation of highly reactive phosphaborenes, RP=BR, and demonstrate their use for the introduction of P=B units into organic systems. Ring opening of a P-B-containing cyclobutene isostere provided access to unique 1,4-boraphosphabutadiene systems with conjugated main-group multiple bonds.

9.
Angew Chem Int Ed Engl ; 55(49): 15356-15359, 2016 12 05.
Artigo em Inglês | MEDLINE | ID: mdl-27860199

RESUMO

An aluminum-catalyzed hydroboration of alkynes using either the commercially available aluminum hydride DIBAL-H or bench-stable Et3 Al⋅DABCO as the catalyst and H-Bpin as both the boron reagent and stoichiometric hydride source has been developed. Mechanistic studies revealed a unique mode of reactivity in which the reaction is proposed to proceed through hydroalumination and σ-bond metathesis between the resultant alkenyl aluminum species and HBpin, which acts to drive turnover of the catalytic cycle.

10.
Dalton Trans ; 45(35): 13695-9, 2016 Sep 21.
Artigo em Inglês | MEDLINE | ID: mdl-27530604

RESUMO

Oxidative addition of inert bonds at low-valent main-group centres is becoming a major class of reactivity for these species. The reverse reaction, reductive elimination, is possible in some cases but far rarer. Here, we present a mechanistic study of reductive elimination from Al(iii) centres and unravel ligand effects in this process. Experimentally determined activation and thermodynamic parameters for the reductive elimination of Cp*H from Cp*2AlH are reported, and this reaction is found to be inhibited by the addition of Lewis bases. We find that C-H oxidative addition at Al(i) centres proceeds by initial protonation at the low-valent centre.

11.
Catheter Cardiovasc Interv ; 88(7): 1027-1033, 2016 Dec.
Artigo em Inglês | MEDLINE | ID: mdl-27037544

RESUMO

INTRODUCTION: Coronary angiography and angioplasty via transradial approach is shown to be associated with significant reduction in access site complications. Due to a lack of sufficient data, the use of the right or left radial approach is still operator-dependent. We performed a meta-analysis of prospective randomized studies to compare right versus left radial artery approach for coronary procedures. METHODS: We found 12 randomized studies meeting the predetermined inclusion criteria. A total of 6,450 patients were included in the meta-analysis of which 3,217 patients underwent coronary procedures via right radial approach and 3,233 patients via left radial approach. The primary endpoint was the comparison of fluoroscopy time, procedure time, contrast use and cross-over rates between two radial approaches. RESULTS: Pooled analysis of the included studies showed a similar rate of cross-over events (4.2% for right radial approach vs. 4.1% for left radial approach, odds ratio (OR)=1.08, P = 0.68), and similar total procedure times (18.8 ± 10.3 min vs. 18.1 ± 10.0 min, standard difference (SD) of the mean = 0.09, P = 0.162) between the two radial approaches. However, the right radial approach was found to be associated with minimally longer fluoroscopy times (5.8 ± 4.4 min vs. 5.3 ± 4.2 min, SD of the mean = 0.157, P < 0.001) and greater contrast use (84 ± 35 mL vs. 82 ± 34 mL, SD of the mean = 0.082, P = 0.003). Access site complications and the incidence of stroke were similar between two radial approaches. CONCLUSION: Our meta-analysis suggests a small but statistically significant difference in terms of contrast use and fluoroscopy time in favor of coronary procedures performed via left radial approach in comparison to right radial approach without any significant difference in access site or other procedural complications between the two radial approaches. © 2016 Wiley Periodicals, Inc.


Assuntos
Cateterismo Periférico/métodos , Angiografia Coronária/métodos , Intervenção Coronária Percutânea/métodos , Artéria Radial , Idoso , Cateterismo Periférico/efeitos adversos , Distribuição de Qui-Quadrado , Meios de Contraste/administração & dosagem , Angiografia Coronária/efeitos adversos , Feminino , Fluoroscopia , Humanos , Masculino , Pessoa de Meia-Idade , Razão de Chances , Intervenção Coronária Percutânea/efeitos adversos , Punções , Doses de Radiação , Exposição à Radiação , Radiografia Intervencionista , Ensaios Clínicos Controlados Aleatórios como Assunto , Fatores de Risco , Fatores de Tempo , Resultado do Tratamento
12.
Chemistry ; 22(18): 6248-52, 2016 Apr 25.
Artigo em Inglês | MEDLINE | ID: mdl-26918876

RESUMO

We report the preparation of N-heterocyclic carbene (NHC)-stabilized compounds containing P=B double bonds. The reaction of the highly functionalized phosphinoborane Mes*(SiMe3 )P-B(Cl)Cp* with Lewis bases allows access to base-stabilized phosphinidene boranes Mes*P=B(L)Cp* (L=4-dimethylaminopyridine (DMAP), NHC) by Me3 SiCl elimination. The formation of these species is shown to proceed through transient borylphosphide anions generated by Me3 Si abstraction.

13.
Angew Chem Int Ed Engl ; 54(2): 683-6, 2015 Jan 07.
Artigo em Inglês | MEDLINE | ID: mdl-25418506

RESUMO

The reactivity of the 2-phosphaethynolate anion (PCO(-)) towards a cyclic trisilene (cSi3(Tip)4) is reported. The result is the net activation of the P-C and Si-Si multiple bonds of the precursors affording a heteroatomic bicyclo[1.1.1]pentan-2-one analogue ([P(CO)Si3(Tip)4](-); 1). This reaction can be interpreted as the formal addition of a phosphide and a carbonyl across the Si-Si double bond. Photolytic decarbonylation of 1 results in the incorporation of the phosphide vertex into the cyclotrisilene scaffold, yielding a congener of the cyclobutene anion with considerable allylic character.

14.
J Phys Chem B ; 119(4): 1416-24, 2015 Jan 29.
Artigo em Inglês | MEDLINE | ID: mdl-25539423

RESUMO

We report on a strategy for using SABRE (signal amplification by reversible exchange) for polarizing (1)H and (13)C nuclei of weakly interacting ligands which possess biologically relevant and nonaromatic motifs. We first demonstrate this via the polarization of acetonitrile, using Ir(IMes)(COD)Cl as the catalyst precursor, and confirm that the route to hyperpolarization transfer is via the J-coupling network. We extend this work to the polarization of propionitrile, benzylnitrile, benzonitrile, and trans-3-hexenedinitrile in order to assess its generality. In the (1)H NMR spectrum, the signal for acetonitrile is enhanced 8-fold over its thermal counterpart when [Ir(H)2(IMes)(MeCN)3](+) is the catalyst. Upon addition of pyridine or pyridine-d5, the active catalyst changes to [Ir(H)2(IMes)(py)2(MeCN)](+) and the resulting acetonitrile (1)H signal enhancement increases to 20- and 60-fold, respectively. In (13)C NMR studies, polarization transfers optimally to the quaternary (13)C nucleus of MeCN while the methyl (13)C is hardly polarized. Transfer to (13)C is shown to occur first via the (1)H-(1)H coupling between the hydrides and the methyl protons and then via either the (2)J or (1)J couplings to the respective (13)Cs, of which the (2)J route is more efficient. These experimental results are rationalized through a theoretical treatment which shows excellent agreement with experiment. In the case of MeCN, longitudinal two-spin orders between pairs of (1)H nuclei in the three-spin methyl group are created. Two-spin order states, between the (1)H and (13)C nuclei, are also created, and their existence is confirmed for Me(13)CN in both the (1)H and (13)C NMR spectra using the Only Parahydrogen Spectroscopy protocol.


Assuntos
Acetonitrilos/química , Ligantes , Espectroscopia de Ressonância Magnética
15.
Dalton Trans ; 44(3): 1077-83, 2015 Jan 21.
Artigo em Inglês | MEDLINE | ID: mdl-25410259

RESUMO

The short lived pincer complex [(C5H3N(CH2P((t)Bu)2)2)Ir(H)2(py)]BF4 is shown to be active for signal amplification by reversible exchange. This catalyst formulation enables the efficient transfer of polarization from parahydrogen to be placed into just a single molecule of the hyperpolarisation target, pyridine. When the catalysts (1)H nuclei are replaced by (2)H, increased levels of substrate hyperpolarization result and when the reverse situation is examined the catalyst itself is clearly visible through hyperpolarised signals. The ligand exchange pathways of [(C5H3N(CH2P((t)Bu)2)2)Ir(H)2(py)]BF4 that are associated with this process are shown to involve the formation of 16-electron [(C5H3N(CH2P((t)Bu)2)2)Ir(H)2]BF4 and the 18-electron H2 addition product [(C5H3N(CH2P((t)Bu)2)2)Ir(H)2(H2)]BF4.


Assuntos
Complexos de Coordenação/química , Irídio/química , Complexos de Coordenação/síntese química , Cristalografia por Raios X , Hidrogênio/química , Espectroscopia de Ressonância Magnética , Conformação Molecular , Piridinas/química
16.
Chemistry ; 20(30): 9221-4, 2014 Jul 21.
Artigo em Inglês | MEDLINE | ID: mdl-24827791

RESUMO

In the presence of a Lewis acid or base, cyclotrisilene cSi3Tip4 (Tip = 2,4,6-triisopropylphenyl) reacts with CO to form stable adducts of a 1,3-disila-2-oxyallyl zwitterion. Coordination of an N-heterocyclic carbene (NHC) to one silicon center results in a localized Si=C bond, whereas B(C6F5)3 bonds to the carbonyl moiety leading to the delocalization of the positive charge over the oxyallyl moiety. Computational analysis of the electronic structure of the Lewis acid adduct reveals a considerable interaction between the two silicon atoms in 1,3-position and thus aromaticity akin to the homocyclopropenium cation.

17.
Magn Reson Chem ; 52(7): 358-69, 2014 Jul.
Artigo em Inglês | MEDLINE | ID: mdl-24801201

RESUMO

Hyperpolarization methods are used in NMR to overcome its inherent sensitivity problem. Herein, the biologically relevant target nicotinamide is polarized by the hyperpolarization technique signal amplification by reversible exchange. We illustrate how the polarization transfer field, and the concentrations of parahydrogen, the polarization-transfer-catalyst and substrate can be used to maximize signal amplification by reversible exchange effectiveness by reference to the first-order spin system of this target. The catalyst is shown to be crucial in this process, first by facilitating the transfer of hyperpolarization from parahydrogen to nicotinamide and then by depleting the resulting polarized states through further interaction. The 15 longitudinal one, two, three and four spin order terms produced are rigorously identified and quantified using an automated flow apparatus in conjunction with NMR pulse sequences based on the only parahydrogen spectroscopy protocol. The rates of build-up of these terms were shown to follow the order four~three > two > single spin; this order parallels their rates of relaxation. The result of these competing effects is that the less-efficiently formed single-spin order terms dominate at the point of measurement with the two-spin terms having amplitudes that are an order of magnitude lower. We also complete further measurements to demonstrate that (13)C NMR spectra can be readily collected where the long-lived quaternary (13)C signals appear with significant intensity. These are improved upon by using INEPT. In summary, we dissect the complexity of this method, highlighting its benefits to the NMR community and its applicability for high-sensitivity magnetic resonance imaging detection in the future.


Assuntos
Análise de Injeção de Fluxo/instrumentação , Hidrogênio/química , Espectroscopia de Ressonância Magnética/instrumentação , Microquímica/instrumentação , Técnicas de Sonda Molecular/instrumentação , Niacinamida/análise , Niacinamida/química , Desenho de Equipamento , Análise de Falha de Equipamento
18.
Expert Rev Cardiovasc Ther ; 12(4): 463-74, 2014 Apr.
Artigo em Inglês | MEDLINE | ID: mdl-24650313

RESUMO

The proper use of antiplatelet agents in the cardiac catheterization laboratory is important for ensuring optimal results in patients undergoing percutaneous revascularization. Understanding the mechanisms by which these drugs exerts their effects is important for both interventional and non-interventional cardiologists. The effects of these agents on platelet function can be assessed and monitored using a variety of commercially available laboratory assays but so far these tests have not been adopted in routine clinical practice. Currently, aspirin, thienopyridines and glycoprotein IIb/IIIa inhibitors are the primary types of antiplatelet drugs being utilized. The use of these drugs and of several newer antiplatelet drugs in the treatment of patients undergoing percutaneous revascularization in the cardiac catheterization laboratory will be discussed, especially in the light of the recently published guidelines.


Assuntos
Síndrome Coronariana Aguda/terapia , Intervenção Coronária Percutânea , Inibidores da Agregação de Plaquetas/uso terapêutico , Aspirina/uso terapêutico , Humanos , Intervenção Coronária Percutânea/métodos , Complexo Glicoproteico GPIIb-IIIa de Plaquetas/antagonistas & inibidores , Resultado do Tratamento
19.
Angew Chem Int Ed Engl ; 53(8): 2216-20, 2014 Feb 17.
Artigo em Inglês | MEDLINE | ID: mdl-24458444

RESUMO

The generation of heavier double-bond systems without by- or side-product formation is of considerable importance for their application in synthesis. Peripheral functional groups in such alkene homologues are promising in this regard owing to their inherent mobility. Depending on the steric demand of the N-alkyl substituent R, the reaction of disilenide Ar2Si=Si(Ar)Li (Ar = 2,4,6-iPr3C6H2) with ClP(NR2)2 either affords the phosphinodisilene Ar2 Si=Si(Ar)P(NR2)2 (for R = iPr) or P-amino functionalized phosphasilenes Ar2(R2N)Si=Si(Ar)=P(NR2) (for R = Et, Me) by 1,3-migration of one of the amino groups. In case of R = Me, upon addition of one equivalent of tert-butylisonitrile a second amino group shift occurs to yield the 1-aza-3-phosphaallene Ar2(R2N)Si=Si(NR2)(Ar)-P=C=NtBu with pronounced ylidic character. All new compounds were fully characterized by multinuclear NMR spectroscopy as well as single-crystal X-ray diffraction and DFT calculations in selected cases.

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