1,354,655 research outputs found
A library approach to the development of chiral supramolecular ligands for asymmetric hydrogenation
No full textThe creation of chiral ligands of simple structure and truly easy preparation is the condition for supramolecular catalysis to become of practical industrial use. To this end, we developed a library of chiral monodentate phosphites, named BenzaPhos,[1] which can be prepared in only two simple steps from readily available compounds. The new ligands, containing a benzoic acid primary amide group capable of hydrogen bonding interactions, showed excellent activity and stereocontrol in the enantioselective hydrogenation of both benchmark substrates and ‘challenging’ olefins.
A series of experiments and computational studies strongly suggest that ligand-substrate H-bonding crucially affects these outstanding catalytic properties.
[1] L. Pignataro, C. Bovio, M. Civera, U. Piarulli, C. Gennari, Chem. Eur. J. 2012, 18, DOI: 10.1002/chem.201201032
Stereoselective aldol reactions of [gamma]-thiobutyrolactone : The benzaldehyde anomaly
No full textDifferent protocols (lithium enolate reactions, fluoride catalyzed and Lewis acid mediated silyl ketene acetal reactions) were studied to achieve stereoselectivity in the aldol reactions of γ-thiobutyrolactone: in all cases benzaldehyde showed a striking peculiarity compared to aliphatic aldehydes
Recent applications of phosphorus reagents: from organic synthesis to stereoselective catalysis
No full textThis chapter will focus on the use of phosphorus (III) compounds in organic synthesis, as reagents, organic catalysts, as well as ligands for transition metal catalysis. The versatility of these compounds stems from the great variety of their steric and electronic properties, which can be conveniently tuned for different applications. For this reason, after a brief presentation of the nomenclature of organic phosphorus compounds, we will discuss the methods for the determination of the steric and electronic paramenters of mainly phosphorus(III) compounds as a means to interpret their reactivity. The reactivity of PIII derivatives will be presented, focussing on a selection of the most recent achievements in the area of organic synthesis and stereoselective organic and transition metal catalysis
The Four-electron Oxidation of meso-Octaethylporphirinogen via a Metal-mediated Dealkylation Process: Formation of [RuL(PhCN)2] [H2L = 5,15-dihydro-5,5,10,15,20-hexaethylporphyrin].
No full textA four-electron oxidation of meso-octaethylporphyrinogen (H4L') involving the dealkylation of two meso carbons has been observed in the reaction between [Ru(Cl)2(PhCN)4] and [Li4L'(THF)4] leading to [RuL(PhCN)2] which contains two hemiporphyrin fragments joined by two sp3 carbons, as shown by an X-ray analysis
Rh-Catalyzed Enantioselective Conjugate Addition of Arylboronic Acids with a Dynamic Library of Chiral Tropos Phosphorus Ligands
No full textA library of 19 chiral tropos phosphorus ligands, based on a free-torotate (tropos) biphenol unit and a chiral P-bonded alcohol (11 phosphites, 1-P(O)2O to 11-P(O)2O) or secondary amine (8 phosphoramidites, 12-P(O)2N to 19-P(O)2N), were screened, individually
and in combinations of two, in the rhodium-catalyzed asymmetric conjugate addition of arylboronic acids to enones and enoates. High enantioselectivities (up to 99% ee) and excellent yields were obtained in the addition to either cyclic or acyclic substrates. The flexible biphenolic P ligands outperformed the analogous rigid binaphtholic P ligands. Variable-temperature
31P NMR studies revealed that the biphenolic
ligands are tropos even at low temperature. Only below 190 K was a coalescence observed; upon further cooling, two atropisomers were detected.
The Rh homocomplexes ([Rh(La)2]+) were also studied: in general, a single doublet (P–Rh coupling) was observed in the case of the biphenolic phosphite ligands, over the temperature range 380–230 K, demonstrating
their tropos nature in the rhodium complexes even at low temperatures.
On the other hand, the phosphoramidites showed different behaviors depending on the structure of the ligand and on the nature of the rhodium source. The spectrum at 230 K of the mixture of [Rh(acac)(eth)2] (eth=C2H4) with phosphite 6-P(O)2O and phosphoramidite 19-P(O)2N (the most enantioselective ligand combination in the conjugate addition reaction) revealed the presence of four homocomplexes (total approximately 40%: [Rh-
{6-P(O)2O}2], [Rh{(aR)-19-P(O)2N}2], [Rh{(aS)-19-P(O)2N}2], [Rh{(aR)-19-P(O)2N}{(aS)-19-P(O)2N}]) and one heterocomplex, [Rh{6-P(O)2O}{(aR)-19-P(O)2N}] (approximately 60%) In the heterocomplex, the biphenol-derived
phosphite is free to rotate (tropos) while the biphenol-derived phosphoramidite shows a temperaturedependent tropos/atropos behavior (coalescence temperature=310 K)
Effect of multichannel transcranial direct current stimulation to reduce hypertonia in individuals with prolonged disorders of consciousness: A randomized controlled pilot study
No full textBackground: Spasticity management in severely brain-injured patients with disorders of consciousness (DOC) is a major challenge because it leads to complications and severe pain that can seriously affect quality of life. Objectives: We aimed to determine the feasibility of a single session of transcranial direct current stimulations (tDCS) to reduce spasticity in chronic patients with DOC. Methods: We enrolled 14 patients in this double-blind, sham-controlled randomized crossover pilot study. Two cathodes were placed over the left and right primary motor cortex and 2 anodes over the left and right prefrontal cortex. Hypertonia of the upper limbs and level of consciousness were assessed by the Modified Ashworth Scale (MAS) and the Coma Recovery Scale-Revised (CRS-R). Resting state electroencephalography was also performed. Results: At the group level, spasticity was reduced in only finger flexors. Four responders (29%) showed reduced hypertonicity in at least 2 joints after active but not sham stimulation. We found no behavioural changes by the CRS-R total score. At the group level, connectivity values in beta2 were higher with active versus sham stimulation. Relative power in the theta band and connectivity in the beta band were higher for responders than non-responders after the active stimulation. Conclusion: This pilot study highlights the potential benefit of using tDCS for reducing upper-limb hypertonia in patients with chronic DOC. Large-sample clinical trials are needed to optimize and validate the technique
Neural Correlates of Non-ordinary States of Consciousness in Pranayama Practitioners: The Role of Slow Nasal Breathing
No full textThe modulatory effect of nasal respiration on integrative brain functions and hence consciousness has recently been unambiguously demonstrated. This effect is sustained by the olfactory epithelium mechanical sensitivity complemented by the existence of massive projections between the olfactory bulb and the prefrontal cortex. However, studies on slow nasal breathing (SNB) in the context of contemplative practices have sustained the fundamental role of respiratory vagal stimulation, with little attention to the contribution of the olfactory epithelium mechanical stimulation. This study aims at disentangling the effects of olfactory epithelium stimulation (proper of nasal breathing) from those related to respiratory vagal stimulation (common to slow nasal and mouth breathing). We investigated the psychophysiological (cardio-respiratory and electroencephalographic parameters) and phenomenological (perceived state of consciousness) aftereffects of SNB (epithelium mechanical – 2.5 breaths/min) in 12 experienced meditators. We compared the nasal breathing aftereffects with those observed after a session of mouth breathing at the same respiratory rate and with those related to a resting state condition. SNB induced (1) slowing of electroencephalography (EEG) activities (delta-theta bands) in prefrontal regions, (2) a widespread increase of theta and high-beta connectivity complemented by an increase of phase-amplitude coupling between the two bands in prefrontal and posterior regions belonging to the Default Mode Network, (3) an increase of high-beta networks small-worldness. (4) a higher perception of being in a non-ordinary state of consciousness. The emerging scenario strongly suggests that the effects of SNB, beyond the relative contribution of vagal stimulation, are mainly ascribable to olfactory epithelium stimulation. In conclusion, slow Pranayama breathing modulates brain activity and hence subjective experience up to the point of inducing a non-ordinary state of consciousness
A Long Journey through Organic Chemistry – Celebrating Cesare Gennari's 70th Birthday
No full textCesare Gennari has contributed to many research areas in organic chemistry. On the occasion of his 70th birthday, his friends, colleagues and former students contributed to a special collection which covers his main fields of interest
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