1,720,969 research outputs found
Experimental challenges in organic synthesis for the evaluation of molecular structures and for the study of non-covalent interactions
Get PDFIn the last decades, the awareness of the importance of organic chemistry has been growing
amongst researchers working in the most diverse fields of science. Indeed, much of the current welfare
of advanced societies is due to recent huge leaps in the comprehension of the behaviour of matter. In
particular, material and life sciences have been undergoing an impressive development, and their
academic results keep on having a steady growing impact on our daily routine, albeit most people are not
aware of the prompt repercussions of academic research on their own lives. Carbon plays a pivotal role
in material and life sciences, hence nowadays both skills and knowledge of organic chemists are
fundamental. However, the interpretation of experimental data is often non-trivial, and a theoretical
analysis – able to summarize, predict and understand the plethora of experimental data – is important.
Hence, computational studies are highly recommended to guide it, since they can (i) anticipate otherwise
unpredictable results, (ii) indicate new directions for further investigation, and (iii) readjust
misconceptions and false perceptions.
The three main aims of this thesis are:
i. the exploitation of interdisciplinary approaches involving organic chemistry and state-of-the-art
computational methods to gain deeper insights into the studied molecular systems, with special
attention to molecular structures;
ii. the assessment of the importance of organic chemistry in two apparently unrelated fields, namely
astrochemistry and biochemistry;
iii. the evaluation of the effects of non-covalent interactions on molecular geometry, whose detailed
study is important for both astrochemical and biochemical purposes, but also in the field of
catalysis.
The presentation of the previous goals will be as unitary as possible, since such aims are intimately
interconnected with each other.
Finally, some interesting results concerning photocatalysis – a rapidly growing research field of organic
chemistry – will be discussed. In the next years, this field is expected to gain unprecedented development
with the fruitful interplay between experimental and theoretical chemistry. In particular, the composition
of the reaction mixture is known to play a pivotal role in the outcome of the process, due to the reciprocal
interactions at the molecular level between the reaction partners. Furthermore, such interactions are also
strongly affected by the nature of the solvent.
More specifically, three main projects will be discussed.
Project 1: Phenylmethanimine The successful endeavour enabling the generation and characterization of an elusive aromatic imine, i.e.
phenylmethanimine (PMI), will be described. PMI was chosen because it is expected to be an
astrochemically relevant species.
Project 2: Fluorothreonine
A full account concerning synthesis and characterization of the only fluoro amino acid of natural origin
discovered so far, namely 4-fluorothreonine, will be reported. Fluorothreonine was chosen as a test case
for the evaluation of the effects of fluorination on molecular properties. Indeed, fluorothreonine is
expected to feature interesting conformational behaviours due to peculiar non-covalent interactions
involving fluorine.
Project 3: Bismuth-mediated photocatalysis
A bismuth-mediated photocatalytic reaction in aqueous conditions will be described. Bismuth was chosen
as a heavy member of the pnictogens, which are known to be involved in interesting non-covalent
interactions
Ultraviolet Photoprocessing of Glycine Adsorbed on Various Space-Relevant Minerals
Get PDFThe discovery of amino acids such as glycine on meteorites and comets confirms the role of small bodies as transport and delivery vehicles of building blocks of life on Earth and possibly on other planetary bodies of our Solar System. Glycine is quite interesting because it is the simplest of the 20 biogenic amino acids, from which complex organic molecules might have originated in our evolved Solar System. To investigate the possible chemical evolution of this molecule in space, it is important to consider how the interaction with mineral matrices influences its photostability. Indeed, the presence of minerals can mediate the effects of electromagnetic radiation, catalyzing photoreactions, or protecting molecules against degradation. Such interactions are responsible for the preservation/degradation mechanisms of organic molecules in space environments. Laboratory simulations of UV processing may provide key insights into the survival of organic molecules in space environment and rocky surfaces, which is of particular relevance for current missions of sample return from asteroids, such as NASA OSIRIS-REx and JAXA Hayabusa 2, and in particular, upcoming space exploration missions on planetary surfaces, such as ESA-Roscosmos ExoMars 2022 and NASA Mars 2020. In this article, we report a laboratory study of UV irradiation of glycine adsorbed on various space relevant minerals: forsterite, antigorite, spinel, and pyrite. We monitored possible changes of glycine functional groups due to UV irradiation through in situ infrared (IR) spectroscopic analysis. Results show that degradation of glycine occurs with a half-life of 0.5–2 h depending on the mineral substrate. Appearance of new IR bands suggests the occurrence of catalytic reactions mediated by minerals and UV
Al(Salen) Metal Complexes in Stereoselective Catalysis
Get PDFSalen ligands are a class of Schiff bases simply obtained through condensation of two molecules of a hydroxyl-substituted aryl aldehyde with an achiral or chiral diamine. The prototype salen, or N,N′-bis(salicylidene)ethylenediamine has a long history, as it was first reported in 1889, and immediately, some of its metal complexes were also described. Now, the salen ligands are a class of N,N,O,O tetradentate Schiff bases capable of coordinating many metal ions. The geometry and the stereogenic group inserted in the diamine backbone or aryl aldehyde backbone have been utilized in the past to efficiently transmit chiral information in a variety of different reactions. In this review we will summarize the important and recent achievements obtained in stereocontrolled reactions in which Al(salen) metal complexes are employed. Several other reviews devoted to the general applications and synthesis of chromium and other metal salens have already been published
Diastereoselective and enantioselective photoredox pinacol coupling promoted by titanium complexes with a red-absorbing organic dye
Get PDFThe pinacol coupling reaction, a reductive coupling of carbonyl compounds that proceeds through the formation of ketyl radicals in the presence of an electron donor, affords the corresponding 1,2-diols in one single step. The photoredox version of this transformation has been accomplished using different organic dyes or photoactive metal complexes in the presence of sacrificial donors such as tertiary amines or Hantzsch's ester. Normally, the homo-coupling of such reactive ketyl radicals is neither diastereo- nor enantio-selective. Herein, we report a highly diastereoselective pinacol coupling reaction of aromatic aldehydes promoted by 5 mol% of the non-toxic, inexpensive and available Cp2TiCl2 complex. The key feature that allows the complete control of diastereoselectivity is the employment of a red-absorbing organic dye in the presence of a redox-active titanium complex. Taking advantage of the well-tailored photoredox potential of this organic dye, the selective reduction of Ti(IV) to Ti(III) is achieved. These conditions enable the formation of the D,L (syn) diastereoisomer as the favored product of the pinacol coupling (d.r. > 20 : 1 in most of the cases). Moreover, employing a simply prepared chiral SalenTi complex, the new photoredox reaction gave a complete diastereoselection for the D,L diastereoisomer, and high enantiocontrol (up to 92% of enantiomeric excess)
Asymmetric Reactions Enabled by Cooperative Enantioselective Amino- and Lewis Acid Catalysis
No full textOrganocatalysis - the branch of catalysis featuring small organic molecules as the catalysts - has, in the last decade, become of central importance in the field of asymmetric catalysis, so much that it is now comparable to metal catalysis and biocatalysis. Organocatalysis is rationalized and classified by a number of so-called activation modes, based on the formation of a covalent or not-covalent intermediate between the organocatalyst and the organic substrate. Among all the organocatalytic activation modes, enamine and iminium catalysis are widely used for the practical preparation of valuable products and intermediates, both in academic and industrial contexts. In both cases, chiral amines are employed as catalysts. Enamine activation mode is generally employed in the reaction with electrophiles, while nucleophiles require the iminium activation mode. Commonly, in both modes, the reaction occurs through well-organized transitions states. A large variety of partners can react with enamines and iminium ions, due to their sufficient nucleophilicity and electrophilicity, respectively. However, despite the success, organocatalysis still suffers from narrow scopes and applications. Multicatalysis is a possible solution for these drawbacks because the two different catalysts can synergistically activate the substrates, with a simultaneous activation of the two different reaction partners. In particular, in this review we will summarize the reported processes featuring Lewis acid catalysis and organocatalytic activation modes synergically acting and not interfering with each other. We will focus our attention on the description of processes in which good results cannot be achieved independently by organocatalysis or Lewis acid catalysis. In these examples of cooperative dual catalysis, a number of new organic transformations have been developed. The review will focus on the possible strategies, the choice of the Lewis acid and the catalytic cycles involved in the effective reported combination. Additionally, some important key points regarding the rationale for the effective combinations will be also included. π-Activation of organic substrates by Lewis acids, via formation of electrophilic intermediates, and their reaction with enamines will be also discussed in this review
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Oxidative polymerization of hydroxylated naphthalene derivatives: Modeling free radical coupling pathways to PAH-derived melanins of biological and astrochemical relevance
No full textThe oxidative polymerization of hydroxylated naphthalene derivatives, including chiefly 1,8-dihydroxynaphthalene (1,8-DHN), is an important process of broad biological, environmental and astrochemical relevance, being involved for example in the biosynthesis of black melanin pigments in certain pathogenic fungi (Aspergillus fumigatus) and halophilic ascomycetous black yeasts, in the combustion of organic matter, in the metabolic evolution of xenobiotics in water and soil pollution, and in the complex transformations of polycyclic aromatic hydrocarbons (PAHs) in the interstellar medium (ISM). The latter processes, especially, attract growing interest in relation to the abiogenetic theory of the origin of life. PAHs are widely diffuse in the ISM and more than 20% of the carbon in the universe may be associated with PAHs, thus serving as precursors to life molecules. Dust grain chemistry in particular could be central to PAHs reprocessing, since icy matrices can trap several astrochemically relevant CHON-bearing molecules, and mineral catalysis could have played a significant role in prebiotic chemistry. Studies on PAHs reactivity under astromimetic conditions have resulted in the identification of several hydroxylated products, which may represent the initial products of reprocessing. In addition, growing interest is currently focusing on the possible role of PAHs and their oxygenated derivatives as the main determinants of infrared emission features seen in different astrophysical environments. Early experiments showed that exposure of naphthalene to ultraviolet radiation in ice under astrophysical conditions leads to the generation of phenolic and quinone derivatives, allowing specific prediction of the existence and relative abundances of various oxidized naphthalenes in meteorites. However, apart from those studies, the mechanisms of conversion of PAHs under conditions mimicking those occurring in the ISM are largely uncharted, which makes it difficult to probe the actual role of this chemistry in the reprocessing of carbon compounds.
Aim of this thesis is to investigate the oxidative pathways of hydroxylated naphthalenes, including 1-naphthol, 2-naphthol, 1,8-DHN, 1,6-DHN and 2,6-DHN, as representative in order to gain a first insight into the mode of coupling of these compounds, the nature and properties of the resulting polymers and their actual formation under astrochemically relevant conditions. To this aim, the thesis developed along three main phases:
a) the isolation and characterization of the main oligomer intermediates in the oxidative conversion of the selected hydroxynaphthalenes;
b) the synthesis and characterization of the resulting melanin-like polymers;
c) the characterization of the species formed by UV-induced oxidation of hydroxynaphthalenes following adsorption on astrochemically-relevant minerals, by comparison with chemically-produced model polymers.
The main outcome of these studies was:
a) the elucidation of the reaction pathways and mode of coupling of hydroxylated naphthalenes;
b) the definition of basic structure-property relationships in DHN-derived polymers;
c) the characterization of melanin-type materials generated via solid state polymerization on mineral surface.
Overall, these results fill a gap in the chemical literature on the oxidative polymerization of hydroxylated naphthalenes and provide an improved background to inquire into the mechanisms of PAH reprocessing in the ISM and other astrochemically relevant environments
Photoredox Propargylation of Aldehydes Catalytic in Titanium
Get PDFA practical and effective photoredox propargylation of aldehydes promoted by 10 mol % of [Cp2TiCl2] is presented. No stoichiometric metals or scavengers are used for the process. A catalytic amount of the cheap and simply prepared organic dye 3DPAFIPN is used as the reductant for titanium. The reaction displayed a broad scope, and no traces of allenyl isomers were detected for simple propargyl bromide, whereas mixtures of propargyl and allenyl isomers were observed for substituted propargyl bromides
- …
