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Photogenerated Quinone Methides as Useful Intermediates in the Synthesis of Chiral Binol Ligands
No full textThe photoinduced synthesis of chiral 3,3'-CH2X-disubstituted BINOL ligands (X ) NR2, SR, OH) has been achieved with excellent ee by UV-visible activation of BINOLAMs bearing L-proline ester arms. Quinone methides, detected by laser flash photolysis, are the key intermediates involved in such a synthetic protocol, which undergo reversible nucleophilic conjugate additions by a great variety of nitrogen nucleophiles (amines and R-amino acid derivatives) with complete configuration retention of the BINOL moiety
Photoarylation of Alkenes and Heteroaromatics by Dibromo-BINOLs in Aqueous Solution
No full textThe photochemistry of 6,6’-dibromo-BINOLs (BINOL=2,2’-dihydroxy-1,1’-binaphthyl) under mild conditions has been investigated in neat and aqueous acetonitrile through product distribution analysis and laser flash photolysis. Arylation and alkylation have been successfully achieved in the presence of allyltrimethylsilane, ethyl vinyl ether, pyrrole, pyridine, thiophene, benzene, and indole. Such a photoreactivity offers a metal and protecting group free synthetic protocol toward mono- and disubstituted 6-aryl/alkyl BINOLs, since the BINOL chirality is preserved in the photoactivation process
Selective Arylation, Alkenylation, and Cyclization of Dibromonaphthols, Using Visible Light, via Carbene Intermediates
No full textThe photoreactivity of several 3-substituted-1,6-dibromo-2-naphthols has been investigated in neat acetonitrile in the presence of diluted Et3N and in aqueous buffered acetonitrile (pH 8, phosphate buffered), using visible light (450 nm). Hydrobromic acid loss in the presence of the base, for the unsubstituted naphthol, or heterolytic C-Br cleavage directly from the naphtholates, for the more acid 3-substutited naphthols (R=COOCH3, CONH2, CONMe2), generates electrophilic carbene
intermediates, which have been successfully trapped by molecular oxygen, pyrrole, acrylonitrile, ethyl vinyl ether, and allyltrimethylsilane. Product distribution analysis reveals three types of products arising from (i) arylation, (ii) alkenylation, and (iii) cyclization reactions. The generation and the reactivity of a ketocarbene intermediates, as electrophilc diradicals, has been supported by laser flash photolysis, with the detection of both the carbene (λmax 510 nm) and 1,2-naphthoquinone-O-oxide (R=CONMe2, λmax 600 nm) in the presence of oxygen
Photoarylation/Alkylation of Bromo-Naphthols
No full textThe photochemistry of 6-bromo-2-naphthols has been studied in acetonitrile, aqueous acetonitrile, and isopropyl alcohol in the absence and in the presence of triethylamine by product distribution analysis, laser flash photolysis (LFP), fluorescence, phosphorescence, electrochemical measurements, and DFT calculations. Hydrobromic acid loss in the presence of Et3N occurs from the triplet state of 6-bromo-2-naphthol, generating an electrophilic carbene intermediate, which has been successfully trapped by oxygen, allyltrimethylsilane, 2,3-dimethylbut-2-ene, pyrrole, acrylonitrile, 1,4-dimethoxybenzene, and also pyridine.
The generation and the reactivity of a triplet carbene intermediate has been supported by LFP, with the
detection of 2,6-naphthoquinone-O-oxide (530-650 nm) in the presence of O2. The electrophilic diradical character of the carbene has been supported by DFT calculations, using the B3LYP, PBE0, and MPWB1K functionals, with the 6-31+G(d,p) basis set and PCM solvation model
Protecting Group Free Synthesis of 6-Substituted Naphthols and Binols
No full textA straightforward route for the preparation of 6-substituted
naphthols and 6,6'-disubstituted binols (binol=2,2'-dihydroxy-1,1'-binaphthyl) is presented. The synthesis has been accomplished by a one-step procedure starting from 6-bromo derivatives via direct lithiation with n-BuLi, followed by the addition of several electrophiles
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
Substituted Heterocyclic Naphthalene Diimides with Unexpected Acidity. Synthesis, Properties, and Reactivity
No full textNaphthalene bisimides (NDIs) with a heterocyclic 1,4-dihydro-2,3-pyrazinedione moiety have been synthesized from both 2,6-dibromonaphthalene and 2,3,6,7-tetrabromonaphthalene bisanhydrides by means of a stepwise protocol including imidization, nucleophilic displacement of the bromine atoms by ethane-1,2-diamine, in situ reductive dehalogenation, and further oxidation. These heterocycles (R = n-pentyl, cyclohexyl) are yellow dyes with green emission in organic solvent,
where the acid form dominates. The orange nonfluorescent conjugate base can be generated quantitatively by CH3COONBu4 addition in DMSO, where it exhibits a pKa=7.63. The coniugate base becomes the only detectable species (by UV-vis spectroscopy), in water solution, even under acid conditions (pH 1). In aqueous DMSO the acid/base equilibrium is a function of the DMSO/ water ratio. The unexpected acidity of these heterocyclic NDIs, which justifies the reactivity with CH2N2, has been rationalized by DFT computational means [PBE0/6-31+G(d,p)] in aqueous solvent (PCM models) as a result of a strong specific solvation effect, modeled by the inclusion of three water molecules
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
Bipyridyl Ligands as Photoactivatable Mono- and Bis-Alkylating Agents Capable of DNA Cross-Linking
No full textThe photoactivation of 6,6-bis-CH2X-[2,2]bipyridinyl-5,5-
diol ligands as mono and bis-alkylating agents has been investigated, detecting transient heterocyclic quinone methide
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