Search results for "Isomerization"
showing 10 items of 281 documents
Azo-hydrazo conversion via [1,5]-hydrogen shifts. A combined experimental and theoretical study
2012
Abstract Azoalkenes 6e , 6g , 6h , and 8c underwent an easy azo-hydrazo conversion via a [1,5]-hydrogen shift yielding α,β-unsaturated hydrazones. The isomerization products were characterized through spectroscopic and spectrometric techniques. In order to understand the nature of the mechanism of these [1,5]-hydrogen shifts, the transition state structures of the reactions were theoretically studied at the B3LYP/6-31G(d,p) level. Substitution effects in the propenylazo system on the kinetic and thermodynamic parameters were analyzed. An electron localization function (ELF) analysis of the electronic structure of the transition state structure associated with the azo-hydrazo conversion of t…
Isomerization and Application of Aroylnorbornenecarboxylic Acids for Stereoselective Preparation of Heterocycles
2002
When boiled in acidic or basic solution, diendo-3-aroylbicyclo[2.2.1]heptane-2-carboxylic acids (1 and la) isomerize to exo-3-aroylbicyclo[2.2.1]heptane-endo-2-carboxylic acids (2 and 2a). Similar endo→exo and even exo → endo isomerization of the aroyl group occurred when the Diels-Alder product containing a mixture of 3-exo-p-toluoylbicyclo[2.2.1 ]hept-5-ene-2-endo-carboxylic acid (4) and 3-endo-p-toluoylbicyclo[2.2.1]hept-5-ene-2-exo-carboxylic acid (5) was reacted with bifunctional reagents: o-aminothiophenol, 3-amino-1-propanol, 1,4-diaminobutane or diexo-3-hydroxymethylbicyclo[2.2.1]heptane-2-amine. All the reactions yielded mixtures of norbornene diendo- and diexo-fused heterocycles (…
Photochemistry of 1,2,4-oxadiazoles. A DFT study on photoinduced competitive rearrangements of 3-amino- and 3-N-methylamino-5-perfluoroalkyl-1,2,4-ox…
2006
The photoinduced competitive rearrangements of 5-perfluoroalkyl-3-amino(N-alkylamino)-1,2,4-oxadia- zoles have been investigated by DFT calculations and UV-vis spectroscopy. The observed product selectivity depends on the number of hydrogen atoms present in the amino moiety and involves two or three possible routes: (i) ring contraction-ring expansion (RCRE), (ii) internal-cyclization isomerization (ICI), or (iii) C(3)-N(2) migration-nucleophilic attack-cyclization (MNAC). UV absorption and fluorescence spectra of the reactants, and vertical excitation energy values, calculated by time dependent DFT, support the involvement of a neutral singlet excited state in the photoexcitation process. …
Photochromic System among Boron Hydrides: The Hawthorne Rearrangement
2019
Photoswitchable molecules have attracted wide interest for many applications in chemistry, physics, and materials science. In this work, we revisit the reversible photochemical and thermal rearrangements of the two B20H182– isomers reported by Hawthorne and Pilling in 1966, whose mechanism had not been understood so far. We investigate the rearrangements by means of a joint experimental and computational study with the outcome that B20H182– represents the first boron-based photochromic system ever reported. Both photochemical and thermal isomerizations occur through the same intermediate and involve a diamond–square–diamond (DSD) mechanism. Given the absence within boron chemistry of named …
Optical suppression of ferroelectricity in polysiloxane copolymers with chiral and photochromic side groups
1998
The ferroelectricity of surface-stabilized aligned films of polysiloxane copolymers containing chiral esters and photochromic azobenzene moieties within the side groups is suppressed on UV irradiation and is rebuilt on visible irradiation. The liquid crystalline polymer (Sc* 64 SA 80 i) shows ferroelectric behavior below 64°C. The photoinduced modification of supramolecular order and the corresponding changes in its dynamics are studied by polarized UV/vis spectroscopy and broad band dielectric spectroscopy (10−2–105 Hz). The dielectric measurements show that UV irradiation causes a loss of ferroelectricity within an interval of 10 K below the phase transition temperature. The photochemical…
Conformational preferences and synthesis of isomersZandEof oxazole-dehydrophenylalanine
2016
Dehydrophenylalanine, ΔPhe, is the most commonly studied α,β-dehydroamino acid. In nature, further modifications of the α,β-dehydroamino acids were found, for example, replacement of the C-terminal amide group by oxazole ring. The conformational properties of oxazole-dehydrophenylalanine residue (ΔPhe-Ozl), both isomers Z and E, were investigated. To determine all possible conformations, theoretical calculations were performed using Ac-(Z/E)-ΔPhe-Ozl(4-Me) model compounds at M06-2X/6-31++G(d,p) level of theory. Ac-(Z/E)-ΔPhe-Ozl-4-COOEt compounds were synthesized and the conformational preferences of each isomer, Z and E, were investigated using FTIR and NMR-NOE in solutions of increasing p…
A theoretical study of the lowest electronic states of azobenzene: the role of torsion coordinate in the cis-trans photoisomerization
2003
In the present paper we report the results of a multiconigurational computational study on potential- energy curves of azobenzene along the NN twisting to clarify the role of this coordinate in the decay of the S2(pp*) and S1(np*) states. We have found that there is a singlet state, S3 at the trans geometry, on the basis of the doubly excited coniguration n 2 p* 2 , that has a deep minimum at about 90 of twisting, where it is the lowest excited singlet state. The existence of this state provides an explanation for the short lifetime of S2(pp*) and for the wavelength-dependence of azobenzene photochem- istry. We have characterized the S1(np*) state by calcu- lating its vibrational frequencie…
Photochemical formation of [4.4.4](1,3,5)cyclophanes from 1,3,5-tris(3-phenylpropenoyl)benzenes
2004
Abstract Irradiation of 1,3,5-tris(3-phenylpropenoyl)benzene ( 1a ) yields in solution a dimer 2a by a threefold head-to-head/anti [2π+2π]cycloaddition. The stereochemistry of this [4.4.4](1,3,5)cyclophane was determined by 1H and 13C NMR studies including NOE measurements and a calculation of the AA′MM′ spin pattern of the methine protons. In contrast to the solution photochemistry, which is presumably controlled by the arrangement of an excimer, the irradiation in the crystalline state leads by a topochemical control to a dimer 3a , which contains a single four-membered ring.
Pyrene-benzoylthiophene Bichromophores as Selective Triplet Photosensitizers.
2006
Combination of the pyrene and benzoylthiophene units constitutes an interesting approach to design bichromophoric photosensitizers with increased intersystem crossing quantum yield and enhanced selectivity. The potential of this strategy has been illustrated in the present work by using a model photoisomerization reaction.
Highlights of metal ion-based photochemical switches
2014
Abstract Selected metal complexes behaving as photochemical switches, mostly operating by light emission/absorption without the intervention of any added chemical species, are reviewed. First, the switching mechanisms are analyzed to offer readers an easier understanding of the described systems. Examples of photochemical switching based on complexation/decomplexation processes and photo-bistable systems, comprising metal complexes with azobenzene, azopyridine, diarylethene, spirocyclic and rhodamine ligands, make the central body of the review. These examples were chosen to provide an overview of the evolution of metal-ion based photochemical switches, collecting together historical cases …