Search results for "Isomerization"
showing 10 items of 281 documents
Mechanism of excited state deactivation of indan-1-ylidene and fluoren-9-ylidene malononitriles.
2016
Herein, we report complementary computational and experimental evidence supporting the existence, for indan-1-ylidene malononitrile and fluoren-9-ylidene malononitrile, of a non-radiative decay channel involving double bond isomerisation motion. The results of UV-Vis transient absorption spectroscopy highlight that the decay takes place within hundreds of picoseconds. In order to understand the related molecular mechanism, photochemical reaction paths were computed by employing multiconfigurational quantum chemistry. The results indicate that the excited state deactivation occurs via concerted double bond twisting of the dicyanovinyl (DCV) unit coupled with a pyramidalisation of its substit…
MoVReagents in Organic Synthesis
2016
The use of MoV reagents, and in particular MoCl5, in organic synthesis is surveyed. The oxidative treatment of aromatic substrates is the most common application. The unique properties of these reagents are due to their high oxidative power combined with exquisite Lewis acid properties. In several examples MoV reagents outperform other common oxidative coupling reagents. C–C bond formation through inter- and intramolecular oxidative coupling can lead to selective formation of five- to eight-membered ring systems. Mechanistic investigations of the courses of reactions involving MoV reagents and aromatic substrates indicate that radical cations are initially formed, entering the oxidative cou…
Selective isomerization of a trans -butanediacetal derivative of tartaric acid with differentiating substituents at C-2 and C-3
2017
Abstract A trans-disubstituted butanediacetal derivative with two different substituents (ester and thioester) – methyl (2R,3R,5R,6R)-3-ethylsulfanylcarbonyl-5,6-dimethoxy-5,6-dimethyl-1,4-dioxane-2-carboxylate, was selectively converted to the cis derivative – methyl (2S,3R,5R,6R)-3-ethylsulfanylcarbonyl-5,6-dimethoxy-5,6-dimethyl-1,4-dioxane-2-carboxylate in high yield on a multigram scale. The product of this reaction offers the possibility for selective modification of one of the substituents, which was demonstrated by Fukuyama reduction of the thioester group.
Optical data storage in Langmuir-Blodgett-Kuhn multilayer assemblies of azo-dye side chain LC-polymers
1991
We describe the preparation of LB-films of novel polymers containing azobenzene dye side chains. X-ray refection measurements, surface-plasmon experiments were used to determine the structure of LB-films which had been transferred onto a substrate. Illumination with UV-light causes trans-cis isomerisation of the azobenzene side groups. This effect can be used for optical data storage and will be discussed in detail.
Photoreactions in Langmuir-Blodgett-Kuhn multilayer assemblies of liquid crystalline azo-dy side-chain polymers
2007
Reversible and persistent optical data storage in Langmuir-Blodgett-Kuhn liquid crystalline systems has been reported recently. Depending on the molecular structure, the trans-cis isomerization in these systems (induced by illumination with UV- and VIS-light) leads to different macroscopic behavior of the chromophore percentage on the photo-induced structural changes in a series of statistical copolymers.
Base-catalyzed isomerization of 2-isoxazolines enables a two-step enantioselective synthesis of β-hydroxynitriles from enals.
2010
The asymmetric synthesis of β-hydroxynitriles remains a challenge in organic synthesis. Herein we report a convenient synthesis of β-hydroxynitriles from enantiomerically enriched 3-unsubstituted 2-isoxazolines via a base-catalyzed ring-opening reaction that takes place without loss of enantiopurity. In combination with organocatalytic enantioselective synthesis of 3-unsubstituted 2-isoxazolines, the ring-opening enables a short 2-step synthesis of β-hydroxynitriles from α,β-unsaturated aldehydes in high enantiomeric purity.
Reactions of α-diimino ligands with the chloro-bridged dimer [RhCl(COD)]2(COD=1,5-cyclooctadiene)
1988
Abstract The reactions of α-diimino ligands N - N ′ [ N - N ′= 2,2′-bipyridine (bipy), C 5 H 4 N2CHNR (R= C 6 H 4 OMe- p , PyCa), RNCHCHNR (R=C 6 H 4 - OMe- p , DAB)] with [RhCl(COD)] 2 give rise to stoichiometry, solvent, ligand, and temperature dependent equilibria. In general, the 1/1 ligand/dimer reaction yields the ionic product [Rh(COD)( N - N ′)] [RhCl 2 (COD)], at room temperature. For N - N ′=DAB, the ionic form is in equilibrium with the binuclear compound [{RhCl(COD)} (μ-DAB){RhCl(COD)}] (containing a σ σ,'- N , N ′ bridging α-diimine), which becomes the predominant species at low temperatures. In [Rh(COD)( N - N ′)] [RhCl 2 (COD)], a fast exchange of the Rh(COD) unit betwe…
Cycloalkin-vinylidencycloalkan-umlagerungen
1989
Abstract Flash pyrolysis of the strained cyclic alkynes 1 leads to ring systems 2 with an exocyclic allene group and in a further isomerization step to the conjugated vinyl compounds 3 .
Coordination chemistry of N,N′-bis(coordinating group substituted)oxamides: a rational design of nuclearity tailored polynuclear complexes
1999
Abstract The coordinating properties of N , N ′-bis(coordinating group substituted)oxamides have been thoroughly investigated both in aqueous solution and in the solid state. The easy cis – trans isomerization equilibria that they exhibit together with the great variety of N , N ′-substituents which can be used to play on the overall charge, complexing ability and polarity, make them very suitable ligands in designing homo- and heterometallic species. The knowledge of their complex formation in aqueous solution by potentiometry and using the hydrogen ion concentration as a probe, allowed us to settle the basis of a rational design of oxamidate-containing polynuclear species whose nuclearity…
Temperature and light sensitive copolymers containing azobenzene moieties prepared via a polymer analogous reaction
2009
Abstract Four different series of polyacrylamides containing different amounts of azobenzene moieties have been synthesized via a polymer analogous reaction of poly(pentafluorophenylacrylate) (PPFPA). All copolymers were designed to exhibit a lower critical solution temperature (LCST) in aqueous solution, which was dependent on (i) the amount of incorporated chromophoric azobenzene groups and (ii) the isomerization state of the respective azobenzene group. Higher LCST values were measured for UV-irradiated solutions of the copolymers in comparison to the non-irradiated copolymer solutions. A maximum difference in the LCST of up to 7 °C was found for the copolymer poly(N,N-dimethylacrylamide…