Search results for "Selectivity"

Synthesis of vanillin in water by TiO2 photocatalysis

2012

Abstract The photoproduction of vanillin is studied in aqueous medium starting from trans-ferulic acid, isoeugenol, eugenol or vanillyl alcohol by using both commercial and home prepared TiO2 samples as photocatalysts and batch Pyrex photoreactors. The photo-oxidation at room temperature of these compounds produces vanillin with a selectivity ranging from 1.4 to 21 mol% with respect to the converted substrate. An investigation on the intermediates was performed in the case of trans-ferulic acid; for this substrate the most important intermediates were homovanillic acid, vanillyl mandelic acid, trans-caffeic acid, formic acid, acetic acid, and oxalic acid. The carbon mass balance, including …

Green Oxidation of Alcohols to Carbonyl Compounds by Heterogeneous Photocatalysis

2010

ChemInform Abstract: Stereocontrolled Approach to δ- and γ-Lactones and 1,3-Diols. The Role of X- Ion in the Selenolactonization.

2010

Abstract Two complementary approaches have been realized for the stereoselective synthesis of 4,6-disubstituted δ-lactones and 1,3- anti and syn diols. In the 6- endo selenolactonization the role of the X − ion on the stereoselectivity is shown. Moreover, the highly stereoselective transformation of the 6- endo δ-lactone to the 5- exo γ-lactone is reported.

Mg/BOX complexes as efficient catalysts for the enantioselective Michael addition of malonates to β-trifluoromethyl-α,β-unsaturated ketones and their…

2021

Abstract Magnesium (II)-BOX complexes catalyze the enantioselective Michael addition of malonates to β-trifluoromethyl enones and their N-sulfonyl imines to give ketones or (E)-enamines bearing a trifluoromethylated stereogenic center, respectively, with good yields and high enantiomeric excesses. Magnesium complexes proved to be more active and stereoselective than zinc and copper analogues in these reactions.

Halogen‐Imparted Reactivity in Lithium Carbenoid Mediated Homologations of Imine Surrogates: Direct Assembly of bis‐Trifluoromethyl‐β‐Diketiminates a…

2020

The selective formal insertion (homologation) of a carbon unit bridging the two trifluoroacetamidoyl chlorides (TFAICs) units is reported. The tactic is levered on a highly chemoselective homologation-metalation-acyl nucleophilic substitution sequence which precisely enables to assemble novel trifluoromethylated β-diketiminates within a single synthetic operation. Unlike previous homologations conducted with LiCH2Cl furnishing aziridines, herein we exploit the unique capability of iodomethyllithium to act contemporaneously as a C1 source (homologating effect) and metalating agent. The mechanistic rationale grounded on experimental evidences supports the hypothesized proposal and, the struct…

Understanding the stereoselectivity in Brønsted acid catalysed Povarov reactions generating cis/trans CF3-substituted tetrahydroquinolines: a DFT stu…

2016

The Bronsted acid (BA) catalysed Povarov reactions of (E)-1-phenyl-N-(4-(trifluoromethyl)phenyl) methanimine with 1-vinylpyrrolidin-2-one (VPO) and with allyltrimethylsilane (ATMS), affording CF3-substituted cis/trans tetrahydroquinoline (THQ) derivatives, have theoretically been studied using the M06-2X functional together with the standard 6-31G(d) basis set. These BA catalysed Povarov reactions are multistep processes initialised by the nucleophilic attack of the non-substituted carbon atom of the nucleophilic ethylene on the imine carbon atom of the super-electrophilic protonated imine, yielding the corresponding cationic intermediate along the endo/exo stereoisomeric approach mode. Upo…

An Unprecedented Iridium(III) Catalyst for Stereoselective Dimerisation of Terminal Alkynes

2008

A novel iridium(III) hydride complex, IrHCl(TIMP 3 ) {HTIMP 3 = tris[1-(diphenylphosphino)-3-methyl-lH-indol-2-yl]methane) was prepared and fully characterized in both the solid state and in solution. Chloride abstraction by silver cations provides a more reactive compound, [IrH-(TIMP 3 )][BF 4 ], which can react with pyridine (py) and phenylacetylene to yield the complexes [IrH-(TIMP 3 )(py)][BF 4 ] and [Ir(PhCH=C-CH=CHPh)-(TIMP 3 )][BF 4 ], respectively. Interestingly, IrH-(TIMP 3 )(py)][BF 4 ] efficiently catalyses the stereoselective dimerisation of model terminal alkynes to the 1,4-disubstituted (E)-but-1-en-3-yne only.

Allylester als selektiv abspaltbare Carboxyschutzgruppen in der Peptid- undN-Glycopeptidsynthese

1983

Allylester eignen sich als Schutz der Carboxyfunktion bei Peptid- und N-Glycopeptidsynthesen und konnen von Aminosauren einfach hergestellt werden. Unter den Bedingungen der Boc- und Z-Gruppenabspaltung im Sauren sind die Allylester bestandig. Durch Behandlung mit katalytischen Mengen von Tris(triphenylphosphan)rhodium(I)-chlorid in Ethanol/wasser (9:1) werden sie im neutralen bis schwach basischen Milieu schonend gespalten. Der N-terminale Schutz und die N-glycosidische Bindung bleiben dabei unverandert erhalten. Die dadurch mogliche selektive Deblockierung der α-Carboxyfunktion geschutzter N-glycosylierter Asparagin-Derivate 15 wird zur gezielten C-terminalen Kettenverlangerung zu N-Glyco…

Dodecyloxy-substituted 2,4,6-tris(styryl)pyridines

2008

(E,E,E)-2,4,6-Tris(styryl)pyridines 2a-c with 3, 6 or 9 dodecyloxy substituents were prepared by the highly stereoselective condensation reaction of collidine (8) and the corresponding phenyl-(1-phenyl-methylidene)amines 7a-c (Siegrist reaction). In contrast to the corresponding compounds with a benzene or a 1,3,5-triazine core, 2a-c do not show any hints for the formation of thermotropic liquid crystals. The major application of such star-shaped systems is in the field of nonlinear optics.

The selective oxidative dehydrogenation of ethane over hydrothermally synthesised MoVTeNb catalysts

2002

Mo–V–Te–Nb metal oxide catalysts prepared by hydrothermal synthesis and heat-treated in N2 at high temperatures (600–700 °C) show high activity and selectivity for the oxidative dehydrogenation of ethane to ethene. Yields of ethene of 75% have been obtained at 400 °C on the best catalysts. Dejoz Garcia, Ana Maria, Ana.M.Dejoz@uv.es