In situ XAFS investigation of palladium species present during the Heck reaction in room temperature ionic liquids

The Heck reaction, performed in room temperature ionic liquids, has been studied by in situ XAFS, which indicates that palladium clusters of 0.8-1.6 nm diameter are the main species present during reaction.

The Heck reaction in ionic liquids: A multiphasic catalyst system

[GRAPHICS]

Palladium-Catalyzed Oxidative Synthesis of Highly Functionalized Ortholactones

A palladium-catalyzed oxidative reaction is reported which converts dihydropyrans to their corresponding ortholactone. The products are formed in good to excellent yields with a very high level of chemoselectivity and functional group tolerance. Mechanistic studies confirm that the reaction proceeds by a Wacker-type mechanism.

A concise and convergent (formal) total synthesis of huperzine A

The first convergent synthesis of the tricyclic skeleton of huperzine A is described and includes, as the key step, an efficient regioselective intramolecular Heck reaction of 2-(tert-butyldimethylsillyoxymethyl)-6-(2-methoxy-5-bromopyridin-6-yl)methylcyclohex-2-enol.

Mechanistic studies of amide bond scission during acidolytic deprotection of Pip containing peptide.

Unusual TFA catalyzed cleavage reaction is reported for peptide containing pipecolic acid residues. Although the use of TFA under standard cleavage conditions is sufficiently mild to prevent degradation of the desired products. the amide bond between consecutive pipecolic acid residues is unexpectedly hydrolyzed by standard TFA treatment. The hydrolysis is proposed to proceed via an oxazolinium ion intermediate, This mechanism is supported by H/D exchange as observed by ESI-MS and NMR experiments.

Utilisation of ionic liquid solvents for the synthesis of Lily-of-the-Valley fragrance {beta-Lilial (R); 3-(4-t-butylphenyl)-2-methylpropanal}

The Heck arylation of 2-methylprop-2-en- I -ol in ionic liquids and organic solvents is reported using a range of homogeneous and heterogeneous palladium catalysts. Higher activity is observed in the ionic liquid media compared with N-methyl pyrrolidinone and under solventless conditions. The ionic liquid-catalyst system may be recycled easily with little loss in activity, although significant palladium leaching from the heterogeneous catalyst was observed. In the case of Trans-bis(2,3-dihydro-3-methylbenzothiazole-2-ylidene)diiodopalladium (11) reported to be highly active for this transformation, significant induction petiods were observed indicating that nanoparticles may be responsible for the catalysis. Using the ionic liquid Heck reaction, a recyclable synthesis for the fragrance beta-Lilial((R)) has been developed. (c) 2004 Elsevier B.V. All rights reserved.

Synthesis of the putative structures of homopumiliotoxins 235C and 233F

A novel approach to diene based quinolizidines, using an intramolecular Heck reaction in which the vinyl bromide double bond undergoes inversion of configuration, is reported. These quinolizidines have previously been proposed as tentative structures for homopumiliotoxin alkaloids 233F and 235C. The mass spectral data of the synthetic materials were different to those of the natural products confirming that the original structures need to be revised. (C) 2004 Elsevier Ltd. All rights reserved.

H+/2e- stoichiometry of the NADH : ubiquinone reductase reaction catalyzed by submitochondrial particles

Nitochondrial NADH:ubiquinone-reductase (Complex I) catalyzes proton translocation into inside-out submitochondrial particles. Here we describe a method for determining the stoichiometric ratio (H) over right arrow (+)/2e(-) (n) for the coupled reaction of NADH oxidation by the quinone accepters. Comparison of the initial rates of NADH oxidation and alkalinization of the surrounding medium after addition of small amounts of NADH to coupled particles in the presence of Q(1) gives the value of n = 4. Thermally induced deactivation of Complex I [1, 2] results in complete inhibition of the NADH oxidase reaction but only partial inhibition of the NADH:Q(1)-reductase reaction. N-Ethylmaleimide (NEM) prevents reactivation and thus completely blocks the thermally deactivated enzyme. The residual NADH:Q(1)-reductase activity of the deactivated, NEM-treated enzyme is shown to be coupled with the transmembraneous proton translocation (n = 4). Thus, thermally induced deactivation of Complex 1 as well as specific inhibitors of the endogenous ubiquinone reduction (rotenone, piericidin A) do not inhibit the proton translocating activity of the enzyme.

Optimization of heat transfer characteristics, flow distribution, and reaction processing for a microstructured reactor/heat-exchanger for optimal performance in platinum catalyzed ammonia oxidation

The present work is focused on the demonstration of the advantages of miniaturized reactor systems which are essential for processes where potential for considerable heat transfer intensification exists as well as for kinetic studies of highly exothermic reactions at near-isothermal conditions. The heat transfer characteristics of four different cross-flow designs of a microstructured reactor/heat-exchanger (MRHE) were studied by CFD simulation using ammonia oxidation on a platinum catalyst as a model reaction. An appropriate distribution of the nitrogen flow used as a coolant can decrease drastically the axial temperature gradient in the reaction channels. In case of a microreactor made of a highly conductive material, the temperature non-uniformity in the reactor is strongly dependent on the distance between the reaction and cooling channels. Appropriate design of a single periodic reactor/heat-exchanger unit, combined with a non-uniform inlet coolant distribution, reduces the temperature gradients in the complete reactor to less than 4degreesC, even at conditions corresponding to an adiabatic temperature rise of about 1400degreesC, which are generally not accessible in conventional reactors because of the danger of runaway reactions. To obtain the required coolant flow distribution, an optimization study was performed to acquire the particular geometry of the inlet and outlet chambers in the microreactor/heat-exchanger. The predicted temperature profiles are in good agreement with experimental data from temperature sensors located along the reactant and coolant flows. The results demonstrate the clear potential of microstructured devices as reliable instruments for kinetic research as well as for proper heat management in the case of highly exothermic reactions. (C) 2002 Elsevier Science B.V. All rights reserved.

Highly efficient asymmetric hetero-Diels-Alder reactions of carbonyl compounds catalyzed by Lewis acid platinum complexes of conformationally flexible NUPHOS-type diphosphines

Conformationally flexible NUPHOS-type diphosphines have been resolved as their diastereopure platinum BINOLate complexes delta- and lambda-[(NUPHOS)Pt{(S)-BINOL}] and the corresponding enantiopure Lewis acids delta- and lambda-[(NUPHOS)Pt(OTf)(2)], being generated by protonation with trifluoromethanesulfonic acid, act as highly efficient catalysts for the hetero-Diels-Alder reaction of nonactivated conjugated dienes with aryl glyoxals and glyoxylate esters, giving ee's as high as 99%.

Kinetics of Aqueous Phase Dehydration of Xylose into Furfural Catalyzed by ZSM-5 Zeolite

ZSM-5 zeolite in H+ form with an average pore size of 1.2 nm was used for aqueous phase dehydration of xylose to furfural at low temperatures;, that is, from 413 to 493 K. The selectivity in furfural increased with the temperature to a value of 473 K. Beyond this temperature, condensation reactions were significant and facilitated by the intrinsic structure of ZSM-5. A reaction mechanism that included isomerization of xylose to lyxose, dehydration of lyxose and xylose to furfural, fragmentation of furfural to organic acids, oligomerization of furfural to bi- and tridimensional furilic species, and complete dehydration of organic acids to carbonaceous deposits was developed, and the associated kinetic parameters were estimated. The rate of furfural production was found to be more sensitive to temperature than the rates of side reactions, with an estimated activation energy of 32.1 kcal/mol. This value correlated well with data in the literature obtained by homogeneous catalytic dehydration.

Enantioselective Rhodium-Catalyzed Alkylative Desymmetrization of 3,5-Dimethylglutaric Anhydride

A rhodium-catalyzed enantioselective cross-coupling of sp³ organozinc reagents and 3,5-dimethylglutaric anhydride has been developed to afford the corresponding products, syn-deoxypolypropionates, in excellent yields and enantioselectivities. This reaction has been developed so that both commercially available and in situ prepared organozinc reagents are competent coupling partners.

A study of Heck cyclization reactions to form phenanthridine ring systems

A survey of conditions for the palladium catalyzed intramol. Heck cyclization of protected amines has shown that the Herrmann-Beller palladacycle can be exploited under 'cationic' conditions to provide a robust and rapid route (

Thermochemistry of Ionic Liquid-Catalyzed Reactions: Theoretical and Experimental Study of the Beckmann Rearrangement-Kinetic or Thermodynamic Control?

A thermodynamic analysis of the experimental conditions of the Beckmann rearrangement reaction of oximes into amides has been undertaken to examine whether the reaction is under thermodynamic or kinetic control. To answer this question, the thermodynamic properties of the typical Beckmann rearrangement reactions in the ideal gaseous state-cyclohexanone oxime to caprolactam and 2-butanone oxime to N-methylpropanarnide-were studied by using the quantum mechanical method. Gibbs energy and equilibrium constants of the Beckmann rearrangement have been assessed in the gaseous and the liquid phases. Results of the thermodynamic analysis have shown that Beckmann rearrange ments are kinetically controlled. Thus, a search for possible active ionic liquid based catalysts for the mild reaction conditions has been performed.