Temperature microwave

Larhed et al. investigated enantioselective Heck reactions with 2,3-dihydrofuran as alkene [86]. In the coupling with phenyl triflate, conditions previously reported by Pfaltz [87] were attempted under microwave irradiation. Interestingly, the catalytic system Pd2(dba)3/(4S)-4-t-butyl-2-[2-(diphenylphosphanyl)phenyl]-4,5-dihydro-l,3-oxazole, identified by the Swiss team, was found suitable for high-temperature microwave-assisted enantioselective Heck reactions (Scheme 76). Using a proton sponge as a base and benzene as a solvent gave the best conversions (Scheme 76). At tempera-... 

Based on the properties of ionic hquids in high-temperature microwave-enhanced reactions, the authors chose l-butyl-3-methylimidazolium tetraflu-orophosphate ([bmimjPFe) as the suitable ionic liquid (Scheme 23). The addition of 0.15 mmol of [bmimjPFe to a reaction in 2.0 mL of DCF was found to increase the reaction rate dramatically and a set-temperature of 190 °C was reached in a mere 1 min, while the reactions programmed at 190 °C, in the absence of the ionic liquid, reached only 170 °C in 10 min. The reactions were finished in a mere 18-25 min of irradiation time, including the hydrolysis of the sensitive imidoyl chloride moiety with water. The formed bis-lactams were isolated in good yield and purity. 

Direct high-temperature microwave-assisted aminations of a variety of aryl halides [257] or aryl triflates [258] under transition metal-free conditions have also been reported, probably involving a benzyne mechanism. 

Although microwave activation of catalytic reactions has been the subject of many studies (Sects. 10.3.1 and 10.3.2), the mechanism of these reactions is not yet fully understood. In heterogeneous catalytic liquid/solid and gas/solid systems many results have revealed significant differences between the rates of conventionally and microwave heated reactions. As a rule, at the same temperature microwave heated reactions were faster than conventional and their rate enhancement was over one or-... 

Reverz, R. and Hasty, E. (1987) Recovery study using an elevated pressure temperature microwave dissolution technique. Paper presented at the Pittsberg Conference and Exposition on Analytical Chemistry and Applied Spectroscopy, March 1987. 

Despite the extensive use of microwave-enhanced reactions in order to effect syntheses, very little has been explored in the synthesis of thieno[3,2-i pyrimidines. One example of this newer technology being applied to heterocyclic systems involves the conversion of the formamidine 479 into 480 when treated with an amine under high-temperature microwave conditions (Equation 180) <20040L1523>. 

The sample must be in the gas phase. Microwave absorption lines are considerably broadened by molecular collisions at intermediate and high pressures at atmospheric pressure, microwave absorption lines are tens of thousands of megacycles wide. Hence the gas is kept at low pressure, typically 0.01 to 0.1 torr under these conditions, line widths run about MHz. The compound studied need not be a gas at room temperature, but it must have sufficient vapor pressure to give detectable absorption. To study involatile compounds such as the alkali halides, the waveguide must be heated to 500-1000°C high-temperature microwave spectroscopy presents great experimental difficulties, but it has been used to study most of the alkali halides. 

Abstract The Zr (Ti)-Si-Al HDN Catalytic Materials have been synthesized rapidly with a new route heated by microwave. The synthesis conditions such as synthesis temperature, microwave oven pressure, pH value of synthesis solution and raw material were examined by experimentation. The thermostability, pore volume, surface area, surface Si/Al and hydrodenitrogen activity of the synthesis samples were also characteristiced. 

Florian, D. and G. Knapp. 2001. High-temperature, microwave-assisted UV digestion A promising sample preparation technique for trace element analysis. Anal. Chem. 73 1515-1520. 

The 1,2,4-triazine core is a synthetically important scaffold because it could be readily transformed into a range of different heterocyclic systems such as pyridines (Sect. 3.1) via intramolecular Diels-Alder reactions with acetylenes. 1,2,4-Triazines have been synthesized by the condensation of 1,2-diketones with acid hydrazides in the presence of NH4OH in acetic acid for up to 24 h at reflux temperature. Microwave dielectric heating in closed vessels allowed the reaction to be performed at 180 °C (60 °C above the boiling point of acetic acid). As a result, the reaction time was reduced to merely 5 minutes. Subsequently, a 48-membered library of 1,2,4-triazines was generated from diverse acyl hydrazides and a-diketones [139]. Two thirds of the desired heterocycles precipitated from the reaction mixture upon cooling with > 75% purity, while the remaining part of the library was purified by preparative LCMS (Scheme 56). 

Sometimes one has to live with this problem, and some of the finest work, particularly on transient molecular species, has been achieved using source modulation. This includes some of the earliest work on neutral free radicals, by Kewley, Sastry, Winnewisser and Gordy [5], who studied the SO and CS species. With the benefit of hindsight, we know these radicals to be very long-lived. We shall illustrate the details of source modulation by describing two other experiments. The first is the classic work of Woods [6] and Dixon and Woods [7] who obtained the first microwave spectrum of a molecular ion, namely CO+. The second is an example of a high-temperature microwave cell for the study of refractory materials. 

During the past five years two research disciplines of optical spectroscopy and magnetic resonance have merged when it became evident that at low temperatures, microwave radiation of resonance frequencies with the zero-field (zf) transitions of the lowest triplet state could have observable effects on the phosphorescence intensity as well as the spectrum. Quantitative information can then be obtained from these phosphorescence-microwave multiple-resonance experiments from which the magnetic, the radiative, and the nonradiative as well as the structural properties of the triplet state can be determined. 

Levine, K.E., Batchelor, J.D., Rhoades, C.B., Jones, B.T. Evaluation of a high-pressure, hlgh-temperature microwave digestion system. J. Anal. At. Spectrom. 14, 49-59 (1999)... 

Optimization of the MAE procedure consists in choosing an appropriate solvent (or solvent mixture) and others factors such as solvent-to-feed ratio, extraction time and temperature, microwave power, and matrix characteristics (including water content). 

Recent research on the preparation of BN thin films has concentrated on the use of plasmas to reduce the deposition temperature. Other research has focused on the use of an alternate nitrogen source, such as NF3, with BX3 (X = F or Cl) and B2H6 precursor gases to further reduce the deposition temperature. Microwave plasmas have also been used to deposit BN with borane-dimethyl amine and nitrogen gas. ... 

For high-temperature microwave ceramic processing, the refractory specimen enclosure (the casket) serves as both i) a thermal insulator and ii) a microwave susceptor. The susceptor allows the microwave energy to couple with the material at high temperature. The thermal insulation diminishes the dissipation of thermal energy. On occasion, the caskets are referred to as susceptors, which does not fully depict the dual role of the specimen enclosure or casket as both a thermal insulator and a microwave susceptor (absorber). 

Bykov, Y.V. Rybakov, K.I. Semenov, V.E. High-temperature microwave processing of materials. J. Phys. D. Appl. Phys. 2001, 34, R55-R75. 

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