Irradiation procedures, general

The chemical reactions induced by ultrasonic irradiation are generally influenced by the irradiation conditions and procedures. It is suggested that ultrasound intensity , dissolved gas , distance between the reaction vessel and the oscillator and ultrasound frequency are important parameters to control the sonochemical reactions. 

Preparation of metal fluorides under microwave irradiation A general procedure for the synthesis of metal fluoride is as follows, BMIBF4 (l-butyl-3-methyfimidazolium tetrafluoroborate), the ionic liquid (IL) solvent (precursor for fluoride ions), and the different metal salts at a weight ratio of 10 1, are mixed in a round-bottom flask fixed with a water condenser. All the reaction mixtures were heated in the above-mentioned... 

NAA method has been apphed to study the concentration distributions of typical toxic heavy metals in small amounts of industrial type air particulate fractions. Generally, toxic components are distributed in two forms in the particulates, e.g., in a matrix involved form and in a surface deposited form of which the latter one can be considerably enriched. Bnme (1973) has hsted various methods for the determination of interference-free trace elements in water, biological specimens, and food sources by means of NAA involving the use of a cold neutron irradiation procedure. 

The carboxylic function is generally protected by the benzyl protecting group. After the reaction sequence, the deprotection of benzyl ester is carried out by using potassium carbonate aluminium chloride Na-NH3 etc. Most of the deprotection procedures give moderate yields and a longer reaction time is required. The microwave irradiation procedure is completed in 3-10 min and yields are high (89-92%) (Scheme 16). 

Figure 32-8 is a block diagram showing the flow of sample and standards in the two most common types of activation methods, destructive and nondestructive. In both procedures the sample and one or more standards are irradiated simultaneously with neutrons (or other types of radiation). The samples may be solids, liquids, or gases, although the first two are more common. The standards should physically and chemically approximate the sample as closely as possible. Generally, the samples and standards are contained in small polyethylene vials heat-scaled quartz vials are also used on occasion. Care must be taken to ensure that the samples and standards are exposed to the same neutron flux. The time of irradiation depends on a variety of factors and often is determined empirically. Frequently, an exposure time of roughly three to five times the half-life of the analyte product is used (see Figure 32-7). Irradiation times generally vary from a few minutes to several hours.

Reaction that can be carried out by the oxidative coupling of radicals may also be initiated by irradiation with UV light. This procedure is especially useful if the educt contains oleflnic double bonds since they are vulnerable to the oxidants used in the usual phenol coupling reactions. Photochemically excited benzene derivatives may even attack ester carbon atoms which is generally not observed with phenol radicals (I. Ninoraiya, 1973 N.C. Yang, 1966). 

Irradiation of the Condensed Pyridine A -Ethoxycarbonylimines 13a-f General Procedure 145... 

The glass plate was exposed to microwave irradiation, eluted, and viewed by standard TLC visualization procedures to assess the results of the reaction. In this particular example, the synthesis of an arylpiperazine library (Scheme 4.25) was described, but the simplicity and general utility of the approach for the rapid screening of solvent-free microwave reactions may make this a powerful screening and reaction optimization tool. The synthesized compounds were later screened for their antimicrobial activity without their removal from the TLC plate utilizing bioautogra-phical methods [84],... 

The solvent-free preparation of 1,2,3-trisubstituted imidazolidin-4-ones from aldehydes and N-substituted a-amino acid amides has been reported by Pospisil and Potacek (Scheme 6.202) [365], The general procedure simply involved heating equimolar mixtures of the aldehyde and amine building blocks under open-vessel micro-wave irradiation for 5 min at 200 °C. After cooling to room temperature, the imida-zolidin-4-one products were purified by flash chromatography. 

The first investigations in the 1960s [11,12] established the base-induced isomerization of alkyne precursors as the most practical and general route for the synthesis of alkoxy-and aryloxyallenes. In the meantime, a number of monosubstituted allenes 8 bearing an achiral or a chiral group R is smoothly accessible by this efficient procedure (Scheme 8.5) [1, 2,13-19]. Beside the most commonly used base potassium tert-butoxide, other bases, e.g. n-butyllithium, are also applicable for this isomerization. Recently, the yields of alkyne-allene isomerizations could be significantly increased, in particular with aryloxy-substituted allenes, by using microwave irradiation (Eq. 8.1) [20]. 

The simplicity of the two-phase modification of the Gabriel synthesis of primary amines, via the N-alkylation of potassium phthalimide, makes the procedure considerably more convenient than the traditional method, which normally requires the use of anhydrous dipolar aprolic solvents. The reaction can be conducted under solid liquid conditions using potassium hydroxide in toluene [25], or with preformed potassium phthalimide [26, 27] (cf ref. 28). As is normal for acylation reactions, relatively mild conditions are required for the preparation of the A-ethoxycarbonyl derivative [29], whereas a reaction temperature of 100°C is generally used for N-alkylation (Table 5.16). The reaction time for the soliddiquid two-phase system can be reduced dramatically with retention of the high yields, when the reaction mixture is subjected to microwave irradiation [30]. 

---