Microwave focused

Novel solvent-free organic reactions using focused microwaves with formation of heterocycles 98S1213. 

In some cases, no cycloalky ladon is observed by the reacdon of uittomethane v/ith electron-deficient olefins v/ilh cyano and methoxycarbonyl groups The reacdon affords new, highly fiincdonalLzed cyclohexenes in the presence of catidydc amount of piperidine under solvent- free conchdons v/ith focused microwave irradiadon fEq 7 41 ... 

The synthesis of 4-unsubstituted DHPs in a focused microwave reactor has been reported using alkyl acetoacetates and hexamethylenetetramine 19 as the source of both formaldehyde and ammonia, with additional ammonium acetate to maintain the stoichiometry [57], Irradiation for 100 s under solvent-free conditions gave, for example, 1,4-DHP 20 in 63% isolated yield (Scheme 5). 

Finally, the introduction of focused microwave instruments further enabled to speed up the synthesis of libraries, by reducing the actual time needed for reaction [28-34]. The combination of using dedicated microwave instruments and sohd or solution phase tagging subsequently became a very powerful tool for PASP and SPOS apphcations [25,26,33-51]. 

The open-vessel MAE approach can be contrasted to closed-vessel MAE (Section 3.4.5.1). Focused microwave-atmospheric pressure technology was reviewed by Mermet [473]. 

Matusiewicz [63,68] has reported the development of a high-pressure, high-temperature, focused-microwave-heated acid (HNO3) digestion system. This microwave technique requires only about 3 % of the time necessary for the thermal high-pressure (HPA) technique. The technique of microwave heating samples in sealed containers to speed up acid digestion has been in widespread use for the past few years [69,70]. 

With the exception of GC-MIP-AES there are no commercial instruments available for speciation analysis of organometallic species. Recently, a prototype automated speciation analyser (ASA) for practical applications was described [544,545], which consists of a P T system (or focused microwave-assisted extraction), multicapillary GC (MC-GC), MIP and plasma emission detection (PED). MCGC-MIP-PED provides short analysis times ([Pg.676]

FMASE Focused microwave-assisted Soxhlet GPEC Gradient polymer elution... 

FMW Focused microwave-enhanced GSSMS Gliding spark source mass... 

In some cases, no cycloalkylation is observed by the reaction of nitromethane with electron-deficient olefins with cyano and methoxycarbonyl groups. The reaction affords new, highly functionalized cyclohexenes in the presence of catalytic amount of piperidine under solvent-free conditions with focused microwave irradiation (Eq. 7.41).42... 

A. Jurado Lopez, M.D. Luque de Castro, Optimisation of Focused Microwave Digestion of Proteinaceous Binders Prior to Gas Chromatography, Talanta, 65, 1059 1062 (2005). 

Alkylations in dry media of the ambident 2-naphthoxide anion were performed under the action of focused microwave activation. Whereas the yields were identical to those obtained under the action of A for benzylation, they were significantly improved under microwave irradiation conditions for the more difficult n-octylation (a less reactive electrophilic reagent). No change in selectivity was observed, however, indicating the lack of influence of ionic polarization [94],... 

Knoevenagel condensation reaction of creatinine with aldehydes occurs rapidly under solvent-free reaction conditions at 160-170 °C using focused microwave irradiation (Scheme 6.18) [66],... 

KMn04 impregnated alumina oxidizes arenes to ketones within 10-30 min under solvent-free conditions using focused microwaves [111]. /i,/i-Disubstituted enamines have been successfully oxidized into carbonyl compounds with KMn04-Al203 in domestic (255 W, 82 °C) and in focused microwave ovens (330 W, 140 °C) under sol-vent-free conditions by Hamelin et al. [112]. The yields are better in the latter case. When the same reactions are conducted in an oil bath at 140 °C, no carbonyl compound formation is observed (Scheme 6.36). 

A comparative study [12] of the reactivity of the oxalimide 16 in a variety of solvents (xylene, chlorobenzene, toluene) and of methylphosphinite 17 was performed with the focused microwave Synthewave 402 reactor (Merck Eurolab, div. Prolabo, France), using different conditions of power and exposure time (Scheme 8.8). In all experiments yields were better than those of previous procedures with classical heating (Tab. 8.2), and the authors wrote it is dear that microwave technique is applicable to highly functionalized compounds containing stereogenic centers without appreciable modification of these centers . 

The reaction was studied under the action of focused microwaves and conventional heating and it is noteworthy that elimination is more efficient than Michael addition (leading to aziridines) under irradiation, the conditions being the same. 

In 1996, the condensation of orthoesters with ortho-phcnylcnediamines leading to benzimidazoles [48] was achieved either in toluene under reflux or without solvent by use of focused microwaves (Scheme 8.30). 

Addition of lithium chloride is necessary for the reaction to proceed. Whereas no reaction was observed under salt-free conditions, the lactone is obtained in 90% yield within 5 min in the presence of LiCl impregnated with KF on alumina (KF-LiCl-Al203 = 1 1 4 by weight) under the action of focused microwave irradiation at 150 W. With conventional heating no reaction occurred. 

Bazureau described a tandem conjugate addition-intermolecular hetero-Diels-Al-der reaction of perimidine derivatives under solvent-free conditions with focused microwave irradiation [77]. The main feature of this reaction was attainment of complete addition in less than 8 min. When the reaction mixture was heated in an oil bath set at the same temperature for the same time the results were analogous (Scheme 9.27). 

Jimenez et al. studied the asymmetric Diels-Alder reactions of 1-aryl-1,2-diaza-l,3-butadienes 114, heterodienes derived from sugars, with diethyl azodicarboxylate (115), a heterodienophile [85]. The reactions were performed without solvent in a focused microwave reactor for periods of a few hours. The reaction is stereoselective... 

The same authors also investigated the reaction of the imidate 120 and aldehydes 123 with dry acetic acid as a catalyst in the absence of solvent. The mixture was immediately submitted to focused microwave irradiation to afford compounds 125 together with alcohols, ethyl acetate and a small amount ofimine 124 (Scheme 9.37) [88],... 

Likewise, Loupy showed the utility of focused microwaves as an energy source in the 1,3-dipolar cycloaddition of N-methyl-a-phenylnitrone (152) with fluorinated di-polarophiles in solvent-free conditions (Scheme 9.47) [98],... 

The one-pot synthesis of /1-lactams containing the ferrocene moiety 226 (Scheme 9.73) was described by Bonini [124], The reactions were performed under classical heating (3.5-24 h) and/or by using a focused microwave reactor (4—6 min). In this case, the stereochemical outcome of the reaction is the same with both procedures and microwave irradiation does not necessarily improve on the yields obtained by classical heating. 

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