Synthesis microwave-assisted

If the microwave-assisted densification step is performed with precursors differing from those used for the seed layer particles, multicomponent films can be prepared. For example, it is possible to use Sn ZnO nanoparticles as a seed layer followed by the deposition of Al ZnO nanoparticles inside the pores, resulting in a homogeneous Sn ZnO/Al ZnO film [104]. A seeded growth process for nanoparticles was proposed by Yang and coworkers for the coating of commercially available titania powder (P25) with flower-like titania nanostructures using the microwave-assisted reaction of TiCLi in benzyl alcohol [105]. 

and coworkers presented an in-depth and critical examination of the specific role of microwave dielectric heating diuing the synthesis of ZnO microrods in a mixture of water and ethylene glycol as well as of ZnO nanoparticles in benzyl alcohol. Carefully performed control experiments clearly demonstrated that in both reaction systems no differences between conventional and microwave heating existed. The ZnO particles had the same crystal structure, primary crystallite size, shape, and size distribution, independent of the heating technique [109]. 

Recently, the synthesis of crystalline tungsten oxide nanoparticles within benzyl alcohol droplets was achieved by using microwave dielectric heating of non-aqueous droplets in a microfluidic device with a reaction time of just 64 ms [112]. The microwave-assisted nonaqueous routes are also relevant for the synthesis of non-oxide nanomaterials such as metals and metal chalcogenides [113]. 

Microwave-assisted synthesis provides an environmentally friendly route for synthesis as it reduces the reaction time from days and hours to minutes and increases product yield [19] consequently, this method is also applied to the synthesis of amphiphilic 

Two commonly used modes for microwave-assisted synthesis are pulsed power mode and dynamic mode. One is a temperature-controlled mode and the other uses power as well as the temperature control mode [24]. 

Mix ethanol (12 g), a CS solution (25 ml, around 15 mg of CS per ml of 0.2 M acetic acid) and an octanal solution in ethanol. Transfer the reaction mixture to a 100 ml round-bottom flask, capped with a rubber septum. Then add excess sodium cyanoboro-hydride to the flask. Irradiate the reaction mixture with microwave irradiation at 25 to 40 °C for 1 min to 1 h. Progress of the reaction can be checked by thin layer chromatography using a suitable solvent. The product is precipitated 

MW-HT techniques can shorten the reaction time. For example, Ivanov et al. [38] synthesized ZnO nanocrystals in 10 min with the assistance of microwave hydro-thermal methods. Huang et al. [39] also demonstrated the use of microwave radiation in the hydrothermal synthesis of ZnO complex nanostructures. These nanocrystals showed high photocatalytic activity in a model reaction of Methyl Orange photodegradation. Li et al. [40] synthesized W03 nanorods in 20 min and showed high 

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Scheme 8 Microwave-assisted synthesis of imidazo-annulated heterocycles...

This transformation can also be carried out under solvent-free conditions in a domestic oven using acidic alumina and ammoniiun acetate, with or without a primary amine, to give 2,4,5-trisubstituted or 1,2,4,5-tetrasubstituted imidazoles, respectively (Scheme 15A) [69]. The automated microwave-assisted synthesis of a library of 2,4,5-triarylimidazoles from the corresponding keto-oxime has been carried out by irradiation at 200 ° C in acetic acid in the presence of ammonium acetate (Scheme 15B) [70]. Under these conditions, thermally induced in situ N - O reduction occurs upon microwave irradiation, to give a diverse set of trisubstituted imidazoles in moderate yield. Parallel synthesis of a 24-membered library of substituted 4(5)-sulfanyl-lff-imidazoles 40 has been achieved by adding an alkyl bromide and base to the reaction of a 2-oxo-thioacetamide, aldehyde and ammonium acetate (Scheme 15C) [71]. Under microwave-assisted conditions, library generation time was dramatically re-... 

Scheme 23 Microwave-assisted synthesis of benzo-derivatives...

Scheme 32 Microwave-assisted synthesis of a [3-1-2] cycloaddition library...

Microwave-Assisted Synthesis of Sulfur and Nitrogen-Containing Heterocycles... 

One of the first published microwave-assisted synthesis of benzothiazoles is the condensation of a dinucleophile such as 2-aminothiophenol, with an ortho-ester (neat) in the presence of KSF clay in a mono-mode microwave reactor operating at 60 W under a nitrogene atmosphere [ 12] (Scheme 12). Traditional heating (oil bath, toluene as solvent and KSF clay) gave the expected products in similar yields compared to the microwave experiments but more than 12 h were required for completion. Solvent-free microwave-assisted syntheses of benzothiazoles was also described by attack of the dinucleophiles cited above on benzaldehydes and benzaldoximines [13] (Scheme 12). This methodology was performed in a dedicated monomode microwave reactor... 

Potent antimicrobial l,2,4-triazolo[3,4-fc]-l,3,4-thiadiazepines derivatives were prepared from readily accessible substituted 2-mercapto-l-aminotria-zoles and substituted chalcones on basic alumina in a solvent-free microwave-assisted synthesis (Scheme 28). Exposure of the reaction mixtures to microwaves led to an important decrease of the reaction time, which has been brought down from hours to seconds, accompanied by improved yields as compared with conventional heating [36]. This facile, rapid, and economic... 

The strategies explored and defined in the various examples presented open a way for wider application of microwave chemistry in industry. The most important problem for chemists today (in particular, drug discovery chemists) is to scale-up microwave chemistry reactions for a large variety of synthetic reactions with minimal optimization of the procedures for scale-up. At the moment, there is a growing demand from industry to scale-up microwave-assisted chemical reactions, which is pushing the major suppliers of microwave reactors to develop new systems. In the next few years, these new systems will evolve to enable reproducible and routine kilogram-scale microwave-assisted synthesis. 

Cellulose was the first type of solid support introduced for SPPS [91 ] however, the scope of its use is limited by low loading capacity ( 0.1 mmol/g) and chemical stability. In spite of these drawbacks, microwave-assisted synthesis was successfully performed on cellulose membranes [92-94] and beads [95]. 

Since 1986, when the very first reports on the use of microwave heating to chemical transformations appeared [147,148], microwave-assisted synthesis has been shown to accelerate most solution-phase chemical reactions [24-27,32,35]. The first application of microwave irradiation for the acceleration of reaction rate of a substrate attached to a solid support (SPPS) was performed in 1992 [36]. Despite the promising results, microwave-assisted soHd-phase synthesis was not pursued following its initial appearance, most probably as a result of the lack of suitable instriunentation. Reproducing reaction conditions was nearly impossible because of the differences between domestic microwave ovens and the difficulties associated with temperature measurement. The technique became a Sleeping Beauty interest awoke almost a decade later with the publication of several microwave-assisted SPOS protocols [37,38,73,139,144]. There has been an extensive... 

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