Microwave-assisted organic synthesis and transformations using benign reaction media

Microwave-assisted synthesis of new N₁,N₄-substituted thiosemicarbazones

We present an efficient procedure for the synthesis of thirty-six N₁,N₄-substituted thiosemicarbazones, including twenty-five ones that are reported for the first time, using a microwave-assisted methodology for the reaction of thiosemicarbazide intermediates with aldehydes in the presence of glacial acetic acid in ethanol and under solvent free conditions. Overall reaction times (20-40 min when ethanol as solvent, and 3 min under solvent free conditions) were much shorter than with the traditional procedure (480 min); satisfactory yields and high-purity compounds were obtained. The thiosemicarbazide intermediates were obtained from alkyl or aryl isothiocyanates and hydrazine hydrate or phenyl hydrazine by stirring at room temperature for 60 min or by microwave irradiation for 30 min, with lower yields for the latter. The preliminary in vitro antifungal activity of thiosemicarbazones was evaluated against Aspergillus parasiticus and Candida albicans.

Microwave-assisted partial hydrogenation of citral by using ionic liquid-coated porous glass catalysts

The microwave-assisted hydrogenation of citral (3,7-dimethylocta-2,6-dienal) to citronellal with molecular hydrogen as the reducing agent was investigated. Several polar and non-polar solvents were screened and imidazolium-based ionic liquids were applied as modifiers for the palladium-containing porous glass catalysts (Pd/TP). The best results were obtained with N-ethyl-N'-methylimidazolium dicyanamide, N-ethyl-N'-methylimidazolium acetate, or N-ethyl-N'-methylimidazolium trifluoroacetate, which were used to prepare supported catalysts with an ionic liquid layer (SCILL) on Pd/TP by wet-impregnation. The influence of pressure and temperature when using these ionic liquid-containing catalysts, as well as their long-term stabilities, were examined. Working with microwave-assisted heating, high yields of citronellal were achieved under mild conditions within short reaction times. Catalyst characterization was carried out by means of BET measurements, X-ray photoelectron spectroscopy (XPS) and thermo-gravimetric analyses. The influences of the ionic liquid layer were derived from experiments carried out before and after the reactions.

Alternative energy input: mechanochemical, microwave and ultrasound-assisted organic synthesis

Microwave, ultrasound, sunlight and mechanochemical mixing can be used to augment conventional laboratory techniques. By applying these alternative means of activation, a number of chemical transformations have been achieved thereby improving many existing protocols with superior results when compared to reactions performed under traditional conditions. The purpose of this critical review is to highlight the advances in this general area by presenting such newer applications in organic synthesis (175 references).

Microwave-Assisted Synthesis of N-Phenylsuccinimide

A microwave-assisted synthesis of N-phenylsuccinimide has been developed for the second-semester organic teaching laboratory. Utilizing this procedure, N-phenylsuccinimide can be synthesized by heating a mixture of aniline and succinic anhydride in a domestic microwave oven for four minutes in moderate yields (40-60%). This technique reduces the reaction time as compared to the traditional synthesis by several hours, which allows the preparation to be achieved in a single organic chemistry laboratory period. This reaction is performed in the absence of solvent, is energy efficient, and is atom economical; therefore, it represents a "greener" preparation than the traditional synthesis of N-phenylsuccinimide.

Microwave-assisted tandem transformation on an ionic-liquid support: efficient synthesis of pyrrolo/pyridobenzimidazolones and isoindolinone-fused benzimidazoles

A tandem transformation that involves the formation of three bonds and two heterocyclic rings in a one-pot fashion through amino-alkylation of an ionic-liquid-immobilized diamine with keto acids followed by successive double intramolecular cyclizations to afford a tricyclic framework has been explored. This tandem cyclization has been utilized to develop a rapid and efficient method to synthesize various pyrrolo[1,2-a]benzimidazolones and pyrido[1,2-a]benzimidazolones on an ionic-liquid support by using focused microwave irradiation. The application of this tandem cyclization was further extended to the aromatic keto acids to provide isoindolinone-fused benzimidazoles, a structurally heterogeneous library with skeletal diversity. The outcome of the cascade reaction was confirmed by the X-ray crystallographic study of the product directly attached to the ionic-liquid support. Use of the ionic liquid as a soluble support facilitates purification by simple precipitation along with advantages like high loading capacity, homogeneous reaction conditions, and monitoring of the reaction progress by regular conventional spectroscopic methods, whereas application of microwave irradiation greatly accelerates the rate of the reactions.

Microwave-Assisted Synthesis and Antibacterial Activity of Novel Chenodeoxycholic Acid Thiosemicarbazone Derivatives

A rapid and efficient method for the synthesis of novel chenodeoxycholic acid thiosemicarbazone derivatives under solvent-free conditions using microwave irradiation is reported. Ten novel compounds have been synthesised in good yields. Their structures were elucidated by 1H NMR, IR, ESI-MS spectra and elemental analysis. Preliminary results showed that some of these compounds possess inhibitory effects against S. typhimurium and E. coli.

Microwave-assisted green synthesis of silver nanostructures

Over the past 25 years, microwave (MW) chemistry has moved from a laboratory curiosity to a well-established synthetic technique used in many academic and industrial laboratories around the world. Although the overwhelming number of MW-assisted applications today are still performed on a laboratory (mL) scale, we expect that this enabling technology may be used on a larger, perhaps even production, scale in conjunction with radio frequency or conventional heating. Microwave chemistry is based on two main principles, the dipolar mechanism and the electrical conductor mechanism. The dipolar mechanism occurs when, under a very high frequency electric field, a polar molecule attempts to follow the field in the same alignment. When this happens, the molecules release enough heat to drive the reaction forward. In the second mechanism, the irradiated sample is an electrical conductor and the charge carriers, ions and electrons, move through the material under the influence of the electric field and lead to polarization within the sample. These induced currents and any electrical resistance will heat the sample. This Account summarizes a microwave (MW)-assisted synthetic approach for producing silver nanostructures. MW heating has received considerable attention as a promising new method for the one-pot synthesis of metallic nanostructures in solutions. Researchers have successfully demonstrated the application of this method in the preparation of silver (Ag), gold (Au), platinum (Pt), and gold-palladium (Au-Pd) nanostructures. MW heating conditions allow not only for the preparation of spherical nanoparticles within a few minutes but also for the formation of single crystalline polygonal plates, sheets, rods, wires, tubes, and dendrites. The morphologies and sizes of the nanostructures can be controlled by changing various experimental parameters, such as the concentration of metallic salt precursors, the surfactant polymers, the chain length of the surfactant polymers, the solvents, and the operation reaction temperature. In general, nanostructures with smaller sizes, narrower size distributions, and a higher degree of crystallization have been obtained more consistently via MW heating than by heating with a conventional oil-bath. The use of microwaves to heat samples is a viable avenue for the greener synthesis of nanomaterials and provides several desirable features such as shorter reaction times, reduced energy consumption, and better product yields.

Synthesis and Characterisation of Novel 1,3-Bis{(6-Aryl-1,2,4-Triazolo [3,4-b][1,3,4]Thiadiazol-3-yl)-Methoxy}benzenes under Microwave Irradiation

Eleven novel 1,3-bis{(6-aryl-1,2,4-triazolo[3,4-b][1,3,4]thiadiazol-3-yl)-methoxy}benzenes have been synthesised in high yields by the condensation of 5,5'-[1,3-phenylene-bis(oxymethylene)]bis(4-amino-3-mercapto-1,2,4-triazole) with substituted benzoic acids in the presence of phosphorus oxychloride under microwave irradiation. Compared with the conventional heating method, this method is facile, rapid and efficient. The structures of all new compounds were characterised by 1H NMR, IR, MS and elemental analysis.

Synthesis of novel pyrazolo[1,5-a]pyrazin-4(5H)-one derivatives and their inhibition against growth of A549 and H322 lung cancer cells

A series of substituted pyrazolo[1,5-a]pyrazin-4(5H)-one was synthesized by the reaction of ethyl 1-(2-oxo-2-phenylethyl)-3-phenyl-1H-pyrazole-5-carboxylate derivatives and 2-(2-aminoethoxy)ethanol or 2-morpholinoethanamine in the condition of microwave-assisted one-step and solvent-free in a good yield. The structures of the compounds were determined by IR, (1)H NMR and mass spectroscopy. In addition, a representative single-crystal structure was characterized by using X-ray diffraction analysis. Preliminary biological evaluation showed that the compounds could inhibit the growth of A549 and H322 cells in dosage-dependent manners.

Synthesis of New Ferrocene bis Thiocarbazones under Solvent-Free Conditions using Microwave

A simple efficient method for the synthesis of new ferrocene bis thiocarbazones via microwave irradiation under solvent-free conditions has been developed. Compared to the conventional method, its major advantages are short reaction times, good conversions and actually accorded the green synthesis technology. The structures of target compounds were confirmed by 1H NMR, IR, ESI-MS spectra data and elemental analysis. Preliminary bioassay results showed that some of these compounds possess inhibitory effects against S. aureus, S. pyogenes and E. coli bacteria.

Experimental evidence of selective heating of molecules adsorbed in nanopores under microwave radiation

We have performed in situ quasielastic neutron scattering (QENS) measurements on zeolite-guest systems under microwave irradiation, for comparison with corresponding simulations. Both experiment and simulation reveal selective heating of methanol in silicalite, but little to no heating of benzene in silicalite. Effective translational and rotational temperatures extracted from QENS data under microwave heating were found to depend on microwave power. In agreement with simulation, QENS rotational temperatures significantly exceed translational ones at high microwave power, thus providing the first microscopic proof for athermal effects in microwave-driven nanopores.

Synthesis of New Deoxycholic Acid bis Thiocarbazones under Solvent-Free Conditions using Microwave Irradiation

A simple method for the synthesis of new deoxycholic acid bis thiocarbazones under solvent-free conditions using microwave irradiation has been developed. Its main advantages are short reaction times, good conversions and the environmentally friendly nature of the process. Their structures were characterised by 1H NMR, IR, ESI-MS spectra and elemental analysis. The preliminary results indicate that some of these compounds possess inhibitory effects against Escherichia coli.

Microwave-assisted synthesis, crystal structure of pyrazolo[1,5-a]pyrazin-4(5H)-ones and their selective effects on lung cancer cells

A series of novel pyrazolo[1,5-a]pyrazin-4(5H)-one derivatives with hydrophilic group was synthesized under general heating condition and microwave-assisted condition. The structures of compounds were determined by IR, 1H NMR and HRMS, moreover, representative crystal structures were characterized by using X-ray diffraction analysis. Preliminary biological evaluation showed that some compounds could inhibit the growth of A549, H322 and H1299 cells in dosage dependent manners. The compounds could inhibit growth of A549, H322 and H1299 cells in different mechanism. Compounds 3e-h inhibited growth of A549 cells by inducing a strong G1-phase arrest. Whereas these compounds inhibited growth of H1299 and H322 cells by inducing apoptosis.

Water-Promoted Palladium-Catalysed Heck Cross-Coupling Reactions of Aryl Halides with Alkenes in TBAB

Water was found to promote the ligand-free Pd(OAc)2-catalysed Heck cross-coupling reaction. In the presence of 3 mol% Pd(OAc)2, 2 equiv. of Cs2CO3 and 15 mg of water, the coupling of a wide variety of aryl halides with alkenes proceeded smoothly in TBAB ( n-Bu4NBr) in moderate to excellent yield. The Pd(OAc)2/TBAB system could be recovered and reused at least three times without loss of catalytic activity.