Atmospheric pressure microwave assisted heterogeneous catalytic reactions

The purpose of the study was to investigate microwave selective heating phenomena and their impact on heterogeneous chemical reactions. We also present a tool which will help microwave chemists to answer to such questions as "My reaction yields 90% after 7 days at reflux; is it possible to obtain the same yield after a few minutes under microwaves?" and to have an approximation of their reactions when conducted under microwaves with different heterogeneous procedures. This model predicting reaction kinetics and yields under microwave heating is based on the Arrhenius equation, in agreement with experimental data and procedures.     

Microwave assisted solid additive effects in simple dry chlorination reactions with n-chlorosuccinimide

Solid additives participate in the dry microwave assisted chlorination reaction of N-chlorosuccinimide with the xylenes affecting both yields and chemoselectivities. Total yields can be increased up to nine times for simple alkylaromatics and chemoselectivities can be altered according to the desired ring or α-side chlorination product by choosing the appropriate additive. We believe that in these reactions the solid additives play a very important role by increasing yields and affecting chemoselectivities, as well as behaving as microwave energy absorbers that consequently aid the transfer of heat to the active reagents.     

Microwave‐accelerated Conjugate Addition of 2‐Arylindoles to Substituted β‐Nitrostyrenes in the Presence of Ammonium Trifluoroacetate: An Efficient Approach for the Synthesis of a Novel Class of CB1 Cannabinoid Receptor Allosteric Modulators

2‐Arylindoles, in general, exhibit reduced reactivity towards the conjugate addition to substituted nitrostyrenes, when compared with indoles. We report here an efficient, expeditious, and high‐yielding conjugate addition of 2‐arylindoles to substituted β‐nitrostyrenes in the presence of ammonium trifluoroacetate under microwave irradiation. This method is mild with high and reproducible yields and is selective for addition of β‐nitrostyrenes when compared with other electrophiles. The results obtained from the optimized microwave method consistently provided improved yields in a shorter time compared with those of the conventional heating synthetic route.     

Solvent- and metal-free selective oxidation of thiols to disulfides using I2/DMSO catalytic system

We describe herein a simple, fast and inexpensive protocol for the oxidative coupling of thiols employing a stoichiometric amount of DMSO and iodine as the catalyst. Various aromatic disulfides were obtained in good to excellent yields in short reaction times at room temperature, while aliphatic disulfides were achieved in good yields when the reactions were conducted under microwave irradiation.     

Microwave Intensified Synthesis: Batch and Flow Chemistry

Many studies have been conducted on organic and inorganic synthesis by microwave heating owing to its special heating mechanism, leading to improved reaction rate, higher purity and yields. We specifically demonstrated microwave heating in the fabrication of nanoparticles and polyester. By fine‐tuning the microwave and experimental parameters, the materials prepared have shown excellent physical and bio‐properties, e. g. narrow particle size distribution, controlled morphology, varied molecular structure and so forth. We further highlight the recent procedure of using fluidic reactors on preparing both metals and metal oxides nanoparticles. The experimental design strategies and fundamentals of the microwave interaction with chemicals are presented. Furthermore, the key factors and issues facing in this area are also discussed.     

Suzuki coupling of aryl chlorides with phenylboronic acid in water, using microwave heating with simultaneous cooling

[reaction: see text]. We present here a methodology for the Suzuki coupling of aryl chlorides with phenylboronic acid using Pd/C as a catalyst, water as a solvent, and microwave heating. We show that simultaneous cooling in conjunction with microwave heating prolongs the lifetime of the aryl chloride substrates during the course of the reaction and, as a result, yields of the desired biaryl as well as overall recovery of material can be increased.     

Microwave-assisted expeditious and efficient synthesis of cyclopentene ring-fused tetrahydroquinoline derivatives using three-component Povarov reaction

We report here an efficient and expeditious microwave-assisted synthesis of cyclopentadiene ring-fused tetrahydroquinolines using the three-component Povarov reaction catalyzed by indium (III) chloride. This method has an advantage of shorter reaction time (10–15 min) with high and reproducible yields (up to 90%) and is suitable for parallel library synthesis. The optimization process is reported and the results from the microwave route are compared with those of the conventional synthetic route. In almost all cases, the microwave acceleration consistently provided improved yields favoring the cis-diastereomer.     

Solid supported synthesis of structurally diverse dihydropyrido[2,3-d]pyrimidines using microwave irradiation

We have synthesized a library of 16 dihydropyrido[2,3-d]pyrimidines in high yields (82-92%) on solid support using microwave irradiation.     

Highly efficient microwave-accelerated preparation of β-ketoimines

We present here a new and efficient methodology for the β-ketoimine ligand with microwave heating system. The new method showed faster reaction rates, higher yields, and selectivities of the desired compounds in the absence of solvents.     

Solventless lactam synthesis by intramolecular cyclizations of alpha-iminoester derivatives under microwave irradiation

We have previously reported a new synthesis of amides from esters and amines under microwave irradiation, offering much higher yields than those achieved with conventional heating [1]. We have now extended these studies to the ring closure of neat iminoesters I2, I3 and I4-I6 to give five- and six-membered ring lactams L5, L6 and larger lactams L7-L9 (where I means imine and L means lactam), respectively, under both classical heating conditions and microwave irradiation.     

RAPID CARBOHYDRATE PROTECTING GROUP MANIPULATIONS ASSISTED BY MICROWAVE DIELECTRIC HEATING

The protocols for oligosaccharide synthesis are often tedious due to extended synthetic routes and reaction times. We herein describe methods assisted by microwave dielectric heating, which enable very short reaction times and high yields for the introduction and removal of eleven of the most commonly used protecting groups in carbohydrate syntheses. Several examples are reported, where solid supported reagents in combination with microwave dielectric heating have been used. This results in both faster and easier synthesis and purification.     

p-Toluenesulfonic acid-promoted selective functionalization of unsymmetrical arylalkynes: a regioselective access to various arylketones and heterocycles

Regioselective hydration of a wide range of internal alkynes has been afforded in high to good yields by using PTSA in EtOH. The scope of the reaction of alkynes has been delineated. Arylaliphatic alkynes and diarylalkynes were regioselectively hydrated in good to excellent yields and short reaction times when the reaction was achieved under microwave irradiation. Moreover, diarylalkynes, arylenynes as well as diaryldiynes bearing a methoxy- or a thiomethyl substituent on the ortho position underwent a regioselective 5-endo-dig-cyclization to give a variety of 2-aryl- and 2-styrylbenzofuran or benzothiphene derivatives. We believe that, this new environmentally metal-free procedure combined to microwave irradiation would be in importance in the search of green laboratory-scale synthesis.     

Intramolecular cyclization of delta-iminoacetylenes: a new entry to pyrazino[1,2-a]indoles

[reaction: see text] The synthesis of the pyrazino[1,2-a]indole nucleus was achieved by intramolecular cyclization of several 2-carbonyl-1-propargylindoles in the presence of ammonia. The reaction conditions were optimized using microwave heating and a pool of catalysts. Cyclization of 1-alkynylindole-2-carbaldehydes was easily accomplished under standard heating conditions, whereas microwave heating contributed to reduced reaction times and improved overall yields. Moreover, a fine-tuning of the microwave irradiation time made possible the selective synthesis of both pyrazino[1,2-a]indole isomers. TiCl4 proved the catalytic system of choice to achieve pyrazinoindoles in satisfactory yields starting from 1-alkynyl-2-acetylindoles and 1-alkynyl-2-benzoylindole derivatives. Also in these cases, microwave heating contributed to faster reactions and improved yields. The uncatalyzed versus catalyzed reaction mechanism is discussed.     

Microwave Flow: A Perspective on Reactor and Microwave Configurations and the Emergence of Tunable Single‐Mode Heating Toward Large‐Scale Applications

Microwave heating in chemical reactions was first reported in 1986. There have since been many reports employing microwave heating in organic chemistry, where microwave heating has afforded higher yields of products in shorter time periods. However, such reactions are challenging to scale in batch due to the limited penetration depth of microwaves as well as the wave propagation dependence on cavity size. Continuous flow has addressed both these issues, enabling scalability of microwave processes. As such, a host of reports employing microwave flow chemistry have emerged, employing various microwave heating and reactor configurations in the context of either custom‐built or commercial apparatus. The focus of this review is to present the benefits of microwave heating in the context of continuous flow and to characterize the different types of microwave flow apparatus by their design (oscillator, cavity type and reactor vessel). We advocate the adoption of tunable, solid‐state oscillator single‐mode microwave flow reactors which are more versatile heaters, impart better process control and energy efficiency toward laboratory and larger‐scale synthetic chemistry applications.     

Palladium-catalyzed microwave-assisted amination of 1-bromonaphthalenes and 5- and 8-bromoquinolines

[reaction: see text] 1-Aminonaphthalenes and 5- and 8-aminoquinolines were rapidly prepared from the respective aryl bromides in good yields by Pd-catalyzed aryl amination under microwave conditions. Consistent improvements in yields over those obtained under standard conditions were seen with quinoline substrates. In the cases where 5-bromo-8-cyanoquinoline was used as a substrate, no desired products were obtained under standard conditions with a number of different primary and secondary amines. However, microwave conditions provided the desired products in good to excellent yields.     

A facile stereoselective synthesis of julolidine hybrid analogs via domino knoevenagel intramolecular hetero Diels–Alder reaction

We have reported here a facile stereoselective synthesis of julolidine hybrid analogs by employing domino Knoevenagel intramolecular hetero Diels–Alder reaction on symmetrical 1,3-diones and unsymmetrical 1,3-diones It was found that the cycloaddition proceeded efficiently under microwave irradiation to afford highly stereoselective cycloadducts in good yields     

Magnetic isotope effect on kinetic parameters and quantum beats of radical pairs in micellar solution studied by optically detected esr using pulsed microwave

We investigated the quantum beats, the oscillation between singlet and triplet states of radical pairs induced by the microwave field resonant to one of the component radicals. They were observed as the alternation of the yields of the component radicals by a nanosecond time-resolved optical absorption with the X-band (9.15 GHz) resonant microwave pulse. This technique was applied to the photochemical reaction of benzophenone, benzophenone-d(10), and benzophenone-carbonyl-(13)C in a sodium dodecylsulfate micellar solution with a step-by-step increase of the resonant microwave pulse width. The yields of the component radicals showed alternation with an increase of the microwave pulse width. This indicates that the radical pair retains spin coherence in the micellar solution. The magnetic isotope effect on the amplitude of the quantum beat was observed. The MW effect on the quantum beat of BP-(13)C decreases from 80% to 60% of that of BP by irradiation of the pi-pulse MW due to spin-locking. The kinetic parameters were also determined using the X- or Ku-band (17.44 GHz) region. They are almost similar to each other except for the intersystem recombination rate in the system of BP-(13)C, which may be slightly higher than those in other systems.     

An efficient one-step regiospecific synthesis of novel isoxazolines and isoxazoles of N-substituted saccharin derivatives through solvent-free microwave-assisted [3+2] cycloaddition

Novel isoxazolines and isoxazoles of N-substituted saccharin derivatives are synthesized in good yields by 1,3-dipolar cycloaddition of N-allyl or propargyl N-substituted saccharin with arylnitrile oxide under solvent-free microwave irradiation. In this process, the yields were significantly improved over conventional heating, without alteration of the selectivity. The regioselectivity as well as the nonthermal specific microwave effect are discussed.     

Tricholoroisocyanuric Acid as a Useful Reagent in Microwave‐assisted Aromatization of 1,3,5‐Trisubstituted 2‐Pyrazolines

An efficient aromatization of 1,3,5‐trisubstituted 2‐pyrazolines to their corresponding pyrazoles has been performed by tricholoroisocyanuric acid [TCCA] under microwave irradiation in excellent yields. It has been observed that the reactions occur more rapidly under microwave irradiation conditions, and the amount of the reagent TCCA consumed is considerably reduced to afford better yields when compared with conventional thermal conditions at the same temperature.     

Fast oxidation of secondary alcohols by the bromate-bromide system using cyclic microwave heating in acidic water

We report an improved, gentle, cyclic microwave activation technique for the oxidation of secondary alcohols using nonhazardous hypobromous acid (BrOH) as the reagent in acidic water. Several aliphatic and aromatic secondary alcohols were successfully oxidized to the corresponding ketones using this technique in high yields and with only minor amounts of side products.