Microwave-induced catalytic transformation of 2-<Emphasis Type="Italic">tert</Emphasis>-butylphenol at low temperatures

Catalytic transformation of 2-tert-butylphenol (2TBP) on a heterogeneous K10 catalyst has been studied in the temperature range from +75 to –176°C under microwave and conventional conditions. The reaction was carried out with or without solvents and the kinetic results were described using the method of initial reaction rates. Reaction rates were recorded under microwave conditions and compared to conventional ones. Transformation of 2TBP did not proceed below 0°C under conventional conditions, whereas under microwave irradiation significant initial reaction rates were observed. Relatively high initial reaction rates at low temperatures were most probably caused by superheating of catalyst particles. The superheating temperature was calculated to be between 80 and 115°C above the temperature of the bulk.     

Silica‐supported synthesis of some 1,3,5‐trisubstituted 2‐pyrazolines under solvent‐free and microwave irradiation conditions

A simple method for the synthesis of 2‐pyrazolines is described which occurs on silica surface under solvent‐free conditions within 110‐180 sec using microwave irradiation. The results obtained indicate that the use of silica gel as a support in pyrazoline formation reactions can have a profound effect on reaction rates and yields and cause cleaner reaction conditions.     

Kröhnke reaction in aqueous media: one-pot clean synthesis of 4′-aryl-2,2′:6′,2″-terpyridines

A clean aqueous Kröhnke reaction process has been accomplished via a one-pot procedure of 2-acetylpyridine with aromatic aldehyde and ammonium acetate under microwave irradiation or conventional heating conditions. This method is convenient, economic and environmental friendly.     

One-Pot Synthesis of 1-Aminophosphinic acids using 50% Hypophosphorus Acid under microwave irradiation

A simple and efficient method has been developed for the synthesis of α-aminophosphinic acids from hypophosphorus acid under solvent-free conditions using microwave irradiation. α-Aminophosphinic acids were obtained in high yield under mild conditions by reaction of hypophosphorus acid with aldehydes in the presence of amines under microwave irradiation.     

微波-超声复合场辅助水介质砜类化合物合成

发展了一种微波-超声复合场下绿色高效合成砜类化合物的简便方法。 以芳甲基氯和苯亚磺酸钠为原料,以水为反应介质,在微波-超声复合场辅助下合成了一系列芳甲基苯基砜。 通过考察微波功率、超声功率、原料投料比、溶剂体积和反应时间等因素的影响,得出了苄基氯与苯亚磺酸钠之间模型反应的最优反应条件:微波功率为40 W,超声功率为50 W,苄基氯与苯亚磺酸钠的摩尔配比为1:3,反应时间为5 min。 在此条件下合成的苄基苯基砜产率为83%。 相比常规油浴条件,在微波-超声复合场的强化下反应速率提升了约42倍。 该方法具有较好的底物适应性,成功合成了23种砜类化合物。     

Microwave-assisted synthesis of α-aminophosphinic acids from hypophosphorus acid salts under solvent free conditions

A simple, efficient, and general method has been developed for the synthesis of α-aminophosphinic acids from hypophosphorus salts under solvent-free conditions using microwave irradiation. α-Aminophosphinic acids were obtained in high yield under mild conditions by reaction of the amine salts of hypophosphorus acid with aldehydes in the presence of microwave irradiation.     

Microwave‐Assisted Solvent‐Free Synthesis of Highly Functionalized Pyrimidine Derivatives

We here described an efficient method for the synthesis of a series of highly functionalized pyrimidines via the addition and condensation reaction of ketene dithioacetals with guanidine carbonate or amidine hydrochlorides by microwave irradiation under solvent‐free conditions in the absence of a catalyst, giving the products with good yields (79–98%).     

A Green,Solvent‐Free,Microwave‐Assisted,High‐Yielding YbCl3 Catalyzed Deprotection of THP/MOM/Ac/Ts Ethers of Chalcone Epoxide and 2′‐Aminochalcone and Their Sequel Cyclization

Under microwave and solvent‐free conditions, YbCl₃ efficiently catalyzed the deprotection of tetrahydropyran‐2‐yl, methoxymethyl (MOM), acetyl, and tosyl groups and sequel cyclization of chalcone epoxide to 2‐hydroxyindanone and 2′‐aminochalcone to aza‐flavanone. The reaction afforded the products in excellent yield (78–99%) at 850 W microwave heating within 1–5 min under eco‐friendly conditions. The merits of the presented protocol include high yield, use of microwave irradiation, solvent‐free condition, catalyst reusability, and no need for purification with column chromatography. The present method is very much milder but more advanced than those reported earlier.     

Investigating the existence of nonthermal/specific microwave effects using silicon carbide heating elements as power modulators

The use of passive heating elements made out of chemically inert sintered silicon carbide (SiC) allows microwave transparent or poorly absorbing reaction mixtures to be heated under microwave conditions. The cylindrical heating inserts efficiently absorb microwave energy and subsequently transfer the generated thermal energy via conduction phenomena to the reaction mixture. In the case of low to medium microwave absorbing reaction mixtures, the addition of SiC heating elements results in significant reductions (30-70%) in the required microwave power as compared to experiments performed without heating element at the same temperature. The method has been used to probe the influence of microwave power (electromagnetic field strength) on chemical reactions. Six diverse types of chemical transformations were performed in the presence or absence of a SiC heating element at the same reaction temperature but at different microwave power levels. In all six cases, the measured conversions/yields were similar regardless of whether a heating element was used or not. The applied microwave power had no influence on the reaction rate, and only the attained temperature governed the outcome of a specific chemical process under microwave conditions.     

Microwave‐Assisted Solid‐Phase Synthesis of a 1,2‐Disubstituted Benzimidazole Library by Using a Phosphonium Linker

An efficient and rapid microwave‐assisted solid‐phase method for the synthesis of 5‐methyl‐1,2‐disubstituted benzimidazoles derivatives has been developed. The phosphonium linker, obtained by reaction between polymer‐supported triphenylphosphine and 4‐fluoro‐3‐nitrobenzyl iodide, underwent aromatic substitution with primary amines, followed by one‐pot reaction with aldehydes in the presence of SnCl₂·2H₂O, yielded the benzimidazole system under microwave irradiation. The final products were released from the resin with NaOH under microwave irradiation and were obtained in high purity and good overall yield.     

Microwave‐assisted Regioselective Synthesis of Novel Bis(azoles) and Bis(azoloazines)

We introduce efficient regioselective synthesis of a series of novel bis(pyrazoles) and bis(isoxazoles) via 1,3‐dipolar cycloaddition reaction under microwave irradiation. Unequivocal structural assignment of the obtained regioisomers was determined utilizing ¹H‐¹³C HMBC NMR techniques as a valuable tool. A comparative study of the aforementioned reactions was carried out under conventional method as well as under microwave irradiation conditions.     

A novel synthesis of bis-(α-hydroxyalkyl)phosphinic acids involving microwave irradiation

A simple, efficient, and general method has been developed for the synthesis of bis-(α-hydroxyalkyl)phosphinic acids from hypophosphorous acid using microwave irradiation. Bis-(α-hydroxyalkyl)phosphinic acids were obtained in high yield under mild conditions by reaction of hypophosphorous acid with aldehydes under microwave irradiation.     

Microwave‐assisted suzuki coupling reaction for rapid synthesis of conjugated polymer–poly(9,9‐dihexylfluorene)s as an example

Long reaction period (dozens of hours) is often required for the synthesis of conjugated polymers by palladium‐catalyzed Suzuki polymerization reaction. This work shows that microwave can accelerate Suzuki polymerization to realize the ultra‐rapid synthesis of conjugated polymers, here poly(9,9‐dihexylfluorene)s (PDHFs) as an example. The effects of reaction conditions on the polymerization have been systematically investigated, including the mode of microwave irradiation, microwave power, reaction temperature, reaction time, solvents, catalyst species, and catalyst concentrations. Compared with the conventional heating method (oil bath) for the synthesis of PDHFs (48 h, M_w = 20,000 g/mol), Suzuki polymerization under optimized microwave condition can yield PDHFs with higher molecular weight (M_w = 40,000 g/mol) in a much shorter time (14 min). The structures of obtained PDHFs samples are fully characterized spectroscopically, demonstrating well‐defined PDHFs have been prepared through microwave‐assisted (MA) Suzuki polymerization reaction. In addition, the mechanism of MA Suzuki polymerization is proposed preliminarily. © 2013 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Synthesis of alkyl α‐aminomethyl‐phenylphosphinates and N,N‐bis(alkoxyphenylphosphinylmethyl)amines by the microwave‐assisted Kabachnik–Fields reaction

As a novel extension, the Kabachnik–Fields reaction was applied to the synthesis of alkyl α‐aminomethyl‐phenylphosphinates, and the double phospha‐Mannich reaction was utilized in the preparation of bis(alkoxyphenylphosphinylmethyl)amines. A total of 27 new aminophosphinate derivatives were synthesized by the microwave‐assisted solvent‐free condensation of alkyl phenyl‐H‐phosphinates, paraformaldehyde, and primary or secondary amines. The starting P‐species were also prepared under microwave conditions. The formation of the N‐methylated aminomethyl‐phenylphosphinate by‐products was also investigated.     

A Multicomponent,Catalyst‐free,One‐Pot Synthesis of Functionalized 1,4‐Dihydroquinolines and Their Antimicrobial Studies

An atom‐efficient, catalyst‐free, and environmentally friendly approach towards the synthesis of substituted 1,4‐dihydroquinolines ( 5a , 5b , 5c , 5d , 5e , 5f , 5g , 5h , 5i , 5j ) through a one‐pot multicomponent reaction involving resorcinol, aromatic aldehyde, acetoacetanilide, and ammonium acetate has been developed. The reactions were carried out under both conventional and microwave irradiation conditions. In general, improvement in rate and yield were observed when reactions were carried out under microwave irradiation compared with classical conditions. Most of the new compounds possessed moderate to significant antibacterial and antifungal activities.     

One‐Pot Microwave‐Assisted Synthesis of Benzopyrano[2,3‐c]pyrazol‐3‐one Derivatives

A convenient one‐pot microwave‐assisted synthesis of Benzopyrano[2,3‐c]pyrazol‐3‐one derivatives was developed, where Knoevenagel condensation and hydrazinolysis reactions were continuously performed under microwave irradiation without separation and purification of the intermediates until the last cyclocondensation reaction except the substrates addition. The one‐pot procedure exhibited shorter reaction times and higher yields (25–55%) of the objects than the conventional heating method.     

Microwave Accelerated Polymerization of 2‐Phenyl‐2‐oxazoline

Summary: We studied the cationic ring‐opening polymerization of 2‐phenyl‐2‐oxazoline under microwave irradiation. A comparison with thermal heating shows a great enhancement in the reaction rates while the living character of the polymerization is conserved. The polymerizations were performed at the temperature of boiling butyronitrile (123 °C). The polymerization of 2‐phenyl‐2‐oxazoline under microwave conditions, described herein for the first time, is shown to be a rapid and environmentally friendly alternative to the classical methods.

Schematic of the activation of the reactive site by microwave irradiation.     

Microwave-Induced Solvent-free Synthesis of β-Keto Esters Using Montmorillonite KSF and K10 Clays as Efficient and Recyclable Heterogeneous Solid Acids

Montmorillonite KSF and K10 clays catalyzed effectively the reaction of methyl diazoacetate with various aldehydes using microwave irradiation under solvent-free conditions affording the corresponding β-keto esters in good yields and short reaction time. The present method is an improvement for the previous known synthetic methods and has many obvious advantages compared to them including the high efficiency, generality, high yields, operational simplicity, environmental benignity, and possibility of recycling the natural solid clays.     

Microwave‐Assisted Free Radical Polymerizations and Copolymerizations of Styrene and Methyl Methacrylate

Summary: Radical homopolymerizations and copolymerizations of styrene were performed in toluene and N,N‐dimethylformamide (DMF) as solvents using different initiators with and without microwave irradiation. Only the homopolymerization of styrene under microwave irradiation in DMF with DtBP showed significantly enhanced styrene conversion whereas other initiators resulted in no or only slight increase of styrene conversion under microwave irradiation. In any case, DMF was required to gain in styrene conversion under microwave irradiation. Significantly higher monomer conversions were observed under otherwise comparable conditions in the copolymerization of styrene and methyl methacrylate (MMA) in DMF. Microwave‐induced selectivity of monomers was not observed in copolymerizations.

Yield of styrene polymerizations under varying reaction conditions initiated by DtBP.     

A green and selective synthesis of 2‐aryloxazines and 2‐aryltetrahydropyrimidines

An efficient method for the selective synthesis of 2‐substituted oxazines and tetrahydropyrimidines by the reaction of arylnitriles with 3‐amino‐1‐propanol and 1,3‐diaminopropane in the presence of montmorillonite K‐10 and KSF as inexpensive, environmentally benign, and reusable catalysts under classical heating conditions and microwave irradiation is reported. J. Heterocyclic Chem., (2011).