Organic syntheses using microwave irradiation solvents

Wittig olefination is a well-known reaction for constructing carbon-carbon bonds in organic synthesis [28]. A few solvent-free Wittig reactions [29, 30] have been developed which are explored later in this chapter in conjunction with microwave irradiation [31, 32] or using a special stirring device such as a high-speed steel mill... 

Alternatively, microwave-assisted synthesis has been carried out using standard organic solvents under open-vessel conditions. If solvents are heated by microwave irradiation at atmospheric pressure in an open vessel, the boiling point of the solvent typically limits the reaction temperature that can be achieved. In order to none-... 

Several microwave-assisted protocols for soluble polymer-supported syntheses have been described. Among the first examples of so-called liquid-phase synthesis were aqueous Suzuki couplings. Schotten and coworkers presented the use of polyethylene glycol (PEG)-bound aryl halides and sulfonates in these palladium-catalyzed cross-couplings [70]. The authors demonstrated that no additional phase-transfer catalyst (PTC) is needed when the PEG-bound electrophiles are coupled with appropriate aryl boronic acids. The polymer-bound substrates were coupled with 1.2 equivalents of the boronic acids in water under short-term microwave irradiation in sealed vessels in a domestic microwave oven (Scheme 7.62). Work-up involved precipitation of the polymer-bound biaryl from a suitable organic solvent with diethyl ether. Water and insoluble impurities need to be removed prior to precipitation in order to achieve high recoveries of the products. 

Numerous reactions in organic synthesis can be achieved under solid-liquid PTC and with microwave irradiation in the absence of solvent, generally under normal pressure in open vessels. Increased amounts of reactants can be used to ensure better compatibility between the in-depth penetrability of materials and the radiation wavelength. 

The kinetics of the acid-catalyzed esterification reaction of 2,4,6-trimethylbenzoic acid in i-PrOH under microwave irradiation have been investigated [84], A simple and practical technique for MW-assisted synthesis of esters has been reported wherein the reactions are conducted either on solid mineral supports or by using a phase transfer catalyst (PTC) in the absence of organic solvents [85], The esterification of enols with acetic anhydride and iodine has also been recorded [86],... 

In microwave-assisted synthesis, a homogeneous mixture is preferred to obtain a uniform heating pattern. For this reason, silica gel is used for solvent-free (open-vessel) reactions or, in sealed containers, dipolar solvents of the DMSO type. Welton (1999), in a review, recommends ionic liquids as novel alternatives to the dipolar solvents. Ionic liquids are environmentally friendly and recyclable. They have excellent dielectric properties and absorb microwave irradiation in a very effective manner. They exhibit a very low vapor pressure that is not seriously enhanced during microwave heating. This makes the process not so dangerous as compared to conventional dipolar solvents. The polar participants of organic ion-radical reactions are perfectly soluble in polar ionic liquids. 

The appropriateness of using of microwave-assisted organic synthesis to obtain pyridinecarboxylic acids and pyrimidinecarboxylic acids was also described in [202]. The application of microwave irradiation reduces significantly the reaction time from 2 h to 10 min, increases yields of the target compounds and also allows high-boiling and hard-to-remove solvents (DMF and acetic acid) to be replaced by ethanol. 

Another method involves microwave irradiation. It has been described for the synthesis of 1,3-dialkylimidazolium tetrachloroaluminates [40]. This method precludes the use of volatile organic solvents and is faster, more efficient and also ecofriendly, affording high yields of the desired products. 

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