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Solvent and Solvent Free (Neat) Reactions in Microwave Synthesis
The two main types of conditions used for chemical reactions, those run in the presence of solvent and those run in a solventless environment, are equally important and both can benefit from microwave heating.
Microwave irradiation is not only applicable to standard solution-phase work, but to solid-phase and solvent-free systems as well. Many synthetic methods can be executed by at least one of these systems, though one may have to experiment in order to find the optimal conditions. This chapter discusses the different types of reaction conditions that can be used successfully with microwave irradiation and should be utilized in conjunction with Chapter 4 for synthetic applications. Note: The reader should assume that all reaction schemes shown in this chapter utilize microwave irradiation. In multi-step schemes, the use of microwaves is indicated by the word “microwaves” on the arrow.
Reactions in the presence of solvent
Solution-phase reactions performed in the presence of solvent can be either homogeneous or heterogeneous. Homogeneous reactions include your standard organic reactions in which all reagents are dissolved in the solvent. Microwave irradiation has been used extensively and successfully with homogeneous solution-phase reactions. Chapter 4 provides an in-depth review of the many homogeneous synthetic applications that have been enhanced with microwaves.
Heterogeneous reactions in solution involve insoluble solids that are used as reagents, catalysts, or supports. These include transition-metal and Lewis acid catalysts, non-dissolvable salts, and solid-phase resins (beads, lanterns, crowns, pins). These types of transformations are widely used and highly successful. One of the main disadvantages in traditional heterogeneous reactions are the long reaction times that are required for their completion, which is largely due to the insolubility. Use of microwave irradiation has been shown to drastically speed up these reactions, allowing for productive high throughput synthesis.
The following chapter on synthetic applications extensively covers heterogeneous reactions involving transition-metal catalysts, as well as those that utilize Lewis acids and other insoluble salts. It also provides a few examples of solid-phase reactions.188,204,609,631 The remainder of this section will provide a more detailed review on microwave-enhanced reactions that have been performed on a solid-phase resin.27-42
Combinatorial chemistry on solid-phase supports was first applied to peptide synthesis. It made sense to use peptides in the first microwave-assisted, solid-phase reaction. Traditional peptide hydrolysis requires high temperature 6M HCl for at least 24 hours. In 1988, Yu et al. performed a successful hydrolysis in seven minutes in a domestic microwave oven (Scheme 6).27 The same group, four years later, performed peptide coupling reactions in quantitative yields with microwave irradiation (Scheme 7).28
Scheme 6
Scheme 7
Sigmatropic rearrangements are important pericyclic reactions that involve the formation of new carbon–carbon bonds. Sometimes, conventional methods require very long reaction times. Microwave irradiation has been used to facilitate these transformations, and this is outlined in the next chapter. Claisen rearrangements have been successfully performed on a solid phase resin coupled with microwave heating. Scheme 8 illustrates the rearrangement of resin bound O-allylic aryl ethers to ortho-allylic salicylic acid derivatives, where the allyl, hydroxyl, and carboxylic acid groups are adjacent to each other.38 These compounds are difficult to synthesize with traditional aromatic substitution reactions.
Scheme 8
Strohmeier and Kappe have performed an important example of microwave-assisted, solid-phase parallel synthesis.29 Enones, as well as their 1,3-dicarbonyl intermediates, are important building blocks for heterocyclic scaffolds that are used in pharmaceutical drug design. Conventional solid-phase methods require multiple steps and long high-temperature reaction times. In this microwave-enhanced, two-step procedure, acetoacetylation of a polystyrene Wang (PS-Wang) resin to a functionalized β-ketoester, followed by Knoevenagel condensation with an aldehyde, yields enones in less than one hour (Scheme 9).
Scheme 9
Glass and Combs have also performed rapid parallel synthesis on solid-phase resins with microwave irradiation.31 They have examined the utility of a “safety catch” sulphonamide linker to produce large libraries of diverse amides and ureas. These safety catch linkers are highly stable through a given synthetic sequence until cleavage from the resin is necessary. The linker must first be activated before nucleophilic displacement of the substrate can occur. Conventional displacement of the substrate requires a very strong nucleophile and limits library diversity. The use of microwave irradiation allows for any nucleophile to displace the resin, including weak ones. Scheme 10 shows a facile biaryl urea synthesis, which includes a Suzuki coupling reaction, linker activation via alkylation, and subsequent cleavage with diisopropyl amine (DIPA).
Scheme 10
Microwave-assisted, solid-phase syntheses are not restricted to spherical bead polymer resins. Scharn et al. used a planar cellulose membrane to synthesize a parallel library of 8000 1,3,5-triazines via nucleophilic substitution reactions.40 The planar membrane is composed of an array of “spots” that are individually derivatized. Scheme 11 illustrates how an amino-functionalized spot is doped with cyanuric chloride and then diversified with different amines by microwave heating. The second nucleophilic substitution requires five hours of thermal heat for completion, but with microwave irradiation, an entire library can be synthesized in six minutes.
Scheme 11
Solvent Free (Neat) reactions
Reactions performed in a solvent-free environment are becoming more prevalent in organic chemistry. An increasing need for less hazardous reaction conditions and environmentally safe procedures, or green chemistry, has led chemical synthesis in this direction. Microwave irradiation has been used extensively in solvent-free reactions.5,8,16,20,43-181 There are three main types of solvent free reactions: reaction mixtures adsorbed onto mineral oxides, phase transfer catalysis (PTC), and neat reactions. This section will identify and provide examples for each type. For an overview of a wider range of different chemical transformations that can be performed solventless, the reader should consult Chapter 4, as there are over 100 additional solvent-free references.
An increasingly popular solvent-free method is to adsorb reagents onto mineral oxides. The reagent is first dissolved in an appropriate volatile solvent. After the mineral oxide (alumina, silica gel, clay, or zeolites) is added, the solvent is removed by evaporation. The impregnated solid support is then irradiated with microwaves in “dry media”. Upon completion of the reaction, a solvent is added to extract the product(s) from the support. Choice of solid support depends on the type of reaction a chemist is going to perform. Alumina can act as a base, but if a stronger one is needed, potassium fluoride on alumina is extremely basic. Silica gel naturally acts as a weak acid, while some of the montmorillonite clays provide acidities near sulfuric and nitric acids. As a whole, this solid-state application will greatly reduce the amount of solvent used that eventually needs to be properly disposed of and will minimize potentially hazardous reaction conditions.
Kidwai and co-workers have done extensive research in solvent-free reaction chemistry.37,104-113,276,290 Scheme 12 shows an example of a microwave-enhanced synthesis to N-acylated cephalosporin derivatives.37 Cephalosporanic acid and a heterocyclic carboxylic acid were adsorbed onto basic alumina and irradiated with microwaves for 2 minutes to yield the antibacterials in 82-93% yield. With thermal heat, this reaction can take anywhere from two to six hours and provides much lower yields. Another reaction performed on basic alumina is shown in Scheme 13.105,113 Barbituric and thiobarbituric acid derivatives are adsorbed onto the alumina with substituted arylmercuric chlorides to yield biologically active fungicides.
Scheme 12
Scheme 13
Kabalka and co-workers have also explored solventless, microwave-enhanced reactions on dry media.99-101,628-629 Sonogashira coupling reactions are a palladium-catalyzed reaction between terminal alkynes and an aryl halide. These reactions typically employ a solvent and an amine, which produce environmental burdens. Scheme 14 illustrates a Sonogashira coupling that was performed on potassium fluoride/alumina doped with a palladium/copper iodide/triphenylphosphine mixture. The arylalkynes were synthesized in very high yields (82-97%).99 Another type of coupling reaction that can be performed in a solvent-free environment is Glaser coupling. This copper-catalyzed coupling of two terminal alkynes produces diacetylene derivatives, which are very important in the polymer and material science industries. Phenylacetylene and copper chloride on potassium fluoride/alumina, coupled with microwave irradiation, give diphenylbutadiyne in a 75% product yield (Scheme 15).
Scheme 14
Scheme 15
The Baylis-Hillman reaction is an important carbon–carbon bond forming reaction that forms multifunctional molecules. In this reaction, an aldehyde reacts with an electron deficient alkene to yield allylic alcohol derivatives. Isomerization of acetylated Baylis Hillman adducts will yield (E)-trisubstituted alkenes, which are often difficult to synthesize. Microwave irradiation of the functionalized acetates on montmorillonite K10 clay yields trisubstituted alkenes in 13 minutes (Scheme 16).130 The clay acts as a catalyst, since only starting material is recovered in its absence.
Scheme 16
Another pioneer in microwave-assisted solvent-free reactions is Andre Loupy.5,8,45-63,297,310,351,416,425,439,472,491,501,505-507, 522,568,590, 607,657,708,709 One important reaction that is used frequently in natural product syntheses is the Beckmann rearrangement. This reaction rearranges ketoximes to amides or lactams in the presence of acid. Traditionally, very strong acids are used to promote the rearrangement. Loupy and co-workers have performed facile Beckmann rearrangements on montmorillonite K10 clay under microwave irradiation in high yields (68-96%) (Scheme 17).60 Another microwave reaction performed by Loupy et al. in a solventless environment is carbohydrate glycosylation. Scheme 18 illustrates the glycosylation of peracetylated D-glucopyranose with decanol.54
Scheme 17
Scheme 18
Varma and co-workers have performed extensive research on microwave-assisted, solvent-free reactions in numerous areas including oxidations, reductions, protections, deprotections, and condensations.20,84-98 Many of these are discussed in Chapter 4 and include additional references. Another area of interest is the enamine-mediated approach to isoflav-3-ene synthesis. Enamines are traditionally synthesized via azeotropic removal of water and usually require an initial acid catalyst. Scheme 19 shows a microwave-enhanced, solvent-free, one-pot synthesis to isoflav-3-ene derivatives, which takes place in only seven minutes.98 An efficient microwave-induced tetrahydroquinolone synthesis effected on clay is completed in only two minutes (Scheme 20).96
Scheme 19
Scheme 20
Didier Villemin is yet another researcher who has examined reactions in dry media extensively. Metallophthalocyanines have become important molecules in the material science industries, as they are stable to strong acids and bases, as well as high temperatures. Traditional synthetic routes to phthalocyanines require long reaction times and very high temperatures. Villemin and co-workers have performed one-step metallophthalocyanine syntheses on clay, zirconium phosphate, and encapsulated in zeolite via microwave irradiation (Scheme 21).132 These reactions were completed in only five minutes and in quantitative yields.
Scheme 21
Solid-liquid-phase transfer catalysis is another type of solvent-free reaction. With this method, a reagent acts as both a reactant and an organic phase. Microwave irradiation has been used extensively in these types of reactions.8,50,66,351,472,483,490,491,505,593 An inexpensive and useful phase transfer catalyst (PTC) is polyethylene glycol (PEG). Medium to high molecular weight PEG is a solid at room temperature, but at 50 °C, it melts to become a liquid. At temperatures above 50 °C, derivatized PEG can be used as a soluble polymeric support in the solution phase, but when cooled to room temperature, it becomes solid and provides for simple purification. Scheme 22 exhibits a PEG-supported alkyl-ation of a Schiff base to aminoacid derivatives under microwave irradiation in 75-98% yield.166
Scheme 22
Another useful PTC for microwave-assisted reactions is poly(styrene-co-allyl alcohol) (Ps-OH). This support possesses the properties of both PEG and polystyrene. Vanden Eynde and Rutot rapidly synthesized heterocyclic compounds via supported β-keto esters, with the first step only taking five minutes (Scheme 23).41 The parent polymer can be regenerated from the resulting acylated polymer by saponification.
Scheme 23
Andre Loupy has also done some interesting research involving phase transfer catalysis coupled with microwave irradiation.8,50,62,351,472,491,505,593 β-Elimination of halogenated precursors, with potassium t-butoxide/tetrabutylammonium bromide (KOtBu/TBAB) as the PTC, provides a new route to ketene O,O- and S,S-acetals (Scheme 24).62 Compared to both conventional and ultrasonic methods, microwave irradiation produced much larger product yields.
Scheme 24
Another area of interest includes the synthesis of furan diethers. These types of compounds constitute a large percentage of the derivatives that make up biomass, a renewable source of natural products. Loupy and co-workers developed two methods of microwave-assisted phase transfer catalysis for furan synthesis, solid-liquid PTC (solid KOH and Aliquat 336) and liquid-liquid PTC (aqueous KOH and Aliquat 336).351 Scheme 25 shows the reaction between 2,5-furandimethanol and an alkyl halide by both PTC methods. Phase transfer catalysis can also benefit the reaction between furfuryl alcohol and a dihalide (Scheme 26).
Scheme 25
Scheme 26
Performing a reaction neat under microwave irradiation is the third type of solvent-free reaction. With this method, neither a mineral oxide nor a PTC is used, and the liquid or solid reagents are used directly from their containers with no dilutions. One interesting neat reaction utilizes Lawesson’s reagent, which transforms a carbonyl moiety into its thio analog. Scheme 27 exhibits the microwave-induced conversion of amides to thioamides in six minutes.88,165 An additional microwave example converts coumarins and other lactones to their thio derivatives in only 3 minutes with quantitative product yields (Scheme 28).88
Scheme 27
Scheme 28
Substituted 2-oxazolines are important heterocyclic intermediates used in drug discovery. Classical syntheses of these compounds require high temperatures, azeotropic water removal, and multi-step procedures. With microwave irradiation, 2-oxazolines are synthesized from the cyclodehydration reaction between a carboxylic acid and α,α,α-tris(hydroxymethyl) methyl amine without any solvent or solid support in 2-5 minutes (Scheme 29).57
Scheme 29
Both Diels-Alder and 1,3-dipolar cycloadditions benefit from microwave-assisted neat conditions, as they require long reaction times and very high thermal temperatures. In a solventless environment, vinylpyrazoles react with substituted alkynes to yield non-aromatic cycloadducts via microwave irradiation in 15 minutes (Scheme 30).214 Schemes 31 and 32 illustrate successful 1,3-dipolar cycloadditions that yielded heterocycles in very high product yields.8,364
Scheme 30
Scheme 31
Scheme 32
Thus, the two main types of conditions used for chemical reactions, those run in the presence of solvent and those run in a solventless environment, are equally important and both can benefit from microwave heating. We have seen that microwave irradiation is not only applicable to standard homogeneous reaction mediums, but to solid-phase systems as well. Most synthetic methods can be executed by at least one of these systems. In conjunction with the following synthesis chapter, a chemist can now develop optimal and efficient synthetic routes.
Instruments
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106. Kidwai, M.; Sapra, P. “An expeditious solventless synthesis of isoxazoles.” Org. Prep. Proced. Intl. 2001, 33, pp. 381-86.
107. Kidwai, M.; Sapra, P.; Misra, P.; Saxena, R.K.; Singh, M. “Microwave-assisted solid support synthesis of novel 1,2,4-triazolo[3,4-b]-1,3,4-thiadiazepines as potent antimicrobial agents.” Bioorg. Med. Chem. 2001, 9, pp. 217-220.
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110. Kidwai, M., Bhushan, K.R.; Sapra, P.; Saxena, R.K.; Gupta, R. “Alumina-supported synthesis of antibacterial quinolines using microwaves.” Bioorg. Med. Chem. 2000, 8, pp. 69-72.
111. Kidwai, M.; Venkataramanan, R.; Kohli, S. “Alumina-supported synthesis of β-lactams using microwave.” Synth. Commun. 2000, 30, pp. 989-1002.
112. Kidwai, M.; Misra, P.; Dave, B.; Bhushan, K.R.; Saxena, R.K.; Singh, M. “Microwave-activated solid support synthesis of new antibacterial quinolones.” Monatsh. Chem. 2000, 131, pp. 1207-12.
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122. Maree, M.D.; Nyokong, T. “Solvent-free axial ligand substitution in octaphenoxyphthalocyaninato silicon complexes using microwave irradiation.” J. Chem. Res. (S) 2001, pp. 68-69.
123. Massicot, F.; Plantier-Royon, R.; Portella, C.; Saleur, D.; Sudha, A.V.R.L. “Solvent-free synthesis of tartramides under microwave activation.” Synthesis 2001, pp. 2441-44.
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126. Quiroga, J.; Cisneros, C.; Insuasty, B.; Abonia, R.; Nogueras, M.; Sanchez, A. “A regiospecific three-component one-step cyclocondensation to 6-cyano-5,8-dihydropyrido-[2,3-d]pyrimidin-4(3H)-ones using microwaves under solvent-free conditions.” Tetrahedron Lett. 2001, 42, pp. 5625-27.
127. Rajakumar, P.; Murali, V. “Sodium sulfate supported synthesis of cationic cyclophanes using microwaves.” J. Chem. Soc., Chem. Commun. 2001, pp. 2710-11.
128. Romanova, N.N.; Kudan, P.V.; Gravis, A.G.; Zyk, N.V. “Investigation of the stereochemistry of rapid solvent-free microwave syntheses of β-amino acid esters.” Fifth International Electronic Conference on Synthetic Organic Chemistry (ECSOC-5) 2001, E0018 (www.mdpi.net).
129. Romanova, N.N.; Gravis, A.G.; Kudan, P.V.; Bundel, Y.G. “Solvent-free stereoselective synthesis of β-aryl-β-amino acid esters by the Rodionov reaction using microwave irradiation.” Mendeleev Commun. 2001, pp. 26-27.
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