Solvent-free transformations

According to the atom-economy principle included in the 12 principles of Green Chemistry, protocols where all the materials used in the process are incorporated in the end product are highly desirable. In this context, multicomponent reactions, which perform with good atom economy, are interesting reactions. Gold-catalysed multicomponent reactions under solvent-free conditions have been extensively studied. 

Nolan and coworkers have recently reported the gold-catalysed transformation of allqmes and allylic alcohols to homoallylic ketones. This reaction involved the hydroalkojqrlation of the allq ne followed by Claisen rearrangement. It proceeded in solvent-free conditions, at veiy low catalyst loading. Internal and terminal allq nes were successfully used as well as different allylic alcohols. The recyclability of the catalyst was studied by conducting the reaction with model substrates (diphenyalcetylene and allyl 

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This transformation can also be carried out under solvent-free conditions in a domestic oven using acidic alumina and ammoniiun acetate, with or without a primary amine, to give 2,4,5-trisubstituted or 1,2,4,5-tetrasubstituted imidazoles, respectively (Scheme 15A) [69]. The automated microwave-assisted synthesis of a library of 2,4,5-triarylimidazoles from the corresponding keto-oxime has been carried out by irradiation at 200 ° C in acetic acid in the presence of ammonium acetate (Scheme 15B) [70]. Under these conditions, thermally induced in situ N - O reduction occurs upon microwave irradiation, to give a diverse set of trisubstituted imidazoles in moderate yield. Parallel synthesis of a 24-membered library of substituted 4(5)-sulfanyl-lff-imidazoles 40 has been achieved by adding an alkyl bromide and base to the reaction of a 2-oxo-thioacetamide, aldehyde and ammonium acetate (Scheme 15C) [71]. Under microwave-assisted conditions, library generation time was dramatically re-... 

Tetrasubstituted pyrroles were also obtained in a coupled domino process carried out under solvent-free conditions on silica gel (Scheme 9). The process involved the transformation of the alkynoate 27 into the 1,3-oxazoUdine 28 that could be further rearranged (through loss of one molecule of water)... 

Macromolecular Rapid Communications 22,No.l6, 7thNov. 2001, p.1325-9 ORGANIC SOLVENT-FREE ENZYMATIC TRANSFORMATION OF... 

Moriya et al. first demonstrated that anisotropic structure in blends can be generated by applying ac electric fields to solvent-free blends of PEO and PS [60]. A similar morphology transformation induced by electric fields has been detected in PEO/PS/cyclohexane systems [61-63]. During solvent evaporation,... 

A new synthetic route for functionalized polyhydroxyalkyl-pyrimidines starting from unprotected aldoses and based on montmorillonite K-10 catalysis and solvent-free microwave irradiation conditions, has been reported by Yadav et al,m Thus, reaction of D-glucose and D-xylose with semicarbazide or thiosemicarbazide (186) in the presence of montmorillonite K-10, under microwave irradiation, proceeded via domino cycloisomerization, dehydrazination, and dehydration of the intermediate semi- or thiosemicarbazones (187) to afford l,3-oxazin-2-ones or l,3-oxazine-2-thiones (188) in one single step and in yields between 79% and 85% (Scheme 34). Other mineral catalysts tested, such as silica gel and basic alumina, were far less effective for this transformation and only silica gel was active at all, giving low yields (15-28%) of compounds 188a-d. The l,3-oxazin-2-ones(thiones) thus synthesized were subsequently converted into the target pyrimidines by reaction with aromatic... 

H. Kotsuki, T. Shimanouchi, R. Ohshima, and S. Fujiwara, Solvent-free organic reactions on silica gel supports. Facile transformation of epoxides to /i-halohy-drins with lithium halides, Tetrahedron, 54 (1998) 2709-2722. 

In 2001, Linder and Podlech studied the microwave-assisted decomposition of diazoketones derived from a-amino acids [340]. In the presence of imines, the initially formed ketene intermediates reacted spontaneously by [2+2] cydoaddition to form /3-lactams with a trans substitution pattern at positions C-3 and C-4 (Scheme 6.179) [340], In order to avoid the use of the high-boiling solvent 1,2-dichlorobenzene, most transformations were carried out in 1,2-dimethoxyethane under sealed-vessel conditions. Solvent-free protocols, in which the substrates were adsorbed onto an inorganic alumina support, led only to the corresponding homologated /3-amino acids. Evidently, traces of water present on the support trapped the intermediate ketene. 

Scheme 6.186) [347]. The condensation of O-allylic and O-propargylic salicylalde-hydes with a-amino esters was carried out either in the absence of a solvent or - if both components were solids - in a minimal volume of xylene. All reactions performed under microwave conditions rapidly proceeded to completion within a few minutes and typically provided higher yields compared to the corresponding thermal protocols. In the case of intramolecular alkene cycloadditions, mixtures of hexa-hydrochromeno[4,3-b]pyrrole diastereoisomers were obtained, whereas transformations involving alkyne tethers provided chromeno[4,3-b]pyrroles directly after in situ oxidation with elemental sulfur (Scheme 6.186). Independent work by Pospisil and Potacek involved very similar transformations under strictly solvent-free conditions [348]. 

More recently [29] the microwave-mediated Biginelli dihydropyrimidine synthesis (Eq. 2) was reinvestigated using a purpose-built commercial microwave reactor with on-line temperature, pressure, and microwave power control. Transformations performed with microwave heating at atmospheric pressure in ethanol solution resulted in neither a rate increase nor an increase in yield when the temperature was identical to that used for conventional thermal heating. The only significant rate and yield enhancements were found when the reaction was performed under solvent-free conditions in an open system. 

An efficient ring-expansion transformation has also been described under solvent-free conditions (Scheme 6.26) [92]. This microwave procedure is superior to the same reactions conducted in traditional methanolic solution. 

Modification of Substituents and Functional Groups of Cyclic Nitronates At an ambient temperature, four-membered cyclic nitronate containing the chloronitromethyl group at the C-3 atom is gradually transformed into the corresponding acid chloride even under solvent-free conditions, and treatment of the latter with aqueous ammonia affords amide (168) (Scheme 3.119, Eq. 1). 

Due to its Lewis acidic properties, the use of chloral (trichloroacetaldehyde) in the Beckmann rearrangement was investigated . When a variety of ketoximes is admixed with chloral hydrate and the mixture is heated at low pressure in nitrogen atmosphere, the Beckmann rearrangement afforded the corresponding amides in excellent yields (73-98%). The transformation occurs under neutral, relatively mild and solvent-free conditions. 

The Cannizzaro reaction, that is, the base-catalysed disproportionation of a carbonyl compound to an alcohol and a carboxylic acid, has gained some importance as an economically viable alternative to the reduction with borohydrides. However, the reaction is restricted to carbonyl compounds without any a-hydrogen, which do not undergo competing aldol reactions. Thus, mainly aromatic aldehydes are used for this kind of transformation. The protocols developed for microwave applications typically involve solvent-free conditions using alumina as the solid support. Under these conditions, a significant acceleration of the reaction was achieved. 

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