Catalyst-free reactions

MW irradiation conditions [80]. More recently, however, Varma and Kocevar s group have shown that a solvent-free and catalyst-free reaction of hydrazines with carbonyl compounds is possible upon MW irradiation (Scheme 6.24) [81]. Interestingly, the general reaction proceeds smoothly even for solid reactants and is completed below the melting points of the two reactants possibly via the formation of a eutectic. The reactions have been conducted in household MW oven and the control experiments are conducted concurrently in separate open beakers the reactions can be essentially followed by visual observation when a melt is obtained [82],... 

Kumaravel K, Vasuki G (2009) Four-component catalyst-free reaction in water combinatorial library synthesis of novel 2-amino-4-(5-hydroxy-3-methyl-lH-pyrazol-4-yl) H-chro-mene-3-carbonitrile derivatives. Green Chem 11 1945-1947... 

The catalytic action of the sodium salicylate can be clearly shown by the increase in brightness of the chemoluminescence when tiny amounts of the basic salt are added to the catalyst-free reaction mixture. The light intensity decreases considerably when the reaction mixture is cooled in an ice bath and increases when it is warmed in a water bath. 

A one-pot solvent and catalyst free reaction of amine, CS2 and alkyl halides at room temperature presents a very simple and green route to the synthesis of dithiocarbamates (Scheme 5.49). Significantly, higher yields were obtained in neat compared to those in organic solvents.Certainly, this procedure is of much importance in pharmaceutical and agrochemical industries. 

Catalyst-free reactions at supercritical conditions have also been employed for transesterification (Bunyaldat et al 2006 Demirbas, 2002 2003 Cao et al., 2005 He et al., 2007a 2007b Kusdiana Saka, 2001 Minami et al., 2006 Varma Madras, 2007 Wang Yang, 2007 Warabi et al., 2004). 

Interesting is the development of concept of copper-free Sonogashira reaction in which Cul catalyst was not used (Scheme 2.15). In these reactions, coupUng products 47 in various yields were still obtained (Table 2.14), probably due to the presence of trace amounts of copper in palladium reagent. When copper catalyst-free reaction was carried out with copper milling balls as a source of copper, yields increased to 31-88% (Table 2.15). Finally, copper vials in combination with copper balls as a source of copper afforded respectable yields in the range of42-90% (Table 2.16). 

The original Biginelli procedure has been modified extensively to increase the overall yields, including the addition of Lewis acid catalyst,application of polyphosphate ester or fluorous solvent, and introduction of microwaves " and even solvent-free or catalyst-free reaction conditions. 

By supplying heat and an alcohol to a carboxyl acid, an ester can be formed. This catalyst-free reaction is desirable because there is no catalyst to be separated from the reaction medium and products. To boost the reaction and avoid the use of thermal energy, these reactions are commonly catalyzed by acid or base compounds. 

The inverse electron demand Diels-Alder cycloaddition (iEDDA) reaction between strained alkenes and tetrazine derivatives has gained more and more attention for orthogonal labeling of biomolecules in the last years (Fig. IE) [63]. This catalyst-free reaction can be extremely rapid in the case of tranr-cyclooctenes as reactants with second order rate constants of up to 380,000 s in aqueous solutions... 

Four years later, Palasz showed the catalyst-free reaction in aqueous suspension without acetonitrile to be mostly cis-selective at ambient temperature [31]. The reactions of 1,3-dimethyl barbituric acid 61 or thiobarbituric add 65 with... 

Jerome et al. reported the catalyst-free reaction of mostly acyclic 1,3-dicarbonyl compounds with formaldehyde 72 and styrene derivatives in water at elevated temperatures... 

A reaction that is set to gain momentum within the field of macromolecular science over the next few years is the cycloaddition of nitrile oxides with alkynes and norbornenes to form isoxazoles and isoxazolines respectively (Scheme 2.6). Whilst strictly a catalyst-free reaction, the in situ generation of the requisite nitrile oxide from the respective oxime precursor typically makes use of a mixture of a dipole generating agent (such as Chloramine-T) and a weak base (such as aqueous NaHC03). 

It is worth noting that the reaction of metals with functional alcohols forming stable complexes with metal cations is most often much more facile than with the aliphatic ones. Both alkaline earth (Mg, Ca, Sr) and rare earth elements do react with alkoxy alcohols such as 2-methoxy-ethanol (now prohibited from use in European Union because of its toxicity) on reflux without the need of addition of a catalyst. Even tin can be converted into Sn(II) alkoxides by a catalyst-free reaction with amino alcohols under high temperatures and pressures [112]. 

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