Reactions without solvent

The solvents used for preparing carbonates from alcohols and CDI are THF, DMF, benzene, toluene, chloroform, 2-butanone, and pyridine. It is also possible to carry out the reaction without solvent. A collection of acyclic carbonates is given in Table 3-12. 

This chapter will deal with applications of microwave irradiation in the synthesis of heterocycles by a variety of means, excluding cycloadditions, which will be described in the next chapter. We have chosen to report first reactions in solution in organic solvents, then heterogeneous reactions without solvent under a variety of conditions, and finally to deal with emerging techniques which employ ionic liquids. 

Quite recently this reaction was revisited by Kappe [91] et al. who reinvestigated the Biginelli synthesis under the action of microwave irradiation under a variety of different conditions. At atmospheric pressure in ethanol solution there is no difference from conventional heating. Under pressure the yield is reduced and byproducts are formed. In an open system rate and yield enhancements are significant and this is rationalized by the rapid evaporation of the solvent which means that this is in fact a solvent-free reaction. This was confirmed by running the reaction without solvent under the action of microwaves and with thermal heating. (Scheme 8.65)... 

Traditional Heck arylation of the corresponding ethyl vinyl ether afforded high yields with most of the aryl bromides investigated (Eq. 11.11). Under continuous singlemode microwave treatment the transformations were complete within 10-12 min [25], Heck reactions without solvent in a domestic microwave oven have been examined by Diaz-Ortiz [26]. The reactions were conducted in closed vessels with reported temperatures of 150 °C. A study was performed in which reactions performed with microwave irradiation were compared with oil-bath-heated reactions with identical reaction times and temperatures. The isolated yields tended to substantially favor the microwave-heated reactions (Eq. 11.12). 

Palkovits et al. took a different approach to the telomerization of 1,3-butadiene with glycerol, screening various phosphine ligands and running the reaction without solvent. The importance of the steric and electronic properties of the phosphine ligand on activity and selectivity of the palladium catalyst has been highlighted in... 

Increasing reaction concentration (and with it plant productivity). An ultimate, if rarely achievable, goal would be to run the reaction without solvents. 

M. R. Naimi-Jamal, etal., Reactive Milling with the Simuloyer Environmentally Benign Quantitative Reactions without Solvents and Wastes, Chem. Engng. Sd., 2002b,... 

Recently, gold has emerged as one of the most active catalysts for alcohol oxidation and is especially selective for poly alcohols. In 2005, Corma [184] and Tsu-kuda [185], independently demonstrated the potential of gold nanoparticles for the oxidation of aliphatic alcohols. For example, in the case of gold nanoparticles deposited on nanocrystalline cerium oxide [184], a TOF of 12 500 h 1 was obtained for the conversion of 1-phenylethanol into acetophenone at 160 °C (Fig. 4.67). Moreover this catalyst is fully recyclable. Another example of a gold catalyst with exceptional activity is a 2.5% Au-2.5% Pd/Ti02 as catalyst [186]. In this case for 1-octanol a TOF of 2000 h-1 was observed at 160 °C (reaction without solvent, Fig. 4.67). 

It is noteworthy that, even in the absence of solvent, the esterification reaction proceeds easily at high temperatures (60-200°C) [74]. The esterification reactions without solvent, as compared with those with the solvent, are industrially advantageous and give esterified woods with a wide range of monoester contents [74]. The hygroscopicity and initial weight loss temperature of the esterified wood meals decrease with increase in the monoester content [76]. 

Zhu A, Jiang T, Han B, Huang J, Zhang J, Ma X (2006) Study on guanidine-based task-specific ionic liquids as catalysts for direct aldol reactions without solvent. New J Chem 30 736-740... 

The results of the hydrosilylation reactions carried out without solvent and in a solution of benzene, indicate that the use of a solvent decrease markedly the turnover rates for homogeneous catalysts, however the reactivity order for different silanes and selectivities founded are not sensibly modified. Thus, for more cile and precise comparison between homogeneous and heterogenised catalysts, in where the reaction without solvent was more difficult to standardise, we have employed 1ml of benzene as solvent in the following reactions. 

Some reactions can be quite exothermic. Before running a reaction without solvent, it would be best to check the heat of reaction by thermal analytical methods. This will avoid runaway reactions. A thermoregulator should also be used to control the temperature. 

Extruders are in common use for the processing of polymers, as in the spinning of synthetic fibers. They are also used for the extrusion cooking of foods, such as ready-to-eat breakfast cereals. They are not part of the standard equipment of the usual chemical laboratory, yet they offer considerable promise for running reactions without solvent on a continuous basis. 

Dibutyltin dichloride can be used as a catalyst in the addition reaction without solvent Trimethylsilyl bis(fluorosulfonyl)imide is also an efficient catalyst promoting the derivatization at -78° (11 examples, 84-98%). ... 

The reduction of a,p-acetylenic ketones is accomplished in 24-48 h at room temperature by using twofold excess of NB-Enantrane and running the reaction without solvent. Both chemical and enantiomeric yields are high (Table 26.14) and provide (S)-propargyl alcohols. The reduction fits the steric model proposed for Alpine-Borane reduction [3]. Nopol benzyl ether liberated after the reduction maybe easily isolated during purification of the product and recycled. 

From these reports and by comparison with the conditions known to induce the Amadori rearrangement, it can be concluded that Schiff had already obtained the rearrangement products by carrying out the reaction without solvent and at elevated temperatures. Sorokin, on the other hand,preparing his products in an alcoholic solution and at lower temperatures, was able to isolate the comparably less stable glycosylamines. Sachsse would have found both, the unstable glycosylamine and the stable 1-aminodeoxyketose, depending on the reaction conditions employed. 

Reaction condition [LSHJ/[RGJ = 1/1.5 [NJ/[LSH]=1/10, reaction without solvent at 35°C. All products were converted in Methyl-/3-thiolaurylbutyrate (MTB), and optical activity of this product, reported in the Table, used to compare optical yields. 

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