Heterogeneous reaction conditions

Solid-phase organic synthesis (SPOS) exhibits several shortcomings, due to the nature of the heterogeneous reaction conditions. Nonlinear kinetic behavior, slow reactions, solvation problems, and degradation of the polymer support due to the long reaction times are some of the problems typically experienced in SPOS [2], Any technique which is able to address these issues and to speed up the process of solid-... 

The formation of linear alkylbenzene (LAB) by Friedel-Crafts alkylation is strongly influenced by homogeneous or heterogeneous reaction conditions (Table 3.5).7 The steric bulk of silica reduces over-alkylation and shelf life. Silica also catalyses the formation of ethers. 

The use of polymeric, insoluble protecting groups in combinatorial or parallel synthesis that allow for the facile separation of target compounds from the rest of the reaction mixture under heterogeneous reaction conditions. Final target compounds may be assayed either while still attached to the resin beads or following cleavage... 

Discuss the use of homogeneous versus heterogeneous reaction conditions for the coordination and traditional Ziegler-Natta polymerizations of propene, isoprene, styrene, methyl methacrylate, and n-butyl vinyl ether. 

The low temperature ( 140°C) anionic ring opening polymerization is further complicated by the crystallinity in nylon 6. Magill [66] has reported that the temperature for maximum crystallization rate in nylon 6 is about 140-145°C. The nucleation rate is low above 145°C, whereas viscous effects hinder crystal growth below this temperature. As a result, at about 140-145°C, heterogeneous reaction conditions can be encountered (as we have seen in our studies) if there is simultaneous polymerization of caprolactam and crystallization of the nylon 6 formed. 

Chemat, F., Poux, M. and Galema, S.A., Esterification of stearic acid by isomeric forms of butanol in a microwave oven under homogeneous and heterogeneous reaction conditions, /. Chem. Soc., Perkin Trans. 

Scheme 8.19 Diastereocontrolled Darzens condensations under heterogeneous reaction conditions.

In water, the highest stereoselectivities were obtained by using the fourth-generation amphiphilic dendrimer, whereas, under heterogeneous reaction conditions in THF as solvent, the third-generation dendritic catalyst proved to be the most selective. 

Rhodium, " titanium,and tungsten " complexes have also been used for this reaction. The reaction can be promoted photochemically and the rate is enhanced by the presence of primary amines.Coordinating ligands also accelerate the reaction,polymer-supported promoters have been developed " and there are many possible variations in reaction conditions.The Pauson-Khand reaction has been done under heterogeneous reaction conditions, and with... 

Styrene only forms an isotactic polymer under heterogeneous reaction conditions. This is the case with sodium or potassium alkyls [185,186]. It has been reported that the lithium alkyls produce isotactic polystyrene under homogeneous reaction conditions, but later investigation showed that the presence of lithium hydroxide or oxide is necessary [187]. 

The ferrio-siloxanes 3a,b can be easily transformed into the corresponding bis-metalated species 8a,b by treatment with a second equivalent of sodium ferrate 2a under heterogeneous reaction conditions. 

Two methods which reduce or avoid 0-alkylation and employ heterogeneous reaction conditions include (i) the heating of crystalline thallium(I) enolates of B-keto esters or B-diketones with primary or secondary alkyl iodides and (ii) the heating of stabilized sodium enolates with methyl- or ethyl-sulfo-nium salts.2 However, the reproducibility and generality of the former method has been questioned. ... 

Little information is available on the mechanism of the traditional Ullmann condensation reactions. The difficulties in drawing mechanistic conclusions arise from the reactions being conducted at high temperatures (>150 °C) and the typically heterogeneous reaction conditions. Because the mechanism is not yet known in detail, improvements in this reaction have been gained empirically, instead of from theory. The mechanistic proposals for these reactions remain speculative. 

Examples of high polydispersity ratios and bimodal distributions have been previously rationalized by heterogeneous reaction conditions. Tirrell et al.(19) found Mw/Mn values well over 2.0 and low molecular weight shoulders in slow, low temperature RIM polymerized urethanes based on polycaprolactone diols. Xu et al. 

FIGURE 4.8. By-products formed in a Fischer indolization due to heterogeneous reaction conditions... 

IV. C. Choosing Solvents to Provide Heterogeneous Reaction Conditions... 

In this context our interest was focused on sulfonic acid esters of starch, derived fh)m the reaction of organic sulfonyl chloride (e.g. benzene, methane, or toluene sulfonyl chloride). These derivatives can be employed as partially protected and reactive intermediates. Up to now, especially the reaction of starch with p-toluenesulfonyl chloride in pyridine was studied, i.e. under heterogeneous reaction conditions, which may be accompanied by several side reactions. Alternatively, a homogeneous procedure was published using dimethyl sulfoxide as solvent for starch. However, the sulphur contents of the products were very low and an extensive degradation of the polymer occured . The extent of / toluenesulfonyIation of primary and secondary groups was determined by the iodination method. It was revealed that the reaction proceeds faster at 0-6 than at 0-2 and 0-2... 

As discussed above, the preparation of cellulose derivatives historically, and at present at least on a commercial scale, involve heterogeneous reaction conditions. This was due principally to the intractable nature of cellulose, its insolubility in polar solvents and that the original solvents for cellulose did not yield tree solutions. 

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