Homogeneous reaction conditions

The main focus of this account is to review some aspects of the chemistry of cellulose esters. Emphasis is placed on the esterification reaction, carried out under the homogenous reaction conditions (HRC) scheme. Unconventional methods for the synthesis of cellulose derivatives, e.g., esters and ethers... 

Ruiz N (2004) Acylation of Cellulose Under Homogeneous Reaction Conditions. MSc Thesis, University of Sao Paulo, BrazU... 

The results are comparable to those of homogeneous reaction conditions (Table 41.10), and recycling of the catalyst was successful with constant ee-values over five cycles, even though conversion decreased. Amazingly, the catalyst was still active, despite being stored under atmospheric conditions for 24 h (Table 41.10, entry 7). 

A library of compounds prepared under homogeneous reaction conditions without the aid of polymeric supports... 

The idea to use solvent systems enabling homogeneous reaction conditions at elevated temperatures and liquid/liquid phase separation at lower— preferably room—temperature seems to be obvious. Nevertheless, it is only recently that thermomorphic solvent systems gain attention [30-33] for product separation or multiphase catalysis [34,35]. The main reasons for the delayed engagement is that an efficient choice of a useful solvent system is not easy to achieve. There is also a lack of experience with thermomorphic systems in general. Reactions are optimized to be carried out in solvents having certain distinct solubility and polarity characteristics. A thermomorphic solvent system of choice will have to fulfill these requirements and to show the thermomorphic effect in addition. 

The reaction of aldehyde hydrazones is more complex and requires further study [103]. It is usually carried out with a 90% solution of lead tetraacetate in acetic acid with sufficient additional solvent to bring about homogeneous reaction conditions initially. 

The ring-opening of oxiranes, leading to the formation of isomeric carbonyl compounds by the action of acid catalysts as a result of rearrangement, is of continuing interest (refs. 1-4). However, most of these studies focus mainly on the transformations of terpene oxides or oxiranes with other functional groups in the liquid phase, under homogeneous reaction conditions. 

Likewise, yet in a more pronounced manner, a kaolinite, another clay with a strongly acidic surface (Ho = -3 to -6) (ref. 2), also favors ipso nitration the yield of p-nitroanisole i rises to 27%, boosted by a factor 20 as compared to homogeneous reaction conditions ... 

Another important aspect of our acceptor-bound strategy was the delivery of donors in solution. In selecting donors we considered their stability, reactivity, ease of synthesis, and activation under homogeneous reaction conditions. The use of insoluble reagents (such as powdered molecular sieves) would not be compatible with an automated instrument. 

Whereas the results summarized in Scheme 4.32 were achieved under homogeneous reaction conditions, Colonna et al. reported the use of chiral phase-transfer catalysts for asymmetric addition of benzyl mercaptan and thiophenols to cyclohexenone and derivatives [55b], The best result was 85% yield and 36% enantiomeric excess in the addition of thiophenol to cyclohexenone, catalyzed by ca 0.4 mol% N-(o-nitrobenzyl)quininium chloride at 25 °C. In this experiment, CCI4 served as solvent and solid KF as the base. Finally, Aida et al. reported in 1996 that chiral... 

To explain this different behaviour, mechanistic studies of the system were performed with the aid of molecular modelling, 13C NMR spectroscopy, induced circular dichroism, a systematic variation of the reaction parameters, and variation of the molecular structure of the sugar moieties and of the linking units. These studies established that under homogeneous reaction conditions (water), the main factor influencing the enantioselectivity is probably the ordering and specific orientation of the ketone at the chiral interface. Under heterogeneous conditions in THF the situation appears to be more complex. 

Dendritic polymers feature homogeneous reaction conditions and enable the application of standard analytical techniques (TLC, IR, NMR, MALDI-TOF etc.) as well as the orthogonal use of insoluble reagents. One drawback of... 

Intramolecular heterolysis of H2 with elimination of HX is commonly observed under homogeneous reaction conditions as shown in Eq. (13) (55-57). 

Thus polystyryl carbanions and polyacrylonitrile carbanions prepared by anionic polymerization were reacted with cellulose acetate or tosylated cellulose acetate in tetrahydrofuran under homogenous reaction conditions. The carbanions displaced the acetate groups or the tosylate groups in a S v2-type nucleophilic displacement reaction to give CA-g-PS and CA-g-PAN. Mild hydrolysis to remove the acetate/tosylate groups furnishes the pure cellulose-g-polystyrene (Figure 3). 

In an extension of this work, either zinc(II), palladium(II), rhodium(I) or copper(I) salts were immobilised in an ionic liquid film (SILP, vide supra) onto diatomic earth and the catalysts tested for activity in the reaction between phenylacetylene and 4-isopropyl-phenylaminc.1 391401 The supported rhodium, ruthenium and zinc complexes afford higher rates and selectivities relative to their use under homogenous reaction conditions. Lower rates are, however, observed with the copper salt, which is rationalised by strong complexation of the ionic liquid to the Cu(I) centre. 

The previous chapters have shown how combinatorial technologies have always been associated with chemistry in the SP and how this chemistry is flexible enough to provide libraries in many formats. More recently, there has been growing interest in libraries of small organic molecules prepared under homogeneous reaction conditions. These libraries are usually called solution-phase libraries and will be discussed in this chapter to demonstrate how they present a useful alternative to SP libraries and how they can provide the chemist with more options with regards to the choice of library format. 

Resin beads, which can be considered as individual microreactors, allow the use of mix-and-split techniques, which are not applicable when using homogeneous reaction conditions. 

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]. 

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